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Production of Butyl Acetate by Catalytic Distillation. Theoretical and Experimental Studies Ajay Singh, R. Hiwale, S. M. Mahajani,* and R. D. Gudi Department of Chemical Engineering, Indian Institute of Technology, Bombay, Mumbai, India
J. Gangadwala and A. Kienle Max Planck Institute for Dynamics of Complex Technical Systems, Magdeburg, Germany
The esterification reaction of acetic acid with n-butanol in a continuous catalytic distillation system has been studied. The products of esterification reactionsviz, water and butyl acetates are separated by distillation during the course of the reaction, to overcome the equilibrium limitations. A 3-m-tall column with reactive and nonreactive zones, packed with a commercial catalytic packing (KATAPAK-S) and noncatalytic wire gauze packing, was used for this purpose. For a feed concentration corresponding to that obtained in a one-stage continuously stirred tank reactor at reaction equilibrium, conversion of ∼100% was realized, with selectivity on the order of 99%. Interestingly, in most of the experiments, the bottom stream contained butyl acetate that was almost free of acetic acid. The influence of various operating parameters, such as feed flow rate, feed composition, feed location, and boil-up rate, on the conversion, selectivity, and separation was studied. A dynamic equilibrium stage model was developed and solved to predict the transient and steady-state results. Reasonably good agreement between the experimental and simulation results was realized. 1. Introduction Reactive distillation (RD) is the combination of reaction and distillation in a single vessel; therefore, it enjoys many specific advantages over the conventional sequential approach of reaction followed by distillation or other separation techniques. Improved selectivity, increased conversion, better temperature control, effective utilization of reaction heat, scope for difficult separations, and the avoidance of azeotropes are a few of the advantages that are offered by RD. The introduction of an in situ separation process in the reaction zone, or vice versa, leads to complex interactions between vapor-liquid equilibrium, mass-transfer rates, and diffusion and chemical kinetics, which presents a great challenge for the design and synthesis of these systems. Because RD is a relatively new field, research on various aspects such as modeling and simulation, process synthesis, column hardware design, nonlinear dynamics, and control is underway. The suitability of RD for a particular reaction is dependent on various factors, such as the volatility of reactants and products along with the feasible reaction and distillation temperature.1 Hence, the use of RD for every reaction may not be feasible. Exploration of the candidate reactions for RD itself is an area that requires considerable attention to expand the domain of RD processes. Although invented in 1921, the industrial application of reactive distillation did not take place before the 1980s. Various reviews have appeared recently on the commercial applications of reactive distillation.2,3 The esterification of acetic acid with alcohols such as n-butanol, isobutyl alcohol, and amyl alcohol fall into a * To whom correspondence should be addressed. Tel.: +91-22-25767246. Fax: +91-22-25726895. E-mail: sanjaym@ che.iitb.ac.in.
typical class of reacting systems. The alcohols are sparingly soluble in water, and the ester is almost insoluble. Another interesting feature of this system is that it is associated with the formation a minimum boiling ternary azeotrope of etser, alcohol, and water, which is heterogeneous in nature. Hence, in a typical RD column that consists of both reactive and nonreactive zones, the heterogeneous azeotrope or a composition close to the azeotrope can be obtained as the distillate product. Moreover, the aqueous phase that forms after the condensation of the vapor is almost pure water. It can be conveniently withdrawn as a product, and the organic phase can be recycled back as reflux. The pure ester, which is the least-volatile component in the system, is realized as a bottom product.4,5 In the RD scheme, the option of using a conventional reactor for partial reaction, followed by an RD column, has been reported to offer better economics, e.g., in the MTBE process.6 The simulation studies on the butyl acetate synthesis have also indicated that this option is more promising than conducting the entire reaction in an RD column.7 The purpose of the present work is to perform a detailed experimental investigation on an RD column that is operated in such a mode. 2. Previous Studies Although the reaction of butyl acetate synthesis has been studied quite extensively in a batch mode to determine the reaction kinetics,8 very few papers on experimental studies in a continuous mode via RD have appeared in the literature. Hanika et al.,9 Janowsky et al.10 and Steinigeweg and Gmehling7 recently have published some important information in this regard. Hanika et al.9 used a column (inner diameter (ID) of 81 mm) that consisted of a catalytic zone packed with Katapak-S packing and two separation zones that were
10.1021/ie049659u CCC: $30.25 © 2005 American Chemical Society Published on Web 03/26/2005
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equivalent to 20 theoretical stages. Two different configurations of catalytic distillation have been considered. In the first set, only a catalytic distillation column was used; in the second set, a primary fixed-bed reactor that was packed with ion-exchange resin, followed by a catalytic distillation column, was studied. A mixture of acetic acid and butanol (in excess) was preheated and either fed into the catalytic zone of the column or into the pre-reactor. The output from the prereactor, containing an almost-equilibrium mixture of acetic acid, butanol, butyl acetate, and water, was preheated almost to its boiling point and fed into the catalytic zone of the RD column. Aspen Plus software and its database was used to simulate butyl acetate synthesis via RD. The experimental data were observed to agree well with the model. Janowsky et al.10 performed butyl acetate synthesis experiments to study steady-state column performance at three different pressures over a range of 0.65-1.105 bar. They used a packed column in their experiment, and each section of the column was equivalent to 15 theoretical stages. The stripping section, which was filled with catalyst, acts as a reactive section, and feed that comes from a pre-reactor, containing a slight excess of butyl acetate, was introduced at the top of the reactive section. At higher pressure, they observed a significant amount of 1-butene at the top of the column. Also, the unwanted byproduct di-butyl ether (DBE) was observed, with the main product (butyl acetate) in the bottoms. They were able to eliminate the formation of 1-butene by decreasing the column pressure up to 0.65 bar; however, they were unable to eliminate DBE from the bottoms. Excellent work on the synthesis of butyl acetate has been recently reported by Steinigeweg and Gmehling.7 They studied the thermodynamic properties, reaction kinetics, and RD system through experiments and simulation. Katapak-S (Sulzer ChemTech) filled with strongly acidic-ion-exchange resin (Amberlyst-15) was used as a catalyst. The activity coefficients for the liquid phase were calculated by the UNIQUAC equation. A pseudo-homogeneous activity-based model was used to describe the rate equation. The experiments were performed in a continuous RD column by introducing a fresh mixture of acetic acid and butanol as a feed. The effect of different parameters, such as reboiler duty, feed location, composition, molar ratio, and pressure, was studied. Experiments have shown that the most suitable feed location was the top of the catalyst bed and, by increasing number of reactive stages, conversion increases. A maximum conversion of 98% was realized. All the simulations were performed using a steady-state simulator (Aspen Plus). In addition to the UNIQUAC interaction parameters, data for the column heat loss and reaction kinetics were incorporated into the process simulator. The decanter model of Aspen Plus was used for the decanter simulation. Comparison of the experimental data with simulation results indicated that an equilibrium stage model is capable of describing the column profiles quantitatively. The same model and simulator were used further to predict the performance of the column with feed that contains butyl acetate and water. The authors suggested that a pre-reactor, followed by an RD column, is the best process alternative. The experiments with a four-component feed to the RD column were not performed. Of all three of these groups, only Janowsky et al.10 have reported the formation of
side products such as 1-butene and DBE in the RD process. Although the formation of ether is in very small proportion, it is an important consideration, in view of the desired product specifications. Recently, Gangadwala et al.11 have simulated the performance of the same system using a DIVA simulator by considering their own kinetics8 to explain the results of Hanika et al.9 and Janowsky et al.10 The formation of DBE has been incorporated in their kinetic model, which explains the results of Janowsky et al.10 However, it was realized that the experimental data on the system, especially for a four-component feed to the column, is limited and does not cover a wide range of operating parameters, such as reboiler duty, feed location, feed flow rate, and molar ratio. Moreover, the column composition profiles, which helps explain some important results, have not been reported in their studies. It is with this intention that the present work has been undertaken. All the experiments were performed, keeping in mind the best possible configuration, suggested by Steinigeweg and Gmehling,7 i.e., a prereactor followed by RD column. 3. Experimental Section 3.1. Material and Catalysts. n-Butanol and acetic acid, both analytical reagent (AR) grade as well as commercial grade, were obtained from s. d. Fine Chemicals, Ltd., India. The packings used in the RD column were Katapak-S packed with Amberlyst-15 (Sulzer, Switzerland) and noncatalytic wire mesh distillation packing (Evergreen India, Ltd). 3.2. Apparatus and Procedure. The experimental setup of a laboratory-scale RD plant is shown in Figure 1. A 3-m-tall distillation column (ID ) 54 mm) that operated at atmospheric pressure was used. The reboiler (3-L capacity) was externally heated with the help of a heating mantle. The nonreactive rectifying and stripping sections were packed with Evergreen Hyflux lowpressure-drop structured packing that was composed of fine metallic wires. The middle reactive zone was packed with Sulzer Katapak-S packing embedded with ionexchange resins and Amberlyst-15 as a catalyst. The height of all of the sections was 1 m. Proper insulation with an external wall heating arrangement was provided, to minimize the heat losses to the surroundings. The reaction mixture, which consisted of acetic acid, butanol, butyl acetate, and water or an equilibrium mixture from the batch reactor, was fed continuously into the column through a rotameter. An electronically driven metering pump was used to transfer the liquid from the feed tank to the column. In the condenser, two immiscible phases were formed: an aqueous phase (i.e., almost pure water) and an organic phase containing water, butanol, and butyl acetate. The feed was preheated before it was introduced into the column. A phase separator was used with the condenser, to provide reflux to the column and to continuously withdraw water that formed during the reaction. Temperature sensors (Pt 100) were provided at different locations in the column, to measure these temperatures (Positions 1-8). These temperatures were also transferred to the computer through a data acquisition system and are plotted on the same graph simultaneously during the course of the experiment. 3.3. Analysis. The samples were analyzed using a gas chromatograph (model C-911, Mak Analytica India, Ltd.) was equipped with a thermal conductivity detector
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Figure 1. Experimental setup of the continuous reactive distillation (RD) column at the Indian Institute of Technology in Bombay, India.
(TCD). The column used for the analysis was Porapack-Q with hydrogen as carrier gas at the flow rate of 20 mL/min. The injector and detector were maintained at temperatures of 220 and 150 °C, respectively. The oven temperature was maintained isothermally at 240 °C to get the best resolution in less time. The results obtained via GC were confirmed by independent titrations, using standard sodium hydroxide (NaOH) solution. The analytical uncertainty was
