Strong and weak points of membrane systems applied to gas separation

Membrane operations are being asserted as a tangible option to conventional technologies in gas separation due to an advanced flexibility to tolerate fluctuations in feed composition and flow rate, operational simplicity and reliability, absence of moving parts, particularly when weight and space requirements are important. Most of the commercial synthetic membranes are made of polymers, characterised by specific transport properties according to their physical state. However, in order to keep the economic competitiveness also in aggressive environments and severe operating conditions, it is necessary to manufacture membranes starting from robust materials with intrinsic high permselective properties. Nanoporous inorganic molecular sieves are potential interesting materials for their peculiar transport, mechanical and physical properties significantly higher than polymers. Processing difficulties, reproducibility problems during the preparation step and high costs still hinder their direct application as membranes at an industrial level. Although the development of new permselective materials is fundamental to meet the purification request, the correct combination of multiple membrane stages in integrated systems can improve significantly the performance. Therefore, according to the specific separation requests, it is possible to select the appropriate membrane material arranging a sequence of membrane units that maximises the, purity and the recovery of a component with a right share of the separation load on each stage.