In natural gas filtration, cellulose triacetate (CTA) and cellulose diacetate (CDA) are still the leading polymers, with inherent selectivity for CO2/CH4 of 20-25.
One of the main issues with polymers used in natural gas filtration is plasticization by the acid gases CO2 and H2S. The hydroxyl functionality of CDA can be used to perform chemical modifications to allow cross-linking between the polymer chains. Such modifications can increase the service life of these membranes significantly, albeit at the cost of permeability (as described by Robeson). A graphical depiction of gas (blue spheres) traveling through an Eastman membrane material polymer matrix is shown here.
In this application, both spiral wound and hollow fiber (HF) modules made with CTA/CDA are in commercial use.
The following figure shows a micrograph of an asymmetric HF membrane cellulose. Cellulose acetates are known to produce an open, porous substructure, allowing fast transport in both flat sheet for spiral-wound modules and HF membrane modules.