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The effects of membrane porosity and pore size on CO 2 absorption performance and long-term stability were investigated using asymmetric poly(vinylidene fluoride) hollow fiber membrane contactors, prepared with differing inner surface structures by thermally induced phase separation. Membranes prepared using nitrogen gas as the bore fluid had lower inner surface porosity than membranes prepared with solvent. Monoethanolamine (MEA) solutions were used as absorbents on the tube side, while pure CO 2 was supplied to the shell side. Gas absorption performance and long-term stability of the synthesized membranes were compared with those of a commercial poly(tetrafluoroethylene) membrane. The effect of surface properties on gas absorption performance depended on MEA concentration. A membrane with lower porosity and small pore diameter at the inner surface was stable for 200 h, while a membrane with high porosity and larger pore size was completely wetted within 100 h and flux rate decreased sharply. A mathematical model for pure CO 2 absorption was developed by considering partial wetting of the membrane. The simulation results were discussed in light of the results from long-term stability tests of membranes.

In the present paper, the simultaneous removal of CO2 and H2S from their pressurized mixture with CH4 (5% CO2–2% H2S–93% CH4) using custom made membrane contactors equipped with micro-porous polymeric hollow fibers is described. Two types of commercial hollow fibers were employed and compared, i.e., expanded poly(tetrafluoroethylene) (ePTFE) and poly(tetrafluoroethylene-co-perfluorinated alkyl vinyl ether) (PFA). The feed gas mixture composition was 2% H2S, 5% CO2 in balance of CH4 to mimic the typical natural gas composition. Distilled water, aqueous sodium hydroxide and amine solutions of different concentrations were tested as absorption liquids to achieve the selective removal of the acid gases from the pressurized gas streams. The effect of pressure on the simultaneous CO2 and H2S absorption rates was investigated by the simultaneous pressurization of the absorption liquid and the feed gas (up to 50 bar) and monitoring the residual acid gases in the exit stream. The obtained experimental results indicated that the simultaneous CO2 and H2S fluxes were enhanced by increasing the inlet gas pressure for both physical and chemical absorption solvents. Up to the authors’ best knowledge, this is the first report on (i) the experimental simultaneous removal of CO2 and H2S from pressurized gas streams using HFM contactors and absorption solvents up to 50 bar and (ii) the utilization of PFA fibers in custom made high pressure-membrane modules. PFA fibers exhibited impressive higher fluxes (9–10 times) for CO2 and H2S than those obtained with the common ePTFE fibers which could be attributed in part to the associated higher mass transfer coefficient. The anticipated CO2/H2S flux ratio of 2.5 (which matches the ratio of their inlet concentrations in the feed gas) was obtained whenever the co-absorption was unhindered into absorption solvent of sufficient capacity, e.g., 2.0 M NaOH solution. Analysis of the mass transfer coefficients showed that while the overall mass transfer coefficients are determined by the liquid phase mass transfer coefficients at low pressures, gas phase mass transfer resistance contributes considerably to the overall resistance at high pressures.

Seven kinds of asymmetric poly(vinylidene fluoride) (PVDF) hollow fiber membranes with considerable different structures at the outer surfaces were prepared via thermally induced phase separation (TIPS) method and applied for CO 2 absorption as gas–liquid membrane contactors. A commercial microporous poly(tetrafluoroethylene) (PTFE) hollow fiber membrane was also used as a highly hydrophobic membrane. Experiments on the absorption of pure CO 2 into monoethanolamine (MEA) solutions were performed and the effects of membrane structure and inner diameter on the membrane performance were investigated. The module efficiencies were compared for the two patterns of operation, i.e. case 1 operation where liquid flows in the tube side while gas in the shell side and case 2 operation where liquid flows in the shell side while gas in the tube side. CO 2 absorption fluxes observed in case 1 operation were inversely proportional to the inner diameter of the hollow fiber membrane, and CO 2 absorption rate per fiber was almost the same for all membranes. CO 2 absorption rates observed in case 2 operation were much smaller than those in case 1 operations. The performances of the PVDF membranes were comparable with that of the commercial PTFE membrane. A mathematical model for pure CO 2 absorption in a membrane contactor, which assumes that the membrane resistance is negligibly small and the total membrane area is effective for gas absorption, was proposed to simulate CO 2 absorption rates. Experimental results in case 1 operation were satisfactorily simulated by the model for all membranes with different structures. Experimental results obtained in case 2 operation were also simulated by the model except for a membrane with a very low surface porosity on its outer surface. CO 2 absorption flux increased with increasing the MEA concentration up to the concentration of 2 mol/dm 3 , however, the CO 2 absorption flux hardly increased with further increase in MEA concentration. This behavior is discussed based on the decrease in the effective gas–liquid contacting area with increasing MEA concentration. A method to estimate CO 2 solubility and diffusivity in MEA solutions, which are essential for calculating CO 2 absorption flux by the model, is also described.

Polymer blend hollow fiber membranes were prepared via thermally induced phase separation (TIPS). Poly(vinyl butyral) (PVB) and poly(ethylene-co-vinyl alcohol) (EVOH) were used as polymers and diluent was polyethylene glycols (PEG) with the molecular weight of 200. The phase diagrams obtained in the polymer blends system indicated both UCST (upper critical solution temperature) and LCST (lower critical solution temperature) behaviors. For LCST behavior, there were two kinds of phase separations. It was considered that the phase separation occurred at higher temperature was between PVB and EVOH, and that occurred at lower temperature was between polymer and diluent. The addition of EVOH had an eminent effect on the hollow fiber membrane structure and membrane performance. With the increase of EVOH content in the polymer blend system, the membrane became more hydrophilic, and the water permeability became higher due to the enlargement of the pores. The addition of EVOH was also effective to improve the mechanical property of hollow fiber membrane.

Energy consumption for the separation of CO 2 in flue gas by a novel hollow fiber facilitated transport membrane module was evaluated and compared with those by conventional separation processes such as gas absorption and polymeric membrane separation. In the novel facilitated transport membrane system, both a feed gas (CO 2 /N 2 mixture) and a carrier solution (aqueous diethanolamine solution) are supplied to the lumen side (feed side, high pressure side) of the hollow fiber ultrafiltration membrane module and flow upward. The carrier solution, which contains dissolved solute gas, CO 2 in the present case, permeates the membrane to the permeate side (low pressure side, shell side), where the solution liberates dissolved gas to become a lean solution and the lean solution is returned to the lumen side by a pump. The feed side pressure was atmospheric and the permeate side pressure was controlled in the range from 10 to 27 kPa. CO 2 in the feed gas consisting of 10% CO 2 and 90% N 2 was concentrated to higher than 99%. The effects of various system parameters, such as permeate side pressure, temperature, gas and liquid flow rates and the inner diameter of hollow fiber membrane, on the energy consumption for CO 2 recovery were investigated. Energy consumption decreased with increasing temperature and had a minimum at an optimum permeate side pressure. The minimum energy consumption was estimated as 0.211 kWh kg-CO 2 −1 when the inner diameter of the hollow fiber was 0.8 mm. The energy consumption increased with feed gas flow rate, however, it was little influenced by liquid flow rate. The energy consumption decreased remarkably with increasing the inner diameter of the hollow fiber, and it was estimated as 0.072 kWh kg-CO 2 −1 when hollow fibers of 1.4 mm in inner diameter were used. This value is the lowest among those reported so far.

Poly(ethylene-co-vinyl alcohol) (EVOH) is a copolymer of ethylene and vinyl alcohol with an excellent combination of chemical, thermal and mechanical properties. EVOH hollow fiber membranes were successfully prepared by adding solvent (glycerol) to the cooling water bath in thermally induced phase separation (TIPS) process. The effect of glycerol:water ratio on membrane structures and properties including water permeability, solute rejection, and membrane strength were studied. EVOH crystallization is so slow that glycerol in the quench bath was allowed to flow into the membrane, thereby achieving high porosity at the outer surface. Therefore, adding solvent (glycerol) and increasing its content brings about an increase in the water permeability and a decrease in solute rejection. However, the membrane becomes more symmetric and weaker with increasing glycerol:water ratio due to the higher porosity.

Hydrophilic microporous hollow fiber membranes were prepared via thermally induced phase separation (TIPS). Poly(vinyl butyral) (PVB) and three kinds of polyethylene glycols (PEG) with the molecular weight of 200, 400 and 600 were used as polymer and diluents. In all cases, liquid–liquid (L–L) phase separation occurred and asymmetric cellular structures with the smaller pores at the outer surface were obtained. The properties of membranes such as pore size, water permeability, solute rejection and intensity were investigated by changing molecular weight of PEG, concentration of PVB, composition of the bath and air gap distance. The smaller molecular weight of PEG, the more PEG in the bath and the shorter air gap distance resulted in the higher water permeability, the lower solute rejection property and the lower mechanical strength.

Novel hollow fiber facilitated transport membrane modules were used for ethylene/ethane separation using an aqueous silver nitrate solution as a carrier solution. In this membrane system, a feed gas consisting of 80 mol% C 2 H 4 and 20 mol% C 2 H 6 and an aqueous 4 M silver nitrate solution are supplied to the lumen side (high pressure side, feed side) of a hollow fiber ultrafiltration membrane module and flow upward. In the lumen, the solution absorbs C 2 H 4 selectively to become a rich or laden solution. Some part of the rich solution permeates the membrane to the permeate side (low pressure side, shell side) and other part of the solution leaves the module from its top and then it is supplied to the top of the shell side, where the absorbed gas is stripped from the rich solution and recovered as enriched ethylene. The lean solution is circulated to the lumen of the hollow fiber module by a pump. The feed side pressure was from 200 to 500 kPa and the permeate side pressure was atmospheric. Three modules with different liquid permeability and design were used in experiments. The module, in which the carrier solution permeated the membrane at the upper part of the module, had higher performance than the module in which the solution permeated at the lower part. The module, in which the solution did not permeate the membrane, also had high performance although a large gas–liquid separator was needed downstream of the module. The module efficiency, which is defined as the ratio of the experimentally observed ethylene recovery to the recovery calculated by assuming gas–liquid phase equilibria at both sides of the membrane, was from 61 to almost 100%. Ethylene in the feed gas was concentrated from 80 to 99.8 mol%. The maximum C 2 H 4 permeance was 1.1 × 10 −4 mol m −2 s −1 kPa −1 (3.3 × 10 −4 cm 3 cm −2 s −1 cm Hg −1 ) and the selectivity of C 2 H 4 over C 2 H 6 was about 375 at the ethylene partial pressure of 164 kPa. The maximum ethylene recovery was 87% when the mean residence time of the feed gas in the module was 4.9 s. The membrane was confirmed to be stable during a series of experiments.

Mesoporous silica membranes were prepared by solvent evaporation methods using tetraethyl orthosilicate (TEOS) as a silica source and hexadecyltrimethylammonium chloride (CTAC) as a template and applied to filtration membranes which enable to separate solutes by nano-order size. Effect of the membrane thickness on structure formation was studied. The final membrane thicknesses were controlled from 0.43 μm to 127.7 μm by several deposition methods, such as spin-coating, casting by applicator, and pouring in Petri dish. The mesophase of the membrane changed from a cubic phase to a one-dimensional hexagonal (1d-H) phase with increasing membrane thickness. The membrane thickness was one of the key factors to control the mesostructure formed in the solvent evaporation method. In the monitoring of the structural growth process, it was found that 1d-hexagonal structure was initially obtained on a glass substrate, and this structure was changed to cubic structure. This result can be explained by changing the surfactant concentration, which decides the mesostructures. For filtration application, the silica membrane with the cubic structure was deposited by spin-coating on the porous alumina substrate. The rejection coefficient of Vitamin B 12 with the molecular weight of 1355 was about 0.8.

High-density polyethylene (HDPE) hollow fiber membranes were prepared via thermally induced phase separation (TIPS). The pore size at the outer surface was quite smaller than that at the inner surface and the membrane structure was asymmetric. A membrane module was prepared by the obtained hollow fiber membranes. Experiments on the carrier-mediated transport of Ce(III) through supported liquid membranes (SLMs) were performed using the membrane module. The SLMs consisted of organic solutions of octyl(phenyl)-N,N-diisobutylcarbamoylmethyl phosphine oxide (CMPO) and tributyl phosphate (TBP) in dodecane absorbed into the microporous HDPE hollow fiber membrane. Effect of the aqueous strip solution flow rate on the membrane performance was investigated.

A novel facilitated transport membrane for gas separation using a capillary membrane module is proposed in which a carrier solution is forced to permeate the membrane. Both a feed gas and a carrier solution are supplied to the lumen side (high pressure side, feed side) of the capillary ultrafiltration membrane and flow upward. Most of the carrier solution which contains dissolved solute gas, CO 2 in the present case, permeates the membrane to the permeate side (low pressure side, shell side), where the solution liberates dissolved gas to form a lean solution. The lean solution is circulated to the lumen side. This type of capillary membrane module was applied to the separation of CO 2 from model flue gases consisting of CO 2 and N 2 . Monoethanolamine (MEA), diethanolamine (DEA) and 2-amino-2-methyl-1-propanol (AMP) were used as carriers or absorbents of CO 2 . The feed side pressure was atmospheric and the permeate side was evacuated at about 10 kPa. CO 2 in the feed gas was successfully concentrated from 5–15% to more than 98%. The CO 2 permeance was as high as 2.7×10 −4 mol m −2 s −1 kPa −1 (8.0×10 −4 cm 3 cm −2 s −1 cmHg −1 ) when the CO 2 mole fraction in the feed was 0.1 and temperature was 333 K. The selectivity of CO 2 over N 2 was in the range from 430 to 1790. The membrane was very stable over a discontinuous one-month testing period.

A novel facilitated transport membrane, a bulk flow liquid membrane (BFLM), was applied to ethylene/ethane separation using silver nitrate as a carrier. In this membrane system, a carrier solution is supplied to the feed gas side (high pressure side) of a UF membrane and is allowed to permeate to the permeate side (low pressure side). The transferred carrier solution is recycled to the feed side. The effect of such convective flow of the carrier solution through the membrane on ethylene permeance was investigated using a dead-end type filtration cell equipped with an ultrafiltration membrane. The C2H4 permeance, RC2H4, increased with increasing the volumetric permeation flux of the carrier solution u, and when u was 4×10−5m s−1 and the C2H4 partial pressure was 9 kPa, RC2H4 was 4×10−5 mol m−2 s−1 kPa−1 and the selectivity of C2H4 over C2H6 was 1100. This value was ≈30 times that at u=0. This type of membrane was very stable during intermittently performed experiments for more than 2 months.

The effect of convective flow of a carrier solution through a facilitated transport membrane on the CO 2 permeation behavior was investigated using diethanolamine (DEA) as a carrier of CO 2 . A dead-end type filtration cell, equipped with an ultrafiltration membrane, was used as a gas permeation cell. A carrier solution was supplied to the feed side (high pressure side) of the cell and was allowed to permeate to the receiving side (low pressure side) and was circulated between the receiving and the feed side by a pump. The feed gas was a mixture of CO 2 and CH 4 . The convective flow of the carrier solution through the membrane enhanced the gas permeance. For example, the CO 2 permeance, R CO 2 , increased with increasing the volumetric permeation flux of the carrier solution u and when u was 1.9×10 −5 m s −1 and the CO 2 partial pressure was 4.0 kPa, R CO 2 was 6.23×10 −5 mol m −2 s −1 kPa −1 . This value was about ten times that at u =0 with no convective flow of the carrier solution across the membrane. The selectivity of CO 2 over CH 4 was as high as 1970. This type of membrane was very stable during the experiments for more than 2 months.

A novel liquid membrane for gas separation is proposed in which a membrane liquid is continuously supplied to the feed side (high pressure side) of a supported liquid membrane (SLM) and is forced to permeate through the membrane from the feed to the permeate side (low pressure side). The membrane liquid is circulated between the feed and the strip side of the membrane. In this mode of operation, gas permeation rate is enhanced by the convective or bulk flow of the membrane liquid dissolving gas. In addition, the fouling of membrane caused by the loss of the membrane liquid from the membrane due to drying-up and transmembrane pressure difference can be prevented. Experiments on the separation of CO 2 from CH 4 were performed using water as a membrane liquid. It was demonstrated that the convective flow of water through the membrane enhanced the gas permeabilities remarkably. For example, CO 2 permeability when the water permeation rate was 1.2 l/(m 2 min) was about 20 times that without water permeation. The effect of convective flow on the gas permeability was discussed on the basis of a simple permeation model. It was found that if the parameter (Peclet number) m = uL / D e ( u is the linear velocity of water through membrane or volumetric water flux, L the membrane thickness and D e is the effective diffusivity in membrane) is large, convective water transport considerably enhances gas permeability. Experimentally observed gas permeance was approximately simulated by the proposed model.

As a part of the investigation on the treatment of low-level radioactive wastewater using supported liquid membranes, a method for stabilizing a supported liquid membrane (SLM) is proposed. Aqueous Ce(III) solutions containing sodium nitrate and nitric acid were used as the simulated low level radioactive wastewater. The SLM consisted of octyl(phenyl)-N,N-diisobutylcarbamoylmethylphosphine oxide (CMPO) as a carrier of Ce(III), tributyl phosphate (TBP) as a modifier and dodecane as a solvent. The strip solutions were aqueous solutions of chelating agents such as trisodium citrate and disodium hydrogen citrate. A small plate-and-frame type SLM module was used and circulation mode for both the feed and the strip solutions was adopted. The SLM was unstable and water permeation through the SLM occurred, suggesting replacement of the organic membrane solution in the pores of a support membrane by the aqueous feed solution. However, the SLM was stabilized by adding an organic membrane solution to the reservoir of the strip solution and by circulating the membrane solution through the strip side of the SLM module so that the SLM could frequently contact the membrane solution dispersed as droplets in the strip solution. By this method, the SLM was not degraded by repeated replacements of both the feed and the strip solutions. The effect of chelating agent in the strip solution on the permeation rate of Ce(III) was also investigated.

Facilitated transport of CO 2 through a thin solution-cast perfluorosulfonic acid ionomer membrane has been studied. The CO 2 carrier was monoprotonated ethylenediamine and was immobilized in the ionomer membrane by electrostatic force. The cast membrane with membrane thickness 22 μm showed much higher CO 2 permeance than the commercial Nafion ® 117 membrane of thickness 186 μm. However, selectivity of CO 2 over N 2 was lower in the cast membrane. The effects of the membrane thickness and annealing temperature in the membrane preparation on membrane performance were investigated. CO 2 permeability and the selectivity decreased with decreasing membrane thickness and with increasing annealing temperature. Dynamic mechanical studies were done to study the membrane structures. The cast membrane annealed at 200°C showed two peaks similar to the commercial membrane. In contrast to this, the peak assigned to the polar region was small in the cast membrane annealed at a lower temperature, which suggested that clear clusters do not form in the membrane.

Facilitated transport of SO2 through supported liquid membranes containing water as a carrier was studied experimentally and theoretically in the range of SO2 partial pressure, pF,SO2, from 3.5 Pa to 2 kPa. The permeance of SO2 at pF,SO2=3.5 Pa was as high as 6×10−4 mol m−2 s−1 kPa−1, and the contribution of gas–phase mass transfer resistance to the overall mass transfer resistance was found to be significant. The observed SO2 permeance was analyzed on the basis of the theory of facilitated transport which had been developed by the present authors for the facilitated transport of CO2 using amine as a carrier. The theory could quantitatively explain the effect of various experimental conditions such as the SO2 partial pressure and the membrane thickness on the permeance of SO2. Comparison of the present facilitated transport membrane with polymeric membranes showed that the present membrane is much more effective for the removal of SO2 at low partial pressures encountered in flue gas desulfurization processes.

In order to examine the feasibility of treating low level radioactive wastewater by supported liquid membranes (SLMs), experiments were performed using a stirred permeation cell. Aqueous solutions of nitric acid and/or sodium nitrate containing about 550 ppm Ce(III), 490 ppm Fe(III), 320 ppm Cr(III) and 330 ppm Ca(II) were used as a simulated low level radioactive wastewater. The liquid membrane consisted of octyl(phenyl)- N , N -diisobutylcarbamoylmethylphsophine oxide as a carrier of cerium, tributylphosphate as a modifier and dodecane as a solvent. The strip solutions were water and aqueous sodium citrate solutions. The effects of various experimental conditions such as concentrations of cerium, HNO 3 and NaNO 3 in the feed solution, liquid membrane composition and temperature were investigated. Rapid transport and enrichment of cerium were realized by adding sodium citrate to the strip solution as the masking agent of cerium and also using a feed solution of low nitric acid concentration and high sodium nitrate concentration. Increasing the temperature from 298 to 318 K was very effective in enhancing the permeation rate of cerium. The membrane area required for removing 99.9% of cerium from the feed solution containing about 550 ppm Ce was estimated as 3.3 m 2 at 1 m 3 day −1 of the treatment rate.