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Abstract:
Membrane reactors belong to the most innovative concepts in chemical reaction engineering capable to intensify a multitude of processes and to improve selectivities and yields with respect to valuable intermediate products. Typical examples where the latter aspect is of importance are partial oxidations or hydrogenations.
Profound knowledge exists about the modelling of conventional tubular catalytic fixed-bed reactors where the reactants are fed together at the reactor inlet. The application of simplified 1D isothermal models is state of the art, e.g. [1]. Less studies were oriented on a more detailed mathematical modelling (2D, non-isothermal). Main difficulties usually arise in particular for low aspect ratios. Then radial distributions of porosities, flow rates and transport coefficients have to be taken into account [2]. The situation is even more complex if reactants are also dosed along the reactor axis.
Purpose of this contribution is the formulation, solution and application of a non-isothermal 2D membrane reactor model (steady state, pseudohomogeneous). The radial and axial velocity field is quantified by a modified Navier Stokes equation (containing a side friction term). A critical and novel issue is the proper formulation of boundary conditions at the reactor wall in the mass and energy balances considering mass transfer through the wall. For validation of the 2D model a parallel experimental study of the highly exothermic oxidative dehydrogenation of ethane to ethylene applying a VOx/γ-Al2O3-catalyst was performed a) in a one stage, b) in a cascade of three membrane reactors and c) in the established conventional fixed bed reactor [3,4].
Main focus of the parametric study presented is the estimation of the internal temperature profile, the internal molar fraction profiles for reactions and the mathematical description of the different velocity fields in fixed bed and membrane reactors. The performance of 1D and 2D membrane reactor models will be compared.
In a second part the potential of various types of stage-wise temperature profiles in combination with corresponding optimised dosing profiles is presented. The concept allows to improve significantly the selectivity and yield of an intermediate product in parallel-series reactions. The concrete results depends on the ratios of the rates of desired and undesired reactions. Thus, the temperature impact is determined by the specific activation energies of all reactions. Results for three conceivable constellations are presented (activation energy of the desired reaction can be the largest, smallest or equal to the activation energy of the undesired reactions). An optimised stage-wise temperature profile is proven to be a powerful way to improve performance parameters (e.g. conversion). Hot spots and runaways can be effectively suppressed by an adjusted cooling and/or heating. Further, improved performance can be achieved by combining the optimised stage-wise temperature profile with a stage-wise dosing of one or several of the reactants [5], respectively.
Literature:
[1] Westerterp, K. R., van Swaaij, W. P. M., Beenackers, A. A. C. M., Chemical reactor design and operation, John Wiley & Sons, 1984
[2] Winterberg, M., Tsotsas, E., Krischke, A., Vortmeyer, D., Chem. Eng. Sci., 55 (2000) 967
[3] Klose F., Joshi M., Hamel C., Seidel-Morgenstern A., Selective oxidation of ethane over a VOx/γ-Al2O3 Catalyst-investigation of the reaction network, Appl. Catal. A.-Gen.
[4] Klose F., Wolff T., Thomas S., Seidel-Morgenstern A., Operation modes of packed-bed membrane reactors in the catalytic oxidation of hydrocarbons, Appl. Catal. A.-Gen.,
[5] Hamel C., Thomas S., Schädlich K., Seidel-Morgenstern A., Theoretical analysis of reactant dosing concepts to perform parallel-series reactions, Chem. Eng. Sci., 58, 2003, 4483 - 4492
