Mass transfer effects on the residue curve maps for membrane reactors

Membrane technology is of great industrial interest, as in many cases it has successfully replaced traditional, pollution-prone and energy consuming separation processes. Furthermore, a hybrid process combining chemical reactions and membrane separation, conventionally termed as "membrane reactor", has attracted more and more attention due to the potential improvements achieved by overcoming chemical equilibrium limitations, by enhancing the reaction kinetics, or by in-situ purification, etc. To design membrane reactors, one needs methods to analyse the feasibility of membranes for a considered reaction system; despite its increasing importance, however, the systematic analysis of membrane reactors has been addressed only to a small extent. Therefore, our research aims at evaluating membrane diffusion effects on the feasible products. In this work, the model equations for a batch membrane reactor (as Fig. 1) has been developed to study the effects of membrane diffusion. With a certain heating policy, the kinetic effect of chemical reaction can be characterized by a single parameter, the Damköhler number Da. An isobaric gas permeation process is considered here so that direct comparison with reactive distillation [1,2] can explicitly show the effects of incorporating membranes. An example of technical relevance, the effect of a Knudsen membrane on the cyclisation reaction of 1,4-Butanediol, is adopted to illustrate the utility of the model. The diagrams (in the poster) show that both chemical reaction and membrane diffusion could result in the appearance or elimination of singular points. This implies: for a considered reaction, different feasible products are attainable by utilizing suitable membranes. For the singularity analysis of reactive membrane separation processes, we propose a new set of transformed composition variables in analogy to the transformation first introduced by Barbosa et al [3]. This new set of transformed composition variables can be applied to equilibrium-controlled processes, as well as to flux-controlled processes. According to the proposed transformation, a hypothesis concerning singular points in membrane reactors is consequently inferred. The hypothesis can give the potential singular point curves [4], which provide more insight into the effects of membrane diffusion (cf. Fig. 2, extracted from the poster). Furthermore, the singular points in membrane processes can be denoted as "arheotrope", because they are flux-controlled, while the well-known "azeotropes" are fully equilibrium-controlled ones. Finally, it is worth while notifying that the proposed model and hypothesis are generally valid not only for membranes of veritable bodies, but also for reactive stripping, where an inert gas acts in the same way as a membrane [6]; or for reactive distillation affected by interfacial mass transfer. Reference [1] Venimadhavan, G., Buzad, G., Doherty M. F., & Malone, M. F. (1994). Effect of kinetics on residue curve maps for reactive distillation. A.I.C.H.E. Journal, 40, 1814-1824. [2] Qi, Z., Kolah, A., & Sundmacher, K. (2002). Residue curve maps for reactive distillation systems with liquid-phase splitting. Chemical Engineering Science, 57, 163-178. [3] Barbosa, D., & Doherty, M. F. (1988). The simple distillation of homogeneous reactive mixtures. Chemical Engineering Science, 43, 541-550. [4] Qi, Z., & Sundmacher, K. (2002). Bifurcation analysis of reactive distillation systems with liquid-phase splitting. Compt. Chem. Eng., 26, 1459-1471. [5] Wade, J., & Merriman, R. W. (1911). Influence of water on the boiling point of ethyl alcohol at pressures above and below the atmospheric pressure. J. chem. Soc., Trans. 99, 997-1011. [6] Schlünder, E.-U. (1977). Über den Einfluß der Stoffübertragung auf die Selektivität der Thermischen Trennverfahren - dargestellt am Beispiel der Schleppmitteldestillation. vt, Verfahrenstechnik, Nr.11, 682-686. Abstract of the poster submitted to ISMR3/CCRE18, 15.05.200