The electret effect in cellulose acetate reverse osmosis membranes

The electret potentials developed by reverse osmosis electret membranes help control the undesirable deposition of charged colloidal particles on the membrane surfaces during membrane desalination. These antifouling electret membranes should help prevent the costly flux declines normally associated with deposition of colloidal iron oxides on the reverse osmosis membrane surfaces. Homocharge and heterocharge behavior of cellulose acetate membrane electrets have been studied. Asymmetric reverse osmosis membranes and dense membrane films were studied. The homocharge and heterocharge of cellulose acetate reverse osmosis electret membranes have been explained.     

Hydrodynamic resistance and flux decline in asymmetric cellulose acetate reverse osmosis membranes

Long-term flux-versus-time measurements were made on a series of homogeneous, dense cellulose acetate (39.8% acetyl) membranes. All tests were carried out at 1500 psi applied pressure on a 3.5% NaCl feed. These tests show that essentially all the resistance to solvent (water) flow in the typical asymmetric reverse osmosis membrane is concentrated in the thin dense layer. Further, the evidence strongly suggests that the longterm flux decline noted in asymmetric membranes is a result of viscoelastic deformation of the dense layer.     

醋酸纤维素衍生物反渗透膜对低分子量有机物分离特性研究

研究用醋酸纤维素衍生物氰乙基醋酸纤维素、羟丙基醋酸纤维素和醋酸纤维素反渗透膜对有机醇、有机酸及有机胺类水溶液的分离特性,考察操作压力对反渗透分离特性的影响,并应用不可逆热力学过程得出的Spiegler—Kedem膜输送方程解析实验数据,获得了反射系数σ、溶质渗透系数ω和溶剂水渗透系L_p等膜参数。     

Development and characterization of cellulose acetate benzoate flat osmotic membranes

Different parameters of casting solutions and casting conditions were studied for the development of cellulose acetate benzoate flat osmotic membranes. Casting solutions were prepared with different concentrations of the polymer, the additive, and the solvent; viscosity of the casting solution; and the thickness of the membrane developed. The membranes were given different evaporation periods and annealing temperatures under different RH. Different annealing baths were also used. Based on these, conditions were optimized for the development of cellulose acetate benzoate flat osmotic membranes. These membranes were characterized with respect to bound water content, specific water content, transport properties by direct osmosis, salt intake by direct immersion, water permeability coefficient of the dense membrane, diffusion coefficient, salt permeability, and salt distribution by electrical conductivity. Also, cellulose acetate benzoate membranes were compared with conventionally used cellulose acetate membranes. © 1994 John Wiley & Sons, Inc.     

Improving permeation flux by pulsed reverse osmosis

Pulsed and nonpulsed reverse osmosis experiments were conducted with a sucrose solution in cylindrical cellulose acetate membranes. For pulsing frequencies up to 1 Hz, permeation increases of more than 70 per cent were observed. A simple quasisteady-state theory for concentration polarization based on a film model of mass transfer describes the increase.     

Cellulose membranes for reverse osmosis part II. Improving RO membranes prepared from non-woody cellulose

Experiments were carried out to minimize the stages of preparing reverse osmosis (RO) desalination membranes at economical cost and to improve the transport properties of RO membranes prepared from non-wood fibrous materials (cotton linters and bagasse pulp) to approach those prepared from imported viscose pulp and purchased cellulose acetate (see Part I). Further study was carried out on examining the effects of long-term testing and subsequent washing of membranes by a special mixture. Finally the quality of the product water reverse osmosis was examined by comparing its water analysis with standard specification for drinking and domestic water.     

Preparation of ultrathin reverse osmosis membranes and the attainment of theoretical salt rejection

Cellulose acetate reverse osmosis membranes, 600–2800 A. thick, have been prepared on glass surfaces by dipping a clean glass plate into a dilute solution of cellulose acetate. After drying, the membranes are floated of onto a water surface and placed on molecular filter supports. Theoretical salt rejections, as calculated from the solution-diffusion model of membrane transport for cellulose acetate, were obtained with imperfection-free membranes.     

High flux freeze-dried cellulose acetate reverse osmosis membranes as microporous barriers in gas permeation and separation

The permeations of helium, hydrogen, methane, ethylene, nitrogen, and argon, and helium–methane, nitrogen–ethylene, oxygen–nitrogen, and sulfur dioxide–nitrogen mixtures have been studied using freeze-dried porous cellulose acetate reverse osmosis membranes. The results illustrate the existence of mobile and immobile sorbed layers and the governing influence of surface flow in gas-phase reverse osmosis separations. Preshrunk freeze-dried porous cellulose acetate membranes seem to offer a practical means of utilizing the reverse osmosis process for recovering helium from natural gas and separating sulfur dioxide from flue gases.     

Performance of solution-cast membranes of poly(hydroxythyl methacrylate) in osmosis and reverse osmosis

Noncrosslinked copolymers of hydroxyethyl methacrylate (HEMA) with ethyl methacrylate (EMA) were prepared and investigated as candidate materials for reverse osmosis membranes. Water and salt flux were calculated from the results of osmosis experiments, compared with cellulose acetate and found to be somewhat smaller. The mobility of water varies by several orders of magnitude between a high flux cellulose acetate membrane and a low flux HEMA-EMA membrane.     

Microstructure and ordering in cellulose acetate and reverse osmosis membranes

The performance of cellulose acetate reverse osmosis membranes for desalination or purification is greatly affected by the microstructure of the membrane. It is, therefore, highly desirable to characterize the microstructure and its dependence on preparation conditions and past history. In this study, various types of cellulose acetate powders, flakes, and solvent cast films have been characterized by differential scanning calorimetry and thermo-optical analysis. It is shown that ordered microstructures exist in many of these samples and that this ordering can be intensified or diminished by suitable treatments. It is conjectured that a similar microordering occurs in the dense layer of asymmetric cast membranes as a result of solvent evaporation, gelation and annealing and that the extent of orientation and chain packing in the ordered regions greatly affects the performance of reverse osmosis membranes.     

Precise measurement of membrane constants of cellulose acetate membranes by direct osmosis tests

A method for the determination of membrane constants by direct osmosis tests was studied using cellulose acetate membranes. A countercurrent type osmosis cell was designed and made for this study, and a method for precise measurements of permeated water and solutes through the membrane was established. Based on the membrane constants derived from direct osmosis tests, membrane performances of cellulose acetate membranes under pressure of 40 atm were predicted. The predicted values were in good agreement with the observed values in reverse osmosis experiments and it was confirmed that membrane performances under pressure could be predicted by the direct osmosis with considerably good accuracy.     

Novel polymer alloy membranes composed of poly(4-vinyl pyridine) and cellulose acetate. I. Asymmetric membranes

The preparation of a novel asymmetric membrane cast from a homogeneous mixture of poly(4-vinyl pyridine) and cellulose acetate is described. The two polymers were found to be compatible in the presence of Lewis acids, yielding hydrophilic homogeneous alloys when properly prepared. Asymmetric membranes were cast from a six-component dope mixture containing chloroform, methanol, acetone, and formamide. The as-cast solution, when coagulated in water at ambient temperature, yields a highly plasticized anisotropic matrix which solidifies into a glassy state as the chloroform slowly diffuses into the water. The membranes, following this stage, exhibit a semipermeable dense skin that produces high fluxes and salt rejections when tested in a high-pressure reverse osmosis mode (no annealing is required). The anisotropic morphology was confirmed by scanning electron microscopy studies which revealed a dense skin resting on a highly porous, open-celled foamlike structure. This structure does not collapse upon drying and retains its original wet dimensions. Brief attempts to quaternize the matrix are also reported.     

Water and solute transport across cellulose acetate membranes in the treatment of soybean whey by reverse osmosis

In processing full-fat soy flour to produce an acid-precipitated lipid protein concentrate, there results a by-product whey fraction which, because of its high biological oxygen demand, represents a serious disposal problem. Processing of food waste streams by reverse osmosis has received considerable attention because of its low theoretical energy requirement, since no phase change is involved. A series of statistically designed and analyzed experiments were conducted on a pilot-plant reverse osmosis unit to study the effect of the operating parameters on solute and solvent transport in cellulose acetate membranes. Sucrose and sodium chloride solutions were tested in addition to soybean whey to relate the mixed solute system in whey to that of single-solute organic and inorganic feed solutions. Water flux was shown to have an Arrhenius dependency on temperature, and some membrane compaction was observed with the more porous membrane. Concentration polarization for sucrose and sodium chloride solutions increased linearly with water flux. Solute flux for soybean whey solutions decreased with molarity and was independent of pressure, whereas solute rejection increased with temperature and pressure and was independent of molarity. Good agreement was obtained using the derived parameters A, B, and τ for soy whey in the diffusion transport model when compared to the observed experimental values.     

New polymer materials for reverse osmosis membranes

The results of investigations on new polymer materials suitable for reverse osmosis are reported. The studies particularly concerned polypiperazinamides derived from differently structured bicarboxylic acids. Such polyamides show a water permeability that may vary from 0.09 μg cm^?1 sec^?1 to 2.3 μg cm^?1 sec ^?1 depending on the structure.The results obtained from reverse osmosis tests on dense and homogeneous films are described. Anisotropic membranes were prepared from poly(trans-2,5-dimethyl-piperazin-thiofurazanamide). Their properties very closely approach those of the corresponding cellulose acetate membranes. The anisotropic structure of the membranes has been evidenced by stereoscan electron microscope analysis.     

Concentration and pressure dependence of rate of membrane permeation

The permeation of water and sodium chloride in cellulose acetate membranes has been examined over a wide range of concentration and pressure. The results obtained from reverse osmosis experiments have been used to evaluate relations derived on the assumption that permeation of both solution components takes place by diffusion down a concentration gradient in the membrane. With the aid of equilibrium and raidoactive tracer measurements, most of the deviations could be attributed to nonconstancy of the diffusion, and, especially, the distribution coefficients of water. A comparison of the net flux in reverse osmosis with the rate of tracer permeation in the same membrane provided positive evidence to show that hydrodynamic flow under pressure cannot account for the water flux through the membrane. Differences in the shape of the distribution isotherms for salt and water between solution and membrane provide an explanation for the high selectivity of cellulose acetate membranes in favor of water.     

Synthesis and characterization of an interpenetrating polymer network composed of poly(methacrylic acid) and poly(vinyl alcohol)

Temperature and pH‐responsive interpenetrating polymer network (IPN) hydrogels, constructed with poly(methacrylic acid) (PMAA) and poly(vinyl alcohol) (PVA), by a sequential IPN method, were studied. The characterization of IPN hydrogels was investigated by Fourier‐transform infrared spectroscopy, differential scanning calorimetry (DSC) and swelling under various conditions. The IPN hydrogels exhibited relatively high swelling ratios, in the range 230–380 %, at 25 °C. The swelling ratios of the PMAA/PVA IPN hydrogels were pH and temperature dependent. DSC was used for the quantitative determination of the amounts of freezing and non‐freezing water. The amount of free water increased with increasing PMAA content in the IPN hydrogels. Copyright © 2004 Society of Chemical Industry     

Study of the structure and transport of asymmetric polyamide membranes for reverse osmosis using the electron spin resonance (ESR) method

The electron spin resonance technique (ESR) was used to study the structure and transport of asymmetric aromatic polyamide membranes. TEMPO (2,2,6,6-tetramethyl-1-piperridinyloxy free radical) was used as a spin probe that was brought into the membrane either by (a) immersion ofthe membranes in aqueous TEMPO solutions, (b) reverse osmosis (RO) experiments with feed solutions involving TEMPO or (c) blending TEMPO in casting solutions. The membranes were further tested for the separation of sodium chloride and TEMPO from water by RO. It was concluded that aromatic polyamide membranes contain water channels in the polymer matrix like cellulose acetate membranes. The presence of such water channels allows aromatic polyamide membranes to be used as RO membranes. The diffusion of organic solutes through the water channels seems much slower in aromatic polyamide membranes than in cellular acetate membranes, which probably causes a higher separation of organic solutes by aromatic polyamide membranes than cellulose acetate membranes. A comparison was made with other RO membranes (cellulose acetate, CA) and ultrafiltration membranes (polyethersulphone, PES). It was observed that the ESR technique can be used to study the structure of OF and RO membranes. The presence of water channels in the polymer matrix seems indispensable for the RO membrane.     

Silylcellulose Acetate Membrane for Desalination of Water

Abstract

Commercial cellulose acetate (39.9% acetyl) was partially modified with different proportions of Trimethylchlorosilane. The physical properties of modified polymers were studied. The membranes prepared from these polymers were evaluated for desalination of water by reverse osmosis. The thermal stability of modified cellulose acetate was also investigated.     

The use of direct osmosis tests as complementary experiments to determine the water and salt permeabilities of reinforced cellulose acetate membranes

In this article the use of the direct osmosis test as a complementary experiment to the reverse osmosis test is described. The membranes investigated are cellulose acetate membranes reinforced with mineral fillers. In both tests the same influence of the type of fillers on the water permeability coefficients is found. The salt permeability coefficients indicate the presence of pressure sensitive defects in the reinforced membranes. The direct osmosis test is found to be a suitable test to confirm and predict the membrane properties under reverse osmosis conditions.