Ind. Eng. Chem. Res. 2000, 39, 2601-2611
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Reaction Kinetics and Chemical Equilibrium of Homogeneously and Heterogeneously Catalyzed Acetic Acid Esterification with Methanol and Methyl Acetate Hydrolysis T. Po1 pken, L. Go1 tze,† and J. Gmehling* Carl von OssietzkysUniversita¨ t Oldenburg, Technische Chemie, P.O. Box 2503, D-26111 Oldenburg, Germany
The reaction kinetics and chemical equilibrium of the reversible esterification of methanol with acetic acid were investigated. This system is of major importance as a model reaction for reactive distillation. The reaction has been catalyzed both homogeneously by acetic acid itself and heterogeneously by an acidic ion-exchange resin (Amberlyst 15). The chemical equilibrium composition was measured for various temperatures and starting compositions of the reactants and products. Kinetic information was obtained at temperatures between 303.15 and 343.15 K at various starting compositions covering concentration ranges from the stoichiometric regime to the dilute regions. Both the esterification and the hydrolysis reaction were investigated to yield a model which is applicable for any starting composition. The homogeneous reaction has been described with a simple power-law model. The use of activities in the kinetic model instead of mole fractions results in a much smaller residual error. To compare pseudohomogeneous and adsorption-based kinetic models for the heterogeneously catalyzed reaction, independent binary liquid adsorption experiments were used to fit the adsorption constants to keep the number of adjustable parameters the same for each model. The use of activities instead of mole fractions results in a slight improvement of the kinetic model only, while incorporating adsorption information into the kinetic model results in a much better fit. The chemical equilibrium composition calculated from the kinetic model is in agreement with the measured chemical equilibrium. Introduction Methyl acetate synthesis by esterification of acetic acid with methanol, and the backward reaction, the hydrolysis of methyl acetate, have recently received growing interest as a model reaction for reactive distillation.1 Reactive distillation offers distinct advantages over the conventional approach of performing the reaction and separation sequentially. Examples are reduced capital and operating costs due to higher conversion and reduction of the extent of parallel and consecutive reactions resulting in a higher selectivity. Its major drawback is that the chemical reaction has to have significant conversion at distillation temperatures, which limits the range of applicability somewhat. Reactive distillation has been identified as being suitable for the methyl acetate system for different processes, namely, the synthesis of methyl acetate,2,3 the hydrolysis of methyl acetate,4-6 and the recovery of dilute acetic acid from wastewater.7,8 The methyl acetate process not only has been the first process employing the concept of reactive distillation9 when esters gained commercial importance in the late 1920s10 but also was the first process being adapted to a large industrial scale.2 Today, the hydrolysis of methyl acetate to acetic acid and methanol is of major importance because methyl acetate is a byproduct during the synthesis of poly(vinyl alcohol) * To whom correspondence should be addressed. Phone: +49-441-7983831.Fax: +49-441-7983330.E-mail: gmehling@tech. chem.uni-oldenburg.de. † Present address: Sulzer Chemtech Ltd., Separation and Reaction Technology, P.O. Box 65, CH-8404 Winterthur, Switzerland.
(PVA), and acetic acid and methanol can be recycled on site into the process. Although the reaction is known to proceed rather slowly, most reactive distillation modeling and simulations considering this reaction have been performed assuming chemical equilibrium. It has been shown by Bessling et al.1 that the error introduced by assuming infinitely fast reaction kinetics is relatively small at atmospheric pressure and high catalyst mass. By operating reactive distillation columns at reduced pressure and thus reduced temperature, one is able to study the effect of reaction kinetics on the performance of a reactive distillation column and gain more insight into the process. This knowledge is of great importance for the design of reactive distillation columns for esterification reactions, especially for higher esters, where reaction kinetics is usually slower than that for the methyl acetate system. Another advantage of the methyl acetate system is that the miscibility gap in the methyl acetate-water system disappears upon the addition of small amounts of methanol and/or acetic acid, which results in the absence of a second liquid phase in the reactive distillation column under normal operating conditions. This simplifies column modeling and allows the investigation of the backward reaction kinetics under homogeneous conditions. Because in modern reactive distillation processes a structured or random packing is employed, it is desirable to use a rate-based model. To apply a rate-based model for the simulation and design of a reactive distillation process, a reliable knowledge of the reaction
10.1021/ie000063q CCC: $19.00 © 2000 American Chemical Society Published on Web 06/17/2000
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Ind. Eng. Chem. Res., Vol. 39, No. 7, 2000
kinetics and the chemical equilibrium is required besides the mass-transfer characteristics, vapor-liquid equilibrium, and the various pure component properties. When the equilibrium-stage model is used, knowledge of reaction kinetics is desirable to relax the often-made simplifying assumption of chemical equilibrium. The kinetics of this esterification reaction has been investigated to some extent in the past. The reaction has been studied in terms of heterogeneous catalysis by various ion-exchange resins7,8,11-13 or homogeneous catalysis by strong acids, like hydrogen chloride14 or hydrogen iodide.15 To the best of our knowledge, all investigations published either covered only a limited temperature range,13 covered a limited concentration range,8,14 or suffered from side reactions with the catalyst.15 In some of the published models, the backward reaction has not been considered, which is a valid assumption if one operates with a large excess of one reactant7,8,14 and at low conversion, but not when one operates at nearly stoichiometric concentrations, as in the case of reactive distillation processes for the synthesis or the hydrolysis of methyl acetate. Our group already reported kinetic data for this system in the past,3,16,17 which were fitted to a considerably smaller database than that in the present work. In this work, we extend the range of the data from the stoichiometric regime into the more diluted regions, as they are encountered in reactive distillation processes, where the reactants are fed in a countercurrent fashion into the column. Also, a comparison of different kinetic models and an independent investigation of chemical equilibrium is presented. This work should therefore ensure a reliable modeling of reaction kinetics in the methyl acetate system catalyzed by Amberlyst 15 or similar ionexchange resins. Experimental Section Chemicals. Acetic acid and methanol used were of analytical grade (99.8%, Scharlau), the water was bidistilled, and the methyl acetate was reaction grade (99.7% purity, as determined by gas chromatography, Riedel-de-Hae¨n). For the titrimetric analysis, Titrisol (1.0 and 0.1 N sodium hydroxide, E. Merck) was used. The chemicals were used without further purification except for drying over a molecular sieve. Catalyst. The macroreticular ion-exchange resin Amberlyst 15 (wet) (Rohm and Haas Co.) was selected as the catalyst. The catalyst was washed once with methanol and then several times with water until the supernatant liquid was colorless to remove impurities. In some experiments, the catalyst was used wet, while in others it was used after being dried. In all cases, the water content of the catalyst was determined after the kinetic experiment by drying of the catalyst at 90 °C under vacuum until the mass remained constant (usually 2 days). Drying at higher temperatures involves the risk of loosing sulfonic acid sites in the form of SO3 because of desulfonization of the polystyrene matrix of the catalyst. The ion-exchange capacity of the resin was determined by total exchange with sodium chloride solution and subsequent titration. A value of 4.77 ( 0.01 mequiv of H+‚g-1 of dry catalyst was obtained, which compares well with the value of 4.75 mequiv‚g-1 given by the manufacturer. A sieve analysis of the commercial resin using mesh widths of 1.20, 1.12, 0.80, 0.63, 0.40, and 0.20 mm gave the distribution of pellet diameters (Table
Table 1. Size Distribution of Commercial Amberlyst 15 diameter range (mm) >1.12 1.00...1.12 0.80...1.00 0.63...0.80 0.40...0.63 0.20...0.40
