Transport Phenomena in Porous Membranes and Membrane Reactors

Abstract

IntroductionMembrane reactors offer the possibility to integrate dosing, separation and reaction processes in a single apparatus. Their effi cient operation depends on the effectiveness of the catalyst and the corresponding reaction kinetics as well as on the transport processes in the reactor. The diffusive and convective mass and energy transfer can be systematically infl uenced by the reactor geometry, the membrane morphology and the operation conditions. Hence, there is a high optimization potential for these reactors. Due to the large number of parameters, the numerical simulation is increasingly used for reactor sizing and design. The applied models and tools for this purpose are presented in Chapter 2 , where different modeling depths are taken into account. Chapter 3 provides details about the kinetics of an example reaction (oxidative dehydrogenation of ethane), which are necessary for the study of the catalysts, membranes and processes in the framework of this project. The aim of the current chapter is the analysis of the transport phenomena, in particular the superposition of convection and diffusion processes. It focuses on fi xed -bed or packed -bed reactor s ( PBR ), packed -bed membrane reactor s ( PBMR ) and catalytic membrane reactor s ( CMR ), which are investigated numerically by a pseudo -homogeneous model approach. Concerning the superposition of convective and diffusive processes, some aspects of the numerical solution of the corresponding differential equations are discussed from a mathematical point of view in Section 4.2 .The complexity of the simulation model and the numerical effort depend on the model dimension and especially on the manner of description of the velocity profi les. Through meaningful simplifi cations in the modeling, based on a balance between expenses and benefi ts, computing effort can often be signifi cantly reduced. Plug fl ow conditions or developed velocity profi les are frequently used as model assumptions. For example in PBRs, developed velocity profi le can be reliably used for the momentum analysis without great deviation to the full -order model, (Hein, 1999 ), due to the setting of this profi le after a very short entrance length. But, no developed profi le can be obtained in the membrane reactor due to