(77a) Porous Crystalline Molecular Sieve Membranes for Kr/Xe Separation

Chemical separations account for approximately half of US industrial energy use, and up to 15% of the nation’s total energy consumption. Non-thermally driven membrane based separations can make industrial relevant separations more energy efficient. In particular, the separation of Krypton (Kr) from Xenon (Xe) is an industrially challenging separation. Kr and Xe are widely used in fluorescent light bulbs. High-purity Xe, has been used in commercial lighting, medical imaging, anaesthesia and neuroprotection. Effectively separating Kr from Xe in nuclear reprocessing plants, would lead to a considerable reduction in storage costs, and in potential revenue generated from the sale of pure Xe. The conventional method to separate these two gases is fractional distillation at cryogenic temperatures, which is an energy intensive process. In this respect, membrane technology could play a key role in making this separation less energy intensive and therefore economically feasible. Membrane separation processes have several advantages over conventional fractional distillation; for instance, it is a viable energy-saving method, since it does not involve any phase transformation, furthermore, the required membrane process equipment is simple, easy to operate, control and scale-up. In particular, if prepared in membrane form, microporous crystalline molecular sieves (such as zeolites, metal organic frameworks, and aluminophosphates) combine highly desirable properties, such as uniform micropores, high surface areas, and exceptional thermal and chemical stability, making them ideal candidates for challenging molecular gas separations, such as Kr/Xe separation. In this presentation, I discuss three recent successful examples of crystalline microporous molecular sieve membranes (SAPO-34, ZIF-8, and AlPO-18) that effectively separate Kr/Xe gas mixtures. To our best knowledge these membranes are the first examples of any microporous crystalline membrane displaying effective separation for Kr/Xe gas mixtures at industrially relevant feed compositions. Fundamentally, I discuss the separation mechanisms involved: molecular sieving, competitive adsorption, and difference in diffusivities.