Gas separation processes/pervaporation/vapor permeation

Pervaporation (PV) is a fractionation process in which a liquid mixture is maintained at atmospheric pressure on the feed side of the membrane, while the permeate is removed as a vapor. Transport is induced by using a vacuum pump at the permeate side or by cooling the permeate vapor to create a partial vacuum.

Most PV membranes consist of a dense top layer and an open porous support. The transport of the
feed through the top layer follows the solution-diffusion model, involving three steps:

• Selective sorption of the liquid mixture on the feed side (most dominant);

• Selective diffusion through the membrane;

• Desorption into the vapor phase on the permeate side.

The separation capacity of a PV membrane is primarily a function of the membrane material and the feed species (interaction of each feed component with the polymer). Feed temperature, feed composition (interaction between different feed components) and permeate pressure have a secondary influence.

Three different types of PV membranes can be distinguished:

• hydrophilic membranes;

• organophilic or hydrophobic membranes ;

• organoselective membranes.

The choice of membrane type depends strongly on the type of application. Hydrophilic membranes are mostly used for organic solvent dehydrations, organophilic membranes to eliminate organic species from aqueous or gaseous effluents, and organoselective membranes for the separation of purely organic mixtures.

PV can be a good alternative to more traditional techniques such as vacuum distillation and solvent extraction. The process has advantages over these techniques with respect to energy saving, process simplicity and lower capital costs. Overall, PV can offer a solution in cases where conventional separation processes fail or result in high energy consumption, such as the separation of azeotropic mixtures.

Despite its high potential, the industrial development of PV units is still very limited. The main application of PV in industry is the dehydration of organic liquids with hydrophilic membranes, usually made of polyvinyl alcohol (PVA). Organophilic membranes (mostly PDMS membranes) are used for the recovery of hydrocarbons from air or gases (petrochemical industry, natural gas industry).

In gas separation (GS), both the feed and the permeate consist of a gas; in vapor permeation (VP), these consist of a vapor. The rate of gas/vapor permeation depends on the partial pressure difference across the membrane.

Most membranes used for GS can be categorised into two types:

• porous ceramic membranes: highly voided structure with randomly distributed interconnected pores. Separation is a function of the permeate character and membrane properties. Different transport mechanisms can be involved;

• non-porous polymeric membranes: gas transport is described by the solution-diffusion mechanism, determined by molecular interactions between the gas molecules and the membrane.

Within polymer membranes, a distinction can be made between polymers in a rubbery state and those in a glassy state. With glassy polymers, diffusivity is dominant due to the rigid nature of the polymer chains. Therefore, small molecules permeate more easily. In rubbery polymers, solubility is dominant. In this case, permeability increases with increasing permeate size. In general, rubbery polymers exhibit high permeabilities and low selectivities. Glassy polymers show higher selectivities but much lower permeabilities.

The most important ceramic GS membranes are based on microporous silica, zeolite or microporous carbon. Gas separation membranes can be an alternative to more conventional separations such as cryogenic distillation, absorption processes and pressure swing adsorption. Because no phase change is needed, energy costs are lower. Operating simplicity, intrinsic modularity and the absence of additional chemicals are additional advantages. At the moment, GS membranes are applied at an industrial scale in the following areas:

• Air separation (production of nitrogen- or oxygen-enriched air);

• Hydrogen separation and recovery;

• Natural gas separations;

• Air dehydration;

• Organic vapor recovery.

New developments

Although membrane GS is already used on an industrial scale, many challenges must be overcome before this technology becomes more widely adapted. New generations of polymeric, ceramic and mixed matrix membranes need to be developed with higher thermal and chemical stability, higher selectivity and improved resistance to fouling and corrosion.

Research is being conducted on many different applications. One of these applications is the reduction or elimination of CO2 emissions from electricity power plants fuelled by coal or gas. Special membranes are needed due to the large volume flow rates, low CO2 concentrations and process parameters such as temperatures and pressures. Three different CO2 capture techniques have been identified:

• Post-combustion capture (separation of CO2 from exhaust gas);

• Pre-combustion capture (H2/CO2 separation);

• Oxyfuel combustion (supply of pure oxygen for combustion: O2/N2 separation).

For PV, much research is conducted on the use of organophilic membranes which can be used for the removal of dilute organic compounds from aqueous streams.