Mass transport with vaying diffusion- and solubility coefficient through a catalytic membrane layer
Advancing the chemical engineering fundamentals
Membranes and Membrane Science - I (T2-8a)
Keywords: catalytic membrane layer, dispersed catalyst particles, variable diffusion coefficient, nonlinear mass transport, external mass transfer resistance
Mass transport with varying diffusion- and solubility coefficient through a catalytic membrane layer
Endre Nagy
University of Pannonia, Research Institute of Chemical and Process Engineering, P.O.Box 158, 8201 Veszprém, Hungary, e-mail: nagye@mik.vein.hu
The catalytic membrane reactor is widely recommended to apply for heterogeneous reactions. This reactor with segregated feed of reactant(s), and/or with separation of products improves safety, selectivity and efficiency of the catalytic processes. Thus, the hydrogenation, dehydrogenation, oxidation reactions, etc., with different membranes, are the most common examples of the conversion enhancement [1, 2]. To accomplish these, one could use either an intrinsically catalytic membrane (e.g. zeolite) or a membrane that has been made catalytic through, e.g., catalytically active particles, as metallic complexes, activated carbon or metallic clusters, dispersed throughout the membrane phase.
There are only few results in the literature that investigate the mass transfer rate through catalytic membrane layer. Recently Nagy [3] developed mathematical models which define the concentration distribution and the mass transfer rate in membrane layer with dispersed catalyst particles. Depending on the catalyst particle size, heterogeneous- (for the case of micro-sized particles) and pseudo-homogeneous models (for sub-micron particles) were recommended. For first-order reaction it will be developed an analytical solution for the mass transport. This model assumes constant diffusion in both membrane layer and catalyst particles and constant solubility coefficient of the reactant in the catalyst particles.
In the reality, both parameters might depend on the concentration and/or on the inhomogeneity of the membrane layer. E.g., in the case of zeolite catalyst particles or membrane, the mass transport could be described by the Maxwell-Stefan approach [4]. Its combination with the Langmuir adsorption equation, a strong concentration dependency of the diffusion coefficient and the adsorption equilibrium can be obtained even for the mass transfer of a single reactant.
General mathematical models were developed to describe the mass transport in such nonlinear system taking into account the external mass transfer resistances as well. The concentration distribution and the mass transfer rates are defined by closed, explicit mathematical expressions for pseudo first-order reaction. Both the heterogeneous and the pseudo-homogeneous model [3] will be applied. The last model can be easily used to describe the mass transport through intrinsically catalytic, e.g., zeolite, membrane.
The mass transfer rate should be replaced into the differential mass balance equation given for the tube (or shell) side of the membrane layer and the effect of the concentration dependency on the outlet concentration will be shown. As case study the oxidation of alkenes by means of peroxide (t-BuOOH) will be studied.
The results presented could significantly improve the description of the catalyst processes in membrane reactor.
[1] Markano,, J.G.S., Tsotsis, T.T., Catalytic Membranes and Membrane Reactors., Wiley-VCH, 2002
[2] Saracco, G. Neomagus, H.W.J.P., Versteeg, G.F., van Swaaij, W.P.M., Chem. Engng. Sci. 54 (1999) 1997.
[3] Nagy E., Ind. Eng. Chem. Res., (April, 2007)
[4] Krishna, R., Wesselingh, J.A., Chem. Engng. Sci. 52 (1997) 862.
See the full pdf manuscript of the abstract.
Presented Tuesday 18, 15:40 to 16:00, in session Membranes and Membrane Science - I (T2-8a).
