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.     

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.     

Improved cellulose acetate membranes for reverse osmosis

Performance of cellulose acetate membranes in reverse osmosis varies with the conditions under which they are cast. By varying casting solution composition and holding time in a systematic way, improvement in water flux at a given level of salt rejection has been obtained. Statistically designed experiments have been helpful in optimizing these two variables. A phase diagram of the cellulose acetateformamide-acetone casting system has been determined which gives the region of natural solubility of this three component system.     

Evaporation rate and equilibrium phase separation data in relation to casting conditions and performance of porous cellulose acetate reverse osmosis membranes

Quantitative data on solvent evaporation rates and equilibrium phase separation compositions corresponding to different film casting conditions are given, and the results are discussed with reference to reverse osmosis data on membrane performance. A typical evaporation rate curve plotted on a semilog scale consists of an initial linear part from whose slope evaporation rate constants b have been determined. For a given casting solution composition and temperature, an optimum value of b exists for best membrane performance, and identical values of b result in same membrane performance. Treating the casting solution as a polymer–solvent–nonsolvent ternary system, the data on equilibrium phase separation compositions have been plotted in a triangular diagram for solution temperatures of ?10°, 0°, and 24°C. The data indicate that even small changes in solution temperature can bring about significant changes in solution structure and hence membrane performance, and useful conclusions on solution structure can be drawn from such phase equilibrium diagrams. The data and correlations presented here offer a quantitative confirmation of the governing significance of the solution structure-evaporation rate concept relating to the mechanism of phase separation and pore formation in the surface region during the process of making reverse osmosis membranes.     

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.     

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.     

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.     

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.     

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.     

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     

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.     

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.     

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.     

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.     

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.