274808 Gas Transport of Paraffins and Olefins in Liquid Crystal Membranes

Tuesday, October 30, 2012: 4:15 PM
403 (Convention Center )
Feras Rabie, Chemical Engineering, Virginia Polytechnic Institute and State University, Blacksburg, VA and Stephen M. Martin, Chemical Engineering, Virginia Tech, Blacksburg, VA

Gas Transport of Paraffins and Olefins in Liquid Crystal Membranes

Abstract

A solubility induced phase separation (SIPS) method was used to form 8CB and MBBA liquid crystal domains in a polysulfone matrix.  The membranes were characterized for their temperature dependent phase behavior and structural characteristics using DSC, POM, and FE-SEM.  Paraffin and olefin separations were investigated at different temperatures and different liquid crystal phases using a mixed gas permeation apparatus.  Propane and propylene were chosen to minimize size effects of diffusion and sorption.

            We studied the effects of permeation and selectivity on intermolecular ordering of the small molecular liquid crystal.  The liquid crystal phase plays an important role for diffusion and solubility on the solute.  The crystalline state exhibited the lowest permeability with an increase in both propylene transport and propylene selectivity over propane as the crystalline domain sizes increased.  Transport through the liquid crystals at their mesophase resulted in both an increase in permeability and selectivity.  The contributing factor to this phenomenon was concluded to be solubility selectivity of propylene over propane due to enhanced intermolecular interactions between the ordered domains and the more polar propylene solute.  A further increase in temperature resulted in a two fold magnitude increase in permeability while maintaining a selectivity of 4.

            A cyanobiphenyl liquid crystal attached to a butadiene backbone with a flexible alkane spacer was synthesized for the investigation of its gas transport properties.  Room temperature transport studies showed a helium selectivity of 60 over methane.  This was primarily due to size selective diffusion across the membrane with very little solubility or sorption.  The effect of propane and propylene transport on pressure and phase dependence was tested.  Transport for temperatures under the LCPs glass transition was dominated by diffusivity.  A transition to a smectic liquid crystal phase resulted in an increase in permeability and selectivity.  The contributing factor was the increase of propylene solubility compared to that of propane.  A feed pressure increase resulted in an increase in permeability selectivity, which was also due to an increase in solubility selectivity.  Mixed gas transport experiments correlated well with single gas transport.

            A series of methacrylate liquid crystal polymers with nitro azobenzene and cyano biphenyl mesogens were synthesized and characterized.  These liquid crystal monomers were copolymerized with 2-ethylhexyl acrylate (2-EHA) using a free radical polymerization method.  Liquid crystal phases were present for LC content of over 50 mol% for the cyanobiphenyl and 60 mol% for the nitro azobenzene.  Mesophase stability increased with increasing LC content.  A method to form acrylate networks for the fabrication of stable membranes was developed and partially characterized.  We will test its transport properties in the future.


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See more of this Session: Membranes for Gas Separations II
See more of this Group/Topical: Separations Division