What’s Membrane Distillation?

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Membrane distillation is a process used to separate or purify liquids. The technology works by creating a temperature gradient across the membrane, with one side being hotter and the other side being colder. Vapors rising from the heated liquid pass through the membrane to the cooler area of ​​lower pressure, where they condense into a purified form of the liquid. This type of distillation finds application in industrial plants, where certain liquids must be separated from a mixture, and in desalination plants, where fresh water must be obtained from salt water.

When a liquid is heated, it eventually begins to evaporate or boil. The heat causes the molecules of the liquid to acquire energy and rise to the vapor state. An example of this is steam rising from hot water.

In membrane distillation, the vapor is held in a chamber and increases in pressure, forcing the molecules through a porous medium: the membrane. The type of membrane that is used in this distillation process only lets the steam through, eliminating other impurities. After the vapor has passed through the membrane, it encounters a cold area or surface which causes it to condense back to a liquid state. The resulting liquid consists only of the molecules that were in the vapor, and any salt or other unevaporated impurities are still in the original mixture.

There are a few types of membrane distillation processes. Direct contact membrane distillation, commonly used in desalination, keeps the quench solution in contact with the opposite side of the membrane. Air-gap membrane distillation separates the coolant from the membrane by a cavity and a hard surface on which the vapor can condense. Other types of processes, such as sweep gas and vacuum membrane distillation, actively draw vapor through the membrane using a gas or vacuum. The latter techniques are useful for removing unwanted compounds from the original blend.

The uses of membrane distillation are diverse. It can be used to remove salt and other mineral impurities from water, but is also useful for removing specific chemicals or creating concentrated blends for industrial or commercial use. As the vapor distills across the membrane, it is removed from the original mixture, making that original mixture more concentrated.

Membrane distillation has become a desirable solution for desalination because it requires relatively little energy to operate. It can be powered by solar heat or waste heat from other operations. The small amount of heat required for the process makes it a possible solution for setting up small-scale plants in remote areas that may not have access to extensive energy resources.

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