Chapter 12 - Vacuum Membrane Distillation

Abstract

The chapter discusses vacuum membrane distillation (VMD) as there are two membrane processes, pervaporation (PV) and VMD, that are similar and often confusion occurs between these processes. In both techniques, vacuum or low pressure is applied on the permeate side to establish the driving force. Vacuum membrane distillation (VMD) is another variant of membrane distillation (MD). In this configuration low pressure or vacuum is applied on the permeate side of the membrane module by means of vacuum pumps. The applied permeate pressure is lower than the saturation pressure of volatile molecules to be separated from the feed solution and condensation takes place outside the membrane module at temperatures much lower than the ambient temperature. In VMD, because of the hydrophobic nature of the used membranes, the feed cannot penetrate inside dried membrane pores unless a trans membrane hydrostatic pressure exceeds the “liquid entry pressure of water (LEP),” which is characteristic to each membrane. This condition results in the formation of liquid/vapor interfaces at the entrances of the membrane pores and, because of the applied low pressure on the permeate side, molecules evaporate from the feed side of the membrane, cross the pores in vapor phase, and condense outside the membrane module by means of external condensers. In VMD, the driving force is maintained by applying a continuous vacuum at the permeate side (i.e. downstream pressure) below the equilibrium vapor pressure.