Gas Separation Membranes

‌Development Status

Gas separation membranes are a rapidly developing technology. Different polymer membranes have varying permeability and selectivity for different gas molecules, allowing for the selective separation of specific gases from mixtures. For example, they can be used to collect oxygen from air, recover hydrogen from ammonia synthesis tail gas, and separate hydrogen and carbon monoxide from petroleum cracking gas.

Chemists at the University of California, Los Angeles, have created a conductive organic film made from polyaniline. This polymer can incorporate charged atoms, altering the film’s permeability based on the amount of dopant. This film allows oxygen to pass through faster than nitrogen, carbon dioxide faster than methane, and hydrogen faster than nitrogen, making it cost-effective for producing oxygen and nitrogen. It can also potentially be used to remove pollutants from automotive and industrial exhaust gases.

Research on gas separation membranes mainly focuses on oxygen-enriched membranes. These polymers need to have both high permeability and high selectivity. General Electric has developed a copolymer of polycarbonate and silicone for separation membranes, which can produce 40% oxygen-enriched air in a single stage. Using oxygen-enriched air instead of regular air can significantly improve the efficiency of combustion devices and reduce pollution. Additionally, there is ongoing development of an underwater breathing apparatus that extracts dissolved oxygen directly from seawater.

Working Principle

When the concentration of VOCs (volatile organic compounds) in gas is very high (10,000 ppm), membrane separation is an effective and feasible emission control method. The process typically involves compression condensation and membrane separation. After the gas is compressed and condensed, the exhaust enters the membrane separation unit. Here, the non-condensed organic gases pass through a semi-permeable membrane, which is designed to be 10-100 times more permeable to VOC vapors than to air. The concentrated gas is then recycled back into the system for re-compression and condensation, while the purified gas is discharged.

Advantages

Membrane separation can handle various pollutants, including benzene, toluene, xylene, methyl ethyl ketone, chloroform, trichloroethylene, dichloromethane, and vinyl chloride. This technology is particularly promising for purifying low-boiling-point organic compounds and chlorinated organic compounds that are difficult to condense or adsorb with activated carbon. Unlike carbon adsorption, membrane separation eliminates the need for desorption and further treatment of concentrated gas. Initially used for concentrations above 1% and gas volumes less than 350 m³/h, this technology is expected to be applicable to a wider range of VOC concentrations and gas volumes.