Effect of non-solvent additives on the structure and performance of PVDF hollow fiber membrane contactor for CO2 stripping

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

Development of Porous Asymmetric Polyamide–Imide Torlon® Membranes for Physical CO2 Absorption and Separation

Porous flat-sheet polyamide–imide (PAI) membranes were prepared via a phase inversion method to evaluate CO2 absorption performance in the gas-liquid membrane contactors. Different amounts of polyethylene glycol (PEG-600) were introduced into the polymer solution to investigate the structure and performance of resulted membranes. The membranes were characterized in terms of gas permeation, contact angle measurement and CO2 absorption flux. By introducing 6 wt.% PEG into the polymer dope, N2 permeance of the membrane was significantly improved from 482 to 1320 GPU. Mean while, the effect of PEG on the measured water contact angle was in significant. From CO2 absorption test, the developed membrane presented about 90% higher CO2flux compared to the plain membrane at water flow rate of 70 ml/min. In conclusion, by introducing a polymeric non-solvent additive into the polymer dope, it is possible to enhance surface porosity (permeability) of PAI membranes, which is a key factor for CO2 absorption test

Influence of membrane morphology on characteristics of porous hydrophobic PVDF hollow fiber contactors for CO2 stripping from water

Wetting resistance and gas permeability are the main factors for membrane contactor applications, which can be optimized according to the membrane morphology. In present study, three different types of the membrane morphology were obtained via a dry–wet spinning technique. By measuring cloud point data and viscosity, the polymer dope composition was adjusted to produce the different morphologies. The membranes with large finger-like, small finger-like and almost sponge-like morphology were obtained. The plain PVDF membrane with large finger-likes morphology presented the higher N2 permeance, lower wetting pressure and larger mean pore size (0.08 µm). By addition of phosphoric acid into the spinning dope, the prepared sponge-like morphology resulted in the high surface porosity with small pore sizes, which demonstrated good permeability and wetting pressure. It was found that the mean pore size measured by gas permeation method was approximately three times larger than those from FESEM examination. CO2 stripping from water was conducted through the gas–liquid membrane contactors. The membranes with smaller pore sizes and higher wetting pressure presented higher stripping performance. In conclusion, a structurally developed PVDF hollow fiber membrane for gas–liquid contactor applications can be achieved by controlling the membrane morphology

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

Porous polyvinylidenefluoride (PVDF) hollowfibermembranes were fabricated via a wet phase-inversion method. Ortho-phosphoric acid and lithium chloride monohydrate (LiCl·H2O) were used as phase-inversionpromoteradditives 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 hollowfibermembranes 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 hollowfibermembrane 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 CO2absorption in gas–liquid membrane contactors

Preparation and characterization of porous PVDF hollow fiber membranes for CO2 absorption: effect of different non-solvent additives in the polymer dope

Different types of non-solvent additives were introduced into the polyvinylidene fluoride (PVDF) dope to investigate improvement of the hollow fiber membrane structure for CO2 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). CO2 absorption by distilled water was conducted through the gas–liquid membrane contactors. The PVDF/glycerol membrane demonstrated higher CO2 absorption flux than the other membranes. At the absorbent flow rate of 280 ml/min, CO2 flux of 7.8 × 10-4 mol/m2 s was achieved, which was approximately 30% higher than CO2 flux of the plain PVDF membrane. In conclusion, a developed membrane structure prepared by controlled phase-inversion process can be a promising alternative for CO2 capture in gas–liquid membrane contactors

Effect of additives on the structure and performance of polysulfone hollow fiber membranes for CO2 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 (CEPw) and CO2 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 CEPw and CO2 absorption rate than the other PSf hollow fiber membranes

Preparation of Microporous PVDF Hollow Fiber Membrane Contactors for CO2 Stripping from Diethanolamine Solution

Microporous polyvinylidene fluoride (PVDF) hollow fiber membranes were fabricated via a wet-spinning process. The prepared fibers were characterized in terms of contact angle, gas permeability, wetting pressure and morphology. CO2 stripping from preloaded aqueous diethanolamine solution was conducted through the gas–liquid membrane contactors. The effects of lithium chloride (LiCl) concentration on the membrane properties and CO2 stripping performance were investigated. The addition of different LiCl concentration resulted in a less finger-like structure, with highly effective surface porosity and high wetting pressure. Conversely, N2 permeability, contact angle value and membrane pore size were reduced as the concentration of the additives were increased. For stripping flux performance, membrane with 5 wt% LiCl demonstrated the highest flux of 1.61 × 10−2 mol/m2 s at 110 ml/min of the liquid flow rate. It was found that the membrane with less finger-like structure and higher effective surface porosity exhibited higher stripping efficiency. These results suggest that developed membrane structure can be an alternative for the CO2 stripping through gas–liquid membrane contactors