Critical Water Entry Pressure Research Articles

Liquid-liquid extraction of phenolic wastewater through a surface modified poly(vinylidene fluoride-co-hexafluoropropylene) hollow fiber membrane contactor and process optimization

BackgroundStrict environmental regulations have been recognized for phenolic wastewater treatment due to toxicity of the phenolic compounds. In this study, solvent extraction of a phenolic wastewater was investigated via a liquid-liquid membrane contactor system. MethodsPorous poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) hollow fiber membranes were modified by coating an ultra-thin layer of hydrophobic surface modifying macromolecule (SMM). The membranes were characterized by various tests such as SEM-EDS, nitrogen permeation, overall porosity, contact angle and critical water entry pressure (CEPw). Significant findingsFrom SEM, an open structure with combination of finger-like and sponge-like structure was produced. The membrane exhibited high surface hydrophobicity (water contact angle of 109°) with good wetting resistance, overall porosity of 82% and mean pore size of 18 nm. From response surface methodology (RSM), the maximum phenol extraction flux of 2.02 × 10−3 kg/m2s was estimated by the model at the optimum process variables as phenol concentration of 448 mg/L, feed temperature of 42 °C, aqueous phase flowrate of 196 mL/min and organic phase flowrate of 117 mL/min. The coefficient of determination (R2) of 0.972 showed a close proximity between the model and the experimental data. The membrane showed a reasonable flux that can be a favorable alternative for industrial phenolic wastewater treatment.

Hydrophobic Modification of Polyetherimide Hollow Fiber Membrane Contactor by 2-(Perfluoroalkyl) Ethanol Coating for Carbon Dioxide Absorption

Highly porous polyether imide (PEI) hollow fiber membranes were fabricated through a phase-inversion process. Surface modification of the membranes was performed by coating an ultra-thin layer of 2-(perfluoroalkyl) ethanol (ZONYL). The properties of the prepared membranes were evaluated through field emission scanning electronic microscopy (FESEM), water contact angle, N2 permeation, overall porosity, critical water entry pressure (CEPw) and collapsing pressure experiments. From FESEM, the PEI membrane showed an open structure with large finger-likes cavities. By coating the ZONYL layer, the modified membrane presented improved outer surface water contact angle (105 °), CEPw (450 kPa), and collapsing pressure (750 kPa). The improved PEI-ZONYL membrane presented an almost stable CO2 absorption flux (6.8×10-4 mol/m2 s) during 72 h of the gas-liquid membrane contactor operation while a significant flux reduction was observed for the plain PEI membrane. Therefore, the improved membrane with high surface hydrophobicity and porosity can be a promising candidate for high performance gas-liquid membrane contactors.

Extraction of penicillin-G from pharmaceutical wastewaters via a developed hydrophobic PVDF-HFP hollow fiber membrane contactor and process optimization

Discharge of pharmaceutical wastewaters can cause substantial environmental impacts. Since the conventional wastewater treatment processes are not efficient for comprehensive separation of pharmaceutical substances, a non-dispersive solvent extraction via a hollow fiber membrane contactor can be a promising alternative. In the present study, a structurally developed poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) hollow fiber membrane was fabricated via a non-solvent induced phase separation (NIPS) process to enhance extraction of penicillin-G (Pen-G) from the wastewater. Response surface methodology (RSM) was used for optimization of the main operating variables in the liquid–liquid membrane contactor process. From FESEM examination, the prepared membrane showed an open structure with large finger-likes, ultra-thin outer skin, and inner skinless layer. The membrane presented a narrow pore size distribution with mean pore size of 0.038μm, water contact angle of about 94°, critical water entry pressure of 350 kPa, and overall porosity of about 84 %. Extraction of Pen-G from the aqueous phase by 5 wt% organic solution of Aliquat 336 was performed. The feed concentration of 218 ppm, temperature of 41 °C, flowrate of 51 ml/min, and the organic phase flowrate of 144 ml/min were the optimum values predicted by the model. A maximum Pen-G extraction flux of 1.55×10−3 kg/m2 s was predicted at the optimum operating conditions. The value of error between the experimental and predicted fluxes was about 3.23% which confirmed accuracy of the model. Due to the developed membrane structure, the prepared PVDF-HFP hollow fiber membrane showed a considerably higher Pen-G extraction flux compared to the PP commercial membranes.

Surface modified porous polyetherimide hollow fiber membrane for sweeping gas membrane distillation of dyeing wastewater

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.

Improvement of porous polyvinylidene fluoride-co-hexafluropropylene hollow fiber membranes for sweeping gas membrane distillation of ethylene glycol solution

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 N 2 permeation, overall porosity, critical water entry pressure (CEP w ), 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 N 2 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 . CEP w 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.

Influence of Air-Gap Length on CO2 Stripping from Diethanolamine Solution and Water Performance of Surface Modified PVDF Hollow Fiber Membrane Contactor

Surface Modifying Macromolecule (SMM) blended PVDF hollow fibers (HFs) were spun at different air-gaps (o to 20 cm) and used for CO2 stripping from aqueous DEA solution and water. The manufactured fibers were firstly subjected to various characterization tests such as contact angle and critical water entry pressure measurement to evaluate the HF hydrophobicity and wetting resistance, respectively. The pure helium permeation experiments were also conducted to obtain membrane pore size and effective porosity. Morphology of the HFs was investigated by Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM). The SEM images showed that both outer and inner diameters of HFs decreased significantly by increasing air-gap length which mainly because of elongation of HF caused by gravity while travelling through the air-gap. Also, the gradual decrease in roughness on the external surface of the produced HFs was observed from the AFM images. It was found that the increase of liquid velocity enhances the CO2 stripping flux. It was found that 10 cm air-gap gave maximum stripping flux of 3.34 x 10(-2) and 1.34 x 10(-3) (mol/m(2)s) for DEA solution and water, respectively. The increase in gas velocity, on the other hand, did not affect the stripping flux significantly. It was observed that the increase of temperature from 25 to 80 degrees C led to the marked enhancement of stripping flux from 6.30 x 10(-3) to 3.34 x 10(-2) (mol/m(2)s) and 6.5 x 10(-5) to 1.34 x 10(-3) (mol/m(2)s), for DEA solution and water, respectively. Furthermore, the increase in DEA concentration from 0.25 to 1 mol/L, led to the enhancement of the stripping flux from 6.84 x 10(-3) to 3.34 x 10(-2) (mol/m(2)s) at a liquid velocity of 0.7 m/s. It was concluded that the HF spun at 10 cm air-gap exhibited the best stripping performance among all fabricated HFs.

Fabrication and Characterization of Polyetherimide Hollow Fiber Membrane Contactor for Carbon Dioxide Stripping from Monoethanolamine Solution

Article history: In this research, process asymmetric polyetherimide hollow fiber membranes using ethanol (0, 2 and 4 wt%) as non-solvent additive in the polymer dope via phase inversion method were fabricated. Aqueous solution of 1-methyl-2-pyrrolidine (NMP) (90%) was applied as a bore fluid to avoid inner skin layer formation and water was used as the external coagulant. The morphology of fabricated membranes was examined using field emission scanning electron microscope (FESEM). A gas permeation test was conducted using Nitrogen. Fabricated membranes were characterized in terms of pore size, critical water entry pressure, water contact angle and collapsing pressure. The performance of fabricated membranes for carbon dioxide stripping from monoethanolamine solution using a gas - liquid membrane contactor system was studied. The results showed that carbon dioxide stripping flux and efficiency increased by increasing liquid velocity. Also, enhancement of stripping flux by increasing gas velocity was negligible. By increasing MEA solution temperature, stripping flux increased; therefore, liquid phase temperature is a key parameter which needs to be controlled.

Surface modification of polysulfone hollow fiber membrane spun under different air-gap lengths for carbon dioxide absorption in membrane contactor system

Surface modified polysulfone (PSf) hollow fiber membranes (HFMs) using surface modifying macromolecules (SMM) were spun with air-gaps of 0–50cm. Morphological analysis of the prepared fibers were carried out using scanning electron microscopy (SEM) and atomic force microscopy (AFM). The wetting resistances of the membranes were determined by critical water entry pressure (CEPw) and contact angle (CA) measurements. The HMFs were further subjected to CO2 capture by membrane contactor (MC) where water was used as absorbent. The test results indicated that both CEPw and CA showed a maximum value at 15cm air-gap distance. The maximum He permeation was also achieved at 15cm air-gap. The CO2 flux of prepared membranes showed a maximum of 4.79×10−3mol/m2s at the absorbent flow rate of 300ml/min. It was deduced that both He permeation and CO2 absorption flux were governed by the HFM surface porosity. The long-term stability test revealed the reduction of 25% in CO2 flux during the first 55h of operation for the PSf membrane prepared at 15cm air-gap. This study indicated that the surface modified HFMs prepared using an appropriate air-gap could be a promising option to increase membrane performance in MC systems.

Study on the effect of air–gap length on properties and performance of surface modified PVDF hollow fiber membrane contactor for carbon dioxide absorption

Surface modified polyvinylidene fluoride (PVDF) hollow fiber membranes (HFMs) were spun via dry–wet spinning technique at different air–gap lengths (0–20cm). The morphology of prepared membranes was evaluated by scanning electron microscopy (SEM) and atomic force microscopy (AFM). Membranes were also characterized in terms of gas permeation, overall porosity, collapsing pressure, critical water entry pressure (CEPw) and contact angle. To determine the CO2 absorption flux of HFMs, a gas–liquid membrane contactor system was used. Experimental results of this study revealed that by increasing the air–gap distance from 0 to 20cm, wetting resistance and contact angle of fabricated membranes increased due to enhancement of membrane surface hydrophobicity. The highest helium (He) permeation was achieved for the spun fiber at the air–gap of 10cm. From CO2 absorption experiment it was found that the hollow fiber spun at the air–gap of 10cm had the maximum CO2 absorption flux of 1.41×10−3mol/m2s at the absorbent flow rate of 300ml/min, which was significantly higher than CO2 absorption flux obtained by other researchers. It was also found that both highest He gas permeance and CO2 absorption flux were controlled by the surface porosity of the hollow fiber due to the maximum values obtained. Thus, the choice of an appropriate air–gap distance for fabrication of surface modified membranes was found to be a promising method to improve CO2 removal in membrane contactor systems.

Effect of SMM concentration on morphology and performance of surface modified PVDF hollow fiber membrane contactor for CO2 absorption

Surface modified polyvinylidene fluoride (PVDF) hollow fiber membranes were fabricated via a dry–wet phased inversion process. Surface modifying macromolecules (SMMs) (0, 2, 4 and 6wt.%) were used as additives in the spinning dope. During phase inversion SMM migrates to the membrane surface, resulting in different surface morphology and surface chemistry. The surface modified PVDF membranes showed the larger pore size, higher gas permeance, effective surface porosity, contact angle and overall porosity but lower critical water entry pressure compared to the PVDF hollow fiber membrane without SMM. The performance of the surface modified membrane in contactor application for physical CO2 absorption was investigated by the fabricated gas–liquid membrane contactor module, where distilled water was used to dissolve CO2. It was found that the liquid phase resistance was dominant in the absorption experiment. The results show that the surface modified PVDF membrane has a higher performance compared to control PVDF membrane. By increasing SMM concentration in the spinning dope, the CO2 absorption flux increased significantly. With the membrane prepared from 6wt.% of SMM in the spinning dope, a maximum CO2 absorption flux of 5.4×10−3mol/m2s was achieved at 300mlmin−1 of absorbent flow rate, which was almost 650% more than the fabricated membrane without SMM.

Effect of novel surface modifying macromolecules on morphology and performance of Polysulfone hollow fiber membrane contactor for CO2 absorption

A novel surface modified Polysulfone (PSf) hollow fiber membrane was fabricated via dry–wet phased inversion process. surface modifying macromolecule (SMM) was used as additive in the spinning dope. The surface modified membrane showed large pore size, higher effective surface porosity, contact angle and porosity but lower critical water entry pressure (CEPw) compared to Polysulfone hollow fiber membrane without SMM. The performance of surface modified membrane in contactor application for CO2 absorption via distilled water as absorbent was studied. The results show that surface modified membrane has higher performance compared to plain Polysulfone membranes. With the membrane prepared from SMM in the spinning dope a maximum CO2 flux of 5.8×10−4mol/m2s was achieved at 300ml/min of absorbent flow rate, which was almost 76% more than the other membrane. In a long-term stability study, the initial flux reduction was found to be about 18% within the 50h of operation for surface modified membrane.

Effect of different additives on the physical and chemical CO2 absorption in polyetherimide hollow fiber membrane contactor system

Porous asymmetric polyetherimide (PEI) hollow fiber membranes were fabricated via a phase-inversion method using ethanol, glycerol and acetone as the additives in the spinning dope. Also, hollow fiber PEI membrane without additives was fabricated. An aqueous solution of 1-methyl-2-pyrrolidone (80wt.%) was used as bore fluid to prevent forming of an inner dense skin layer. The precipitation rate of the polymer dopes with the different additives was studied using cloud point measurement. The effect of the additives on the resulting membrane structure, surface porosity, pore size, critical water entry pressure, collapsing pressure and physical and chemical CO2 absorption performance by distilled water and NaOH (1M) solution in a gas–liquid membrane contactor system were investigated and compared. Cloud point diagrams indicated that the precipitation rate of the polymer dopes increased following the trend of ethanol>acetone>glycerol. Results of gas permeation tests showed that ethanol and glycerol as additives provided the membranes with the largest and smallest pore size, respectively. Moreover, all the additives resulted in an increase in the effective surface porosity. The cross-section of the membranes was examined via a scanning electron microscopy. Ethanol in the spinning dope provided the membrane structure with a sublayer with finger-like macrovoids, originating from the inner and outer surfaces of the hollow fiber and extending to the middle section of the hollow fiber wall, which resulted in a larger pore size and higher CO2 absorption rate than the other PEI hollow fiber membranes.

A novel surface modified polyvinylidene fluoride hollow fiber membrane contactor for CO2 absorption

A novel surface modified polyvinylidene fluoride (PVDF) hollow fiber membrane was fabricated via a dry–wet phased inversion process. A surface modifying macromolecule (SMM) was used as an additive in the spinning dope. During phase inversion SMM migrates to the membrane surface and functions as both a pore former and surface modifier. The surface modified PVDF membrane showed large pore size, higher effective surface porosity, contact angle and porosity but lower critical water entry pressure compared to the PVDF hollow fiber membrane without SMM. The performance of the surface modified membrane in contactor application for CO2 absorption via distilled water as absorbent was studied. The results show that the surface modified PVDF membrane has higher performance compared to control PVDF membranes. With the membrane prepared from SMM in the spinning dope a maximum CO2 flux of 7.7×10−4 mol/m2 s was achieved at 300 ml/min of absorbent flow rate, which was almost 93% more than the other membrane. In a long-term stability study, CO2 flux was decreased only about 7.7% by using surface modified PVDF membrane during 150 h operation.

Effect of bore fluid composition on structure and performance of asymmetric polysulfone hollow fiber membrane contactor for CO2 absorption

Porous asymmetric polysolfone (PSf) hollow fiber membranes were fabricated using wet spinning process. Glycerol (4wt.%) was used as additive. Aqueous solutions of 1-methyl-2-pyrrolidone (NMP) with different composition (0, 50, 70 and 90wt.% NMP) were used as bore fluid. Distilled water was used to preparing aqueous NMP solutions. Structure and performance of resulting membranes in measuring of surface porosity, pore size, critical water entry pressure (CEPw) and CO2 absorption in hollow fiber membrane contactor were investigated. The cross-section, the inner skin layer and the inner surface of the membranes were examined via scanning electronic microscopy (SEM). Using distilled water as bore fluid provided the membrane structure with a finger-like macrovoids which extend from the inner and outer surfaces of the membranes to the middle of the hollow fiber wall. Other hollow fiber membranes formed with a finger-like structure near the other surface and a sponge-like structure with holes beneath, which resulted in a high CEPw and CO2 absorption rate. In addition, since a mixture of distilled water and NMP was used as the neutral bore fluid, the membranes showed a skinless inner surface. Results of gas permeation tests showed with increasing NMP concentration in the bore fluid the N2 permeance decrease significantly and the wetting resistance decrease slightly. CO2 absorption by distilled water was conducted through the gas–liquid membrane contactors. By employing 90% NMP in the bore fluid, the PSf membrane showed a CO2 flux of approximately 110% higher than the PSf membrane fabricated with distilled water as the internal coagulant at the absorbent velocity of 7.9×10−4m/s.

Development of asymmetric polysulfone hollow fiber membrane contactor for CO2 absorption

Porous asymmetric polysulfone (PSf) hollow fiber membranes were fabricated via wet spinning process. A mixture of 1-methyl-2-pyrrolidone (NMP) and distilled water (90wt.% NMP) was used as bore fluid to make hollow fiber membranes without inner skin layers, whereas glycerol with 2 and 4wt.% was used as additives to the spinning dope. The effect of the additives concentration on the structure, surface porosity, pore size, critical water entry pressure (CEPw), and CO2 absorption flux of the fabricated membranes were investigated. The cross-section and inner surface of the membranes were examined via scanning electronic microscopy (SEM). Using glycerol as the additive in the spinning dope provided the membrane structure with a thin finger-like and a thick sponge-like layer, which resulted in a high CEPw and CO2 absorption rate. Results of gas permeation tests showed with increasing glycerol concentration the N2 permeance increase significantly and the wetting resistance decrease slightly. CO2 absorption by distilled water was conducted through the gas–liquid membrane contactors. By using 4% glycerol in the spinning dope, the PSf membrane showed a CO2 flux of approximately 57% higher than the PSf membrane with 2% glycerol at the absorbent flow rate of 200ml/min.

Novel chemical surface modification to enhance hydrophobicity of polyamide-imide (PAI) hollow fiber membranes

Membrane wetting is a major concern for the application of membrane contactors. In order to develop a novel hydrophobic membrane that can be used in membrane contactors, a hydrophobic surface modification method for Torlon ® polyamide-imide (PAI) membrane was explored, which has not been reported previously. Asymmetric microporous hollow fiber membranes were fabricated using PAI material followed by a simple surface modification using a silane coupling agent, 3-aminopropyltrimethoxysilane (APTMS), and a perfluro-compound (Fluorolink S10) to produce PAI hollow fiber membranes with a hydrophobic surface. The reactions between PAI and the modification reagents have been confirmed by experiments. Optimal modification conditions were investigated by varying the modification parameters, such as APTMS treatment temperature, APTMS concentration, APTMS treatment time, and Fluorolink S10 treatment time. The modified PAI membranes were, subsequently, characterized by a series of standard protocols in terms of membrane morphology, chemical structure, contact angle, porosity and critical water entry pressure. A preliminary study of using the modified membranes in a membrane contactor for CO 2 absorption was also carried out. The newly developed PAI hollow fiber membranes showed much higher hydrophobicity, the contact angle was improved from 76° to 108–117° and the critical water entry pressure was improved from 0.7–0.8 bar to 2.1–4.0 bar. Moreover, the modified PAI hollow fiber membranes showed reasonably high CO 2 flux by using water as a physical absorption liquid. The results indicate that the modified PAI membranes have great potential to be used in membrane contactor applications.

A developed asymmetric PVDF hollow fiber membrane structure for CO 2 absorption

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.

A comparative study on the structure and performance of porous polyvinylidene fluoride and polysulfone hollow fiber membranes for CO 2 absorption

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 CO 2 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 CO 2 flux compared to the commercial asymmetric PVDF membrane. The CO 2 flux of 8.20 × 10 −4 mol/m 2 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 CO 2 absorption and separation through membrane contactors.

Preparation of polyvinylidene fluoride hollow fiber membranes for CO2 absorption using phase-inversion promoter additives

Abstract 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.

Effect of additives on the structure and performance of polysulfone hollow fiber membranes for CO 2 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.