Nature of flow on sweeping gas membrane distillation

The process of sweeping gas membrane distillation (SGMD), with the liquid feed and the sweeping gas counterflowing in a plate and frame membrane module, has been studied. A theoretical model, which was presented in a previous paper and permitted to obtain the temperature profiles inside the fluid phases, has been developed in order to analyse the physical nature of the transmembrane water flux. Two porous hydrophobic membranes have been studied in different experimental conditions. The influence of some relevant parameters, such as the inlet and outlet temperatures or the circulation velocities of the fluids, has been studied. The experimental results have been analysed according to the model and the conclusion is that the water transport takes place, apparently, via a combined Knudsen and molecular diffusive flow mechanism. From the temperature profiles, a local temperature polarisation coefficient may be defined. From this local value, an overall one for the whole system is then defined. The new theoretical predictions have been applied to the obtained results and the accordance may be considered good.     

Characterization of ion-exchange membrane materials: properties vs structure

This review focuses on the preparation, structure and applications of ion-exchange membranes formed from various materials and exhibiting various functions (electrodialytic, perfluorinated sulphocation-exchange and novel laboratory-tested membranes). A number of experimental techniques for measuring electrotransport properties as well as the general procedure for membrane testing are also described. The review emphasizes the relationships between membrane structures, physical and chemical properties and mechanisms of electrochemical processes that occur in charged membrane materials. The water content in membranes is considered to be a key factor in the ion and water transfer and in polarization processes in electromembrane systems. We suggest the theoretical approach, which makes it possible to model and characterize the electrochemical properties of heterogeneous membranes using several transport-structural parameters. These parameters are extracted from the experimental dependences of specific electroconductivity and diffusion permeability on concentration. The review covers the most significant experimental and theoretical research on ion-exchange membranes that have been carried out in the Membrane Materials Laboratory of the Kuban State University. These results have been discussed at the conferences "Membrane Electrochemistry", Krasnodar, Russia for many years and were published mainly in Russian scientific sources.     

Interaction of ionic compounds with multilamellar liposomes. An electrokinetic model

We have proposed a theoretical model of interaction of ionic compounds (ionic adsorbate or ionic drugs), with multilamellar-liposomes, by means of their electrokinetic property variation with the ionic compound concentration. In this work, we show the complete development of the model proposed. Its theoretical results have been analysed to study the influence on the zeta-potential value of the following: number of membranes, size, shear-plane situation, critical concentration of formation (ccf), and the ionic compound concentration which annuls the zeta-potential value of the multilamellar-liposomes formed.     

Application of charged membranes in water softening: modeling and experiments in the presence of polyelectrolytes

In this paper theoretical and experimental results for water softening by bipolar membranes in the presence of polyelectrolytes have been presented. A modified capillary model is introduced for flow of a multiion solution through the membrane. The most important modifications are: an integral boundary condition in Poisson equation and a coupling coefficient in the convective molar flux by using the modified Faxen equation. In addition, electrical permittivity of solution has not been considered as a constant. The predictions of the model were compared well with the experimental data obtained by using bipolar membranes in a recirculating test system in the presence of polyelectrolytes.     

Activity coefficients and Donnan coion exclusion in charged membranes with weak-acid fixed charge groups

We have studied theoretically the effects that the dissociation equilibrium of weak-acid fixed charge groups (e.g. carboxyl groups) exerts on the mean activity coefficients in charged membranes using a Donnan formalism. The model calculations indicate that unless carbon dioxide is excluded from the external aqueous solution, the pH of the membrane solution can be low enough to affect significantly the effective fixed charge concentration and the coion exclusion when the membrane fixed charge concentration is high compared with the external solution salt concentration. Although this problem was already pointed out in previous studies, the possibility that the pH and salt concentration effects appeared in the mean activity coefficient was not considered. We have analyzed quantitatively these effects here and shown that they will eventually be reflected in the effective mean activity coefficients obtained experimentally unless an appropriate theoretical model incorporating the dissociation equilibrium equation explicitly is used to analyze the experimental data.     

Kinetics of immobilized hydrogenase catalysis

Hydrogenase from phototrophic bacteriaThiocapsa roseopersicina has been immobilized in polyacrylamide gel membranes. Stability of the preparations thus obtained in the reaction of hydrogen evolution in the system sodium dithionite-methyl viologen-hydrogenase has been studied. Kinetics of immobilized hydrogenase action is discussed. Hydrogen flux values have been determined as functions of sodium dithionite and methyl viologen concentrations and plate thickness. The results obtained are interpreted in terms of a theoretical model of two consecutive heterogeneous reactions. The theory is compared with the experiment; a number of parameters describing enzyme action in membranes are determined from the kinetic data. The kinetic data and the data of other independent experiments have been used to determine the same parameters in order to test model applicability. Good agreement between the theoretical model and the experimental data is observed. Author to whom correspondence should be addressed. Department of Chemical Enzymology.     

Modeling proton transfer in imidazole-like dimers: a density functional theory study

A detailed theoretical study of proton transfer reaction in protonated imidazole, 1,2,3-triazole, and tetrazole dimers, the basic components of polymeric membrane used in proton exchange membranes fuel cells, has been carried out. In particular, several approaches based on density functional theory have been considered and their results compared with those provided by post-HF methods. From a computational point of view, these molecules appear to be a very challenging playground also for robust and recent functionals. Indeed none of the considered approaches provide results in close agreement with the reference post-HF data and a combined BMK//B3LYP model seems the only approach able to reproduce both the energetic and the structural features. From a chemical point of view, two new mechanisms of proton transfer in tetrazole dimers have been investigated and found to be more favorable than that previously hypothesized in literature. At the same time, the theoretical results show a direct connection between the obtained proton transfer barrier and the charge localized on the transferred hydrogen.     

Enrichment of 13C by chemical exchange between CO2and amine carbamate in nonaqueous solvents

Enrichment of ¹³C by chemical exchange between CO₂ and amine carbamate in nonaqueous solvents has been mathematically modelled in two ways. The height equivalent to a theoretical plate and steady-state separation, based on the two models, have been obtained. If only the isotopic exchange between CO₂ gas and amine carbamate is considered, the model can estimate the process performance for pressures close to the atmospheric one and room temperature. For process analysis at pressures higher than the atmospheric one and lower temperatures, a two-step model has been used. Using the two models the effects of pressure have been studied.     

Concentration polarization in gas permeation

The aim of this article is to show the importance of concentration polarization effects in the separation of gas–vapor mixtures using membranes. In the experimental part of this work, gas mixture measurements are conducted with a specially designed test cell. The experimental data are analyzed using a two-resistance model for the transport through the membrane, which is derived in the theoretical part of this work. The two resistances considered are the transport through the boundary layer on the feed side of the membrane and through the separation layer. For the transport through the separation layer an extended free volume model is derived. This model considers not only the feed side but also the influence of the permeate side on the separation properties of the membrane. The results of the measurements show the influence of concentration polarization effects and their dependencies on feed pressure, membrane thickness, and feed flow rate.     

Passive Transport of Ionic Drugs through Membranes with pH-Dependent Fixed Charges

We have studied both theoretically and experimentally the passive transport of ionic drugs through membranes with pH-dependent fixed charge. The system considered constitutes a simplified model for pH-controlled drug delivery through membranes of biochemical and pharmaceutical interest. The theoretical approach employed is based on the Nernst-Planck flux equations and all of the species present in the system (the neutral or ionic drug and the hydrogen and hydroxide ions) have been taken into account together with a Langmuir-type isotherm for the adsorption of the ionic drug onto the membrane surface. The membrane permeabilities of cationic, anionic, and neutral drugs through porous membranes with graft-polymerized weak polyelectrolytes have been measured as a function of the external pH. According to the nature of the grafted polyelectrolyte, the ionized membrane fixed groups can be negative or positive. For the amphoteric membrane, both fixed charge groups are present in the grafted chains. In all cases, the ionization state of the weak polyelectrolyte fixed groups changes with the local pH within the membrane. A comparison of the theoretical results with the experimental data allows one to explain qualitatively the changes of the membrane flux with the external pH and gives new physical insights into the transport problem. Copyright 2000 Academic Press.     

Wetting Study of Hydrophobic Membranes via Liquid Entry Pressure Measurements with Aqueous Alcohol Solutions

A new concept of liquid entry pressure measurements is applied to study the hydrophobicity of microporous membranes for aqueous alcohol solutions. The effects of alcohol concentration, type of alcohol, and temperature on liquid entry pressure of the membrane have been studied. Two theoretical equations for the determination of membrane pore size have been proposed. The former equation was developed taking into account the deviation from the Laplace–Young equation due to the membrane structure by means of the structure angle. The latter equation was established considering only the range of alcohol concentration in which the dispersion component of liquid surface tension remains practically constant. Hydrophobicity has been expressed in terms of wetting surface tension, γ_L^w. Based on these measurements, the maximum concentration before the spontaneous wetting occurs would be predicted.     

Quantitative studies of interaction between complementary polymers and oligomers in solutions

A theoretical model for interpolymer reaction of the type: matrix + oligomer ? polycomplex has been considered. Matrix and oligomer are complementary macromolecules with the matrix chain length substantially greater than that of the oligomer chain. Theoretical relations connecting the conversion degree in the interpolymer reaction with temperature, oligomer length and oligomer concentration are developed. The limits for application of the considered theoretical model are indicated. High selectivity of the interpolymer reaction with respect to the oligomer chain length is predicted theoretically. The validity of the theoretical relations is demonstrated by the reactions between polycarboxylic acids and polyethylene glycol in aqueous and water-methanol media. The selectivity of the interpolymer reactions with respect to chain length is experimentally proved for the example of fractionated relatively low molecular poly-N-vinyl pyrrolidone complexing with polyacrylic acid.     

新型亲和膜色谱用于去除胆红素的研究

 以纤维素膜为亲和基质 ,接枝后分别以聚赖氨酸和季铵盐为配基 ,制备了两种亲和膜介质 ,用于去除磷酸缓冲液 (pH 7 4)及人血白蛋白 (HSA)溶液中的胆红素。实验结果表明 ,该方法对磷酸缓冲液中胆红素的去除率能达到 70 %以上 ;对HSA溶液中的胆红素 ,其去除率稍低 ,但在较低浓度的HSA溶液中 ,也能获得高于 5 0 %的去除率。并对亲和膜与胆红素间的亲和作用进行了研究 ,考察了温度、流速、HSA浓度及胆红素初始浓度等相关条件对去除效果的影响。结果表明 ,在较高的温度条件下 ,去除效果较佳 ;在较高的初始浓度情况下 ,胆红素的吸附速率较大。     

Mass transfer in the membrane concentration polarization layer under turbulent cross flow : II. Application to the characterization of ultrafiltration membranes

Eleven commercial membranes with MW cut-off values between 5,000 and 25,000 daltons have been characterized using a 0.5% Dextran T10 solution as a test substance. In order to predict the concentration at the membrane wall, Sherwood correlations based on various turbulence models have been used. The effects of the roughness of the membrane and viscosity changes due to concentration polarization and suction through the membrane have also been considered. The results have been compared with an uncorrected film theory model in a Reynolds interval of 10000-20000. It is difficult to test the models unless a direct method of estimating the concentration at the membrane wall is developed. However, some conclusions can be drawn about the applicability of the suggested models, according to the type of membrane used. It was also noted that the differences between the models tended to diminish with increasing Reynolds numbers.     

Boron reduction performance of reverse osmosis seawater desalination process

In recent years, seawater desalination systems using reverse osmosis (RO) membranes have been constructed to settle the lack of drinking water. RO desalination membranes have high rejection for most of solutes in seawater. Japanese drinking water standards for the water quality of the permeate can be achieved except for boron. Therefore, the boron rejection needs to be considered in the design of the RO process and during the operation of the plant. Luckily, there is a simple and easy method to estimate boron concentration.In this paper, we report measured boron permeabilities and their relation to salt permeabilities using cross-linked polyamide membranes. Chemical degradation of the membranes affected these permeabilities to different degrees. Boron concentrations in the permeate were then calculated using a computer program that was based on the boron permeabilities calculated from the measured salt permeabilities. Results obtained were compared with actual data taken at a RO plant of Toray Industries, Inc., Ehime. The model data fitted the experimental result, well. It was also found that a relationship existed in the permeate between salt and boron concentrations and that the boron concentration can be obtained from measurement of the salt concentration.     

Statistical model of the localized electron in dilute ionic solutions

A theoretical model of the localized excess electron in dilute ionic solutions is presented. The model is based on the ion-pair theory of ionic solutions. The giant quasi-dipoles built of separated cation-anion pairs have been considered as electron traps. The statistical distribution of the depth of the electron traps and the optical absorption spectrum of the localized electron have been calculated. The trap distribution results from the statistical distribution of the distances separating two ionic partners of opposite charge and depends on the solvation degree of the ions. Numerical calculations of the electron absorption spectra have been performed for dilute NaX and LiX solutions (X stands for single-charged anion) in tetrahydrofuran at room temperature.     

Isotopic effects in hydrogen-bonded crystals with order–disorder type phase transitions

The influence of geometric isotopic effects on the Curie temperature of hydrogen-containing crystals was considered. The experimental pressure and concentration dependencies of the Curie temperature have been explained in a framework of the pseudo-spin Ising model, by taking into account the changes in geometry of hydrogen bonds under pressure and due to isotopic exchange.     

SELECTIVE SEPARATION OF WATER-ETHANOL MIXTURES THROUGH COPOLYMERIC MEMBRANES: Ⅰ. ACRYLIC ACID AND ACRYLONITRILE COPOLYMER AND ITS IONIZED MEMBRANES

The copolymer of acrylic acid and acrylonitrile has been synthesized and pervaporation properties of the copolymeric membranes have been investigated. In order to elucidate the influence of membrane-permeate interaction on the pervaporation of water-ethanol mixtures and to prepare much improved membranes, the membranes have been treated with alkali metal, alkali earth metal and transition metal salt aqueous solutions. The treated membranes (ionized membranes) exhibited higher separation factors than the untreated membranes. The separation factors of various alkali metal cation membranes decreased in the following order : Li~+>Na~+>K~+, and the permeation rates showed an opposite tendency. The dependence of pervaporation behavior on the copolymer composition ,feed concentration and operating temperature have been studied with both ionized and non-ionized membranes. The apparent activation energies of water and ethanol permeation were calculated.     

SELECTIVE SEPARATION OF WATER—ETHANOL MIXTURE THROUGH COPOLYMERIC MEMBRANES.Ⅱ.ACRYLONITRILE AND MALEIC ANHYDRIDE COPOLYMERIC MEMBRANES

Acrylonitrile(AN) and maleic anhydride(MA) copolymer has been synthesized by radical polymerization using ammonium persulfate and sodium bisulfite as initiator.The pervaporation properties of the copolymeric membranes prepared have been investigated for the first time. The dependences of pervaporation characteristics on coplymer composition,feed concentration and operating temperature have been studied.In order to improve the separation properties of the copolymeric membranes,the membranes were hydrolyzed with 10 wt% soldium hydroxide or potassium hydroxide aqueous solution.The hydrolyzed membranes containing more than 0.069MA mol fraction showed higher tensile strength and separation properties than the original membranes.     

(Series Editor) Transform Techniques in Chemistry,P.R. Griffiths,D. Hercules (Eds.), in: Modern Analytical Chemistry Series,Plenum, New York,NY (1978), 385 pp. $47.40

Concentration potentials in zinc sulphate, copper sulphate, nickel sulphate and cobalt sulphate solutions across liquid membranes based on myristates and laurates of zinc, copper, nickel and cobalt and caprate and caprylate of copper have been studied in concentration cells in which the liquid membrane separates the two solutions of the same electrolyte. The behaviour of the cell with heavy metal soap liquid ion-exchange membranes becomes more and more nernstian as the number of carbon atoms in the hydrocarbon chain of the fatty acid soap increases. Concentration potential values with copper, zinc, nickel or cobalt soap follow the order of their insolubility in water. The failure to record the theoretical potentials as calculated by Nernst's equation has been explained by considering the transport of water across organic phase by hydrated ions and the dissolution of heavy metal soaps in aqueous solutions. Liquid membranes based on heavy metal myristate soaps when used in concentration cells could measure the activities of zinc, copper, nickel and cobalt ions in the concentration range of 10^?4–10^?1M.