Water vapor permeation through cellulose acetate membranes and its impact upon membrane separation performance for natural gas purification

Cellulose acetate is the predominant material used in membrane separation of acid gases from natural gas and biogas. However, the sensitivity of these membranes to water vapor is not well understood. In this work, flat-sheet membranes of two different degrees of acetylation, were exposed to both dry and humidified CH4 and CO2/CH4 mixtures. Positron Annihilation Lifetime Spectroscopy experiments showed that the number of free volume elements decreased as water concentration increased, indicating pore filling effects. The size of the free volume elements declined initially, followed by an increasing trend at vapor partial pressures greater than 2.5 kPa, indicating polymer swelling. Gas permeabilities of CH4 and CO2 followed a similar trend, with an initial decline due to hindered diffusion and competitive sorption, followed by an increase as the humidity exceeded 2.5 kPa. Water vapor permeabilities increased from 11,000 to 27,000 Barrer as the water activity increased but a change in the rate of increase was also noted at 2.5 kPa. At humidities in excess of 0.8, the extent of membrane swelling was such that equilibrium was not established even after 8 h of operation. Importantly, plasticization had significantly less impact on the polymer with a higher degree of acetylation.