Apr. 22, 2002 Polymer membranes that "simultaneously and surprisingly" improve permeability while favoring bigger molecules over smaller ones have been developed by a team of researchers that includes Dr. Richard J. Spontak, associate professor of chemical engineering at North Carolina State University.
The "ultrapermeable, reverse-selective, nanocomposite membranes" are described in a paper published in the April 19 issue of the journal Science. According to the authors, the membranes should be useful in environmental remediation, seawater desalinization, biological purification and other molecular separations - including gas and petroleum production.
At NC State, the research team included Drs. Timothy Merkel and Benny Freeman, who with Spontak conducted this work in the Department of Chemical Engineering. Merkel is now at the Research Triangle Institute and Freeman has since joined the University of Texas at Austin.
The membranes are called reverse-selective because, contrary to expectations, they permit the preferential passage of larger molecules - such as butane - relative to small molecules, such as methane.
"In conventional membranes, increasing permeability invariably leads to decreased selectivity," said Spontak. "To use an analogy, if you make the holes in a net big enough, ping-pong balls, tennis balls, and basketballs will all make it through. With our membranes, we can achieve both high permeability and reverse selectivity - in other words, our net preferentially lets basketballs get through."
The research team achieved their results by embedding very fine silica particles into high-free-volume, glassy polymer membranes. As the researchers discovered, the resulting membranes behave unlike similar membranes embedded with metal oxides, carbon black or other nanoscale particles. Instead of the reduced permeability typical of "filled" membranes, the chemical engineers found both dramatically increased permeability and enhanced selectivity.
"The fact that both permeability and vapor selectivity increase when we add fumed silica to the polymer," said Merkel, "indicates that these particles modify transport properties without introducing gross defects or selectivity-destroying gaps into the membrane."
The research should benefit industries like natural-gas suppliers and petroleum processors now struggling with energy-intensive and expensive methods of separating gases.
For examples of reverse-selective membrane applications, the authors cite "the removal of higher hydrocarbons from methane in the purification of natural gas, organic monomer separation from nitrogen in the production of polyolefins, and hydrocarbon removal from hydrogen in refineries."
The NC State research team worked with Zhenjie He and Dr. Ingo Pinnau, both of Membrane Technology and Research in Menlo Park, Calif., and Drs. Pavla Meakin and Anita Hill at the Commonwealth Scientific and Industrial Research Organization in Clayton, Australia.
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