Rajeev presents his paper entitled, "Plasticization-resistant membranes for CO2 removal from natural gas" at the 2004 North American Membrane Society (NAMS) meeting in Honolulu, Hawaii. Co-authors Rajeev Prabhakar and Benny D. Freeman (UT Austin), and I. Roman (MEDAL L. P. / Air Liquide).
Abstract
Polymer membrane-based separation of CO2 from natural gas is gaining attention due to the compact size, low energy requirement, ease of use and scale-up, and potential for offshore installation of membrane systems. Current membranes, however, suffer losses in separation performance under field conditions due to plasticization of the polymer. This is caused mainly by sorption of CO2 and higher hydrocarbons into the polymer membrane in amounts sufficient to increase polymer chain mobility and reduce its ‘size-sieving’ ability. The result is a loss of the product, methane, from the feed stream into the low pressure permeate stream. This requires either a second membrane module to recover the lost product and recompress it to pipeline specifications, or to simply accept the loss – both of these can be expensive options.
Efforts have been made to suppress plasticization of current polymer membranes by blending with other polymers, thermal treatment of membranes and crosslinking of polymer chains.1-6 While some success has been achieved in delaying the onset of plasticization to higher partial pressures of the plasticizing penetrants, these approaches essentially “treat the symptom” rather than the fundamental cause. An alternate approach is to address the core issue of high solubility of higher hydrocarbon compounds by considering polymeric materials with inherently low solubility for these compounds. Completely fluorinated polymers can exhibit low solubility for higher hydrocarbons. In this presentation, the experimentally-determined pure and mixed gas transport properties of these materials will be presented. Also, their potential as plasticization-resistant membranes will be discussed based on CO2-CH4 mixed-gas permeation results up to industrially-significant CO2 partial pressures. For example, Hyflon AD 80, which is a copolymer containing 80 mole% 2,2,4-trifluoro-5-trifluoromethoxy-1,3- dioxole (TTD) and 20 mole% tetrafluoroethylene (TFE), exhibits very high CO2 permeability (of the order of 200 Barrers) and reasonable CO2/CH4 selectivity (in the range of 9 to 12) for feed streams up to 800 psia containing 20% CO2.
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