Influence of the flow direction on the mass transport of vapors through membranes consisting of several layers

• The flow of vapors through asymmetric porous membranes is theoretically investigated.
• Capillary condensation and the Joule–Thomson effect is taken into account.
• The mass flux for the two orientations of the membrane may differ.
• The mass flux difference may be large if condensation occurs.

The flow of vapors with possible condensation is described, taking into account the energy balance, the Joule–Thomson effect and possible capillary condensation. For vapors close to saturation, the mass flux may be several times larger in one flow direction than in the opposite flow direction through an asymmetric membrane. As an example, the flow of isobutane through a porous ceramic membrane consisting of three different layers is theoretically investigated. The individual layers have different thicknesses with pore sizes of 10 nm for the separation layer, 100 nm for the middle layer and 6 μm for the support layer. For a small ratio of the upstream pressure to the saturation pressure, the fluid does not condense within the porous membrane, and the mass flux is a few percent larger for the flow direction from the support to the separation layer than for the other flow direction. For larger upstream pressures, the fluid may condense and the mass flow can become about 7 times larger for one flow direction than for the opposite flow direction.