The principle is that some components of the raw gas could be transported through a
thin membrane (!1 mm) while others are retained. The transportation of each
component is driven by the difference in partial pressure over the membrane and is
highly dependent on the permeability of the component in the membrane material. For
high methane purity, permeability must be high. Solid membrane constructed from
acetate–cellulose polymer has permeability for CO2 and H2S up to 20 and 60 times,
respectively, higher than CH4. However, a pressure of 25–40 bar is required for the
process [15].