Vacuum membrane distillation (VMD) is a separation process based on the use
of microporous hydrophobic membranes. The membrane is located between an a
queous phase and a permeate, which is kept under vacuum at pressure values
below the equilibrium vapor pressure of the feed. The liquid stream vaporiz
es at one side of the membrane, and the vapors diffuse through the gas phas
e inside the membrane pores. The process rate and performance are affected
highly by the transport phenomena both in the liquid phase and through the
membrane. Heat- and mass-transfer resistance in the liquid phase, as well a
s mass-transfer resistance through the membrane, play an important role in
determining the process performance. Based on VMD experimental data for sev
eral binary aqueous mixtures containing volatile organic compounds, a simpl
e criterion to investigate the role of each transport resistance on the sep
aration efficiency is discussed.