Effect of additives concentration on performance of cellulose acetate and polyethersulfone blend membranes

Asymmetric micro porous membranes have been prepared successfully from blending of cellulose acetate (CA) and polyethersulfone (PES) by the phase inversion method with N, N-dimethylformamide (DMF) as solvent. Two additives were selected in this study, including polyethylene glycol 600 (PEG 600) and polyvinylpyrrolidone (PVP). The effects of concentration of additives on CA/PES blend membrane performance and cross-section morphology were investigated in detail. CA/PES membranes were compared with CA/PES/PEG and CA/PES/PVP membranes in the performance such as pure water flux, membrane resistance, porosity and cross-section morphology. The resulting blend membranes were also carried out the rejection and permeate flux of Egg Albumin (EA) proteins with molecular weight of 45 Da. The membranes thus obtained with an additive concentration of 5 wt% of both PEG and PVP exhibited superior properties than the 80/20% blend composition of CA and PES membranes. The permeate flux of protein was increased from 44 to 134 lm² h with increase in concentrations of both PVP and PEG in 80/20% blend composition of CA and PES membranes. Cross-sectional images from scanning electron microscopy showed larger macropores in the bottom layer of the membranes with increasing additives content. Observations from scanning electron microscopy provided qualitative evidence for the trends obtained for permeability and porosity results.     

Effect of secondary additives in casting solution on the performance of porous cellulose acetate reverse osmosis membranes

The solution structure and evaporation rate constant can be varied by changing the temperature of the casting solution and the temperature of the casting atmosphere for a given film-casting solution composition. The effects of the two temperature changes can be simulated (without changing the two temperatures) by replacing a small part of the solvent (acetone) by a secondary additive in the casting solution. The effect of 20 different secondary additives in the batch 316-type casting solution has been studied and is discussed. Porous cellulose acetate reverse osmosis membranes, capable of giving a 20% to 25% increase in productivity at a 90% level of solute separation for a 3500 ppm NaCl–H₂O feed solution at 250 psig, have been produced using 5 wt-% ethyl ether as the secondary additive in the above casting solution. The use of secondary additives offers a new flexibility in the choice of film-casting conditions and in the general development of reverse osmosis membranes.     

Hydrogen/carbon monoxide separation with cellulose acetate membranes

Hydrogen/carbon monoxide ratio adjustment is of fundamental interest to the petrochemical and energy industries for the production of C₁ chemicals. Cellulose acetate membranes have been found to be effective for this service and can offer improvements over other gas purification techniques in several respects including reduced capital and operating costs. Membrane and cryogenic processes are compared for the specific case of manufacture of a 1/1 H₂/CO syngas for oxoalcohol production. A particular advantage of the membrane process is its flexibility. This has benefited a recently installed facility for the Central Research Institute of the Electrical Power Institute of Japan for variable adjustment of the hydrogen to carbon monoxide ratio at a test facility demonstrating coal gasification/ gas turbine power production. Design and operating details of this facility are discussed. The membrane process may also be used to produce a pure CO product. The performance of polysulphone and cellulose acetate are compared for this application.     

Pervaporation separation of water/isopropanol mixtures through crosslinked carboxymethyl chitosan/polysulfone hollow‐fiber composite membranes

Carboxymethyl chitosan (CMCS)/polysulfone (PS) hollow‐fiber composite membranes were prepared through glutaraldehyde (GA) as the crosslinking agent and PS hollow‐fiber ultrafiltration membrane as the support. The permeation and separation characteristics for dehydration of isopropanol were investigated by the pervaporation method. Pure chitosan, carboxymethyl chitosan, and crosslinked carboxymethyl chitosan membranes were characterized by Fourier transform infrared (FT‐IR) spectroscopy and X‐ray diffraction (XRD) to study the crosslinking reaction mechanism and degree of crystallinity, respectively. The effects of feed composition, crosslinking agent, membrane thickness, and feed temperature on membrane performance were investigated. The results show that the crosslinked CMCS/PS hollow‐fiber composite membranes possess high selectivity and promising permeability. The permeation flux and separation factor for isopropanol/water is 38.6 g/m²h and 3238.5, using 87.5 wt % isopropanol concentration at 45°C, respectively. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 1959–1965, 2007     

Reverse osmosis separation of binary organic mixtures using cellulose acetate butyrate and aromatic polyamide membranes

A systematic investigation has been conducted to demonstrate applicability of reverse osmosis (RO) fractionation of organic liquid mixtures by laboratory-prepared cellulose acetate butyrate (CAB) and aromatic polyamid (PA) membranes. The determination of preferential sorption was also conducted by using liquid chromatography technique. It was found that reverse osmosis was applicable to the fractionation of organic liquid mixtures. It was also found that the component of the binary mixture that is enriched in the membrane permeate can be predicted by considering the preferential sorption and Stokes' radius for each constituent of the feed mixtre.     

Synthesis and characterization of polyacrylamidegrafted sodium alginate membranes for pervaporation separation of water + isopropanol mixtures

Polyacrylamide‐grafted‐sodium alginate copolymers were prepared by persulfate‐induced radical polymerization by using polymer‐to‐monomer ratios of 2 : 1 and 1 : 1. Polymers were characterized by Fourier transform infrared spectroscopy, differential thermal analysis, and viscosity. Membranes were prepared from the polymers, crosslinked with glutaraldehyde, and used in the pervaporation separation of water + isopropanol mixtures at 30°C. Equilibrium swelling experiments were performed for mixtures containing 10 to 80 mass % of water in the feed. Both the grafted copolymer membranes were ruptured while separating 10 mass % of water in the feed mixture. However, beyond 20 mass % of water in the feed mixture, flux increased with increasing grafting ratio, while selectivity decreased. Pervaporation separation experiments were carried out at 30, 40, and 50°C for 20 mass % of water in the feed mixture. By increasing the temperature, flux increased, whereas selectivity decreased. Arrhenius activation parameters for pervaporation and diffusion decreased with increasing grafting ratio of the membranes. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 2030–2037, 2004     

Pervaporation of water/alcohol mixtures through chitosan/cellulose acetate composite hollow‐fiber membranes

For the purpose of separating aqueous alcohol by the use of pervaporation technique, a composite membrane of chitosan (CT) dip‐coated cellulose acetate (CA) hollow‐fiber membranes, CT‐d‐CA, was investigated. The effects of air‐gap distance in the spinning of CA hollow‐fiber membranes, chitosan concentration, and sorts of aqueous alcohol solutions on the pervaporation performances were studied. Compared with unmodified CA hollow‐fiber membrane, the CT‐d‐CA composite hollow‐fiber membrane effectively increases the permselectivity of water. The thickness of coating layer increases with an increase in chitosan concentration. As the concentration of chitosan solution increased, the permeation rate decreased and the concentration of water in the permeate increased. In addition, the effects of feed composition and feed solution temperature on the pervaporation performances were also investigated. The permeation rate and water content in permeate at 25°C for a 90 wt % aqueous isopropanol solution through the CT‐d‐CA composite hollow‐fiber membrane with a 5‐cm air‐gap distance spun, 2 wt % chitosan dip‐coated system were 169.5 g/m² h and 98.9 wt %, respectively. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 94: 1562–1568, 2004     

Effects of polymer solvents on the performance of cellulose acetate membranes in methanol/methyl tertiary butyl ether separation

The performances of cellulose acetate membranes prepared with casting solutions, with acetone, dimethylformamide (DMF), and N‐methylpyrrolidone (NMP) as solvents, were studied in a series of methanol/methyl tertiary butyl ether separation experiments. The flux and selectivity of the membrane samples were affected by the type of solvent used to prepare the casting solution. The sample with DMF consistently gave the highest selectivity and lowest flux, followed by the samples with NMP and acetone. The differences in the performances were attributed to the effects of the volatility and evaporation rates of the solvents. Scanning electron microscopy and atomic force microscopy techniques were used for comparing the morphologies of the membranes. In addition, we used Raman spectroscopy as a novel technique to study the sorption selectivities of the membrane samples prepared with the three different solvents. In a parallel study, the relation between the polymer concentration in the casting solution and the morphology and performance of the membrane samples was studied. Under similar preparation conditions, the morphology of the membrane changed from being porous to being dense when the membrane was prepared with casting solutions with increasing polymer concentration. Also, the selectivity increased and the permeability decreased with increasing polymer concentration in the casting solution. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 2882–2895, 2001     

ZSM-5沸石填充聚氨酯膜的制备及渗透汽化分离水中乙酸异丙酯

采用两步法制备了ZSM-5沸石填充的疏水性端羟基聚丁二烯基聚氨酯（PU）膜,用以分离水中芳香性有机物乙酸异丙酯。对该膜的化学结构、形貌及热稳定性进行了表征,并研究了ZSM-5沸石填充的PU膜的溶胀度及渗透汽化性能。结果表明：添加ZSM-5沸石后,膜的热稳定性明显提高,沸石与膜的相容性较好,且随着添加量的增加,膜的溶胀度降低,分离因子先升后降。在303 K、料液浓质量分数为1%的条件下,ZSM-5添加量为20%（质量分数）时,分离因子达到最高;同时随着料液浓度及操作温度的上升,通量和分离因子都增加。在333 K、料液质量分数为1%的条件下,PU-ZSM-5-20膜的分离因子及通量最高可达288.72 g/（m2·h）和53.21 g/（m2·h）。     

Pervaporation separation of isopropanol/benzene mixtures using inorganic–organic hybrid membranes

A series of pervaporation hybrid membranes were prepared from polyethylene glycol (PEG) and phenylaminomethyl trimethoxysilane (PAMTMS) based on the sol‐gel process, in which PEG was used as an organic moiety to improve the affinity for organic alcohols and silicone of PAMTMS was used as inorganic moiety to increase the permeation flux of organic species. Their application to separate isopropanol/benzene mixtures was investigated. FTIR spectra confirmed the reaction products. DSC measurement revealed that the influence of PEG content on the T_g and thermal behavior of membranes A, B, and C. FE‐SEM images exhibited that phase‐separated structure has occurred when the PEG content elevated to some extent. Pervaporation experiments showed that the permeation flux increased and the separation factor decreased with an increase in isopropanol (IPA) content in feed at 30°C. Meanwhile, the separation factor increased with an increase in feed temperature at 60 vol % IPA content. Moreover, it was found that the permeation flux was independent of the feed temperature, suggesting that feed temperature has little impact on the thermal motion of polymer chains. The increasing cross‐linking degree in hybrid matrix might be responsible for such trend. Based on these findings, it can be concluded that these pervaporation hybrid membranes have potential applications in the separation of isopropanol/benzene binary mixtures. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Freezing and nonfreezing water in cellulose acetate membranes

The relative amounts of freezing and nonfreezing water in various cellulose acetate (CA) membranes were determined by differential scanning calorimetry. It was found that: (1) A significant fraction (17–40%) of the water (1.0–3.1 g H₂O per gram dry CA) in any membrane does not freeze at temperatures as low as ?60°C. (2) The amount of nonfreezing bound water (0.4–0.7 g nonfreezing water per gram dry CA) depends upon the nature of the membrane and is significantly higher than the total amount of water (all of which is nonfreezing) absorbed from liquid water by a dense film of the same polymer (～0.18 g water per gram dry CA). The structures of the membranes were studied by scanning electron microscopy. The results suggest that the amounts of nonfreezing water in cellulose acetate membranes decrease with the increase in the packing density (compactness) of the polymer within the membrane. In dense films, the extent of polymer–polymer interactions within the polymeric matrix is high, and therefore the macromolecular chains are less accessible to bind water.     

Permselectivity of cellulose acetate membranes; separation of H2/CO mixtures and the effect of added transition metals

Cellulose acetate membranes permit permeation of H₂ and CO₂ but are relatively impervious to CO and N₂. Permselectivity was demonstrated by effective separation of H₂ from its mixtures with CO. The presence of RuCl₃ in the membrane does not result in any appreciable change in permselectivity. The exposure of RuCl₃- and RhCl₃-modified membranes to H₂/CO mixtures results in the formation of RuCl₂(CO)_x and [RhCl(CO)₂]₂, respectively. However, these complexes are not covalently anchored to the cellulose acetate matrix and apparently function only as additives that block access to the press and channels in the cellulose acetate membrane.     

Studies on the solvent exchange technique for making dry cellulose acetate membranes for the separation of gaseous mixtures

The effect of the pore size on the surface of the water-wet reverse osmosis membrane and the solvents used in the solvent exchange process on the pore size of the resulting dry cellulose acetate membrane was studied with respect to the separation of a CO₂/CH₄ mixture. It has been found that there is a critical pore size on the surface of wet membrane that results in the smallest pore size on the dry membrane and, consequently, in the highest separation factor. Such critical pore sizes become greater when the boiling point of the second solvent is increased.     

Influence of the dilute‐solution properties of cellulose acetate in solvent mixtures on the morphology and pervaporation performance of their membranes

The pervaporation performance of cellulose acetate (CA) membranes prepared from acetone (AC), acetone/tetrahydrofuran (AC/THF), acetone/chloroform (AC/CF), and acetone/cyclohexane (AC/CYH) was studied for separating MeOH/MTBE (methyl tert‐butyl ether) mixture with 5 (wt) % MeOH. The dilute‐solution properties and Huggins constant (K_H) of CA dissolved in AC and AC/solvent mixtures with 15 vol % of the second solvent (tetrahydrofuran, chloroform, or cyclohexane) were examined. J and α of the CA membranes were affected by the types of solvent mixtures used to prepare the casting solutions. Under the same conditions, the membrane with AC/CYH had the highest J value and the lowest α value, and it was followed by the membranes with AC/CF, AC/THF, and AC. The increasing value of J and decreasing value of α for the CA membranes from different solvent mixtures were in good agreement with the increasing value of K_H of CA in corresponding solvent mixtures. Furthermore, differences in the morphology from scanning electron microscopy images of the cross sections or from atomic force microscopy photographs of the surfaces of the membranes existed, and they provided proof of the different pervaporation performances of the CA membranes prepared from AC and AC/solvent mixtures. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 97:1891–1898, 2005     

The states of water in cellulose acetate membranes

Differential scanning calorimetric melting endotherms of wet and half-dried cellulose acetate membranes and salt distribution coefficients were studied to clarify the states of water in membranes. We have suggested that (a) there are four states of water in cellulose acetate membranes; (b) these states of water are those of completely free water, free water very weakly interacting with polymer, bound water which can contain salts, and bound water which rejects salts; (c) the semipermeability of membranes depends on the ratio of four states of water in membranes.     

Effect of casting conditions on the performance of porous cellulose acetate membranes in reverse osmosis

Several sets of porous cellulose acetate membranes were made using the same casting solution composition and gelation conditions but varying the casting solution temperature and solvent evaporation conditions. The films were tested in reverse osmosis experiments at 250 psig using aqueous feed solutions containing 3500 ppm NaCl. The results show that the product rate obtained at a given level of solute separation is independent of evaporation time in the range tested and, for a given casting solution composition, the temperature of the casting solution and conditions of solvent evaporation during film formation together constitute an important interconnected variable governing the porous structure of the resulting membranes. These results offer a new approach to the problem of developing more productive reverse osmosis membranes and have led to a new class of porous cellulose acetate membranes capable of giving product rates 100% to 150% higher than those of the best membranes reported, at any given level of solute separation under the experimental conditions used. These results are of practical importance in low-pressure reverse osmosis applications.     

The thermodynamic state of water in cellulose acetate membranes

Using experimental sorption data and corresponding experimental results from calorimetric investigations, the state of water in cellulose acetate (CA) membranes is discussed by applying a theoretical treatment of sorption reported previously (1–3). The sorption of water can be attributed to a gain in surface energy at the polymer/vapor interface. Using differential thermodynamic potentials of sorbed water together with experimentally determined heat capacities of sorbed water, the thermodynamic potentials G, H, and S of sorbed water are estimated for the temperature interval ?40 to + 40°C. At constant temperature, each thermodynamic potential depends on the water content. The resulting distribution function of G indicates that the sorbed water exists in different states. Comparing the Gibbs free energy of sorbed water with that of ice or liquid water at the same temperature leads to the conclusion that none of the sorbed water freezes to ice within the temperature interval used. Based on the Gibbs free energy of water in electrolyte solutions and the distribution function of G for sorbed water, partition coefficients of salts within CA membranes may be estimated. The results are in good agreement with experimentally determined partition coefficients which are available from the literature. As the partition coefficient of a salt is directly related to the salt rejection of the membrane, this provides a method of estimating the desalination performance of a membrane from its water sorption isotherm.     

Pervaporation performance and Transport phenomenon of PVA blend membranes for the separation of THF/water azeotropic mixtures

Summary Dense polymer membranes were made by mixing aqueous solutions of hydrophilic polymers poly(vinyl alcohol) (PVA) and polyethyleneimine (PEI) in different ratios for investigating the separation of Tetrahydrofuran (THF)/water azeotropic mixtures by pervaporation (PV). In order to gain a more detailed picture of the molecular transport phenomenon, we have performed sorption gravimetric experiments at 30 °C to compute diffusion, swelling, sorption and permeability coefficients of PVA/PEI membranes in the presence of THF and water. The membranes were found to have good potential for breaking the azeotrope of THF at 6% concentration of water. An increase in PVA content in the blend caused a reduction in the flux and an increase in selectivity. Among the blends tested in the study, the 5:1 PVA/PEI blend membrane showed the highest separation factor of 181.5, exhibited a flux of 1.28 kg/m²h for THF respectively at azeotropic feed composition.     

Effect of surface-modifying macromolecules and solvent evaporation time on the performance of polyethersulfone membranes for the separation of chloroform/water mixtures by pervaporation

Polyethersulfone (PES) membranes using different evaporation periods were fabricated by the phase-inversion method. Pervaporation experiments were conducted for chloroform/water mixtures to determine the selectivity of the PES membranes. It was found that chloroform could be concentrated in the permeate from chloroform/water binary feed mixtures by PES membranes prepared using longer evaporation periods, and that the selectivity of PES membranes in pervaporation could be reversed by shorter evaporation periods. This study also showed that by adding surface-modifying macromolecules (SMM) up to 1 wt % into the casting solution, chloroform enrichment in the permeate could be increased by 50%. Chloroform enrichment increased with increasing SMM concentration until an optimal value, after which the enrichment decreased. © 1994 John Wiley & Sons, Inc.