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This new polymer membrane mimics naturally
occurring pores found within cell membranes. The unique hourglass
shape effectively separates molecules based on their shape.
Separation is more efficient, requiring less energy. Applications
include water and gas purification. The separation of carbon
dioxide (gray and red) from methane (gray and white) is
illustrated.
Image courtesy of Commonwealth Scientific
and Industrial Research Organization (CSIRO).
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�If this material was used instead of
conventional cellulose acetate membranes, processing plants would
require 500 times less space to process natural gas for use because of
the membranes� more efficient separation capabilities, and would lose
less natural gas in their waste products,� said Freeman, noting that,
pound for pound, natural gas has a worse global warming impact on the
atmosphere than carbon dioxide.
When developed for commercial use, the plastic could also be used to
isolate natural gas from decomposing garbage, the focus of several
experimental projects nationally. The TR plastic described in
tomorrow�s issue of Science could also help recapture carbon dioxide
being pumped into oil reservoirs in West Texas and elsewhere, where it
serves as a tool for removing residual oil.
Freeman is a co-author on the Science article about the research. He
holds the Kenneth A. Kobe Professorship and Paul D. and Betty
Robertson Meek & American Petrofina Foundation Centennial
Professorship of Chemical Engineering. Elizabeth Van Wagner, a
graduate student in chemical engineering, also is a co-author in
Austin.
Park, lead author of the article, initially engineered the membrane
while at Hanyang University in Korea. As a research assistant in the
lab of Professor Young Moo Lee, Park investigated whether plastics
made of rings of carbon and certain other elements could work well at
separating carbon dioxide out of gas wastes produced by power plants.
Separating the greenhouse gas from other gases at power plants must
occur at high temperatures, which usually destroy plastic membranes.
Lee and Park not only found that the TR plastic could handle
temperatures above 600 degrees Fahrenheit, but that the heat
transformed the material into the better performing membrane described
in Science. That membrane breaks a performance barrier thought to
affect all plastic membranes.
�I didn�t expect that the TR plastic would work better than any other
plastic membranes because thermally stable plastics usually have very
low gas transport rates through them,� Park said. �Everyone had
thought the performance barrier for plastic membranes could not be
surpassed.�
Park joined Freeman�s laboratory in Austin because of the professor�s
expertise in evaluating membranes. Park then verified that the TR
plastic separated carbon dioxide and natural gas well. Natural gas
that is transported in pipelines can only contain 2 percent carbon
dioxide, yet often comes out of the ground with higher levels of the
gas, requiring this separation step.
�This membrane has enormous potential to transform natural gas
processing plants,� Freeman said, �including offshore platforms, which
are especially crunched for space.�
To better understand how the plastic works, Dr. Anita Hill and her
group at Australia�s national science agency analyzed the material
using positron annihilation lifetime spectroscopy. The method used at
the Commonwealth Scientific and Industrial Research Organization
suggested the hour-glass shape of the pores within the plastic, which
are much more consistent in size than in most plastics.
The pores appear and disappear depending on how often the chains of
chemicals that make up the plastic move.
�The plastic chains move, and as they do, they open up gaps that allow
certain gas molecules to wiggle through the plastic,� Freeman said.
Freeman and Park intend to learn more about how these mobile pores
behave as they develop the TR plastic for commercial purposes.
Park said, �These membranes also show the ability to transport ions
since they are doped with acid molecules, and therefore could be
developed as fuel cell membranes. However, a lot of research still
needs to be done to understand gas and ion transport through these
membranes.�
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