Polymer-based membranes applied to gas separation: material and engineering aspects

Polymers are widely used as membrane material to perform the separation of various gaseous mixtures due to their attractive permselective properties and high processability. Although the development of new robust materials is fundamental to enlarge the economic competitiveness in aggressive environments and severe operating conditions, the correct combination of multiple membrane stages in appropriately designed systems can improve significantly the performance, achieving the separation target by using existing materials. In fact, a single membrane stage requires the lowest membrane area but also produces the lowest purity in the permeate stream, whereas multiple membrane stages provide higher purity values with higher membrane surface areas. According to the specific transport properties of rubbery and glassy polymer membranes, it is possible to select an appropriate material in a sequence of membrane units that maximises the purity and the recovery of a species by means of a right share of the separation load on each stage. In this framework, some examples of gas separations of industrial interest are discussed to support this methodology.

In gas separations where the most permeable species has to be recovered, multi-stage sequences allow to obtain higher purity values with appropriate combinations of material permselectivity and applied driving force. Membrane surface requirements also depend on the distribution of the separation load. At low stage cut values, the purity of the most permeable component (CO2) in the ternary mixture increases for low θ1 values, it reaches a maximum (θ1 around 0.2) then starts to decrease at higher θ1. Membrane surface area always increases with the recovery in the first membrane stage. As higher θ values are analysed (high recovery is the separation target), the optimal performances for two membrane stages system are achieved if θ1 = θ2 condition is realised. This trend is further confirmed, since the CO2 concentration in the feed stream increases (e.g. moving from 10% to 40%). However, in these circumstances, the differences between single and two membrane stages in cascade are less significant about CO2 purity, but they remain important with reference to membrane area. The results of this study at the different investigated operating conditions have a general character and they can be used as guideline for the design of membrane stage sequences.