Membrane distillation is a thermally driven desalination process. A hydrophobic membrane displays a barrier for the liquid phase, allowing the vapor phase (e.g. water vapor) pass through the membrane pores. The driving force of the process is the partial vapor pressure difference created to the temperature difference. MD has different configurations including direct contact membrane distillation (DCMD), Air gap membrane distillation (AGMD) and Vaccum membrane distillation (VMD).
In DCMD, both sides of the membrane are charged with liquid- hot feed water on the evaporator side and cooled permeate on the permeate side. The condensation of the vapor passing through the membrane happens directly inside the liquid phase at the membrane boundary surface. Since the membrane is the only barrier blocking the mass transport, relatively high surface related permeate flows can be achieved with DCMD. A disadvantage is the high sensible heat loss, as the insulating properties of the single membrane layer are low. However, a high heat loss between evaporator and condenser is also the result of the single layer. This lost heat is not available to the distillation process whereby its efficiency is lowered. Working of DCMD is shown in this video.