The Impact of Impurities on the Performance of Cellulose Truacetate Membranes for CO2 Separation

THE IMPACT OF IMPURITIES ON THE PERFORMANCE OF CELLULOSE TRUACETATE MEMBRANES FOR CO2 SEPARATION
Hiep Lu, Colin Scholes, Shinji Kanehashi, Sandra Kentish

Cellulose triacetate (CTA) membranes have been widely applied in natural gas processing and have competitive CO2/N2 selectivity for both pre and post-combustion carbon capture. This project has investigated the effects of a range of gas impurities on CTA membrane performance in these applications. The membranes were relatively stable when exposed to liquid water at pH 3 and pH 7 with a 30% increase in CO2 and N2 permeability and no loss in CO2/N2 selectivity. However, the membrane failed at pH 13 due to hydrolysis of the CTA polymer chains. Similarly, the membrane performance declined significantly when exposed to 0.74 kPa NOx at 22oC over a 120 day aging period. This was due to the reaction of trace NO2 in the gas mixture with the alcohol functional groups within the membrane structure.

In natural gas processing, the performance of CTA membranes can also be affected by ethylene glycol, which can be entrained into the membrane separation unit from the upstream dehydration unit. It was found that the glycols initially absorbed into the membrane reducing the permeation of He, CO2 and CH4 by a “pore-blocking” mechanism, but after a period of time, plasticised the membranes and enhanced the transport of CO2 and CH4. The membrane performance recovered when the glycols were removed from the polymer using a methanol wash. Hydrogen sulphide in the raw natural gas might also affect CTA membrane performance in both natural gas processing and pre-combustion carbon capture applications. The permeability of H2S across a range of partial pressures (up to 0.75 kPa) and temperature (22oC - 80oC) was measured. The membrane was found to be stable at 0.75 kPa H2S at 22oC for up to 300 days. At low CO2 partial pressure (0.75 bar), the permeation of CO2 and CH4 through the CTA membrane declined when adding toluene and xylene of up to 0.5 vapour activity. However, the CTA membrane was plasticised when toluene vapour activity increased above 0.5 activity. At high CO2 pressure (7.5 bar), the membranes were plasticised by both hydrocarbons at 0.3 vapour activity. The PhD thesis on this topic was submitted in January 2018.