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.     

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.     

Water and solute transport through cellulose acetate reverse osmosis membranes

Reverse osmosis data on two different cellulose acetate membranes using seven organic solutes of varying molecular weight have been obtained.A combined viscous-flow and frictional model is presented and used to estimate the maximum retention, the friction between solute and membrane, the distribution coefficient for solute and the pore radius.The calculated values of the maximum retention and distribution coefficient have been compared with the Ferry-Faxen equation. For the more open membrane these are in good agreement. The tighter one, however, shows a greater interaction between solute and membrane than predicted by the Faxen equation.Some data on two-solute systems are presented and shown to give variation in the retention, which can be explained from the convection term.Furthermore, for experiments with dextran the permeate shows a significant reduction in both $\text{M}$_n and $\text{M}$_t     

Modification of cellulose acetate reverse osmosis membranes by radiation grafting

Large excesses of a chain transfer agent, carbon tetrachloride, were introduced to a recipe for the mutual radiation grafting of styrene to cellulose acetate film. The effect of the carbon tetrachloride on the molecular characteristics as well as the reverse osmosis and time dependent mechanical properties of resulting graft copolymers was determined. Extremely short side chains were generated as a consequence of the high concentrations of chain transfer agent and the composite results further suggest that the morphology of the grafted films is best described as “destructured” or internally plasticized consequent to grafting in the presence of CCl₄. Reverse osmosis fluxes increased with percent graft; salt rejection was high and unaffected by per cent graft up to 40% graft; and the tensile creep under wet conditions was significantly retarded by the grafting. These effects were shown to accrue from grafting per se by control experiments involving α-methylstyrene which will not propagate to form a polymer under these conditions. These results are compared and contrasted with earlier work on grafting in the absence of CCl₄ where long side chains of polystyrene were generated resulting in a structuring of the polymer involving domains of polystyrene-rich material and domains of cellulose acetate rich polymer.     

Parameters for prediction of reverse osmosis performance of cellulose acetate propionate membranes

Data on reverse osmosis separations have been obtained for 12 alkali metal halide solutes and 24 organic solutes (including eight alcohols, four aldehydes, seven ketones, and five ethers) with cellulose acetate propionate (CAP) membranes using single-solute dilute aqueous feed solutions at 250 psig. From the analysis of these data, the parameters and correlations needed to calculate the values of solute transport parameter D_AM/Kδ for the above classes of inorganic and organic solutes for a CAP membrane of any surface porosity from data on D_AM/Kδ for NaCl only have been generated. These parameters and correlations enable one to predict reverse osmosis separations of different solutes included in the classes of compounds studied in this work, from a single set of experimental data on membrane specifications given in terms of pure water permeability constant and D_AM/Kδ for NaCl. The reverse osmosis characteristics of CAP material lie intermediate between those of cellulose acetate and aromatic polyamide materials reported in the literature.     

Development and performance of some porous cellulose acetate membranes for reverse osmosis desalination

The effects of casting solution composition and evaporation period on the performance of resulting porous cellulose acetate membranes have been studied, and the results are discussed in terms of casting solution structure, solvent evaporation rate during film formation, and the film shrinkage temperature profile. The development of Batch 316-type porous cellulose acetate membranes is reported. At 90% level of solute separation and feed flow conditions corresponding to a mass transfer coefficient of 45 × 10^?4 cm/sec, the productivities of the above membranes are 21.5 gallons/day/ft² at 250 psig using 3500 ppm of NaCl in the feed, and 53.9 gallons/day/ft² at 600 psig using 5000 ppm of NaCl in the feed.     

Apparatus for rapid casting of tubular cellulose acetate reverse osmosis and ultrafiltration membranes

An improved apparatus is described for making tubular cellulose acetate membranes for reverse osmosis and ultrafiltration applications. The incorporation of an adjustable centering-bob and a sleeve in the design of the casting-bob housing, and the inclusion of an automatically controlled electrical water probe at the bottom of the casting-bob are the novel features of the apparatus. The adjustable centering-bob offers the capability of regulating the passage for the flow of the casting solution during film casting; this capability makes the casting-bob housing useful for a wide range of casting solution viscosities necessary for making both reverse osmosis and ultrafiltration membranes. The sleeve incorporated in the design makes it possible to switch from one tube-casting to the next immediately without any need for the intermediate time consuming operation of cleaning up the casting-bob system. Thus a single casting-bob housing is sufficient for making a plurality of membranes, one after another, with little loss of time between castings. The water probe maintains in the casting tube any desired length of air-zone for the freshly cast membrane. The operation of the apparatus is amenable to a high degree of automation. These features make the apparatus particularly suitable for industrial utilization.     

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.     

Improvement of porous cellulose acetate reverse osmosis membranes by change of casting conditions

The effects of temperature of casting solution in the range ?10° to 15°C, that of casting atmosphere in the range 10° to 30°C, relative humidity of casting atmosphere in the range 35% to 75%, and solvent evaporation period in the range 0.5 to 3 min were studied on shrinkage temperatures, solute separations, and product rates of Loeb-Sourirajan-type cellulose acetate membranes in reverse osmosis experiments. The composition of casting solution used was as follows: cellulose acetate, 17; acetone, 69.2; magnesium perchlorate, 1.45; and water, 12.35 wt-%. Best performance was obtained with membranes cast under the following conditions: temperature of casting solution, 10°C; temperature of casting atmosphere, 30°C; relative humidity of casting atmosphere, 65%; and solvent evaporation period, 1 min. For a 90% level of solute separation, the productivities of the above type of membranes were 22.9, 61.4, and 64.5 gallons/day-ft² at 250, 600, and 1500 psig using 3500 ppm NaCl–H₂O, 5000 ppm NaCl–H₂O, and 28395 ppm NaCl–H₂O feed solutions, respectively. In all cases, the feed flow rates corresponded to a mass transfer coefficient of 45 × 10^?4 cm/sec on the high-pressure side of the membrane. The general specifications of the above type of membranes are given for the operating pressures of 250, 600, and 1500 psig. The effects of the above casting condition variables on the surface pore structure during film formation are discussed.     

Cellulose membranes for reverse osmosis Part I. RO cellulose acetate membranes including a composite with polypropylene

With the aim of obtaining RO membranes for brackish water desalination from purified celluloses (cotton linters and bleached bagasse pulp), two reactions (heterogeneous and homogeneous) were applied for the synthesis of cellulose acetate (CA). The efficiency of the membranes was measured and compared with those prepared from purchased CA and prepared CA by acetylation of imported high-grade viscose wood pulp. The effect of blending CA with polypropylene (PP), on the efficiency of the prepared RO membranes was also studied. Results showed that the method of preparation of CA plays a profound effect on the salt rejection and water flux of the RO membranes. The efficiencies of RO membranes formed from heterogeneously acetylated celluloses are higher than those prepared from homogeneous ones. Blending the acetylated cellulose with 9% PP wastes improves the efficiency of membranes prepared from the homogeneously acetylated celluloses.     

Reverse osmosis characteristics of cellulose acetate butyrate membranes

Cellulose acetate butyrate membranes were cast from five different formulations. The pure water flux through the membrane increased with evaporation period. The separation of 4000 ppm NaCl aqueous solution remained unchanged until it reached a critical flux; at that point, separation decreased inversely proportional to the flux. Scanning electron microscope photography of the membranes corresponding to each evaporation period is reported.     

Permeability of phenol through cellulose acetate membranes by reverse osmosis in various alcoholic systems

Reverse osmosis separation of phenol in various alcoholic solutions using porous cellulose acetate membranes was investigated. The permeation behavior of phenol was measured for cellulose acetate membranes having various pore size distributions which were prepared by annealing at four different temperatures. Some differences were found between the aqueous and the alcoholic solutions in solute permeabilities and product rates. Membranes annealed at 90°C showed higher permselectivity than membranes annealed at lower temperatures. The pore character was classified into two types according to the relation of the product rate of 1-propanol and that of water. It was found in a series of alcoholic solutions that the permeability of phenol, the product rate, and the apparent partition coefficient are closely related to the carbon number of the alcohols, but the values of J_v × η (ca. 1.25 × 10^?4 poise·m³/m²·day) and of the permselectivity coefficient (ca. 0.83) remain constant. The result was analyzed by using the three-dimensional solubility parameter to obtain some information for the partition mechanism of solutes in aqueous and alcoholic solutions.     

Structure of porous cellulose acetate membranes and a method for improving their performance in reverse osmosis

Experimental data support the hypothesis that the surface layer of the asymmetric Loeb-Sourirajan type porous cellulose acetate membranes has a heterogeneous microporous structure. A general method is proposed for improving the performance of the above membranes in reverse osmosis, by which product rates are increased without decreasing solute separation. The method consists in pumping pure water past the back side of the membrane under just enough pressure for a sufficiently prolonged period of time; after such pretreatment, the membrane is used in the reverse osmosis experiments in the normal manner with the surface layer facing the feed solution. Back-pressure treatment at 400 psig for 85 hr on preshrunk and normally pressure-treated membranes increases the product rate by over 20% without decreasing solute separation in reverse osmosis experiments at 600 psig with the use of 0.5 wt-% NaCl–H₂O feed solutions; with a different sequence of back-pressure treatment, similar results have been obtained in reverse osmosis experiments at 1500 psig also. The compaction effect of a normal membrane and that of a back pressure treated membrane are the same during continuous reverse osmosis operation under 600 psig; the effects of back-pressure treatment on a normal membrane and a compacted membrane are also the same. The pure water permeability data obtained in cyclic experiments show that the smaller pores on the surface layer are opened more than the bigger ones during the back side operation. The probable structural changes taking place in the film during back-pressure treatment are discussed.     

Prediction of reverse osmosis performance of cellulose acetate membrane

This study illustrates the analytical techniques involved in specifying the membrane and outlines the procedure for predicting the reverse osmosis (RO) performance of these membranes using feed solutions, containing either single solutes or mixed electrolytes having a common ion. The scientific basis for such specification and prediction techniques has been extensively discussed in the literature. In the present work, the governing transport equations for RO systems, involving preferential sorption of water at the membrane–solution interface, are utilized.     

Permselectivities of some aromatic compounds in organic medium through cellulose acetate membranes by reverse osmosis

Reverse osmosis of some aromatic compounds in 1-propanol solution using porous cellulose acetate membranes and some factors which influence the organic solute permeability were investigated. Reverse osmosis data for a number of benzene derivatives showed that only phenol was rejected and the others were enriched. For the compounds with various substituent groups, the solute permeabilities have the following order; OH < CH₃ < H < Cl < NH₂ < NO₂. In the series of benzene, naphthalene, and anthracene, the permeability is related to the molar volume of solutes and varied as follows: benzene < naphthalene < anthracene. Generally speaking, there are two factors—a partition coefficient and a diffusion coefficient—for governing the permeation behavior through the membrane. Under the condition that no steric effect is exerted, the partition coefficient exclusively is a dominant factor. A good correlation between the partition coefficient of a particle and that of the membrane was pound. The partition coefficients are closely related to the dipole moment of solutes.     

The effect of precipitating media on the performance of porous cellulose acetate reverse osmosis membranes

A two-stage method of making semi-permeable high flux reverse osmosis membranes was developed using water-ethanol mixtures to precipitate the cellulose acetate. This eliminated the need for heat treatment and produced membranes with fluxes up to 5 m³/m² day and sodium chloride rejections up to 85 %. Their properties are compared with membranes made by the three-stage method involving heat treatment and show more sensitivity to the effects of pressure.