Chapter 2 - Membranes Used in MD and Design

Abstract

It is generally admitted that the membrane distillation (MD) permeate flux increases with the increase of the pore size and/or porosity. The choice of a membrane for MD applications is a compromise between a low heat transfer flux by conduction achieved using thicker membranes and a high permeate flux achieved using thin membranes having large pore size, low pore tortuosity, and high porosity. It is agreed upon that in MD the membrane itself acts only as a physical barrier sustaining the liquid vapor interfaces formed at the entrances of the membrane pores. Volatile compounds present in feed solution are transported across the membrane pores according to the vapor/liquid equilibrium (VLE) principle, and both heat and mass transfer occur simultaneously through the membrane. Many MD researchers believe that selective properties in MD process are not governed by the membrane transport phenomena. Membrane surface modification using different technologies such as grafting, coating, or blending fluorinated surface-modifying macromolecules (SMMs) with hydrophilic polymers are also tested for different MD systems and configurations. The study also discusses commercial membranes; commercial microporous hydrophobic membranes made of PP, PVDF, or PTFE (Teflon), available in tubular, capillary, or flat sheet forms.