18th European Symposium on Computer Aided Process Engineering – ESCAPE 18
Bertrand Braunschweig and Xavier Joulia (Editors)
© 2008 Elsevier B.V./Ltd. All rights reserved.
Heterogeneous Kinetics and Residue Curve Map
Determination for Ethyl tert-Butyl Ether Synthesis
via Reactive Distillation Using Ion Exchange Resin
Catalysts
Muhammad Umar*, A.R.Saleemi, Muhammad Faheem, Suleman Qaiser
Department of Chemical Engineering, University of Engineering & Technology
Lahore, 54890, Pakistan
Abstract
Use of oxygenates such as Ethyl Tert-Butyl Ether (ETBE) is growing in Europe and
United States for octane rating augmentation as well as adding to the oxygen content of
gasoline fuels. This study deals with determining the eexperimental residue curve maps
(RCM) using the most suitable catalyst, found during batch kinetic studies, at wide
range of molar feed ratio of reactants. RCM experiments revealed that there was a
quaternary reactive azeotrope of water-TBA-EtOH and ETBE existing at 346 K. Molar
feed ratio of 1:2 (TBA:EtOH) was found optimum as it yielded the minimum
concentration of desired product (ETBE) in the residue which suggest that maximum of
ETBE was in distillate. Results of batch kinetic and RCM studies are used to simulate a
purpose built reactive distillation column in CHEMCAD ® software.
Keywords: Heterogeneous kinetics, residue curve map, ETBE synthesis, Ion exchange
resin, Reactive distillation
1. Introduction
The phasing out of lead compounds as octane rating boosters in 1970’s has steered a
new era of research for the alternative gasoline additives. The instant solution for the
problem was to raise octane number with increasing concentrations of butanes and
aromatics in gasoline. The more promising solution was the use of a new class of highoctane oxygen containing compounds, called oxygenates. Primarily, light alcohols like
methanol, ethanol and some alkyl ethers like methyl tert-butyl ether (MTBE) were
considered as useful compounds [1]. Among the tertiary alkyl ethers, ETBE
outperformed other oxygenates by virtue of its better blending and environmental
properties. It has higher octane rating of 111, lower oxygen contents of 15.7 %, low
water solubility of 23.7 mg/l and low blending Reid vapor pressure of 25.7 KPa as
compared to its competitors [2, 3].
*Corresponding author e-mail addres: bazaidumar@gmail.com
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M.Umar et.al
ETBE has been synthesized predominately using ethanol and iso-butylene (IB) in 80’s
and 90’s [4-5]. In late 90’s replacement of IB with tert-butyl alcohol (TBA) was
emphasized due to its low cost, easy availability and handling. Moreover, liquid phase
reaction is relatively easy to control as compared to gas phase reactions. Various authors
have investigated the synthesis of ETBE from ethanol and TBA using different catalysts
like ion exchange resins, Amberlyst-15 [6], S-54 and D-72 [2] and β-zeolite [7].
This work is extension of our previous work regarding the ETBE synthesis using seven
different ion exchange resin catalysts namely CT-124, CT-145H, CT-151, CT-175, CT275, Amberlyst-15 and Amberlyst-35 [8]. After characterization of catalysts, most
suitable catalyst was experimentally found and used in further studies. Residue curve
map (RCM) is considered to be very useful technique for viability and feasibility of
reactive distillation [9]. So far to the best of our knowledge and information there is
hardly any study available in open literature on the experimental residue curve map
determination for ETBE synthesis using EtOH and TBA. Few studies exist in literature
for RCM of ETBE synthesis but that involve iso-butene and EtOH as reactant.[10].
Therefore main aim of this study was to experimentally determine the residue curve
maps for ETBE synthesis using EtOH and TBA on ion exchange resin catalyst found
most suitable in batch kinetic studies. This data is then utilized in reactive distillation
column.
Most of the work in literature pertains to ETBE synthesis in reactive distillation
columns using commercially available simulators like ASPEN PLUS® and HYSYS®. In
present work, CHEMCAD® is used as simulation tool. A purpose built packed reactive
distillation column is used to generate experimental data for ETBE synthesis with EtOH
and TBA on ion exchange resin catalyst CT-145H.
2. Experimental
2.1. Materials and methods
All chemicals, TBA (99.5% GLC), EtOH (99.8%, GC), ETBE (97% GC), iso-propanol
(99.5% GC) were purchased from local market manufactured by Fisher UK and their
purity was verified by gas chromatography. These were used without further
purification. Ion exchange resin catalysts of CT brand were supplied by courtesy of M/S
Purolite® UK, while Amberlyst-35wet and Amberlyst-15 were provided by M/S Rohm
and Haas® France.
2.2. Apparatus
A three neck, round bottom; glass reaction vessel of 5.0 x10 -4 m3 volume was used to
carry out RCM experiments, similar to that used by Song et. al. [11]. Hot plate with
temperature control unit was used as heat source. This hot plate was also equipped with
variable speed magnetic stirrer to shake the contents of reaction flask at desired speed.
2.3. Procedure
For each RCM experimental run, predetermined and measured quantities of each
reactant (TBA and EtOH) in specified mole ratios were fed in to reaction vessel.
Catalyst was added to reaction mixture as weight percent and the contents of reaction
flask were continuously stirred and heated. A constant heat input and fixed stirring
Heterogeneous Kinetics and Residue Curve Map Determination for Ethyl tertButyl Ether Synthesis via Reactive Distillation Using Ion Exchange Resin Catalysts
speed were maintained throughout the experimentation. Initially, all the vapors formed
were condensed and refluxed back to reaction flask. When temperature of liquid and
vapor phases became stable then all the vapors formed were totally condensed and
collected in receiving flask. Liquid samples from the distillation flask were taken
sporadically and these were analyzed for their composition. Experiment was stopped
when there was not enough liquid left in the flask to take the sample. Next experimental
run was started with initial composition as close as possible to the end point of previous
run. By doing this we can plot the significant portion of each residue curve [11].
2.4. Analysis
Samples were analyzed by Perkin-Elmer Clarus 500 gas chromatograph (GC) equipped
with Porapak-Q (80/100) column of 2.0 m length and 3.175 x 10 -6m diameter using
thermal conductivity detector (TCD). High purity (99.999%) helium gas was used as
mobile phase at 3.0 Kg/cm2 pressure and flow rate of 35-ml/min. Injector and detector
temperatures were maintained isothermally at 458 K. Iso-propanol was used as internal
standard.
3. Results and Discussions
3.1. Catalyst characterization and batch kinetic
Seven different ion exchange resin catalysts were used in this study. These were
characterized by various techniques. Detail characterization results, physical properties
of all catalysts and batch kinetic studies and kinetic modeling results are given in our
previous work [8]. CT-145H was found better than other catalysts; hence it was used for
RCM experiments.
3.2. Residue Curve Map (RCM) Studies
In order to find the technical viability of reactive distillation for any particular reaction,
RCM is considered as the most use full tool. It can be determined theoretically as well
as experimentally in a simple distillation assembly. There is hardly any experimental
RCM study available in literature for this particular reaction system, so temptation was
to explore its behavior. Due to the importance of molar feed ratio (MFR) of reactants, a
wide range of initial MFR’s were used in these experiments. The ratios used were
(TBA:EtOH) 1:0.5, 1:0.8, 1:1, 1:1.3, 1:1.8, 1:2, 1:5, 1:8. In RCM, all curves ought to
originate from lowest boiling component called the unstable node and then tend to move
towards highest boiling component called the stable node. The RCM for etherification
reaction under study can be represented in a quaternary diagram (Fig. 1). In this figure
each corner represents a pure component and each edge represents a binary non-reactive
mixture of one reactant and a product. To draw the diagram, ETBE was taken as
reference component and independent composition of all other components are
represented in the form of transformed variables [12]. These transformed variables are
expressed in Eq. 1-3.
XA = xEtOH+xETBE
(1)
XB = xTBA+xETBE
(2)
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M.Umar et.al
XC = xwate r -xETBE
(3)
Where XA, XB and XC are transformed variables whilst x is the liquid phase mole fraction
of the respective component written in the subscript. It was observed during
experimentation that equilibrium temperature of vapors and liquid residue was 349 K.
At this temperature a quaternary azeotrope of water-EtOH-TBA-ETBE was observed
having mole fraction of, water = 0.0342, EtOH = 0.7277, TBA = 0.1373 and ETBE =
0.1009. It is concluded from the results of RCM experiments that increase in the molar
feed ratio beyond 1:2 (TBA:EtOH) resulted in more un-reacted ethanol and TBA
present in the top product which of course is not desirable in a reactive distillation
column. Therefore, molar feed ratio between 1:1.3 and 1:2 was considered most suitable
in these experiments because the concentration of desired product i.e. ETBE was
minimum in residue and maximum in top product.
Although the composition of liquid distillate is normally not measured in RCM, but to
track the concentration of desired product, distillate composition was measured for each
molar feed ratio. Variation of ETBE concentration in distillate with various molar feed
ratios revealed that maximum ETBE concentration was found for MFR between 1:1.3
and 1:2.Further increase in MFR resulted in alleviation of ETBE concentration in
distillate. It was also observed that even towards the end of each experimental run, some
concentration of ETBE was present in residue suggesting the existence of azeotrope. It
is also to be mentioned that there was no reference work available in open literature to
compare the results with this study.
TBA (355.5 K)
1
Water (373 K)
1:0.5
0.8
1:0.8
1:1
1:1.3
0.4
1:1.8
1:2
XB
0.6
1:5
0.2
0
ETBE (341 K) 0
1:8
0.2
0.4
0.6
0.8
1
EtOH (351.04 K)
XA
Figure 1 RCM of ETBE synthesis on
CT-145H catalyst
Figure 2. Reactive Distillation Column (RDC)
assembly for ETBE synthesis
A small scale reactive distillation column as shown in figure 3 was built and used to
optimize the operating parameters of ETBE synthesis on ion exchange resin catalyst
Purolite® CT-145H. RD columns are normally simulated with commercially available
software like ASPEN PLUS® or HYSYS®. In this study, CHEMCAD® has been used as
simulation tool. The column was simulated at the conditions found most suitable in
Heterogeneous Kinetics and Residue Curve Map Determination for Ethyl tertButyl Ether Synthesis via Reactive Distillation Using Ion Exchange Resin Catalysts
batch kinetic and RCM studies. Heat input to the reboiler for simulation was fixed at
300 Watts. Reboiler duty was not varied during the simulation.
70
60
Mole fraction
Liquid Temperature 0 C
80
50
40
30
20
10
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
0
0
2
4
6
8
10
Segment No
12
14
1
16
3
5
7
9
11
13
15
Segment No
EtOH
Water
T BA
(a)
ET BE
(b)
Figure 3. Liquid temperature (a) and concentration (b) profile for ETBE synthesis
Column characteristics and some simulation conditions are presented in table 1. Column
temperature profile and concentration profile of each component are ilustrated in fig.
3(a,b).Temperature profile shows the impulsive rise after first segment and it levels off
at about 346 K, which closely matches the equilibrium temperature in RCM studies.
The liquid concentration profile shows that the conversion of limiting reactant is not
appreciable at these conditions which may be due to insufficient liquid hold up in the
reactive section of the column. This study also elucidate that there should be very
careful choice of operating conditions of reboiler duty and reflux ratio in reactive
distillation column. This is due to the fact that a subsequent separation unit will be
inevitable for distillate to attain the desired product purity if the manipulated variables
are not adequately controlled.
Table 1. Specifications and simulation conditions for RD column
Parameters
Section
Rectification &
Stripping
Height= 0.30m (4 segments in each section), Dia.= 0.05m
Packing = Protruded metal packing PRO-PAK® 0.24” (SigmaAldrich), sp. surface 1233 m2/m3, free area 96 %
Reactive Section
Height
=
0.61m ( 8 segments), Dia. = 0.05m
Packing= Structured (wire mesh pockets filled with catalyst), sp.
surface 1235 m2/m3, free area 97%
Condenser
Total condenser
Reboiler
Partial Reboiler with variable power input (0-800W), Simulated
at 300 Watts
Reflux Ratio
5.0
IB
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M.Umar et.al
4. Conclusions
Synthesis of ETBE from TBA and EtOH on ion exchange resins CT-124 and CT-145H
is found to be favorable as compared to the other catalysts used in this study. Maximum
TBA conversion and ETBE selectivity attained with CT-145H were 81.58 % and 62.86
% respectively. Experimental RCM study shows that synthesis of ETBE in reactive
distillation column is feasible with CT-145H as catalyst. Simulation of small scale
reactive distillation column with CHEMCAD shows the approximate TBA conversion
of 40 % at the conditions found optimum during batch studies and residue curve map
determination. In next phase of this work, experimental results will be compared to
those obtained from simulation of the actual column.

References
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