AUSTIN, Texas—A cheaper, “greener” method for purifying natural gas could result from groundbreaking membrane research by a University of Texas at Austin engineering team.
Dr. Benny Freeman, professor of chemical engineering, and Dr. Ingo Pinnau, director of materials and membrane development at Membrane Technology and Research, Palo Alto, Calif., are principal investigators for the project. Their work, which fuses nanotechnology with polymer chemistry, is reported in the April 19, 2002 issue of Science.
Natural gas — a primary heating and cooking fuel staple in millions of households across the United States — consists of methane mixed with a percentage of unwanted hydrocarbon by-products, including butane and propane. The impurities, collectively called natural gas liquids, are heavier than methane and can condense into liquid within the pipes to cause clogging. Costly and energy intensive condensation processes are used to remove them from the mix.
Freeman and Pinnau, who earned his doctor’s degree in chemical engineering from the university, use a membrane to do the same job. Their membrane employs the concept of diffusion, a process where molecules in a gaseous mixture or a liquid solution disperse, moving from regions of higher concentration to lower concentration.
The established capacity to fine-tune membranes that weed out high molecular weight undesirables opens the door to future industrial applications. One possibility under study by the California team is a process to separate out multiple impurities from natural gas as it flows through a system of pipes at high pressure.
Such a refining system would have great advantages over the processes in use, said Freeman. It is more environmentally friendly because it’s energy-efficient. Moreover, membranes are relatively inexpensive to build, compact and far less expensive to maintain. “It has no moving parts,” he said.
Ordinarily, when a membrane partition separates the high and low concentration regions, small molecules will diffuse across the barrier more readily than larger ones. But a special category of polymer membranes, called reverse selective membranes, has the opposite property of selectively passing bigger molecules. The most highly reverse-selective class of membranes — the substituted polyacetylenes — also is the most permeable. They are composed of hydrocarbon chains having a loose-packed structure that enables large vapor molecules to easily pass through them.
The researchers have succeeded in drastically improving the performance of poly(4-methyl-2-pentyne) (PMP) — a rigid, bulky substituted polyacetylene with large side-groups that prevent close packing — by introducing fumed-silica “nanospacers” during the formulation process. The extremely minute (about 1/4,000 the diameter of a human hair) silicon dioxide particles, when mixed with PMP during production, pile up in the empty spaces between the polymer chains, pushing the chains even farther apart. Membranes made from the silica-treated material are three times as permeable and twice as selective as those made from regular PMP, Freeman said.
“In traditional systems, the addition of inorganic filler decreases permeability,” he said. Previous work has focused on using large inorganic particles — at least 1,000 times bigger than fumed silica — to modify membrane properties.
“That would be like throwing in boulders,” Freeman said. “It introduces production problems. We wanted to use something comparable in size to the polymer molecules we were working with.”
The tiny, solid fumed silica not only had the right dimensions, but also enhanced PMP’s reverse selective properties beyond expectations. The researchers were able to effectively separate gaseous n-butane from a mixture of 98 percent methane and 2 percent butane.
Though the process by which it works is not entirely understood, the researchers believe the fumed silica particles occur in small enough sizes and great enough numbers to disrupt chain packing throughout the entire membrane, leaving it more open and free-flowing.
Freeman and Pinnau received the 2002 Cooperative Research Award in Polymer Science and Engineering, sponsored by the Eastman Kodak Company and presented by the American Chemical Society, in recognition of outstanding collaborative efforts between academia and industry. Their work was funded by the National Science Foundation and the U.S. Department of Energy.
For more information contact Becky Rische (512) 471-7272.
