Pervaporation

The pervaporation is a technical membrane processes for purification of liquid mixtures. For each application, a membrane must be selected through which the contaminating mixture component diffuses much better than the valuable substance or the component present in excess. After penetrating the membrane, the contamination evaporates on its back. The steam ( permeate ) is then drawn off and condensed elsewhere. The concentrated solution ( retentate ) remains on the inside of the membrane . Since the transport process through the membrane layer is a slow process, the need for membrane surface increases with the concentration of the contamination. Pervaporation is only an economical alternative to other separation processes if the differences in solubility differ by orders of magnitude and the retentate component is in excess relative to the starting solution.

Functional principle of pervaporation

The physical processes occurring in the membrane material can be described with the solution-diffusion model. According to the model, one component of the mixture to be separated preferably penetrates the membrane and is adsorbed on its inner surface or dissolved in the entire membrane material. Towards the outside of the membrane, the concentration of the readily soluble component is lower because it evaporates on the membrane surface and is thus continuously withdrawn. A concentration gradient is thus established across the membrane cross-section, which is the driving force for the diffusion of the permeate through the membrane. The different solubility of plastic membranes is based on the chemical interaction between the membrane material and the liquid molecule. In the case of ceramic membranes based on zeolite, there is also the fact that only the smaller liquid molecules can pass through the narrow channels in the membrane (see also 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 during pervaporation is relatively small. It is usually in the range of one bar, with almost atmospheric pressure being applied on the side of the retentate and the space of the permeate being operated at negative pressure or vacuum. The partial pressure of the permeate on the gas side is reduced by the negative pressure and the back diffusion into the membrane is minimized. Alternatively, the permeate partial pressure can also be reduced by purging with an inert gas (usually nitrogen ).

Because of the cooling ( evaporative cooling ) caused by evaporation, it must be compensated by adding heat. This is done either by preheating the inlet or by heating between individual membrane modules. If a purge gas is used, this can also be used for heating.

Membrane materials

In contrast to other membrane processes, dense membranes are used in pervaporation. Compared to other membrane processes, the membranes used for pervaporation are particularly tight. The plastic membranes consist of a layered composite material in which a synthetic fiber fabric is embedded for the required stability. The actual, dense separating layer is either applied as a thin film to the carrier membrane or covalently anchored directly in the pores by means of pore filling. Ceramic membranes have a ceramic support body on which the zeolite layer is applied.

Membrane materials for separating water and organic liquids are technically mature, for example for:

To separate water from organic liquids, hydrophilic polymers such as polyvinyl alcohols , polyimides and cellulose acetate or sodium zeolite A membranes are used.
Purification of water contaminated with organic liquids with the aid of hydrophobic or organophilic membrane materials, in particular polydimethylsiloxane or polyoctylmethylsiloxane.

With the help of innovative membrane materials specially tailored to the separation problem, it is also possible to selectively separate individual organic compounds from organic mixtures, for example:

Reduction of the aromatic content in refinery streams, e.g. separation of benzene from a mixture of aliphatics .
Breaking azeotropes
Conditioning of liquid hydrocarbon streams
Purification of extraction agents
Purification of organic solvents

Technical applications

Technically significant separation tasks of pervaporation are:

Removal of water from organic solvents which form an azeotrope with water , for example ethanol , n-propanol , isopropanol , n-butanol , ethyl and butyl acetate , propanone , tetrahydrofuran , 2-butanone .
Increase in the conversion in polycondensation reactions in that the chemical equilibrium is shifted towards the product side by separating off the released water .
Removal of methanol and ethanol from mixtures with esters
Removal of aromatics from mixtures with aliphatics

Differentiation from other membrane separation processes

Pervaporation differs from reverse osmosis and vapor permeation in the state of aggregation on both sides of the membrane:

Table 1: Differentiation from other membrane separation processes
	Physical state
Membrane process	Permeate	Retentate
Pervaporation	gaseous	liquid
Reverse osmosis	liquid	liquid
Vapor permeation	gaseous	gaseous