Equilibrium model, dehydrogenation

T. Kokugan, A. Trianto, and H. Takeda, Dehydrogenation of pure cyclohexane in the membrane reactor and prediction of conversion by pseudo equilibrium model, J. Chem. Eng. Jpn. 31,596-603 (1998). 

Due to its complexity (conversion and separation in the same unit) and because this system has been most widely studied experimentally, CMRs for dehydrogenation (or more generally for equilibrium-restricted reactions) have been the subject of modeling approaches [6, 54-59]. The modeling of CMRs requires mass and energy balances in both feed and permeate sides of the reactor (plug-flow behavior is always assumed) and appropriate boundary conditions. Generally these models fit the experimental data well. 

Preliminary results obtained in an effort to model the dehydrogenation of ethylbenzene to styrene in a "membrane reactor" are described below. The unique feature of this reactor is that the walls of the reactor are conprised of permselective membranes through which the various reactant and product species diffuse at different rates. This reaction is endothermic and the ultimate extent of conversion is limited by thermodynamic equilibrium constraints. In industrial practice steam is used not only to shift the ec[uilibrium extent of reaction towards the products but also to reduce the magnitude of the ten erature decrease which accon anies the reaction when it is carried our adiabatically. 

The macroscopic mass balance model by Tsotsis et al. [1992], when applied to the reaction of ethane dehydrogenation, compares well with experimental data and both show higher conversions than the corresponding equilibrium values based on either tube-or shell-side conditions (pressure and temperature). This is clearly a result of the equilibrium displacement due to the permselective membrane. The conversion, as expected, increases with increasing temperature. 

Mohan and Govind [1988c] applied their isothermal packed-bed porous membrane reactor model to the same equilibrium-limited reaction and found that the reactor conversion easily exceeds the equilibrium value. The HI conversion ratio (reactor conversion to equilibrium conversion) exhibits a maximum as a function of the ratio of the permeation rate to the reaction rate. This trend, which also occurs with other reactions such as cyclohexane dehydrogenation and propylene disproportionation, is the result of significant loss of reactant due to increased permeation rate. This loss of reactant eventually negates the equilibrium displacement and consequently the conversion enhancement effects. 

For hydrogenolysis, Leclercq, Leclercq, and Maurel (67) tried to account for the change in order with changing hydrogen pressure by the Cimino model (Scheme 14). They supposed that the rate-determining step involves molecular hydrogen, and they replaced all the equilibrated dehydrogenation steps with an overall equilibrium, with an equilibrium constant 2 ... 

Figure I Model discrimination in butene dehydrogenation. Operability region, equilibrium surface, location of preliminary and designed experiments at 525°C.

A large number of hydrogenation and dehydrogenation reactions were tested in the early studies of dense-metal membrane reactors (see listing in Shu et al. [34], Hsieh [35], and Gryaznov and Orekhova [36]). Many works tested the dehydrogenation of cyclohexane to benzene as a model reaction since it can be carried out at low temperature with no side reactions and no deactivation a conversion of 99.5% was achieved with a palladium membrane, compared with 18.7% at equilibrium, at 200°C [31]. 

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