Membranes and Membrane Technologies最新文献

Effect of the content of tetrafluoroethylene groups on the gas transport properties of vinylidene fluoride-tetrafluoroethylene copolymers has been studied. The experimental values of permeability coefficients P and diffusion coefficients D for gases H₂, He, N₂, O₂, and CO₂ as well as lower hydrocarbons CH₄, C₂H₄, and C₂H₆ are measured, and their solubility coefficients S are calculated. It is shown that the values of the solubility coefficients of СО₂ and С₂Н₄ deviate from the direct correlation dependence of lоg S on the Lennard-Jones potential, and this effect is explained in terms of facilitated transport models. It is demonstrated that an increase in the content of TFE groups leads to a significant rise in the permeability coefficients of the studied penetrants mainly due to an increase in their diffusion coefficients. For example, the permeability coefficient of helium and hydrogen increases by almost 2.5 times, carbon dioxide by 3 times, argon, oxygen, methane and ethylene by 3.5 times, and nitrogen and ethane by 4.4 times, respectively. These gas separation parameters in combination with good film-forming properties and commercial availability make it possible to consider the studied VDF-TFE copolymers to be promising materials for the fabrication of composite gas separation membranes.

The progress of modern technologies and the requirements imposed on the production ecology demand the development of new ion-exchange membrane polymer materials with a set of desired properties. These materials are used in liquid and gas separation and purification systems, chemical and electrochemical syntheses, and alternative energetics. Membrane materials based on perfluorosulfonic acid polymers (PFSA) possess a set of characteristics necessary for their practical application: high ionic conductivity and selectivity and good chemical stability, strength, and elasticity. This review addresses the microstructure of PFSA membranes and its change induced by water and solvent uptake and discusses the features of ion and gas transport, mechanical properties, and the dependence of a number of parameters on polymer chain length and ionic form.

Commercially available hollow fiber membranes made of two polymers, namely, polysulfone and poly(phenylene oxide), are studied experimentally. The main task of this study is to estimate the gas transport characteristics of these membranes in relation to air components and noble gases. Therefore, the values of permeability of the membranes for nitrogen, oxygen, helium, argon, xenon and krypton are measured. Particular attention is paid to the xenon-containing air mixture, since the capture of medical xenon is an urgent chemical and technological problem due to a high cost of the process of obtaining this gas. The values of permeability of the two membranes for individual gases are determined, and the values of ideal selectivity are calculated. For example, the values of membrane permeability for argon, krypton, and xenon are 20.8, 8.4, and 6.8 GPU for the polysulfone membrane and 19.5, 6.2, and 4.8 GPU for the poly(phenylene oxide) membrane, respectively. It is found that the xenon permeability of these membranes in the case of separation of the gas mixture composed of nitrogen, oxygen, and xenon is 5.9 and 4.1 GPU for polysulfone and poly(phenylene oxide). It is also shown that the performance of membrane modules based on polysulfone and poly(phenylene oxide) depends on the total membrane area.

This work is devoted to the removal of dissolved oxygen from a model absorbent based on monoethanolamine (MEA) to prevent its oxidative degradation during the absorption purification of flue gases from carbon dioxide. Composite membranes based on porous ceramic and polymeric supports with a thin selective layer of poly[1-(trimethylsilyl)-1-propyne] or its blend with polyvinyltrimethylsilane are developed, and gas-liquid membrane contactors are created on their basis. It is shown that the use of these contractors in the vacuum mode allows the removal of up to 60% of dissolved oxygen from the model sorbent.

The paper suggests exact formulae for calculating the electromigration, diffusion, and convective transference numbers of counterions in the cell model of a charged membrane depending on the physicochemical parameters and equilibrium concentration of the electrolyte. The cell model was previously developed to calculate all the kinetic coefficients of the Onsager matrix, and the asymmetry of the cross coefficients was established. A limiting case of an ideally selective membrane is studied in detail, for which approximate formulae for transference numbers are obtained. The obtained dependences are illustrated by graphs by way of example of an MK-40 cation-exchange membrane after conditioning at room temperature. The proposed calculation procedure for transference numbers is applicable to any single-layer membranes in solutions of a binary electrolyte.

The results of a theoretical analysis of the influence of the electroosmotic flow on the electromigration and convective transport of competing ions separated by the electrobaromembrane process are presented. Separated ions of the same charge sign move in an electric field through the pores of a track-etched membrane to the corresponding electrode, while a commensurate convective counterflow being created by the pressure drop across the membrane. A simplified model based on the convective electrodiffusion equation and the Hagen–Poiseuille equation allows the analysis of experimental data using only the effective transport numbers of ions in the membrane as fitting parameters. Using a 2D mathematical model described by the system of Nernst–Planck, Navier–Stokes, and Poisson equations, it is shown that the electroosmotic flow can cause the effective transport numbers of competing ions to exceed their values in solution, even if these ions are coions for the membrane.

The paper presents the results of a study of water uptake, ionic conductivity, and Donnan potential in systems with perfluorosulfonic acid membranes in the H⁺, Li⁺, Na⁺, and K⁺ ionic forms and solutions of inorganic electrolytes. The properties of commercial membranes Aquivion E87-05S and Nafion 212, as well as membranes obtained from dispersions of Nafion 212 in solvents of various nature (N,N-dimethylformamide, 1-methyl-2-pyrrolidone, mixtures of isopropyl alcohol with water in a volume ratio of 80–20) have been studied. The effect of the number of functional groups, the length of the side chain of polymer macromolecules, and the morphology of the polymer in membranes on their equilibrium and transport properties depending on the nature of the counterion has been determined. The effect of relaxation and electrophoretic factors on the transfer of alkali metal ions through the system of pores and channels of perfluorosulfonic acid membranes is discussed. The slope of the concentration dependences of the Donnan potential for all highly hydrated membranes in the H⁺ form has been found to be close to the Nernstian one, while the selectivity to alkali metal ions increases for membranes with the highest ion exchange capacity or the lowest amount of sorbed water and diffusion permeability due to the exclusion of co-ions from the membrane phase.

Amino acids that are ampholytes can be effectively separated and purified by the method of neutralization dialysis (ND), whose advantage is the ability to control the pH value of the solution without adding reagents. An important task is to optimize the parameters of the ND process to ensure minimal losses of amino acids during their isolation from mixed solutions. An experimental study of the process of demineralization of the phenylalanine and sodium chloride equimolar mixture by the ND method is carried out. It is established that varying the concentration and flow rate of acid and alkali solutions in the corresponding compartments of the dialysis cell allows for regulating the pH value of the solution being desalted and controlling the amount of amino acid losses. Halving the acid concentration (from 0.10 to 0.05 M) allowes reducing the losses of phenylalanine from 18.3 to 16.4%, and using a lower solution flow rate in the acid compartment (0.75 instead of 1.50 cm s^–1) makes it possible to reduce these losses to 14.2%. At the same time, in all experiments, the electrical conductivity of the solution being desalted decreases by 90%, which suggests a high demineralization rate and the effectiveness of the method used to isolate phenylalanine from the mixed solution.

Pd60%Cu40% alloy membranes are modified with nanostructured coatings to intensify the low-temperature (25–100°С) transport of hydrogen. Classical palladium black and filamentous particles are deposited as surface modifiers by electrodeposition. The experimental data confirm that the deposition of the modifying layer on both surfaces of the Pd60%Cu40% alloy membranes can considerably reduce surface limitations for the process of hydrogen transfer. In the low-temperature hydrogen transport processes, the developed membranes demonstrate high and stable fluxes up to 0.36 mmol s^–1 m^–2 and high hydrogen permeability up to 1.33 × 10^–9 mol s^–1 m^–2 Pa^–0.5. For the Pd60%Cu40% alloy membranes modified with nanofilaments hydrogen permeability is up to 1.3 times higher compared with the membranes modified with classical black and up to 3.9 times compared with the uncoated membranes. The Pd60%Cu40% alloy membranes also exhibit a high level of H₂/N₂ selectivity, up to 3552. The strategy of surface modification of palladium-based membranes can shed new light on the development and manufacture of high-performance and selective membranes for ultrapure hydrogen production units.

In this work, the production of sodium hydroxide by the method of bipolar electrodialysis from a solution of sodium carbonate using bipolar membranes MB-3 has been studied. For research, a laboratory electrodialyzer-synthesizer with a three-chamber unit cell has been used. The membrane package of the electrodialyzer has contained five elementary cells, the active area of each membrane being 1 dm². To compare the obtained mass transfer characteristics, the process of preparation of sodium hydroxide from sodium sulfate has been additionally studied. It has been shown that the use of sodium carbonate as the initial solution makes it possible to increase the concentration of the resulting alkali from 0.92 to 1.7 M under comparable process conditions compared to the preparation of sodium hydroxide from a sodium sulfate solution. When sodium carbonate is used, the alkali current efficiency is more than 70% in all experiments, while when alkali is obtained from a sodium sulfate solution, the current efficiency drops sharply to 0.4–0.5% when the concentration exceeds 0.8 M NaOH. The energy consumption for the transfer of one kg of alkali is in the range of 2.8–13.9 kWh/kg at operating current densities of 1–3 A/dm².