Polymer Electrolyte Membranes for Liquid Olefin-Paraffin Separation
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Olefin/Paraffin separation, traditionally carried out by cryogenic distillation, is difficult to achieve due to the similar size and volatility of the components. Recently, many studies have explored membrane separation methods that utilize a metal ion to facilitate preferential olefin transport across the membrane. However, much of this work focuses on smaller molecules, C2-C3, which are gaseous at room temperature, while little work has been done studying separation of larger molecules, C5 and greater, which are generally liquid at room temperature. The processes developed to separate small molecules are not necessarily directly applicable to separate larger molecules. A polymer electrolyte membrane consisting of an active layer of polyethylene oxide (PEO) and silver tetrafluoroborate (AgBF4) has shown high selectivity for separating gaseous olefin/paraffin mixtures. The current project investigates the feasibility of applying this membrane to the separation of pentene and pentane (liquid C5 olefin and paraffin). Process variables investigated are the: pure component permeability ratio, equilibrium sorption uptakes, pure component diffusivities, and stable membrane lifetime. Permeation tests on individual species (n-pentane and 1-pentene) were performed in two operating modes with membranes of varying silver concentrations: direct liquid contact to the membrane, and vapour contact to the membrane. The vapour contact mode showed improved membrane stability in comparison to the liquid contact mode. The olefin/paraffin permeability ratio increases with increasing silver content in the membrane, however, the membrane selectivity is much lower than that achieved with smaller olefin/paraffin pairs. Selective chemical interactions between pentene and the membrane were observed, as the pentene sorption uptake is higher than that of pentane. In addition, a residual fraction is observed – a fraction of the pentene does not desorb from the membrane at ambient conditions – indicating a permanent or semi-permanent interaction. Desorption of pentane is determined to follow a Fickian diffusion model, while desorption of pentene appears to be governed by pseudo-second order kinetics.