Ind. Eng. Chem. Res. 2007, 46, 1099-1104
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Cyclohexanol Production via Esterification of Cyclohexene with Formic Acid and Subsequent Hydration of the EstersReaction Kinetics Frank Steyer† and Kai Sundmacher*,†,‡ Max Planck Institute for Dynamics of Complex Technical Systems, Sandtorstraβe 1, 39106 Magdeburg, Germany; and Process Systems Engineering, Otto-Von-Guericke-UniVersity Magdeburg, UniVersita¨tsplatz 1, 39106 Magdeburg, Germany
An alternative to current production routes to cyclohexanol, the esterification of cyclohexene with formic acid and subsequent splitting of the ester can overcome many of the drawbacks associated with conventional processes. The process that is being considered could be carried out in a reactive distillation column. To develop such a process, reliable data on liquid-liquid and vapor-liquid phase-splitting behavior and on reaction kinetics is of high importance. The current publication focuses on the reaction kinetic aspects of such a process. Reaction kinetic data for the three equilibrium reactions are being presented Introduction Cyclohexanol is a bulk chemical that is needed as an intermediate in nylon production. It is being produced in the million-ton-per-year scale. The production of cyclohexanol is currently based on the oxidation of cyclohexane, the hydrogenation of phenol, or the direct hydration of cyclohexene. Of these, the cyclohexane oxidation route is still by far the largest. However, these three conventional processes all have drawbacks.1 The oxidation process suffers from a high safety risk associated with the formation of explosive mixtures when mixing cyclohexane and air for oxidation. It also has a fairly low selectivity, even at very low conversions, and is very energy consuming because of the high hydrogen consumption in the production of cyclohexane from benzene. Phenol hydration mainly suffers from high phenol prices as compared to benzene and again has the drawback of high hydrogen consumption. The comparatively new Asahi process of cyclohexene hydration overcomes these problems but suffers from the very slow reaction rates of the direct hydration of cyclohexene. The low reaction rates lead to very high catalyst amounts needed and to very specialized zeolite catalysts of the HZSM5-type. The catalysts used are typically very fine-grained and are being used in high concentrations in a slurry-type reactor with two liquid phases. The organic product coming out of the reactor is separated and distilled; the catalyst-containing aqueous phase is recycled to the reactor. Besides the fairly large amount of catalyst needed, this process also suffers from fairly low equilibrium conversion, leading to high recycle amounts that have to be distilled. Because of impurities in the cyclohexene feed, a part of the unreacted cyclohexene is also lost as part of a purge stream that is needed to avoid accumulation of cyclohexane and benzene in the recycle. Theoretically, the Asahi process would seem ideally suited to be carried out in a reactive distillation column because of its moderately exothermic character and its chemical equilibrium limitation. The reactive distillation column would internalize the recycle streams into the reactor/separator and would * To whom correspondence should be addressed. E-mail:
[email protected]. † Max Planck Institute for Dynamics of Complex Technical Systems. ‡ Otto-von-Guericke-University Magdeburg.
theoretically allow complete conversion of cyclohexene, thus allowing it to be reactively separated from any cyclohexane/ benzene impurities in the feed. The impurities could then be recovered at the top of such a reactive distillation column; the cyclohexanol would leave the column at the bottom because of its much higher boiling point. Even though the process seems attractive, it is extremely hard to implement for two reasons. The first reason is the large amount of catalyst needed to achieve acceptable reaction rates. Since catalyst loading in reactive distillation equipment is severely limited because of the need for low-to-moderate pressure drops within the column, the high amount of catalyst needed would lead to very large and expensive columns. The second drawback lies in the particle size of the zeolite catalysts used. Since these are typically in the submicron range, there is no easy way to fix them in the column. The alternative of using typical reactive distillation catalysts such as Amberlyst 15 leads to an additional increase in the catalyst amount needed because of lower catalyst effectivity. There has been some effort of growing zeolite catalysts on conventional nonreactive packings,2 but such an approach can only lead to very low catalyst loadings in the column. The alternative of simply suspending the catalyst in the aqueous phase in the column (similar to a suggestion made for a different system3), leading to a pseudohomogeneous catalyst behavior, has the drawback of not being able to limit the catalytic part of the column toward the bottom, which in turn leads to similar recycle problems as in the conventional Asahi process. Also, there exists a certain risk of clogging the column should the catalyst adhere to the column internals. Because of these problems, a new approach was chosen for the production of cyclohexanol in a reactive distillation setting. It is known from the literature4 that the reaction of cyclohexene with formic acid is a fast reaction, even with conventional reactive distillation catalysts such as Amberlyst 15. When considering reactions as being fast, this refers to reactions that can realistically reach complete conversion within the few minutes of residence time within a reactive distillation column. In this case, the reaction leads to formic acid cyclohexyl ester (FCE), which can then be split with water into formic acid and cyclohexanol. The reaction scheme that is being suggested is shown in Figure 1. To be able to design a reactive distillation system for such a complex system, precise data is needed for the phase behavior
10.1021/ie060781y CCC: $37.00 © 2007 American Chemical Society Published on Web 01/20/2007
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Figure 1. Suggested reaction scheme.
including liquid-phase splitting and for reaction kinetics. Our previous publications5,6 have already given vapor-liquid and liquid-liquid equilibrium data as well as a consistent nonrandom two-liquid (NRTL) parameter set to describe the fairly complex phase behavior. This publication will focus on the determination of the temperature-dependent reaction kinetic constants. Also, some consideration will be given to their dependence on catalyst particle size and external mass transfer resistances. Materials Used The reaction system under study consists of five components, namely, cyclohexene, cyclohexanol, water, FCE, and formic acid. Of these, the water was taken from a deionizer (type Millipore Milli-Q Gradient). Cyclohexene, cyclohexanol, and formic acid were acquired from VWR in synthesis quality (>99%) and used as delivered, except for the cyclohexene, which was distilled twice to remove some high-boiling stabilizers. Amberlyst 15 from Rohm & Haas was used as a catalyst and acquired from VWR. It was washed repetitively with water and isopropanol until no further discoloration of the solvent could be observed. The activity of the catalyst was determined by titration after the washing procedure to be slightly below 5 (mmol H+)/g, as expected. FCE on the other hand had to be synthesized because we were not able to find a vendor for it. The synthesis was performed by reacting cyclohexene and formic acid with Amberlyst 15 as a heterogeneous catalyst. The precise synthesis route has been published in a previous paper.6 The FCE produced this way had a purity > 98% (GCMSD based on area percentages). The main impurities were cyclohexanol and traces of cyclohexene.
type cc075 for adjustment and a hall sensor for stirrer speed measurement of type sm94). It allows adjusting stirring speeds from
