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.     

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.     

The effect of short air exposure periods on the performance of cellulose acetate membranes from casting solutions with high cellulose acetate content

Highly productive cellulose acetate membranes were cast under conditions of very short air exposure periods from cellulose acetate–acetone–formamide casting solutions having a high cellulose acetate (CA) content and lying close to the phase boundary. Air exposure periods as short as 0.05 sec were used with CA content up to 32 wt-%. Membranes from a casting solution containing 30 wt-% cellulose acetate (E-398-3), 45 wt-% acetone, and 25 wt-% formamide perform as well as membranes from other compositions at all salt rejection levels for a 0.5 wt-% NaCl feed at 600 psig. Partial replacement of acetone by dioxane in the casting solution substantially increases the water flux from membranes cast with short air exposure periods at any given salt rejection level below 96% salt rejection. Addition of small amounts of ZnCl₂ to nondioxane casting solutions with 32 wt-% CA improves membrane performances remarkably for lower salt rejection levels, while the improvement in performance of membranes from 30 wt-% CA casting solutions with dioxane due to ZnCl₂ addition is marginal. Variation in air exposure from 0.05 to 2 sec results in minor performance variations in the membranes having any of these compositions. With air exposure periods beyond 2–3 sec, membrane fluxes drop drastically. The concept of a thinner skin satisfactorily explains the improvement in mixed solvent systems, whereas ZnCl₂ acts as a swelling salt. A Kimura-Sourirajan-type membrane performance plot indicates that for a 0.5 wt-% NaCl feed at 600 psig, membranes of the present work perform as well as the best performing membranes reported in the literature for conversion of brackish water.     

Effect of ethanol–water mixture as gelation medium during formation of cellulose acetate reverse osmosis membranes

By using ethanol–water mixtures in a wide range of alcohol concentrations and temperatures, cellulose acetate membranes with a wide range of surface porosities can be obtained. Two different casting solution compositions were used, involving cellulose acetate, acetone, and aqueous magnesium perchlorate (composition I) or formamide (composition II). All reverse osmosis experiments were carried out at 250 psig using a 3500 ppm NaCl–H₂O feed solution at laboratory temperature. The effective area of film surface was 12 cm² in all cases. With composition I, with pure water gelation medium at 0°C, the resulting membrane gave a solute separation of 5% and product rate of 220 g/hr, whereas with 95% alcohol as gelation medium, the resulting membrane gave a solute separation of ～1% and product rate of 1240 g/hr under otherwise identical experimental conditions. With composition II membranes, the maximum product rate of 360 g/hr with the corresponding minimum solute separation of ～1% was obtained with 71.2% alcohol–water gelation medium at 0°C. Increase in the temperature of the gelation medium in the range 12° ?25°C tends to increase the average size of pores on the membrane surface. These results offer a basis for the development of cellulose acetate ultrafiltration 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.     

An approach to the development of cellulose acetate ultrafiltration membranes

The film-casting solution consisted of a mixture of cellulose acetate, acetone, and aqueous magnesium perchlorate [Mg(ClO₄)₂:H₂O = 1:8.5], designated as polymer P, solvent S, and nonsolvent N, respectively. Using the composition P:S:N = 17: 69.2: 13.8 as reference, films were obtained from 19 different casting solutions in which the weight ratios S/P, N/S, and N/P were varied in different directions. The casting solution temperature was 0°C, and solvent evaporation period during film formation was minimum in most cases. The effects of variations of casting solution temperature and solvent evaporation period were also briefly studied. Reverse osmosis experiments with resulting membranes were carried out at 100 psig using 200 ppm NaCl–H₂O as the feed solution. Decrease in S/P, increase in N/S, and increase in N/P in the casting solution, decrease in temperature of the casting solution, and increase in solvent evaporation period tend to increase the size of pores on the surface of resulting membranes in the ascast condition. Increase in S/P in the casting solution, and increase in the temperature of the casting solution tend to increase the effective number of pores on the membrane surface. These results offer definitive physicochemical criteria in terms on solution structure–evaporation rate concept for developing useful cellulose acetate ultrafiltration membranes.     

Studies on syntheses and permeabilities of special polymer membranes. V. Effect of casting solution composition on permeation characteristics of cellulose acetate membranes in separation of aqueous polymer solutions

The permeation characteristics of cellulose acetate membranes in separation of polymers from their aqueous solutions were investigated by changing the preparation conditions of the membranes, that are the solvent evaporation period and the casting solution composition consisting of a mixture of cellulose acetate (CA), acetone (A), and formamide (FA). The rates of pure water permeability were influenced remarkably by the solvent evaporation period and the casting solution composition. When the solvent evaporation period was short, the rates of pure water permeability increased with a decrease in A/CA, increase in FA/A, and increase in FA/CA in the casting solution. From the experimental results using poly(vinyl alcohol) as poly(ethylene glycol) as feed solute, it was seen that the changes of solvent evaporation period and casting solution composition related to the change of microporous structure of the resulting membranes. The effect of feed concentration and operating pressure on the permeation characteristics were also studied. There was found a concentration polarization of poly(vinyl alcohol) molecules on the surface of the membrane, and a compaction of the membrane occurred under pressure.     

Cellulose acetate ultrafiltration membranes

Twenty-three different casting solution compositions involving cellulose acetate E-938-3 (polymer, P), acetone (solvent, S), and aqueous magnesium perchlorate (Mg(ClO₄)₂:H₂O = 1:8.5) (nonsolvent, N) were studied for making ultrafiltration membranes. These compositions (expressed in weight units) involved N/P ratios of 0.817 to 1.3 and S/P ratios of 3.5 to 5.33. It was found that by adjusting the values of S/P and N/P ratios in the casting solution composition, temperature of the casting solution, temperature of the casting atmosphere, and solvent evaporation period during film formation, a wide variety of cellulose acetate membranes useful for both ultrafiltration and reverse osmosis applications could be obtained. The results of a continuous test run at 50 psig for a period of 250 hr with a typical set of membranes, and their separation characterstics for a group of solutes ranging in molecular weight from 58 to 160,000, are presented.     

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.     

A rationale for the preparation of Loeb-Sourirajan-type cellulose acetate membranes

An attempt has been made to rationalize the variables in the preparation procedure of Loeb-Sourirajan-type reverse-osmosis membranes. The quaternary phase diagram of the system cellulose acetate–acetone–formamide–water was determined and has proved a useful tool in the discussion of membrane structures and properties. A mechanism based on differences in the precipitation rate of the polymer during the membrane formation process has been suggested to explain the observed asymmetry in the membrane structure. The porosity of the membrane has been ascribed to the relative rates of water entering and solvent leaving the cast film. The effects of the casting solution composition, the evaporation time, the wash bath temperature, and the annealing procedure have been studied. X-Ray diffraction and electron microscopy were used to supplement flux and retention data of membranes made from a cellulose acetate–formamide–acetone casting solution.     

Physicochemical processes occurring during the formation of cellulose diacetate membranes. Research of criteria for optimizing membrane performance. IV. Cellulose diacetate-acetone-organic additive casting solutions

Asymmetric membranes were prepared from cellulose diacetate using a mixture of acetone and formamide, DMF, DMSO, NMP, dioxane or acetic acid as a basic solvent and water as the precipitation agent. The membrane water flux and salt rejection were determined by reverse osmosis tests. The change in mobility of the polymer chains during the evaporation and coagulation phases was studied by fluorescence depolarization. The main parameters such as casting solution composition, evaporation time, water concentration required for polymer precipitation, speed of coagulation and their effect on membrane structure and performance are discussed.     

Cellulose acetate reverse osmosis membranes: Optimization of preparation parameters

Asymmetric cellulose acetate based membranes usually employed in reverse osmosis as well as in separations in aqueous systems can possibly be applied in the so‐called salinity process of energy generation. For these applications, membranes with a relatively high water permeability (sometimes also called water flux) and low salt permeability (or high salt rejection) are required. In this study the authors present the optimization of such membranes, which concerns the preparation parameters. The membranes studied were prepared from a solution whose composition were previously optimized.⁴ The authors concluded that the optimum preparation parameters are as follows: thickness of the liquid film of 100 μm; 30 s allowed for evaporation of the solvent; and temperature of coagulation bath of 0–4°C and 80–85°C as annealing temperature. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 134–139, 2007     

Preparation and properties of poly(acrylic acid) composite membranes

A new type of membrane has been prepared for hyperfiltration (reverse osmosis) desalination that is essentially a very thin polyelectrolyte membrane. It is prepared by casting an aqueous solution of a polyelectrolyte, specifically poly(acrylic acid) (PAA), directly on one surface of a finely porous support membrane. In hyperfiltration tests, these composite membranes exhibit desalination performance comparable in dilute solutions to that observed with cellulose acetate membranes of the Loeb-Sourirajan type. The water flux through these membranes is linear in the pressure up to 100 atm. Salt rejection is a function of pressure; it is also a function of the concentration of the feed solution and the charge of the counterion, in qualitative agreement with the Donnan ion-exclusion mechanism. Typical long-term results range from water fluxes of 2 × 10^?3 g/cm²-sec (50 gal/ft²-day) and 80% salt rejection to 0.2 × 10^?3 g/cm²-sec (5 gal/ft²-day) and >99.5% salt rejection at 1500 psi with 0.3 wt-% NaCl. These membranes appear to be useful for brackish water desalination.     

The role of silver nanoparticles on mixed matrix Ag/cellulose acetate asymmetric membranes

Mixed matrix asymmetric membranes were prepared by the addition of silver nanoparticles to cellulose acetate/acetone/formamide casting solutions with ratios acetone/formamide varying from 1.44 to 2.77 to prepare ultrafiltration/nanofiltration membranes covering a wide range of hydraulic permeabilities. Binding of the silver nanoparticles to the polymer matrix is revealed through comparison of the FTIR spectra of the cellulose acetate and the Ag/cellulose acetate membranes. In the later, there is a decrease of the ratio between the bands intensities at 2,000–2,500 cm⁻¹. Membrane surface charge of the mixed matrix membranes varies with the pore size and pH, and when compared with cellulose acetate membranes there is a decrease of the negative surface charge densities. The silver nanoparticles in all mixed matrix membranes results in an enhancement of the hydraulic permeabilities, ranging from 10.8 kg m⁻² h⁻¹ bar⁻¹ to 67.1 kg m⁻² h⁻¹ bar⁻¹. POLYM. COMPOS., 38:32–39, 2017. © 2015 Society of Plastics Engineers     

Determination of concentration changes on membrane surface as function of time during evaporation of reverse-osmosis film casting solutions

Data on the activity of acetone for the binary system acetone-formamide, and a specific ternary system cellulose acetate-acetone-formamide are given for the temperature range 0 to 30°C. A method of determining concentration changes occurring on the membrane surface during the evaporation stage in the process of making cellulose acetate reverse osmosis membranes is illustrated.     

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.     

Cellulose acetate reverse osmosis membranes: Optimization of the composition

Membranes based on cellulose acetate for reverse osmosis can possibly be applied to the so‐called salinity process of energy generation and water desalinization. The requirements for membranes for these two different applications are a relatively high water flux and low salt permeability. In this article, we present the optimization of the composition of such membranes. We started by producing membranes with a patented casting solution with the following composition: 45.77 wt % dioxane, 17.61 wt % acetone, and 8.45 wt % acetic acid (solvents); 14.09 wt % methanol (nonsolvent); and 7.04 wt % cellulose diacetate and 7.04 wt % cellulose triacetate. The membranes produced with this solution were analyzed comparatively, with the membranes obtained by the introduction of modifications to the following parameters: the solvent mix, the nonsolvent mix, the proportion of cellulose diacetate and cellulose triacetate in the casting solution, and the addition of reinforcing cellulose fibers. The results led us to conclude that the best membrane formulation had the following composition: 45.77 wt % dioxane, 17.61 wt % acetone, and 8.45 wt % acetic acid (solvents); 4.22 wt % cellulose triacetate and 9.86 wt % cellulose diacetate (polymers); 14.09 wt % methanol (nonsolvent); and 0.5 wt % cellulose fibers (with respect to the total polymer content). © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 4052–4058, 2006     

Investigation of the effect of tetrahydrofuran and acetone as cosolvents in acrylonitrile–butadiene–styrene–based nanofiltration membranes

In this study, novel nanofiltration membranes were prepared with acrylonitrile–butadiene–styrene (ABS)–poly(ethylene glycol)–N,N ‐dimethylacetamide–[tetrahydrofuran (THF)–acetone] as a cosolvent. All of the membranes were prepared by the phase‐inversion method and a casting solution technique. The effects of the cosolvent concentration in the casting solution and the evaporation time before the immersion/precipitation step on the membrane performance and properties were investigated. The prepared membranes were characterized through their permeation flux, salt rejection, and phase‐inversion time values. The salt rejection was increased from 53% for the bare ABS membrane to 73% for the membrane prepared with 40 wt % THF as a cosolvent. The water flux was decreased from 4345 to 1121 cc m^?2 h^?1 with the addition of THF to the casting solution. The addition of acetone to the casting solution improved the water flux from 4345 to 5607 cc m^?2 h^?1 and reduced the salt rejection from 53 to 36%. The evaporation time of THF and acetone led to similar effects on flux and rejection. However, with evaporation time, membranes prepared with acetone were denser than those prepared with THF; this was due to the lower boiling point and higher boiling rate of acetone at the same temperatures. This resulted in greater effects on the ABS performance and structure. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134 , 44993.     

三乙酸纤维素正渗透膜的制备与性能

以三乙酸纤维素（CTA）为膜材料,1,4-二氧六环、丙酮为溶剂,甲醇、乳酸为添加剂,采用相转换法制备了三乙酸纤维素正渗透膜。研究了不同1,4-二氧六环/丙酮配比、添加剂乳酸含量、挥发时间、膜厚度、热处理温度条件下正渗透膜性能的变化规律。研究表明,当采用纯水为原料液,0.56mol/L CaCl2为汲取液时,优化制备的CTA正渗透膜的水通量达到14.10L/(m2?h),溶质反扩散量为0.031mol/(m2?h);采用0.1mol/L NaCl为原料液,4mol/L葡萄糖为汲取液时,优化制备的CTA正渗透膜的水通量保持在5L/(m2?h)以上,对NaCl的截留率大于99%。CTA正渗透膜相比于HTI膜,具有较高的亲水性、水通量、截留率,稳定性更好。     

Membranes composites preparees a partir d'alcool polyvinylique et de diisocyanate de toluylene destinees a l'osmose inverse

Composite reverse osmosis membranes have been prepared by coating a Millipore filter with an ultrathin layer of polyvinyl alcohol crosslinked by toluene diisocyanate. The membranes present a good selectivity towards aqueous salt solutions (for instance water flux of 500 liters/m²-day at 28°C, salt rejection of 98% with a 3.5% NaCl solution under a pressure of 100 bars) and have better temper-ature and pressure stabilities than cellulose acetate membranes. Their resistance to acid and alkaline hydrolysis is satisfactory. The influence of different preparation factors on the osmotic properties of the membranes was examined.