Effect of applied pressure on performance of PTFE membrane in DCMD

Abstract

In membrane distillation (MD), mass and heat transfer occurs simultaneously through the membrane. According to the theory for mass and heat balance, if the membrane is compressible, total pressure in the pore and the thermal resistance of the membrane will change and affect the performance of the membrane. To verify this theory, direct contact membrane distillation (DCMD) experiments were conducted in which the absolute pressures applied on both sides of membranes were controlled identically and varied independently of the flowrates. Both hollow fibre and flat-sheet membrane modules were employed. The hollow fibre membrane had a rigid structure that was not deformable over the range of experimental pressures (1–50 kPa), while the composite flat-sheet membrane was composed of a scrim support layer and a sponge-like PTFE active layer that was compressible over the range of pressures considered. When pressures were applied on both sides of the compressible membrane, the volume of active layer will become smaller. As the active layer was compressed in the tested pressure range, the void volume (porosity) and the thickness of the active layer was reduced, which resulted in the reduction of the thermal resistance of the membrane and so increased the heat conduction loss across the membrane. Furthermore, if there is no air escaping out of the membrane pores, the air pressure within the pores will increase due to the reduction of void volume (porosity), leading to an increase of the mass transfer resistance. The effect of pressure on the flux was also considered at different velocities (0.095 and 0.114 m/s) and temperatures (50 and 70 °C). For the composite flat-sheet membrane, there was a 15–39% flux reduction when the gauge pressure was increased from 1 to 45 kPa at constant inlet temperatures and flowrates. However, there was no noticeable change of flux with pressure for the incompressible hollow fibre module.