Fixed-Bed Reactor for studying the Kinetics of
Methane Oxidation on Supported Palladium
Objectives:
1.
The general goal is to understand:
a)
b)
the influence of the presence of catalysts on the mechanism and rate of chemical reaction
the catalytic materials, their activities and the chemical and physical nature of their
surfaces.
2.
The specific goals are:
a)
To understand the kinetics of the catalyzed oxidation of methane in a gradientless
(perfectly mixed) reactor by:




b)
Determining the rate constant
Finding the activation energy
Evaluating the order of the reaction through experiments
Thus obtaining the rate expression
To apply the chemical engineering concepts to compare and interpret the observed
performance of Fixed-Bed Reactor with and with out backmixing (PFTR & R-R).
Theoretical backgrounds
The oxidation of Methane is envisioned as a single step reaction
Pd
CH4  2O2  CO2  2H2O
The global rate of a solid catalyzed reaction is normally expressed
as:
Moles of reactant consumed per unit time per unit mass of catalyst
(or per unit volume of the reactor containing the catalyst)
r(C, T)  k(T )C  k0 e
n
E
RT
Tr n n
[ ] Cr
T
(1)
Tr is the reference temperature, taken to be 250C
How to measure the global reaction rate [r(C,T)]?
We need to start from species mass balance equation for
particular reactor.
accumulation = flow in - flow out +generation by reaction
PFR
PFR-R
(2)
Equations For a Gradientless Reactor (CSTR or PFRR)
Gradienless reactor(Perfectly mixed) in which all physical rate
limitations are neglected, and uniform temperature and constant
pressure
(3)
qr Cor  qrCr  Vr
qr
r(C, T) 
(Cor  Cr )
V
CSTR
qr
V
PFRR
qr
can be
m
qr
r(C, T) 
Cor X
m
(4)
PFR-R
Where m is the weight of the catalyst
(5)
Where X, the fractional conversion of the reactant, is given by
X  1
Cr
Cor
(6)
Substitution from eq (1) for r(C,T) and from eq (5) for Cr
casts eq (4) in the following form, which relates the reactant
(Methane) conversion to rate constant and operating
conditions
X
m n1 Tr n
 Cor ( ) k0e
n
(1 X)
qr
T
E

RT
(7)
Co
An Ideal Plug Flow Reactor
Co
This model can be described by the following
differential equation for the methane material balance
dC
m Tr

k( T)C n
d
qr T
(8)
C(o )  Co r

C
where  is the axial position, made dimensionless by the reactor length. The
initial condition for Eq 8. is the concentration at  = 0.
Tr
T
C

(9)
For the constant temperature case, Eq. (8) is readily integrated to give the concentration as a
function of position. Application of Eq. (9) and substitution of the fractional conversion x
(now the conversion at  = 1) from Eq. (6) leads to the following equation for the reactor
conversion.
Cr
X  1
Cor
nm 1 Tr n
X  1 [1
(
) k(T)]
qr Cor T
(6)
1
n
(10)
If the reaction order (n) is 1/2then the fractional conversion
of Methane will be in the following form
1
2
m 1 Tr
2
X  1 [1
(
) k(T)]
2qr Cor T
11
Vent
Flow
Controll er
Pre-heater
Reactor
Gas
Chromatograp
h
Integr ator
He
O2
He
CH4
R.P.
T
T
T
Temperature
Controll er
EXPERIMENT SETUP
The Experimental Setup
Key words
1 = rotameters
2 = bubble flowmeter
3 = control valves
4 = 4-way valve
5 = mass flowmeters
6 = temperature indicators controls
7 = thermocouples
8 = recycle pump
9 = preheating zone
10 = furnace
11 = catalyst section
12 = moisture trop
13 = gas chromatograph
Experiment Procedure
First session
Run the reactor with zero recycle (PFR)
Second session
Run the reactor with recycle flow above 7 liters/min
Third session
Run the reactor as in the second session with different
Methane concentration.
P.S. Keep same feed composition in the first and second
sessions
Feed gases are:
Oxygen
Flows at rate of(30ml/min),
Mixture of 2% (by volume) Methane in Helium
Flows at different rates (30,25, 20, or 16 ml/min)
Helium for balance
Total flow to the reactor is 100ml/min
CATALYST DEACTIVATION DIAGRAM
Pd Sites
Al2O3
A
Fresh Catalyst (high dispersion; high surface area)
Pore cintering
Cintered Pd
Al2O3
B
Old Catalyst
Low dispersion (low ac tivity)
-Al2O3
C
Old catalyst
Low surface area (low activity)
Activation energy
without catalyst
Activation energy
with catalyst
Product
Reactant
Progress of the reaction
How to analyze our data
2.
Apply the data on Eq. (6) to calculate experimental conversion (x) for PFR and R-PFR.
Plot x vs T (reaction temperatures) for both reactors for same initial concentration of
CH4.
To evaluate the reaction order (use the experimental data of R-PFR):
A) Construct curves by plotting x versus Cor (initial concentration of CH4) for
varying temperatures, see if the slopes of these lines show negative, zero or
positive values.
B) Use Eq. (5) [r(C,T)=qr/mCorx] to calculate r(C,T) (reaction rate) as a function of
reaction temperature for all initial concentrations of methane.
C)
Plot rate data versus C (i.e., CrTr/T) for varying temperatures on logarithmic
coordinates. According to Eq. (1) the straight lines should have slope equal to the
R-R order n and the intercept equal to k(T).
reaction
R-R
T1 T
2
R-RPFTR
x
x
1.
1<n<1
r
T
Cor
T5
n & k (T)
Cr Tr/T
3.
Use k(T) values and corresponding temperatures to create an Arrhenius
plot. The ko and E/R (activation energy) values can be find from the
intercept and the slope of the straight line.
4.
To test the kinetic model, Eq. (7) and, n=1/2, ko and E/R values can be used to
calculate the conversion x as a function of the reactor temperature. Plot
x(Cor/1-x)1/2 vs T of observed and predicted . See if all data points fall on same
line, if so then the model is correct.
5.
To test the consequences of varying the initial concentrations of methane on
the reaction
R-Rrate, plot r versus T for various Cor.
R-R
R-R = -E/R
Slope
Intercept=ko
If n=1/2
Ln k r
1/T
T
T
6.
Use Eq. (11) to calculate conversion x for PFTR.
1
m
1 Tr 2
X  1 [1
(
) k(T)]2
2qr Cor T
7.
Plot experimental and calculated conversion values versus T for both
reactors (P.S. for same initial feed concentration).
R-R
PFTR
x
T
R-R
PFTR
x
T
R-R
T1
R-R
x
1<n<1
Cor
T2
r
T5
n & k (T)
CrTr/T
R-R
Slope = -E/R
Intercept=ko
Ln k
1/T
R-R
If n=1/2
T
R-R
r
T
R-R
PFTR
x
T