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.     

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.     

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.     

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.     

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.     

Reverse osmosis with a cellulose acetate membrane in the reverse orientation

Salt rejection curves of CA membranes placed with the porous layer towards the influent side and the skin towards the effluent side show typical maxima. Analysis of these maxima yields information on the salt permeabilities of the porous layer (ω_b) and of the skin (ω_a) in a rather direct way.     

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.     

Short cut design methods for reverse osmosis separation with tubular modules

The basis for obtaining explicit flux expressions in reverse osmosis separations with concentration polarization has been illustrated for tubular modules with turbulent flow. Various analytical equations developed thereby to predict the required number of tubes and the permeate solute concentrations are described and compared with one another as well as with the currently used numerical methods. The utility of such simple design procedures are pointed out. The possibilities and difficulties of adopting these procedures for designing spiral-wound modules have been discussed. The analytical equations can be adopted to “tapered-flow” reverse osmosis without much difficulty.     

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     

Effect of cellulose acetate/cellulose triacetate ratio on reverse osmosis blend membrane performance

Surface functionalization and modification including the grafting process are effective approaches to improve and enhance the reverse osmosis (RO) membrane performance. This work is aimed to synthesize grafted/crosslinked cellulose acetate (CA)/cellulose triacetate (CTA) blend RO membranes using N-isopropylacrylamide (N-IPAAm) as a monomer and N,N-methylene bisacrylamide (MBAAm) as a crosslinker. The morphology of these membranes was analyzed by scanning electron microscopy and their surface roughness was characterized by atomic force microscopy. The performance of these membranes was evaluated through measuring two major parameters of salt rejection and water flux using RO unit at variable operating pressures. It was noted that the surface average roughness obviously decreased from 148 nm for the pure CA/CTA blend membrane with 2.5% CTA to 110 nm and 87 nm for the grafted N-IPAAm and grafted/crosslinked N-IPAAM/MBAAm/CA/CTA-RO membranes, respectively. Moreover, the contact angle decreased from 51.98° to 47.6° and 43.8° after the grafting and crosslinking process. The salt rejection of the grafted CA/CTA-RO membrane by 0.1% N-IPAAm produced the highest value of 98.12% and the water flux was 3.29 L/m²h at 10 bar.     

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.     

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.     

反渗透膜的污染与清洗

针对国电邯郸热电厂反渗透系统污堵的情况,结合原水水质及预处理工艺的特点,对反渗透膜污堵原因进行分析认为:微生物滋生和过量FPS是造成污染的主要原因,同时磷系阻垢剂的使用加剧了微生物的滋生。通过采取不同的方法对反渗透膜进行化学清洗试验,确定最佳的清洗方案是草酸、NaOH+十二烷基苯磺酸钠+阳离子表面活性剂1227。     

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.     

The effect of pressure on the bulk polymer microstructure in cellulose acetate reverse osmosis membranes

The results of the investigations of the pressure effect on the bulk polymer microstructure using Differential Scanning Calorimetry (DSC) are presented. The samples of both “dense” CA-398-3 membranes and Kesting dry asymmetric CA-398-3 membranes have been investigated. The results show that the pressure treatment is also a kind of an “annealing” process that orders the polymer microstructure. The effect takes place both in the case of the originally more ordered (from a DSC point of view) polymer microstructure of “dense” CA membranes and the originally less ordered (almost completely amorphous) polymer microstructure of the dry asymmetric CA membranes investigated. DSC curves on the thermoanalytical diagrams of the pressure treated “dense” CA membranes show mainly a shift of the endothermic peak (melting process?) to higher temperatures, and DSC curves of the pressure treated Kesting dry asymmetric membrane samples show both a shift of the endothermic peak to higher temperatures and quite an enlargement of the area under the peak. It could be inferred from the above that pressure action orders (densifies) the bulk polymer microstructure of both the skin and porous layer of an asymmetric CA reverse osmosis membrane.     

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.