Liquid, Vapor, and Gas Transport Properties in Membranes and Films

J. Pellegrino, X. Yi, J. Portnoy, K. Nerbonne, and T. Rasco; O. Stange (GKSS Research Center); and M. Guiver (National Research Council of Canada)

Objective: To develop improved measurement methods for obtaining diffusion and solubility of liquids, vapors, and gases in membranes and films, to elucidate transport mechanisms and quantitative structure/transport property prediction methods for membrane materials (especially polymeric) based on high quality measurements of sorption and transport in several well-characterized systems, and to compile property data on industrially important materials used for membrane-based separations.

Problem: Although polymeric and inorganic materials are used in membrane and adsorptive separation processes, a significant barrier to the optimum use of existing materials and development of new materials is the lack of predictive capabilities for the transport properties of mixtures in any selected material. Improved processes for obtaining high- purity oxygen and nitrogen from air, processing natural gas, recovering hydrogen from refinery streams, recovering and purifying olefin streams, and purifying water are examples of important industrial uses of membranes.

Approach: This program has measurement, modeling, and database components. Measurements of liquid, vapor, and gas diffusion and sorption in thin layer films are critical for development of techniques to predict membrane transport properties. These measurements provide a means to include the effects of both chemical and structural subgroups in the material, and ultimately, to delineate rational design criteria for separations. Through our collaborations we have access to materials for which chemistries are well studied and/or can be varied in well- defined ways. In addition, the polymers currently under study (polypropylene, cellulose acetate, polysulfones, polyperfluorosulfonic acid, polytrimethylsilyl-propyne, and polyaniline) represent both commercial and newly developed materials. This research program also includes the development of an internet-accessible database of polymeric material properties important for membrane separation design.

Results and Future Plans: A flow cell equipped with attenuated total reflectance (ATR)-FTIR and an accurate flow control and measurement system has been developed to measure diffusion of multicomponent mixtures in films. Two techniques for making measurements on pre-made films (necessary for making measurements on commercial membranes) have been developed. Using one of the techniques, measurements of water and acetone mixtures diffusing from the liquid state into a commercial polypropylene (PP) film have shown that (1) water must lose its H-bonding before entering PP and (2) in a mixture, acetone diffusion is coupled with water and speeds up the process. The second technique, using a thin (<0.5 µm) adhesive layer (e.g., of a mineral or fluorochemical oil) to maintain good optical contact between a pre-made film and the ATR crystal, will facilitate measurements with gas mixtures. The sorption program includes four sorption apparatus (based on pressure decay methods) that, during the past year, were used to determine the film density of polyvinyl alcohol-modified with cyclodextrin side groups. A surface acoustic wave device will be brought into service during the coming year with the ultimate objective of combining it with the ATR-FTIR flow cell for multicomponent gas and vapor transport measurements. An initial version of the membrane technology database, containing unevaluated gas transport properties on hundreds of polymers, has been completed and is accessible via the internet.* This work begins to address a need within the chemical engineering community for comprehensive, critically evaluated information on separation membranes, and how these membranes interact with important chemical feedstock components. The database includes permeability, solubility, and diffusion coefficients; ideal and mixed gas separation factors; temperatures; primary reference; monomer repeat unit structure; and common names and abbreviations. Future work will expand the number of polymers and include predictive models.