CFD simulations have been carried out to describe pervaporation of organic-water mixtures with a tubular membrane. The calculated density and temperature profiles have been compared with experiments. The profiles have been measured by ultrasound computer tomography (UCT) for pervaporation of pure water and an ethanol-water mixture. Good agreement between the measured and calculated density and temperature profiles has been found. Several flow patterns have been detected depending on the density ratio between the non-permeating and the permeating component in the mixture. For light organic-water mixtures an inversion point has been found at the membrane surface below which the flow moves downwards and above which the flow moves upwards. The inversion point is caused by the opposite effects of water concentration and temperature on the mixture density and consequently on the natural convection. For heavy organic-water mixtures the inversion point disappears and the flow on the membrane surface is only downward. The flux through the membrane tube increases as the density ratio increases. For the design of multi-tubular pervaporation modules, the results suggest that for separation of heavy organic-water mixtures the flux is maximized with a squared configuration. On the other hand, a triangular configuration is preferred for dewatering of light organic-water mixtures. (c) 2006 Elsevier B.V. All rights reserved.