A.F. Ismail

A review on the latest development of carbon membranes for gas separation

Inorganic membranes have been developed before 1945. The earlier application of inorganic membranes was primarily concentrate on military purpose. Carbon membrane is one type of porous inorganic membrane. Although the concept of carbon membrane for gas separation has been found in the early 1970, the interest to develop carbon membrane only increased, since Koresh and Soffer successfully prepared apparently crack-free molecular sieving hollow fiber carbon membranes. Nowadays, plenty of researchers have used different polymeric materials; including polyimides, to prepare carbon membranes by using pyrolysis. In general, carbon membranes can be divided into four major configurations: flat sheet, membrane supported on tube, capillary, and hollow fiber. Permeation properties of carbon membranes have been improved greatly in these 20 years. Carbon membranes offer advantages over polymeric membranes especially in terms of selectivity as well as thermal and chemical stability. More attention will be paid to carbon membranes in this century. This paper will review the development of carbon membranes in the last 30 years and give a clear future direction in research for carbon membrane.

Penetrant-induced plasticization phenomenon in glassy polymers for gas separation membrane

Penetrant-induced plasticization of gas separation membranes has been the subject of a number of recent investigations. The plasticization of the polymer matrix by penetrant gases can be attributed to the swelling stresses on the polymer network. It is well known that sorption of carbon dioxide in glassy polymers can facilitate the local segmental organization with a reduction in the permselectivity and substantially affect membrane morphological performance. Hence, plasticization is a phenomenon that most frequently encountered in polymer–gas systems for commercial CO2/CH4 separation applications where the membranes are exposed to high CO2 concentration in the feed stream. This paper attempts to present an overview of the plasticization phenomenon and the alteration of physical properties resulting from the polymer plasticization by the sorbed penetrant molecules. The plasticization suppression methods in polymeric membrane for gas separation and the future direction of research in this area are also given.

Influence of the thermastabilization process and soak time during pyrolysis process on the polyacrylonitrile carbon membranes for O2/N2 separation

The hollow fiber membranes were fabricated using the dry/wet spinning process while polyacrylonitrile (PAN) carbon hollow fiber membranes were produced using inert gas pyrolysis process. The influences of the thermastabilization conditions and soak time on carbon membrane performance were investigated. It was observed that oxidative thermastabilization atmosphere produced carbon membranes with high stability while longer soak time improved the membrane selectivity and reduced the membranes permeability.

Optimization of cellulose acetate hollow fiber reverse osmosis membrane production using Taguchi method

Cellulose acetate hollow fiber membranes for reverse osmosis (RO) were spun using a forced convection technique. In this study, a systematic experimental design based on Taguchis method (which is a fractional factorial method) has been employed for discussing the relationship between the rejection rate coefficient, permeation rate and the dry-wet spinning conditions for making cellulose acetate hollow fibers for RO. The factors considered in the experimental design included the polymer contents (PCs), the ratio of the solvent (acetone) to swelling agents (formamide) in the dope solution, the dope extrusion rate (DER), the type of bore fluid (BF), the residence time (RT) and the nitrogen gas flushing rate (GR). The results indicate that the BF and the DER are the two most important factors in determining the performance of the RO membranes.

A study of extrusion shear and forced convection residence time in the spinning of polysulfone hollow fiber membranes for gas separation

Polysulfone hollow fiber membranes for gas separation were spun using a forced convection technique. Experiments were designed to decouple the effect of extrusion shear from forced convection residence time in the dry gap allowing both factors to be investigated. The main objective was to study the pure influence of shear and its capacity to increase membrane selectivity. The results suggested that extrusion shear influences phase inversion dynamics. Increasing shear decreased active layer thickness and increased pressure-normalized flux. This was discussed in terms of thermodynamic instability and polymer precipitation/coalescence speed. Increasing shear was found to increase selectivity to levels greater than the intrinsic value for the amorphous membrane polymer. This may be as a result of induced molecular orientation in the active layer. However, a critical shear rate existed beyond which selectivity deteriorated. This was attributed to the development of surface pores as the active layer thins. Membranes spun at intermediate forced convection residence times exhibited the highest selectivities. Skin formation must be complete, but excessive residence time allows deleterious non-solvent encroachment from the lumen. The results indicate that if enhanced selectivity and high flux are to be achieved, membranes should be spun at a high shear rate and an optimized residence time in order to minimize surface defects, increase the critical shear rate, decrease active layer thickness and heighten molecular orientation.

Fabrication of polydopamine functionalized halloysite nanotube/polyetherimide membranes for heavy metal removal

Polydopamine modified halloysite nanotubes (HNTs) were synthesised through a one step facile procedure and employed as a well dispersed hydrophilic additive to enhance the filtration properties of polyetherimide (PEI) membranes. The nanocomposite membranes were prepared by an immersion precipitation method with different amounts of modified HNTs (MHNTs) in the casting solution. The good dispersion of MHNTs throughout the membrane matrix was confirmed by elemental mapping analysis. The prepared nanocomposite membranes were extensively studied in terms of their porosity, morphology, membrane hydraulic resistance and hydrophilicity. The permeation experiments showed that the modified membranes exhibited higher water flux than a pristine PEI membrane. The antifouling and anti-biofouling behaviour of the modified membranes was investigated in detail. The results revealed that a membrane with a 3 wt% MHNT dosage showed a higher Fouling Resistance Ratio (FRR) of 74.5% with reversible membrane fouling of 64.3%. Moreover, the membrane showed excellent resistance to microbial growth on the membrane surface. The well performing membrane was subjected to heavy metal ion rejection. Results indicated that membranes had the capacity to adsorb Pb2+ and Cd2+. Overall, PEI–MHNTs nanocomposite membranes could have great potential to improve antifouling, anti-biofouling and filtration properties.

Suppression of plasticization in polysulfone membranes for gas separations by heat-treatment technique

polysulfone (Udel P1700). Two types of membranes with heat treated and untreated were prepared to study the effect of heat-treatment process on its permeation properties. For untreated membrane, CH4 permeation rate remained almost constant with applied gas pressure. CO2 permeation rate exhibited an increase with increasing feed pressure, indicating that CO2 plasticized the membrane material. The asymmetric membranes were given different heating temperature and different treatment durations to investigate the suppression of undesirable CO2 plasticization. Permeation rates were reduced with the intensity of heat treatment. Experimental results showed that the membranes were stabilized against CO2 plasticization after heat-treatment process, especially at temperature of 140 °C. Heating temperature has tremendous effect on the gas transport properties for treated membrane at 180 °C, giving an unusual behavior of CO2. Plasticization is a pressure-dependent and time-dependent phenomenon. For the original membrane, the relaxation permeation rate of CO2 is linearly related to the logarithmic experimentation time. The CO2 permeation rate for treated membrane maintains at steady state over the applied feed pressures. Thus, the heat-treatment method to suppress CO2 plasticization and achieve better asymmetric membrane performance was effective.

Study of shear rate influence on the performance of cellulose acetate reverse osmosis hollow fiber membranes

The effect of shear rate on the separation performance of reverse osmosis hollow fiber membrane is discussed. Experiments involving six different dope extrusion rates (DERs) (ranging 2.5-5 ml/min) are performed with the other process factors set at the optimum conditions determined by the Taguchi analysis. This will enable an assessment to be made on the relationship between the DER and the rejection rate. The regression method is used to analyse the experimental results and an empirical model has been developed. Simultaneously, it is found that there is a fairly strong correlation between extrusion shear rate and the rejection rate of the membranes, whereby as the shear rate increases, the rejection rate increases until a critical level of shear is achieved, beyond which reverse osmosis membrane performance deteriorates, suggesting that there exists an optimum shear rate which yields optimal membrane morphology for reverse osmosis hollow fiber membranes.

Effects of shear rate and forced convection residence time on asymmetric polysulfone membranes structure and gas separation performance

The objectives of this study are to illustrate the effects of shear rate and forced convection residence time on asymmetric polysulfone membrane structure and gas separation performance. The membranes were produced by a simple dry/wet phase inversion technique using a pneumatically-controlled flat sheet membrane casting system. Varying the casting speed varied shear rate. Rheologically induced molecular orientation in membranes during casting was measured directly using plane polarized reflectance infrared spectroscopy technique. The highly sheared asymmetric membranes tend to exhibit greater molecular orientation in the skin layer. Thus, a high pressure-normalized flux and selectivity were obtained. The mean pressure-normalized fluxes of O2 and CO2 were about 5.05 and 11.41 GPU, respectively. The selectivity of O2/N2 and CO2/CH4 were approximately 6.72 and 32.63, respectively, at shear rate of 367 s−1. However, increasing forced convection residence time in the dry phase inversion step resulted in lower pressure-normalized flux but higher selectivity membrane. The best membrane performance obtained based on the trade-off between pressure-normalized flux and selectivity was observed at forced convection residence time of 20 s and at 367 s−1 shear rate.

Effect of polymer concentration on the structure and performance of PEI hollow fiber membrane contactor for CO2 stripping

A series of polyetherimide (PEI) hollow fiber membranes with various polymer concentrations (13-16 wt.%) for CO2 stripping process in membrane contactor application was fabricated via wet phase inversion method. The PEI membranes were characterized in terms of liquid entry pressure, contact angle, gas permeation and morphology analysis. CO2 stripping performance was investigated via membrane contactor system in a stainless steel module with aqueous diethanolamine as liquid absorbent. The hollow fiber membranes showed decreasing patterns in gas permeation, contact angle, mean pore size and effective surface porosity with increasing polymer concentration. On the contrary, wetting pressure of PEI membranes has enhanced significantly with polymer concentration. Various polymer concentrations have different effects on the CO2 stripping flux in which membrane with 14 wt.% polymer concentration showed the highest stripping flux of 2.7 × 10(-2)mol/m(2)s. From the performance comparison with other commercial membrane, it is anticipated that the PEI membrane has a good prospect in CO2 stripping via membrane contactor.