Recovery of Acetic Acid by Reactive Distillation: Parametric Study and

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Recovery of Acetic Acid by Reactive Distillation: Parametric Study and Nonlinear Dynamic Effects Ajay Singh, Anand Tiwari, Vineet Bansal, Ravindra D. Gudi, and Sanjay M. Mahajani* Department of Chemical Engineering, IIT-Bombay, Powai, Mumbai 400 076, India

The paper presents a parametric study for the design of a reactive distillation column used to recover acetic acid from dilute aqueous solution (30% w/w) through the formation of methyl acetate. The parameters such as feed molar ratio, feed location, reflux ratio, and reboil ratio are varied by a one-parameter continuation method, and the best possible configuration is suggested. Close to quantitative recovery may be obtained by a proper choice of parameters, and reactive distillation can be successfully used for the recovery process. An experimental support is provided to the recommended configuration. The system is highly nonlinear, and solution multiplicity is realized in a certain parametric space. Apart from input parameters, a possible role of modeling assumptions, kinetics, and phase equilibrium models on multiplicity has been discussed. Introduction Recovery of acetic acid from its dilute aqueous solutions is a challenging task as normal distillation is expensive if the concentration is less than 30% w/w. One needs to remove water, which has high latent heat, as an overhead product. Moreover, since the relative volatility is very small, the requirement of high reflux ratio further increases the energy consumption. Among the known alternate methods, reactive distillation (RD) appears to be a promising alternative and a number of studies have appeared in the recent past.1-6 In reactive distillation, acetic acid is made to react with alcohols such as methanol and the ester thus formed is separated during the course of the reaction by distillation. In our previous work, we studied this process in detail through experiments and simulation and also proposed various process alternatives.6 The most attractive of all these alternatives is the use of a single reactive distillation column that offers methyl acetate-water azeotrope (92% methyl acetate) as an overhead product, which may directly be subjected to hydrolysis to obtain pure acetic acid. The azeotrope can be conveniently broken by nondistillative techniques such as pervaporation,24 if production of pure methyl acetate is the objective. The configuration is shown in Figure 1. As reported by Tang et al.,4 it is not possible to obtain pure methyl acetate as distillate due to the large amount of water present in the feed. Among the other studies, although nearly quantitative conversion of acetic acid has been reported,3 the methanol to acid ratio used is much higher and there is not much information on the further processing of the top stream, which may suffer due to formation of an azeotrope between methyl acetate and methanol. It is known that advanced distillation like extractive distillation, in such a case, may not be a viable alternative from an economic point of view. The proposed alternative can circumvent this problem and offer close to quantitative recovery giving methyl acetate-water azeotrope instead of methanol-methyl acetate azeotrope as an overhead product, keeping the molar ratio of acetic acid to methanol close to 1. The dilute acetic acid feed may be introduced in the upper nonreactive rectifying section so as to provide the solVent effect for the in situ separation of methanol from the minimum boiling methyl acetate-methanol azeotrope which is otherwise heading * To whom correspondence should be addressed. Tel.: +91-2225767246. Fax: +91-22-25726895. E-mail: [email protected].

Figure 1. Proposed configuration for recovery of acetic acid by reactive distillation.6

toward the top. The internal recycle of methanol to the reactive zone is thus achieved, resulting in a close to stoichiometric requirement of methanol. This strategy, used in the commercial process for the synthesis of methyl acetate,7 if applied judiciously, is also expected to work well in the case of recovery wherein the feed has a large amount of water present. A systematic parametric study of such a configuration is therefore necessary to suggest an optimal process design, and an attempt has been made in the present work in this direction. For the best configuration the predictions are supported through experiments on a laboratory-scale reactive distillation column. The system is highly nonlinear, and one may realize steadystate multiplicity in the simulation under certain conditions. As pointed out by Kienle and Marquadt,8 multiplicity has a special place in designing a control strategy; hence, we throw some light on this aspect. Nonlinear dynamics giving rise to multiple steady states (MSS) and oscillations for the reactive distillation system has been studied by many authors in the past. MSS were reported9,10 for the synthesis of methyl tert-butyl ether (MTBE), which later became a model system for the studies in nonlinear dynamics of reactive distillation (e.g., refs 11 and 12). Nonlinear dynamic study of another fuel ether, tert-amyl methyl ether (TAME), was also studied on similar lines and the occurrence of MSS has been reported through experiments and theoretical

10.1021/ie071070i CCC: $37.00 © 2007 American Chemical Society Published on Web 11/23/2007

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Figure 2. Effect of height of reactive zone on conversion of acetic acid. Reflux ratio, 7.

analysis.13-16 Oscillations have also been reported for the synthesis of fuel ethers like MTBE and TAME by reactive distillation.17-19 Apart from the fuel ethers, only a few other examples showing such nonlinear dynamic effects in RD columns have been reported.20-22 While the nonlinear dynamics of reactive distillation systems for the synthesis of fuel ethers and esters has been studied in the past, no such work has been reported for the recovery of acetic acid through esterification with methanol in RD. As mentioned earlier, reactive distillation is also successfully used for synthesis of methyl acetate with pure acetic acid feed. However, the nonlinear dynamic behavior of recovery process in RD can be different from that used for the synthesis of methyl acetate, in spite of the fact that we conduct the same reaction in a reactive distillation column. The recovery process in RD is associated with a large amount of water in the column. The present work aims at studying this RD-based process through the effect of different design and operating parameters. Base Configuration for Parametric Study The column configuration that is used here for the parametric study is shown in Figure 1. As mentioned before, this is the most promising alternative proposed in our earlier work.6 The column has three sections (i.e., reactive, rectifying, and stripping), and the height of each section is equal to 1 m. The nonreactive rectifying and stripping sections have eight stages per meter and the reactive zone has three stages per meter. The total number of stages is 21. The feed to the column is dilute aqueous solution of acetic acid (30% w/w). The configuration suggested has many parameters, and a systematic parametric study is necessary to determine the conditions for the best possible performance. An experimentally validated equilibrium-stage model for reactive distillation is used in a DIVA simulation environment to predict the performance and to perform a parametric study by one parameter continuation. The simulation results were also verified with a commercial simulator, ASPEN PLUS, for a few representative cases, and the results were found to be in agreement. The column model uses both kinetics and vapor-

Figure 3. Effect of molar ratio of methanol/acid on the conversion profile of acetic acid. Molar ratio: - ‚‚‚ -, 1.0; ‚‚‚, 1.5; - -, 2.0; s, 2.5. O, Unstable points.

liquid equlibrium (VLE) described in our earlier work,6 unless otherwise mentioned. In the base case, the dilute acid feed (30% w/w) was introduced at the top of the reactive zone and pure methanol feed was introduced at the bottom of the reactive zone. A higher molar ratio of methanol to acid (2.5) was used to ensure the complete conversion of acetic acid. Figure 2 shows the effect of reboiler duty at various heights of the reactive zone for a fixed reflux ratio of 7. It can be seen that the conversion increases with an increase in the height of the reactive zone. As high as 95% conversion is achievable for a reactive zone height of 2 m. Increasing the reactive zone height has a 2-fold effect: conversion enhances due to the increase in residence time and, second, the products separate efficiently in the column with increased height. Although slightly better conversion can be obtained by further increasing the height of the reactive zone, a 24-stage column with 2 m height of reactive zone (24-stage column: eight stages in rectifying zone, six stages in reactive

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Figure 4. Concentration profiles of all the components until and after the maximum conversion shown in Figure 3. The upper row indicates the behavior until the maximum, and the lower row indicates the behavior after the maximum. The arrow direction indicates increase in reboiler duty. Reactive stages 9-15.

zone, and eight stages in the stripping zone, one condenser, and one reboiler) is used as a base case for further parametric study. A plot of conversion vs reboiler duty was chosen to study the effect of different parameters. Effect of Molar Ratio of Methanol to Acid. The molar ratio of methanol to acid was varied from 1 to 2.5 by keeping the feed location at the top of the reactive zone. The corresponding variation in the conversion is shown in Figure 3. As the molar ratio of methanol to acid increases, the methanol concentration in the reactive zone increases to enhance the forward reaction. Therefore, a large molar ratio of methanol to acid was used by many researchers in the past.2,3 However, for a given molar ratio, beyond a certain reboiler duty, there is a sudden drop in the conversion. This is a peculiar behavior and can be explained with the help of Figures 4 and 5, which show the shift in the steady-state column profiles of concentration and temperature with an increase in reboiler duty under otherwise similar conditions. At low reboiler duties the conversion increases with reboiler duty, mainly due to an increase in methanol concentration and a reduction in methyl acetate concentration in the

reactive zone (see the upper row of plots in Figure 4). However, with further increase in reboiler duty beyond a limit, the region of high methanol concentration moves in the upward direction toward the condenser and eventually moves out of the reactive zone (see the lower row of plots in Figure 4), which results in a reduced reaction rate in spite of the rise in temperature of the reactive zone that is evident from Figure 5b. The system was also found to show multiple steady states in a narrow range of reboiler duty, especially at a lower methanol to acid feed ratio. It can be seen that, by increasing the molar ratio of methanol to acid, the multiplicity region disappears and conversion can be enhanced up to 96%. Methanol forms a minimum boiling azeotrope with methyl acetate which is a possible overhead composition, and a nearly pure water or a water-methanol mixture can be obtained in the bottom stream of the column. Although a high methanol to acid ratio has been recommended in previous studies, no guidelines are given on how to process (1) the methanol-methyl acetate azeotropic mixture that is obtained as the overhead product and (2) the bottom stream that contains the excess unreacted methanol.


Figure 5. Temperature profiles of all the components until (a) and after (b) the maximum conversion shown in Figure 3. The arrow direction indicates increase in reboiler duty. Reactive stages 9-15.

There is an extra cost associated with this processing. Extractive distillation technique can be used to break the azeotrope, but it may not be economical. Using a large molar ratio of methanol to acid may also be uneconomical due to large recycle and energy costs. These are the issues that motivate us to further modify the design by changing the parameters, especially the relative positions of the feeds, and to explore the possibility of operating the column with a lower feed molar ratio and still get the desired recovery. Effect of Feed Stage Location. It is known that a commercial reactive distillation column used for the purpose of synthesis receives the pure acid feed in the nonreactive rectifying section, thus providing a solvent effect for the separation of methanol and methyl acetate azeotrope.7 Hence, in the recovery column we examine the effect of shifting the aqueous acid feed location in the upward direction. In the base case, this feed is introduced at the top of the reactive zone. Now we move it in the nonreactive rectifying section. The effect of acid feed location

on the conversion is shown in Figure 6. The location was varied from stage 15 (top of the reactive zone) to stage 23 (top of the column). The molar ratio of methanol to acid was kept constant at 1.0. The continuation analysis was performed with respect to reboiler duty at different acid feed locations. It can be seen that under the operating conditions of interest (reflux ratio 7), as the acid feed location is shifted from the top of the reactive zone to the top of the reactive distillation column (moving toward the overhead condenser), the maximum achievable conversion increases. Methanol-methyl acetate azeotrope is a minimum boiling azeotrope that has a tendency to move upward. Acetic acid introduced, in the form of aqueous solution, at the top of the column acts as a solvent that selectively separates methanol from the methanol-methyl acetate mixture. It has a dual advantage. First, it helps separate methyl acetate from methanol, and second, it results in the effective internal recycle of methanol to the reactive zone thereby maintaining a sufficiently high concentration of methanol in the reactive zone. The presence of a large amount of methanol and a lesser amount of methyl acetate in the reactive zone enhances the forward reaction, resulting in an increase in conversion. This effect has been explained systematically with the help of residue curve maps by Doherty and Malone23 for the synthesis of methyl acetate with pure acetic acid as feed. Our study shows that the best acid feed location for acetic acid recovery is near the top of the column rather than the top of the reactive zone. Acid feed contains a large amount of water. Introducing acid feed near the top of the column results in realization of methyl acetatewater as the top product. Methyl acetate-water azeotrope as a distillate is much more convenient from a process point of view compared to methanol-methyl acetate azeotrope because it contains less water and a larger amount of methyl acetate (composition 0.92/0.08) that can be easily separated by using nondistillative techniques such as pervaporation,24 or else the mixture can be subjected to hydrolysis if pure acetic acid is the desired product. Thus it can be seen that the problem encountered due to a higher feed molar ratio can be circumvented by

Figure 6. Conversion of acetic acid with respect to reboiler duty. Acid feed location on - ‚‚‚ -, 15; ‚‚‚, 17; - -, 19; s, 21; - ‚ -, 23. O, Unstable points. Reflux ratio, 7.

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Table 1. Operating Design Parameters Used for Results Presented in Figure 7 acetic acid (30%) feed rate methanol feed rate reboiler duty catalyst loading per stage number of stages (nonreactive) number of reactive stages dilute acid feed location methanol feed location

180 g/h 60 g/h 850 W (including heat losses of 80%) 42 g 14 (1 m rectifying section + 0.5 m stripping section) 6 (2 m) 17th stage from bottom fifth stage from bottom

introducing the feed near the top of the column. Another advantage is that the system does not show any output multiplicity when acid is fed at the top of the column (see Figure 6; acid feed location, stage 23) and such a situation is desirable from an operational point of view. Therefore, the best feed location for the aqueous solution of acid is the top of the column, i.e., a stage below the condenser. Experimental Validation The model used here has been validated using the data generated for the continuous reactive distillation experiments in our earlier work.6 The parametric study presented here reveals that the proper location of acid feed helps improve the performance significantly to give close to quantitative conversion and recovery. We now present an experimental proof by conducting a run in the same column, with suitable modification, based on the results of the parametric studies. The acid feed location is moved to the top of the column, and the height of the reactive zone was increased from 1 to 2 m. The operating and design parameters used for the experiments are given in Table 1. The detailed description with the schematic of the experimental setup and the analytical method may be found in ref 6. Figure 7 shows the comparison of the model prediction and newly generated experimental data. The results are in agreement. The recovery is close to 100%, and the acetic acid concentration in both top and bottom streams was below the detectable limits of our analytical method. To the best of our knowledge, this is the first time the quantitative recovery is being reported

experimentally, at a relatively small feed ratio of methanol to acid. It should be noted that due to height constraint we could not perform the experiment under the conditions such that methyl acetate-methanol azeotrope is obtained at the top with total internal recycle of methanol as described in the earlier section. Hence, methanol is used in slight excess (1.8 times) to obtain the desired recovery. However, this molar ratio is much less compared to that used in the reported experiments in the literature (e.g., 3) including our own work.6 A sufficiently tall column, we believe, would offer a quantitative recovery with even a close to stoichiometric feed molar ratio as predicted by the experimentally validated model. Multiple Steady States. It was seen in the parametric studies that, under certain conditions such as close to stoichiometric feed molar ratio, the reactive distillation column tends to show output multiplicity. Though it is difficult to reproduce such an effect experimentally in a laboratory column, it is an important consideration and is a highly relevant aspect of the column design, operation, and control. It is therefore desirable to study this nonlinear dynamic effect in detail and identify the parameter space wherein the effects are predominant. The system under consideration is highly complex with more than 75 state variables interacting with each other in a nonlinear fashion. It is known that the nonlinearity in reactive distillation may originate from a vapor-liquid equilibrium relationship, reaction kinetics, and the functional relationship of the physical properties with temperature and compositions.8 A thorough mathematical treatment based on the stability analysis is difficult in such a case and is beyond the scope of this work. Nevertheless, a relatively qualitative approach of examining the effect of modeling assumptions and input parameters has been presented to get a better insight into the behavior of the column. Effect of Reaction Kinetics. The reaction kinetics plays an important role in the column performance and behavior. Here, three different reaction kinetic models, including our own,6,24,25 are used to study the nonlinear behavior of the column with respect to the reaction kinetics. The rate equations are summarized in Table 2. The molar ratio of methanol to acid was close to 1. Analysis shows that the reaction rates have a significant impact on the presence of multiplicities. Continuation

Figure 7. Comparison of experimental and predicted profiles for continuous reactive distillation performed under the conditions mentioned in Table 1.

Ind. Eng. Chem. Res., Vol. 46, No. 26, 2007 9201 Table 2. Different Kinetic Models Used for the Simulation in the Present Work no.

kinetic model

1

LHHW

2

pseudohomogeneous

3

pseudohomogeneous (concentration-based model)

model equation

(

ref

r ) mcat kfa′acoha′meoh - kba′meoaca′water / (a′acoh + a′meoh + a′meoac + a′water) where a′ ) Kiai / Mi r ) kf(aacohameoh - ameoacawater / Keq) where kf ) 2.7 × 105 exp( - 6287.7 / T) and Keq ) 2.32 exp(782.98 / T) r ) V(kfxacohxmeoh - kbxmeoacxwater) where kf ) kf,0 exp( - Ef / RT) and kb ) kb,0 exp( - Ef / RT)

)

2

24 25

6

analysis and stability analysis show that different kinetic models show different multiplicity behaviors. The kinetic models proposed by Po¨pken et al.24 and Song et al.25 show both input and output multiplicities and our model shows only input multiplicity, as is evident in Figure 8. The models by Po¨pken et al.24 and Song et al.25 predict relatively large rates compared to our model under the conditions of interest. We increase the reaction rate of Po¨pken et al.24 model by a factor of 10, or in other words, the simulations were performed at 10 times the value of the Damkohler number (Da), which is defined as the ratio of the forward rate constant to the total feed flow rate. The results were insensitive to the increase in Da, which indicates that the column operates near the reaction equilibrium, and the kinetic model proposed by Po¨pken et al.24 predicts a very high reaction rate. These results are similar to those obtained by Chen et al.,18 who studied the effect of the Damkohler number on the solution multiplicity of MTBE synthesis. It was reported that the Damkohler number plays an important role in the multiplicity behavior. The multiplicity observed at a very large value of the Damkohler number (close to chemical equilibrium condition) disappears at a low value of the Damkohler number (kinetically controlled regime). When we use our own kinetics, the reaction rate is low and the column operates in a kinetically controlled regime. In this case Da varies from 3 to 5 from top to bottom. The reaction rate obtained by our kinetics6 was increased by a factor of 10 (Da ) 30-50). Figure 9 shows that the actual kinetics does not show the multiplicity. However, multiplicity appears when the reaction rate is increased, i.e., at higher values of Da. From the foregoing discussion it is clear that the column can show output multiplicity under an equilibrium controlled regime that can disappear under a kinetically controlled regime. The

mathematical forms of the kinetics equations (see Table 2) used are different, and the observation that multiplicity exists in all cases, at a large Da number, indicates that the multiplicity is certainly not because of the nonlinearity in the kinetics. It is the extent of reaction and hence the order of magnitude of rate that influence the presence of multiplicity under the conditions of interest. Hence, to study this effect further, we perform continuation analysis for the fast reaction kinetics. In practice, one may come across these conditions in the presence of a very active catalyst. In the following sections we use the fast kinetics by Popken et al.24 to further study the nonlinear dynamic behavior of the process. Similar results would be obtained by other kinetic models at a sufficiently large Damkohler number. Effect of Reflux and Distillate to Feed (D/F) Ratios. Po¨pken et al.24 reported the effect of reflux ratio on the conversion at a fixed value of D/F ratio for the synthesis of methyl acetate. We present a similar study for the recovery of acetic acid system. A 24-stage column is used for this purpose, and the column configuration is as given in Figure 1. The acid feed is introduced at the 19th stage (rectifying section), and the methanol feed is introduced at the ninth (bottom of reactive zone) stage. The molar ratio of methanol to acid is assumed to be close to 1. The continuation analysis was performed using DIVA simulation environment by using the reflux ratio as a continuation parameter. We did not observe any special types of nonlinearity for the synthesis of methyl acetate (i.e., with pure acetic acid as feed) under the operating region of interest. However, for the recovery of acetic acid, i.e., with dilute acetic acid as feed, a peculiar nonlinear behavior was observed that has not been reported before. The effect of reflux ratio on the conversion of acetic acid at different values of D/F ratio is shown in Figure 10. It can be

Figure 8. Effect of reaction kinetic models on conversion profile of acetic acid. Kinetic models: ‚‚‚, Singh et al.;6 - -, Song et al.;25 s, Po¨pken et al.24 O and ], Unstable points.

Figure 9. Conversion vs reboiler duty predicted by the kinetic model by Singh et al.6 at different values of Damkohler number. ‚‚‚, Da × 1, s, Da × 5; - -, Da × 10. O, Unstable points.

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Figure 10. Conversion of acetic acid with respect to reflux ratio at various distillate to feed (D/F) ratios. -O-, with energy balance; ‚‚‚]‚‚‚, without energy balance - ‚ 0ThinSpace‚ -, without energy balance and ideal vapor phase. O, ], and 0, Unstable branch.

seen that the column shows multiplicity in a large operating region of reflux ratio and D/F ratio. As the value of the D/F ratio increases, isolated solutions (isola) are realized. With a further increase in the D/F ratio, the region of isola shrinks and finally disappears at a larger value of the D/F ratio. A still further increase in the value of the D/F ratio results in a single solution. A similar behavior was observed as the value of the D/F ratio decreases from its nominal value (D/F ) 0.1085). It should be noted here that such a type of multiplicity has not been observed for the synthesis of methyl acetate, i.e., with pure acetic acid as feed. Chen et al.18 have reported the formation of isola for MTBE synthesis under chemical equilibrium conditions that disappear in the kinetic regime. In order to confirm the results obtained by the continuation analysis and stability analysis, dynamic simulation was performed at D/F ratio ) 0.111 (Figure 10), which shows an isolated solution at a value of the reflux ratio of 4. A perturbation was introduced in the reflux ratio from 4 to 20 and again from 20 to 4, and a corresponding variation in outgoing acetic acid is shown in Figure 11. This shows that the system attains a different steady state under the same operating condition after

Figure 11. Transient response of conversion predicted by dynamic simulation of acid recovery system at the given values of operating parameters: D/F ) 0.111. Step changes in the reflux ratio 4-10-4.


Figure 12. Two stable steady states profiles for methanol concentration and temperature. D/F ratio, 0.111; reflux ratio, 4.

Figure 13. Conversion of acetic acid with respect to reflux ratio. Distillate to feed (D/F) ratio, 0.1085. s, Wilson model; ‚‚‚, UNIQUAC model; O and ], unstable branch.

Figure 14. Multiple steady states in conversion of a nine-stage reactive distillation column (number of reactive stages, 1). s, Stable branch; O, unstable branch.

the said perturbation. The column profiles of methanol concentration and temperature corresponding to the two different stable steady states are shown in Figure 12. The upper temperature profile corresponds to lower steady-state conversion, and the lower temperature profile corresponds to higher steadystate conversion. At higher temperature, the methanol concentration in the reactive zone is very low, and hence conversion is less. On the other hand, the higher conversion steady state corresponds to higher methanol concentration in the reactive zone. Effect of Nonlinearity in the Energy Balance. Here, we remove the energy balance from the model and repeat the simulations. The corresponding bifurcation analysis with respect to the reflux ratio is shown by dotted lines in all the plots in Figure 10. It can be seen here that the column shows multiplicityas well as isola formation without energy balance as well. This means that the nonlinearities in the heat effects do not play a role in the observed multiplicity.

Effect of Nonlinearity in the VLE Model. It has been reported in the literature that, in some cases, the multiplicity behavior depends on the VLE model itself. The reason for this effect may be the inaccuracies in predictions by the VLE models like UNIQUAC in the region of a possible liquid-phase split. The Wilson model, which does not consider the liquid-liquid split, may be used in such a case to verify this effect. Hence, the simulations were performed using the Wilson model with parameters reported in ref 23, and the results are shown in Figure 13. Although the MSS/isola region shifts, the Wilson model also predicts similar nonlinear behavior (isola and MSS prediction) that has been predicted by the UNIQUAC model. Hence, the multiplicity observed is not due to the use of the UNIQUAC model and possible phase splitting. In all the simulations reported earlier, the liquid-phase nonideality was represented by the UNIQUAC VLE model and the vapor phase was assumed as ideal except for the dimerization of acetic acid. The dimerization constant and other parameters of incorporating the dimerization in the vapor phase were taken

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from ref 26. In the next step, the vapor-phase dimerization was removed from the VLE model, and the corresponding profiles are shown as the - ‚ - lines in Figure 10. The system still shows the MSS/isola. Number of Stages. In a further attempt to identify the cause of MSS, we tried to simplify the configuration by reducing the number of stages and hence the number of state variables. It was observed that, as the number of stages decreases, the multiplicity region shrinks. Figure 14 shows the multiplicity in a nine-stage column (including condenser and reboiler) with only a single reactive stage. This means that only a sufficiently tall column can exhibit the solution multiplicity under the conditions of interest. From the foregoing discussion on the effect of the modeling assumptions and the design parameters on the multiplicity behavior, we may conclude that there is no single cause which is responsible for the observed multiplicity behavior. The nonlinearity associated with the VLE of the system combined with the reaction leads to occurrence of MSS. Conclusions Acetic acid from its dilute aqueous solution can be effectively recovered in the form of methyl acetate by reactive distillation. The suitable location of acid feed is the top of the nonreactive rectifying section providing an extractive distillation effect to break methanol-methyl acetate azeotrope, which allows the internal recycle of methanol to the reactive zone, thereby increasing the conversion level. Experiments in a laboratory column with the suggested configuration were performed, and the desired recovery level was obtained. The recovery process that takes place in the presence of a large amount of water exhibits steady-state multiplicity which does not exist for a synthesis process under otherwise similar conditions. The large reaction rates, near-stoichiometric feed molar ratio, and sufficiently large number of stages favor the multiplicity behavior. The kinetic model, nonlinearity in energy balance, and that in vapor-phase dimerization of acetic acid do not influence the appearance of multiple steady states. This observation leads us to conclude that the main cause is the nonlinearity in VLE, which in combination with reaction results in such behavior. Literature Cited (1) Neumann, R.; Sasoon, Y. Recovery of dilute acetic acid by esterification in a packed chemorectification column. Ind. Eng. Chem. Process Des. DeV. 1984, 23 (4), 654-659. (2) Xu, Z.; Afacan, A.; Chuang, K. T. Removal of Acetic Acid from Water by Catalytic Distillation. Part 1: Experimental Studies. Can. J. Chem. Eng. 1990, 77, 676-681. (3) Scates, M. O.; Parker, S. E.; Lacy, J. B.; Gibbs, R. K. Recovery of acetic acid from dilute aqueous streams formed during a carbonylation process. U.S. Patent 5,599,976, 1997. (4) Tang, Y. T.; Hung, S. B.; Chen, Y. W.; Huang, H. P.; Lee, M. J.; Yu, C. C. Design of Reactive Distillations for Acetic Acid Esterification with Different Alcohols. AIChE J. 2005, 51, 1683-1699. (5) Hung, W.; Lai, I.; Chen, Y. W.; Hung, S. B.; Huang, H. B.; Lee, M. J.; Yu, C. C. Process Chemistry and Design Alternatives for Converting Dilute Acetic Acid to Esters in Reactive Distillation. Ind. Eng. Chem. Res. 2005, 45, 1722. (6) Singh, A., A.; Tiwari, A. A.; Mahajani, S. M.; Gudi, R. D. Recovery of Acetic acid from Aqueous Solutions by Reactive Distillation. Ind. Eng. Chem. Res. 2006, 45 (6), 2017-2025.

(7) Agreda, V. H.; Partiu, L. R.; Heise, W. H. High Purity Methyl Acetate via Reactive Distillation. Chem. Eng. Prog. 1990, 40, 40-46. (8) Kienle, A.; Marquadt, W. Non-linear Dynamics of Reactive Distillation In ReactiVe Distillation: Status and Future Directions; Sundmacher, K., Kienle, A. Eds.; Wiley-VCH: Weinheim, Germany, 2003. (9) Jacobs, R.; Krishna, R. Multiple Solutions in Reactive Distillation for Methyl tert-Butyl Ether Synthesis. Ind. Eng. Chem. Res. 1993, 32, 1706. (10) Nijhuis, S. A.; Kerkhof, F. P. J. M.; Mak, A. N. S. Multiple steady states during reactive distillation of methyl tert-butyl ether. Ind. Eng. Chem. Res. 1993, 32 (11), 2767-2774. (11) Guttinger, T. E.; Morari, M. Predicting Multiple Steady States in Distillation: Singularity Analysis and Reactive Systems. Comput. Chem. Eng. 1997, 21, S995-S1000. (12) Sneesby, M. G.; Tade, M. O.; Smith, T. N. Steady-state Transitions in the Reactive Distillation of MTBE. Comput. Chem. Eng. 1998, 22, 879892. (13) Mohl, K. D.; Kienle, A.; Gilles, E. D. Multiple Steady States in a Reactive Distillation Column for the Production of TAME I. Theoretical Analysis. Chem. Eng. Technol. 1998, 21 (2), 133-136. (14) Mohl, K. D.; Kienle, A.; Gilles, E. D.; Rapmund, P.; Sundmacher, K.; Hoffmann, U. Nonlinear Dynamics of Reactive Distillation Processes for the Production of Fuel Ethers. Comput. Chem. Eng. 1997, 21, 989994. (15) Mohl, K. D.; Kienle, A.; Gilles, E. D.; Rapmund, P.; Sundmacher, K.; Hoffmann, U. Steady-state Multiplicities in Reactive Distillation Columns for the Production of Fuel Ethers MTBE and TAME: Theoretical Analysis and Experimental Verification. Chem. Eng. Sci. 1999, 54 (8), 1029-1043. (16) Baur, R. R.; Higler, A. P.; Taylor, R.; Krishna, R. Comparison of Equilibrium Stage and Nonequilibrium Stage Models for Reactive Distillation. Chem. Eng. J. 1999, 76, 3347. (17) Sundmacher, K.; Hoffmann, U. Multicomponent Mass and Energy Transport on Different Length Scales in a Packed Reactive Distillation Column for Heterogeneously Catalysed Fuel Ether Production. Chem. Eng. Sci. 1999, 49 (24A), 4443-4464. (18) Chen, F.; Huss, R. S.; Malone, M. F.; Doherty, M. F. Multiple Steady States in Reactive Distillation: Kinetic Effects. Comput. Chem. Eng. 2002, 26 (1), 81-93. (19) Katariya, A. M.; Moudgalya, K. M.; Mahajani, S. M. Nonlinear Dynamic Effects in Reactive Distillation for Synthesis of TAME. Ind. Eng. Chem. Res. 2006, 45 (12), 4233-4242. (20) Gehrke, V.; Marquardt, W. A. Singularity Theory Approach to the Study of Reactive Distillation. Comput. Chem. Eng. 2005, 21, S1001S1006. (21) Kumar, A.; Daoutidies, P. Modelling, Analysis and Control of Ethylene Glycol Reactive Distillation Column. AIChE J. 1999, 45 (1), 5168. (22) Huss, S. R.; Chen, M. F.; Doherty, M. F.; Malone, M. F. Reactive Distillation for Methyl Acetate Production. Comput. Chem. Eng. 2003, 27 (12), 1855-1866. (23) Doherty, M. F.; Malone, M. F. Conceptual Design of Distillation Systems; McGraw-Hill: New York, 2001. (24) Po¨pken, T.; Gotze, L.; Gmehling, J. Reaction Kinetics and Chemical Equilibrium of Homogeneously and Heterogeneously Catalyzed Acetic Acid Esterification with Methanol and Methyl Acetate Hydrolysis. Ind. Eng. Chem. Res. 2000, 39 (7), 2601-2611. (25) Song, W.; Venimadhavan, G.; Manning, J. M.; Malone, M. F.; Doherty, M. F. Measurement of Residue Curve Maps and Heterogeneous Kinetics in Methyl Acetate Synthesis. Ind. Eng. Chem. Res. 1998, 37 (5), 1917-1928. (26) Barbosa, D.; Doherty, M. F. The Simple Distillation of Homogeneous Reactive Mixtures. Chem. Eng. Sci. 1988, 43 (3), 541-550.

ReceiVed for reView August 6, 2007 ReVised manuscript receiVed October 2, 2007 Accepted October 5, 2007 IE071070I

Recovery of Acetic Acid by Reactive Distillation: Parametric Study and

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