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The phase separation phenomena in ternary solutions of polysulfone (PSI) in mixtures of a solvent and a nonsolvent (N,N-dimethylacetamide (DMAc) and water, in most cases) are investigated. The liquid-liquid demixing gap is determined and it is shown that its location in the ternary phase diagram is mainly determined by the PSf-nonsolvent interaction parameter. The critical point in the PSf/DMAc/water system lies at a high polymer concentration of about 8~o by weight. Calorimetric measurements with very concentrated PSf/DMAc/water solutions (prepared through liquid-liquid demixing, polymer concentration of the polymer-rich phase up to 60%) showed no heat effects in the temperature range of -20°C to 50°C. It is suggested that gelation in PSf systems is completely amorphous. The results are incorporated into a discussion of the formation of polysulfone membranes.

Low natural gas prices are contributing to rapid growth in natural gas combined cycle (NGCC) power production in the United States. CO2 capture from the exhaust gas of these plants is complicated by the relatively low CO2 concentration in this flue gas (3%–4%). A membrane process using incoming combustion air as a sweep stream in a selective exhaust gas recycle configuration can be used to preconcentrate CO2 from 4% to 15%–20% with almost no energy input. Depending on the process configuration, the selective recycle membrane design reduces the minimum energy of a CO2 capture step by up to 40%. An all-membrane design using a capture step in series with a selective recycle membrane can capture 90% of CO2 from an NGCC power plant using less energy and at a lower cost than the base-case amine process analyzed by the U.S. Department of Energy. The current state-of-the-art membranes for use in this process have a CO2 permeance of 2200 gpu and a CO2/N2 selectivity of 50. Higher CO2 permeance will improve the economics and reduce the footprint of a membrane CO2 capture system, while higher CO2/N2 selectivity is of less benefit, because the process is limited by the affordable pressure ratio.

The characteristic permeate flux behaviour in ultrafiltration, i.e., the existence of a limiting flux which is independent of applied pressure and membrane resistance and a linear plot of the limiting flux versus the logarithm of the feed concentration, is explained by the osmotic pressure model. In the mathematical description presented here, a quantity Ann/(R,k) is introduced which is the ratio of the resistance caused by the osmotic pressure and the resistance of the membrane itself. For high values of this quantity (> 19) the flux is practically limited by the osmotic pressure. Factors leading to high values of the quantity Ann/(R,k) are discussed and it is concluded that in the ultrafiltration of medium molecular weight solutes (10,000 to 100,000 daltons) osmotic pressure limitation is more likely than gel layer limitation.

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Cellulose acetate and polysulfone casting solutions were coagulated in water/solvent mixtures with differing solvent content. Precipitation in pure water yielded skinned membranes, Precipitation in water/solvent mixtures with solvent concentration exceeding a certain minimum value (which is different for different systems) resulted in microporous membranes. This phenomenon has been explained in terms of the model description for the formation of asymmetric membranes as adopted in our laboratory. In this model, the skin formation is related to gelation and the formation of the porous substructure to liquid-liquid phase separation. It is made plausible that the addition of solvent to the coagulation bath favours nonsolvent inflow and hence liquid-liquid demixing in the precipitating film.

SummaryThe influence of concentration polarization on the permeate flux in the ultrafiltration of aqueous Dextran T70 solutions can be described by (i) the osmotic pressure model and (ii) the boundary layer resistance model. In the latter model the hydrodynamic resistance of the non-gelled boundary layer is computed using permeability data of the Dextran molecules obtained by sedimentation experiments. It is shown both in theory and experiment that the two models are equivalent.