We study the dehydrogenation of iso-butane in a membrane reactor with carbon membranes; these are molecular sieves with pores of molecular dimensions. To provide information on the transport laws of the various components, for reactor design purposes, we studied the separation in a membrane module either by maintaining the shell-side under lower pressure (or under vacuum) or by sweeping it with an inert diluent stream. The purpose of this work is to derive multi-component flux expressions for single-file diffusion in these two modes of separation, using molecular mechanics calculations of the thermodynamics of molecular adsorption into, diffusion within and desorption from the pores, and apply them for reactor design. In the process we calibrate these expressions with integral measurements of separation in a membrane module. Good predictions of reactor performance are obtained for a reaction coupled with separation by sweeping the hydrogen with nitrogen but poor predictions were achieved for a reaction coupled with vacuum-driven separation. Reactor performance in the former mode is better due to excellent transport selectivity, which we attribute to mutual blocking of counter-diffusion by nitrogen and hydrocarbons. (C) 2004 Elsevier Ltd. All rights reserved.