Pervaporation

Pervaporation is a membrane process for the technical purification of liquid mixtures. For each application, a membrane must be chosen by the contaminating component mixture diffuses through much better than the value of the material or component present in excess. After the penetration of the membrane contamination evaporated on the back. Of steam ( the permeate) is then drawn off and is condensed in another place. On the inside of the membrane, the concentrated solution ( retentate) remains. Since the transport process through the membrane layer is a slow process, the need for membrane surface area increases with the concentration of the impurity. Pervaporation only represents an economic alternative to other separation methods when the differences in solubility differ by orders of magnitude and the retentate component based on the starting solution is present in excess.

Operating principle of pervaporation

The processes involved in the membrane material physical processes can be described by the solution-diffusion model. According to the model concept, a component of the mixture to be separated preferably penetrates into the membrane and is adsorbed on the inner surface thereof or dissolved throughout the membrane material. To the outside of the membrane, the concentration of the readily soluble component is smaller, since it is vaporized at the membrane surface, thus continuously withdrawn. About the membrane cross-section arises as a concentration gradient a, which is considered as the driving force for the diffusion of the permeate through the membrane. The difference in solubility is based on polymer membranes to chemical interaction between membrane material and liquid molecule. For ceramic membranes, zeolite is added to only the smaller liquid molecules can pass through the narrow channels in the membrane (see, zeolites and molecular sieves).

Technical variants

In contrast to the pressure-driven membrane separation process, the pressure difference between the two sides of the membrane in pervaporation is relatively low. It is usually in the range of one bar, which is present on the side of the retentate is almost atmospheric pressure and the permeate chamber is operated at reduced pressure or vacuum. By the negative pressure, the partial pressure of the permeate is lowered to the gas side, and minimizes the back diffusion into the membrane. Alternatively, the Permeatpartialdruck can also be lowered by washing with an inert gas (usually nitrogen).

Because of the caused by the evaporation cooling ( evaporative cooling ) must be compensated by the application of heat. This is done either by preheating the feed or heating between individual membrane modules. When using a purge gas and this can be used for heating.

Membrane materials

Unlike other membrane processes dense membranes used in pervaporation. Compared with other membrane processes, membranes used for pervaporation are particularly tight. The polymer membranes consist of a layer -by-layer laminated material in which a synthetic fiber woven fabric is embedded for the required stability. The actual, dense separation layer either as a thin film applied to the support membrane, or directly anchored by means of pore -filling in the pores covalently. Ceramic membranes have a ceramic support body on which the zeolite layer has been applied.

Technically perfected are membrane materials for the separation of water and organic liquids, for example, to:

Separation of water from organic liquids can be used hydrophilic polymers such as polyvinyl alcohols, polyimides, and cellulose acetate or sodium zeolite A membrane.
Cleaning polluted with organic liquids water using hydrophobic or organophilic membrane materials, in particular polydimethylsiloxane or Polyoctylmethylsiloxan.

Using new, specially tailored to the separation problem membrane materials, including the selective separation of individual organic compounds from organic mixtures is possible, for example:

Reduction of the aromatics content in refinery streams, such as the separation of benzene from a mixture of aliphatic compounds.
Breaking of azeotropes
Conditioning of liquid hydrocarbon streams
Purification of extractants
Purification of organic solvents

Technical Applications

Industrially important separation processes pervaporation are:

Removal of water from organic solvents which form an azeotrope with water, for example ethanol, n -propanol, isopropanol, n-butanol, ethyl acetate and butyl acetate, propanone, tetrahydrofuran, 2 -butanone.
Future revenue growth for polycondensation reactions by the chemical equilibrium is shifted toward the product side by removing the released water.
Removing methanol and ethanol, mixtures of esters
Removal of aromatics from mixtures with aliphatic

Separation from other membrane separation processes

Pervaporation is different from the reverse osmosis and Vaporpermeation by the physical state of the both sides of the membrane: