The influence of boundary layers on multi-component mass exchange in a FricDiff-module

Abstract

This paper describes the separation of vaporous alcohol–water mixtures using the FricDiff separation technology. This separation process is based on a difference in interspecies friction between the components of a feed mixture and a sweep gas. A comparison is made between the performances of tubular and plate FricDiff-modules for several dimensions of the module using the finite element program COMSOL Multiphysics TM [Comsol, 2006. COMSOL Multiphysics User's Guide, version 3.3]. It is shown that these configurations give different results when the fluxes through the porous barrier (which separates the feed mixture compartment from the compartment where the sweep gas is flowing) increase. These differences can be attributed to the resistances to mass transfer located in flow channels adjacent to the porous barrier. These resistances are different for the two configurations and when they become dominant, the tubular and plate FricDiff-modules behave differently. Sherwood correlations are derived to describe the resistances to mass transfer in the flow channels. Both the developing and fully developed boundary layer regimes are taken into account. In the developing regime, the derivation is based on that of a pseudo-binary system, consisting of one of the feed components and the sweep gas. In the fully developed regime the true multi-component behavior of the system is taken into account. It is shown that these Sherwood correlations give a very accurate description of the separation process in a FricDiff-module.