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Dr. Ho Bum Park, a Post-doc, publishes a Science paper

New Membrane Strips Carbon Dioxide from Natural Gas Faster and Better

A modified plastic material greatly improves the ability to separate global warming-linked carbon dioxide from natural gas as the gas is prepared for use, according to engineers at The University of Texas at Austin who have analyzed the new plastic’s performance.

Like a sponge that only soaks up certain chemicals, the new plastic permits carbon dioxide or other small molecules to go through hour-glass shaped pores within it, while impeding natural gas (methane) movement through these same pores. The thermally rearranged (TR) plastic works four times better than conventional membranes at separating out carbon dioxide through pores.

Dr. Ho Bum Park, a postdoctoral student in the laboratory of Professor Benny Freeman, also found that TR plastic membranes act quicker. They permit carbon dioxide to move through them a few hundred times faster than conventional membranes do – even as they prohibit natural gas and most other substances from traveling through their pores for separation purposes.

“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.”

(Author: Barbra Rodriguez, Engineering School)

Roy Raharjo wins 2007 AIChE Separations Division Graduate Student Awards

Chemical Engineering doctoral student recognized by AIChE

Roy Raharjo, a chemical engineering doctoral student, received the American Institute of Chemical Engineers (AIChE) Separations Division Graduate Student Award, which annually recognizes six outstanding graduate students from around the country. A student of Dr. Benny Freeman, the Kenneth A Kobe Professor in Chemical Engineering, Raharjo was awarded for his research in the competitive effects of vapor / gas permeation in solubility selective polymers. Raharjo is the third student from Freeman’s research group to receive the award.

Dr. Freeman wins the University CO-OP’s Hamilton Award

On March 28, 2007, Professor Freeman (center) accepted the University CO-OP’s Hamilton Award for the Best Research Paper published by a UT Faculty member in 2006. This honor, given to only one UT faculty member per year, was in recognition of the paper published by Dr. Freeman’s group in Science: Lin, H., E. van Wagner, B.D. Freeman, L.G. Toy, and R.P. Gupta, “Plasticization-Enhanced H2 Purification Using Polymeric Membranes,” Science, 311(5761), 639-642 (2006).

 Professor Freeman (center)
Professor Freeman (center)

ACS Annual Conference, San Francisco, CA. 10 – 14 September 2006

ACS 2006
San Francisco, CA, 10-14 September
Preprints of Attendees

Bryan McCloskey’s Abstract

Amphiphilic Graft Copolymers as Antifouling Coatings for Water Purification Membranes

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Hao Ju’s Abstract

Synthesis and Characterization of Surface Coated Ultrafiltration Membranes to Enhance Oil/Water Fouling Resistance

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Hobum Park’s Abstract

Water and Salt Transport Behavior through Hydrophilic-Hydrophobic Copolymer Membranes and Their Relations to Reverse Osmosis Membrane Performance

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Roy Raharjo’s Abstract

A Fundamental Study of Mixture Permeability, Solubility, and Diffusivity in Vapor Selective Polymers

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Scott Kelman’s Abstract

Enhancing the Chemical Resistance of High Fractional Free Volume Vapor-Selective Polymers

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Xiaoyan Wang’s Abstract

Effect of Free Volume on Gas Sorption in Polymeric Membranes

Xiao-Yan Wang, University of Texas at Austin
B. Freeman, University of Texas at Austin
I. C. Sanchez, University of Texas at Austin
A. J. Hill, CSIRO-CMIT, Australia

Poly(1-trimethylsilyl-1-propyne) (PTMSP), a solubility selective membrane, has the potential to be used in the natural gas separation. PTMSP ages rapidly as evidenced by the density increase with time; then the free volume decreases with time. Cavity size (free volume) distributions of PTMSP are calculated at different densities using an energetic based cavity-sizing algorithm. Free volume of PTMSP decreases with increasing densities. We also simulate solubility of CO2 and CH4 in PTMSP at different densities. Solubility shows no significant change as the density increases. This result is in contradictory with the prediction from the scale particle theory, but it is in agreement with experimental observation.

Key word: Solubility, free volume, molecular simulation, scale particle theory.

NAMS Annual Conference, Chicago, IL. 13 – 17 May 2006

North American Membrane Society Annual Meeting 2006
Chicago, IL, 13th – 17th May
Abstracts of Attendees

Brandon Rowe’s Abstract

Tracking Physical Aging of Thin Glassy Polymer Films by Ellipsometry

B. Rowe, University of Texas
D. Paul, University of Texas
B. Freeman, University of Texas

Current gas separation membranes are typically formed from glassy polymers because of their exceptional permeability-selectivity properties. Glassy polymers are non-equilibrium materials whose properties (e.g., density, permeability, etc.) spontaneously, but usually slowly, evolve over time towards an equilibrium state. This process is known as physical aging. Interestingly, the physical aging rate becomes orders of magnitude more rapid if the thickness of the film is decreased below about one micron[1]. This phenomenon is an intrinsically fascinating scientific issue, and understanding physical aging is valuable for the gas separation industry.

Commercially used membranes for gas separation have a thickness of approximately 0.1 microns or less and may continue to decrease as the technology for making these films develops. Films ranging from 400 nm to 1000 nm have recently been shown to have rapid and thickness dependent aging rates as determined by gas permeability measurements. In addition to studying the gas transport properties of polymer films, physical aging is being examined by tracking the change in density of polymer films over time using high precision ellipsometry. Ellipsometry is a sensitive optical technique for determining properties of surfaces and thin films by measuring the changes in the polarization state of light when it is reflected from a sample. The refractive index of a polymer sample, measured by ellipsometry, is related to the polymer density through the Lorentz-Lorenz equation. Studies show an increase of polymer density with time, which correlates with the reduction in size and/or concentration of free volume elements in the sample. The effect of humidity on ellipsometry measurements has been studied and a direct relationship between refractive index and relative humidity has been found. The influence of humidity on physical aging has also been investigated.

1. Huang, Y. and D.R. Paul, Physical aging of thin glassy polymer films monitored by gas permeability. Polymer, 2004. 45(25): p. 8377-8393.

Hobum Park’s Abstract

Novel Sulfonated Poly(arylene ether) Copolymer Membranes and Their Use in Reverse Osmosis: Fundamental Water and Salt Transport Study, Chlorine Stability and Anti-Fouling Characteristics

H. Park, University of Texas at Austin
B. Freeman, University of Texas at Austin
J.E. McGrath, Virginia Tech University
Z. Zhang, Virginia Tech University
G. Fan, Virginia Tech University
M. Sankir, Virginia Tech University
A. Roy, Virginia Tech University
H.S. Lee, Virginia Tech University
A Badami, Virginia Tech University

Recently, the interest in preparing new RO materials having excellent chlorine tolerance, similar or higher water flux and rejection and markedly reduced fouling relative to the conventional RO membranes is increasing. In the late 1980s, a few sulfonated polymers were reported to have promising RO properties and excellent chlorine tolerance, but the lack of control of the sulfonation process made the preparation of reproducible the materials challenging1. Recently, a systematic series of sulfonated poly(arylene ether) copolymer membranes developed at Virginia Tech have been prepared by directly copolymerizing a disulfonated monomer into the polymer backbone2. The present study focuses on exploring fundamental structure-property relationships in these materials. These polymers have excellent chlorine tolerance, low fouling in oily or protein-containing waters, and high water flux with moderate salt rejection. A fundamental study of salt diffusivity, solubility and permeability in these sulfonated copolymer membranes is providing basic information useful for molecular engineering these materials towards better property profiles.

1. R.J. Petersen, “Composite Reverse Osmosis and Nanofiltration Membranes” J. Membrane Sci. (1993), 83, 81-150. 2. M.A. Hickner, H. Ghassemi, Y.S. Kim, B. Einsla and J.E. McGrath, “Alternative Polymer Systems for Proton Exchange Membranes”, Chem. Rev. (2004), 104, 4587-4611.

Roy Raharjo’s Abstract

A Fundamental Study of Mixture Permeability, Solubility, and Diffusivity in Vapor Selective Polymers

R. Raharjo, University of Texas at Austin
B. Freeman, University of Texas at Austin
E. Sanders, Air Liquide, Newport, DE

Membrane separation technology has recently emerged as a potential alternative technique to remove higher hydrocarbons (C3+) from natural gas1. The material that could be suitable, and, in fact, is currently being considered, for such separation is silicone rubber or poly(dimethylsiloxane) (PDMS). Aside from its high gas permeabilities (i.e., highest among rubbery polymers), this polymer is more permeable to the larger, more condensable higher hydrocarbon than to smaller, less condensable permanent gases, such as methane. As a result, methane, the major constituent in natural gas, can be kept at high pressure, which eliminates the cost of recompression that would be incurred if a methane-selective membrane were used for this separation.

The gas sorption and transport properties in PDMS have been studied in the past. Most of the references in the literature, however, report only its pure gas properties; only a few actually investigate its mixture properties. To accurately estimate the membrane separation performance, the use of mixtures in the study is required. This paper presents the n C4H10/CH4 mixed gas permeability, solubility, and diffusivity in PDMS at various temperatures from -20oC to 50oC. The dilation isotherms of the mixture are also reported to complement the mixture sorption data.

The CH4 permeability, solubility, and diffusivity increase as the n C4H10 fugacity in the mixture increases. The n C4H10 sorption and transport properties in mixtures are unaffected by the presence of CH4 and are similar to those in pure gas conditions. The overall n C4H10/CH4 selectivity increases as n C4H10 activity in the mixture increases and temperature decreases. This report represents the first combined presentation of gas mixture permeability, solubility, and diffusivity in PDMS, which is the most widely used vapor separation polymer.

Another polymer whose properties will be discussed is poly(1-trimethylsilyl-1-propyne) (PTMSP), an ultra-high free volume, solubility selective, glassy polymer. These studies will help illustrate the fundamental mechanism of the competitive permeation, sorption, and diffusion in glassy polymers.

Xiaoyan Wang’s Abstract

Physical aging in poly[1-(trimethylsilyl)-1-propyne] (PTMSP)membranes: Effects on free volume and gas permeability

X. Wang, University of Texas at Austin
R. Raharjo, University of Texas at Austin
B. Freeman, University of Texas at Austin
I. Sanchez, University of Texas at Austin

Poly(1-trimethylsilyl-1-propyne) (PTMSP), the most permeable polymer known, is a fast aging material. The permeability of PTMSP to gases and vapors decreases dramatically with physical aging. Cavity size (free volume) distributions were calculated in as-cast and aged PTMSP using an energetic based cavity-sizing algorithm. The large cavities found in as-cast PTMSP disappear in aged PTMSP, which is consistent with the positron annihilation lifetime spectroscopy (PALS) measurements. We also characterized the connectivity of cavities both in as-cast and aged PTMSP membranes. Cavities are more connected in as-cast PTMSP than in aged PTMSP. The average cavity sizes calculated from computer simulation are in good agreement with PALS measurements. The transport and sorption properties of gases in as- cast and aged PTMSP are also measured by molecular simulation. Computer simulations showed the decrease of permeability and the increase of permeability selectivity in PTMSP membranes with physical aging, which agrees withe experimental observations. The reduction in gas permeability with physical aging results mainly from the decrease of diffusion coefficients. Solubility coefficients show no significant changes with physical aging.