Thin-film Membranes promise a dramatic reduction in Energy Use

Researchers at the University of Minnesota (Minneapolis/St. Paul; www.umn.edu) have produced thin-film zeolite membranes that are only 200 nm thick, compared to 5–10 μm for conventional zeolite membranes. The new membranes could cut energy and capital costs by 90% for the production of petrochemicals, says Michael Tsapatsis, a professor of chemical engineering and materials science.

Instead of 1,000 m2 of membrane you would need only 100 m2 to do the same job, he says.

As in the case of conventional membranes, the new membranes consist of a layer of zeolite on a porous-metal support structure. Also, the synthesis is similar in that zeolite crystals are grown hydrothermally, in an aqueous solution, by mixing silica and a structure-directing agent (SDA) — an organic cation or surfactant that forms pores in the zeolite.

The SDA is then removed by calcination to obtain porous crystals.

The difference, says Tsapatsis, is that the crystal growth is arrested before crystal formation is completed, leaving layers of porous clay floating in solution. Each layer is about 1-μm square and 2-nm thick. These layers are filtered from the water, suspended in a solvent (for example, toluene), and separated by a combination of centrifugation and sonification.

The layers are deposited on a porous metal support, then calcined at about 500°C to remove all organic material.

So far, the researchers have produced membranes with pore sizes of 3 and 6 Å.

They have used the former to separate hydrogen and helium and the latter to recover p-xylene and o-xylene from mixed xylenes. “Our next step,” says Tsapatsis, “is to develop membranes between three and six Angstroms for the separation of other petrochemicals, such as ethylene and ethanol.”

Ref: CHEMICAL ENGINEERING - WWW.CHE.COM - NOVEMBER 2011