E. Korngold

Effect of porous support fabric on osmosis through a Loeb-Sourirajan type asymmetric membrane

Abstract Commercially available asymmetric membranes of the Loeb-Sourirajan (L-S) type comprise a support fabric, bonded to the porous substructure. The influence of this fabric on osmotic permeation flux was examined, mostly with a Toray CA-3000 membrane from which, with care, it was possible to remove the support fabric. In osmosis experiments with 12% MgCl2 solution on one side (either side) and 6% solution on the other, the permeation flux (J1) was of the order of 0.01 and 0.06 m3/m2 d with and without fabric, respectively. These results could be generalized by considering the resistivity to solute diffusion in the non-skin part of the membrane. This resistivity term averaged 104 and 17 d/m for membranes with and without fabric, respectively, and in further tests without fabric, it was between 15 and 25 d/m over a wide range of MgCl2 concentrations. Four other L-S membranes, all with support fabric, were tested in osmosis experiments. Their resistivity values were similar to or higher than those of the Toray membrane with fabric, but, with one of the four, the results were affected by switching the location of the high and low concentration solutions. It was concluded that existing commercially available L-S membranes are not appropriate for large-scale osmosis applications because their support fabric decreases permeation flux excessively.

Removal of arsenic from drinking water by anion exchangers

Abstract Selective removal of arsenic(V) from drinking water was carried out with strong-base anion-exchange resins. The influence on the efficiency of the process of parameters such as type of anion-exchange resin and water composition was investigated. Recycling of the sodium chloride regenerant solution was carried out by precipitating the As with FeCl3.

Water desalination by pervaporation with hollow fiber membranes

Abstract A new desalination process consisting of air humidification by pervaporation through hydrophilic or microporous hydrophobic hollow fibers followed by dehumidification by cooling water was investigated. In this system hot water is passed through hollow fibers in a recycled air-sweep pervaporation process. The water is heated by waste heat or solar energy or by any other cheap source of energy. The flux of water through the hollow fibers is in the range of 1.5–3.0 l/m2h, when water temperature is 45–65°C. The energy requirement for recycling hot water depends on water temperature as well as on the diameter and length of the hollow fibers. The energy requirement for air recycling depends on the air temperature and on the pressure drop of the system. The calculated energy requirement for pumping air and water in a pilot plant unit of a capacity of 6.3 l/h with 4 m2 of anion-exchange hollow fibers was about 2 kWh/m3, when hot water temperature was 60°C.

Diffusion of water and ethanol in ion-exchange membranes: limits of the geometric approach

Diffusion coefficients of water as a solvent and ethanol as a tracer in sulphonated polyethylene (SPE) were measured by the pulsed-gradient spin-echo NMR method. Comparison with the data available for a number of ionic and non-ionic polymers and solvents shows that, except for Nafion, all the materials exhibit a fairly universal dependence of diffusivity on the volume fraction of the solvent. This result is explained on the basis of the geometric obstruction approach and general conditions are formulated for the observed universal dependence. Explanations are proposed for the peculiar behaviour of Nafion and for the low diffusivity of ethanol within the same framework.

Removal of heavy metals from tap water by a cation exchanger

Abstract Selective removal of heavy metals such as Cu, Ni, Co, Mn, Cd, and Pb from tap water that contains relatively high concentrations of calcium and magnesium was carried out with a cation-exchange resin possessing a chelating iminodiacetic acid group. When the concentration of each of these metals in tap water was of the order of a few ppm, their leakage fell below the permitted level in drinking water. Removal of the heavy metals from the resin was successfuly perfomed with 3 M HCl or HNO3.

Preferential sorption in ion-exchange pervaporation membranes: sorption of water-ethanol mixture by sodium polyethylene sulphonate

Sorption of water and ethanol from aqueous ethanol solutions by sulphonated polyethylene membrane is studied both theoretically and experimentally, particular attention being paid to the effect of ionogenic-group content (capacity) on the sorption and sorption selectivity. A theoretical model is proposed based on models of salting-out and swelling of ion-exchangers. Examination of the experimental results obtained using the isopiestic method reveals a satisfactory agreement with the model, i.e. with a modified BET equation for sorption of water and a modified Setschenow equation for salting-out. Based on the results of this study, the following conclusions may be drawn: sorption selectivity toward water increases as capacity increases; the per equivalent uptake of water is determined primarily by water activity in the outside solution and the presence of ethanol plays only a minor role; the osmotic effect associated with deformation of the matrix has a negligible influence on selectivity. The model makes it possible to estimate sorption selectivity and may have applications in the calculation of pervaporation membrane performance. The sorption of water by the polymer was found to be far more pressure-dependent than that expected from thermodynamics, presumably due to the nonelastic behavior of the noncrosslinked polyethylene matrix.

Hydrophilic hollow fiber membranes for water desalination by the pervaporation method

Abstract Hydrophilic ion-exchange membranes based on sulfonated polyethylene hollow fibers were manufactured, and their suitability for a water pervaporation process was studied for possible application in water desalination systems. The effects of the following parameters on the average water flux were determined: membrane properties (diameter (0.4–1.8 mm) and wall thickness (0.05–0.18 mm)); charge density (0.6–1.2 meq g −1 ); and operating conditions (brine inlet temperature (30–68°C), air sweep velocity (0–6 m s −1 ), and salt concentration in the feed brine (0–3 M)). A water flux of 0.8–3.3 kg m −2 h −1 was obtained using this type of hollow fiber with an inlet brine temperature of 25–65°C. It was found that, for our application, the optimal specifications for the ion-exchange hollow fibers were an outside diameter of 1.2 mm, a wall thickness of 0.1 mm, and an ion-charge density of about 1.0 meq g −1 . This information is required as basic data for the design of a prototype water desalination system based on a pervaporation system that uses this type of ion-exchange hollow fiber membrane.

Influence of specially modulated ultrasound on the water desalination process with ion-exchange hollow fibers

Abstract The enhancing effect of specially modulated ultrasound signals (SMUS) on the process of water desalination by means of ion-exchange hollow fibers was investigated. For Na+ - H+ ion exchange the enhancement effect increased with increasing power of the ultrasound, reaching about 40% at a signal power of 61 W. Depending on the concentration of the solutions and hydrodynamic conditions, ultrasound enhanced different steps of the overall ion-exchange process. In the range of 20–50°C, the effect increased with temperature from 40% to 60%. We also demonstrated the feasibility of utilizing SMUS for improvement of a cavitation process and thus for improving the performance of our desalination system.

Transport mechanism in ion-exchange pervaporation membranes: Dehydration of water-ethanol mixture by sodium polyethylene sulphonate membranes

Pervaporation of a water-ethanol mixture with sulphonated polyethylene membranes of various capacities at various temperatures and for various feed compositions was studied both theoretically and experimentally. A model is proposed that accounts both for the effects of partial immobilization of water in the membrane due to adsorption by the groups and for the obstruction effect of the polymer matrix on the basis of percolation arguments. Analysis of the pervaporation data against the results of previous sorption experiments showed that the model is in fair agreement with the experiment. A number of peculiarities not predicted by the model and not observed for ethanol transport, such as enhanced diffusivity and selectivity and reduced apparent activation energy, were observed concerning the transport of water. These deviations are explained by assuming the existence of a selective facilitated mechanism of water transport. Other possible mechanisms are also discussed.

Influence of complexing agents on the removal of metals from water by a cation exchanger

Abstract Selective removal of metals such as Cu, Ni, Co and Cd from tap water containing small amounts of complexing agents such as humates and salts of carboxylic acid was carries out with a cation-exchange resin possessing a chelating imonodiacetic group. Some of the complexing agents investigated had little effect on leakage, and eluate residues remained below levels permitted for drinking water; other agents (e.g., EDTA) affected the process significantly. Removal of the metals from the resin was successfully performed with 3N HCl or HNO 3 .