Mixed Gas Transport Study Through Polymeric Membranes: a Novel Technique

Abstract

The gas transport and separation properties of polymers have been successfully exploited in commercial ventures. Industrial applications employing membrane processes range from production of pure gases to barrier coatings for protection against environmental elements. Membrane separations are simple, energy efficient processes, which can be economically competitive with traditional separation technologies. <P> Membrane separation and permeation characteristics for a particular mixed gas system is typically calculated from single-component transport parameters, namely, diffusion coefficients and solubility constants. In certain gas systems involving gaseous or vapor mixtures, where mass transport is affected by coupling effects or competition between penetrants for unrelaxed free volume, such calculations can lead to erroneous estimates of the membrane separation efficiency. Attempts to study the true transport phenomena effective during mixed gas permeation through membranes have been restricted due to experimental limitations. Also, the absence of rigorous theoretical models hinders the complete understanding of the transport phenomena. <P> The current research involved design and development of an experimental set-up for observing mixed gas permeation through non-porous membranes with real time resolution. The technique employs a gas chromatograph as the selective detector for monitoring the variation in gas concentration, as the gases permeate through the membrane. The same set-up can also be used for conducting single gas permeation experiments. The novelty in the experimental set-up is the In-line sampling interface, used for injection of permeate gases in the GC without introducing any leaks in the permeate volume. Also, a novel data cropping technique is used to elucidate the transport properties of gases through membranes under mixed gas permeation conditions. Mixed gas feed concentration studies performed on a rubbery polymer (PDMS: poly dimethyl siloxane) showed no coupling effects. However, with a glassy polymer (NEW TPI: thermoplastic polyimide), the synergistic effects of gases is observed to play a major role in altering the gas transport and separation properties of the membrane.