Improving blowout pressure in supported ionic liquid membranes for light paraffin fractionation

Abstract

Gas from shale reservoirs provides the U.S. with relatively clean-burning fuel and important precursors for petrochemicals. However, due to the similar thermophysical properties between shale gas components, energy-intensive cryogenic distillation is used to separate the heavier hydrocarbons from methane. Compared to distillation, membrane-based technology could yield an order of magnitude improvement in energy efficiency. Nevertheless, this requires membranes capable of operating at high transmembrane pressures. Here, we report supported ionic liquid membranes (SILMs) that operate within industrially relevant operating pressures (i.e., 7–30 bar) without experiencing blowout—a loss of ionic liquid (IL) and subsequent membrane defects at elevated transmembrane pressures. By considering the effect of the pore size distribution on capillary pressure, described by the Young-Laplace equation, we developed polyethersulfone-based SILMs that operate above 16 bar. To our knowledge, this is the highest reported blowout pressure for a SILM using a commercial membrane support. Pure-gas permeation experiments indicate promising C₃H₈/CH₄ permselectivity values as high as 4 for SILMs with larger pores (30–100 nm). Yet, this reverse-selectivity is not observed for SILMs with smaller pores (4 nm), partly due to their lower surface porosity, which results in significantly higher mass transfer resistance from the polymeric support and, consequently, reduced permselectivity. Additionally, trends in glass transition temperature and melting point of the imbued ILs, as well as increased CO₂/N₂ and CO₂/CH₄ permselectivity suggest that nanoconfinement effects may also play a significant role in the separation performance of these membranes.