Concepts and Analysis of Membrane Reactors

The potential of using various types and configurations of membranes in reactors attracts already a long time the interest of chemical reaction engineers. The most obvious and most intensively studied possibilities of membrane reactors are: (1) Retention of homogeneous catalysts and enzymes in continuously operated reactors (2) Optimising the contact of reactants and performing reactions within membranes (3) Selective withdrawal of products formed in reversible reactions via membranes (4) Controlled dosing of reactants using membranes in order to improve the selectivity In all these fields systematic academic studies contributed in the last decades to analyse the principles theoretically and to demonstrate their applicability in the lab scale. As concerns industrial applications option (1) is clearly the most successful. In contrast, there is not yet a significant industrial breakthrough of membrane reactors based on options (2), (3) and (4). One of the most important reasons for the limited success of these promising concepts is the fact that it is not an easy task to develop membranes and catalysts performing together efficiently at the same operating conditions. In the first part of the lecture basic aspects are revised related to quantify rates of chemical reactions and transport processes through different types of inorganic temperature resistant membranes. For many heterogeneously catalysed reactions the currently available selective dense membranes do not offer sufficient fluxes and are thus not compatible enough to the reaction kinetics. In contrast, more permeable porous membranes do typically not provide the required separation factors. There is a clear need for a joint development of catalysts and membranes. The intention of the second part of the lecture is to discuss the potential of membrane reactors for application (3). Results of selected case studies will be presented. It is elucidated that an efficient withdrawal of products formed in equilibrium limited reactions requires typically significant amounts of sweep gas. This additional effort needs to be included in a critical and meaningful process evaluation. The demand for highly selective and still sufficiently permeable membranes will be again emphasised. In the third part of the lecture application (4) will be analysed. The oxidative dehydrogenation of ethane will be considered as a case study. It will be demonstrated that a successful operation of a suitable membrane reactor requires to adjust carefully the amounts of oxygen dosed in accordance with the specific reaction rates. Simplified mathematical models will be presented in order to generalise the results obtained. Finally, some practical problems of realising membrane reactors will be addressed and general conclusions concerning their future potential will be drawn