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Separation of carbon dioxide from a humid mixture of CO2−N2 through membranes containing immobilized solutions of Na2CO3−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 Na2CO3 concentration, CO2 partial pressure, and feed stream relative humidity (RH) were investigated. The carbonate concentration was in the range of 0−4.0 mol/dm3. The feed gas RH range was 49−100%; the sweep gas was dry helium. CO2 partial pressure (p CO 2 ,f) range was 0.007−0.77 atm. Addition of Na2CO3 increased the CO2 permeability drastically at lower carbonate concentrations; at higher Na2CO3 concentrations, this permeability increase is partly compromised by increased solution viscosity and salting-out effect. N2 permeability coefficient decreased with an increase in Na2CO3 concentration. Very high CO2/N2 selectivities were observed at high Na2CO3 concentrations. Higher CO2/N2 selectivities were observed at lower CO2 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 CO2/N2 selectivity values were observed at lower feed stream RHs. When the feed RH = 50.7%, p CO 2 ,f = 0.007 atm and the Na2CO3 concentration was 1.0 mol/dm3; the separation factor α(CO2/N2) 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.

A generation 0 poly(amidoamine) (PAMAM) dendrimer in the immobilized liquid membrane (ILM) configuration was studied using flat films and hollow fibers for CO 2 -N 2 separation. This dendrimer as a pure liquid functions as a CO 2 -selective molecular gate with highly humidified feed gas (Kovvali et al. J. Am. Chem. Soc. 2000, 122, 7594). The present work broadens the range of relative humidity of the feed gas stream (RH f ) by adding a small amount of glycerol to the pure dendrimer liquid. A 75% dendrimer-25% glycerol ILM was found to increase the operating range of RH f substantially while maintaining the CO 2 permeance and the selectivity (R CO 2 /N 2 ) close to the levels observed with a pure dendrimer ILM. The performances of pure and 75% dendrimer ILMs were found to be superior or comparable to the highest reported R CO 2 /N 2 's. This behavior is explained in terms of the charged environment in the dendrimer liquid membrane under humidified feed conditions and facilitated transport of CO 2 .

Glycerol carbonate is studied as a new physical solvent for carbon dioxide separation from CO 2 / N 2 mixtures. CO 2 -selective behavior of glycerol carbonate is explored in an immobilized liquid membrane (ILM) configuration. Pure glycerol carbonate ILMs retained their CO 2 /N 2 selectivity around 80-130 over a large range of CO 2 partial pressures. Its CO 2 selectivity was not affected by the absence of humidity in feed and sweep streams. When exposed to dry feed gas conditions, the CO 2 permeability through pure glycerol carbonate ILMs was about 100 barrer, which increased to about 350 barrer in the presence of humidified feed streams. Addition of small amounts of facilitating carriers such as poly(amidoamine) dendrimer (generation zero) and sodium glycinate appears to significantly help CO 2 facilitation at low CO 2 partial pressures. However, at high CO 2 feed partial pressures, there was a loss of selectivity with the addition of the carriers. The potential for glycerol carbonate based ILMs in CO 2 separation from different CO 2 -containing gas mixtures is discussed and compared with existing solvent/membrane configurations.

Membrane reactors have found utility in a broad range of applications including biochemical, chemical, environmental, and petrochemical systems. The variety of membrane separation processes, the novel characteristics of membrane structures, and the geometrical advantages offered by the membrane modules have been employed to enhance and assist reaction schemes to attain higher performance levels compared to conventional approaches. In these, membranes perform a wide variety of functions, often more than one function in a given context. An understanding of these various membrane functions will be quite useful in future development and commercialization of membrane reactors. This overview develops a functional perspective for membranes in a variety of reaction processes. Various functions of the membranes in a reactor can be categorized according to the essential role of the membranes. They can be employed to introduce/separate/purify reactant(s) and products, to provide the surface for reactions, to provide a structure for the reaction medium, or to retain specific catalysts. Within these broad contexts, the membranes can be catalytic/noncatalytic, polymeric/inorganic, and ionic/nonionic and have different physical/chemical structures and geometries. The functions of the membrane in a reaction can be enhanced or increased also by the use of multiple membrane-based schemes. This overview develops a perspective of each membrane function in a reactor to facilitate a better appreciation of their role in the improvement of overall process performance.