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Ind. Eng. Chem. Res. 2004, 43, 762-768
Simulation Studies of Catalytic Distillation for Removal of Water from Ethanol Using a Rate-Based Kinetic Model Christina B. Dirk-Faitakis and Karl T. Chuang* Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta T6G 2G6, Canada
A new approach based on catalytic distillation (CD) technology was proposed to remove water from ethanol, by reaction with isobutylene to form tert-butyl alcohol (TBA) and ethyl tert-butyl ether. Reaction within the CD column was calculated using a rate-based kinetic model with the simulation package Aspen Plus. A sensitivity analysis of the effect of the key design and operating factors on the column performance was performed. The most important factors affecting water conversion, TBA selectivity, and reboiler duty are the operating pressure and reaction temperature, distillate-to-feed ratio, amount of catalyst, and feed and reaction stage location. Optimization of operating variables found that the CD process offers potential advantages of reduced energy consumption and reduced capital cost over the traditional approaches for the removal of water from ethanol. Introduction Demand for ethanol as an automotive fuel alternative continues to be high as countries are pushing for greater self-reliance and cleaner burning fuels. As well, oxygenated gasoline additives play an increasingly important role as governments look to introduce legislation requiring gasoline containing a minimum oxygen content. Ethers such as methyl tert-butyl ether (MTBE), ethyl tert-butyl ether (ETBE), and tert-amyl methyl ether (TAME) and alcohols such as ethanol, methanol, and tert-butyl alcohol (TBA) are examples of oxygenates that have been used to increase the oxygen content of gasoline. In the United States, the U.S. Environmental Protection Agency regulates the amount of oxygenates that can be added to gasoline. It is up to fuel producers to decide which oxygenates to use and in what proportions to blend in order to meet the requirements. The standard specifications for automotive spark-ignition engine fuel allow for a maximum oxygen content of 2.7 wt % for gasoline-oxygenate blends containing aliphatic ethers and/or alcohols (including TBA).1,2 ETBE has superior oxygenating properties such as high octane number and low vapor pressure and would be classified as a semirenewable resource when derived from ethanol obtained from the fermentation of biomass. U.S. ethanol production has been steadily increasing since the 1980s, with a reported 2.13 billion gallons of fuel ethanol produced in 2002.3 One of the largest costs involved in the production of ethanol is the energy required for product purification to remove water from the ethanol. To enrich ethanol from the azeotrope at 95% (weight percent) ethanol to 100%, it requires about half the energy as is required in enriching ethanol from an initial 10% ethanol mixture to the azeotrope point. This study proposes a process whereby water in a near-azeotropic ethanol-water mixture is reacted with isobutylene (IB) in a catalytic distillation (CD) column to form TBA and a portion of the ethanol is reacted to form ETBE. The * To whom correspondence should be addressed. Tel.: (780) 492-4676. Fax: (780) 492-2882. E-mail: karlt.chuang@ ualberta.ca.
final product is a mixture of ethanol, TBA, and ETBE suitable for use as an oxygenated fuel additive. Background By simple distillation, an ethanol-water mixture can be enriched to 95 wt % ethanol. This process cannot achieve a higher purity ethanol solution because of the presence of the binary ethanol-water azeotrope (95.6 wt % ethanol). Further enrichment of ethanol must obviate the azeotrope point. Conventionally, this can be done by azeotropic distillation, in which a third component is added to break up the binary ethanol-water azeotrope; however, this process is very energy-intensive because ethanol must be distilled twice to recover the added third component.4 Other technologies available to purify ethanol include molecular sieve technology and pervaporation. Molecular sieve technology operates on the principle of pressure-swing adsorption and is characterized by low steam and power consumption but also requires high capital investment. Pervaporation is a membrane process and is a rather costly process because of the necessary reheating of the feed stream and cooling of the permeate. To reduce energy and capital costs, new ways must be designed to remove water from ethanol. CD, which has been proven to be a very high efficiency process to produce ether oxygenates,5 seems to be very promising for this application. CD can be defined as a process in which a heterogeneously catalyzed chemical reaction and separation of the products and reactants occur simultaneously within a single distillation column.6 By combination of chemical reaction and separation into one unit, CD can significantly reduce capital and energy costs. Inside the CD column, the solid catalyst is surrounded by boiling liquid; therefore, the heat released by exothermic reaction can be efficiently used by distillation to generate more vapors, thus reducing the reboiler duty. CD is most suitable for equilibrium-limited reactions because the reaction products formed can be distilled away from the reactants, thus shifting the chemical equilibrium toward 100% conversion. To date, CD has been used for the production of fuel ethers such as MTBE, ETBE, and
10.1021/ie034123e CCC: $27.50 © 2004 American Chemical Society Published on Web 12/30/2003
Ind. Eng. Chem. Res., Vol. 43, No. 3, 2004 763 Table 1. Selected Azeotropic Data for the ETBE-TBA-EtOH-H2O System system ETBE (1)-ethanol (2)
Taz (°C)
24.8 50 60 65 90 ethanol (1)-water (2) 78.2 109.0 TBA (1)-water (2) 79.91
Table 2. Rate Constants Used in the Kinetic Model
xaz,1 ) yaz,1 xaz,1 ) yaz,1 mole fraction mass fraction 0.790a 0.695a 0.645b 0.637a 0.510a 0.895 0.886 0.6459
0.893 0.835 0.801 0.796 0.698 0.956c 0.952c 0.882d
Paz (kPa) 18.14 53.66 78.89 94.69 220.61 101.33 303.98 101.33
a Reference 11. b Reference 12. c CRC Handbook of Chemistry and Physics, 59th ed.; CRC Press: Boca Raton, FL, 1978. d Perry’s Chemical Engineers’ Handbook, 6th ed.; McGraw-Hill Book Co.: New York, 1984.
TAME.7,8 The objective of this research was to examine the feasibility of using CD to dehydrate ethanol by converting water to TBA. This paper examines how the CD process, using a kinetic reaction model to model the hydration and etherification reactions, can be used to remove water from ethanol-water mixtures. CD Model Modeling of the CD process requires simultaneous prediction of reaction and separation, and several model options are available. This work uses a nonequilibrium kinetic model to describe reactions that occur in the catalytic zone of the column and an equilibrium model to describe the separation in the column. An equilibrium model is most frequently used for the simulation of distillation columns. This approach has been used for over 40 years and is close to reality when reliable vapor-liquid equilibria (VLE) are available. The chemical reaction was simulated with a rate-based kinetic model because this allows design calculations to be performed to determine the size of the reaction section. Earlier work simulated the process as an equilibrium (chemical and physical) system.9 The equilibrium model assumed that reaction equilibrium conditions were achieved in the reaction zone, and this model showed that it was not possible to break the ethanol-water azeotrope because equilibrium conditions favored ETBE over TBA. Water could only be completely removed using a large excess of IB, in which case ethanol was also completely converted to ETBE. Physical Equilibrium Model. An important consideration in any distillation design is the ability to predict multicomponent VLE and liquid-liquid equilibria (LLE). Reliable VLE and LLE are needed to establish distillation boundaries and to determine if and where azeotropes and phase separations occur. Table 1 summarizes azeotropic data for the system. Simulation studies were performed with the Aspen Plus10 RADFRAC column, which is a rigorous model for simulating multistage vapor-liquid fractionation operations. The UNIQUAC activity coefficient model for the liquid phase and the Redlich-Kwong equation of state for the gas phase were used as the equilibrium models. UNIQUAC binary interaction parameters were taken from Aspen’s database, regressed from binary literature data,11-13 or estimated by Aspen Plus using UNIFAC. Validation of the equilibrium model was performed by comparing model data to experimental ternary data14,15 and binary azeotropic data.11 Kinetic Reaction Model. ETBE is produced by the etherification reaction of ethanol and IB over an acid
parameter
value
kTBA kETBE KW KEtOH KTBA KC
5.4995 × 104e-4305.5/T 1.2053 × 109e-7746.8/T 2.9088 × 10-3e3801.9/T 0.34277 × 100e1323.1/T 2.9134 × 108e-6602/T 5.58316 × 10-3e2822.7/T
catalyst. TBA is produced by the hydration of IB and water over an acid catalyst. Both reactions are exothermic, reversible, and equilibrium-limited. Equilibrium calculations at 60 °C and P ) 5 atm show that the etherification reaction is favored over the hydration reaction by a factor of 16 to 1. Several authors have reported reaction kinetics for the liquid-phase synthesis of ETBE.16-19 Kinetic data from our laboratory, for both the etherification and hydration reactions, were determined using the ion-exchange catalyst Amberlyst 35. This study shows that the hydration reaction that produces TBA is favored at low temperatures.20 The expressions for the equilibrium constants and the rate equations for the liquid-phase synthesis of ETBE and TBA are based on the Langmuir-Hinschelwood-Hougen-Watson model. The rate equations used in this work are given below, and the rate constants are given in Table 2.
Hydration reaction: 1 kTBA
H2O + (CH3)2CdCH2 79 8 (CH3)3COH -1 kTBA water IB TBA Etherification reaction: CH3CH2OH + ethanol 1 kETBE
(CH3)2CdCH2 79 8 (CH3)3COCH2CH3 -1 kETBE IB ETBE kTBAKTBACTBA KC rTBA ) 1 + KWCW + KEtOHCEtOH + KTBACTBA kTBACIBKWCW -
rETBE )
kETBECIBKEtOHCEtOH 1 + KWCW + KEtOHCEtOH + KTBACTBA KC )
k1TBA k-1 TBA
K ) K0e-∆HA/RT k ) k0e-∆Ea/RT The main side reactions are the dimerization of IB to form diisobutylene and the dehydration of ethanol to form diethyl ether. However, both side reactions are essentially eliminated at lower reaction temperatures21 and are ignored in this work. Results and Discussion Correlation and Prediction of Equilibrium Data. Figure 1 shows the ternary diagram for water-ethanol-ETBE as calculated with Aspen Plus using the UNIQUAC model. The model results compare reasonably well with the experimental ternary data, which is
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Figure 3. Schematic of the CD column configuration used in the simulations (base case).
Figure 1. Ternary diagram for the H2O-EtOH-ETBE system at P ) 1 atm and T ) 35 °C predicted by Aspen Plus using the UNIQUAC model.
Figure 4. Effect of the column pressure on the water conversion, TBA selectivity, and temperature in stage 3 (reaction stage) of the column. Table 3. Parameter Values for the Base Case Used in Aspen Plus Simulations value parameter
Figure 2. Experimental ternary data for the H2O-EtOH-ETBE system at P ) 1 atm and T ) 35 °C: (b) aqueous phase from ref 15; (O) organic phase from ref 15; (2) aqueous phase from ref 14; (4) organic phase from ref 14.
shown in Figure 2. As well, azeotropic data for the ethanol-water and ethanol-ETBE binaries are well predicted. Simulation of Operating Variables. The first part of the simulation runs involved varying the values of key operating conditions and observing the effect on the water conversion, separation efficiency, and reboiler duty. Figure 3 shows a typical configuration of a CD column used in the simulations. The column is divided into three sections: rectification section, reaction zone, and stripping section. The CD column includes a total condenser and a partial reboiler. The distillate (IB) is cooled and recycled to the front of the column. The operating parameters and their values for the base case are shown in Table 3. All feed streams enter at 25 °C. (a) Effect of the Pressure. Operating pressure is one of the key operating parameters of a distillation column. The choice of operating pressure for a CD column depends on many considerations such as the overhead temperature, bottom temperature, reaction
column pressure (atm) distillate-to-feed ratio (D/F) reflux ratio (RR) total number of stages reaction stage location feed stage location (above stage) molar composition of ethanol-water feed molar feed ratio of IB(fresh)-water catalyst weight (kg)
base case
design A
design B
4 0.2 5 10 3 3
6 0.2 6 10 3 and 5 3
6 0.2 9 10 3 and 5 3
0.88:0.12
0.88:0.12
0.60:0.40
5:1
3.48
1.5
40
40
60
temperature, and relative volatilities of the components in the system. The column pressure sets the lower and upper bounds of the temperature within the column. The operating pressure should be chosen within a range such that water can be used as a coolant for the overhead condenser and steam can be used as a heating medium for the reboiler. Within this range, the reaction temperature mainly determines the operating pressure. Because chemical reactions take place within the liquid phase, the reaction temperature is close to the boiling point of the liquid phase flowing around the catalyst. Therefore, the reaction temperature will increase with the operating pressure. Figure 4 shows the effect of the column pressure on the water conversion, TBA selectivity, and temperature on stage 3 (reaction stage) of the column. As the pressure increases from 2 to 10 atm, the reaction stage temperature increases linearly from 22
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Figure 5. Effect of the molar feed ratio of IB to water on the water conversion and TBA selectivity.
to 146 °C. Intuitively, one would also expect that as the reaction temperature increases, higher rates of TBA production would be predicted. However, Figure 4 shows that initially water conversion increases, reaches a maximum, and then decreases again. TBA selectivity decreases with an increase in the temperature. This suggest that as the temperature increases, higher reaction rates may be expected but that at the higher temperatures the ETBE reaction product is favored. (b) Effect of the Molar Feed Ratio of IB to Water. It was assumed that the composition of the ethanolwater feed is fixed near the azeotropic composition at a molar ratio of 88:12, so the only feed ratio to be varied is the stoichiometric ratio of IB to water. Figure 5 shows the effect of the molar feed ratio of IB to water on the water conversion and selectivity of TBA production. It can be seen that as the IB-water ratio increases, both water conversion and TBA selectivity increase. The water content in the bottoms product reflects this trend and shows a decrease with increasing IB-water feed ratio. For all three curves, a plateau region is reached at IB-water ratios greater than 5. The optimum stoichiometric ratio of IB to water lies between 4 and 5 and strikes a balance between good water conversion and TBA selectivity. This large stoichiometric excess of IB is needed because part of IB will be consumed by the etherification reaction to produce ETBE. The presence of TBA and ETBE in the final product should not cause any problems because they are themselves good octane oxygenates. (c) Effect of the Distillate-to-Feed Ratio. From the simulation runs, it was found that the distillate rate or the distillate-to-feed ratio (D/F) is very critical, having a significant effect on the overall conversion of the olefin hydration reaction as well as the reboiler heat duty. Figure 6 shows the effect of the distillate-to-feed ratio on the reboiler duty and water conversion. With an increase in D/F, the duty of the reboiler increases significantly because more unreacted IB and ethanol will escape from the top of the column. This increases the energy cost per kilogram of bottoms product. On the other hand, as the D/F ratio is decreased, the column begins to operate more under total reflux conditions. This forces IB back to the reaction zone to react with water and ethanol. The reboiler duty decreases significantly as the D/F ratio drops to 0.05. Although water conversion is still relatively high, indicating that TBA is still being produced, the etherification reaction dominates at these conditions and a significant portion of the ethanol is converted to ETBE. This can be seen in Figure 7, which shows TBA selectivity as a function of the D/F ratio. As the D/F ratio increases, the TBA
Figure 6. Effect of the distillate-to-feed ratio on the reboiler duty and water conversion.
Figure 7. Effect of the distillate-to-feed ratio on the TBA selectivity.
Figure 8. Effect of the reflux ratio on the water conversion, IB in the distillate, TBA selectivity, and reboiler duty.
selectivity increases, with values as high as 70%. This is higher than that of a previous simulation study performed in our laboratory, where reaction equilibrium was assumed. In this work, the highest TBA selectivity that could be achieved was 35%, with a maximum water conversion of 33%.9 (d) Effect of the Reflux Ratio. The purpose of the reflux ratio is to help increase the concentration of the component that is concentrated at that stage. The effect of the reflux ratio is shown in Figure 8. The concentration of IB in the distillate is quite insensitive to the reflux ratio over the range considered. Increasing the reflux ratio also increases the ethanol content in the bottoms product (data not shown). The effect of increasing the reflux ratio on the reboiler duty is significant. Increasing the reflux ratio from 2 to 10 requires a 3-fold increase in the reboiler duty. The reflux ratio also has an effect on the selectivity of the reaction products. Increasing the reflux ratio increases the TBA selectivity. The rate of TBA formation is greater than the rate of
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Table 4. Effect of Feed and Reaction Stage Location
run E1 E2 E3 E4 E5 F1 F2 F3 F4a F5a a
feed location (above stage) EtOH-H2O IB 3 3 3 7 2 3 3 3 3 3
3 7 2 3 3 3 3 3 3 3
reaction location (Treactor, °C) 3 (46.5) 3 (55.9) 3 (46.5) 3 (35.0) 3 (45.8) 3 (46.5) 2 (35.5) 5 (46.5) 2 (35.6) 3 (45.7)
total stages
water conversion (%)
H2O
10 10 10 10 10 10 10 10 10 10
84.0 83.9 84.0 0.0 83.4 84.0 1.8 83.9 66.3 91.4
0.5 0.5 0.5 2.8 0.5 0.5 2.8 0.5 1.0 0.2
3 (45.7) 5 (46.5)
bottom composition (wt %) EtOH TBA 47.8 44.4 47.8 53.1 48.0 47.8 52.3 47.8 50.5 48.0
9.8 9.8 9.8 0.0 9.8 9.8 0.2 9.8 7.7 10.7
ETBE 11.8 19.3 11.8 0.0 11.4 11.8 1.8 11.8 5.7 11.3
The total catalyst weight remains the same, only equally split between both reaction stages.
ETBE formation because increasing the reflux ratio increases the concentration of IB and decreases the concentration of ethanol and TBA on the reaction stage. A lower concentration of TBA on the reaction stage drives the forward hydration reaction. Furthermore, a decrease in the concentration of ethanol on the reaction stage decreases the rate of the etherification reaction. Although a reflux ratio over 2 seems to offer no additional benefit to separation within the CD column, it does affect the distribution of TBA and ETBE produced and a minimum reflux ratio of 5 is required to achieve a TBA selectivity of 50%. (e) Effect of the Feed and Reaction Stage Location. The functions of the rectification and stripping sections are to purify the overhead and bottom products and recycle the unconverted reactants back to the reaction zone in a CD column. The number of separation stages and the location of the feed and reaction zone(s) must be adjusted to offer the best separation and the most favorable reaction conditions. Simulations were performed to determine the effects of the location of the feed and reaction stage(s) and the effect of multiple reaction stages. The results are summarized in Table 4. Comparing runs E1-E5 shows that the location of the ethanol-water feed is more critical than the IB feed stage. Moving the IB feed stage above or below the reaction stage has no significant effect on the water conversion although the TBA selectivity decreases somewhat when IB is introduced lower in the column as a higher temperature is realized on the reaction stage, and this favors ETBE production. When the ethanol-water feed is introduced below the reaction stage (compare E4 and E1), no reaction occurs. This is because ethanol-water, being the heavier components of the feed, will travel down upon entering the column, whereas IB, being the lightest component, will travel up the column. This is also seen in run F2, where the reaction stage has been moved up one stage in relation to the feed stage. Runs F1-F3 show the effect of moving the reaction stage in relation to the feed stage. Locating the reaction stage below the feed stage (F3) has no significant effect on water conversion and selectivity, whereas locating the reaction stage above the feed stage (F2) results in essentially no reaction occurring because most of the liquid feed is bypassing the reactor. Runs F4 and F5 compare multiple reaction stages. Simulation run F5 shows an overall increase in the water conversion when there are two reaction stages in the CD column compared to a single reaction stage (E1). Not only is more water converted to TBA but the selectivity of TBA increases slightly as well. This shows how separation (between two reaction stages) can effect the reaction.
Figure 9. Effect of the catalyst weight on the water conversion and TBA selectivity.
(f) Effect of the Catalyst Weight. Catalyst is used to influence the selectivity and to accelerate reactions. For a reaction that is not limited by equilibrium, increasing the amount of catalyst increases conversion. Figure 9 shows the effect of the catalyst weight on the water conversion and TBA selectivity. As the amount of catalyst on the reaction stage increases, the amount of water converted to TBA increases as well. However, the TBA selectivity decreases with an increase in the catalyst weight. Whereas the TBA selectivity continues to decrease linearly with increasing catalyst weight, water conversion begins to plateau at higher catalyst weights, indicating that the reaction is approaching equilibrium-limited conditions. Other hydrodynamic factors and column specifications will also need to be considered in deciding on the minimum and maximum amounts of catalyst that can be accommodated on a reaction stage. Simulation Optimization. After the sensitivity analysis of the key operating variables was performed, further simulations were performed to demonstrate the feasibility of design, by showing that water could be removed from the ethanol-water mixture with an energy cost lower than that of traditional separation technologies. An optimized column configuration was developed based on the following objectives: maximize the water conversion, maximize the TBA selectivity, and minimize the reboiler duty. Figure 10 is a flow diagram of the CD process. Feeds F1 and F2 enter at the top of the column above stage 3. F1 is the ethanol-water feed, and F2 is the IB feed stream. The CD column operates at a pressure of 6 atm, and the two reaction zones, located on stages 3 and 5, operate at 54.9 and 56.9 °C, respectively. Operating at a distillate-to-feed (D/F) ratio of 0.2 and a reflux ratio of 6 forces unreacted IB out through the top of the column, where it is recycled back to the column feed via stream R. Before being recycled
Ind. Eng. Chem. Res., Vol. 43, No. 3, 2004 767 Table 5. Comparison of Energy Requirements for Ethanol-Water Distillation final ethanol-water composition (mass fraction of EtOH)
energy requirement [kW/kg of producta (Btu/lb)]
0.949 0.940 0.915 0.856 0.803
1.9 (2950) 1.7 (2640) 1.5 (2330) 1.3 (2020) 1.26 (1950)
a 1 kg of product is defined based on the final ethanol-water composition (e.g., 1.26 kW/kg of product, where the composition is 0.803 kg of ethanol and 0.197 kg of water).
Figure 10. Flow diagram of the optimized CD process (design A).
Figure 11. Column profiles of the optimized CD process (design A).
to the front of the column, stream R passes through a heat exchanger. In this way IB losses from the vapor stream of the flash drum (L) are minimized. The product (ethanol, ETBE, TBA, IB, and water) leaves the flash drum at ambient pressure and temperature. If the product can be cooled to 20 °C in the flash drum, then there are no losses of product from the flash drum (L). Figure 11 shows the composition profile along the length of the column. The parameter values for this process (design A), based on a molar feed stream of ethanolwater of 88:12, are shown in Table 3. An alternative recycle scheme (figure not shown) involved operating the CD column under total reflux (F/D ) 0) and recycling the IB from the flash drum back to the CD column. However, this scheme is not feasible because the vapor exiting the flash drum also had a significant amount of TBA and ETBE that would also need to be recycled. The IB recycle from the top of the column has a much higher IB purity. (a) Energy Considerations. The majority of the energy cost in operating a CD column is required in the reboiler for heating and vaporization requirements. The energy requirements of the CD column itself requires Qr ) 1472.2 kW for the reboiler and Qc ) -1302.9 kW for the condenser. With a bottoms production rate of 6615 kg/h, this translates into a reboiler duty of 0.22 kW/kg of bottoms product. This is much lower in comparison to the energy cost required for azeotropic distillation, which is on the order of 0.9 kW/kg of
ethanol.4 In addition to this, cooling of the bottoms stream from the column may be required. Heat exchanger symbols indicate where energy may be recaptured and returned to the process to heat alternate streams. All values are based on the basis of an ethanol-water feed rate of 100 kmol/h. There are many advantages to operating the CD column at low pressure and mild temperatures: (1) Operating at low pressures results in a low reaction stage temperature; this is beneficial for the life of the catalyst, which has a maximum operating temperature of 150 °C. (2) Operating at low temperatures favors the production of TBA over ETBE. In this way water can be removed from the ethanol-water mixture without significant loss of ethanol to ETBE production. For additional energy savings (preprocess savings), it is also possible to feed a lower purity ethanol-water stream at F1. This also offers the benefit of more product yield for the producer in comparison to producing an anhydrous ethanol product. Furthermore, because IB is readily available at refineries, ethanol can be shipped directly to refineries via pipelines and there is no worry about water contamination. Energy savings would be realized during the first part of the distillation process, in bringing the ethanol from 10 to 80 wt % instead of from 10 to 95 wt %. Table 5 is a comparison of simulation results for reboiler energy requirements in distilling an ethanol-water mixture starting from an initial ethanol content of 10 wt %. Simulations are based on a 40-stage (100% efficiency) distillation column with a constant D/F ratio of 0.046 and variable reflux ratio. In this scenario (design B, Table 3), a higher reflux ratio and a higher IB feed rate result in slightly higher energy costs for the CD process, namely, 0.32 kW/kg of product. Also, both the column size and the amount of required catalyst increase. A sensitivity analysis and cost optimization would need to be performed to determine the advantages and benefits for this scheme. Some of the factors affecting this decision include (1) cost of IB, (2) cost of catalyst, (3) product requirements, (4) heating costs, and (5) capacity. Conclusions A novel process scheme was developed to remove water from an ethanol-water mixture by reaction in a CD column. A sensitivity analysis of the key operating parameters of the CD column shows that the most important factors affecting water conversion, TBA selectivity, and reboiler duty are the operating pressure and reaction temperature, distillate-to-feed ratio, amount of catalyst, and feed and reaction stage location. Using a rate-based kinetic model, it was shown that removal of water from an azeotropic ethanol-water mixture was
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possible by conversion to TBA. Optimization of operating variables found that the CD process offers potential advantages of reduced energy consumption and reduced capital cost over the traditional approaches for the removal of water from ethanol. It was also shown that additional energy savings (preprocess savings) could be realized by using a lower purity ethanol-water feed stream to the CD column. Other advantages in using a lower grade ethanol-water stream as the feed include a higher product yield and the possibility of transportation through refinery pipelines without worrying about water contamination. Further work is needed to determine fuel properties (such as MON, RON, blending Reid vapor pressure, oxygen content, and energy content) of the mixture and to develop an optimized formulation. Literature Cited (1) Standard Specification for Automotive Spark-Ignition Engine Fuel; Annual Book of ASTM Standards, D 4814-01a; American Society for Testing and Materials: West Conshohocken, PA, 2002. (2) Oxygenated Unleaded Automotive Gasoline Containing Ethanol; Canadian General Standards Board, CAN/CGSB-3.511-93; Canadian General Standards Board: Ottawa, Ontario, Canada, 1993. (3) Renewable Fuels Association. Fuel Ethanol: Industry Guidelines, Specifications, and Procedures, RFA 960501 (revised May 2002). (4) Parkinson, G. Battelle maps ways to pare ethanol costs. Chem. Eng. 1981, June, 29. (5) Fair, J. R. Design aspects for reactive distillation. Chem. Eng. 1998, Oct, 158. (6) Podrebarac, G. G.; Ng, F. T. T.; Rempel, G. L. More uses for catalytic distillation. Chemistry 1997, May, 37. (7) De Garmo, J. L.; Parulekar, V. N.; Pinjala, V. Consider reactive distillation. Chem. Eng. Prog. 1992, 88, 43. (8) Ignatius, J.; Ja¨rvelin, H.; Lindquist, P. Use TAME and heavier ethers to improve gasoline properties. Hydrocarbon Process. 1995, 2, 51. (9) Dirk-Faitakis, C.; Chuang, K. T. Simulation Studies of the Removal of Water from Ethanol by a Catalytic Distillation Process. Chem. Eng. Commun. 2003, submitted for publication. (10) Aspen Plus, version 11.1-0; Aspen Technology Inc.: Cambridge, MA, 1996.
(11) Rarey, J.; Horstmann, S.; Gmehling, J. Vapor-Liquid Equilibria and Vapor Pressure Data for the Systems Ethyl tertButyl Ether + Ethanol and Ethyl tert-Butyl Ether + Water. J. Chem. Eng. Data 1999, 44, 532. (12) Oh, J.-H.; Park, S.-J. Isothermal Vapor-Liquid Equilibria at 333.15 K and Excess Molar Volumes at 298.15 K of Ethyl tertButyl Ether (ETBE) + Alcoh-1-ol (C1-C4) Mixtures. J. Chem. Eng. Data 1998, 43, 1009. (13) Leu, A.; Robinson, D. B. Vapor-Liquid Equilibrium for Four Binary Systems. J. Chem. Eng. Data 1999, 44, 398. (14) Fandary, M. S.; Aljimaz, A. S.; Al-Kandary, J. A.; Fahim, M. A. Liquid-Liquid Equilibria for the System Water + Ethanol + Ethyl tert-Butyl Ether. J. Chem. Eng. Data 1999, 44, 1129. (15) Quitain, A. T.; Goto, S. Liquid-Liquid Equilibria of Ternary ETBE-EtOH-H2O and Quaternary ETBE-EtOH-H2OTBA Mixtures. Can. J. Chem. Eng. 1998, 76, 828. (16) Fite´, C.; Iborra, M.; Tejero, J.; Izquierdo, J.; Cunill, F. Kinetics of the Liquid-Phase Synthesis of Ethyl tert-Butyl Ether (ETBE). Ind. Eng. Chem. Res. 1994, 33, 581. (17) Assabumrungrat, S.; Kiatkittipong, W.; Sevitoon, N.; Praserthdam, P.; Goto, S. Kinetics of Liquid-Phase Synthesis of Ethyl tert-Butyl Ether from tert-Butyl Alcohol and Ethanol Catalyzed by β-Zeolite Supported on Monolith. Int. J. Chem. Kinet. 2002, 34, 292. (18) Zhang, T.; Jensen, K.; Kitchaiya, P.; Phillips, C.; Datta, R.; Liquid-Phase Synthesis of Ethanol-Derived Mixed Tertiary Alkyl Ethyl Ethers in an Isothermal Integral Packed-Bed Reactor. Ind. Eng. Chem. Res. 1997, 36, 4586. (19) Franc¸ oisse, O.; Thyrion, F. C.; Kinetics and Mechanism of Ethyl tert-Butyl Ether Liquid-phase Synthesis. Chem. Eng. Process. 1991, 30, 141. (20) Meng, N.; An, W.; Chuang, K. T.; Sanger, A. R. Removal of Water from a Near-azeotropic Water-Ethanol Mixture by Catalytic Reaction with iso-Butene. Can. J. Chem. Eng. 2003, submitted for publication. (21) Jayadeokar, S. S.; Sharma, M. M. Absorption of Isobutylene in Aqueous Ethanol and Mixed Alcohols: Cation Exchange Resins as Catalysts. Chem. Eng. Sci. 1992, 47, 3777.
Received for review September 11, 2003 Revised manuscript received November 12, 2003 Accepted November 19, 2003 IE034123E
