Experimental Characterization of an Ion Transport
Membrane (ITM) Reactor for Methane Oxyfuel
Combustion
by
Daniel Jolomi Apo
B.Eng. Mechanical Engineering, University of Benin, Nigeria. 2007
Submitted to the Department of Mechanical Engineering
in partial fulfillment of the requirements for the degree of
ARCHIVES
Master of Science in Mechanical Engineering
at the
MASSACHUSETTS INSTITUTE OF TECHNOLOGY
February 2012
@
Massachusetts Institute of Technology 2012. All rights reserved.
Author ................
.............................
Department of Mechanical Engineering
January 3, 2012
, 1 j,
C ertified by .........................
YAV I(
Ahmed F. Ghoniem
Ronald C. Crane ('72) Professor
wisor
.T)S
/A
Accepted by.............................D
o
David E. Hardt
mom
Chairman, Department Committee on Graduate Theses
2
Experimental Characterization of an Ion Transport Membrane
(ITM) Reactor for Methane Oxyfuel Combustion
by
Daniel Jolomi Apo
Submitted to the Department of Mechanical Engineering
on January 3, 2012, in partial fulfillment of the
requirements for the degree of
Master of Science in Mechanical Engineering
Abstract
Ion Transport Membranes (ITM) which conduct both electrons and oxygen ions have been
investigated experimentally for oxygen separation and fuel (mostly methane) conversion
purposes over the last three decades. The fuel conversion investigations typically involve
converting methane to syngas or higher hydrocarbons. Over the past decade, ITMs have
received considerable interest in the industry for oxygen separation and production of syngas. There is also a possibility that the future of ITM industrial use lies with clean power
generation as long as stable ITMs are developed which separate oxygen from air and enable
reaction of methane to produce carbon dioxide, steam, and the heat which powers turbines.
This would hopefully provide CO 2 capture compatibilities at a lower financial and energetic
cost than 'conventional' methods.
In this thesis, an analysis of reported experimental ITM reactors is presented, with a view to
understanding the processes which govern the permeation of oxygen, conversion of methane
and production of desired gas species. The analysis shows that temperature and mass flow
influence the oxygen permeation within the reactor. Also, the influence of fuel/0 2 ratio on
fuel conversion and CO selectivity is discussed. The design and operation of a novel ITM
reactor for the experimental investigation of oxygen separation and oxyfuel combustion (a
relatively new ITM concept) is presented. The ITM reactor was designed with the aim of
providing insight into the use of ITMs for power generation. The reactor has provisions
for optical and spatial analysis. The reactor was used to conduct experiments using a
Lao.gCao. 1 FeO 33 (LCF) membrane. The results of experiments conducted are presented
to show the effects of temperature, mass transfer, and fuel on oxygen permeation, fuel
conversion and species selectivity. A comparison is made between the observed results and
reported values in literature.
Thesis Supervisor: Ahmed F. Ghoniem
Title: Ronald C. Crane ('72) Professor
Acknowledgments
I would like to thank my advisor, Professor Ahmed Ghoniem, for his guidance during my
Masters program. His advice and encouragement always proved invaluable for my success in
research.
I am grateful to my master's degree research mentor, Dr. Patrick Kirchen, from whom
I have gained a lot of research knowledge. I would also like to thank him for doing the
background work and design of the reactor on which this thesis is based.
I am grateful to my research colleagues in the reacting gas dynamics laboratory and other
friends at MIT for all the good times.
I'm grateful to Dr. Susumu Imashuku and Dr. Lei Wang of the MIT electrochemical
energy laboratory for their help with the initial testing of the membrane on which this thesis
is based.
I would like to thank my research and fellowship sponsors - KAUST, KFUPM, Ceramatec,
and TOTAL.
I'm grateful to my parents and siblings. They were my source of strength during difficult
times.
Finally, I'm grateful to GOD.
6
Contents
1 Introduction
1.1 The CO 2 Problem. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.2 Solutions Proffered . . . . . . . . . . . . . . . . . . . .
1.3 Thesis Focus . . . . . . . . . . . . . . . . . . . . . . . .
19
19
2 State of the Art
. . . . . . . . . . . . . . . . . ..
2.1 ITM Background.
2.1.1 Membranes and Oxygen Transport Membranes
2.1.2 Mixed Ionic-Electronic Conducting Membranes
2.1.3 ITM Concepts
2.1.4 Usage history and applications . . . . . .
2.1.5 Fundamentals of Oxygen Flux . . . . . .
2.2 Reactive ITM Applications . . . . . . . . . . . .
2.2.1 Oxidative Coupling of Methane . . . . .
2.2.2 Syngas Production . . . . . . . . . . . .
2.2.3 Oxymel Combustion . . . . . . . . . . .
2.2.4 Comparison of ITM methane applications
2.2.5 Reactive ITM challenges.. . . . . . ..
2.3 Methodology for ITM Reactor Characterization
2.3.1 Sealing and Leak Detection . . . . . . .
2.3.2 Experimental Methodology . . . . . . . .
2.3.3 Methods for Analysis . . . . . . . . . . .
2.4 Analysis of ITM Experimental Investigations . .
2.4.1 Temperature Analysis . . . . . . . . . .
2.4.2 Mass Transfer Analysis . . . . . . . . . .
2.4.3 Reactive Analysis . . . . . . . . . . . . .
2.5 Conclusions . . . . . . . . . . . . . . . . . . . .
25
26
26
29
32
34
37
45
46
47
49
50
51
52
53
55
57
60
65
69
72
74
.
3
.
.
.
.
.
.
.
Experimental Approach
3.1 Reactor Design and Installation
3.1.1 Reactor . . . . . . . . .
3.1.2
3.2
Reactor Enclosure.
. . .
3.1.3 Reactor Sealing . . . . .
Membrane . . . . . . . . . . . .
3.2.1
Membrane Details
. . .
. . .
89
3.3
3.4
3.5
3.6
3.2.2 Pre-operation Membrane Analysis . . .
Reactor Process Control and Instrumentation
3.3.1 Plumbing . . . . . . . . . . . . . . . .
3.3.2 Instrumentation . . . . . . . . . . . . .
3.3.3 Gas Chromatography . . . . . . . . . .
3.3.4 Heating . . . . . . . . . . . . . . . . .
3.3.5 Safety Provisions . . . . . . . . . . . .
Experimental Methodology and Procedures .
3.4.1 Overview of experimental procedures
3.4.2 Temperature Control . . . . . . . . . .
3.4.3 Inlet Gas Control . . . . . . . . . . . .
Methodology for Analysis . . . . . . . . . . .
3.5.1 Permeation-only Analysis . . . . . . .
3.5.2 Reactive Analysis . . . . . . . . . . . .
Conclusions . . . . . . . . . . . . . . . . . . .
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
90
92
96
97
103
104
107
110
112
113
113
114
115
117
123
4 Results and Analysis
4.1 Tem perature . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.2 M ass Transfer . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.3 Reactive Analysis . . . . . . . . . . . . . . . . . . . . . . . . .
4.3.1 Effect of Fuel Inlet Fraction. . . . . .
. . . . . . ..
4.3.2 Oxymel Analysis . . . . . . . . . . . . . . . . . . . . .
4.3.3 Influence of Reactive (reducing) Sweep Gas on Oxygen Flux
4.4 Comparison with other ITM Experimental Investigations . . .
4.4.1 Temperature Comparison. . . . . . . . . . .
. . ..
4.4.2 Mass Transfer Comparison . . . . . . . . . . . . . . . .
4.4.3 Reactive Comparison . . . . . . . . . . . . . . . . . . .
4.5 Conclusions...... . . . . . . . .
. . . . . . . . . . . . .
125
126
128
130
130
134
135
136
136
138
139
140
5 Conclusions
5.1 Summary .............
5.2 Outlook and Future Work . .
5.2.1 Global Measurements .
143
143
144
144
146
5.2.2
Spatial analysis . . . .
Appendices
157
A Reactor within Enclosure
159
B Methodology for Leak Quantification (under non-permeation conditions) 161
C Flow Procedures
163
List of Figures
CO 2 Capture in Power Plant Systems: Comparison of Different Natural Gas
Technologies. Adapted from Kvamsdal et. al [1] . . . . . . . . . . . . . . . .
20
Four common membrane concepts for oxygen ion conduction: (a) solid electrolyte membrane, (b) dual phase membrane, (c) mixed ionic-electronic conducting membrane (MIEC), and (d) asymmetric porous membrane with graded
porosity. Adapted from Bouwmeester et al. [2] . . . . . . . . . . . . . . . . .
27
.. .... .... ... ..... ... ...
30
32
33
2-18
2-19
2-20
. . . . . . . . . . . . . . . . .
Common membrane configurations.. . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . .
ITM operation.. . . .
AZEP process flow sheet, showing the flux of oxygen in the ITM (or MCM)
reactor. Adapted from Griffin et al. [4] . . . . . . . . . . . . . . . . . . . . .
Oxygen migration in ion transport membranes. Adapted from Liu et al. [5]
Important sections in oxygen permeation through ion transport membranes.
pO2 is the chemical potential of 02. Adapted from Sunarso et al. [6] . . . . .
Effect of membrane thickness on the limiting step during oxygen permeation.
Adapted from Sunarso et al. [6] . . . . . . . . . . . . . . . . . . . . . . . . .
Surface reaction using a catalyst layer [7] . . . . . . . . . . . . . . . . . . . .
Reactions during oxidative coupling of methane [8] . . . . . . . . . . . . . .
Production of syngas using an ITM reactor [9] . . . . . . . . . . . . . . . . .
Reported effects of temperature on oxygen permeation flux for: (a) separationonly cases; and (b) reactive cases. . . . . . . . . . . . . . . . . . . . . . . . .
Ratio of reactive/non-reactive pre-exponential factors . . . . . . . . . . . . .
Normalized fluxes as a function of temperature for: (a) separation-only cases;
and (b) reactive cases. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Reported effects of Temperature on CH 4 conversion . . . . . . . . . . . . . .
Non-reactive oxygen flux dependence on Reynolds number . . . . . . . . . .
Reactive oxygen flux dependence on: (a) Reynolds number; and (b) residence
time. The sweep gas is a mixture of an inert gas (He or Ar) and CH 4 . . . . .
Dependence of CH 4 conversion on sweep mass flow rate . . . . . . . . . . . .
Dependence of CH 4 conversion on fuel/0 2 ratio. . . . . . . . . . . . . . . . .
Dependence of CO selectivity on fuel/0 2 ratio. . . . . . . . . . . . . . . . . .
3-1
3-2
3-3
Basic schematic of the ITM reactor . . . . . . . . . . . . . . . . . . . . . . .
The ITM Reactor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . .
Reactor front cross-section.. . . . . . . . .
79
80
81
1-1
2-1
2-2 An ideal perovskite structure
2-3
2-4
2-5
2-6
2-7
2-8
2-9
2-10
2-11
2-12
2-13
2-14
2-15
2-16
2-17
[3].
36
38
39
40
46
46
48
66
67
68
69
70
71
71
73
74
3-4
3-5
3-6
3-7
3-8
3-9
3-10
3-11
3-12
3-13
3-14
3-15
3-16
3-17
3-18
3-19
3-20
3-21
3-22
3-23
3-24
3-25
4-1
The ITM Reactor within its insulation.. . . .
. . . . . . . . . . . . . . .
The different sealants used for the ITM reactor . . . . . . . . . . . . . . . .
Sealant performance tests . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Sealant performance results: quantified in pmol.s- 1 of oxygen (from the air
which leaks into the reactor sweep side) . . . . . . . . . . . . . . . . . . . . .
Comparison of total leak at 500sccm air and 500sccm C0 2 , to the expected
oxygen permeation flux (assumed to be in the order of 1umol.cm- 2 .s- 1 ) . . . .
Reactor top cross-section . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Pre-operation EDX graph . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Pre-operation XRD graph . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Pre-operation SEM image . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Reactor Process Control and Instrumentation (PC&I) . . . . . . . . . . . . .
Measurement locations on the ITM reactor . . . . . . . . . . . . . . . . . . .
Reactor Plumbing Schematic . . . . . . . . . . . . . . . . . . . . . . . . . . .
The major reactor instrumentation . . . . . . . . . . . . . . . . . . . . . . .
Reactor thermocouples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Membrane temperature measurement configuration . . . . . . . . . . . . . .
Pyrometer calibration data: (a) membrane temperature vs pyrometer voltage
output; (b) membrane emissivity and pyrometer current output vs membrane
temperature (E= (62.5 * I + 323) /T) . . . . . . . . . . . . . . . . . . . . . .
Oxygen sensor calibration: (a) calibration duct; (b) calibration data . . . . .
Reactor Gas Chromatography . . . . . . . . . . . . . . . . . . . . . . . . . .
Reactor Heating Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
The Enclosure Heaters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Heating tests with the cartridge and enclosure heaters (SP = Set Point). N.B:
The green vertical lines indicate the cartridge heater locations . . . . . . . .
Reactor Safety Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
82
84
86
86
87
90
91
91
92
93
94
97
97
98
99
100
102
104
105
106
107
108
Dependence of oxygen permeation flux on membrane temperature (non-reactive).
Feed (air) flow = 500sccm, sweep (C0 2) flow = 500sccm. . . . . . . . . . . . 126
4-2 Bulk temperatures within reactor feed and sweep sides . . . . . . . . . . . . 127
4-3 Dependence of oxygen permeation flux on Sweep (C0 2 ) flow (non-reactive).
Membrane temperature = 800'C, feed (air) flow = 500sccm. . . . . . . . . . 128
4-4 Dependence of oxygen permeation flux and sweep 02 partial pressure on the
sweep Reynolds number (non-reactive). Membrane temperature = 800'C, feed
(air) flow = 500sccm. Re = (p#) / (pzDc), De = 2.24cm. . . . . . . . . . . . 129
4-5 Dependence of (a) oxygen permeation flux; (b) sweep oxygen partial pressure,
and fuel/0 2 ratio; on fuel inlet concentration. Membrane temperature =
800 0 C, CO 2 inflow = 500sccm, feed (air) flow = 500sccm. . . . . . . . . . . . 130
4-6 Dependence of fuel conversion and species selectivities on fuel inlet concentration. Membrane temperature = 800 0 C, CO 2 inflow = 500sccm, feed (air)
flow = 500secm . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131
4-7 Possible interaction of fuel with permeated oxygen in the reactor sweep side
132
4-8
4-9
4-10
4-11
4-12
4-13
Analysis of oxymel products obtained from reaction between fuel and 02 in
the ITM reactor. Membrane temperature = 800'C, CO 2 inflow = 500sccm,
feed (air) flow = 500sccm . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Influence of fuel addition on oxygen permeation flux in the ITM reactor. Membrane temperature = 800'C, feed (air) flow = 500sccm, sweep flow = 500sccm
CO 2 + variable CH 4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Oxygen flux dependence on temperature (non-reactive) for the ITM reactor
compared with reported investigations. . . . . . . . . . . . . . . . . . . . . .
Normalized oxygen flux dependence on temperature (non-reactive) for the
ITM reactor compared with reported investigations. See section 2.4.1 for
. . . . . . . . . . .
normalization methodology.. . . . . . . . . . . . . .
Normalized flux dependence on sweep mass flow (non-reactive) for the ITM
reactor compared with reported investigations. See section 2.4.1 for normal. . . . . . . . . . . . . . . .
ization methodology.. . . . . . . . . . . . .
Normalized flux dependence on Residence time (non-reactive) for the ITM
134
135
137
137
138
reactor compared with reported investigations. rres = V/V. See section 2.4.1
for normalization methodology. . . . . . . . . . . . . . . . . . . . . . . . . . 139
4-14 Dependence of CH 4 conversion on fuel/0 2 ratio for the ITM reactor compared
with reported investigations. . . . . . . . . . . . . . . . . . . . . . . . . . . . 140
A-1 The ITM reactor within its enclosure in the laboratory. . . . . . . . . . . . . 160
C-1 Reactor flow diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 163
12
List of Tables
2.2
Comparison between ITM investigations in oxidative coupling of methane
(OCM), syngas production and oxymel combustion. . . . . . . . . . . . . . .
Experimental investigations from literature considered for analysis . . . . . .
3.1
3.2
Process Control Equipment and Instrumentation . . . . . . . . . . . . . . . . 95
. . . . . . . . . . . . . . . . . . . . 112
ITM reactor measurements.. . . . .
4.1
4.2
Experimental points considered for analysis . . . . . . . . . . . . . . . . . . . 126
Mass flow rates at different planes below the membrane (inlet sweep velocity
= lcm/s). Results obtained from 2-D numerical analysis of the reactor . . . 133
5.1
Experimental points for future consideration . . . . . . . . . . . . . . . . . . 145
C.1
Flow procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 164
2.1
50
62
14
Nomenclature
Abbreviations
DAQ
data acquisition system, page 94
EDX
energy-dispersive x-ray spectroscopy, page 57
GC
gas chromatograph, page 103
ITM
ion transport membrane, page 21
MIEC
mixed ionic-electronic conductor, page 26
0CM
oxidative coupling of methane, page 47
PC&I
process control and instrumentation, page 22
POM
partial oxidation of methane, page 47
sccm
standard cubic centimeters per minute, page 80
SEM
scanning electron microscopy, page 57
XRD
x-ray diffraction, page 57
Symbols
uhi
ni
i
mass flow rate of gas i, page 96
molar flow rate of gas i, see equation (3.11), page 117
volume flow rate of gas i , see equation (2.12), page 54
A
membrane surface area exposed to gases on one side, page 117
AO
pre-exponential factor for flux normalization, see equation (2.20), page 65
D*
tracer diffusion coefficient, see equation (2.2), page 40
De
characteristic reactor length, see equation (2.22), page 70
Ds
self-diffusion coefficient, see equation (2.2), page 40
Dv
vacancy diffusion coefficient, see equation (2.8), page 44
Ea
activation energy, see equation (2.20), page 65
F
Faraday constant, see equation (2.3), page 41
h*
electron hole, page 38
balanced surface exchange rate at equilibrium, see equation (2.7), page 43
e
J
2
normalized oxygen permeation flux, see equation (2.21), page 66
Jo 2
oxygen flux, page 33
k,
forward surface reaction rate constant, see equation (2.8), page 44
kr
reverse surface reaction rate constant, see equation (2.8), page 44
ks
surface exchange coefficient, see equation (2.2), page 40
L
membrane thickness, page 40
Le
characteristic thickness, page 34
MA
molar mass of gas i, page 115
mfj
mass fraction of gas i, page 116
n
number of carbon atoms in the molecule of a carbon-containing product i , see
equation (2.19), page 60
00
lattice oxygen, page 38
P0
low oxygen partial pressure, page 28
high oxygen partial pressure, page 28
R
universal gas constant, see equation (2.3), page 41
Rm
log-mean radius, see equation (2.9), page 44
Ro
outer radius, see equation (2.9), page 44
Rin
inner radius, see equation (2.9), page 44
Re
Reynolds number, see equation (2.22), page 70
S,
selectivity of carbon-containing product p , see equation (2.0), page 34
T
membrane temperature, see equation (2.3), page 41
t el
electronic transfer number, see equation (2.6), page 43
16
tel
electronic transference number, see equation (2.1), page 37
tion
ionic transfer number, see equation (2.6), page 43
tion
ionic transference number, see equation (2.1), page 37
V
volume, see equation (2.23), page 70
V**
oxygen vacancy, page 38
Xi
molar fraction of gas i, see equation (2.15), page 58
Xr
conversion of reactant gas r, see equation (2.0), page 34
Y
yield of carbon-containing product p , see equation (2.0), page 34
C2
organic compound containing two carbon atoms, page 34
e-
electron, page 26
Greek
p
viscosity, see equation (2.22), page 70
p
density, see equation (2.22), page 70
o-e_
partial electronic conductivity, see equation (2.3), page 41
o-ion
partial ionic conductivity, see equation (2.3), page 41
o-total
total ionic conductivity, see equation (2.6), page 43
7-
timescale, see equation (2.23), page 70
Subscript
in
gas flows into the reactor, see equation (2.15), page 58
leak
leakage of gas into reactor sweep side, see equation (2.12), page 54
meas
gas concentration measured at sweep side exit by gas chromatograph or oxygen
sensor, page 118
out
gas flows out of the reactor, see equation (2.15), page 58
perm
gas permeation across membrane from feed side to sweep side, page 120
s
property of reactor sweep side, see equation (2.15), page 58
18
Chapter 1
Introduction
1.1
The CO
2
Problem
The general concern about carbon dioxide (C0 2 ) levels in the atmosphere is in its ability
to make the earth warmer. Climate model studies indicate that temperatures everywhere on
earth would increase by 1.5 - 4.5'C if the concentration of CO 2 is doubled in the atmosphere.
The increase in temperatures globally appear to support the results of these studies. Even
though some skeptics remain doubtful of the influence of CO 2 on global warming, it is
commonly agreed that further increases in CO 2 emissions and global warming need to be
forestalled [10].
One important source of CO 2 emission is natural gas (95% methane) combustion, and it
is currently a very important energy resource around the world. According to the United
States Energy Information Administration, natural gas contributed 5.93% of the world's
energy-related carbon dioxide emissions in 2007. The projection for natural gas contribution
in 2035 is 8.59% [11]. Furthermore, it is expected in the future that the use of natural gas will
19
be limited by rigorous emissions policies, unless carbon capture and sequestration provide
a favorable competition with low-carbon energy sources [12]. Therefore, studies into CO
2
emissions reduction would help provide efficient and clean ways to use natural gas (especially
in power plants), and go a long way to help reduce global warming and aid the capture and
reuse of carbon dioxide.
1.2
Solutions Proffered
There are several technologies currently used in natural gas power plants to facilitate
carbon capture, as shown in figure 1-1. Pre-capture, post-capture, and oxymel techniques
are currently being used even though the technologies for their implementation are relatively
new.
ATR
MSR-H 2
Autothermal reforming +
water-gas shift
Water cycle
Chemical looping
combustion
Membrane H2 separation
AZEP
Advanced Zero Emission
WC
CLC
50.0% - -
4-
*
*47 % 47 %
45
45.0%
40.0%
o
L
di
35.0%
0_0
X_g#
C~i
,
(ITM)
Process
100% CO2 Capture
-5%C02 Capture
SOFC
GT
Solid Oxide Fuel Cell
Gas Turbine
**Coo
.*
ITM: Ion Transport Membrane
Figure 1-1: CO 2 Capture in Power Plant Systems: Comparison of Different Natural Gas
Technologies. Adapted from Kvamsdal et. al [1]
Pre-capture involves the removal of CO 2 after carrying out steam reforming of fuel before
20
the combustion process. Post-capture involves the removal of CO 2 after the combustion
process. Oxymel techniques involve the combustion of fuel and oxygen. For the pre-capture
and post-capture cases however, separating CO 2 from other pollutants using solvents remains
problematic considering the overall efficiencies of these plants.
Oxymel combustion is the best method to burn natural gas cleanly. If the equivalence
ratio during combustion is 1, the products of combustion would ideally be carbon dioxide
(C02 ) and water (H2 0), and carbon dioxide can easily be captured after condensation of
the water. However, the costs of obtaining oxygen by cryogenic distillation or adsorption
techniques are generally high. For example, cryogenic distillation facilities account for 50% of
the construction costs of some syngas plants [13,14]. A novel approach is to use ion transport
membranes (ITM) for separation of oxygen from air before combustion with natural gas (e.g
AZEP in Fig 1-1), with efficiency projections currently above 50%.
ITMs are metallic oxide membranes permeable only to one gas specie (oxygen in this
case) when subjected to high temperatures (> 700) and a partial pressure gradient of the
gas. The use of ITMs for separation of oxygen from air, and combustion with fuels are
not new concepts. However, their use for oxymel combustion is relatively unexplored. Two
limitations currently reported with the use of ITM reactors for oxymel combustion are [15]:
" formation of C 2
4
, C2H, and CO, making CO 2 capture difficult, and
* low oxygen flux and methane conversion in the absence of a catalyst.
In addition to the above limitations, there is a distinct lack of reports on spatial analysis
or optical analysis of the reaction zone within ITM reactors which would aid the study
of different gradients and species formation in the reaction zone. It is therefore necessary
21
to explore the use of ITM reactors for separation of oxygen from air, complete oxidation
of natural gas, as well as the various factors that influence the effective operation of ITM
reactors macroscopically and spatially.
1.3
Thesis Focus
This report covers the design and development of an ITM reactor, and the use of said
reactor for experimental investigation of oxygen separation from air and methane oxymel
combustion.
The experimental approach (see chapter 3) covers:
1. Design of an ITM reactor with stagnation flow configuration and optical access (to aid
spatial and reaction zone analyses),
2. Design of a complete process control and instrumentation (PC&I) system for the reactor, and
3. Development of reactor-centric procedures and equations to aid understanding of the
reactions and processes observed.
The goals for this work are:
1. Fundamental investigation into the effects of operating parameters (temperatures, inlet
gas flow rates, and fuel/diluent ratio) on oxygen flux and the macroscopic thermochemical processes (e.g. yield of reaction products).
2. Investigation into the use of the ITM reactor for methane oxymel combustion. The
oxygen flux and the extent of the different reactions (oxidative coupling, syngas pro22
duction, or oxymel) observed will provide an insight into the suitability of ITM reactors
for methane oxymel combustion. Oxidative coupling produces C2 H4 and C 2H6 ; syngas
production is the partial oxidation of methane into CO and H2 ; and oxymel combustion
produces CO 2 and H2 0.
It should be noted that the overall focus/aim of this work is not to provide a detailed
insight into the use of ITM reactors for power generation or any other full scale application,
but to characterize the ITM reactor at a fundamental level.
Chapter 2 provides a review of the major concepts regarding ITMs as well as insight
into their current usage. An overview of current ITM reactor applications and experimental
methodologies is presented. The two major ITM methane applications are oxidative coupling
and syngas production. Furthermore, an analysis of reported ITM experimental investigations is conducted. The reported data from the investigations are analyzed to understand
some of the underlying governing processes for ITM reactor operation.
In chapter 3, the experimental approach for the ITM reactor is explained. The design of
the ITM reactor is presented along with an explanation of the membrane used. The design of
the reactor PC&I system is explained. Reactor-centric experimental procedures and analysis
equations are also presented.
An analysis of experimental results is presented in chapter 4 with a view to understanding
the current reactor operating regimes and to provide insight for reactor optimization. The
effects of membrane temperature and inlet mass flow rate are analyzed based on their impacts
on oxygen permeation. Also, an analysis of the effects of fuel use in the reactor is conducted.
The main focus in the reactive analysis is on oxygen permeation, fuel conversion, and overall
23
CO 2 yield.
Chapter 5 provides a summary of the contribution of this work and recommendations for
future research.
Chapter 2
State of the Art
The use of ion transport membrane (ITM) reactors for oxygen separation and methane
combustion is a novel approach, the main benefit being low cost of oxygen separation at
relatively high power plant efficiencies. The approach also allows for a wide range of applications. These include the formation of ethylene via oxidative coupling (for polyethylene
formation process), production of syngas via partial oxidation (e.g. for the fischer-tropsch
process), and formation of carbon dioxide, water vapor and heat via complete oxymel combustion (for carbon capture and heat supply to turbines in power plants). Although the
focus of this investigation is oxymel combustion, other modes of ITM operation (oxidative
coupling and syngas production) are also discussed here as they provide the majority of the
current ITM reactor literature.
As seen earlier from Figure 1-1, the efficiencies that can be derived from applying ITM
technology to oxymel combustion and power generation are promising compared to other
carbon capture technologies. However, it is necessary to understand the concept of an ITM,
the characteristics of currently used ITMs, the ITM governing processes, and the principles
25
of oxygen permeation. It is also important to understand the effects of operating parameters
on oxygen permeation and the formation of combustion products in order to optimize the
ITM reactor and aid complete capture of CO2-
2.1
ITM Background
Ion transport membranes (ITMs) are fabricated from ionic and mixed-conducting ceramic
oxides that conduct oxygen ions (02-) and electrons (e-)
at elevated temperatures. What
follows, is a background discussion of the different oxygen transport membrane types, mixed
ionic-electronic conducting membranes (abbreviated as MIEC, but also called ITM), ITM
concepts and applications, and the governing equations and fundamentals of oxygen flux.
2.1.1
Membranes and Oxygen Transport Membranes
In this report, the term 'membrane' refers to a layer of material between two gas streams
which allows a gas (e.g. oxygen) to permeate across it due to a driving force. The driving
force is the difference in the partial pressures of the permeating gas, or the electric potential gradient across the membrane. Figure 2-1 shows four common concepts for membranes
permeable to oxygen. They are [2]: (a) solid electrolyte membrane, (b) dual phase membrane, (c) mixed ionic-electronic conducting membrane (MIEC), and (d) asymmetric porous
membrane with graded porosity.
Solid electrolyte membrane systems involve sandwiching a solid oxide electrolyte between
two gas-permeable electrically conductive electrodes (Figure 2-la). This configuration incorporates the diffusion of oxygen ions into the electrolyte, and catalytic reaction with fuel on
cathode
node
e
e
(b)
A
(a)
P'o2
02
- 2-
e
<-
P"02
(d)
(c)
Figure 2-1: Four common membrane concepts for oxygen ion conduction: (a) solid electrolyte membrane, (b) dual phase membrane, (c) mixed ionic-electronic conducting membrane (MIEC), and (d) asymmetric porous membrane with graded porosity. Adapted from
Bouwmeester et al. [2]
the anode/cathode interface, while giving off water vapor, carbon dioxide, heat, and electrons. The electrons transport from the anode through the external circuit and back to the
cathode, providing a source of useful electrical energy in an external circuit (i.e. oxygen
pump). Solid electrolyte membranes can produce oxygen at high pressures. This provides
an advantage for high pressure applications, since compressors will not be needed [16].
A dual phase membrane, shown in figure 2-1b, is regarded as a metallic phase distribution into the cloud or matrix of an oxygen ion-conducting material/oxide. For example,
yttria-stabilized zirconia (YSZ) can be doped with titania [17, 18], ceria [19, 20], or palladium [21].
An increase in dopant concentration reportedly leads to an increase in the
electronic conductivity of dual phase membranes [2]. However, the range of solid solubility
27
of these multivalent membranes may limit the increase in electronic conductivity. These
membranes broaden the dense ceramic membrane spectrum by adding the solid oxide electrolyte alternative. However, without the application of internal or external circuitry, most
dual phase membranes cannot be used practically since they exhibit very low oxygen flux
when operated within commonly used ranges of temperature (< 1000C) and oxygen partial
pressure
(<
0.21 atm). Also, continuous electronic conduction with accompanying oxygen
permeability can be damaging to the membrane as evidenced in their applications in fuel
cells and oxygen sensors [22].
A mixed ionic-electronic conducting membrane (MIEC) is depicted in figure 2-1c. MIECs
typically conduct both oxygen ions and electrons thereby eliminating the need for electrodes
or external circuitry. They are dense and impermeable to gas molecules, and allow only
oxygen ions and electrons permeate across them. The chemical potential is the driving force
for the overall oxygen flux across the membrane.
Oxygen dissociates and ionizes at the high pressure membrane surface (feed side, P 2 ) by
associating with electrons in accessible surface electronic states. Electronic charge carriers
flow simultaneously to compensate for the flux of oxygen ions within the membrane. When
oxygen ions reach the low pressure surface (sweep side, P02 ), they recombine and release
electrons to form oxygen molecules.
The oxygen molecules on the sweep surface then flow into the sweep stream [2]. MIECs are
very attractive for practical applications since they conduct both oxygen ions and electrons
without the use of external circuitry.
Figure 2-1d shows an asymmetric porous membrane with graded porosity. This is usually
made of materials such as a-A12 0 3 , 7-Al 2 03, TiO 2 and SiO 2 . Its main advantages are [23]
28
(a) high surface area to volume ratio, and (b) enhanced mechanical support.
2.1.2
Mixed Ionic-Electronic Conducting Membranes
As discussed in section 2.1.1, MIECs conduct oxygen ions and electrons without the use
of external circuitry. The most commonly used MIECs have been shown to exhibit either a
perovskite structure (empirical formula ABO 3 ) or fluorite structure (empirical formula A0 2 ,
e.g. ZrO 2 ) [6]. A & B are cations: the A-site is usually occupied by rare earth materials
while the B-site contains transition metals with mixed valency [24]. Perovskites are used
more in practical applications because of their high oxygen fluxes. A MIEC is termed a
perovskite if it has a similar crystal structure to calcium titanium oxide (CaTiO 3 , also called
the 'perovskite structure') [25]. The name 'perovskite' was coined in honor of Count Lev
Alekseevich Perovskii (1792 - 1856), a Russian mineralogist from St. Petersburg, Russia [26].
It should be noted that in the rest of this report, the term 'ion transport membrane' (ITM)
is used to refer to MIECs.
Composition
Perovskites may contain a mixture of two or more elements in the A-site and/or the B-site,
e.g. Bao.5 Sro.5 Co0o.Fe0 .2O36 . One version of a perovskite structure is shown in figure 2-2. The
A-cations combine with oxygen ions to form a lattice structure with octahedral interstitial
sites occupied by the B-cations [27]. The structure is a octahedra with oxygen ions linked to
all corners. The A-cations fill the dodecahedral holes while the B-cation fills the octahedral
holes. The coordination number of the A-cation is 12, while that of the B-cation is 6. The
29
coordination number of an ion in a crystal is the total number of ions it holds as its nearest
neighbors [28].
Active
Top
x
x
F
Right
@02.
*A
*B
Figure 2-2: An ideal perovskite structure [3].
Perovskite membranes have been shown to exhibit higher oxygen fluxes than flourites,
even though they are structurally less stable [6]. Teraoka et al. [29-31] showed that oxygen
permeation fluxes through LaCoO 3-based perovskite-type membranes were 2-4 orders of
magnitude higher than those of stabilized zirconia (a fluorite) at the same temperatures.
ITMs which are made to take advantage of the best traits of perovskites and fluorites, have
been reported. An example is 40 vol.% Lao.sSro 2 MnO 3 - mixed with 60 vol.% yttria-stabilized
zirconia (YSZ) [23].
The metals in perovskite structures can be substituted to enhance performance. It is
important to note that the B-site cation improves the oxidation catalytic activity of per30
ovskites [32].
Lin et al. [33] showed that by substituting Fe with Co, Lao.2 Sro.8 CoO 3-3
(LSC) showed better C 2 selectivity than SrCo 0 .sFeO.2 0 3 _o (SCF) during oxidative coupling of
methane (OCM). Although A-site cations are generally inactive catalytically, they strongly
affect the oxygen non-stoichiometry of the oxide and the flux of oxygen ions across the membrane [34]. I.e. A-site cations influence the oxygen flux while the B-side Cations influence
the ITM's catalytic properties.
Configurations
There are three broad MIEC configurations used in reactors [35] - planar/disk, tubular,
and hollow fiber, as shown in Figure 2-3. However, commercial large scale applications utilize
multi-planar or multi-tubular MIEC reactor configurations [8].
Hollow fiber membranes
in particular, are becoming popular due to their much larger membrane surface area to
equipment volume ratio than planar/disc and tubular membranes [36].
However, hollow
fiber membranes have less thickness and thus lower mechanical strength compared to tubular
membranes leading to difficulties during assembly into membrane reactors [35].
Design variations for porous layers and asymmetric membranes are common. Typically,
porous layers are added to membranes to improve the bending strength and surface reactions
while catalysts can be added to on any given surface to either improve oxygen flux or improve
the results of a desired reaction. Also, the presence of porous layers allows the use of much
thinner dense membranes designed to enhance oxygen flux. In the case of tubular or hollow
fiber membranes, the inside of the tubes are usually packed with a catalyst to improve desired
reactions in the case of a reactive ITM [37,38].
31
feed
side
feed out
feed i
planar/disk
cross-section
02 flux
s-swi
-sweepin
sweep out
e
de
diameter 210mm
thickness 0.5mm)
(a) planar/disk
feedUnt
O flux,
feed side
outlet
'sweep side
Ne717
Neep et
sweep outlet
internal dia.> o.1smm
t*ickness>0.2Smm
(b)tubular
feed inlet
02
flux---,
tubular
cross-section
feed side
d outlet
(c)hollow fiber
sweep outlet
hollowfiber
cross-section
0
internal dia. < 0.15mm
thicness< 0.25mm
Figure 2-3: Common membrane configurations
2.1.3
ITM Concepts
Figure 2-4a shows the application of a two-dimensional planar ITM purely for separation
of oxygen from air. High temperatures and high partial pressure gradient of oxygen are
required for oxygen flux across the membrane. Under these conditions, some oxygen can be
separated from the air on the feed side and permeated to the sweep side where it is 'swept'
by a sweep gas with lower partial pressure of oxygen (e.g. C0 2 )In figure 2-4b, the separation process is used in combination with a reactive process. In
this case an oxymel reaction occurs in the sweep side resulting in the formation of carbon
dioxide and water vapor as products. The use of a reactive sweep gas (e.g. CH 4 ) enhances the
oxygen flux beyond the separation-only case. This is believed to be due to the consumption
of oxygen in the sweep side during reaction, thereby maintaining a large partial pressure
gradient of oxygen across the membrane.
Some important terms regarding ITM use for oxygen separation and fuel combustion are:
32
Feed side - high 02 partial pressure (P'o2 )
air
M
4cl
%W4 o 10CO
4
*1
ITM Temperature Ze700"C
l
Ni
CO
0 2-depleted air
CO2 &
permeated 02
Sweep side - low 02 partial pressure (P"o2)
(a) separation only
air
$$
$0
Feed side - high 02 partial pressure (P'o2 )
CH4
2-depleted
air
C02 & H20
CH4 +202 -.. ' C02 + 2H20
Sweep side - low 02 partial pressure (P"o2 )
(b)separation coupled with oxyfuel reaction
Figure 2-4: ITM operation.
Oxygen flux (J0 2 ): The amount of oxygen passing though the membrane per unit membrane surface area and time [39].
Oxygen partial pressure: This is equivalent to the mole fraction of oxygen in a gaseous
mixture. For ITMs, P
denotes the higher oxygen partial pressure (on the feed side),
while P02 represents the lower oxygen partial pressure (on the sweep side)
Characteristic thickness (Lc): The membrane thickness at which oxygen permeation is
equally determined by the surface-exchange kinetics and bulk-diffusion [6].
Surface
exchange refers to the reactions between oxygen molecules or ions and electrons on the
membrane surface while bulk diffusion refers to the transport of oxygen ions within
33
the membrane.
Conversion (X): The conversion of a reactant r (e.g. 02, CH 4 ) is the percentage of the
total supply of reactant r (total permeated in the case of 02) that is converted to
products during a reaction.
Selectivity (S): The selectivity of a carbon containing product p (e.g CO, C0 2 , C2 H4 ,
C2 H) is the percentage of converted fuel (e.g. CH 4 ) that forms product p [40].
Yield (Y): The yield of a carbon containing product p, is the percentage of total methane
that forms product p. It is the product of the fuel conversion and the selectivity of p,
i.e. Y = Sp
x
X, [40].
C 2 : This notation refers to products with two carbon atoms (mainly C 2H4 and C 2 H6 ).
Feed side: This is the side of the membrane that is exposed to feed gas.
Sweep side: This is the side of the membrane that is exposed to permeated oxygen, sweep
gas and/or fuel.
2.1.4
Usage history and applications
Since the mid 1960s, when rare earth aluminates were first reported to exhibit elevated
levels of oxide ion conductivity [41, 42], the development of oxygen-deficient mixed ionicelectronic conducting membranes has become a popular trend [43]. Teraoka et al. [29-31]
(1985 - 1991) reported the use of cobalt-rich perovskites with very high oxygen fluxes. These
perovskites exhibited high anion deficiencies when subjected to decreased oxygen partial
pressures and increased temperatures. By mid 1990s, extensive research in the development
34
of acceptor-doped perovskites with the general formula LajxAxCoiyByO 3 _6 (A = Sr, Ba, Ca
and B = Fe, Cu, Ni) were widely common [2].
From mid 1990s, two trends began to emerge in the use of ITMs for fuel conversion
purposes.
The earliest trend was oxidative coupling for the conversion of methane and
ethane to ethylene. Ten Elshof et al. [44] carried out oxidative coupling of methane (OCM)
to form ethylene between 800-900C using a Lao0 6 Sr. 4 Co0 .Feo.20
furnace.
3
membrane in a quartz
Lin et al. [40, 45, 46] carried out oxidative coupling of methane and ethane us-
ing Lao.sSro. 2 Co
Fe
0
.4 0 3 _
3
(LSCF) and Bii.5 Y0 .3 Sm 0 .20 3 (BYS) membranes respectively be-
tween 750 - 950C. The methane experiments gave C2 (ethane and ethylene) yields between
10 - 18% while the ethane experiments gave an ethylene yield of 56%. According to Lu
et al. [37], a similar experiment using the barium based membrane BaCeo.sGdo.203_6 gave
16.5% C2.
The second trend was the partial oxidation of methane to syngas (CO + H2 ). Balachandran et al. [47] carried out the partial oxidation of methane to syngas using La-Sr-Fe-Co-O
membrane tubes (each 30cm long) in a reactor at 850'C, obtaining 99% methane conversion.
Ikeguchi et al. [48] reports the use of a disk-type SmO4 Bao.6 Co 0 .2Feo.0 O 3-3 membrane for
partial oxidation of methane to syngas, with 98% CO selectivity being observed at 900C.
Recently, similar experiments were reported by Wang et al. [49] and Takahashi et al. [13]
using membranes made of BaCoFeyZrO 3 _o (BCFZ, x + y + z = 1) and La_,SrTijyFeyO3-6
(LSTF, x = 0.1-0.8, y = 0.6-1.0) respectively. BCFZ gave 95.8% CH 4 conversion and 96.6%
CO selectivity at 925C, while LSTF gave 60% CH 4 conversion and 99.9% CO selectivity at
9000 C.
Of high importance is the work reported by Tan et al. [15].
35
Apart from working on
oxidative coupling of methane [50] and syngas production [35], methane oxymel experiments
were carried out using four hollow fiber Lao. 6Sro. 4 Co0 .2 Fe 0.sO3 6 (LSCF6428) membranes in
a tubular reactor. The aim was to provide an insight into ITM use for oxycombustion based
heat/power generation.
Over the past 10 years, the most commonly reported ITMs have been LSCF [51-54] and
BSCF [55-57].
BSCF exhibits the highest oxygen flux generally reported, but is mostly
limited to separation-only applications because of its chemical and structural instability in
reducing environments [58,59]. LSCF is the most widely used for reactive ITM applications
because of its good flux and acceptable chemical and structural stability [39].
The most
commonly used fuel for ITM reactors is methane (CH 4 ). However, the use of ethane as fuel
has also been reported [60].
ITM reactor
Steam turbine
Bleed
gas
HX
Combustor:
Cooling Water
Natural gsCO2
compressor
CO2
H20
H20
........
.........
.....................
HRSG
Air
...... ....
CO2
Oxygen depleted air
s....
Gas turbine
Figure 2-5: AZEP process flow sheet, showing the flux of oxygen in the ITM (or MCM)
reactor. Adapted from Griffin et al. [4]
Today, ITM use has evolved to a large extent due to the need for less expensive techniques
for oxygen separation in power plants and engines [61]. There are now many different ITM
36
configurations, alternative composite ITM materials [62] and documented industrial applications such as the Zero Emissions Ion Transport Membrane Oxygen Power (ZEITMOP) [63],
Zero Emissions Membrane Piston Engine System (ZEMPES) [51,64,65], and Advanced Zero
Emissions Power Plant (AZEP) cycles [4,66]. Figure 2-5 shows an AZEP process flow sheet.
It can be seen that air is compressed, oxygen is separated from the air by a membrane, and
the separated oxygen is reacted with natural gas. The oxygen depleted air (expanded after
separation) and flue gas are passed through a heat recovery steam generator (HRSG) to help
drive a steam turbine.
2.1.5
Fundamentals of Oxygen Flux
The primary purpose of an ion transport membrane is to separate oxygen from a feed
gas (e.g. air). It is therefore important to understand the processes that drive the oxygen
flux across the membrane in order to properly design and improve membrane performance.
The flux of oxygen across an ITM involves the flux of oxygen ions which are charge
compensated by a simultaneous flux of electrons in the opposite direction. At any total
conductivity, the maximum oxygen flux is said to occur when the transference numbers
(ionic and electronic) are the same, i.e.
tion = tei=
0.5
(2.1)
The transference number of an ion or electron is the fraction of the total current that is
carried by that ion or electron in passing through an electrolytic solution or solid [2].
37
ITM
High 02 partial
pressure side
Oo + 2h'
00000
(P02 )
(4)
O' 4.O
1
02
+ V3*
==
O
0=>+2h'
02 + V'
(5)
-- +02
Sweep
|gas
2(2)
02 --
>
Feed
gas
(P0 2 )
Low 02 partial
pressure side
Figure 2-6: Oxygen migration in ion transport membranes. Adapted from Liu et al. [5]
Figure 2-6 shows a schematic diagram describing the mechanism for oxygen permeation
through an ion transport membrane. Krbger-Vink notation is used to define the charged
defects. 00 stands for lattice oxygen, V ' for oxygen vacancy, and h* for positive electron
hole. The process of oxygen permeation from the feed side (high PO2 ) to the sweep side (low
P0 2 involves a series of steps: [5]:
1. Mass transfer of oxygen gas from the feed gas stream to the surface of the membrane
(high P' 2 side);
2. Reaction on the surface of the membrane (high Po2 side) between oxygen vacancies
and oxygen molecules.
3. Bulk diffusion of oxygen ions by membrane vacancies;
4. Reaction on the surface of the membrane (low P02 side) between electron holes and
oxygen ions; and
5. Mass transfer of oxygen gas from the surface of the membrane to the sweep gas stream
(low P , side).
Generally, the resistances between the gas phase and membrane are usually small and
negligible. Only membrane bulk diffusion (step 3) and surface exchanges (steps 2 and 4)
are taken into consideration in developing transport equations [5,67]. The surface exchanges
involve a series of sub processes as explained by [68]. Figure 2-7 shows the important oxygen
permeation steps. It can be seen that the chemical potential of oxygen drops from the feed
side to the permeate side. This drop in chemical potential is the main driving force for
oxygen flux.
Surface
Exchange
zonel
Bulk
Diffusion
02
Surface
Exchange
zonell
2
Feed side
Sweep side
Figure 2-7: Important sections in oxygen permeation through ion transport membranes. p10 2
is the chemical potential of 02. Adapted from Sunarso et al. [6]
The overall oxygen permeation rate is expected to be limited by the slowest moving
species or the slowest process. However, as the membrane thickness decreases towards the
characteristic thickness (i.e. as L -s Lc), the diffusion resistance decreases and the controlling
step changes from bulk diffusion to surface kinetics [69-72]. Figure 2-8 shows a schematic
39
of the limiting steps. It should be noted that reducing the membrane thickness will improve
oxygen flux. However, once the membrane thickness is reduced below Lc, only marginal (if
any) increases in oxygen flux can be obtained [69,70,73].
Kinetics-controlled
Diffusion-controlled
Lc
Membrane thickness
Figure 2-8: Effect of membrane thickness on the limiting step during oxygen permeation.
Adapted from Sunarso et al. [6]
The characteristic thickness (Lc) of an ITM , is the membrane thickness at which the
oxygen flux is equally determined by bulk diffusion and surface exchange kinetics. According
to Bouwmeester et al. [2], the characteristic thickness can be expressed theoretically as:
Lc
-
D
D
ks
-
D*
k8
ks
(tel =
1)
(2.2)
where ks is the surface exchange coefficient, D, is the self-diffusion coefficient of oxygen anions
with valence charge zo
(=
-2) and D* is the tracer diffusion coefficient. Ds is assumed to be
identical to D* if the correlation between D* and ks can be neglected. D* and k, values can
be obtained from the 180160 isotope exchange, permeation and/or coulometric titration
experiments [6,74]. For equation 2.2 to be valid, a small oxygen partial pressure gradient is
assumed across the membrane, and electronic conduction is assumed to be predominant.
Bulk Transport
The Wagner theory (eqn. 2.3) is widely used to depict bulk diffusion for a relatively thick
membrane (i.e. L > Lc) [2].
Jo
=--
RT
42 F 2 L
6
Jin'
PO2
del
+
dn P0 2
"l"P5
(2.3)
Cion
Jo2 is the oxygen flux, R is the universal gas constant, T is the membrane temperature, F
is the Faraday constant, and L is the membrane thickness. P
and P" represent the high
and low partial pressures of oxygen, respectively. o-e and o-in are the partial electronic and
ionic conductivity, respectively. Equation 2.3 has been derived assuming:
" The overall oxygen permeation rate is determined by the diffusion of oxygen ions
through the membrane lattice or the transport of electronic charge carriers within the
bulk oxide [2]; and
" Two charged species exist in local equilibrium with a hypothetical neutral species
within the bulk oxide. An example is oxygen ions and electrons in equilibrium with
molecular oxygen [6].
Bulk diffusion is influenced by a minimum of three species. They are (a) oxygen vacancies,
41
(b) electrons, and (c) electron holes. Any of them could be the slowest moving species and
hence have the strongest influence on the oxygen flux. A detailed analysis of these species
is given by Surnaso et al [6].
Surface Exchange
Surface exchange reactions on the membrane surface are two-fold. On the feed side of the
membrane, molecular oxygen combines with available oxygen ion vacancies to give oxygen
ions and electron holes thus:
1
-02 +V"
2
-
(2.4)
0 0 +2h'
On the sweep side, the surface reaction is the reverse of that observed on the feed side.
The oxygen ions and electron holes combine to give molecular oxygen leaving oxygen ion
vacancies in the membrane thus :
0 0 + 2h'
1
-02+ V**
2
(2.5)
The transport of oxygen ions within the bulk phase can be improved by a reduction
in membrane thickness or an increase in either conductivities (ionic or electronic) of the
membrane. If this occurs, the limiting step becomes surface exchange kinetics and a new
expression is required for the oxygen permeation flux
al.
[70],
[75,76].
According to Bouwmeester et
the Wagner equation is not suitable for a surface exchange-limited process of oxygen
permeation through the membrane. Instead, the expression becomes
[2]:
1
teltiono-total
o 1 + (2Le/L)
tel and tio
42 F 2
APo2(
L
are the electronic and ionic transfer numbers, respectively. o-total is the total
conductivity. In deriving equation 2.6, identical surface exchange rates were assumed for
both the feed and sweep sides. The membrane characteristic thickness (Lc) is expressed
as [2]:
L =
'
where
je
RT
teltionTtotal
j.o
Jex
42F2
(2.7)
[mol.cm- 2 s-1 ] is the balanced surface exchange rate when oxygen potential gradients
are absent, and its value can be obtained from 10-6 0 isotopic exchange data.
The mechanism of surface exchange is expected to be a combination of sub steps in sequence which may have the capacity to individually limit the overall oxygen permeation rate.
On the feed side, these sub steps include adsorption of oxygen molecules to the membrane
surface, and the transfer of charges between adsorbed species and other species within the
membrane surface. The reverse of these sub steps will occur on the sweep side [68]. A detailed analysis of possible mechanisms to incorporate the surface reaction kinetics into the
bulk-diffusion mechanism is given by Du et al. [71].
43
Overall Oxygen Flux Equations
Several research groups have developed single expressions for the overall oxygen permeation flux across perovskite membranes [67,77,78]. These expressions incorporate all possible
limiting processes during oxygen permeation. According to Liu et al. [5], the flux relations for
planar/disk-shaped and tubular perovskites are given in Equations 2.8 and 2.9 respectively.
krDv (P
Jo
2LKf
Dv
R kkr
[ (B
(Ro - Ri) -k
)
2 )0.5
5(Pb
+ Rm/
2
0.5]
_
(
-
(P2)
)05]
P2
(p2)0.51
Rin -D (Pb)
(2.8)
-P2
(Pb
-
2
(
P2
(0bP )05 + D
k,(P0P
02 =2
-
2
0 5
05
(2.9)
+ Rm/Ro -D (Ps
Dv is the diffusion coefficient of oxygen vacancies, K, and Kf are the experimentally obtained
reverse and forward surface reaction rate constants, respectively. R0 , Ri, and Rm are the
outer, inner and log-mean radii, respectively. Rm = (Ro - Rin)/ln(Ro - Rin).
According to Surnaso et al. [6], the assumptions that make Equations 2.8 and 2.9 valid,
are:
1. Since the ionic conductivity of a perovskite is much less than its electronic conductivity,
the flux of oxygen vacancies is assumed to govern the rate of oxygen permeation.
2. Electron holes move fast enough to make the electric field gradient non-steady (i.e.
electronic conductivity is governed by the electron holes).
44
3. The gas phase exhibits the behavior of an ideal gas.
4. For a tubular membrane, radial diffusion is negligible and the oxygen vacancy diffusion
coefficient is assumed constant.
5. Equation 2.4 represents the feed side surface exchange reaction while equation 2.5
represents the sweep side surface exchange reactions. Also, the forward and reverse
surface exchange rate constants, kf and k, (given in equations eq:transport-equationdisk,eq:transport-equation-tube), are assumed to be equal and applicable to the surface
reactions.
6. The membrane electron hole concentration is assumed constant for both surfaces. The
other species (O, 02, V"*) are analyzed with the law of mass action.
2.2
Reactive ITM Applications
ITM reactors have been used for different reactive purposes. Membrane technology has
advanced beyond separation of oxygen only.
Researchers have reported the coupling of
oxygen separation and reactive process in a single step in ITM reactors. The advantage of
the coupled process is that the mechanisms for oxygen separation and fuel reaction can be
studied in a dependent-form analysis [79].
In this section, the main reactive applications for ITM reactors, which use methane as
fuel, are discussed. A comparison of these applications as well as the challenges associated
to them, are also presented. In general, the three reactive ITM mechanisms (for methane
conversion) differ as follows:
Oxidative Coupling of Methane: Takes place ON the ITM surface.
CH 4 reacts with
oxygen ions to form CH 3 , which couples to produce C2 H and C2 H4 . The ITM is
used to limit availability of 02 and prevent oxidation to CO or CO .
2
Syngas Production: Partial oxidation (fuel rich) of CH 4 with oxygen (though usually
with the aid of a catalyst) to produce CO and H2 . The reaction can also occur on the
reaction surface with molecular oxygen.
Oxymel Combustion: Oxymel combustion requires sufficient 02 for complete oxidation
to CO 2 and H2 0. Catalysts will improve conversion efficiency at lower temperatures.
2.2.1
Oxidative Coupling of Methane
Oxidative coupling of methane is the formation of C2 products by reacting CH 4 and
oxygen ions. Although C2 H6 and C2 H4 are the main C2 products, C24 is mostly desired [8].
All permeated lattice oxygen is consumed on the membrane surface and gaseous oxygen is
absent in the reaction side as shown in Figures 2-9 and 2-10.
CH4
Air Side
C2 , COx
--
2V0+02(g)
2o"+4k
Membrane
V;I oI
Reaction side
Z Mt
aTy-st
Reacdon Side
layer
-*
H 2 0+2VG+2CHc
oxygensiFe
0+2k'+2CH4 (s)
)(3)
C2HI(9)
+h U)+
02(g)
Cil
4(8)
TI
02
1120 + C 2 H 4 (g) + V; -3C
Figure 2-9: Surface reaction using a catalyst
layer [7]
2 H1
(j) + 00'+ k
Figure 2-10: Reactions during oxidative coupling of methane [8]
Three trends are common with most oxidative coupling of methane (OCM) experiments:
46
1. There is preference for tubular membranes with OCM-enhancing catalyst (usually Labased) packed in the tube side [38].
2. Use of a reaction inhibitor to prevent total oxidation catalytic activity of the membrane.
A good example is presented by Lu etal.
[37]
in which BaCeo. 6 SmO. 4 0 3 _6 catalyst was
used to prevent the total oxidation catalytic activity of a SrFeCoo. 5 0 3 6 membrane
reactor, by coating the inside of the membrane tube with the catalyst.
3. The use of membranes with good catalytic properties for OCM at lower oxygen partial
pressure. Good results for C2 yield (17%) were obtained with B1 . 5Y 0 3 SmO. 2 0 3 _s (BYS)
and BaCeo.8 Gdo.203
2.2.2
membranes [37, 45, 46].
Syngas Production
Of all the potential applications for mixed conducting ceramic membranes, partial oxidation of methane (POM) to syngas (CO + H2 ) is thought to be one of the most commercially
important applications [80]. The partial oxidation of methane is a promising alternative for
syngas generation because it is a mildly exothermic reaction and could produce syngas with
the H2 /CO ratio of 2:1, which is a preferable feed stock for methanol synthesis or the FischerTropsch reaction [49]. Combining oxygen separation and partial oxidation of methane into a
single step is therefore important for cost reduction of syngas generation and the processes
which use syngas [81].
Production of syngas in an ITM reactor involves fuel rich reactions which favor the
formation of CO and H2 [8]. One side of the membrane is exposed to an oxidizing atmosphere
47
(usually air) and the other side is exposed to a reducing gas mixture (CH 4 , CO, H2, C0 2,
H2 0 and so on). Therefore, it is crucial that the membrane materials should have a stable
lattice structure under a wide range of oxygen partial pressure and high resistance to the
highly reducing atmosphere besides possessing the desired oxygen permeability [8].
As with OCM experiments, three trends are common with most syngas experiments:
1. Preference for hollow fiber membranes [35]. A major advantage of this configuration
is its relative large surface area to volume ratio.
2. Use of reforming catalyst on sweep side and reduction catalyst on feed side (Fig 2-11).
Perovskite films and Ni-based catalysts (such as Ni/Al 2 0 3 ) are commonly used for
reforming [35]. Platinum is commonly used as oxygen reduction catalyst.
3. Use of membranes with high oxygen permeability to further reduce the ITM reactor
volume and cost [8].
Syngas
(CO + H2)
Oxygendepleted air
CH4
(steam)
Ai
Air
Reforrning
catalyst
02 reduction
catalyst
ITM
Figure 2-11: Production of syngas using an ITM reactor [9]
Oxymel Combustion
2.2.3
Methane oxymel combustion involves reacting methane with permeated oxygen to ideally
produce carbon dioxide, water vapor and heat thus:
CH 4 + 202
-9
CO 2 + 2H2 0 (g) + heat
(2.10)
The reaction is exothermic and the products can easily be separated by condensing the water
vapor thereby capturing carbon dioxide for reuse in the fuel stream or sequestration. Oxymel
combustion requires sufficient oxygen for complete oxidation which in turn requires the
appropriate CH 4 /0
2
molar ratio (i.e. 1/2). Catalysts may also be used to improve conversion
efficiency at lower temperatures, as lower temperatures improve membrane stability. The
use of ITMs for methane oxymel combustion reactions is a relatively new concept.
et al.
[15]
Tan
carried out oxymel combustion in a hollow fiber catalytic LaO.6 Sro. 4 Co 0 .2 Feo. 8 03-6
(LSCF6428) membrane reactor.
Tan et al. prepared a hollow fiber membrane reactor (HFMR) using four LSCF6428
hollow fiber membranes (1.1mm I.D, x 27cm) with the insides packed with catalyst made
of the granular powder (0.6mm dia.) of the membrane material. The membranes had an
asymmetric structure (i.e. a thin separating dense layer integrated with porous layers on
one or both sides). A comparison is also made with the same configuration that includes
the effects of platinum catalyst on the air feed side (outside of the membranes). During the
experiments, the methane feed concentration was kept at a constant 9.75% vol. (CH 4 with
balance He).
2.2.4
Comparison of ITM methane applications
It is important to point to the major differences in the ITM investigations discussed
earlier in order to further understand their purpose and uses. Table 2.1 outlines the major
differences.
Table 2.1: Comparison between ITM investigations in oxidative coupling of methane (OCM),
syngas production and oxymel combustion.
Criteria
Reaction location
OCM
Preferred
membrane
geometry
Catalyst for fuel
conversion
P02 (Partial pressure of oxygen on
the
Tubular
Syngas
Mostly in the gas
phase
Hollow fiber
Hollow fiber
La-based (e.g
La/MgO)
Lower than syngas
case
Ni-based (e.g
Ni/Al 20 3 )
Higher than Oxymel
and OCM cases
Granular form of
membrane in use
Lower than syngas
case
800 - 950 0 C
> 850 0 C
> 850 0 C
Membrane surface
Oxymel
Gas phase
fuel side)
Currently used
operating
temperatures
CH 4 inlet
concentration
Desired products
Challenges
Lower than syngas
Higher than Oxymel
case
and OCM cases
C 2 HA, C2 H6
CO, H2
e Increase of temper- e Developing ITMs
ature beyond 950'C with high 02 flux and
leads to COx prod- long term phase staucts
bility
Lower than syngas
case
C0 2 , H2 0
e Development of
ITMs suitable for,
and stable at power
plant operating
temperatures
0 Higher methane * Chemical deposiconversion rate usu- tion and membrane
ally leads to lower C2 instability have been
selectivity
reported
('-'-i
1400-C)
* Developing catalysts that aid complete combustion of
methane in the gas
lys
phase
It can be seen that unlike OCM, syngas production and oxymel combustion requires
reactions that occur mostly in the gas phase.
Oxymel combustion is similar to syngas
production in terms of membrane geometry preferred and best operating temperatures. It
should also be noted that the methane feed concentration and partial pressure of oxygen on
the fuel side are usually higher in syngas production than in oxymel combustion and OCM.
2.2.5
Reactive ITM challenges
The future of reactive ITMs looks promising based on early results and projections.
However, there are challenges that remain which must be overcome. According to Dyer et
al. [82] the challenges for ITM applications in general include:
e the development of systems that can incorporate the ITM technology;
e the fabrication of mechanically, chemically, and thermally stable ITMs;
e the development of mechanically, chemically, and thermally stable sealants with excellent sealing performance;
e the development of economically-viable technology for ITM fabrication; and
e long-term performance and dependability.
The challenges facing reactive ITMs on the fundamental level are explained below:
Oxidative Coupling: A major challenge for OCM reactions is the C2 yield. Although
OCM experiments have been ongoing for over two decades and efforts have been put
into developing novel catalysts, C2 yields are mostly reported to be < 25% [8]. Although an increase in temperature would favor the formation of more ethylene (C2H4 ),
too high temperature (>950'C) would introduce more oxygen into the gas phase leading to the formation of COx products [50].
51
Syngas Production: Very high temperatures are harmful to reforming catalysts [35]. Also,
researchers have reported membrane contamination instability during syngas reactions [8].
The latter is mainly due to exposure of the membrane to an oxidizing
atmosphere (usually air) on the feed side a reducing gas mixture (CO + H2 ) on the
permeate side.
Oxymel Combustion Little work has been done on oxymel combustion. Therefore the
challenges that lie ahead would likely be more than those currently faced during OCM
or syngas reactions. One major challenge that would be encountered during oxymel
combustion is the selectivity of CO 2. Since carbon capture is one of the main goals
of oxymel combustion, the presence of C2 products and/or CO (however little) in the
product stream would limit the overall success of any oxymel reaction since carbon capture would no longer be possible by simply condensing the water vapor after reaction.
Another challenge is the unsuitability of currently used ITMs for the temperatures
(~ 1400'C)
2.3
used in natural gas power plant combustors.
Methodology for ITM Reactor Characterization
The use of ITMs within reactors for oxygen separation and reaction with gaseous fuels
is a developing trend. Recently, researchers have developed well defined methods for ITM
reactor characterization. The methods for sealing and leak detection are well documented
while experimental methodologies which govern the reactor purging, temperature, inlet gas
flow rates, fuel dilution and post-operation membrane analysis are well understood. The
methodology for analysis involves the calculation of oxygen flux, methane conversion, and
the selectivities or yields of reaction products. This section serves to provide an insight into
the methods in current use for ITM reactor characterization.
2.3.1
Sealing and Leak Detection
The sealing of the ITM reactor is an important step in preparation for experiments
mainly because the oxygen flux expected is very low (in the order of 0.1pmol.cm--2.-1), and
as such, any unexpected gas species within the reactor would invalidate the results obtained.
Sealing is however a major challenge in ITM reactors partly because of the general lack
of reliable seals at the very high temperatures required for oxygen flux, and the differing
thermal expansion coefficients between membranes and supports. However, some progress
has been made in optimization of reactor sealing and reduction of leaks across the seals.
ITM reactors reported in literature tend to be small-scale laboratory setups. As a result,
the areas that require sealing - reactor connecting parts and the membrane - are few and
small in size. The common trend is to apply custom-made gold, silver, or glass-based rings
to these areas and achieve sealing at a high temperature, close to the melting point of the
sealing material [83].
In most cases, an almost perfect sealing of the reactor is possible. The leak across the
membrane seal is generally quantified by calculating the leakage of air or oxygen based on
the concentration of nitrogen or other trace gas detected on the sweep side at temperatures
varying from room temperature to 900'C
[50].
The value of nitrogen leak (as part of air)
into the reactor sweep side is usually obtained using a gas analysis equipment such as a gas
chromatograph. The values of the oxygen (as part of air) and total air leaks are obtained
53
using equations similar to 2.11 and 2.12 respectively as shown below:
(
0.21
V0
2
,leak =
.
Vair,leak =
7 9 VN 2,leak
1
VN
0.79
(-
(.2
2,leak
(.2
where it has been assumed here that the molar fractions of oxygen and nitrogen in air are 0.21
and 0.79 respectively. V0 2 ,leak, VN2 ,leak, and Vair,leak stand for the leak flow rates of oxygen,
nitrogen, and air respectively. The quantified oxygen leak is considered when determining
the oxygen flux (see section 2.3.3).
Zhu et al. [84-87] reports the use of gold rings to seal a dual-phase membrane reactor
at 1040*C for POM experiments, and silver rings to seal membranes at 960'C for oxygen
permeation experiments. Schlehuber et al. [88] reports that when gold rings were used for
sealing a disk-shaped Lao.5 8 Sro. 4 Co 0 .2 Feo.8 03-3 (LSCF) membrane in a reactor which operated at 800'C for 3000 hours, the leakage level of oxygen was less than 5% of the oxygen
permeation flux.
Tan et al. [15] reports the use of high temperature water-based glass-ceramic sealant to
achieve sealing in a hollow fiber membrane reactor (HFMR) which uses a Lao.6 Sro. 4 Coo. 2 Feo.sO
(LSCF6428) membrane and achieves oxymel combustion. This results in a leak which is 0.4%
of the oxygen permeation flux. Wang et al. [38] reports <0.2% leak using a ceramic glass
binder, while Zhang et al. [89] reports that leakage was below the detection limit when glass
rings - produced from glass with a high melting point - were used. However, glass seals
have been reported to become highly reactive at high temperatures leading to failure of the
seal after several hours of operation [90]. Glass seals could also diffuse into membranes at
36
high temperatures thereby contaminating parts of the membrane and causing errors in the
calculation of oxygen flux and other parameters (e.g. fuel conversion) [29,91,92].
2.3.2
Experimental Methodology
Some experimental methods are common during ITM reactor operation. They include
the techniques developed for membrane heating rate and temperature range, flow rate of
inlet gases, fuel dilution, and post-operation membrane analysis.
Temperature
Temperature control is intrinsic to the operation of any experimental ITM setup. It is
generally achieved by placing the ITM setup in a temperature controlled furnace (generally
tubular). The rate of increase of membrane temperature is expected to be limited by the
thermal expansion of the various reactor components, as well as the volume of reactor material to be heated. If considerable time is required for heat equilibration, the heating of
the membrane and the reactor will need to be slow to avoid high temperature differences
within the setup. Most heating rates reported in literature tend to be relatively slow - the
most commonly reported heating rates are between 1 - 50Cmin-1 [15,48,93]. The typically
preferred temperature range is 700 - 900'C.
Inlet Gases
Due to the low oxygen flux derived in ITM experiments (in the order of 0.l 1pmol-cm-2.s
1
),
the flow rate of inlet gases are relatively low. They are controlled using mass flow controllers
55
upstream of the reactor inlets. In some cases, the inlet conditions are kept constant, but in
most reactive cases, the feed and sweep flow rates are varied to study their effects on oxygen
flux and the thermochemical processes. The flow Reynolds number (Re) range is typically
1 to 25.
Fuel
The fuel (i.e. methane) used in ITM reactors is usually diluted with an inert gas (usually Helium or Argon) to control the temperature and stoichiometry during reaction. This
is usually done by mixing the fuel and diluent streams after setting the needed flow rates
through their respective mass flow controllers. Reported fuel inlet concentrations vary from
0% to 100%. In most cases where the fuel concentration is kept constant during experiments [15,46,50], the value is usually <10%. Some cases however exist in which the sweep
side gas is always 100% fuel for reactive experiments [85, 94]. The most common trend is
to vary the fuel inlet concentration in order to study its effects on oxygen flux and the
macroscopic thermochemical processes.
Post-operation Membrane Analysis
In most cases, membranes are analyzed for phase, structure and surface changes after
their use. The techniques commonly used are X-ray Diffraction (XRD), Scanning Electron
Microscopy (SEM), and Energy-Dispersive X-ray spectroscopy (EDX). These techniques have
helped identify membranes which are stable in reactive environments and also to study the
effects of surface changes on the type of reactions which occur.
56
XRD yields the atomic/crystalline structure of materials and is based on the elastic
scattering of X-rays from the electron clouds of the individual atoms in the membrane. SEM
is used study the membrane surface topography and conductivities. The composition of the
membrane is determined using EDX.
2.3.3
Methods for Analysis
In order to obtain the values of oxygen flux and other macroscopic thermochemical processes, researchers tend to use gas analyzers and a mass balance analysis based on the type
of experiment.
Some researchers use an oxygen sensor/analyzer to determine the oxygen concentration in
gas streams [50]. However, the most commonly used gas analyzer is the Gas Chromatograph
(GC). GCs use a capillary column, a detector, and carrier gases to separate and quantify the
components of a gas mixture. The concentrations obtained from oxygen meters and GCs
can be used to calculate the actual values of oxygen flux and other processes as needed. The
most commonly values are oxygen flux, conversion of reactants, and the selectivity or yield
of products:
Oxygen Flux
The oxygen flux (Jo 2 ) calculations are usually based on the type of experiment/reaction
- separation, oxidative coupling, syngas, and oxymel. For oxygen permeation experiments
in which leakage through the membrane seal is considered [88], equation 2.13 is used. Here,
the oxygen leak has been incorporated into the flux equation.
57
In experiments for oxidative coupling of methane and syngas production [48, 50], equations similar to 2.14 are used. In this case, the oxygen flux is simply evaluated based on
the change in oxygen concentration in the air stream and its increase in the sweep stream.
For oxymel experiments, the equations would be similar to other reactive cases except for
the fact that the removal of water vapor before entry into the GC complicates the analysis.
Therefore the equation for oxygen permeation in oxymel reactions is estimated based on
the stoichiometric coefficients of the reactions to the carbon oxides and C2 products. An
example is shown in equation 2.15 [15]. The unit of the oxygen flux in the equations below
is cm 3 (STP)/min.
(
J o2
J0
2
Jo2
where
Xo
0.21
0
= 0. 7 9 7feed,in X
.79
1
= VSout (Xo 2 ,S,out+
XN 2 ,s,out
leakage,
-
sout
2
(2.13)
Vsweepin
N2
Sout
(1
-
(XO2 ,8,OUt/100)
-
(XN 2 ,s,out100))
(2.14)
X0 2 ,a,out
-
X0
2 ,a,out
2 xco
2
,S,oUt
+ 1.5xco,out
+ XC 2 H 4 ,S,OUt
+ 0.5xc 2 H6
,S,OUt)
(2.15)
is the measured molar fraction of nitrogen on the reactor sweep side indicating
,S,out is the measured molar fraction of oxygen on the reactor sweep side, X0 2 ,a,out
is the measured molar fraction of oxygen on the reactor air side, and Vsweep,out is the flow
rate of the product gas stream.
Conversion of Methane and Oxygen
The conversion of fuel is one parameter of interest for an ITM reactor since a high
conversion rate would usually indicate that the oxygen flux obtained is near ideal for a given
fuel flow rate. It can be easily calculated as shown in equation 2.16 [15]. The conversion
rate of oxygen is however a bit more complicated.
It is calculated based on the oxygen
concentrations in the air and sweep exit streams as shown in equation 2.17
XCH4=
(2.16)
XswepouCHsou
X
100%
Vweep,inxCH4,s,inX0
-
2 ,aout)
X02 =
100
[50]
1
-
sweepinX
Veein
0. 21 Vfeca,in
21
0.79Vfeed,inXO 2 ,a,out/ (1-
2
sout (1
-
202o)
(2.17)
where
XCH 4
XCH 4 ,s,in
represents the methane conversion, X0 2 represents the oxygen conversion, and
represents the mole fraction of methane in the sweep gas inlet stream.
Product Yields and Selectivities
Depending on the type of desired reaction, the selectivity and yield of a certain specie are
commonly used parameters for characterizing the ITM reactor performance. In the case of
0CM, C 2H4 is generally considered while for syngas production and oxymel combustion, CO
and CO 2 are considered respectively. The analysis is based on the assumption that methane
is the fuel feedstock.
Equations 2.18 and 2.19 [50] represent the yield (Yi) and selectivity (Si) of any carbon59
containing specie i.
Y = 100 x
i"s'o't
(2.18)
XCH 4 ,s,in
S, = 100 x
*
= 100 X
XCH 4
i ,sout
XCH 4 ,s,in
-
(2.19)
XCH 4 ,s,out
where ni is the number of carbon atoms in the molecule of a carbon-containing product i.
2.4
Analysis of ITM Experimental Investigations
ITM reactor technology has developed impressively since the mid-1980s. As discussed
in sections 2.1 and 2.2, there are many different membranes, membrane geometries and
structures, and applications. As a result of these developments, common trends are observed
with ITM reactor operating methods and processes.
The differences in membrane types, operating conditions, reactor geometries, inlet gases,
and applications present a set of challenges when an analysis into the overall governing
processes is to be conducted. Some of these challenges include large differences in oxygen
permeation flux (up to 3 orders of magnitude difference) and a lack of obvious trends for
fuel conversion and species selectivity for reactive analysis.
This section serves to provide an in depth analysis of ITM reactor governing processes.
The analysis is focused on normalization of oxygen flux using temperature and an effective
activation energy, to consider the effects of temperature and sweep side mass transfer, as well
60
as fuel conversion and CO selectivity in the case of reactive sweep gases. Thirty experimental
investigations have been studied in the development of the analysis in this section as shown
in table 2.2. An absolute (total) pressure of latm is assumed in both the feed and sweep
sides for all the investigations.
Table 2.2: Experimental investigations from literature considered for analysis
No.
Author
Year
Experiment
ITM Name
(geome
(geometry)
1998
0CM
LSCF-8264 (disk)
2
1
Zeng et al. [40]
2
Lu et al. [37]
2000
0CM
3
Zeng et al. [45]
2000
0CM
SFC (tubular)
BY25
(disk)
Feed Gas
ITM Sealant
V.G
& Silver
50% 02 + 50% N2
Helium
Air
Helium
Pyrex glass
Bi2 03 -based
Inert
Se
Sweep Gas
50% 02
+ 50% N2
Helium
T (reactive and non-reactive)
T
b
c
BSCF (disk)
2001
0CM
BSCF (disk)
Gold
2001
0CM
BYS (disk)
Glass & BYS powder
2002
Syngas
BCFZO (disk)
BCFZO (disk)
Gold
[95]
2001
5
Shao et al.
[96]
6
Zeng et al.
[46]
Tong et al.
[97]
SCFO
a
7
ONV
UNK3
Syngas
Dong et al.
4
c
50% 02
b
Yaremchemko et al.
[98]
10
Va,in = 200ml
V0
'in
(reactive)
(reactive)
Air
Helium
T (reactive)
Helium
T (reactive and non-reactive)
Helium
T (reactive and non-reactive)
Vs,in (reactive)
+ 50% N2
T
(disk)
Air
2003
Syngas
LSCG (disk)
UNK
Air
Helium
2004
Syngas
LCFC (disk)
Gold
Air
Argon
(non-reactive)
[99]
SCFO (ITM)
BCFZO (ITM)
BCFZO (ITM)
T (reactive)
(reactive)
VTi
T (non-reactive)
Diethelm et al.
= 300ml
&
XCH 4,in
zGH4,in
disk ITM
tubular ITM
a
T (non-reactive)
b
Va,in (non-reactive)
T = 800"C
c
Va,j,
(non-reactive)
T = 850*C
d
Va,in (non-reactive)
T = 875*C
a
(non-reactive)
V,
Vs,in (non-reactive)
T
T = 875'C
f
Wang et al.
[100]
2004
Separation-only
BSCF (tubular)
Air
Ceramic Glass
Helium
=
800*C
T = 850"C
g
Vs,in (non-reactive)
a
T (non-reactive)
b
T (reactive)
1% CH4
c
T (reactive)
3% CH4 at inlet
11
Ikeguchi et al.
[48]
2005
Syngas
SBCF (disk)
Pyrex glass
Air
T (non-reactive)
Vn
b
12
at inlet
XCH 4 ,on
Argon
a
13
V 0 ,i,
T (reactive)
Helium
a
b
T (reactive)
Air
a
8
(non-reactive)
T (reactive and non-reactive)
a
3
Other
Varied Parameter
Liu et al.
[101]
2005
cV
Separation-only
BSCF (hollow fiber)
Air
Silver
Helium
0 i
Vsi,
(non-reactive)
T = 850*C
(non-reactive)
(non-reactive)
T = 900*C
T = 950*C
a
T (reactive and non-reactive)
no catalyst
b
c
CHnno
T(ctiv
(reactive)
catalyst
catalyst
L
LaSr/Ca catalyst
Wang et al.
[38]
2005
0CM
BSCF (tubular)
Ceramic glass
Air or 0 2 /N2 mixture
Continued on Next Page ...
Helium
o
No.
Author
Year
ITM Name
Experiment
ITM Sealant
Feed Gas
(geometry)
Inert
Varied Parameter
a
T (non-reactive)
hb
t
e
(non-reactive)
HFMR(A), T =850*C
0
(non-reactive)
HFMR(A), T = 900-C
T
f
h
ST
Liu et al.
14
[771
2006
Separation-only
BSCF
Silver
(hollow fiber)
Air
Helium
1
V,,j, (non-reactive)
V,,,i, (non-reactive)
(non-reactive)
1,
a
b
c
Tam et at.
(5O]
2006
16
h
Zhu et at. [86]
2006
0CM
LSCF-6428 (hollow fiber)
Syngas
BCF1585 (disk)
hV,,j
Ceramic
glass
Gold
Air
Argon
V0 , ,_
V0 i.
Air
Helium
T
HFMR(C)
HFMR(C), T =850*C
HFMR(C), T =900*C
HFMR(C), T =950*C
(reactive)
(reactive)
(reactive)
rective
HM() T = 900*C
T (non-reactive)
L
0.4mm
b
Vs,,.,(non-reactive)
L
0.4mm
Ti (reactiecie)FRB)
T(o-ecie
Vs,in (non-reactive)
L
.m 0*
0.8mm
c
d
Zhamg et at.
[69]
2007
a
Syngas
LCCF (disk)
S
a
Glass
Air
Helium
0 ,i,
ds)V
V 0 ,i (reactive)
(reactii~e)
V V,
'in i (nonreactive)
V0 ,i (reactive)
d
e
f
b
a
b9
V0 ,in =l0ml
V0 ,,
20m1
T (reactive)
c
18
diskB)T =95*
(reactive)
(reactive)
T
b
Tam et at.
[15]
2008
Oxymel
Zu
[86]1
20106
Syngas
et al.
a
LSCF-6428
(hollow fiber)
BF58
Ceramic glass
Air
Argon
Vi,,
(reactive)
V,,.,
(reactive)
(disk)
Gold
Air
Helium
V0 ,in (reactive)
VnI(ecive
T (reactive) CHn
Vr,i,
a
Diethelm et at.
21
aa
Kozhevnikov at at.
[102]
22
b
Wamg et at.
2009
Separation-only
BSC(tubular)
Glass
Air
Argon
845*C
T =845
T
Pt
-4*C
T
catls
950"C
=
2009
Syngas
LSFTa1 (disk)
Gold
Air
Argon
2009
Syngas
LSF (ubula)
Glass
Air
Argon
Vs,,,
2009
Syngas
8CFZ (hollow fiher)
Gold
Air
Helium
T
(reactive)
T
(reactive)
(non-reactive)
T(ecie
dVContin(reactivex)
[103]
[49]
T
a
b
c
T
T =900*C5"
HM()
T =950*C
T (non-reactive)
Buysse et at. [56]
20
24
HFMR(B), T = 950*C
ha
a
16
= 850oC
HFMR(B), T =9OO*C
T (reactive)
15
17
HFMR(B)
HFMR(B), T
(non-reactive)
k
1
HFMR(A), T =950*C
(non-reactive)
Vj, (non-reactive)
V,,i. (non-reactive)
V0,,j
0 (non-reactive)
g
HFMR*(A)
0,
V,,(non-reactive)
d
1
Other
Sweep Gas
Va,i
Tianm et al. [105]
2010
Syngas
SC-LSC
(tubular)
Glass
Air
UNK
(reactive)
(reactive)
(reactive)
(reactive
m
V,in
)
(reactiven)Te=845*
Tag=.90"
tubula IT
HFMr(C)
No.
Year
Author
Experiment
ITM Name
Inert
Feed Gas
ITM Sealant
Varied Parameter
T (non-reactive)
a
b
Va, j
(non-reactive)
T = 750*C
c
V 0 ,i,
(non-reactive)
T = 800*C
d
Va,in
(non-reactive)
T = 850*C
e
Vain
(non-reactive)
T = 900*C
f
V,j
(non-reactive)
T = 950*C
g
h
Vs,in
(non-reactive)
T = 750*C
V,
(non-reactive)
T = 800*C
V0 ,in (non-reactive)
T = 850"C
(non-reactive)
(non-reactive)
T = 900'C
Wei at al.
25
[106]
2010
Separation-only
BSCF (hollow fiber)
Helium
Air
Glass
V,i,
V,,in
k
Zhu et al.
26
[85]
2010
Syngas
SDCSSF (disk)
Air
Gold
Helium
T
a
(reactive)
XCH
T
c
27
Gong et al.
[107]
2011
Syngas
LBFZ-0.2 (disk)
Silver
Helium
Air
4
,in
(reactive)
XCH
d
4
,in
(reactive)
L = 1mm
L =
mm
L = 0.5mm
L = 0.5mm
10% Ni/ty-A1
2
0 3 catalyst
10% Ni/-y-A1
2
0 3 catalyst
a
T
b
Vin
c
T
(reactive)
-y-A12 03 catalyst
T
(reactive)
LSCF catalyst
d
e
Kniep et al.
[93]
2011
Syngas
SCF (disk)
Silver
20X 02
+ bal.
Argon
(reactive)
Vs,in
Helium
(reactive)
XCH
29
Markov et al. [94]
2011
Syngas
LSF (tubular)
Glass
Air
Argon
b
2011
Syngas
BSCF (disk)
Ceramic Glass
Air
Helium
T
Shen et at.
[108]
4
,in
LSCF catalyst
LSCF catalyst
xCH 4 ,in
a
30
T = 950*C
T (reactive and non-reactive)
b
28
Other
Sweep Gas
(geometry)
T
(reactive)
(reactive and non-reactive)
'Used
as sweep gas in non-reactive experiments and/or as fuel diluent in reactive experiments.
2
V.G = vacuum grease
3
UNK = unknown.
ITM without laser ablation
ITM with laser ablation
2.4.1
Temperature Analysis
The effect of temperature increase on oxygen permeation flux is perhaps the most understood process in ITM reactor operation. Figure 2-12 shows the plots of Log (J0 2 ) vs 1000/T
for reactive and non-reactive analysis.
s seen from the plots, oxygen flux increases with
temperature. It can also be seen that rr ost investigators consider membrane temperatures
above 700'C. The experimental setups which use barium-containing membranes exhibit the
highest oxygen fluxes.
In order to carry out further analysis of flux dependence on reactor operating conditions,
it is necessary to normalize the flux across all investigations.
This is necessary because
the fluxes reported vary up to 3 orders of magnitude, and because of the differences in
each reactor. Since oxygen ion migration through perovskites is temperature activated
[89],
oxygen flux can be related to temperature via:
AO x
J0 2 =
exp
(
(2.20)
where Ea is the activation energy and AO is a pre-exponential factor which accounts for
parameters including the membrane thickness L, oxygen partial pressures (P
2
andP),
surface reaction constants (kf and kr), and conductivities (owrn and -e). E is the slope of
Log (J0 2 ) vs 1/T. Log (AO) is the y-intercept.
For the purpose of flux analysis, normalized oxygen flux J
Jt*>
_
J2
Ao x exp
65
(E.
(-
2
is obtained thus:
(2.21)
----\-----1Or-
0
CO
E
.
E
*Wang
-2 -
0
------
O
0
--
T-
Zeng1998
Lu2000
Zeng2000
Zeng2001
Tong2002 (a)
Tong2002 (b)
Diethelm2004 (a)
2004 (a)
Ikeguchi
2005 (a)
Liu2005 (a)
Wang2005 (a)
Liu2006 (a)
Liu 2006 (e)
Liu2006 (i)
9Zhang 2007 (a)
- Zhang2007 (c)
-
Buysse2009 (a)
- Diethelm2009 (a)
Wei 2010 (a)
Zhu 2010
Shen2011 (b)
-5
-6
.
0.7
0.8
0.9
1
1000/T [K~1]
1.1
1.2
1.3
(a)
3
----- Zenag
1998
2-
0--
.-
-
0.Tan
E
..
J...
Ikeguchi
....
)
E
-1
-2
~.
2 -
0
-j
-
-3-
0.78
5I
0.8
I
0.82
'
I
0.84
0.86
A
Luo
2010 (a)
-
Tian 2010 (a)
Zhu2010
Gong2011 (a)
Gong2011 (c)
Kniep2011 (a)
Kniep2011 (c)
- Kniep 2011 (d)
Shen2011 (a)
-Shen 2011 (b)
i
I
Dong 2001 (a)
Dong 2001 (b)
Shao 2001
Tong2002 (b)
2005 (b)
]keguchi 2005 (c)
Wang 2005 (a)
2006 (a)
Zhu2006 (a)
Tan2008 (a)
Tan2008 (b)
Diethelm2009 (a)
I
0.88
0.9
-
1
0.92
:
I
0.94
'
I
0.96
a
_
0.98
1
1000/T [K ]
(b)
Figure 2-12: Reported effects of temperature on oxygen permeation flux for: (a) separationonly cases; and (b) reactive cases.
Figure 2-13 shows the ratio of reactive to non-reactive pre-exponential factors for five
reported investigations. The ratio is greater than 1 as expected (since the reactive flux is
higher in all cases). In the case of Shen et al. 2011, the non-reactive flux reported barely
increases with temperature while the reactive flux increases by a factor of 2 from 750 - 850*C.
Furthermore, at 850*C, the reactive flux is an order of magnitude more than the non-reactive
Shen 2011 (b)
Zhu 2010
Wang 2005 (a)
Tong 2002 (b)
Zeng 1998
0
0.5
1
AOreactive /
1.5
2
2.5
3
Onon-reactive
Figure 2-13: Ratio of reactive/non-reactive pre-exponential factors
flux. This is most likely responsible for the large pre-exponential factor ratio reported by
Shen et al. 2011.
In figure 2-14, normalized oxygen flux is shown as a function of temperature for both
reactive and non-reactive cases. As expected, the flux values are fairly close to 1 in most cases.
This shows that the normalization method, albeit imperfect, is quite suitable for comparing
investigations with different membranes. The exception occurs with Liu et al. 2006 (a) in
which the flux showed a slow increase up to 700C and a rapid increase immediately after.
This could be due to an order-disorder transition within the membrane oxygen vacancies
as temperature is increased past a certain temperature [91,92,109,110] (700C in this case)
which would cause a change in Ea and is not reflected in figure 2-12.
It should be noted that, for the most part, many reactive ITM processes are yet to be
fully understood. However, of all the reactive processes, the dependence of fuel conversion
on temperature is the most understood. From figure 2-15, it can be seen that the fuel
conversion increases with temperature for all reported cases. In all cases, the fuel inflow was
kept at a constant rate, and oxygen permeation increases with temperature (as discussed
67
1.4
Zeng1998
Lu2000
Zeng 2000
Zeng2001
Tong 2002 (a)
Tong2002 (b)
Diethelm2004 (a)
Wang 2004 (a)
keguchi2005 (a)
Lu 2005 (a)
Wang 2005 (a)
Liu 2006 (a)
3
1.3
---
1
1.1
Liu2006(i)
Zhang2007 (a)
0.9
2007 (c)
-Zhang
Buysse 2009 (a)
Diethelm2009 (a)
Wei 2010 (a)
0.8
-Zhu
2010
Shen 2011 (b)
550
600
650
700
750
800
850
900
950
1000
1050
Temperature, T [0 C]
1.15 -Zang
1998
Dong2001(a)
Dong2001(b)
Shao 2001
1.1
Tong
2002(b)
-Ikeguchi
*
2005(b)
Ikeguchi
2005(c)
" 1.05
*
I
0.95
-
-
0.9
Shen2011(a)
0.85
0.8
700
Wang205(a)
Tan2006(a)
Zhu2006(a)
Tan2008(a)
Tan2008(b)
- Dietheln2009(a)
SLuo 2010 (a)
Tian2010(a)
Zhu2010
Gong2011(a)
Kniep2011(a)
Knip 2011(c)
Kniep2011(d)
Shen2011(b)
750
800
850
900
Temperature, T [*C]
950
1000
1050
(b)
Figure 2-14: Normalized fluxes as a function of temperature for: (a) separation-only cases;
and (b) reactive cases.
earlier). Therefore it can be assumed that the increase in fuel conversion is mainly due to the
increase in permeated oxygen available for conversion of the fuel as temperature increases
(and accelerates oxidation kinetics).
68
80
60
20
-
c
0
700
750
800
850
900
Temperature, T [(C]
950
1000
Figure 2-15: Reported effects of Temperature on CH 4 conversion
2.4.2
Mass Transfer Analysis
Mass transfer effects in ITM reactors has been studied by varying the inlet feed or sweep
flow rates. Most of the analysis however focuses on the sweep side. The general consensus is that an increase in the feed flow (mostly air) which already has the higher oxygen
partial pressure, will barely impact the local feed partial pressure at the membrane surface.
pin
00ea
Therefore, the chemical potential gradient and oxygen permeation flux across the membrane
barely increases with increasing feed flow.
The mass transfer analysis in this section studies the effects of Reynolds number and
residence time of the sweep gas. For the purpose of proper analysis, only reactors with
tubular or hollow fiber membrane configurations were analyzed since characteristic lengths
and active volumes are difficult to estimate (based on published data) for disk JTM setups.
The Reynolds numbers (Re) for the reported investigations are calculated via:
Re-
(2.22)
where p is the gas density, V is the inflow rate, and p is the gas viscosity. De is a characteristic
reactor length which represents the internal diameter for tubular and hollow fiber membrane
reactors. The gas residence time is calculated via:
=.(2.23)
V
where V is the active volume of the reactor: this represents the membrane volume in tubular
or hollow fiber reactors.
--
2.5
Wang 2004 (e)
Wang 2004 (f)
- Wang 2004 (g)
- Liu 2005 (b)
-
2 -*g
-
Liu 2005 (c)
-
-.-.-
-
-7-y-
Liu 2005 (d)
2006 (b)
. . . .Liu
.
o~ 1.5
Liu 2006 (c)
-----
Liu2006(d)
Liu 2006 (f)
Liu 2006 (g)
Liu 2006 (h)
-- V- Liu 2006 (j)
E--
1.-
z
-v-
Liu 2006 (k)
---
Liu 2006 (I)
-
0.5 -
0
2
4
6
10
8
12
14
16
Buysse 2009 (b)
Wei
Wei
Wei
- Wei
Wei
2010 (g)
2010 (h)
2010 (i)
2010 (j)
2010 (k)
ReResweep
H-
Figure 2-16: Non-reactive oxygen flux dependence on Reynolds number
Figure 2-16 shows the effect of sweep Reynolds number on oxygen permeation flux in
non-reactive cases.
Oxygen flux increases with Reynolds number but begins to level off
gradually as Reynolds number increases beyond a value of 5. It can be assumed that the
mass transfer resistance approaches its minimum value after Re ^ 5.
70
7
1.6
1.6
1.4-
1.4
7
1.2
--
Tan
Tan
-A Tan
,-A Tan
-A--Tan
1.2
1
2006 (c)
2008 (f)
2008 (g)
2008 (h)
2008 (i)
0.80.
0
Z
-4-Tan 2006 (c)
0.6
0.4
0.2
0
Tan 2008 (f)
-A-Tan 2008 (g)
-A-Tan 2008 (h)
--A-Tan 2008 (i)
0
5
10
15
Rsweep
20
z
0
0.6
0.4
25
0.20
___, _______........________
1
2
Residence Time (sweep) [s]
(a)
4
(b)
Figure 2-17: Reactive oxygen flux dependence on: (a) Reynolds number; and (b) residence
time. The sweep gas is a mixture of an inert gas (He or Ar) and CH4 .
Figure 2-17a shows the effect of sweep Reynolds number on oxygen permeation flux in
reactive cases. Oxygen flux shows a similar behavior to that observed in the non-reactive
cases. The oxygen flux also begins to level off beyond Re a 10. In this case however, it is
beneficial for the residence time to be higher than the non-reactive case. This is because
the fuel will need to react with the permeated oxygen. As seen in figure 2-17, the residence
times are generally higher than the non-reactive cases.
100
-4-Tan 2006 (c)
Tan 2008 (f)
-A Tan 2008 (g)
80
Tan 2008 (h)
--
Tan 2008 (i)
.2600
o4020
01
0
5
10
15
20
25
30
Resweep
Figure 2-18: Dependence of CH 4 conversion on sweep mass flow rate.
As with the case of temperature, fuel conversion shows a common trend as sweep mass
flow is increased. An increase in the sweep mass flow rate decreases the methane conversion
in all reported cases shown in figure 2-18. This is because the reactant (CH 4 ) residence time
within the reactor becomes shorter with increasing mass flow and increasing fuel/0
hCH4 ,s,in/7h02 ,perm)
2
(i.e.
ratio. Further increase in fuel inflow will increase the oxygen permeation,
but is detrimental to the conversion of the fuel.
2.4.3
Reactive Analysis
ITM reactor analysis under reactive conditions involves a number of factors which cannot
be compared easily (between different setups) by simply studying the effects of temperature,
mass flow, or fuel dilution. Amongst other things, the different fuel flow rates, fuel dilution
percentages, catalysts, and temperatures present different reaction pathways, and mechanisms within the ITM reactors.
However, a study of the effects of the fuel/0
2
ratio for all reactive cases of temperature,
mass transfer, and fuel diluent variations reveals some trends for fuel conversion and the
selectivity of CO (syngas experiments only). The performance of these reactors under reactive conditions can also be studied in comparison with equilibrium. An equilibrium state
implies the reactant and product concentrations do not change over time. Equilibrium was
calculated using Cantera (an open-source chemical kinetics software). For the purposes of
all reactive analyses in this study, the fuel/0
2
ratio is the ratio of fuel inlet molar flow rate
to the molar flow rate of permeated oxygen thus:
72
fuel
_
nCH ,s,in
(2.24)
4
n 0 2,perm
02
Zan1g
1990
2001(a)
099 9 2001(
b)
100
U
Dong
200
1 (d)
90
80
00o 2001
Tong
2002(0b)
Tong
2002c
(a)
D1eft 2004
2000)0)
Ik'g000i
2000(9
Wang
20050(2)l
To,,20060(9)
Tn 2006
(
T20200(
Zhu
2000(2a))
Zhu
2000
(b)
-kogo
40
60
Too
2000
(a)
Ton
2008
(b)
Kno
2000
() "Cd
.0
CD
0
100
5a2
o0
Tan20080(0)
a 2000
(f)
0+
Toan
2000
()
CH
:
40LTo
'r
30
20 21) 00)
Looh 2010
)0
02
L.02010
(0)
L4
00201 ()
Gong2011(0
A
20
9000g2011(d)
KiP21
Knip 201 o
Kniep2011)b
0
00
'
'
'
B'
ShoO()
I
5
10
15
Eqoh0lbiun
00'C
CH 4 :0 2 [-
Figure 2-19: Dependence of CH 4 conversion on fuel/0
2
ratio.
Figure 2-19 shows the dependence of fuel conversion on fuel/0
2
ratio.
Some of the
investigated ITM reactors operate close to equilibrium. In some cases however, the methane
conversion is far below equilibrium. One distinction that can be made is between disk and
tubular/hollow fibers. It can be seen from the plot that most disk reactors tend to achieve
equilibrium state of fuel conversion while tubular/hollow reactors do not. This could be
attributed to the longer fuel residence time in disk reactors.
For Tan et al. 2006 and 2008, it is possible that unknown reactor conditions contribute
to the low fuel conversion. In the case of Kniep et al. 2011 (c), blank '}-Al 2 0 3 was used in
place of high performing catalysts and this contributed to poor methane conversion. In some
other cases (such as Zeng and Wang et al.), OCM experiments were carried out leading to
low fuel conversion (since there is insufficient oxygen in the gas phase).
73
0
0
0
V
100
90
V
80_
Diethelm 2004 (a)
Ikeguchi 2005 (b)
Ikeguchi 2005 (d)
U Zhu 2006 (a)
6 Zhu 2006 (b)
U Kozhevnikov 2009 (a)
* Kozhevnikov 2009 (b)
* Kniep 2011 (a)
* Kniep 2011 (b)
Kniep 2011 (c)
Kniep 2011 (d)
2 Kniep 2011 (e)
* Kniep 2011 (f)
X Markov 2011
0
Equilibrium 850 C
10 -- - Equilibrium 9000 C
X
6050
40
30 20**
10 -'
0
0.1
(a)
(b)
(d)
(b)
(c)
*
-
70-
o
Dong 2001
Dong 2001
Dong 2001
Tong 2002
Tong 2002
1
CH4 : 02
Figure 2-20: Dependence of CO selectivity on fuel/0
2
ratio.
Unlike the methane conversion, reported CO selectivities are mostly close to equilibrium
as shown in figure 2-20. There are some slight overestimates (e.g. Kozhevnikov et al. 2009)
but it can be seen that high CO selectivity is generally achieved with increase in the fuel/0
2
ratio between 1 - 6.
Kniep et al. 2011 (c-f) are the only major exceptions: in these cases, LSCF and -y-A1 0
2 3
catalysts were used instead of the high performing syngas catalyst (Ni/y-A1 2 0 3 ) used in
Kniep et al. 2011 (a-b).
2.5
Conclusions
Significant research has been done in the experimental characterization of ITM reactors
for oxygen separation and fuel combustion over the last three decades. The investigation of
ITM reactors for methane conversion purposes has become a common trend due, in part, to
the benefits of combining oxygen separation and methane combustion in one unit.
74
The three major groups of methane conversion experiments are oxidative coupling of
methane (formation of C2 products), production of syngas (CO + H 2 ), and oxymel combustion (desired products are CO 2 , H2 0 and heat). A common experimental trend involves
studying the effects of important operating parameters - temperature, methane and air flow
rates, and methane feed concentration - on the oxygen flux, selectivities and yields of products, and methane conversion; though the understanding of the underlying processes and
how these parameters impact them is still somewhat open. These operating parameters are
the most important factors in determining the best operating conditions in ITM reactors.
A number of challenges with the use of ITM reactors have been identified. They include,
but are not limited to, low C2 yield in OCM reactions, damage of catalysts and membranes
at very high temperatures and reducing atmospheres (syngas), and difficulty in producing
pure CO 2 stream (oxymel).
An analysis of reported ITM experimental investigations provides an insight into the
governing processes for ITM reactor operation. As expected, oxygen permeation flux is
favored by temperature increase. The reactor sweep inflow rate also improves the flux, but
its potential for increasing the flux further is limited as minimum mass transfer resistance
approached. For reactive mass transfer analysis, increase of fuel flow rate is beneficial to
oxygen permeation but detrimental to fuel conversion as the fuel/0
2
increases. An analysis
of the fuel conversion and CO selectivity shows that some reported investigations operate
close to equilibrium based on the fuel/0
2
ratio. However, factors such as poor catalysts or
lack of sufficient oxygen in the gas phase could limit the reactor performance.
Currently used ITM reactor setups tend to be small-scale laboratory setups. The analysis
carried out is mostly 'global' and focused on the inlet and exit temperatures, concentrations,
75
pressures and flow rates. An in depth study into the effects of the oxygen-fuel ratio, the
specific reactions which occur, and a spatial resolution of the reaction zone will provide
further insight into ITM reactor operation.
Chapter 3
Experimental Approach
The development of the experimental ITM set up was done with the aim of fulfilling the
following goals:
1. Design, construction and assembly of a bi-directional stagnation flow reactor for study
into the response of oxygen flux and other processes (e.g. fuel conversion) to global
reactor operating parameters (e.g. flow rates, temperatures, fuel dilution, etc).
2. Spatial resolution of species within the reactor with the aid of optical and temperature
measurement techniques.
Experiments were carried out in the Reacting Gas Dynamics laboratory, making use of an
ITM reactor. The reactor was designed using commercial software application (SolidWorks).
A MATLAB code was developed for the data acquisition system (National Instruments PCI
6229) which controls and monitors operating conditions of the reactor and records data from
diagnostic equipment. MATLAB was also used for post-processing of data.
77
The experimental approach covered in this chapter provides an insight into the design
of the ITM reactor and its process control, the membrane used, and the methodologies
developed for experimental procedure and analysis.
3.1
Reactor Design and Installation
The most important step in the study of ITM reactors is the proper design and installation
of said reactor. The design of the ITM reactor was done based on the configuration of
Figure 3-1. The design ensures air flows in from the top while the sweep gas flows in through
the bottom of the reactor. As a result, during reactive experiments, the reaction zone occurs
at a stagnation region just below the membrane. After separation/reaction, oxygen-depleted
air flows through the top manifold while the reaction/separation products flow through the
bottom manifold.
The reactor configuration is therefore one-dimensional (along the stagnation streamline),
thereby facilitating a fundamental study of processes involved during separation and/or
reaction. Importantly, the configuration aids cross validation with in-house numerical 1-D
modeling work done along the stagnation streamline.
3.1.1
Reactor
The ITM reactor was designed for laboratory-scale oxygen separation and non-premixed
combustion experiments. It operates at atmospheric pressure and is housed in an enclosure
(discussed in section 3.1.2) to, among other things, minimize heat losses during experiments.
The reactor is made entirely of Inconel 601 - a specialty metal mainly composed of Nickel
78
ITM
ITM
C02,
H20
Optical
access
Reaction
Zone
streamline
CH4, C02
Figure 3-1: Basic schematic of the ITM reactor
(58-63%), Chromium (21-25%), and Iron (remainder). One reason for using Inconel 601
is its stability in reactive reducing environments and at the temperatures needed for ITM
reactions (currently >700'C). The stability of inconel gives it an advantage over stainless
steel which is stable up to 600-700'C and deteriorates/oxidizes under reactive conditions.
Inconel is also relatively easy to maintain and machine (unlike ceramics). Another reason
for designing the entire reactor body with inconel (bolts, nuts, panels, e.t.c) is to prevent
uneven thermal expansion that would lead to cracks, leaks and rust within the reactor.
The reactor (without its enclosure) is shown in Figure 3-2. The maximum dimensions
of the reactor are 49" x 5" x 50 (L x B x H). Air flows in through the top manifold and
fuel/diluent flows in through the bottom manifold while four exhaust manifolds provide the
flow path for the exhaust gases. Mass flow controllers (±1% accuracy) are used upstream
of the reactor inlets to provide a maximum of 2000sccm, 500sccm, and 50sccm for air, CO 2
79
inlet manifold
Pyrometer view access
SamplingAir
connection
(4 sampling points)
Sensor
connection
manifold
Sensorexhaust
Sensor connection
Exhaust manifold for
sepa ration/reaction
products
Cartridge
heater
Optical
access
Fuel/diluent
Inlet manifold
Figure 3-2: The ITM Reactor
(diluent) and CH 4 (fuel) respectively.
There are 10 connection points on the exhaust manifolds for sensors
-
pressure sensors,
thermocouples, oxygen sensor, and gas chromatograph. The exhaust manifolds have 4 points
each. The sampling lines used at these points are of 1/16" tubing (except the oxygen sensor
which is 23mm in diameter). There are four bores for cartridge heaters which provide fine
control of temperature. A sight tube which is 430 from horizontal allows access to the
pyrometer (installed outside the enclosure) to read the membrane temperature at the center
of the membrane.
Figure 3-3 shows the front section of the reactor and its main part (before exhaust).
The inlet manifolds are fitted with flow straighteners to allow for uniformity of flow characteristics along the stagnation streamline. Five sampling connections provide access for
80
Flow Straightener
Cartridge
Membrane
Heater
1.25"P
Sampling connection
(4 sampling points)
0.88"1
0.5"
(For adjustment of
sweep side gap height)
Thermocouples
Figure 3-3: Reactor front cross-section
thermocouples and Gas Chromatograph (discussed in section 3.3.3) sampling lines. The
topside sampling point provides access to analyze the feed composition and temperature,
while the four sampling points on the bottom provide access to the reaction/separation gas
species. Sampling can be done normal to the membrane (between membrane and exhaust
manifolds) on both sides thereby measuring the gas temperatures before separation or reaction. Sampling can also be done anywhere between the flow straighteners and the exhaust
manifolds on the sweep side, thereby aiding spatial resolution and numerical analysis. Each
sampling point can take up to four sampling lines (1/16" tubing) . Four other small inlets
allow for thermocouples to read the temperature of the center plate at four different points.
The gap for the feed side within the reactor is about 1.25" while that for sweep side is
about 0.88" (adjustable to 1.38"). The adjustability of the permeate side allows for studies
81
into the effects of gap height on the type of reaction and species formed. The 0.88" gap also
enhances mass transfer (for the low inlet flow rates used).
A future adjustment can be made to adapt the reactor for parallel flow experiments by
sealing off the current inlets and using one exhaust manifold on each side as inlet for feed
and sweep gases.
3.1.2
Reactor Enclosure
Enclosure
heater
terminal
Pyrometer
access
Exhaust
manifold
Optical
access
Inlet for cartridge
-
Enclosure
insulation
heaters, sensors,
and sampling lines
32"
Figure 3-4: The ITM Reactor within its insulation
As earlier stated, the main function of the reactor enclosure is to prevent heat losses from
the reactor. It is made of Gemcolite@ No-Smoke@ FG23-103 material and is stable up to
1260 0C.
Figure 3-4 shows the reactor within its insulation. The outer dimensions of the enclosure
82
are approximately 32" x 25" x 39" (L x B x H) and it is 4" thick. Just like the reactor,
there are two optical accesses made of quartz glass on opposite sides of the enclosure and an
access duct for the pyrometer. The cartridge heaters, sensors and sampling lines go through
four holes drilled into the insulation. The enclosure also houses the reactor support bricks
and six enclosure heaters. The enclosure heaters provide radiant heating within the enclosure
to the reactor body and membrane (see section 3.3.4).
3.1.3
Reactor Sealing
The sealing of the ITM reactor was one of the most challenging tasks during installation and commissioning. This is mainly due to gases escaping through metal-metal and
membrane-metal joints and partly due to the manifold flanges deforming during bolt tightening. A breakdown of the different sealants used is important and pressure/leak tests which
help verify the integrity/performance of the sealants is presented below.
Sealants
For the ITM reactor, different sealants were used for the membrane, manifolds (connections between manifolds and reactor main body), and other joints.
Membrane: Alumina felt (supplied by fuelcellmaterials.com) which is stable up to 1650'C,
is used to seal the membrane to the reactor center plate. Figure 3-5a shows the sealant
on the topside of the membrane. It is applied top and bottom of the membrane and
installed in the space shown. The sealing of the membrane reduces the available surface
area on both sides from 3.4" x 3.4" to 3" x 3". It should be noted that membrane
83
Membrane installation Point within
Reactor
(a) Membrane
Inlet manifold
Gasket
Reactor
Caulked Joint (ceramic adhesive)
(c) Caulked Joint
(b) Gasket
Figure 3-5: The different sealants used for the ITM reactor
sealing is the most crucial sealing within the ITM reactor because any non-quantified
leaks across the sides of the membrane from feed to sweep side would invalidate the
values of oxygen flux and other variables obtained during experiments.
Manifolds: Gaskets (Flexitallic@ Thermiculite® 867) are used for the connections between
manifolds and reactor main body. The gasket material is composed of a stainless steel
core (support) and mineral coating (sealant). It provides stiff sealing for the reactor
joints. As seen in Figure 3-5b, the gasket is designed with as little surface area as
possible in order to maximize seal preload.
84
Other Joints: A water-soluble ceramic adhesive (Cotronics® Durabond® 952) is used to
'caulk' the ungasketed joints, as seen in Figure 3-5c. It has similar thermal expansivity
to Inconel 601 and is therefore a near-ideal sealant at high temperatures. It takes
about 2 days to fully caulk the reactor: (1) the preparation and application of the
adhesive takes about 10 hours, (2) the caulk is allowed to set for 24 hours at room
temperature, (3) the caulk becomes stronger when allowed to set for 2 hours at 1000C
and an additional 2 hours at 200'C.
The order of overall reactor sealing is thus:
" Using alumina felt, seal the membrane to the center plate;
" Apply ceramic adhesive to the ungasketed joints. This is called caulking;
" Allow the caulk set at room temperature for about 24 hours;
" Assemble the reactor parts with gaskets used at the manifold joints; and
" Heat up the reactor and allow the caulk strengthen by keeping the temperature constant at 100'C and at 200'C for 2 hours each.
Sealant performance tests
Before proceeding to carry out experiments using the ITM reactor, it was important to
test the integrity of the sealants used. A basic pressure test along with the use of leak testing
fluid would help indicate the leak areas along the body of a reactor. However, further leak
tests are required to quantify the actual leaks within the reactor and through the membrane
seal.
Purging
>2000sccm CO2
Infiltration
test 1
02 sensor
Air?
>2000sccm CO2
250sccm CO2
Membrane
seal test
Infiltration
test 2
4"
500sccm Air
Air?
500sccm CO2
500sccm CO2
Figure 3-6: Sealant performance tests
Sealing (at 250C)
After Sealing (at 250 C)
After Sealing (at 210 C)
0.9 -Before
0.8-
- --
0.7
-
-
0
E 0.6-
-
-
-
0.5
S0.40.3
0.2
0.1 --
0
....
Infiltration Test 1
-.....
Infiltration Test 2
Membrane Seal Test
Figure 3-7: Sealant performance results: quantified in ymol.s-' of oxygen (from the air which
leaks into the reactor sweep side)
The tests were carried out at three different stages of reactor sealing development: (A)
before application of the sealants (measurements at room temperature, 25'C); (B) after
application of the sealants (measurements at room temperature, 25*C); and (C) after application of sealants (measurements at 210C). The reactor was purged with pure CO 2 before
each test. The oxygen sensor was used to monitor and measure the increase of oxygen
86
0.9
Membrane Leak Test
nfiltration Test 2
0.8
8 -
0.7 N
E
0.6k
E
0.5
ME 5--
0
0.4
a)
(U
0.3
(21--e
0.2
0.1
0
Before Sealing
(250 C)
After Sealing
(250 C)
After Sealing
(21 00 C)
0
Figure 3-8: Comparison of total leak at 500sccm air and 500sccm C0 2, to the expected
oxygen permeation flux (assumed to be in the order of 1pmol.cm- 2 .s 1)
concentrations on the reactor sweep side caused by any leak of 02 (in air) into the sweep
stream. Therefore, the air that has leaked into the reactor sweep side from the surrounding
environment (infiltration tests 1 & 2), or from the feed side (membrane seal test), can be
detected.
All measurements were carried out after the value of oxygen concentration detected is
equilibrated. The methodology for quantifying the leak (under non-permeating conditions,
at temperatures below 600'C) is explained in appendix B. Figure 3-6 shows the test configurations and the order in which the tests are carried out:
Infiltration test 1: The purpose of this test is to detect air leakage into the reactor at low
reactor flow rates, by quantifying the concentration of oxygen. It is carried out after
both sides of the reactor are completely purged with CO 2 . Thereafter, the sweep side
87
is set to 250sccm CO 2 flow. For this test and infiltration test 2, the feed side is filled
with CO 2 (no flow on the feed side).
Infiltration test 2: This test serves the same purpose as the first infiltration test, but is
carried out with a higher flow rate of CO 2 on the sweep side. Both sides of the reactor
are completely purged with CO 2 , then the sweep side is set to 500sccm CO 2 flow.
Membrane seal test: This test is used to quantify the leakage rate of oxygen across the
membrane seal and is carried out immediately after infiltration test 2. The flow rate
on the sweep side is maintained, while the feed side is set to 500sccm air flow. After
the new increased value of oxygen concentration has equilibrated, the leak across the
membrane seal is quantified by the difference between this new concentration and that
obtained during infiltration test 2.
As seen from the results in figure 3-7), the leak into the reactor is greatly reduced by
application of the sealants [NB: the membrane was always sealed using alumina felt during
all tests, hence the relatively constant value of membrane seal leak]. From infiltration test 2,
we see that the leak into the reactor is improved by a factor of 4 after sealing. The leak into
the reactor is however less predictable at lower flow rates (-
250sccm) as seen in infiltration
test 1. It can also be seen that the membrane seal leak is a factor of 5 less than the reactor
leak.
Figure 3-8 shows the total leak into the reactor sweep side at inlet flow rates of Vair,a,in
500sccm and Vco2,,in
=
500sccm. The leak flux has been divided by the membrane surface
area to enable comparison with expected oxygen permeation flux. The total leak into the
reactor sweep side (infiltration + membrane seal leak) after sealing, is about 0.23% of the
88
expected oxygen permeation rate across the membrane (1pumol-cm--2-1).
3.2
Membrane
The membrane is the most important part of the technology explored in this work.
Without it, this would only be an experimental combustion set-up. The membrane used for
any reactive ITM investigation must be suitable for long term use in reactive environments.
What follows is a brief overview of the membrane details and the pre-operation analysis work
done on the membrane.
3.2.1
Membrane Details
The membrane used for this work is a perovskite mixed ionic-electronic conductor (MIEC)
supplied by Ceramatec Incorporated. Its chemical formula is Laa. 9 CaO.1 FeO 3-6 (LCF)and it
was chosen for its balance between suitable oxygen flux and stability in reacting environments.
According the U.S patent 6,492,290 BI [111], membranes with the chemical formula:
(LnxCai-x)yFeO 3 -5
where Ln is La or a mixture of Lanthanides comprising La, and
1.0 > x > 0.5
1.1 > y > 1.0
have been shown to provide acceptable oxygen flux while being stable for syngas production.
Membrane
(LaO. 9CaO FeO 34 )
Sealing gasket
placed here
Optical Access
Fuel/diluent
Inlet manifold
Figure 3-9: Reactor top cross-section
Figure 3-9 shows the bottom part of the reactor exposing the membrane. The membrane
sits about 1/8" above the optical accesses: there are two optical accesses, made of quartz
glass, on opposite sides of the reactor to facilitate the monitoring, analysis and imagery of
the reaction zone. The membrane surface area used for oxygen flux calculations (section 3.5)
is 9in 2 or approximately 58cm 2 . There are three membrane thicknesses used: 2.2, 1.3 and
0.89mm.
3.2.2
Pre-operation Membrane Analysis
Due to the importance of surface exchange reactions on the membrane surface as well as
membrane stability under reactive conditions, it was necessary to analyze the membrane pre
and post-operation using X-ray Diffraction (XRD), Energy-Dispersive X-ray spectroscopy
(EDX), and Scanning Electron Microscopy (SEM) techniques. The analyses were carried
90
FeK
LaL
LaL
CaK
.710
1.40
2.18
2.83
LaL
4.20
3.58
4.98
5.1
6.30
keV
Figure 3-10: Pre-operation EDX graph
P =peak
P
C,
0
P
p
C
P
P
a)
P P
10
20
30
40
50
60
70
2 theta
80
90
Figure 3-11: Pre-operation XRD graph
out at the MIT Electrochemical Energy Laboratory'.
EDX provides the composition and chemical characteristics of the membrane as shown
in figure 3-10. The EDX technique is based on the interactions between electromagnetic
radiation and the membrane, and the analysis of X-rays emitted by the membrane in response
'The analyses were carried out mainly by Lei Wang.
91
Acc.V
Spot Magn
15.0 kV 3.0
Det WD
10000x SE
-2
Pm
10.0
Figure 3-12: Pre-operation SEM image
to being hit with charged particles.
XRD (fig. 3-11) yields the atomic/crystal structure of the membrane and is based on the
elastic scattering of X-rays from the electron clouds of the individual atoms in the membrane.
There are 7 major peaks observed, which help to identify the orthorhombic crystal structure
of the LCF membrane.
SEM provides an indication of the membrane surface topography and conductivities. The
technique involves using a high-energy beam of electrons to interact with the membrane's
atoms. The 2pm scaled SEM image for the pre-operation membrane analysis is shown in
figure 3-12.
3.3
Reactor Process Control and Instrumentation
This section provides an overview of the reactor plumbing, instrumentation, gas chromatography, heating, and safety. The process control and instrumentation (PC&I) for the
92
ABBREVIATIONS:
3P = Three-Phase
B= Circuit Breaker
Cart. = Cartridge
DAQ = Data Acquisition
Encl. = Enclosure
M = Mass flow controller
MP = Mass flow controller purge
P = Vacuum Pump
P-0 2 = 10% 02, Balance N2
F = Filter
GC = Gas Chromatograph
S = Control Switch
SCR = Silicon-Controlled Rectifier
K = Key Lock
Sol = Solenoid
R = Relay
RO = Rotameter
T = Temperature Controller
VAC - Alternating Voltage
SECTIONS (see line colors)
(1) Plumbing System
(2) Instrumentation
(3) Gas Chromatography
(4) Heating System
(5) Safety Control
Figure 3-13: Reactor Process Control and Instrumentation (PC&I)
reactor was designed to facilitate the optimization of flow and heating processes, sensor
positioning, as well as to aid the safety the reactor as a whole.
The major sections of the reactor PC&I setup (all discussed in this section) are shown in
figure 3-13. Each section can be identified by the color of the lines in the figure, and most
Figure 3-14: Measurement locations on the ITM reactor
sections are interconnected. At the center of control is the Data Acquisition System (DAQ)
which is connected to all sensors, mass flow controllers (MFC), solenoids, cartridge heaters,
the oxygen sensor, and the pyrometer. A computer using an in-house developed Matlab
data acquisition GUI controls the DAQ. The major process control and instrumentation
components of the ITM reactor set up are listed in table 3.1
Figure 3-14 shows a schematic of the major reactor processes and measurements which aid
the development of analysis equations (section 3.5) and the characterization of the reactor.
All temperature measurements (except for the membrane) are done using thermocouples.
94
Table 3.1: Process Control Equipment and Instrumentation
Equipment/
Instrument
Application
Measurement(s)
Manufacturer and
Model
Mass Flow
Controllers
Pyrometer
Inlet flow rate control
Vfeed,in, VCH 4 ,s,in,
Sierra Smart Trak 2
Vco 2 ,s,in
T
Impac IGA300
Thermocouples
Gas and reactor
temperature
Tfeed,in, Tfeed,out,
Tsweep,in, Tsweepout,
measurements
Tenc
Pressure sensors
Gauge Pressure mea-
Pfeed,out, Psweep,owt
Oxygen sensor
surement
Oxygen
concentration
X0
Data Acquisition
System
Cartridge heaters
Instruments and cartridge heater control
Reactor heating
National Instruments
Enclosure heaters
Reactor heating
Temperature
controllers
Silicon-controlled
rectifier (SCR)
Gas Chromatograph
Relay and SCR
control
Enclosure heater
control
Gas concentrations
I SQUARED R ELEMENT SEU Starbar
Omega
CN77R352-C2
Spang Analog
C-Series
Agilent [t-GC 490
Quad
Membrane
temperature
measurement
2 ,a,out,
Xo
2
,S,o
0 t
XH
2
,
X02,
N 2 , iXCH 4 ,
The temperatures of the inlet gases just before the membrane
(Tfeed,out
Omega PX309
Bosch LSU4.9
Dalton Watt-Flex
CO,, I C2 Hz
the exits from the reactor
Omega Type-G
(Tfeed,in
& Tsweep,in) and at
& Tsweep,out) are measured. Also, the temperatures at
different points in the reactor center plate (T 1 _4) and inside the enclosure (Tec) are measured
to control the radiant and enclosure heaters respectively (section 3.3.4). The temperature
of the membrane (T) at its center is measured by the pyrometer.
Pfeed,out
and Psweep,out are the pressures measured at the reactor exits. The oxygen sensor
provides a realtime measurement of the equilibrium oxygen concentration in the sweep side
(x0 2 ,8 ,out) while the gas chromatograph (CC) provides the concentrations of the various
separation/reaction products in the sweep side exit. The mass flow controllers (MFC) control
the inlet flow rates of gases
3.3.1
(7rifeed,in
&
Tmfsweep,in).
Plumbing
The reactor plumbing system is shown in figure 3-15.
It incorporates multiple flow
configurations for:
" the desired experimental flow processes,
" purging of air out of the reactor (using C0 2 ) before experiments,
" purging of the reactor and its enclosure (using CO 2 ) in the event of an emergency, and
" calibrating the oxygen sensor. This is done regularly in-between experimental runs.
The procedures for flowing gases into the ITM reactor are detailed in appendix C.
The mass flow controllers are placed upstream of the reactor inlets to control the inlet
flow rates, and provide a maximum flow rate of 2000sccm, 500sccm, and 50sccm for Air, CO 2
and CH 4 respectively. The CH 4 solenoid ensures that methane flow can be stopped manually
and through the DAQ while the enclosure solenoid provides a safety mechanism for quick
purging of the reactor enclosure with CO 2 in the event of an emergency (see section 3.3.5).
96
Figure 3-15: Reactor Plumbing Schematic
3.3.2
Instrumentation
The reactor instrumentation is composed of pressure sensors, thermocouples, the pyrometer, and the oxygen sensor. The pressure sensors indicate the absolute pressure on each side
and can be used to detect membrane breakage. The other three instruments provide important data for reactor characterization. Figure 3-16 shows the locations where instruments
can be placed in the reactor.
-_I-1
_
Figure 3-16: The major reactor instrumentation
Thermocouples
Thermocouples are used for two purposes within the reactor, (1) to measure the temperature of the reactor center-plate, and (2) for measuring the gas temperatures within the
reactor and at the gas exhaust.
As seen in figure 3-17, four thermocouples (Omega type-G) are placed within the reactor
center-plate, adjacent to the cartridge heaters. These thermocouples provide the temperatures (T1
) that
enable the monitoring of the reactor temperature.
thermocouple
connection
for feed inlet
therm ocoupie
for sweep inlet
membrane
position
cartridge
heater
cente -plate
thermocouples
Figure 3-17: Reactor thermocouples
Figure 3-17 also shows a thermocouple placed to read the temperature of the inlet sweep
stream (Tsweep,in) just before the membrane. This is also done on the feed side of the reactor.
Both measurements aid in the calculation of the change in temperature and heat transferred
to the gases before separation or reaction (for air or fuel/diluent respectively).
temperatures at the exits
(Tfeed,out
The gas
& Tsweep,out) are also measured using thermocouples.
98
It should be noted that thermocouples can also be placed in a variety of places within the
reactor gas streams to enable detailed analysis of localized effects with the specific goal of
providing validation measurements to the numerical work done on the ITM reactor in other
investigations.
Pyrometer
The membrane temperature is an important part of the operation of any ITM reactor. As
discussed in detail in section 2.2, variations in the membrane temperature influence oxygen
permeation rate, reaction thermochemistry and stability of reactive ITMs.
Distance from
membrane to
pyrometer mount
location = 18"
Angle of
pyrometer
Pyrometer
View access
view = 43'
Membrane
Figure 3-18: Membrane temperature measurement configuration
The membrane temperature for the reactor in this work was set by the radiant and
99
enclosure heaters and was measured using a Pyrometer (INFRATHERM pyrometer IGA
300) which has a measurement range of 300 - 1300'C and sends a current output (converted
to voltage) to the data acquisition system. Its accuracy is within 0.8% of the measured
temperature + 1C, i.e. i(0.008*Tmeas. + 1C).
The pyrometer is located outside the enclosure to prevent it from overheating.
It is
mounted at an angle of 430 from horizontal and about 18" away from the membrane as
seen from the side section of the ITM reactor in figure 3-18.
There are three sapphire
glasses between the pyrometer and the membrane (two within the enclosure and one on
Sapphire was chosen because of its spectral
the pyrometer view access on the reactor).
(transmission) properties.
15
0.9
1100
1000
900
CU
E
10 C
CLID800
E0.8-
~
E
700
08
-
C
E
600
a)
500-
-0-
Emissivity
-G-Current
400
1
2
4
5
Output Voltage [V]
3
6
7
0.7
400
500
600
700
800
0
900
5
1000
Membrane Temperature [ C]
(b)
(a)
Figure 3-19: Pyrometer calibration data: (a) membrane temperature vs pyrometer voltage
output; (b) membrane emissivity and pyrometer current output vs membrane temperature
= (62.5 * I + 323) /T)
The pyrometer was calibrated using a furnace and adopting a geometric configuration
similar to its installation configuration within the reactor and using the same membrane
type (Lao.gCao.1FeO 3 -6) used for the experiments. The calibration was carried out using
100
increments of 50'C from 500 - 1000'C.
As seen from the calibration data in figure 3-19a, the membrane temperature is linearly
related to the voltage output of the pyrometer. This calibration data was incorporated into
the Matlab code developed for the ITM reactor thereby enabling the control/monitoring of
the membrane temperature through the data acquisition system. Figure 3-19b shows the
relationship between temperature, pyrometer current and emissivity of the membrane.
The emissivity
(F) of the membrane
drops from 0.884 to 0.768 as the membrane temper-
ature is increased from 500 - 1000'C. The membrane emissivities were obtained, based on
the manufacturer's recommendation thus:
S
T )4
(62.5*1+323)4
(3.1)
(3.2)
where I is the pyrometer current reading in mA, T is the membrane temperature in Kelvin,
and T is the temperature of a black body (in Kelvin) which produces the same pyrometer
current reading as the membrane.
Oxygen Sensor
The oxygen sensor is used to provide the realtime value of oxygen concentration in the
reactor sweep side exit. It is therefore invaluable during reactor leak testing and oxygen
permeation experiments.
The oxygen sensor used is an automotive type Bosch LSU4.9
universal exhaust gas oxygen sensor (UEGO).
The sensor can measure oxygen concentration within the range of 0 - 0.21 bar in exhaust
101
Exit duct
12 1
Oxygen sensor location
#023mm
41'
Entry duct
$ 1/4"
2
, ,
2.5
3
3.5
Voltage Output [V]
4
4
Figure 3-20: Oxygen sensor calibration: (a) calibration duct; (b) calibration data
gases and it operates up to a gas temperature of 1030'C. Its accuracy is within 0.24% of the
measured oxygen concentration.
The sensor is calibrated using a duct with a similar cross-section as the reactor exhaust
manifold (see Fig 3-20a). The method is to flow:
1. pure CO 2 (i.e. 0% 02),
2. nitrogen and oxygen mixture (10% 02), and
3. air (21% 02)
through the duct and then record the equivalent values of voltage output from the oxygen
sensor.
The 0-10% range (shown in figure 3-20b) is the range of oxygen concentration expected
within the reactor sweep side.
As with the case of the pyrometer, the calibration data
was incorporated into the Matlab code developed for the ITM reactor as well as the data
acquisition system.
102
3.3.3
Gas Chromatography
The measurement of species concentrations is intrinsic to the overall goal of ITM reactor
characterization. These concentrations help determine the oxygen flux across the membrane,
fuel and oxygen conversion, and the selectivities/yields of products. The detection of a
certain species is also a general indication of the occurrence of a type of reaction. A gas
chromatograph is used to carry out the concentration measurements.
The Gas Chromatograph (GC) used is an Agilent fp-GC 490 Quad. It is used to characterize the oxygen flux (02) as well as reaction products (02, CH 4 , COX, C2 Hz, H2 ,...) by
reading the concentrations of the gases in the exhaust streams. The GC requires the use of
Ar and He carrier gases and is equipped with a thermal conductivity detector (TCD). The
detector responds to the difference in thermal conductivity between the carrier gas and the
sample components.
The minimum detection limit for the GC is 10-20ppm and the sample time is ~ 30
seconds. It uses three columns:
" Column 1 (molesieve 10m, Ar carrier gas): detects H2
"
Column 2 (molesieve 10m, He carrier gas): detects 02, N2 , Ar, CO & CH 4
* Column 3 (Pora Plot
Q 10m,
He carrier gas): detects other gases (CH 4 , C0 2 , C2 Hx)
Figure 3-21 shows the GC sampling schematic. In order to analyze samples within the
reactor, it uses sampling tubes (1/16" tubing) which can be inserted through any of the sampling points explained in section 3.1.1. Therefore, it is possible to analyze the concentration
of gases in the feed and sweep side exhausts. The sample gas is filtered before entry into the
103
He
Ar
Sample
HF
H20 to Vent
-
FR
caJ
Gas to
Vent
GC
-I
Membrane Filter
1/16" tubing
V
Figure 3-21: Reactor Gas Chromatography
GC to remove any water (liquid or vapor). Data from the GC is read and recorded using
Agilent EZChrom software.
3.3.4
Heating
The heating of the ITM reactor is important to facilitate separation and reaction since
these processes occur only at high membrane temperatures (currently >700'C). The reactor is heated up using two sets of heaters - cartridge heaters and enclosure heaters. The
cartridge heaters ensure fine control of the reactor and membrane temperatures while the
enclosure heaters provide additional uniform heating capacity. Figure 3-22 shows the major
components of both heating systems.
104
-
-
S Key Lock
-
-
-
Silicon-
Distribution
-
SCR Controlled
Block
-c-
Rectifier
Master
R
Switch
Relay
SRCircuit
Breaker
LI
Tmpo .Circuit
Tm'Breaker
Cont120VAC
120VAC
4
Tem p.
Cont.
120VAC
208VAC (3P)
Figure 3-22: Reactor Heating Systems
Enclosure Heaters
The enclosure heaters, supplied by I Squared R Element Company, consist of six silicon
carbide heating bars connected to three-phase power (two heating bars per phase). They
are installed within the reactor enclosure to provide uniform heating to the reactor. As seen
in figure 3-23, the heaters have a cold section supported by the reactor enclosure material,
and a hot section fully exposed within the reactor enclosure. The enclosure heaters alone
provide a total output of 15kW.
The control system for the enclosure heaters, as seen in figure 3-22 consists of the heating
bars, a silicon-controlled rectifier (SCR), and a temperature controller. The temperature
controller provides a set point to the SCR using variable current control. The enclosure
thermocouple (located in the top inside of the reactor enclosure) provides the enclosure
temperature reading to the temperature controller. The SCR, which uses three-phase power,
105
Enclosure
Heater
terminal
Enclosure
Heater
(hot section)
Enclosure
Heater
(cold section)
Enclosure
Figure 3-23: The Enclosure Heaters
is used to control the heaters.
Cartridge Heaters
The cartridge heaters consist of four Inconel Watt-Flex@ heating bars. They are supplied
with single-phase power (120V) and are installed within the reactor center plate as shown
earlier in figure 3-17. The heaters provide a total output of about 2kW, and are operated
using on/off control, with the set point and process values given by the DAQ software. With
the enclosure heaters installed, the role of the cartridge heaters is to provide additional
heating and fine control of membrane temperature (due to their proximity to the membrane).
The control system for the cartridge heaters as shown in figure 3-22 consists of the heating
bars, a solid state relay and a temperature controller which receives the set point via the
106
DAQ GUI. The process value is the average plate temperature as calculated by the DAQ.
300
-&E
280-
Cartridge Heaters Only (SP:2500 C)
Cartridge Heaters (SP:2000 C) + Enclosure Heaters (SP:250oC
260o
240 -
220-
-.
.
.-.
-.
- ..-.-
U)
0.
E 200
180160 140
.
0
'
.
5
10
'
15
'
'
20
'
'
'
'
I
I
25
30
I
I
35
Position [cm]
Figure 3-24: Heating tests with the cartridge and enclosure heaters (SP = Set Point). N.B:
The green vertical lines indicate the cartridge heater locations
Figure 3-24 shows the results of heating tests carried out using the cartridge heaters and
enclosure heaters. The results are shown as a function of position from the first heater (0cm)
to the last (32.5cm). The reactor center-plate temperatures are provided by thermocouples
as shown in figure 3-17. When the cartridge and enclosure heaters are combined, the entire
reactor can be heated almost uniformly at the desired temperature. However, the cartridge
heaters take longer to equilibrate on their own, as seen from the difference in temperatures
between the cartridge heater and center plate locations during the cartridge heater test.
3.3.5
Safety Provisions
In order to ensure safe operation, it was important to design a safety system for the ITM
reactor.
Figure 3-25 shows the safety network designed for the ITM reactor. The main control
points are:
107
Key lock: This is used to arm or disarm the heating system and fuel solenoid. Without
arming the lock with a key, the heaters cannot be operated by using their control
systems or the DAQ. In the same way, the methane solenoid cannot be operated by its
switch or the DAQ system unless the lock is armed.
Master switch: This is used to immediately switch off the heating systems and methane
solenoid in an emergency during reactor operation.
Enclosure purge switch: This enables the flow of CO 2 to the enclosure, thereby facilitating the cooling down of the enclosure area and diluting any methane present, and
ultimately reducing the possibility of any reactions or fire occurring within the enclosure.
Enclosure Purge Switch
Figure 3-25: Reactor Safety Control
108
What follows is a brief discussion of the possible hazards that can be faced while operating
the heating system or flowing methane in the ITM reactor, as well as steps that can be taken
to prevent them.
Heating Hazards
The most likely hazard that can occur with the heating system is overheating. It is possible for the heaters to exceed the set point temperature provided through the temperature
controllers. In this case, simply pressing the emergency switch would shut down the heaters
and the heating systems. Both controllers also have an upper limit alarm to prevent further
heating.
Another hazard, although less likely, is the burning of unwanted objects (e.g. cleaning
cloth) within the reactor or its enclosure. In this case, the safety procedures described under
Methane flow Hazards below should be followed.
Methane flow Hazards
During reactive experiments, it is possible for the methane to leak into the reactor enclosure and react with the air within the enclosure leading to a fire. It is also possible for an
unwanted reaction to occur within the reactor.
The following steps will help eradicate or mitigate any methane hazard within the ITM
reactor setup:
1. Press the master switch. This turns off the heaters and methane solenoid;
2. Turn on the enclosure purge switch. This purges the reactor enclosure with CO 2 (The
109
mixture of CO 2 and other gases is sent into a vent using a vacuum pump); and
3. Purge both sides of the reactor using the purge procedure described in appendix C
3.4
Experimental Methodology and Procedures
This section provides a discussion of the experimental procedures and techniques developed for characterizing the ITM reactor.
The type of combustion that occurs in the ITM reactor is non-premixed in nature. Nonpremixed combustion would be limited by the mixing of the reactant since the chemical
reaction rates are much faster than the diffusion rates. A major advantage of non-premixed
combustion is that the formation of a thin reaction front can be assumed, as the timescale
of the chemical reaction
(Tchem)
becomes smaller than that of diffusion
(Tdfgf).
In the case of the ITM reactor, the mixing of reactants is limited by the oxygen flux
across the membrane and this further increases the diffusion timescale. As a result, unwanted
conditions such as pyrolysis of the fuel or low fuel conversion rates could easily occur if proper
methods are not developed for controlling and measuring the membrane temperature, inlet
gas flow rates, and inlet gas concentrations.
Two types of experiments were carried out using the ITM reactor:
Permeation-only experiments: In this case, air (21% 02) flows through the feed side
inlet while pure CO 2 flows through the sweep side inlet. Oxygen permeates across
the membrane from the feed side to the sweep side due to the difference in chemical
potential (or partial pressure of oxygen) across the membrane. Therefore, the feed side
110
exhaust manifolds would transport oxygen-depleted air while the sweep side exhaust
manifolds would transport a mixture of O2 and CO2Reactive experiments: In this case, air (21% 02) flows through the feed side inlet while
a mixture of carbon dioxide and methane flows through the sweep side inlet. Oxygen permeates across the membrane from the air side to the sweep side as with the
permeation-only case. Ideally, methane reacts with the permeated oxygen on the sweep
side to form carbon dioxide and water vapor according to equation 2.10. However, the
products of combustion may also contain carbon monoxide, hydrogen, unconverted
CH 4 , excess 02, and/or other hydrocarbons (HC). Therefore, the air side exhaust
manifolds would transport oxygen-depleted air while the sweep side exhaust manifolds
would transport the products of combustion.
As discussed earlier, the performance of an ITM reactor for methane combustion is usually
based on the yield or selectivity of a carbon-containing product. Similarly, for the work done
in this study, the primary variables of interest or performance indices are:
1. Oxygen flux (J02) across the membrane;
2. Carbon dioxide yield (YC0 2 ), for reactive experiments only; and
3. Exhaust gas composition.
The methods for deriving of these variables are outlined in section 3.5 (along with the
methods for deriving selectivities and yields of carbon-containing species (C0 2 , CO, C2 ),
and the conversion of methane). The major focus of the work presented in this study, is the
111
effects of temperature, inlet gas flow rates, and methane inlet concentration on the primary
variables of interest.
3.4.1
Overview of experimental procedures
All experiments and measurements are conducted at atmospheric pressure within and
outside the reactor. Before every measurement is conducted, the reactor is purged on both
sides with about 2000sccm CO 2 for about 5 minutes to remove any air within the reactor.
All measurements are conducted at the temperature, flow rates, and CH 4 inlet concentration
desired after allowing the gas concentrations equilibrate within the reactor.
All measurements are conducted at membrane temperature (T) between 700-800'C, sweep
flow rate (Vsweep,in) between 200-500sccm, and methane flow rate (VH 4 ,,,in) between 0-
20sccm as shown in table 4.1. The feed side inlet gas for all experiments is dry air. The sweep
side inlet gas is pure CO 2 for permeation-only experiments, and CO 2 + CH 4 for reactive
experiments. The CO 2 acts as a diluent in the reactive case. Mixing occurs after the mass
flow controllers, and the fuel/diluent ratio is based on the volume flow rates obtained from
the mass flow controllers. It is important to note that when experiments are conducted to
study the effects of one parameter on the primary variables of interest, other parameters are
kept constant.
Table 3.2: ITM reactor measurements
Experiment
T
(0 C)
varies
Vfeed,in
Vsweepin
VCHs,in
Permeation-only
Controlled Parameter
(range)
T (700-800-C)
(sccm)
500
(sccm)
500
-
Reactive
Sweep,in (200 - 500sccm)
VCH4,s,inh (0 - 20sccm)
800
800
500
500
varies
500
112
(sccm)
-
varies
3.4.2
Temperature Control
The heating system for the ITM reactor serves to provide a two-fold heating function.
The enclosure heaters ensure uniform heating within the reactor and its enclosure, while the
cartridge heaters enable fine control. The reactor is subject to a heating rate of 1.2'C min-'
to enable uniform heating of the reactor mass and membrane, and to avoid uneven expansion
which could lead to membrane breakage or reactor leakage.
As heating is carried out within the entire reactor setup, the membrane temperature is
monitored by the pyrometer. Two thermocouples installed just below and above the membrane measure the change in temperature of the inlet gases, thereby aiding the calculation of
the convective heat transferred to the gases along the stagnation streamline, and thus aiding
in-house numerical analysis.
Measurements are carried out between 700-800'C, mainly because the oxygen flux obtained below 700'C is too low. Also, because of the slow heating rate (1.20C/min), it takes
about 10 hours to heat the membrane (and reactor) from room temperature to 700'C.
For experiments which study the effect of the membrane temperature on the primary
variables of interest, measurements are conducted using 50'C increments from 700'C - 800'C
for the membrane temperature.
3.4.3
Inlet Gas Control
The inlet flow rates reported for current experimental ITM reactors are usually very low
(Re = 1 - 50). As discussed earlier (section 2.3.2), the main reason for these low flow rates
is the low oxygen flux expected across the membrane during experiments (in the order of 1
113
ymolcm-2.s-1).
In addition, high flow rates on either side of the reactor could potentially
lead to:
" A high concentration of soot formed from methane pyrolysis, due to exposure of the
methane to high temperatures and low concentration of the oxidant;
" Increase in unreacted methane in the product stream; and
" Breakage of the membrane, especially if the gas pressures on both sides are unequal.
For permeation-only experiments, the sweep inlet flow rate was increased using 100sccm
increments from 200sccm to 500scem. For reactive experiments, the effect of fuel dilution
was studied by using 5sccm CH 4 increments from 10sccm to 20sccm (while keeping the CO 2
inflow rate constant at 500sccm).
3.5
Methodology for Analysis
The methodology for analysis involves the development of a simple set of equations for
the macroscopic thermochemical processes within the ITM reactor. All analyses rely heavily
on the mole fractions of gases measured at the end of the reactor exhaust manifolds. The
mole fractions are measured using a Gas Chromatograph
(GC).
For all calculations, it should
be noted that all H2 0 (vapor or liquid) is condensed out of the product gas mixture, and
the gas mixture is filtered before entry into the GC (see section 3.3.3)
114
3.5.1
Permeation-only Analysis
The main variable to be measured during permeation-only experiments is the oxygen
permeation rate across the membrane, and the nitrogen concentration (for leakage quantification). For oxygen flux calculations to be made, we need to analyze the flow of gases in
and out of the reactor sweep side.
The flow rates of gases into the reactor are given in standard cubic centimeters per minute
(sccm). The standard conditions for these flow rates are:
* standard temperature (Tt) = 273.15K
" standard pressure (Pt) = 101325Pa (or latm)
The mass flow rate (in kg-s-1) of sweep gas (pure CO 2 ) into the reactor is therefore:
Thsweep,in
MCo2
Mo
x
X Vs8weep,in
RXPst Tn
(33)
where R is the universal gas constant and M is the molar mass. We make three considerations
in this analysis:
Firstly, we must take into account the leak into the reactor sweep side. We do this by
measuring the nitrogen concentration
(xN2 ,s,Out)
on the sweep side exit stream. The method-
ology for leak detection and quantification in the reactor has been discussed in section 3.1.3
and appendix B. The leak
(X0
2
,1eak)
for this analysis is quantified using:
0.21
X0 2 ,leak
0.79 zN2,s,out
giving the actual concentration of permeated oxygen
115
(Xo 2 ,perm)
(3-4)
detected in the sweep outlet
X0 2 ,perm = X0 2 ,s,out -
where
X02,S,out
X0 2 ,leak
(3.5)
is the total oxygen concentration detected in the sweep outlet. The average
molar mass (M) of the gas mixture at sweep side exit is:
M=
(x x M)
(3.6)
Therefore, average molar mass (M) of the gas mixture at sweep side exit (permeation-only)
is obtained from
M = (Xo2,Sout x M0 2 ) + (xN2,S,Out
where M is molar mass, and
XCo
2
X MN2 )
+ (XC0 2 ,s,
0 2 x MCo2)
(3.7)
,S,out is the concentration of CO 2 detected at the sweep side
exit. The mass fraction of any product i is obtained from
M.
A
(3.8)
(mfo2 ,perm),
as detected in the sweep side is
mfi'S'Out = xiS,02t x
The mass fraction of oxygen permeated
therefore
Tmfo 2 ,perm =£02,perm X<
M
(3.9)
Secondly, using the law of mass conservation, we assume the mass flow rate of the gas
leaving the reactor sweep side (rhsweep out) is the related to the mass flow rate of the gas
116
entering the reactor sweep side
msweepout
(hsweep,in)
thus:
msweep, in
-i
XO2,8,OUt
-
X
Mo 2
M
--
(3.10)
N2,s,out X
MN 2
R
Thirdly, we quantify the molar flow rate of permeated oxygen into the sweep side thus:
7O 2 ,perm
mfo 2 ,perm X msweep
(3.11)
out
With all the above considerations, the oxygen flux equation for permeation-only experiments can be formulated.
The oxygen permeation rate across the membrane (in
mol-cm-2. s-1) is given by:
JO2
= AO 2,perm
(3.12)
A
Or for direct analysis based on available data,
VCO2,in
JC 2
(XO 2 ,8 ,out
XCO
-
0.1XN 2 ,s'oUt)
2 ,8 ,oUt
_St
Tst RA
(3.13)
where A is the membrane surface area exposed to gases on one side.
3.5.2
Reactive Analysis
For the analysis of reactive experimental results, the focus of the analysis is again on the
sweep side. The analysis must consider the water vapor which is formed during reaction and
117
removed before entry into the GC. The variables to be measured are: (1) oxygen permeation
rate; (2) methane conversion; and (3) the yields and selectivities of specific products (GO ,
2
CO, H2 0, and H2 ).
The overall reaction on the sweep side, if there is a leak and/or equilibrium is not achieved,
can be given as:
nO2 ,perm0
2
+
nCH 4 ,s,inCH 4 + nCo2,s,inCO2
+ h0 2 ,leak
(3.727N 2 + O
Reactants
t
Products
h02,s,0ut0
2
+
hCH4 ,s,outCH 4 + nco 2 ,s,OutCO 2 +
+nCoS,,oUtCO
where
ni,s,in
+
nH02,,utH2
+
nH 2 O,s,outH 2 0
nsoUtC
represents the molar flow rate of reactant i and
+
(g)
(314)
nN2,s,outN2
ni,s,out represents
the molar flow
rate of product i. The 02 on the reactant side is a combination of permeated oxygen and
the oxygen that has leaked into the reactor sweep side.
The GC provides the measured molar fractions
(Xi,s,meas)
of the products in equation 3.14
(except H2 0) if they exist in the product stream. The values of Xi,s,meas will be higher than
the actual mole fractions (xi,8 ,out) because H20 has been removed before entry into the GC.
Using the same concept as in the permeation-only case (see equation 3.3), we obtain the
118
mass flow rates of CO 2 and CH 4 (both in kg-s-1) into the reactor as:
MCO2 X Pt X Vco 2 ,s,in
(3.15a)
R x Tst
mCH 4 ,s,in
MC2X
-
Pst x VCH 4 ,s,in
R
x
X
VCo2,S,in
R
x
(3.15b)
Tst
Similarly, the inlet molar flow rates are:
Pst
riCo 2 ,s,in =
(3.16a)
Tt
x VCH 4 ,s,in
R x Tt
Ps
nCH4,,,in -
(3. 16b)
The mole fraction of any product i (xT,,,aSt) formed in the reactor sweep side is obtained
from:
Xi,s,out = Xi,s,meas X
where
nH o,s,out and Z ni,s,,ut
2
>i nh~s,out - nH
2 O,s
out
(3.17)
1:hi'S Out
are unknown. The average molar mass of products is obtained
from equation 3.6 while the mass fraction of any product i is obtained from equation 3.8.
The mass flow rate of products is
where
h02 ,s,in
in,out
=ms,in
+ nperm +
rIleak
mhst
=ms,in
+ (no0
2 ,s,in
x
M 0 2 ) + (hN 2 ,s,in
(3.18)
X
MN
2
)
(oxygen permeation + leakage) and nN2 ,s,in ( nitrogen leakage, same as
(3.19)
hNs,out
are unknown. Therefore, apart from C0 2, we quantify the molar flow rate of any product
119
gas i (i,,,,ut, in mol-s 1
) as:
me,
hi,s,out = mfi,,,out x
,out
(3.20)
Using these molar flow rates, we can obtain the total molar flow rate of oxygen gas
(permeation and leakage) into the sweep side as:
ho
2 ,s,in =
hO
2 ,S,out
1.
+
1
nco,sout +± ~-nH20,s,out
2
O~~
2
OO2,sout +
(3.21)
Using the leak assumption explained in the permeation-only case, we can obtain the
molar flow rate of permeated oxygen
no2,perm
(n102,perm)
as:
0.21.
--
o
2
,s,in -
0.79N2,sout
(3.22)
With the equations derived so far (eqns 3.17 - 3.22), mass and mole balances are applied
to equation 3.14 (with a Matlab code) until all unknowns are obtained using the 11 equations
and 2 constraints below:
120
Oxygen Balance:
2
2fnO 2 ,S,out + 2 hCO2 ,S,0 Ut + nH2 O,s,out
nO2 ,perm + 2 nCO2 ,s,in + 2 nO2 ,Ieak
nCH4 ,s,in
+
C02,sin
+ hCO,s,out
(323a)
Carbon Balance:
n CH4 ,sout + ftC02,sout + hCO's,out + nC,sout
(323b)
Hydrogen Balance:
4
ACH4 ,s,in
4
-
iCH
,s,out
4
+ 2 nH2 0,s,out +
2
nH ,s,out
2
(3.23c)
Nitrogen Balance:
3 72 7
.
nO2 ,leak =
hN2 ,sout
(3.23d)
Mass continuity:
mCH4 ,s,in
o
2 ,S,outMO2
+
hCH4 ,s,oUtMCH4
+nCO,S,outMCO +
?H
2
nO
(3.727MN2 + M 0 2 ) =
+ hCO2 ,S,outMCO2 + hH2 O,s ,o utMH2 0
+ mCO2,s,in + hO2,permMO2 +
, 8,o&tMH2
2 ,1eak
nC,s,outMC + nN ,S,OUt MN
+
2
2
(3.23e)
Molar flow rate of 02 gas at sweep outlet:
nO ,s,out =
2
X0 2 ,s,meas X Ths,meas
(3.23f)
Molar flow rate of CH 4 gas at sweep outlet:
nCH ,sout =
4
XCH 4 ,s,meas X
ns,meas
(3.23g)
Molar flow rate of CO 2 gas at sweep outlet:
hCO
2
,s,out =
XC02,s,meas X Ths,meas
(3.23h)
Molar flow rate of CO gas at sweep outlet:
nCO,s,out =
XCO,s,meas X
ns,meas
(3.23i)
Molar flow rate of H 2 gas at sweep outlet:
nH2,s,out =
XH 2 ,smeas X fts,meas
(3.23j)
Molar flow rate of N 2 gas at sweep outlet:
hN
121
2
,s,out =
XN 2 ,s,meas X fts,meas
(3.23k)
Constraints:
'CO
2
(3.24a)
(3.24b)
< hC0 2 ,s,in
,sot
nCH ,soUt >
hCH4 ,s,in
4
With all of the above considerations, equations are formulated for the desired variables.
The oxygen permeation rate across the membrane is given by
=
nO2 , sperm
(3.25)
A
J02
where A (constant at 58cm 2 ) is the membrane area exposed to gases on either side.
The methane conversion (XcH4 ) is given by
?iCH4 ,s,out
XCH4
X
100%
(3.26)
hCH4 ,s,in /
The yields (Y) and selectivities (Si) of CO, C2H4, C2H and H2, can be calculated
individually from
Y
ni x
' o
x 100%
(3.27)
100%
(3.28)
nCH4 ,s,in
and
Si =
2
XCH4
x
where ni is the number of carbon atoms in a molecule of product i. For H2,
nH
2
represents
the ratio of number of hydrogen atoms in H2 to the number of hydrogen atoms in CH .
4
Therefore,
nH 2
equals 1/2.
122
The yield (YC0 2 ) and selectivity (Sc0
YCO2 = nC0
2
2
) of CO 2 are calculated from
('nco2,St
n
-
hCO
2 ,s,in)
x
100%
(3.29)
CH4,s,in
and
SCx2
3.6
YC2
X
100%
(3.30)
XCH4
Conclusions
An ITM reactor has been presented for use in conducting oxygen permeation and methane
oxymel combustion experiments. Its current purpose is to investigate the effects of operating
parameters on the macroscopic thermochemical processes which occur during experiments.
The design is based on a stagnation flow configuration to enable studies at the fundamental
level and cross-validation with numerical work.
Reactor sealing and testing mechanisms
have been developed to mitigate and reduce leakage and quantify any leaks that may occur
during experiments.
The membrane used for the ITM reactor is Lao.9 Cao.1FeO 3-6 (LCF).
It is known to provide a balance between favorable oxygen permeation rates and stability
under reactive conditions [82].
Process control and instrumentation systems have been developed for the ITM reactor to
enable proper control and monitoring. There are five major sections - plumbing, instrumentation, gas chromatography, heating, and safety. The plumbing network has been designed to
ensure ease of flow operations, while the instruments used are intended to provide pressure,
temperature and oxygen concentration readings within the reactor. A gas chromatograph
123
provides the concentrations of product gases at the reactor exhausts, while two heating systeins, enclosure and cartridge, ensure uniform and fine control of heating respectively within
the reactor setup. Because of the risks involved in using methane and operating at high
temperatures, a safety system has been developed for the methane flow and heating systems
within the ITM reactor.
During experiments, operating parameters are varied to obtain the primary variables of
interest - oxygen flux and carbon dioxide yield. The experimental methodology needed to
obtain these variables covers the development of reactor-centric flow procedures, heating
procedures and analysis equations. The flow and heating procedures enable the variation of
operating parameters while the analysis equations enable the derivation of the variables of
interest.
124
Chapter 4
Results and Analysis
The ITM reactor described in section 3.1 was used to carry out experiments involving
variations of temperature, inlet sweep flow (mass transfer) and fuel inlet fraction (reactive
analysis). This chapter covers the results and analysis of the experiments conducted.
As explained in section 3.4, the performance indices are mainly oxygen permeation flux
and carbon dioxide yield (oxymel combustion). In order for ITM separation methods to be
competitive with current oxygen separation processes, oxygen permeation flux must be at
least 7.3 tmolcm--2s1 [112,113]. Also, CO 2 capture will be a relatively easy task if only CO 2
and H2 0 are present in the product stream (since the H2 0 can be condensed). The ideal
C0 2 /H 2 0 product ratio under stoichiometric conditions is 0.5. With these considerations in
mind, the results presented in this chapter will provide an insight into the suitability of the
ITM reactor for competitive oxygen separation and oxymel combustion. Table 4.1 shows the
experimental points considered.
125
Table 4.1: Experimental points considered for analysis
Experiment
Controlled Parameter
Permeation-only
Vs,in
(300sccm)
T
("C)
(OC)0 C
700
750 0 C
800 0 C
800 0 C
800 0 C
V7
(400secmr)
Qnor
500
W
40
-
800 0 C
500
500
500
500
500
500
10
15
20
T (700 0 C)
T (750-C)
T (800-C)
Vs,in (200sccm)
s,in
Reactive
4.1
VCH 4 ,s,in (10sccm)
VcH 4 ,s,in (15sccm)
VCH 4 ,,,j (20sccm)
8000 C
8000 C
Vfeed,in
sweep,in
(sccm)
500
500
500
500
500
(sccm)
500
500
500
200
300
VCH 4 ,s,in
(sccm)
Temperature
10-2
5.2
E
0
E
0
E
5.4
0
E
0
0
C
0)
5.6-
0)
0j
5.8-
0
10-3
700
750
Temperature, T [0 C]
-A
800
0.95
1.05
1000/T [K- ]
Figure 4-1: Dependence of oxygen permeation flux on membrane temperature (non-reactive).
Feed (air) flow = 500scem, sweep (GO2 ) flow = 500scem.
Figure 4-la shows the influence of temperature on oxygen permeation flux. The feed
and sweep inflow rates were kept constant at 500sccm each. As expected, increase in temperature from 700 - 800'C leads to a slight increase the oxygen permeation from 4.4 - 4.9
x10--3pmolcm-2s-1.
It is generally believed that an increase in temperature would lead to a higher disorder
126
in the oxygen vacancies [91, 92,109, 110].
This would in turn lead to a sharp increase in
oxygen permeation. Since the oxygen permeation in figure 4-la shows only a slight overall
increase, it can be assumed that the disorder-order transition of oxygen vacancies does not
occur between 700 - 800*C for the LCF membrane considered in this report.
The relationship between oxygen permeation flux and membrane temperature is further
explored in figure 4-1b (Log J0 2 vs 1000/T). An experimental relationship between oxygen
flux and temperature is obtained using regression analysis (see section 2.4.1) as:
Jo2 = -4.868 exp
5.188 xX
RT
)
mol.cm-2 s-
R2 = 0.237
(4.1)
The activation energy for oxygen permeation flux (based on data from T = 700 - 800-C) is
therefore 51.9kJ/mol.
850
-- Feed gas temperature
-+- Sweep gas temperature
CU
800
7/4
CL
E
a)
0)
750
C.
CD
a)
u) 700
~0
a)
650
650
800
750
700
Heater temperature [ C]
850
Figure 4-2: Bulk temperatures within reactor feed and sweep sides
Figure 4-2 shows the bulk temperatures between the feed and sweep sides of the reactor.
As the reactor temperature is increased from 700 - 800*C, the difference in bulk temperatures
127
(ATulk)
reduces by half its value. The change in ATlk suggests that the heat transfer
resistance of the membrane (Rmem) is reduced as the membrane temperature increases. This
change in ATulk could also be due to radiation effects (as the membrane emissivity decreases
with temperature) on the unshielded thermocouples.
4.2
Mass Transfer
The effect of sweep flow variation on oxygen permeation flux was investigated by increasing the sweep inflow from 200 - 500sccm as shown in figure 4-3. The membrane temperature
and feed (air) inflow rates were kept constant at 800'C and 500sccm respectively. The oxygen permeation flux shows a steady increase as sweep inflow is increased. The relationship
in this case is fairly linear. Due to the steady increase observed, it can be assumed that the
mass transfer resistance is minimum at a sweep inflow rate higher than 500sccm.
C.)
E
0
E
p
C)
x
0
x
c
x
0
1
3
100
-
-
200
-
-
.
300
400
Sweep inflow [sccm]
.
-
-
500
-
-
600
Figure 4-3: Dependence of oxygen permeation flux on Sweep (C0 2 ) flow (non-reactive).
Membrane temperature = 800C, feed (air) flow = 500sccm.
128
C-*-
0
0
2
2
6
4-
0
0
2
0
5
10
15
20
Reynolds Number (sweep), Res [-]
25
Figure 4-4: Dependence of oxygen permeation flux and sweep 02 partial pressure on the
sweep Reynolds number (non-reactive). Membrane temperature = 800C, feed (air) flow
500sccm. Re = (pY) / (pDc), De = 2.24cm.
Figure 4-4) shows the influence sweep mass transfer analysis. the reactor gap height (Dc
= 2.24cm) is the distance from flow straightener to the membrane, and the active volume
(V = 33cm 3 ) is the volume under the membrane (i.e membrane area times gap height).
As shown, for a Reynolds number increase from 6.75 - 16.89 (150% increase), the oxygen
permeation flux increases by 25%.
Furthermore, it is obvious that oxygen permeation increases with increasing sweep flow
rate (Vin) because of the simultaneous decrease in oxygen partial pressure in the sweep side
(Pf2 ). Since Jo 2 increases at a slower rate than
Es,in,
the bulk P
2
decreases as a result.
Furthermore, the increase in flux with increasing sweep inflow suggests that the local
P 2 also decreases. Since an increase in sweep mass flow rate means more oxygen is being
swept away from the membrane surface, the chemical potential gradient across the membrane
increases, and oxygen permeation is improved as a result [114].
129
4.3
Reactive Analysis
The results analyzed so far in this chapter are for permeation-only experiments. When
fuel (CH 4 ) is added to the sweep side inflow, it reacts with permeated oxygen to form
products such as C0 2 , CO, H2 0, and H2 .
4.3.1
Effect of Fuel Inlet Fraction
Figure 4-5 shows the dependence of oxygen permeation flux on CH 4 inlet concentration.
The membrane is kept at 800*C while the feed and sweep inflow rates are each 500sccm. As
the CH 4 inlet concentration is increased from 1.96 - 3.85%, the oxygen flux increases rapidly
to 22.8 x 10-3 tmolcm-2s1. This oxygen flux value is almost 400% more than that observed
in the non-reactive case at the same operating conditions (although 10-20sccm CH 4 has been
added to the sweep inflow in the reactive case).
10.
10
7 9
---
2
CH4:02
100
E
U
E
CO
=
0
10
-2
8-
0
Tzzzz~
7-
C).
______________
C)
0
a.0
10-31.5
2
2.5
3
3.5
CH 4 inlet conc. [vol. %]
4
(a)
6
5
1.5
2
2.5
3
3.5
CH4 inlet conc. [vol. %]
4
(b)
Figure 4-5: Dependence of (a) oxygen permeation flux; (b) sweep oxygen partial pressure,
and fuel/0 2 ratio; on fuel inlet concentration. Membrane temperature = 800C, CO 2 inflow
= 500sccm, feed (air) flow = 500sccm.
Figure 4-5 also shows that the sweep side bulk oxygen partial pressure is quite low. This is
130
caused by reaction of oxygen with methane leading to an increased oxygen chemical potential
gradient across the membrane and an increase in oxygen permeation flux. The slight decrease
in oxygen partial pressure and overall increase in fuel/0
2
ratio (molar ratio of inlet CH 4 to
permeated oxygen) also suggest that an increase in fuel inflow rate may be beneficial to
oxygen permeation. However, since the chemical reaction rate is finite in this case, further
increase in the fuel inflow rate (at CO 2 = 500sccm) would reduce the reactant residence time
and further increase the fuel/0
2
ratio, thereby causing low fuel conversion [115-117].
100
d.80
_5
G 60
0
Co
4
2
SH20
aI)
Cl)
L-
S
40
2
202
0
o
X CH
-A-
CSco
8
e
20-
1.5
2
2.5
3
3.5
4
4.5
5
CH4 inlet conc. [vol. %]
Figure 4-6: Dependence of fuel conversion and species selectivities on fuel inlet concentration.
Membrane temperature = 800'C, CO 2 inflow = 500sccm, feed (air) flow = 500sccm.
Figure 4-6 shows the dependence of methane conversion and the selectivities of CO 2 ,
CO, H2 0, and H2 on the methane inlet concentration. The methane conversion is low and it
decreases with increasing fuel inlet concentration (and fuel inflow rate). One possible reason
for the low methane conversion is depicted in figure 4-7.
It is possible that most of the fuel entering the reactor sweep side flows to the reactor
131
02 present in
exit stream
because of
finite rate
02 flux
IT
Dc
Reaction
Zone
Part of sweep
inflow which
does not see02
mixing/active region
CH 4, CO2
Figure 4-7: Possible interaction of fuel with permeated oxygen in the reactor sweep side
exit without reaching the reaction zone, leading to the presence of residual oxygen (about
7% of sweep outflow) in the sweep side even at high fuel/0
2
ratios where complete oxygen
conversion is expected as shown in figure 4-5b. As seen in the plot, the bulk oxygen partial
pressure at the reactor exit remains fairly constant despite the increase in fuel inlet flow rate
beyond the stoichiometric fuel/0
2
ratio (0.5).
Furthermore, an in-house 2-D numerical analysis of the reactor' (using air in both sides)
shows that only about 1 - 12.5% of the total sweep inflow is advected towards the reaction
zone (approximately 1 - 5mm below the membrane) as shown in table 4.2. This result shows
that the primary cause of low methane conversion is the low amount of fuel which reaches
'The analysis was carried out by Prof. R. Ben-Mansour of KFUPM.
132
the reaction zone. Also, we can assume from figure 4-7 and the results in table 4.2 that
a wider membrane would lead to more fuel being converted, since more fuel will reach the
oxygen being permeated across the membrane.
Table 4.2: Mass flow rates at different planes below the membrane (inlet sweep velocity
lcm/s). Results obtained from 2-D numerical analysis of the reactor
Integrated mass
flow rate(kg/s)
Plane location
below membrane (mm)
The fuel-0
2
1
5.25
3
1.90 x10-
5
Fraction of
total flow (%)
0.12
x10-7
5
4.25
5.59
x10-5
12.5
10
1.82
x 10-4
40.7
15
3.10
x10-4
69.33
reaction favors H2 0 over H2 formation as shown in figure 4-6. The selec-
tivity of CO steadily approaches 100% while that of CO 2 approaches 50% as the fuel inlet
concentration reaches 3.85%. However this is likely due to the conversion of residual carbon (formed earlier at lower fuel inlet concentrations) in the reactor sweep side or the very
fuel-rich nature of the reactor at that point.
Furthermore, increased CO formation could be due to the nature of the methane reaction
kinetics. As discussed earlier, the fuel/0
2
ratio in this case is already high (fig 4-5) meaning
there is more fuel than required for complete oxymel combustion.
Since the kinetics of
methane oxycombustion (equation 2.10) is extremely fast, it is possible that the remaining
CH 4 not consumed by the permeated oxygen may react with CO 2 producing H2 and CO via
equation 4.2 below [93]:
CH 4 + CO 2
a
2CO + 2H2
133
AH29 8
247 kJ/mol
(4.2)
4.3.2
Oxymel Analysis
Figure 4-8 shows the dependence of CO 2 yield and C0 2 :H2 0 product ratio on the reactor
fuel input. From the plot, it can be seen that the ITM reactor is not producing a high
amount of oxymel products at the operating reactor conditions. The overall CO 2 yield is
much lower than the desired value of 100%. The yield increases with fuel inflow, but the
highest value obtained during reactive experiments at 800'C is 2.6%. The low yield can
however be attributed to the fact that not much of the fuel reaches the membrane/reaction
zone.
3
.
.,
E3YCo
2
-
2.5 -
CO2:H20
2 2
0
2-r
(N
0
0 1.5
0
(N0.5
0
5
_
. . . . . ._
10
_.
_
15
20
CH inflow rate [sccm]
. . . . ..
25
Figure 4-8: Analysis of oxymel products obtained from reaction between fuel and 02 in the
ITM reactor. Membrane temperature = 800*C, CO 2 inflow = 500sccm, feed (air) flow =
500sccm.
However, the C0 2 /H 2 0 ratio is not far from ideal oxymel reaction regimes (C0 /H 0
2
2
=
0.5). However, this ratio is more likely to be achieved with reduction in the fuel inflow.
The obvious disadvantage of the fuel increase is a simultaneous reduction in fuel conversion.
134
Also, the fuel/0
ratio would increase further leading to the potential formation of more CO
2
and H2 via equation 4.2.
4.3.3
Influence of Reactive (reducing) Sweep Gas on Oxygen Flux
10
Cn
E
0
E
0
10
-2
0)
X
0
10
0
5
15
10
CH4 inflow rate [sccm]
20
Figure 4-9: Influence of fuel addition on oxygen permeation flux in the ITM reactor. Membrane temperature 800 0C, feed (air) flow = 500sccm, sweep flow = 500sccm CO 2 + variable
CH 4.
As shown in figure 4-9, the flux observed in the reactive experiments can be an order of
magnitude higher than that observed in the non-reactive case. As already discussed, this is
mainly due to the further decrease in the sweep side partial pressure of oxygen due to fuel
consumption in the reactive case.
It is fair to assume that much higher fluxes will be observed when sweep inflow rates are
increased at higher temperatures (> 800*C) as widely reported in literature [15,50,77,101,
118-120]. As seen from the oxygen flux trends in figure 4-9, the reactive flux is likely to
135
show steady increase with further increase in temperature as reported in literature.
4.4
Comparison with other ITM Experimental Investigations
The experimental results analyzed so far in this chapter are reflective of the initial operations of the ITM reactor; the reactor is yet to be operated beyond a membrane temperature
of 800*C and inflow rates beyond 500sccm. However, a comparison with results from state
of the art ITM reactors can provide an insight into the regimes within which the ITM reactor would operate best thereby aiding the reactor optimization process. In this section,
comparisons are made with the state of the art ITM results discussed in section 2.4. Oxygen
permeation fluxes have been normalized for comparison as explained in section 2.4.1.
4.4.1
Temperature Comparison
In figure 4-10, a comparison is made with oxygen permeation fluxes reported in literature. The ITM reactor currently produces lower flux than most investigations in literature.
However, the membrane used (LCF) is more suitable for oxygen permeation under reactive
conditions. Also, effects of temperature beyond 800'C are yet to be explored.
Most of the membranes in figure 4-10 are only stable for oxygen permeation and other
non-reactive processes. The reports which utilize reactive-capable membranes (e.g Lu 2000)
show similar or slightly higher fluxes compared to those reported in this study. Figure 4-11
shows normalized flux as a function of temperature for non-reactive cases. The plot shows
136
1 ----Zeng 1998
1
)1(-- Lu 2000
Zeng 2000
Zeng 2001
A
-X-
0--
-\7-Tong 2002 (a)
Tong 2002 (b)
-1 -_-<}-
Diethelm 2004 (a)
-*-Wang 2004 (a)
oE
*--
-2
Ikeguchi 2005 (a)
Liu 2005 (a)
*----
E
-
2
o
0)
Liu 2006 (a)
Liu 2006 (e)
Liu 2006 (i)
----
3
Wang 2005 (a)
4Zhang
0
2007 (a)
2007 (c)
4--Zhang
Buysse 2009 (a)
5~
. '
-6
0.7
Diethelm 2009 (a)
-
' '
'
0.8
.
0.9
.
1
1000/T [K l]
1.1
'
1.3 --
1.2
Wei 20 (a)
*Zhu 2010
Shen 2011 (b)
This Study
Figure 4-10: Oxygen flux dependence on temperature (non-reactive) for the ITM reactor
compared with reported investigations.
-+- Zeng 1998
-*- Lu 2000
- -X--Zeng 2000
Zeng 2001
--V-Tong 2002 (a)
-V-Tong 2002 (b)
1.4
1.31.2-
-<-
Diethelm 2004 (a)
Wang 2004 (a)
Ikeguchi 2005 (a)
Liu 2005 (a)
-*-Wang 2005 (a)
7P
1.1 ----
~-Y1 -
.-
Liu 2006 (a)
Liu 2006 (e)
---
*
0-y--
Liu 2006 (i)
-4-
0.9 --
-*-
0.8-*
0.7
550
Zhang 2007 (a)
4- Zhang 2007 (c)
'
600
650
700
750
800
850
Temperature, T rc]
'
900
'
'
950
'
'
1000
Buysse 2009 (a)
Diethelm 2009 (a)
Wei 2010 (a)
2010
--*Zhu
-
.
105 --
Shen 2011 (b)
This Study
Figure 4-11: Normalized oxygen flux dependence on temperature (non-reactive) for the
ITM reactor compared with reported investigations. See section 2.4.1 for normalization
methodology.
the normalized flux as a function of temperature to be close to 1. Therefore, the fluxes
obtained from the ITM reactor in this study can be normalized using the methods described
earlier in section 2.4.1.
137
4.4.2
Mass Transfer Comparison
In terms of mass transfer, the ITM reactor operates similar to the reported investigations
for non-reactive cases as seen in figure 4-12. The flow Reynolds number is within similar
range and the oxygen flux trend indicates that the mass transfer resistance is close to its
minimum value at the operating membrane temperature (800C). An analysis at lower inflow
rates is more likely to reveal more details at this temperature.
Wang 2004 (e)
Wang 2004 (f)
5-.A5-
.....2--
-2---*-
..
-- 7
--
-
--- 1.5
-'1
. .. .
-
0.5*-
''-Wei
0
2
4
6
8
10
ReRsweep [--
12
14
Liu 2006 (c)
-Liu 2006 (g)
Liu 2006 (h)
Liu 2006 (j)
-v- Liu 2006 (k)
---
0
Wang 2004 (g)
Liu 2005 (b)
Liu 2005 (c)
Liu 2005 (d)
Liu 2006 (b)
Liu 2006 (d)
Liu 2006 (f)
.---
1.5 -
-
16
18 ---
Liu 2006 (1)
Buysse 2009 (b)
Wei 2010 (g)
Wei 2010 (h)
Wei 2010 (i)
Wei 2010 (j)
2010 (k)
This Study
Figure 4-12: Normalized flux dependence on sweep mass flow (non-reactive) for the ITM
reactor compared with reported investigations. See section 2.4.1 for normalization methodology.
A comparison of the sweep flow timescale as shown in figure 4-13 reflects the difference in
the sweep residence time compared with reported investigations. Since the ITM reactor in
this study has an active volume (V
=
33cm 3 ) which is about 1-2 orders of magnitude above
the reported investigations, this result is expected. Also, all the experiments in figure 413 involve tubular or hollow fiber membrane geometries. A direct comparison with similar
configurations (disk ITM reactors) to the ITM reactor in this study would be ideal. However,
138
#
-4-- Wang 2004 (e)
2.5
. . .
',
.
.
.
Liu 2005 (c)
2@-
--7
S0" 1.5 >-y-
LL-y-
V
1
S -..-
Liu 2005 (d)
Liu 2006 (b)
Liu 2006 (c)
- Liu
Liu
Liu
Liu
2006 (d)
2006 (f)
2006 (g)
2006 (h)
Liu 2006 (j)
-v-- Liu 2006 (k)
Z
-
0.5
0
10
(f)
Wang 2004 (g)
-Liu 2005 (b)
-
5
Wang 2004
10
10
Residence Time (sweep) [s]
10
101
Liu 2006 (1)
Buysse 2009 (b)
Wei 2010 (g)
Wei2010(h)
Wei 2010 (i)
Wei 2010 (j)
Wei2010(k)
This Study
Figure 4-13: Normalized flux dependence on Residence time (non-reactive) for the ITM reactor compared with reported investigations. 7-res = V/V. See section 2.4.1 for normalization
methodology.
the active volume of these setups are difficult to estimate as the geometries are generally not
listed in literature.
4.4.3
Reactive Comparison
From figure 4-14, it can be seen that the fuel conversion in the ITM reactor is below equilibrium. However, the trend suggests that a reduction in fuel/0
2
ratio may favor increased
fuel conversion and quasi-equilibrium operation of the ITM reactor. As discussed earlier,
the low fuel conversion could also be due to most of the inlet methane leaving the reactor
sweep side without reaching the reaction zone (figure 4-7). However, while high fuel conversion is important, it should be noted that the ITM reactor in this study was not designed
specifically for high reactive performance, but for spatially resolved measurements.
139
100
Dong
2001
(c)
Dietheg
200()
80
09200 ()
Wag2005a)
70Tan
200()
60 -
(1)
a 2
T10620086(b)
20
C
Tano
2006
(f)
-20064)
L)
.0
40
Fu
>
41
A)
n
Zh22000(b)
2008
008 ,
Kozh-o'20009(
)
L.02010)0)
10T1n
TZ
L4o2010b2
30
on)
o200e
T4020,0(c)
C.Gon
4.5
1420116()
20
Gong
201()
KnLe0
201 ()
L00a2011(b)
Googp
2011
(c)
Concl20us(i)
CH
20
: 02
ratio[-]
Knoeg
2011
(0)
*
0
510
*h
15
Thu-tod
CH 4 :02 ratio[-
Figure 4-14: Dependence of CH4 conversion on fuel/0
with reported investigations.
4.5
2
ratio for the JTM reactor compared
Conclusions
Preliminary experiments have been conducted using the ITM reactor described in section 3.1. Non-reactive experiments were conducted by varying the membrane temperature
and the sweep inflow rate of CO2. Reactive analyses of the effects of fuel inlet fraction
were also carried out. Furthermore, comparisons were made with reported experimental
investigations in literature.
When the membrane temperature was varied under non-reactive conditions, an overall
increase in oxygen flux was observed up to 4.9 x 10- 3 pmolcm- 2s-1 at 800*C. The activation
energy required for oxygen permeation across the membrane was obtained as 51.9kJ/mol.
Further analysis shows that the difference in bulk temperatures between the feed and sweep
gases decreases with increasing temperature. This suggests that the heat transfer resistance
across the LCF membrane decreases with temperature or the decreasing membrane emissivity
140
(with increasing temperature) affects the unshielded thermocouple readings.
The effect of increasing flow rate in the reactor sweep side was studied to provide insight
into its impact on oxygen permeation flux. Oxygen flux shows an increase with sweep mass
flow mainly because of a simultaneous decrease in sweep side oxygen partial pressure. Even
though the increase is not rapid, it can be assumed that the mass transfer resistance is yet
to reach its minimum value. An analysis of the sweep gas timescale reveals that a reduction
in residence time favors oxygen permeation.
A study of the effects of fuel addition to sweep inflow reveals that oxygen permeation is
enhanced beyond that observed in the non-reactive case. However, the sweep side partial
pressure of oxygen in this case decreases gradually suggesting that most of the fuel entering
the reactor sweep side does not reach the reaction zone. In addition, the increase in fuel flow
rate increases the fuel/0
2
ratio. These factors are likely causes for the low fuel conversion
observed. From the species formation values, it is observed that the reactions are favorable
toward CO and H2 0 rather than CO 2 and H2 respectively. Although, in the case of CO
formation, residual CH 4 may interact with CO 2 to form more CO as the fuel/0
2
ratio
increases in the reactor sweep side.
The mass transfer results observed in this study are in agreement with reported data; the
flux increases gradually with mass inflow. However, the residence time within the reactor was
about 2 orders of magnitude higher than reported in literature, due to the correspondingly
large active volume of the ITM reactor. The fuel conversion is low compared to reported
data. However, there is an indication that lower fuel/0
2
ratios will favor fuel conversion.
Overall, the maximum flux observed in this study (22.8 x10-3pmolcm-2s-') is about 2
orders of magnitude less than required to compete with current oxygen separation technolo141
gies. However, this flux is observed at a relatively low temperature (800C) compared to
reported investigations. According to the trends observed in literature, the flux is likely to
increase appreciably if temperature is increased up to 950C. In terms of the oxymel operating capability of the ITM reactor, it is fair to assume that the desired C0 2 :H2 0 can be
achieved at the current operating temperature of 800*C, but high CO 2 selectivity is more
likely to occur with a lower fuel/0
2
ratio
(=
0.5) at higher temperatures and with a more
appropriate flow configuration.
142
Chapter 5
Conclusions
This work provides a report on the development of an ion transport membrane reactor
for use in oxygen separation from air and combustion with methane. The reactor was used
for experiments involving analysis of the effects of temperature, mass flow, and fuel on oxygen permeation and other thermochemical processes. An analysis of reported experimental
investigations in literature was also presented.
5.1
Summary
The design details and analysis of initial operation of a novel ITM reactor have been
presented. The reactor operates with bi-directional stagnation flow and can support optical
analysis of the reaction zone and spatial analysis along the reactor axes. The application
of the reactor is to investigate oxygen permeation across a planar membrane and facilitate
combustion of the permeated oxygen with fuel. Sealing methodologies have been developed
and an effective strategy has been presented.
143
The membrane used for the ITM reactor
experiments was La 0 .9 Cao. 1 FeO 33 (LCF).
Preliminary experiments conducted show that the maximum oxygen flux obtained at a
membrane temperature of 800'C is 2 orders of magnitude less that is required to compete
with current oxygen separation technologies.
However, the trends reported in literature
suggest that much higher fluxes can be obtained at higher temperatures. The conversion
of methane and overall CO 2 yield are also observed to be below low
(
< 10%). However,
improvements may be observed if the fuel inlet flow rate is reduced at higher temperatures,
and a more appropriate flow configuration is used.
5.2
Outlook and Future Work
The work presented in this reported covers the early operation of the ITM reactor. Further analyses, improvements, and modifications can still be obtained with the ITM reactor.
The next steps for the ITM reactor include further global measurements and spatial measurements.
5.2.1
Global Measurements
In order to further understand the capabilities of the ITM reactor, it is important to
carry out further global experiments. From the discussion in chapter 4, it is evident that
certain operating regimes are yet to be explored. The reactor is yet to be operated beyond
800'C. It is safe to assume based on reports in literature, that more improved results for
oxygen flux, fuel conversion and CO 2 yield can be obtained at higher temperatures. Also, it
may be possible to adjust other conditions such as the reactor gap height and flow rates to
144
Table 5.1: Experimental points for future consideration
Experiment
T
(OC)
(sccm)
VCo 2,in
(sccm)
(700-C)
(750-C)
(800-C)
(850-C)
(900-C)
T (950-C)
V,in (100sccm)
Vs8in (200sccm)
V8,in (300sccm)
VSin (400sccm)
700
750
800
850
900
950
950
950
950
950
500
500
500
500
500
500
500
500
500
500
500
500
500
500
500
500
100
200
300
400
V 8 ,in (500sccm)
950
500
500
700
750
800
850
900
950
950
950
950
950
950
950
950
950
950
950
950
500
500
500
500
500
500
500
500
500
500
500
500
500
500
500
500
500
450
450
450
450
450
450
90
180
270
360
450
500
490
480
470
460
450
Controlled Parameter
T
T
T
T
T
Permeation-only
(700-C)
(750-C)
(800-C)
(850-C)
(900-C)
(950-C)
V,in (1OOscem)
(200sccm)
QV'in (300sccm)
V,in (400sccm)
V,in (500sccm)
XCH 4 ,s,in (0%)
XCH 4 ,s,in (2%)
XCH 4 ,s,in (4%)
XCH 4 ,s,in (6%)
XCH 4 ,s,in (8%)
XCH 4 ,s,in (10%)
V,in
Reactive
Vairin
VCH 4 ,s,in
(sccm)
10
20
30
40
50
obtained improved results.
If a wide range of data is obtained, direct comparisons can be made between reactive
and non-reactive conditions based on variations in temperature and flow rate. Furthermore,
the effects of the reactive sweep gas can be better understood if the total sweep gas flow and
fuel dilution ratio are varied separately (keeping every other condition constant each time).
145
Table 5.1 can be used to carry out a wide range of measurements which would produce
enough data for a more complete analysis of the ITM reactor.
5.2.2
Spatial analysis
Based on the importance of localized effects within the ITM reactor (such as local P0 2 ),
the use of optical measurement techniques will aid the study of species formation in the
reaction zone and its controlling parameters (e.g. flow and temperature). Optical equipment
such as a Scanning Infrared Gas Imaging System could be placed outside of the reactor and
aimed through the optical access to detect gas species within the reaction zone. Importantly,
the reactive oxygen species within the reaction zone can be monitored using the hydroxylradical detection method. As a result, oxygen permeation and flow gradient (within the
sweep side can be analyzed).
Also, the gas sampling points along the reactor length can be used to study the effects of
species, temperature and pressure gradients within the reactor. As discussed in section 3.1,
GC measurement and other sensing probes can be placed across the length of the reactor. The
measurements obtained can be compared with the values at the reactor exit. Importantly,
the local oxygen partial pressures on both sides of the reactor can be obtained leading to
a fundamental understanding of the oxygen flux governing processes. This would also aid
the understanding of the relative impacts of surface exchange and bulk processes on oxygen
permeation and facilitate cross-validation with in-house numerical work carried out on the
ITM reactor.
146
Bibliography
[1] Hanne M. Kvamsdal, Kristin Jordal, and Olav Bolland. A quantitative comparison of
gas turbine cycles with CO 2 capture. Energy, 32(1):10-24, 2007.
[2] Burggraaf A.J. Bouwmeester H.J.M. Dense Ceramic membranes for Oxygen Separation
in: CRC Handbook of Solid State Electrochemistry, chapter 14. CRC Press, Inc., 1997.
[3] Crystal Lattice Structures. Center for Computational Materials Science of the United
States Naval Research Laboratory (17 Feb 2007). Retrieved July 21, 2011 from
http://cst-www.nrl.navy.mil/lattice/.
[4] Timothy Griffin, Sven Gunnar Sundkvist, Knut Asen, and Tor Bruun. Advanced
zero emissions gas turbine power plant. Journal of Engineeringfor Gas Turbines and
Power, 127(1), 2005.
[5] Yutie Liu, Xiaoyao Tan, and K. Li. Mixed conducting ceramics for catalytic membrane
processing. Catalysis Reviews: Science and Engineering, 48(2):145 - 198, 2006.
[6] J. Sunarso, S. Baumann, J. M. Serra, W. A. Meulenberg, S. Liu, Y. S. Lin, and J. C.
Diniz da Costa. Mixed ionic-electronic conducting (MIEC) ceramic-based membranes
for oxygen separation. Journal of Membrane Science, 320(1-2):13-41, 2008.
[7] W. Wang and Y. S. Lin. Analysis of oxidative coupling of methane in dense oxide
membrane reactors. Journal of Membrane Science, 103(3):219-233, 1995.
[8] Weishen Yang and Rui Cai. Oxygen-Ion Transport Membrane and Its Applications in
Selective Oxidation of Light Alkanes in: Inorganic Membranes for Energy and Environmental Applications, chapter 14. Springer, Pittsburgh, PA, USA, 2009.
[9] Henny J. M. Bouwmeester. Dense ceramic membranes for methane conversion. Catalysis Today, 82(1-4), 2003.
[10] Robert G. Watts. Global warming: Is it real? Journal of Solar Energy Engineering,
122(3):158-160, 2000.
[11] Projections: EIA, World Energy Projection System Plus (2010). EIA, International
Energy Statistics database (as of November 2009). Retrieved July 6, 2011 from
http://www.eia.gov/oiaf/ieo/emissions.html.
147
[12] Massachusetts Institute of Technology.
MIT, Cambridge MA, 2010.
The Future of Natural Gas [Interim report].
[13] Yosuke Takahashi, Akihiro Kawahara, Takehiro Suzuki, Masayoshi Hirano, and
Woosuck Shin. Perovskite membrane of LajxSrxTijyFeyO3-deita for partial oxidation
of methane to syngas. Solid State Ionics, 181(5-7):300-305, 2010.
[14] Chun Zhang, Xianfeng Chang, Xueliang Dong, Wanqin Jin, and Nanping Xu. The
oxidative stream reforming of methane to syngas in a thin tubular mixed-conducting
membrane reactor. Journal of Membrane Science, 320(1-2), 2008.
[15] Xiaoyao Tan, K. Li, A. Thursfield, and I. S. Metcalfe. Oxyfuel combustion using a
catalytic ceramic membrane reactor. Catalysis Today, 131(1-4):292-304, 2008.
[16] Clark D.J. Suitor J.W. and Losey R.W. Development of alternative oxygen production
source using a zirconia solid electrolyte membrane, in: Technical progress report for
fiscal years 1987, 1988 and 1990, Jet Propulsion Laboratory Internal Document D7790,
1990.
[17] S. S. Liou and W. L. Worrell. Electrical properties of novel mixed-conducting oxides.
Applied Physics A: Materials Science &amp; Processing,49(1):25-31, 1989.
[18] Worrell W.L. Liou, S.S. Mixed conducting electrodes for solid oxide fuel cells. Proc. 1st
Int. Symp. on Solid Oxide Fuel Cells, Singhal, S.C., Ed., The Electrochemical Society,
Pennington, NJ, pages 81-89, 1989.
[19] B. Cals and J.F. Baumard. Electrical properties of the ternary solid solutions. 17:137147, 1980.
[20] B. Cales and J. F. Baumard. Mixed conduction and defect structure of ZrO CeO 2
2
Y2 0 3 solid solutions. Journal of The Electrochemical Society, 131(10):2407-2413, 1984.
[21] T. J. Mazanec, T. L. Cable, and J. G. Frye. Electrocatalytic cells for chemical reaction.
Solid State Ionics, 53-56(Part 1):111 - 118, 1992.
[22] W. C. Maskell and B. C. H. Steele. Solid state potentiometric oxygen gas sensors.
Journal of Applied Electrochemistry, 16:475-489, 1986. 10.1007/BF01006843.
[23] Zhentao Wu, Bo Wang, and K. Li. A novel dual-layer ceramic hollow fibre membrane
reactor for methane conversion. Journal of Membrane Science, 352(1-2):63-70, 2010.
[24] Alexander S. Mukasyan, Colleen Costello, Katherine P. Sherlock, David Lafarga, and
Arvind Varma. Perovskite membranes by aqueous combustion synthesis: synthesis and
properties. Separation and Purification Technology, 25(1-3):117 - 126, 2001.
[25] Paul W. 0. Hoskin. Minerals: Their constitution and origin: by Hans-Rudolf Wenk
and Andrei G. Bulakh. Cambridge University Press, Cambridge, U.K., 2004. 646 pp.,
526 ill + 16 plates. American Mineralogist, 91(7):1210, 2006.
148
[26] Kenneth W. Bladh John W. Anthony, Richard A. Bideaux and Eds. Monte C. Nichols.
Handbook of Mineralogy, Mineralogical Society of America. Chantilly, VA 20151-1110,
USA.
[27] L. L. Hench and L. K. West. Principles of Electronic Ceramics. John Wiley & Sons,
Inc., 1990.
[28] I. D. Brown. What factors determine cation coordination numbers? Acta Crystallographica Section B, 44(6):545-553, Dec 1988.
[29] Zhang H.M. Furukawa S. Teraoka, Y. and N. Yamazoe. Oxygen permation through
perovskite-type oxides. Chem. Lett., pages 1743-46, 1985.
[30] Nobunaga T. Teraoka, Y. and N. Yamazoe. Effect of cation substitution on the oxygen
semipermeability of perovskite oxides. Chem. Lett., pages 503-06, 1988.
[31] Y. Teraoka, T. Nobunaga, K. Okamoto, N. Miura, and N. Yamazoe. Influence of
constituent metal cations in substituted LaCoO 3 on mixed conductivity and oxygen
permeability. Solid State Ionics, 48(3-4):207-212, 1991.
[32] J. G. McCarty and H. Wise. Perovskite catalysts for methane combustion. Catalysis
Today, 8(2):231-248, 1990.
[33] Y. S. Lin and Y. Zeng. Catalytic properties of oxygen semipermeable perovskite-type
ceramic membrane materials for oxidative coupling of methane. Journal of Catalysis,
164(1):220-231, 1996.
[34] Junichiro Mizusaki, Yasuo Mima, Shigeru Yamauchi, Kazuo Fueki, and Hiroaki
Tagawa. Nonstoichiometry of the perovskite-type oxides LapxSrxCoO 3-deita. Journal
of Solid State Chemistry, 80(1):102-111, 1989.
[35] Xiaoyao Tan and K. Li. Design of mixed conducting ceramic membranes/reactors for
the partial oxidation of methane to syngas. AIChE Journal, 55(10):2675-2685, 2009.
[36] Dixon AG in:. Spivey JJ, editor. Catalysis. Vol. 14. London: Royal Society of Chemistry, 1999.
[37] Yaping Lu, Anthony G. Dixon, William R. Moser, Yi Hua Ma, and Uthamalingam Balachandran. Oxidative coupling of methane using oxygen-permeable dense membrane
reactors. Catalysis Today, 56(1-3):297-305, 2000.
[38] Haihui Wang, You Cong, and Weishen Yang. Oxidative coupling of methane in
Bao.5 Sro.5 Co0 .Fe 2 O3_ tubular membrane reactors. Catalysis Today, 104(2-4):160 167, 2005. Catalysis in Membrane Reactors.
[39] McGovern J. Foy, K. Comparison of ion transport membranes. Fourth Annual Conference on carbon capture and sequestration DOE/NETL, 2005.
149
[40] Y. Zeng, Y. S. Lin, and S. L. Swartz. Perovskite-type ceramic membrane: Synthesis, oxygen permeation and membrane reactor performance for oxidative coupling of
methane. Journal of Membrane Science, 150(1):87-98, 1998.
[41] T. Takahashi and Denki Kagaku Iwahara, H., 1967.
[42] T. Takahashi and H. Iwahara. Ionic conduction in perovskite-type oxide solid solution
and its application to the solid electrolyte fuel cell. Energy Conversion, 11(3):105 111, 1971.
[43] B. C. H. Steele. Oxygen ion conductors and their technological applications. Materials
Science and Engineering: B, 13(2):79-87, 1992.
[44] Elshof J. E. Ten, H. J. M. Bouwmeester, and H. Verweij. Oxidative coupling of methane
in a mixed-conducting perovskite membrane reactor. Applied Catalysis A:General,
130(2):195-195, 1995.
[45] Y. Zeng and Y. S. Lin. Oxygen permeation and oxidative coupling of methane in yttria
doped bismuth oxide membrane reactor. Journal of Catalysis, 193(1):58-64, 2000.
[46] Y. Zeng and Y. S. Lin. Oxidative coupling of methane on improved bismuth oxide
membrane reactors. AIChE Journal,47(2):436-444, 2001.
[47] U. Balachandran, J. T. Dusek, R. L. Mieville, R. B. Poeppel, M. S. Kleefisch, S. Pei,
T. P. Kobylinski, C. A. Udovich, and A. C. Bose. Dense ceramic membranes for partial
oxidation of methane to syngas. Applied Catalysis A: General, 133(1):19-29, 1995.
[48] Masayuki Ikeguchi, Tomohiro Mimura, Yasushi Sekine, Eiichi Kikuchi, and Masahiko
Matsukata. Reaction and oxygen permeation studies in Sm 0 .4 Bao.6 Fe0 .sCo0 .2 0 3 - membrane reactor for partial oxidation of methane to syngas. Applied Catalysis A: General,
290(1-2):212-220, 2005.
[49] Haihui Wang, Armin Feldhoff, Jurgen Caro, Thomas Schiestel, and Steffen Werth.
Oxygen selective ceramic hollow fiber membranes for partial oxidation of methane.
AIChE Journal,55(10):2657-2664, 2009.
[50] Xiaoyao Tan and K. Li. Oxidative coupling of methane in a perovskite hollow-fiber
membrane reactor. Industrial and Engineering Chemistry Research, 45(1):142-149,
2006.
[51] Shokotov M. Yantovski, E. Zero emissions membrane piston engine system (ZEMPES).
Proc. 2nd Annual Conference on Carbon Sequestration, 2003.
[52] F. Iguchi, N. Sata, H. Yugami, and H. Takamura. Oxygen permeation properties
and the stability of Lao.6 Sro.4 Feo.8 Coo.20 3 studied by raman spectroscopy. Solid State
Ionics, Diffusion &amp; Reactions, 177(26-32):2281-4, 2006.
[53] Nanping Xu, Shiguang Li, Wanqin Jin, Jun Shi, and Y. S. Lin. Experimental and
modeling study on tubular dense membranes for oxygen permeation. AIChE Journal,
45(12):2519-2526, 1999.
150
[54] Shiguang Li, Wanqin Jin, Pei Huang, Nanping Xu, Jun Shi, and Y. S. Lin. Tubular
lanthanum cobaltite perovskite type membrane for oxygen permeation. Journal of
Membrane Science, 166(1):51-61, 2000.
[55] Kirsten Foy and Jim McGovern. Analysis of the effects of combining air separation
with combustion in a zero emissions (ZEITMOP) cycle. Energy Conversion and Management, 48(11):3046-3052, 2007.
[56] C. Buysse, A. Kovalevsky, F. Snijkers, A. Buekenhoudt, S. Mullens, J. Luyten, J. Kretzschmar, and S. Lenaerts. Fabrication and oxygen permeability of gastight, macrovoidfree Bao.5 Sro.5Coo.sFeo.203-delta capillaries for high temperature gas separation. Journal
of Membrane Science, 359(1-2):86-92, 2009.
[57] Hui Lu, Jianhua Tong, You Cong, and Weishen Yang. Partial oxidation of methane
in Bao. 5 Sro.5CoosFeo.203-delta membrane reactor at high pressures. Catalysis Today,
104(2-4):154 - 159, 2005. Catalysis in Membrane Reactors.
[58] Qing Zeng and Yan-bo Zuo and Chuan-gang Fan and Chu-sheng Chen. C0 2 -tolerant
oxygen separation membranes targeting CO 2 capture application. Journal of Membrane Science, 335(1-2):140 - 144, 2009.
[59] Stefan Engels, Torsten Markus, Michael Modigell, and Lorenz Singheiser. Oxygen
permeation and stability investigations on MIEC membrane materials under operating
conditions for power plant processes. Journal of Membrane Science, 370(1-2):58 - 69,
2011.
[60] F. T. Akin and Y. S. Lin. Selective oxidation of ethane to ethylene in a dense tubular
membrane reactor. Journal of Membrane Science, 209(2):457-467, 2002.
[61] Adrian Leo, Shaomin Liu, and Joao C. Diniz da Costa. Development of mixed conducting membranes for clean coal energy delivery. InternationalJournalof Greenhouse
Gas Control, 3(4):357-367, 2009.
[62] Igor Kul'bakin, Valery Belousov, Sergey Fedorov, and Anatoly Vorobiev. Solid/melt
ZnO-Bi 2O 3 composites as ion transport membranes for oxygen separation from air.
Materials Letters, 67(1):139 - 141, 2012.
[63] E. Yantovski, J. Gorski, B. Smyth, and J. ten Elshof. Zero-emission fuel-fired power
plants with ion transport membrane. Energy, 29(12-15):2077-2088, 2004.
[64] Shokotov M. McGovern J. Vaddella V. Yantovski, E. Zero emissions membrane piston
engine system for a bus. Proc. VAFSEP 2004, 2004.
[65] Shokotov M. Shokotov V. McGovern J. Foy K. Yantovski, E. Elaboration of zero
emissions membrane piston engine system (ZEMPES) for propane fuelling. Proc. 4th
Annual Conference on Carbon Sequestration, 2005.
151
[66] Sven Gunnar Sundkvist, Stein Julsrud, Bent Vigeland, Tyke Naas, Michael Budd, Hans
Leistner, and Dieter Winkler. Development and testing of AZEP reactor components.
InternationalJournal of Greenhouse Gas Control, 1(2):180-187, 2007.
[67] Sherman J. Xu and William J. Thomson. Oxygen permeation rates through ionconducting perovskite membranes. Chemical Engineering Science, 54(17), 1999.
[68] C.S. Chen, B.A. Boukamp, H.J.M. Bouwmeester, G.Z. Cao, H. Kruidhof, A.J.A. Winnubst, and A.J. Burggraaf. Microstructural development, electrical properties and
oxygen permeation of zirconia-palladium composites. Solid State Ionics, 76(1-2):2328, 1995.
[69] G. Z. Cao. Electrical-conductivity and oxygen semipermeability of terbia and yttriastabilized zirconia. Journal of Applied Electrochemistry, 24(12):1222-1227, 1994.
[70] A.J. Burggraaf P.J. Gellings H.J.M. Bouwmeester, H. Kruidhof. Oxygen semipermeability of terbia-stabilized bismuth oxide. Solid State Ionics, 460:53-56, 1992.
[71] S. Dou, C. R. Masson, and P. D. Pacey. Mechanism of oxygen permeation through
lime-stabilized zirconia. Journal of The Electrochemical Society, 132(8), 1985.
[72] Yee Terrence F. Drake Miles P. Thorogood Robert M., Srinivasan Rajagopalan. Composite mixed conductor membranes for producing oxygen, August 1993.
[73] E. Capoen, G. Nowogrocki, R. J. Chater, S. J. Skinner, J. A. Kilner, M. Malys, J. C.
Boivin, G. Mairesse, and R. N. Vannier. Oxygen permeation in bismuth-based materials. Part II: Characterisation of oxygen transfer in bismuth erbium oxide and bismuth
calcium oxide ceramic. Solid State Ionics, 177(5-6):489-492, 2006.
[74] S. Kim, Y.L. Yang, A.J. Jacobson, and B. Abeles. Oxygen surface exchange in mixed
ionic electronic conductor membranes. Solid State Ionics, 121(1-4):31 - 36, 1999.
[75] M.J. Verkerk and A.J. Burggraaf. Oxygen transfer on substituted ZrO 2 , Bi 2 0 3 , and
CeO 2 electrolytes with platinum electrodes I. electrode resistance by D-C polarization.
Journal of the Electrochemical Society, 130:70-78, 1983.
[76] M.J. Verkerk and A.J. Burggraaf. Oxygen transfer on substituted ZrO 2 , Bi 2 03, and
CeO 2 electrolytes with platinum electrodes II. A-C impedance study. Journal of the
Electrochemical Society, 130(1):78-84, 1983.
[77] S. Liu, X. Tan, Z. Shao, and J. C. Diniz da Costa. Bao.5Sro-5 Coo.sFeo.2O3-deIta ceramic
hollow-fiber membranes for oxygen permeation. AIChE Journal, 52(10):3452-3461,
2006.
[78] Xiaoyao Tan and K. Li. Modeling of air separation in a LSCF hollow-fiber membrane
module. AIChE Journal,48(7):1469-1477, 2002.
[79] Jonas Gurauskis, rjan Fossmark Lohne, Hilde Lea Lein, and Kjell Wiik. Processing of
thin film ceramic membranes for oxygen separation. Journal of the European Ceramic
Society, 32(3):649 - 655, 2012.
152
[80] U. Balachandran, J. T. Dusek, P. S. Maiya, B. Ma, R. L. Mieville, M. S. Kleefisch, and
C. A. Udovich. Ceramic membrane reactor for converting methane to syngas. Catalysis
Today, 36(3):265-272, 1997.
[81] Carl A. Udovich, D. Sanfilippo F. Frusteri A. Vaccari A. Parmaliana, and Arena F.
Ceramic membrane reactors for the conversion of natural gas to syngas. In Studies in
Surface Science and Catalysis, volume Volume 119, pages 417-422. Elsevier, 1998.
[82] Paul N. Dyer, Robin E. Richards, Steven L. Russek, and Dale M. Taylor. Ion transport
membrane technology for oxygen separation and syngas production. Solid State Ionics,
134(1-2):21-33, 2000.
[83] S. Diethelm, J. Van herle, P. H. Middleton, and D. Favrat. Oxygen permeation and
stability of Lao.4Cao.6FexCoxO3-de1ta (x = 0, 0.25, 0.5) membranes. volume 118 of
Journal of Power Sources, pages 270-275, Amsterdam, Netherlands, 2003. Elsevier.
[84] Xuefeng Zhu, Haihui Wang, and Weishen Yang. Relationship between homogeneity
and oxygen permeability of composite membranes. Journal of Membrane Science,
309(1-2):120 - 127, 2008.
[85] Xuefeng Zhu, Qiming Li, Yufeng He, You Cong, and Weishen Yang. Oxygen permeation and partial oxidation of methane in dual-phase membrane reactors. Journal of
Membrane Science, 360(1-2):454 - 460, 2010.
[86] Xuefeng Zhu, HaiHui Wang, You Cong, and Weishen Yang. Partial oxidation of
methane to syngas in BaCeo.15 Feo. 5 0 3 membrane reactors. Catalysis Letters, 111(34):179-185, 2006.
[87] Wenliang Zhu, Guoxing Xiong, Wei Han, and Wenshen Yang. Catalytic partial oxidation of gasoline to syngas in a dense membrane reactor. Catalysis Today, 93-95:257-261,
2004.
[88] D. Schlehuber, E. Wessel, L. Singheiser, and T. Markus. Long-term operation of a
Lao. 5sSro.4Coo.2Feo.sO3-delta membrane for oxygen separation. Journal of Membrane
Science, 351(1-2):16-20, 2010.
[89] Wenxing Zhang, J. Smit, M. van Sint Annaland, and J. A. M. Kuipers. Feasibility
study of a novel membrane reactor for syngas production: Part 1: Experimental study
of 02 permeation through perovskite membranes under reducing and non-reducing
atmospheres. Journal of Membrane Science, 291(1-2), 2007.
[90] X. Qi, F. T. Akin, and Y. S. Lin. Ceramic-glass composite high temperature seals for
dense ionic-conducting ceramic membranes. Journal of Membrane Science, 193(2):185
- 193, 2001.
[91] L. Qiu, T. H. Lee, L. M. Liu, Y. L. Yang, and A. J. Jacobson. Oxygen permeation
studies of SrCo0 .sFeo.2 0 3 - delta. Solid State Ionics, 76(3-4):321 - 329, 1995.
153
[92] H. Kruidhof, H.J.M. Bouwmeester, R.H.E. v. Doorn, and A.J. Burggraaf. Influence of
order-disorder transitions on oxygen permeability through selected nonstoichiometric
perovskite-type oxides. Solid State Ionics, 63-65:816 - 822, 1993.
[93] Jay Kniep and Y.S. Lin. Partial oxidation of methane and oxygen permeation in srcofeox membrane reactor with different catalysts. Industrial & Engineering Chemistry
Research, 50(13):7941-7948, 2011.
[94] Alexey Markov, Mikhail Patrakeev, Ilia Leonidov, and Victor Kozhevnikov. Reaction
control and long-term stability of partial methane oxidation over an oxygen membrane.
Journalof Solid State Electrochemistry, 15:253-257, 2011. 10.1007/s10008-010-1113-x.
[95] Hui Dong, Zongping Shao, Guoxing Xiong, Jianhua Tong, Shishan Sheng, and Weishen
Yang. Investigation on pom reaction in a new perovskite membrane reactor. Catalysis
Today, 67(1-3):3 - 13, 2001. Catalysis in Membrane Reactors.
[96] Zongping Shao, Hui Dong, Guoxing Xiong, You Cong, and Weishen Yang. Performance
of a mixed-conducting ceramic membrane reactor with high oxygen permeability for
methane conversion. Journal of Membrane Science, 183(2):181-192, 2001.
[97] Jianhua Tong, Weishen Yang, Rui Cai, Baichun Zhu, and Liwu Lin. Novel and ideal
zirconium-based dense membrane reactors for partial oxidation of methane to syngas.
Catalysis Letters, 78:129-137, 2002. 10.1023/A:1014950027492.
[98] A.A. Yaremchenko, A.A. Valente, V.V. Kharton, E.V. Tsipis, J.R. Frade, E.N.
Naumovich, J. Rocha, and F.M.B. Marques. Oxidation of dry methane on the
surface of oxygen ion-conducting membranes. Catalysis Letters, 91:169-174, 2003.
10. 1023/B:CATL.0000007150.63791.a2.
[99] Stefan Diethelm, Joseph Sfeir, Frank Clemens, Jan Van herle, and Daniel Favrat.
Planar and tubular perovskite-type membrane reactors for the partial oxidation
of methane to syngas. Journal of Solid State Electrochemistry, 8:611-617, 2004.
10.1007/si0008-003-0489-2.
[100] Haihui Wang, Rong Wang, David Tee Liang, and Weishen Yang. Experimental and
modeling studies on Bao. 5 Sro.5 Coo.sFe0 .2O3. (BSCF) tubular membranes for air separation. Journal of Membrane Science, 243(1-2):405 - 415, 2004.
[101] Shaomin Liu and George R. Gavalas. Preparation of oxygen ion conducting ceramic
hollow-fiber membranes. Industrial & Engineering Chemistry Research, 44(20):76337637, 2005.
[102] S. Diethelm, D. Bayraktar, T. Graule, P. Holtappels, and J. Van herle. Improved
stability of Lao. 5Sro. 5 FeO 3 by Ta-doping for oxygen separation membrane application.
Solid State Ionics, 180(11-13):857 - 860, 2009. Electroceramics XI - Ionic, Electronic
and Mixed Conductors, Fuel Cells Symposium.
154
[103]
Victor Kozhevnikov, Ilia Leonidov, Mikhail Patrakeev, Alexey Markov, and Yakov
Blinovskov. Evaluation of La 0 .5 Sr0 .5 FeO 3 membrane reactors for partial oxidation of
methane. Journal of Solid State Electrochemistry, 13:391-395, 2009. 10.1007/s10008008-0572-9.
[104] Huixia Luo, Yanying Wei, Heqing Jiang, Wenhui Yuan, Yangxiao Lv, Jrgen Caro,
and Haihui Wang. Performance of a ceramic membrane reactor with high oxygen flux
Ta-containing perovskite for the partial oxidation of methane to syngas. Journal of
Membrane Science, 350(1-2):154 - 160, 2010.
[105] Tingfang Tian, Wendong Wang, Minchuan Zhan, and Chusheng Chen. Catalytic
partial oxidation of methane over SrTiO 3 with oxygen-permeable membrane reactor. Catalysis Communications, 11(7):624 - 628, 2010. Catalytic partial oxidation
of methane.
[106] Yanying Wei, Hongfei Liu, Jian Xue, Zhong Li, and Haihui Wang. Preparation and
oxygen permeation of U-shaped perovskite hollow-fiber membranes. AIChE Journal,
57(4):975-984, 2011.
[107] Zhengliang Gong and Liang Hong. Integration of air separation and partial oxidation
of methane in the Lao 4 Bao.6 Fe0 .8 Zn0 .2O3 membrane reactor. Journal of Membrane
Science, 380(1-2):81 - 86, 2011.
[108] Zigang Shen, Xiaorui Guo, Wuqing Zhang, and Lijun Pan. Effect of laser ablation
onBao.5 Sr 0 .5 Coo. 8 Feo. 2 0 3 _ membrane for methane conversion. volume 279, pages 61 65, Hangzhou, China, 2011.
[109] Xinzhi Chen, Liu Huang, Yanying Wei, and Haihui Wang. Tantalum stabilized SrCoO 3
perovskite membrane for oxygen separation. Journal of Membrane Science, 368(12):159 - 164, 2011.
[110] Jianxin Yi, Jochen Brendt, Michael Schroeder, and Manfred Martin. Oxygen permeation and oxidation states of transition metals in (Fe, Nb)-doped BaCoO 3 perovskites.
Journal of Membrane Science, 387-388(0):17 - 23, 2012.
[111] P.N. Dyer, M.F. Carolan, D. Butt, R.H.E. Van Doorn, R.A. Cutler. Mixed conducting
membranes for syngas production, US Patent 6492290, December 2002.
[112] Liang Tan, Xuehong Gu, Li Yang, Wanqin Jin, Lixiong Zhang, and Nanping Xu.
Influence of powder synthesis methods on microstructure and oxygen permeation performance of Ba 0 .5 Sro.5 Coo.sFeo. 20 3 _perovskite-type membranes. Journal of Membrane
Science, 212(1-2):157 - 165, 2003.
[113] A. Ellett, D. Schlehuber, L. Singheiser, and T. Markus. Permeation and stability
investigation of Bao. 5Sro 5 Co 0 .sFeo.2 0 3 _ membranes for oxy-fuel processes. volume 30,
pages 121 - 127, Daytona Beach, FL, United states, 2010.
155
[114] M. Pilar Lobera, Jos M. Serra, Sren P. Foghmoes, Martin Sgaard, and Andreas Kaiser.
On the use of supported ceria membranes for oxyfuel process/syngas production. Journal of Membrane Science, 385-386(0):154 - 161, 2011.
[115] Oliver Czuprat, Thomas Schiestel, Hartwig Voss, and Jurgen Caro. Oxidative coupling of methane in a bcfz perovskite hollow fiber membrane reactor. Industrial &
Engineering Chemistry Research, 49(21):10230-10236, 2010.
[116] F.T. Akin and Jerry Y.S. Lin. Oxygen permeation through oxygen ionic or mixedconducting ceramic membranes with chemical reactions. Journal of Membrane Science,
231(1-2):133 - 146, 2004.
[117] Zebao Rui, Yongdan Li, and Y S Lin. Analysis of oxygen permeation through dense ceramic membranes with chemical reactions of finite rate. Chemical Engineering Science,
64(1):172-179, 2009.
[118] Xiaoyao Tan, Yutie Liu, and K. Li. Mixed conducting ceramic hollow-fiber membranes
for air separation. AIChE Journal, 51(7), 2005.
[119] A. Thursfield and I. S. Metcalfe. Methane oxidation in a mixed ionic-electronic conducting ceramic hollow fibre reactor module. Journal of Solid State Electrochemistry,
10(8):604-16, 2006.
[120] Alan Thursfield and Ian S. Metcalfe. Air separation using a catalytically modified
mixed conducting ceramic hollow fibre membrane module. Journal of Membrane Science, 288(1-2):175 - 187, 2007.
156
Appendices
157
158
Appendix A
Reactor within Enclosure
159
feed exhaust manifold
gas
mpling point
feed inlet manifold
mrem brahe pos tion
enclosure heater
opticaI access (n t in
use in this pict )
Figure A-1: The ITM reactor within its enclosure in the laboratory.
160
Appendix B
Methodology for Leak Quantification
(under non-permeation conditions)
The leak results described in section 3.1.3 are quantified (at temperatures lower than the
onset of oxygen permeation) in molar flow rates to enable a comparison with the expected
oxygen permeation flux across the membrane (1pmolcm-2s 1). With the values of Vsweepin
(pure C0 2 ) and
X0
2
,8 ,0
t
supplied by the CO 2 MFC and the GC respectively, the oxygen leak
rate into the reactor can be quantified. There are two main considerations for this analysis.
Firstly, we can assume that the concentrations of N2 (xN 2 ,s,ot) and CO 2 (xCo 2 ,S,out) in
the sweep outlet stream are related to the concentration of 02
(Xo
2
,s,out)
in the sweep outlet
stream thus:
XN
2
0.79(B)
0.9 XO2,sout
,S,OUt
(B.1)
1
Xco 2 ,s,oUt
=
1 - (X0
2 ,S,OUt
+
XN 2,sOUt) =
1
-
02,SOUt
Therefore, the average molar mass of the sweep side exit gas is
161
(B.2)
M = (Xo2,S,out X Mo2 ) + (XN2 ,s,out X MN2) + (XCO2 ,S,OUt X MCO2)
(B.3)
Secondly, we can calculate the mass flow rate of exit sweep gas (rhsout) based on equa-
tions 3.3 & 3.10. The mass fraction of 02 in the sweep side outlet (mfo2 ,S,out) is therefore
M02
mfO2,8 ,out = XO2,s,out X M
M
With the above considerations, the oxygen leak into the reactor or across the membrane
seal as detected on the sweep side outlet
n02 ,leak
(hO2,leak
m4 "ut
x2,Sout
X
mf
=
in mole-s- 1 ) can be quantified by:
Onns~o
M0
Or for direct analysis based on available data,
niO ,leak
2
=
x ro,~u
' 21 Xo 2,S,
0 ~t X
(0.21 X02,8,out)
v00 2 ,s,in
1
1
0
-
162
Pot
PS t
TetR
(B.4)
Appendix C
Flow Procedures
Legend
V= Valve
D = Flow Direction
P = Vacuum Pump
K= Key lock
S = Switch
DAQ = Data Acquisition System
M = Mass Flow Controller
MP = Mass Flow Controller Purge
Figure C-1: Reactor flow diagram
163
Table C.1: Flow procedures
Purpose
Gas Used
1.
2.
1.
2.
3.
1.
2.
Air
Feed side inflow
CO 2
Feed side purgea
-
Turn VI to the D2 direction
Set M1 to desired flow rate
Open V8
Set M2 to desired flow rate
Turn V3 to the D4 direction
Open V8
Turn on M2
3. Turn on MP2
CO 2 (h.pc)
CO 2
CH 4
Sweep side purge
Procedure
CO2 (in)
2
CO 2 (h.p)
Air d
Reactor inflow (both sides)
4. Turn V3 to the D4 direction
Open V2
1. Open V8
2. Set M2 to desired flow rate
3. Turn V3 to the D3 direction
1. Open V9
2. Set M3 to desired flow rate
3. Turn on K1, Si, and S2
1. Open V8
2. Turn on M2
3. Turn on MP2
4. Turn V3 to the D3 direction
Open V5
1. Turn VI to the D2 direction
2. Set M1 to desired flow rate
3. Open V4
1. Open V8
2. Set M2 to desired flow rate
3. Turn V3 to the D3 direction
4.
Continued on Next Page...
164
Open
V4
Purpose
Reactor purge (both sides)
Enclosure purge
Oxygen sensor calibration
Table C.1 - Continued
Procedure
Gas Used
1. Turn VI to the D2 direction
Air~
m2.
Turn on M1
3. Turn on MP1
4. Open V4
1. Open V8
2. Turn on M2
3. Turn on MP2
CO 2 (m-p)
4. Turn V3 to the D3 direction
5. Open V4
Open V2 and V5
CO2 (h.p)
u p1. Turn on S3
CO 2 and Air
2. Turn on P1
Open VI
CO2 (0% 02)
Open P-0 2 cylinder
P-02 (10% 02)
Turn V1 to the D1 direction
Air (21% 02)
aPurging involves using CO 2 to remove air from the reactor.
bMass flow controller purge (total flow rate - 2000 sccm).
cHigh purge (total flow rate > 2000 sccm).
dAir flow in both sides (normal or purge) aids the cooling of the reactor by forced convection.
165