Membrane Distillation

For sea water distillation and desalination membrane distillation is rather new process. The technology uses special membrane for bringing out the water vapor generated from collected seawater. The water vapor is collected as freshwater. The membrane used in the process is porous hydrophobic membrane, in which water vapor is allowed to pass through the membrane but the drops of water cannot. The membrane when placed on the mobile warm water stream, the water vapor passes through the membrane. Water vapour gets condensed on a second surface which is at a lower temperature than that of the feedwater.

Membrane distillation separation depends on relative volatility of various components in the feed solution. The solution is force through the membrane by applying partial pressure difference across the membrane. When vapor from components of higher volatility when passes through the membrane pores by a convective or diffusive mechanism it is then Separation occurs

Process Categorization
Membrane distillation systems can be categorized into two parts:

• Direct-contact distillation
• Gas-gap distillation

Both methods indicate the permeate or condensing side of the membrane. In both methods feed and membrane is in close contact. In direct-contact membrane distillation, liquid phase and the sides of the membrane remain in contact. The liquid that is on the permeate side acts as the condensing medium for the vapors that leaves the hot feed solution. In gas-gap membrane distillation the condensed permeate and membrane are not in direct contact.

Process Description
Cold seawater takes the route of a condenser having non-permeable walls. It reaches into membrane evaporator after passing through a heat exchanger. Evaporators wall consist of a micro-porous hydrophobic membrane. Water vapour diffuses through the membrane whereas liquid water containing dissolved salts is retained.

The difference in vapour pressure of water across the membrane is main driving force for desalination rather than the total pressure which is the feature of other desalination process. When the source water is heated the vapour pressure gradient is created.


Advantages:
Membrane distillation have following advantages

• The membrane distillation system requires hardly any maintenance.
• Fluctuating heat sources usually of low temperature can be used for the units due to which water can be produced by using energies that normally go wasted like waste heat from factories, hot spring heat, solar energy.
• The cost of running the unit is low.
• Operate at ambient pressures.
• Can operate at lower temperatures.
• Fouling is low.