Membrane purifies gas

Researchers have synthesized a membrane that may purify hydrogen more efficiently than conventional chemical methods do.

The chemical reactions that produce hydrogen, used widely by the chemical industry, leave the gas contaminated with carbon dioxide, water, and hydrogen sulfide. Depending on the application, these impurities must be partially or completely removed, says Benny D. Freeman of the University of Texas at Austin.

Using membranes for hydrogen purification could require less energy than conventional methods do, Freeman says. Available membranes pass hydrogen into low-pressure chambers, leaving behind the contaminants. However, industry typically needs the gas at high pressures. The cost to recompress the gas has kept membranes from competing with conventional methods, he says.

Freeman and his colleagues devised a membrane that is permeable to the contaminating gases instead. The researchers used a polymer with chemical groups that attract the contaminants more strongly than they attract hydrogen. For example, this affinity increased carbon dioxide’s permeability relative to hydrogen but wasn’t so strong that it impeded the contaminant’s movement through the membrane, says Freeman.

The researchers placed the membrane between two chambers at different pressures. They then fed the high-pressure side with a mixture of hydrogen and carbon dioxide at temperatures that match those used in other purification methods. At –20°C and 17 atmospheres of pressure, the membrane was 30 times as permeable to carbon dioxide as it was to hydrogen, the researchers report in the Feb. 3 Science. The hydrogen largely remained in the high-pressure chamber, while the carbon dioxide migrated into the low-pressure chamber.

Tests with gas streams that contained water and hydrogen sulfide indicated that the membrane was also more permeable to those contaminants than to hydrogen, says Freeman.

Aimee Cunningham

Aimee Cunningham is the biomedical writer. She has a master’s degree in science journalism from New York University.

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