Heat and Mass Transfer in a Vacuum Membrane Distillation for Water Desalination

Membrane distillation (MD) is a hybrid thermal-membrane desalination process that produces pure water from a hot saline solution using a hydrophobic membrane. MD can be designed and operated under various configurations. Vacuum membrane distillation (VMD), which is a basic MD configuration, allows for the generation of greater permeate fluxes with reduced heat losses by incorporating vacuum on the permeate side. An external condenser is used to condense the vapor flow and collect the distillate. The 2D conservation equations of mass, momentum, energy, and species are developed and solved numerically. Interfacial balance equations and appropriate boundary conditions are applied. Due to the membrane hydrophobicity, slip velocity and temperature jump conditions are also considered. The simulation results concern the spatial variation of the heat and mass transfer coefficients as well as the freshwater production rate as function of various operating and input variables. The effects of process parameters on pure water production are presented and discussed for different values of slip and Kapitza lengths. The impact of slip flow and temperature jump is found to be important on the trans-membrane flux while less important on the heat and mass transfer coefficients.