A Vycor® Membrane with Reduced Size Surface Pores I. Preparation and Characterization

Vycor® membranes are surface-modified by a crosslinked commercial silicone which is subsequently subjected to oxygen plasma and converted to silica dioxide. Samples are examined by integral gas permeability of helium, nitrogen, methane and carbon dioxide, differential permeability of carbon dioxide and relative permeability of helium gas vs. water vapor. The modified surface is found to contain large micropores as well as a population of small nanopores. The new membrane may be appropriate for applications such as gas/vapor separations, reverse osmosis and the low molecular weight end of nanofiltration.     

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.     

Preparation of composite reverse osmosis membranes by plasma polymerization of organic compounds. III. Plasma polymers of acetylene/CO/H2O

Reverse osmosis characteristics of composite membranes prepared by the plasma polymerization of acetylene/CO/H₂O mixtures with various ratios of components are investigated; porous film of cellulose nitrate-cellulose acetate is used as the substrate. This monomer system seems to have the following advantages: (1) A relatively short deposition time (1–2 min) is enough to produce reasonably good reverse osmosis membranes; and (2) good chemical stability of membranes can be obtained, especially in the case of chlorine resistance.     

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.     

Preparation of composite reverse osmosis membranes by plasma polymerization of organic compounds. IV. Influence of plasma–polymer (substrate) interaction

Preparation of composite membranes by plasma polymerization is affected not only by the type of monomer and the mode of discharge but also by the interaction of plasma–polymer substrate. Consequently, the reverse osmosis characteristics of composite membranes are dependent on the combination of substrate and monomer(s). The interactions of plasma and polymer are investigated using porous polysulfone film and cellulose nitrate–cellulose acetate (CNCA) porous film as the substrates, and acetylene/H₂O/N₂ and acetylene/H₂O/CO as the monomer systems. The effects of plasma pretreatment of the substrates on the chlorine resistance of the membranes are also investigated.     

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.     

Posttreatment effects on pore size distribution of loeb-sourirajan-type modified cellulose acetate ultrathin membranes

The nitrogen gas adsorption isotherms at ?195°C on modified cellulose acetate ultrathin membranes were measured, and the surface area of the pores was determined by the method employed by Cranston and Inkley. A relationship between reverse osmosis characteristics and the mean pore radius was correlated, and it was observed that any method (such as longer evaporation period, heat treatment, or reduction of swelling agent) reducing the mean pore radius to below 20–22 Å improves membrane characteristics of reverse osmosis separation significantly.     

Application of system analysis in two-stage reverse osmosis process design for water desalination

The application of the system analysis approach for two-stage reverse osmosis process design is illustrated in detail for the desalination of a NaCl-H₂O feed solution analogous to sea water, using Loeb-Sourirajan type porous cellulose acetate 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.     

Morphological study of cellulose acetate benzoate membranes by scanning electron microscopy

A Scanning Electron Microscope study undertaken to develop a morphological model to aid in predicting new membrane structures suitable for reverse osmosis membranes was undertaken. The cellulose acetate benzoate was studied and compared to cellulose acetate membranes.     

Reverse osmosis separations of polyethylene glycols in dilute aqueous solutions using porous cellulose acetate membranes

Reverse osmosis separations of eight polyethylene glycol (PEG) solutes in the average molecular weight range of 200 to 6750 in single-solute dilute aqueous solutions have been studied using porous cellulose acetate membranes at the operating pressures of 50, 75, and 100 psig. Diffusivity data for the above PEG solutes have also been obtained from experimental data on intrinsic viscosities. From an analysis of all experimental data, numerical values for the parameters representing the polar (?ΔΔG/RT), steric (δ^*ΣE_s), and nonpolar (ω^*Σs^*) forces governing reverse osmosis separations of PEG solutes have been generated. These numerical values are useful for precise characterization of cellulose acetate membranes for whose specifications sodium chloride is not the appropriate reference solute because of its low or practically negligible separation under reverse osmosis operating conditions. This work also illustrates that solute separation in reverse osmosis can predictably increase or decrease with increase in operating pressure depending on experimental conditions.     

Studies on the development of improved reverse osmosis membranes from cellulose acetate–acetone–formamide casting solutions

Improved membranes from cellulose acetate–acetone–formamide casting solutions have been prepared for low-pressure reverse osmosis applications. The film-casting details for one such type of membranes (Batch 400) are as follows. Casting solution composition: cellulose acetate (E-398-3), 17 wt-%, acetone, 56 wt-%, formamide, 27 wt-%; temperature of casting solution, 24°C; temperature of casting atmosphere, 24°C; casting atmosphere, ambient air in contact with 30 wt-% acetone in aqueous solution; solvent evaporation period, 30 sec; gelation medium, ice-cold water. Using aqueous feed solutions containing 3500 ppm of NaCl, the product rates obtained with the above membranes at 95, 90, and 60% levels of solute separation were 15.9, 22.1, and 58.7 gallons/(day ft²), respectively, at 250 psig under feed flow conditions corresponding to a mass transfer coefficient of 45 × 10^?4 cm/sec on the high-pressure side of the membrane. The effects of casting solution composition, presence of acetone in the casting atmosphere, evaporation period, evaporation rate constant, and the remoteness of casting solution composition from the corresponding phase boundary composition on membrane performance and shrinkage temperature profile were found to be similar to those reported earlier for membranes obtained from cellulose acetate–acetone–aqueous magnesium perchlorate casting solutions. The results illustrate the practical utility of the approach based on the solution structure–evaporation rate concept for creating more productive reverse osmosis membranes.     

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.     

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.     

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.     

Reverse osmosis of wash waters from simultaneous SO2/NOx wet removal technique of flue gas

A promising for simultaneous SO₂/NO_x removal from flue gas consists in using Fe (II)—EDTA solutions as absorbing substrate for a coordinative bonding of NO. Its reduction to molecular nitrogen occurs by bisulfite which results from SO₂ absorption. Economic and environmental requirements imply a need for a nearly complete recovery of the complexing agent EDTA. For this purpose reverse osmosis can be used. With porous cellulose acetate membranes, a high enough rejection of EDTA can be achieved as well as the regulation of the concentration of the accompanying chlorides within the scrubber solution. The relevant selectivity of the membranes increases with rising working pressure. The required removal of the accompanying salts can be adjusted by different membrane annealing and by varying the concentration.     

Reverse osmosis rejection of heavy metal cations

The selective rejection of various heavy metal chloride salts by cellulose acetate reverse osmosis membranes is described. A series of membranes exhibiting NaCl rejection levels of from 26 to 91% is employed.The observed selectivity pattern for divalent metals suggests that ion-water rather than ion-membrane interactions are the controlling factors in cation rejections by cellulose acetate membranes. The partial molal free energies of hydration and the entropies of the ions in solution adequately represent these controlling factors.     

Influence of electrolyte on electrical properties of thin cellulose acetate membranes

The electrical behavior of thin cellulose-2.5-acetate membranes for chemical semiconductor sensor applications was investigated. The influence of electrolyte on the electrical membrane properties was studied by means of electrochemical impedance spectroscopy. The polymeric membranes can be described using the electrical model of Cole and Cole. The membrane electrolyte distribution equilibria were investigated by means of an absorption–desorption method using conductivity measurements. The desorption kinetics for thin cellulose acetate membranes differ significantly from those of thicker cellulose acetate reverse osmosis membranes because here the rate of the ionic exchange at the membrane interface cannot be neglected. The experimentally determined distribution coefficients were used for the discussion of the electrolyte influence on the electrical membrane parameters. A functional dependence was found between the specific membrane resistance of cellulose acetate membranes and the ionic hydration enthalpy. The effect of electrolyte on the relative permittivity can be explained by a theoretical model. © 1994 John Wiley & Sons, Inc.     

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.     

Studies on Gas Transport Through Dry Cellulose Acetate Membranes Prepared by Solvent Exchange Technique

Abstract

The mechanism of gas transport through pores on the surface of dry cellulose acetate membranes under pressure was identified for membranes prepared by the solvent exchange technique using pure gas permeation rate data. The pure gases were helium, methane and carbon dioxide. The variables Involved in the membrane preparation are the shrinkage temperature, the first solvent, the second solvent and the combinations thereof. Different conditions of membrane preparation produce different pore sizes. Depending on this pore size, one of the following mechanisms becomes dominant: Knudsen, surface and size exclusion.