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キーワード:
reaction engineering; chemical reactors; porous media; transport processes; selectivity; reactant dosing
要旨:
Membrane reactors are promising candidates for enhancement of productivity in complex reaction networks [1]. Earlier experimental studies on catalytic oxidation of ethane in a tubular packed-bed membrane reactor (PBMR) revealed, that it is possible to maximize selectivity and yield with respect to different products [2]. Due to the significant difference in feeding the reactants between the PBMR and the classical fixed-bed reactor (FBR), it appears to be necessary, to study the extent of radial temperature and concentration profiles on the specific reactor performances. Such effects must be analyzed using non-isothermal 2D reactor models. Consequently, the focus of our work was to formulate and solve such models.
Ethane oxidation was chosen as a suitable exothermic, kinetically controlled model reaction to perform this study. Based on experiments in a laboratory fixed-bed reactor a reaction network considering 5 sub-reactions was proposed. For a quantitative description of this network kinetic parameters were estimated, based on a Mars van Krevelen mechanism for the oxidative dehydrogenation of ethane and Langmuir-Hinshelwood mechanisms for the deep oxidation reactions [3].
Both FBR and PBMR reactors were described by two-dimensional, non-isothermal, pseudo-homogenous models which were implemented in the simulation tool FEMLABTM. To calculate the velocity and pressure fields, at first, the complete set of incompressible Navier Stokes equations were solved. Subsequently, the component and energy balances were calculated separately using the obtained velocity and pressure profiles. This simplification is justified, because frequently significant amounts of inert are present in the reactors. The PBMR was considered as to be operated in a dead end configuration. Furthermore, a constant flux density through the porous reactor wall was assumed in the PBMR simulation. Effective diffusion coefficients and heat conductivities were used based on standard correlations [4, 5].
A selected example of simulation results is given in Fig. 1. Typical tendencies, which can be seen are:
(1) The temperature profile is much more uniform in the PBMR compared to the FBR (Fig. 1 a and b). This smoothing is one of the most attractive advantages of the PBMR concept.
(2) In contrast to the FBR, where the gas velocity is quite constant over the whole catalyst bed, the PBMR might exhibit a significant axial velocity profile (Fig. 1 c and d). Thus, the contact time of the component fed at reactor inlet is longer in the PBMR than in the FBR. This leads to an enhanced conversion.
(3) Local oxygen concentration is lower and radial profiles are more pronounced in the PBMR than in the FBR (Fig. 1 e and f). Under certain conditions this difference can enhance selectivity with respect to an intermediate product.
Based on more systematic simulations, the necessity of the application of a 2D model, compared to a simple 1D model and the potential of the PBMR will be analyzed in detail.
[1] K. K Sirkar, P. V. Shanbhag, A. S. Kovvali, Ind. Chem. Eng. 38 (1999), 3715-3737
[2] F. Klose, T. Wolff, S. Thomas, A. Seidel-Morgenstern, Catal. Today, 82 (2003), 25-40
[3] F. Klose, T. Wolff, S. Thomas, A. Seidel-Morgenstern, Appl. Catal. A.- Gen., in press
[4] R. C. Reid, J. M. Prausnitz, B. E. Poling, Properties of gases and liquids, McGraw-Hill, 1987
[5] P. Zehner and E. U. Schlünder, Chem. Ing. Tech. 44 (1972), 1303
