The impact of impurities on the performance of cellulose triacetate membranes for CO2 separation

Abstract

Natural gas and coal are the essential energy resources that will continue to occupy over 50% the global electricity market in the coming decades. The natural gas streams normally contain several components that require removal so that the fuel meets pipeline specifications. Membrane separation technology is an outstanding approach for gas processing with advantages in land footprint and energy efficiency. In particular, cellulose triacetate (CTA) membrane have been widely applied in natural gas processing for decades and remain the dominant material on the market share due to their competitive gas separation performance and acceptance by industry. However, the raw natural gas streams also contain several impurities that can negatively affect the performance of the CTA membrane unit. Although several studies on gas separation performance of CTA membrane have been conducted, the impact of impurities on the membrane performance is not fully understood. Cellulose triacetate membranes also have competitive CO2/N2 selectivity and are thus a prospective candidate for post-combustion carbon capture. However, studies on the impact of impurities in the flue gas, including liquid water of variable pH, sulphur oxides and nitrogen oxides, on the gas separation performance of CTA membrane are very limited. In this thesis, the impact of solutions of variable pH on CTA membranes was studied by exposing the dense membranes to solutions of pH 3, 7 and 13 solutions for up to 60 days. It was found that the membranes were relatively stable when exposed to 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. Interestingly, the CTA membrane was more selective for SO2 than CO2 and N2 and stable in 0.75 kPa SO2 at 22oC over a 100 day aging period. The results suggest that CTA is a viable membrane material for post-combustion capture if it can form into an ultrathin film to increase permeance. 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. In this thesis, the impact of two common ethylene glycols, monoethylene glycol and triethylene glycol, on the gas separation performance of CTA membranes was investigated. 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. This plasticisation effect had less effect on He, which may be due to the lower solubility of He in these glycols which limited the transport of this gas through the swollen membrane structure. Interestingly, the membrane performance recovered when the glycols were removed from the polymer using a methanol wash. The findings highlighted the potential to recover the membrane performance when glycol flooding occurs in industrial plants. Hydrogen sulfide in the raw natural gas might also affect CTA membrane performance. This impurity is also of concern in pre-combustion carbon capture. To fulfil the gap of knowledge in the literature, this thesis studied the permeability of H2S across a range of partial pressures (up to 0.75 kPa) and temperature (22oC - 80oC). At 0.75 kPa H2S at 22oC, the CTA membrane showed stable CO2 permeability for up to 300 days which confirmed the long-term resistance of this material to the experimental H2S conditions. Other impurities that might challenge the performance of CTA membranes in natural gas processing and pre-combustion capture are condensable aromatic hydrocarbons. In this thesis, the performance of CTA membranes at 35oC in the presence of toluene and xylene with variable vapour activity was studied. At low CO2 partial pressure (0.75 bar), the permeation of CO2 and CH4 through the CTA membrane declined when adding toluene and xylene up to 0.5 vapour activity. However, the CTA membrane was plasticised when toluene vapour activity increased above 0.5 activity. A similar impact was not clearly observed in the case of high xylene vapour activity. At high CO2 pressure (7.5 bar), the membranes were plasticised by both hydrocarbons at 0.3 vapour activity. This finding demonstrated the co-operative effect of CO2 and condensable hydrocarbons on the CTA membrane. In addition, the sorption and permeability of toluene and xylene through the CTA membrane at vapour activity up to 0.8 at 35oC was also recorded. Overall, the thesis demonstrates that cellulose triacetate membrane is an outstanding material for CO2 separation in natural gas processing, pre- and post-combustion capture with high gas selectivity and resistance to most impurities in these industrial gas streams.