Abstract
We present in this chapter an overview and a state of the art on the Membrane Distillation (MD) process. Such a process is a new, rapidly increasing membrane technology, which can be used for desalination and also for recycling. It is a separation process that involves transport of vapor through porous hydrophobic membranes. The principle, the main applications and the benefits of this separation technique are outlined. Approaches to improve the process performance are also discussed.
A detailed theoretical model for the transport phenomena in an Air Gap Membrane Distillation used for desalination was developed. This model is based on the conservation equations for the mass, momentum, energy and species within the feed water solution as well as on the mass and energy balances on the membrane sides.
In order to enhance the MD process performance, a composite hydrophilic-hydrophobic membrane is proposed in an Air Gap Membrane Distillation (AGMD) unit. The configuration considered here is a vertical tubular one with two concentric tubes. Hot saline water flows inside the inner tube which has a porous hydrophilic-hydrophobic wall. The external tube maintained at a constant and low temperature is the condenser.
The results analyzed the effect of the operating parameters such as the velocity and the temperature of the hot feed water at the inlet of the unit, the properties of the membranes, the cooling plate temperature and air gap thickness on the production rate of the vapor and the efficiency of the process. These results show that this desalination process is very efficient. More than 90% of the total heat flux coming from the hot side of the unit (feed water side) is converted into latent heat for the generation of the permeate (water vapor). The process performance is very sensitive to several parameters in particular to the porosity of the hydrophobic membrane. An increase of this parameter induces a fast growth of the mass and temperature boundary layers of the feed solution. A significant improvement of the permeate flux and the thermal efficiency are obtained by the incorporation of the composite hydrophilic-hydrophobic membrane.