Cross-linked copolyimide membranes for the separation of gaseous and liquid mixtures

Cross-linked polymer structures gain increasing attention as membrane materials because they can fullfill the demands for industrial applications. Thereby, not only good separation characteristics but also high temperature stability and chemical resistancy are required. Furthermore, it is important that the membrane materials be plasticization resistant, because it is found that this causes strong increasing permeability with a drastic loss in selectivity. Plasticization effects occur with polyimide membranes in the presence of high CO2 concentrations, hydrocarbons as propylene, propane, or aromatics. Unfortunately, these components are present in mixtures with high relevance being separated economically by membrane units or hybrid processes. In this article, the advantages of cross-linked 6FDA (4,4'hexafluoro isopropylidene diphthalic acid anhydride)-copolyimides are discussed based on experimental results for the separation of propylene/propane, benzene/cyclohexane, and high-pressure CO2/CH4 mixtures. Additionally, opportunities for implementing the membrane units in conventional separation processes are discussed.