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.     

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.     

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.     

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.     

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.     

High flux cellulose acetate ultrafiltration membranes

Conditions for casting high flux tubular and flat cellulose acetate ultra-filtration membranes are given. The usefulness of some of the tubular membranes cast under the above conditions for ultrafiltration applications in industrial water purification and reuse is illustrated. Using ethanol-water mixture as the gelation medium in the temperature range-20° to 30°C, ultrafiltration membranes giving water fluxes in the range <4 to >400 m³/m² day at the operating pressure of 689.5 × 10³ Pa (100 psig) have been obtained.     

Electrical conductivity measurements as a microprobe for structure transitions in polysiloxane derived Si–O–C ceramics

Polysiloxanes [RSiO_1.5]_n with R=CH₃ (PMS) and C₆H₅ (PPS), respectively, were transformed to Si–O–C ceramics of variable composition and structure upon pyrolysis in inert atmosphere at 800–1500°C. The electrical conductivities of the Si–O–C ceramics in air were measured at room temperature by using a shielded two point configuration. In situ measurements of the dc-conductivity during the pyrolytic conversion from the polymer to the ceramic phase were carried out up to 1500°C with four point contacted carbon electrodes in inert atmosphere. During polymer-ceramic conversion excess carbon precipitates above 400°C (PPS)–700°C (PMS). At temperatures above 800°C (PPS) and 1400°C (PMS) coagulation and growth of the carbon clusters results in a percolation network formation. While below the percolation threshold electrical conductivity can be described according to Motts mechanism by variable-range-hopping of localized charge carriers, regular electron band conduction due to the instrinsic conductivity of turbostratic carbon (8×10⁻⁴ (Ωcm)⁻¹) predominates above. Thus, the in situ measurement of non-linear electrical property changes can be used as a microprobe of high sensivity to detect microstructural transformations during the pyrolysis of preceramic polymers.     

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.     

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.     

Hydrogen Permeation Properties and Hydrothermal Stability of Sol–Gel‐Derived Amorphous Silica Membranes Fabricated at High Temperatures

The sol–gel method was applied to the fabrication of amorphous silica membranes for use in hydrogen separation at high temperatures. The effects of fabrication temperature on the hydrogen permeation properties and the hydrothermal stability of amorphous silica membranes were evaluated. A thin continuous silica separation layer (thickness = <300 nm) was successfully formed on the top of a deposited colloidal silica layer in a porous glass support. After heat treatment at 800°C for an amorphous silica membrane fabricated at 550°C, however, it was quite difficult to distinguish the active separation layer from the deposited colloidal silica layer in a porous glass support, due to the adhesion of colloidal silica caused by sintering at high temperatures. The amorphous silica membranes fabricated at 700°C were relatively stable under steam atmosphere (500°C, steam = 70 kPa), and showed steady He and H₂ permeance values of 4.0 × 10^?7 and 1.0 × 10^?7 mol·m^?2·s^?1·Pa^?1 with H₂/CH₄ and H₂/H₂O permeance ratios of ~110 and 22, respectively. The permeance ratios of H₂/H₂O for membranes fired at 700°C increased drastically over the range of He/H₂ permeance ratios by factors of ~3–4, and showed a value of ~30, which was higher than those fired at 500°C. Less permeation of water vapor through amorphous silica membranes fabricated at high temperatures can be ascribed to the dense amorphous silica structure caused by the condensation reaction of silanol groups.     

Reverse osmosis separation of phenols in aqueous solutions using porous cellulose acetate membranes

Reverse osmosis separations of phenol (9.4 to 108 ppm), p-cresol (108 ppm), and p-chlorophenol (129 ppm) were studied using Loeb-Sourirajan-type porous cellulose acetate membranes, and single-solute aqueous feed solutions at 500 psig and the indicated solute concentrations. It was found that, by dissociating the solute by changing the pH of the feed solution, all the above phenols could be separated by reverse osmosis. Solute separation increased with increase in the degree of dissociation of the solute in the feed solution; and, by the appropriate choice of pore size on the membrane surface, separations of phenol approaching the degree of dissociation of phenol in the feed solution could be obtained under the operating conditions used. Similar experiments using aniline (93 ppm) as the solute showed that dissociation of solute molecules in the feed solution could be a technique generally applicable for the reverse osmosis separation of nonionic solutes in aqueous solution. The effects of operating pressure in the range 250 to 1500 psig and pore size on the membrane surface on the separation of un-ionized phenol and p-chlorophenol showed that, with respect to single-solute aqueous feed solutions of phenols, the component whose relative acidity was greater was preferentially sorbed at the cellulose acetate membrane—aqueous solution interface, and the solute concentration in the membrane-permeated product solution was a function of the extent and mobility of each of the sorbed species.     

Some transport characteristics of aromatic polyamide membranes in reverse osmosis

The effects of solute concentration in the range of 0.0013 to 1.051 molality in the feed solution and operating pressure in the range of 100 to 900 psig on solute transport parameter D_AM/Kδ in reverse osmosis have been studied for a class of laboratory-made aromatic polyamide membranes and aqueous sodium chloride feed solutions. The results showed that D_AM/Kδ for NaCl increased both with increase in operating pressure and solute concentration in the concentrated boundary solution on the high-pressure side of the membrane. A general expression for D_AM/Kδ for NaCl including the effects of both the above operating variables is given. These results are different from the corresponding results obtained for cellulose acetate membranes.     

Permeability of silicone polymers to hydrogen

Permeability coefficients, P?, for H₂ in 10 different types of silicone polymer membranes were measured in the temperature range of 10.0–55.0°C and at pressures up to 100 psig (～6.8 atm). The values of P? decrease slightly with increasing Δp, the pressure difference across the membranes. The permeability of silicone polymers to H₂ increases with an increase in temperature; the values of the energy of activation for permeation are in the range of 1.4–4.3 kcal/mol. The substitution of different functional groups in the backbone and side chains of silicone polymers has similar effects on the permeability of the polymers to H₂ as observed in earlier studies with other light gases. P? for H₂ decreases with an increase in the bulkiness of the substituted functional groups. The substitution of Si? O bonds with stiffer Si? C bonds in the backbone chains also results in a considerable decrease in permeability.     

Evaluation and fabrication of pore‐size‐tuned silica membranes with tetraethoxydimethyl disiloxane for gas separation

Organic/inorganic hybrid silica membranes were prepared from 1,1,3,3‐tetraethoxy‐1,3‐dimethyl disiloxane (TEDMDS) by the sol‐gel technique with firing at 300–550°C in N₂. TEDMDS‐derived silica membranes showed high H₂ permeance (0.3–1.1 × 10^?6 mol m^?2 s^?1 Pa^?1) with low H₂/N₂ (～10) and high H₂/SF₆ (～1200) perm‐selectivity, confirming successful tuning of micropore sizes larger than TEOS‐derived silica membranes. TEDMDS‐derived silica membranes prepared at 550°C in N₂ increased gas permeances as well as pore sizes after air exposure at 450°C. TEDMDS had an advantage in tuning pore size by the “template” and “spacer” techniques, due to the pyrolysis of methyl groups in air and Si? O? Si bonding, respectively. For pore size evaluation of microporous membranes, normalized Knudsen‐based permeance, which was proposed based on the gas translation model and verified with permeance of zeolite membranes, reveals that pore sizes of TEDMDS membranes were successfully tuned in the range of 0.6–1.0 nm. © 2011 American Institute of Chemical Engineers AIChE J, 2011     

Effects of Atmospheric Composition on the Molecular Structure of Synthesized Silicon Oxycarbides

The dependence of silicon oxycarbides' chemical composition and molecular structure on their reaction conditions was tested by varying the atmosphere under which pyrolysis was performed. To obtain the silicon oxycarbides, densely cross‐linked silicone resin particles with an averaged diameter of 2 μm were pyrolyzed in various atmospheres of H₂, Ar, and CO₂, in the temperature range 700°C–1100°C. The residual mass of resin after pyrolysis was almost constant at 700°C, although their apparent colors varied distinctly. The sample obtained from the H₂ atmosphere was white, whereas that obtained from the CO₂ atmosphere was dark brown. Fourier‐transform infrared (FT‐IR) spectra of the residues suggested that the Si–O–Si network evolution was accelerated in the CO₂ atmosphere. Beyond 800°C, the chemical compositions of the compounds obtained from a H₂ atmosphere increasingly approached near‐stoichiometric SiO₂–xSiC composition with increasing the pyrolysis temperature. Compounds from a CO₂ atmosphere approached a composition of SiO₂–xC with no free SiC as the pyrolysis temperature increased. In the products from an Ar atmosphere, SiO₂–xSiC–yC compositions were typically obtained. The observed effects of the pyrolysis atmosphere on the resulting chemical compositions were analyzed in terms of thermodynamic calculations. Electron spin resonance (ESR) spectra revealed broad and intense signals from products obtained from either Ar or CO₂. Estimating from the signal intensity, the residual spin concentrations were in the range 10¹⁸–10¹⁹ g^?1. Meanwhile, the spectra from the samples obtained in H₂ showed weak and sharp signals with estimated spin concentrations ranging from 10¹⁶–10¹⁷ g^?1. This signal attenuation may have been due to the hydrogen capping of dangling bond formed during pyrolysis.     

Gas transport and mechanical properties of PDMS-TFS/LDPE nanocomposite membranes

This study investigates the effect of trimethylsiloxy fumed silica (TFS) on the mechanical and gas permeation properties of polymer nano-composite membranes. The membranes were produced by coating TFS incorporated polydimethylsiloxane (PDMS) at different loadings (5, 10 and 15 wt.%) on a porous low density polyethylene (LDPE) substrate which was formed by a melt-extrusion/salt leaching technique. The PDMS-TFS/LDPE membranes were characterized by SEM, TGA and DMTA. The results showed that good affinity between the PDMS treated TFS particles and PDMS matrix was obtained leading to improved mechanical and thermal properties. For gas permeation, CH₄ and C₃H₈ at different upstream pressure (50 to 80 psig) and temperature (27 to 55 °C) were investigated. The results showed that the C₃H₈/CH₄ ideal selectivity (17.6) and C₃H₈ permeability (1.89?×?10⁴ Barrer) through 10 wt.% TFS loaded membranes (PDMS-TFS10%/LDPE) were 41 and 14% higher than the neat membranes (PDMS-TFS0%/LDPE), respectively. The permeation results also indicate that the performance stability under the conditions investigated makes PDMS-TFS/LDPE membranes interesting for industrial applications.     

Preparation of UC ceramic nuclear fuel microspheres by combination of an improved microwave-assisted rapid internal gelation with carbothermic reduction process

Uranium carbide (UC) ceramic microspheres filled into a cladding are a potential nuclear fuel format for nuclear reactors. Uniform sized ceramic UC microspheres with a diameter of 675?±?10?µm were successfully prepared by an improved microwave-assisted rapid internal gelation process combined with carbothermic reduction. First of all, the nanoparticle carbon was dispersed into the HMUR stock solution, and the C-UO₃·2H₂O gelled microspheres were prepared using an improved microwave-assisted internal gelation process without cooling the initial stock solutions. Next, the gelled microspheres were subjected to a carbothermic reduction process to obtain ceramic UC microspheres. TG and XRD investigations indicated that the C-UO₃·2H₂O microspheres were firstly reduced into UO₂ at a temperature of 700?°C, and were further converted into UC at 1500?°C in argon atmosphere. Crack-free ceramic UC microspheres with a smooth and metallic shiny surface were obtained at a sintering temperature of 1500?°C for 5?h with an initial C/U molar ratio of 3.5.     

Epoxidation of propylene by air over modified silver catalyst

Epoxidation of C₃H₆ to C₃H₆O by air was studied over a silver catalyst modified with alkali or alkaline earth chloride salts. The catalyst preparation factors and the operational conditions could affect obviously the catalytic epoxidation property of the silver catalyst. It was shown that, as a promoter of the silver catalyst, NaCl or BaCl₂ is more suitable than LiCl or NH₄Cl. The loading of NaCl should be controlled at about 3.8 wt%. Using a feed gas of 10% C₃H₆/air at a space velocity of 1.75×10⁴ h⁻¹, 18.6% C₃H₆ conversion and 33.4% selectivity to C₃H₆O were obtained at 350°C. Using a feed gas of 5% C₃H₆/air at a space velocity of 2.4×10⁴ h⁻¹, 54.0% C₃H₆ conversion and 26.3% selectivity to C₃H₆O were obtained at 390°C. This revised version was published online in July 2006 with corrections to the Cover Date.     

Removal of organic sulphur from coal gas

Organic sulphur compounds present in coal gas containing 15-20% CO were effectively converted into H₂S over a “Nimox” (nickel-molybdenum) conversion catalyst. H₂S was effectively removed by “Luxmasse”, a prepared iron oxide. The overall removal of organic sulphur depended upon the concentration of thiophene present. With only 10 ppm thiophene in the gas, the conversion of organic sulphur was 97% at 350°C after a single treatment. With six-stage treatment at 350 psig, the final gas contained only 0.2 ppm total organic sulphur in the presence of 4-5% water vapor.     

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.