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Membrane fouling during the filtration process is a perennial issue and could lead to reduced separation efficiency. In this work, poly(citric acid)-grafted graphene oxide (PGO) was incorporated into a matrix of single-layer hollow fibre (SLHF) and dual-layer hollow fibrr (DLHF) membranes, respectively, aiming to improve membrane antifouling properties during water treatment. Different loadings of PGO ranging from 0 to 1 wt% were first introduced into the SLHF to identify the best PGO loading for the DLHF preparation with its outer layer modified by nanomaterials. The findings showed that at the optimized PGO loading of 0.7 wt%, the resultant SLHF membrane could achieve higher water permeability and bovine serum albumin rejection compared to the neat SLHF membrane. This is due to the improved surface hydrophilicity and increased structural porosity upon incorporation of optimized PGO loading. When 0.7 wt% PGO was introduced only to the outer layer of DLHF, the cross-sectional matrix of the membrane was altered, forming microvoids and spongy-like structures (more porous). Nevertheless, the BSA rejection of the membrane was improved to 97.7% owing to an inner selectivity layer produced from a different dope solution (without the PGO). The DLHF membrane also demonstrated significantly higher antifouling properties than the neat SLHF membrane. Its flux recovery rate is 85%, i.e. 37% better than that of a neat membrane. By incorporating hydrophilic PGO into the membrane, the interaction of the hydrophobic foulants with the membrane surface is greatly reduced.

Wetting and fouling have significantly affected the application of membrane distillation (MD). In this work, a dip-coating method was used for improving surface hydrophobicity of the polyetherimide (PEI) hollow fiber membrane. An air gap membrane distillation (AGMD) process was applied for treatment of the methylene blue (MB) solution. The porous PEI membrane was fabricated by a dry-wet spinning process and the hydrophobic 2-(Perfluoroalkyl) ethanol (Zonyl® BA) was used as the coating material. From FESEM, the modified PEI-Zonyl membrane showed an open structure with large finger-like cavities. The modified membrane displayed a narrow pore size distribution with mean pore size of 0.028 μm. The outer surface contact angle of the PEI-Zonyl membrane increased from 81.3° to 100.4° due to the formation of an ultra-thin coated layer. The pure water flux of the PEI-Zonyl membrane was slightly reduced compared to the pristine PEI membrane. A permeate flux of 6.5 kg/m2 h and MB rejection of 98% were found for the PEI-Zonyl membrane during 76 h of the AGMD operation. Adsorption of MB on the membrane surface was confirmed based on the Langmuir isotherm evaluation, AFM and FESEM analysis. The modified PEI-Zonyl membrane can be a favorable alternative for AGMD of dyeing wastewaters. HIGHLIGHTS The porous PEI membrane was modified by surface coating of Zonyl® BA.; A narrow pore size distribution with mean pore sizes of 0.028 μm was observed.; The outer surface water contact angle was increased from about 81° to 100°.; A more stable AGMD of the MB solution was found for the modified membrane.; A lower flux decline ratio was observed for the modified PEI-Zonyl membrane.

Porous polyvinylidene fluoride-co-hexafluropropylene (PVDF-HFP) hollow fiber membranes were fabricated through a wet spinning process. In order to improve the membrane structure, composition of the polymer solution was adjusted by studying ternary phase diagrams of polymer/solvent/non-solvent. The prepared membranes were used for sweeping gas membrane distillation (SGMD) of 20 wt% ethylene glycol (EG) aqueous solution. The membranes were characterized by different tests such as N2 permeation, overall porosity, critical water entry pressure (CEPw), water contact angle and collapsing pressure. From FESEM examination, addition of 3 wt% glycerol in the PVDF-HFP solution, produced membranes with smaller finger-likes cavities, higher surface porosity and smaller pore sizes. Increasing the polymer concentration up to 21 wt% resulted in a dense spongy structure which could significantly reduce the N2 permeance. The membrane prepared by 3 wt% glycerol and 17 wt% polymer demonstrated an improved structure with mean pore size of 18 nm and a high surface porosity of 872 m−1. CEPw of 350 kPa and overall porosity of 84% were also obtained for the improved membrane. Collapsing pressure of the membranes relatively improved by increasing the polymer concentration. From the SGMD test, the developed membrane represented a maximum permeate flux of 28 kg·m−2·h−1 which is almost 19% higher than the flux of plain membrane. During 120 h of a long-term SGMD operation, a gradual flux reduction of 30% was noticed. In addition, EG rejection reduced from 100% to around 99.5% during 120 h of the operation.

Surface modified porous polysulfone (PSF) hollow fiber membrane was prepared by dip-coating of polydimethylsiloxane (PDMS). The prepared membranes were used in an air gap membrane distillation (AGMD) process for dehydration of MEG-water mixtures. The membranes were characterized in terms of Field Emission Scanning Electronic Microscopy (FESEM), N2 permeation, contact angle measurement and wetting pressure. From FESEM images, the prepared membrane showed a porous structure with finger-like cavities. In addition, formation of ultra-thin coating layer on the inner and outer surface was confirmed by FESEM. From N2 permeation test, the modified membrane showed smaller mean pore size (9 nm) and lower surface porosity compared to the plain membrane. The modified membrane showed a good hydrophobicity with water contact angle of 118° and wetting pressure of 800 kPa. The permeate flux of about 26 kg/m2h was obtained for the modified membrane at MEG concentration of 20 wt%, feed temperature of 90 °C and feed velocity of 0.66 m/s. Due to the improved structure, the modified membrane showed a stable AGMD operation for 80 h compared to the plain membrane.

Porous hydrophobic poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) hollow fiber membranes were prepared via a wet spinning process using polyethylene glycol (PEG-400) and ortho-phosphoric acid (PA) in the spinning dope. CO2 stripping from diethanolamine (DEA) solution was conducted through the gas–liquid membrane contactor modules. By obtaining cloud point diagrams, higher phase inversion rate was confirmed for the solution with PA. The smaller finger-like macrovoids were resulted when using the non-solvent additives. The membranes with higher surface porosity and smaller mean pore size were produced when using the additives in the spinning dope. The improved PVDF-HFP/PA membrane had the mean pore size of 0.015 µm and permeability of 13,100 GPU. This membrane showed a minimum mass transfer resistance of 15,982 (m/s)−1 which is about 68% less than the resistance of plain PVDF-HFP membrane. The improved PVDF-HFP/PA membrane showed CO2 stripping flux of about 9 × 10−3 mol/m2 s at the liquid velocity of 0.045 m/s.

Membrane separation is known as an efficient technique for oily wastewater treatment. In the present study, cellulose acetate (CA) was introduced into the polyetherimide (PEI) solution in order to enhance hydrophilicity and the membrane structure for oil–water separation. The hollow fiber membranes were prepared via a phase-inversion process. The membranes were characterized by N2 permeation test, water contact angle, pure water flux and field emission scanning electron microscopy (FESEM). The blend PEI-CA membrane presented larger finger-likes morphology with a thicker outer skin layer. From N2 permeation test, the blend membrane showed effective surface porosity of 697 m-1 and mean pore size of 4.5 nm. The higher water flux and lower resistance of the blend PEI-CA membrane were related to the higher hydrophilicity and the open structure. Due to small pore sizes and enhanced hydrophilicity, the blend membrane showed a stable oil rejection of over 98% and water flux of 18 L/m2 h after 100 min of the separation operation. The developed PEI-CA membrane can potentially be applied in petrochemical and refinery industries for oily wastewater treatment.

Highly porous polyetherimide (PEI) hollow fiber membranes were prepared through a spinning process. The membranes were modified by a dip-coating method using polydimethylsiloxane (PDMS) to increase surface hydrophobicity. The membranes were used in a sweeping gas membrane distillation (SGMD) process for separation of methylene blue (MB) solutions. From FESEM examination, an open structure with large finger-like cavities was observed in the membrane structure. The PEI-PDMS membrane presented a high surface porosity with N2 permeance of 15300 GPU and mean pore size of 72 nm. Formation of an ultra-thin PDMS coated layer resulted in an improved surface hydrophobicity with water contact angle of around 104o. Moreover, the coated layer resulted in the increment of critical water entry pressure from 250 to 400 kPa. The PEI and PEI-PDMS membranes indicated pure water flux of 26.2 and 22.5 kg/m2 h, respectively, through the SGMD process at feed temperature of 60 °C. During 5 days of the SGMD process, the PEI and PEI-PDMS showed an approximate permeation flux reduction of 42% and 19%, respectively. The PEI-PDMS membrane showed a good antifouling property with flux recovery ratio of about 89.4%. Adsorption of MB on the outer surface of the membranes was confirmed by studying the variation of fouling factor vs. MB concentration in the feed solution. The improved PEI-PDMS membrane demonstrated a high permeation flux and antifouling properties which are promising parameters for a prolonged membrane distillation operation.

Blend hydrophilic polyamide imide (PAI)-sulfonated poly (ether ether keton) (SPEEK) hollow fiber membranes were fabricated for oil-water emulsion separation. The structure and performance of the membranes were examined by FESEM analysis, N2 permeation, overall porosity, collapsing pressure, water contact angle, pure water flux, molecular weight cutoff (MWCO), and oil rejection tests. By studying ternary phase diagrams of polymer/solvent-additive/water system, the higher phase-inversion rate was confirmed for the solutions prepared at higher PAI/SPEEK ratio. A more open structure with larger finger-likes was observed by increasing PAI/SPEEK ratio. Mean pore size of 81 nm, overall porosity of 79% and water contact angle of 58° were obtained for the improved membrane prepared by PAI/SPEEK ratio of 85/15. Increasing SPEEK ratio resulted in lower mechanical stability in terms of collapsing pressure. Pure water flux of about 2.5 times of the plain PAI membrane was found for the improved membrane. MWCO of 460 kDa was found for the improved blend membrane. From oil rejection test, all the membranes demonstrated an oil rejection of over 95%. The improved membrane showed a lower rate of permeate flux reduction compared to the plain membrane which was related to the smaller fouling possibility. Less fouling resistance of the improved membrane was related to the higher flux recovery ratio (about 92%). For all the membranes, the dominant fouling mechanism was found to be the cake filtration. The improved PAI-SPEEK hollow fiber membranes was found to be practical for ultrafiltration of oily wastewaters.

A previously developed PVDF hollow fiber membrane was prepared and its stability during CO2 absorption with alkanolamine solutions was investigated. The membrane was used in a gas–liquid membrane contactor module for over 160 h in a continuous operation using monoethanolamine (MEA) and diethanolamine (DEA) solutions (1 M) as the liquid absorbents. Before and after the CO2 absorption tests, the structures of the membranes were investigated in terms of morphology, N2 permeability, wetting resistance and mass transfer resistance, and thermal and mechanical stability. Under AFM and SEM analysis, the used membranes presented lower surface roughness, which was confirmed by outer surface contact angle measurement. From the CO2 absorption test, using MEA solution as the liquid absorbent, a gradual CO2 flux reduction of about 43% was observed until the end of the operation. The results of the permeation tests showed a decrease in the N2 permeance of the used membranes which was related to their smaller pore sizes due to changes in the surface properties. The mean membrane pore size was reduced from 15.8 nm to 6.8 and 2.24 nm for the membranes used with DEA and MEA solutions, respectively. The wetting resistance of the used membranes slightly decreased due to a reduction in surface hydrophobicity. Compared to the fresh membrane, the mass transfer resistance of the used membranes increased by about 16.8% and 20% when the DEA and MEA solutions were used, respectively. In addition, the used membranes showed lower mechanical stability in terms of elongation and tensile strength compared to the fresh membrane. The effect of the MEA solution on the surface properties of the membrane was greater than that of the DEA solution. It can be concluded that the stability of the PVDF membrane, in terms of its compatibility with the liquid absorbent, is a key factor which should be considered from an economic point of view.

In this study, porous hydrophobic polyvinylidene fluoride (PVDF) hollow fiber membranes were fabricated via a wet phase inversion process. In order to improve the phase inversion rate and provide porous membranes, 4 wt.% lithium chloride (LiCl) was used in the spinning dope. The prepared membrane morphology was studied using field emission scanning electron microscopy (FESEM). Chemical CO 2 absorption by NaOH solution (1M) was conducted through the PVDF hollow fiber membrane contactor. The effect of the main operating condition such as absorbent temperature, CO 2 pressure and absorbent flow rate on the performance of CO 2 absorption was investigated. From FESEM examination, the membrane possesses an almost sponge–like structure with ultra thin skin layer. Results of CO 2 absorption test showed that by increasing the absorbent flow rate the CO 2 flux increased which confirmed the existence of liquid side mass transfer resistance. It was found that by increasing the absorbent temperature the CO 2 flux considerably improved. Meanwhile, the effect of CO 2 pressure on the absorption rate was insignificant. Therefore, it can be concluded that by applying a porous hydrophobic membrane with improved structure and optimizing the operating conditions, high CO 2 removal efficiency can be achieved through gas–liquid membrane contactors

Recently, membrane contactors have attracted attentions as an efficient and flexible technology for CO 2 capture. In the present work, blend hydrophobic polyvinylidene fluoride (PVDF) hollow fiber membranes were prepared via a dry–wet phase-inversion process. Surface modifying macromolecule (SMM) was introduced to improve the membrane properties for CO 2 absorption in gas–liquid membrane contactors. The effect of SMM concentration in the polymer dope, air gap distance and bore fluid composition on the structure and performance of the membranes were investigated. By increasing SMM in the polymer dope, the membranes presented smaller mean pore sizes, higher permeability and surface hydrophobicity. Using 2 wt.% SMM, the membranes prepared at 24 cm air gap showed larger mean pore size, higher hydrophobicity and lower permeability compared to those prepared at 0 cm air gap. By using 60 wt.% DMAc aqueous solution as the bore fluid, the membrane permeability and CO 2 flux significantly improved. Maximum CO 2 absorption flux of 6.8 × 10 −4 mol/m 2 s was achieved at absorbent velocity of 0.025 m/s. The surface modified membrane demonstrated about 10% gradual CO 2 flux reduction for over 140 h of the long-term operation. In conclusion, by improving surface hydrophobicity of the membrane, a long-term stable operation can be achieved for practical implication of gas–liquid membrane contactor technology.

In order to develop the structure of microporous PVDF membranes, PEG-400 was introduced into the polymer dope as a non-solvent additive. The hollow fiber membranes were prepared via a wet phase-inversion process and then used in the membrane contactor modules for CO2 stripping from water. By addition of different amounts of PEG-400, cloud points of the polymer dope were obtained to examine phase-inversion behavior. From FESEM analysis, the membrane structure changed from a finger-like to an approximately sponge-like morphology with the addition of 4 wt.% of PEG-400. The prepared membranes presented smaller mean pore size (0.13 μm) and significantly higher wetting pressure (550 kPa) compared to the plain membrane. From CO2 stripping test, at water velocity of 0.4 m/s, the PVDF membranes prepared by 4% PEG-400 demonstrated an approximate CO2 stripping flux of 4.5 × 10−5 (mol/m2 s) which is 125% higher than the flux of the plain membrane. It could be concluded that structurally developed hydrophobic PVDF hollow fiber membranes can be prepared by a controlled phase-inversion process to enhance the performance of gas–liquid membrane contactor.

Microporous polyvinylidene fluoride (PVDF) hollow fiber membranes with various non-solvent additives, i.e. lithium chloride, glycerol, polyethylene glycol (PEG-400), methanol and phosphoric acid, were fabricated for CO2 stripping via membrane contactors. The membranes were characterized in terms of liquid entry pressure, contact angle, gas permeation and morphology analysis. CO2 stripping performance was investigated by using an in-house made stainless steel module with CO2-preloaded aqueous diethanolamine as the liquid absorbent. Hydrophobicity and gas permeability of the membranes reduced with the addition of a non-solvent additive in the polymer dope but increase in liquid entry pressure was observed as more sponge-like structures developed in the inner layer of the fibers. It was found that PVDF/PEG-400 membrane produced the highest stripping flux of 4.03×10−2 mol m−2 s−1 which can be correlated to its high gas permeation and high effective surface porosity. The result of long-term stripping operation indicated an approximatly 80% stripping flux reduction which can be related to the interaction of polymer membrane and amine solution at high temperature.

Gas–liquid hollow fiber membrane contactor can be a promising alternative for the CO 2 absorption/stripping due to the advantages over traditional contacting devices. In this study, the structurally developed hydrophobic polyvinylidene fluoride (PVDF) hollow fiber membranes were prepared via a wet spinning method. The membranes were characterized in terms of morphology, permeability, wetting resistance, overall porosity and mass transfer resistance. From the morphology analysis, the membranes demonstrated a thin outer finger-like layer with ultra thin skin and a thick inner sponge-like layer without skin. The characterization results indicated that the membranes possess a mean pore size of 9.6 nm with high permeability and wetting resistance and low mass transfer resistance (1.2 × 10 4 s/m). Physical CO 2 absorption/stripping were conducted through the fabricated gas–liquid membrane contactor modules, where distilled water was used as the liquid absorbent. The liquid phase resistance was dominant due to significant change in the absorption/stripping flux with the liquid velocity. The CO 2 absorption flux was approximately 10 times higher than the CO 2 stripping flux at the same operating condition due to high solubility of CO 2 in water as confirmed with the effect of liquid phase pressure and temperature on the absorption/stripping flux.

Different types of non-solvent additives were introduced into the polyvinylidene fluoride (PVDF) dope to investigate improvement of the hollow fiber membrane structure for CO 2 absorption. Phase-inversion behavior of the PVDF dopes was studied using cloud points measurements. Glycerol, phosphoric acid, ethanol and polyethylene glycol (PEG-400) were used as non-solvent additives in the polymer dope. With addition of the additives, precipitation of the polymer dopes increased following the trend of phosphoric acid > glycerol > ethanol > PEG-400. From morphology examination, PEG-400, glycerol and phosphoric acid resulted in the membranes with almost sponge-like structure due to high viscosity of the spinning dopes. The low wetting resistance and high permeability of the plain PVDF and PVDF/ethanol membranes were attributed to the large finger-likes structure. Among the additives, glycerol provided the membranes with larger mean pore size (9.62 nm). CO 2 absorption by distilled water was conducted through the gas–liquid membrane contactors. The PVDF/glycerol membrane demonstrated higher CO 2 absorption flux than the other membranes. At the absorbent flow rate of 280 ml/min, CO 2 flux of 7.8 × 10 −4 mol/m 2 s was achieved, which was approximately 30% higher than CO 2 flux of the plain PVDF membrane. In conclusion, a developed membrane structure prepared by controlled phase-inversion process can be a promising alternative for CO 2 capture in gas–liquid membrane contactors.

Porous polyvinylidene fluoride (PVDF) hollow fiber membranes were prepared via wet spinning process. The membranes were characterized in terms of gas permeation, wetting resistance and overall porosity, which are important parameters for gas absorption/stripping applications. CO2 stripping from water was conducted through the gas–liquid membrane contactor. The effect of different operating conditions on the CO2 stripping efficiency of the membrane was investigated. The results indicated a significant improvement in the CO2 stripping efficiency by increasing the liquid phase temperature. On the other hand, increasing the liquid flow rate resulted in a considerable decrease in the CO2 stripping flux. The maximum stripping efficiency of 32% was achieved at 60 ml/min of the liquid flow rate, 60 °C and 0.5 × 105 Pa. It was found that the liquid phase temperature, the gas–liquid contact area and contact time are the key parameters for enhancement of CO2 stripping efficiency.

Carbon dioxide (CO 2 ), the main greenhouse gas, has been associated with global climate change. Therefore, it is important to develop technologies to mitigate this issue. In present study, porous hydrophobic polyvinylidene fluoride (PVDF) hollow fiber membranes with developed structure for CO 2 absorption were prepared via a wet spinning process. The prepared membranes were characterized in terms of morphology examination, gas permeability, critical water entry pressure (CEP w ) and mass transfer resistance. From the morphology examination, the membrane showed an almost sponge-like structure with inner skinless layer and ultra-thin outer skin layer. Results of gas permeation test indicated that the membrane possess very small mean pore size (3.96 nm) with high surface porosity. The CO 2 absorption experiment demonstrated a significant improvement in the CO 2 flux of the prepared PVDF membrane compared to the commercial porous polytetrafluoroethylene (PTFE) hollow fiber membrane. At the absorbent flow rate of 200 ml/min, CO 2 flux of the PVDF membrane (4.10 × 10 −4 mol/m 2 s) was approximately 68% higher than the CO 2 flux of the PTFE membrane. In addition, the results indicated that an approximate 25% CO 2 flux reduction was gradually occurred at initial 26 h, then the CO 2 flux maintained constant over 140 h of the operation.

Effect of lithium chloride (LiCl) concentration as non-solvent additive in the spinning dopes on the structure and CO 2 absorption performance of the polyvinylidene fluoride (PVDF) membranes was investigated. The hollow fiber membranes were prepared via a wet phase-inversion process and characterized in terms of gas permeability, wetting resistance, mass transfer resistance and overall porosity. The morphology study indicated that by increasing LiCl concentration in the spinning dope, the membrane structure changed from the finger-like to the sponge-like. In addition, by increasing LiCl concentration up to 4 wt.% a drastic decrease in the N 2 permeance and a significant increase in the wetting resistance were observed. CO 2 absorption by distilled water was conducted through the gas–liquid membrane contactors. Using LiCl in the spinning dopes, the CO 2 flux of the prepared membranes significantly improved. By introducing 2 wt.% LiCl, the PVDF membrane showed a CO 2 flux of approximately 60% higher than the plain PVDF membrane at the absorbent flow rate of 200 ml/min. It can be concluded that a porous hydrophobic hollow fiber membrane with improved structure can be a productive alternative for CO 2 absorption and separation through gas–liquid membrane contactors.

Porous polyvinylidene fluoride (PVDF) and polysulfone (PSF) hollow fiber membranes were prepared via a wet spinning method. Glycerol was used as phase-inversion promoter additive in the spinning dopes. Cloud point diagrams of polymer/solvent–glycerol/water were obtained to study precipitation rate of the polymers solution. The membrane structure was compared in terms of morphology, gas permeation, critical water entry pressure, collapsing pressure, overall porosity, contact angle and mass transfer resistance. The cloud point diagrams confirmed a significant increase in the precipitation rate of the spinning dopes with addition of glycerol. The PSF membranes indicated more open cross-section structure with smaller pore sizes. However, the PVDF membranes illustrated an ultra thin outer skin layer with high permeability which resulted in significantly lower mass transfer resistance. Physical CO2 absorption with distilled water was conducted through the gas–liquid membrane contactors. With addition of glycerol, the PVDF membrane demonstrated a structure with significantly higher CO2 flux compared to the commercial asymmetric PVDF membrane. The CO2 flux of 8.20 × 10−4 mol/m2 s was achieved by using the absorbent flow rate of 310 mL/min in the shell side of the membrane module. Therefore, using an improved hollow fiber membrane structure can be a promising alternative for CO2 absorption and separation through membrane contactors.

Porous polyvinylidene fluoride (PVDF) hollow fiber membranes were fabricated via a wet phase-inversion method. Ortho-phosphoric acid and lithium chloride monohydrate (LiCl·H2O) were used as phase-inversion promoter additives in the spinning dopes. Ternary phase diagram of the polymer dopes were obtained using cloud point test. The membranes morphology was examined by field emission scanning electron microscopy (FESEM). The prepared membranes were characterized in terms of gas permeation, critical water entry pressure, overall porosity, contact angle and mass transfer resistance. Cloud point data showed that both additives increased precipitation rate of the spinning dopes significantly. From the FESEM examination, the hollow fiber membranes possess an ultra-thin outer skin layer with a thin finger-like layer beneath and a thick sponge-like sub-layer. Results of gas permeation test revealed that phosphoric acid resulted in the membranes with a larger mean pore size than PVDF + LiCl·H2O membrane. In addition, phosphoric acid in the spinning dope provided the hollow fiber membrane structure with significantly enhanced CO2 flux, which is approximately 33% more than the CO2 flux of the PVDF + LiCl·H2O membrane and the commercial PTFE membrane at 320 mL/min absorbent flow rate. Therefore, it can be concluded that an improved PVDF membrane structure prepared by phase-inversion process can be a cost-effective alternative for CO2 absorption in gas–liquid membrane contactors.

Porous polyvinylidene fluoride (PVDF) hollow fiber membranes were fabricated via a wet phase-inversion process and used in the gas–liquid membrane contactor for physical and chemical CO2 absorption. Effect of different operating parameters such as absorbent temperature, CO2 pressure, absorbent flow rate and long-term operation on the CO2 flux of the membrane were investigated. The characterization results showed that the prepared membrane possess small pore size with high surface porosity and wetting resistance, which are favorable for gas absorption application. Results of CO2 absorption experiments revealed that in case of physical absorption with distilled water, CO2 solubility was the key parameter which significantly affected the membrane CO2 flux. A significant increase in the CO2 flux was observed by increasing CO2 pressure and decreasing the absorbent temperature. However, in case of chemical absorption with NaOH (1 M), the reaction rate was dominant, where the CO2 flux was significantly increased by increasing the absorbent temperature. It was also found that when the liquid contacted the outer skin layer of the hollow fiber membrane, the operation was stable at the same gas and liquid pressure without bubble formation in the liquid phase. In addition, the results of long-term study demonstrated that the membrane performance was maintained constant for long-term operation. The initial flux reduction was found to be about 30% within the 23 h of physical absorption, and 20% of gradual reduction within 80 h of chemical absorption.

Porous polysulfone (PSf) hollow fiber membranes were fabricated via a phase-inversion method by using low molecular weight additives in the spinning dopes. Polyethylene glycol (PEG200), ethanol, glycerol and acetic acid were used as the additives. An aqueous 95 wt.% 1-methyl-2-pyrrolidone (NMP) solution was used as neutral bore fluid to fabricate inner skinless hollow fibers membranes. The precipitation rate of the polymer dopes with the different additives was studied using cloud point measurement. Effect of the additives on the resulting membrane structure, surface porosity, pore size, critical water entry pressure (CEP w ) and CO 2 absorption performance were investigated. Cloud point diagrams indicated that the precipitation rate of the polymer dopes increased following the trend of glycerol > acetic acid > PEG200 > ethanol. Results of gas permeation tests showed that glycerol and PEG200 as additives provided the membranes with the largest and smallest pore size, respectively. Moreover, all the additives resulted in an increase in the surface porosity. The cross-section and inner surface of the membranes were examined via a field emission scanning electronic microscopy (FESEM). Glycerol in the spinning dope provided the membrane structure with a thin finger-like and a thick sponge-like layer, which resulted in a higher CEP w and CO 2 absorption rate than the other PSf hollow fiber membranes.

The structure of polyvinylidene fluoride (PVDF) and polyetherimide (PEI) hollow fiber membranes was developed by formulation of the polymer solutions and regulating spinning parameters. The cloud point diagrams of polymer/solvent/non-solvent were obtained to investigate phase inversion behavior of the solutions. The prepared membranes were characterized in terms of morphology, overall porosity, gas permeation, collapsing pressure, wetting resistance and mass transfer resistance. The gas–liquid membrane contactor modules were prepared to study CO2 absorption performance of the membranes. The PEI and PVDF membranes presented finger-like and sponge-like structure, respectively. Both membranes showed good surface porosity due to enhancement of phase inversion by addition of glycerol to the spinning solutions. The PVDF membrane presented mean pore size of 0.09 μm with low mass transfer resistance and high wetting resistance. At water velocity of 0.03 m/s, the PVDF membrane indicated CO2 flux of 1.5 × 10−3 mol/m2 s which was 35 % higher than the flux of the PEI membrane. Due to the improved structure of the membranes, a significantly high CO2 flux was obtained at the same operating condition compared to the reported membranes in the literature. The developed membranes can be used as promising alternatives for cost-effective gas–liquid membrane contactor systems.

Blend PES/CA hydrophilic membranes were prepared via a phase-inversion process for oil–water separation. PEG-400 was introduced into the polymer solution in order to enhance phase-inversion and produce high permeability membranes. A gas permeation test was conducted to estimate mean pore size and surface porosity of the membranes. The membranes were characterized in terms of morphology, overall porosity, water contact angle, water flux and hydraulic resistance. A cross-flow separation system was used to evaluate oil–water separation performance of the membranes. From FESEM examination, the prepared PES/CA membrane presented thinner outer skin layer, higher surface porosity with larger pore sizes. The outer surface water contact angle of the prepared membrane significantly decreased when CA was added into the polymer solution. The higher water flux of the PES/CA membrane was related to the higher hydrophilicity and larger pore sizes of the membrane. From oil–water separation test, the PES/CA membrane showed stable oil rejection of 88 % and water flux of 27 l/m2 s after 150 min of the operation. In conclusion, by controlling fabrication parameters a developed membrane structure with high hydrophilicity, high surface porosity and low resistance can be achieved to improve oil rejection and water productivity.

Microporous PVDF hollow-fiber membranes intended for use in gas absorption were prepared via a dry-wet phase-inversion method. The membranes were characterized in terms of gas permeation, wetting resistance, mass transfer resistance, overall porosity, and outer surface contact angle. When 4 wt% glycerol was used in the polymer dope, FESEM showed that the membrane had an almost sponge-like structure with an ultrathin outer skin layer. High wetting resistance and N2 permeance with an overall porosity of 77 % and a mean pore size of 0.07 μm were observed for the membranes. A gas–liquid membrane contactor module was used to experimentally investigate the effects of the main operating parameters on CO2 absorption with aqueous DEA solution. Increasing the liquid flow rate led to a significant increase in the CO2 absorption flux, whereas increasing the CO2 concentration in the gas feed from 20 % to 100 % barely affected the CO2 flux. These results indicated that the main resistance came from the liquid phase; the resistance of the gas phase was negligible. The results of a long-term experiment indicated that the CO2 flux dropped by 26 % during the initial 10 h, but then remained stable until the end of operation.

Membrane contactors using microporous membranes for acid gas removal have been extensively reviewed and discussed. The microporous membrane acts as a fixed interface between the gas and the liquid phase without dispersing one phase into another that offers a flexible modular and energy efficient device. The gas absorption process can offer a high selectivity and a high driving force for transport even at low concentrations. Using hollow fiber gas-liquid membrane contactors is a promising alternative to conventional gas absorption systems for acid gas capture from gas streams. Important aspects of membrane contactor as an efficient energy devise for acid gas removal including liquid absorbents, membrane characteristics, combination of membrane and absorbent, mass transfer, membrane modules, model development, advantages and disadvantages were critically discussed. In addition, current status and future potential in research and development of gas-liquid membrane contactors for acid gas removal were also briefly discussed.