Two main factors are important for the design of membrane modules used for gas and vapour permeation: concentration polarisation and flow distribution. The former causes a reduced driving force, significantly affecting membrane performance. A uniform flow distribution will ensure that the complete membrane area is utilised. In order to reduce the influence of concentration polarisation and to ensure an even flow distribution spacers located between two membrane surfaces or plates containing flow channels are employed. A comparison between these geometries using computational fluid dynamics (CFD) is presented. For CFD calculations, the commercial solver FLUENT(TM) has been used and the mass transfer through the membrane has been modelled by user defined functions.