Selective CO₂ Separation from CO₂−N₂ Mixtures by Immobilized Carbonate−Glycerol Membranes

Separation of carbon dioxide from a humid mixture of CO 2 −N 2 through membranes containing immobilized solutions of Na 2 CO 3 −glycerol in porous and hydrophilic poly(vinylidene fluoride) (PVDF) substrate was experimentally studied for use as a venting membrane in space-walk applications. The effects of Na 2 CO 3 concentration, CO 2 partial pressure, and feed stream relative humidity (RH) were investigated. The carbonate concentration was in the range of 0−4.0 mol/dm 3 . The feed gas RH range was 49−100%; the sweep gas was dry helium. CO 2 partial pressure ( p CO 2,f ) range was 0.007−0.77 atm. Addition of Na 2 CO 3 increased the CO 2 permeability drastically at lower carbonate concentrations; at higher Na 2 CO 3 concentrations, this permeability increase is partly compromised by increased solution viscosity and salting-out effect. N 2 permeability coefficient decreased with an increase in Na 2 CO 3 concentration. Very high CO 2 /N 2 selectivities were observed at high Na 2 CO 3 concentrations. Higher CO 2 /N 2 selectivities were observed at lower CO 2 partial pressure differentials. Steady-state water content in the hygroscopic immobilized liquid membrane (ILM) increases with an increase in feed stream RH. The water content in the ILM considerably affects its viscosity and the effective concentration of the carriers in the ILM; those factors determine the permeation performances of the ILM. Generally, lower permeances and greater CO 2 /N 2 selectivity values were observed at lower feed stream RHs. When the feed RH = 50.7%, p CO 2,f = 0.007 atm and the Na 2 CO 3 concentration was 1.0 mol/dm 3; the separation factor α(CO 2 /N 2 ) observed was 3440. Prolonged runs lasting 14 days showed that the ILM permeation performances were quite stable. The ILMs were also found to be stable when challenged with feed streams of very low RHs.