Membranes and Membrane Technologies最新文献

The regularities of steam conversion of ethane and methane–ethane mixtures containing 5, 10, and 15% ethane in a reactor with a membrane in the form of a Pd–6% Ru foil with a thickness of 30 μm and a NIAP-03-01 nickel catalyst are investigated. The reaction is studied under the following conditions: 773 and 823 K, feed space velocities of 1800 and 3600 h⁻¹, and steam/feed ratios of 3 and 5. Steam conversion of ethane was studied in a temperature range of 773–853 K. Comparative experiments in a conventional reactor show that, in the membrane reactor, the conversion of the feedstock by the reaction of steam conversion with the formation of H₂ and CO₂ increases and its hydrocracking decreases. Evacuation of the permeate leads to an increase in the yield of H₂ and CO₂. When decreasing the steam/feed ratio from 5 down to 3, hydrocracking of the feedstock and rate of formation of carbon deposits increase. The optimum conditions for steam conversion of ethane and methane–ethane mixtures are T = 773–853 and 773 K, respectively, 1800 h^–1 and steam/feed ratio of 5. The found regularities are similar to those earlier obtained for other types of feedstock (propane–methane mixtures, propane, n-butane, a mixture simulating the average composition of associated petroleum gas) in this membrane reactor.

This work is devoted to the evaluation of xenon permeability coefficients for a wide range of polymeric membrane materials, as well as the primary experimental verification of the calculation results for materials used in the production of gas separation membranes. The solution of the problem of O₂/Xe mixture separation as a base for xenon-containing waste medical gas mixtures where it is possible to recover xenon for its reuse has been emphasized. The xenon permeability coefficients have been evaluated using a correlation approach that relates the molecular properties of a gas to gas permeability, and available literature data on the permeability of other gases. The results obtained make it possible to distinguish two main groups of membrane polymers in the Robeson diagram for O₂/Xe gas pair: xenon-selective (polysiloxane-based rubbers and highly permeable functional polyacetylenes) and oxygen-selective (polyimides, PIMs, perfluorinated polymers). Industrial composite membrane MDK with a selective layer of silicone copolymer and laboratory composite membranes based on PSf and PVTMS have been experimentally investigated. The obtained data demonstrate satisfactory convergence of the experimental values with the estimated ones. Based on the results obtained, MDK membrane can be recommended as xenon-selective for xenon recovery (α(Xe/O₂) = 3.1).

Hybrid organic-inorganic membranes based on tetraethoxysilane and orthophosphoric acid-doped copolymers of 4-vinylpyridine (4-VP) and 2-hydroxyethyl methacrylate (HEMA) have been formed by the sol-gel synthesis method. The membranes are characterized by high values of exchange capacity and proton conductivity. An increase in the proton conductivity of hybrid organo-inorganic membranes compared to the initial copolymer can be associated with the generation of crystallization water during the formation of a silicon dioxide fragment, which follows from quantum-chemical modeling of the local structure of the membrane. The latter includes an organic part from the copolymerization product of 4-VP with HEMA (44 atoms) and an inorganic part of 27 atoms, repeating the structure of the silicon dioxide block.

The article presents the results of a computational and analytical study of hybrid reverse osmosis–membrane distillation (RO–MD) technologies for desalination of Caspian Sea water, suggesting the production of an additional amount of desalinated water by the MD method from RO concentrates heated to 50–80°C by the waste heat of boiler-fuel combustion products. Two options for solving the problem of the CaCO₃ and CaSO₄ scaling of membranes have been studied: with preliminary nanofiltration (NF) or sodium cationization (Na) of seawater, as an alternative to the use of an antiscalant (AS) or acid. The negative environmental effect of most plants (eutrophication of water bodies) and their low efficiency at high concentrations of desalinated water are taken into account. The Langelier saturation index (СаСО₃) and the degree of concentrate saturation (СаSO₄) were used as criteria for membrane scaling. The NF and RO processes were studied using the computer program ROSA, and the MD and Na processes were studied by computer simulation of the corresponding design models. It has been found that at a 70% permeate yield at the NF and RO stages, the possibility of calcium scale deposition on the RO and MD membranes is excluded, but it can occur on NF membranes, thereby requiring the use of antiscalants. At the same time, the additional production of permeate at the MD stage from RO concentrates reaches 40% of the amount of permeate at the RO stage and the total power consumption according to the scheme is 1.88 kWh/m³. Reducing the calcium hardness of sea water to 50 µeq/dm³ by Na cationization makes it possible to refuse both the use of AS and sulfuric acid acidification with additional production of MD permeate of 27% relative to the RO permeate. The power consumption rises to 2.5 kW h/m³. To employ the known advantages of NF without the use of AS, a hybrid Na–NF–RO–MD scheme is proposed. It has been established that at 80% yields of NF and RO permeates, it is sufficient to reduce the hardness of sea water from 16 to 5.5 meq/dm³ to prevent CaSO₄ scaling at all stages of treatment and to exclude CaCO₃ scaling by acidifying the softened water.

New results of studying the one-sided surface modification of polymer films and flat-sheet composite membranes based on poly(vinyltrimethylsilane) using low-temperature plasma are presented. Treatment is carried out by direct current discharge at a cathode and anode, air is used as a working medium, the exposure time is from 10 to 60 s, and the working pressure in a chamber is 15–20 Pa. The structure of the surface layers is analyzed by XPS, AFM, and SEM, and the contact properties of the surface are studied. For cathode-treated PVTMS films the effective permeability coefficients for O₂, N₂, СН₄, СО₂, Не, and Н₂, as well as the effective gas diffusion coefficients, are measured experimentally and the effective gas solubility coefficients are calculated. The permeability coefficients of the studied gases for cathode- and anode-modified composite membranes with a selective PVTMS layer are determined. It is shown that the choice of electrode significantly affects not only the chemical structure of surface and near-surface PVTMS layers but also the gas-transport parameters of the modified samples. It is found that, in the case of cathode-modified homogeneous films, the values of permeability, diffusion, and solubility coefficients of gases are higher while the values of selectivity are lower compared with the anode-modified films. At the same time, the treatment of PVTMS films at the cathode for 30 s makes it possible to increase O₂/N₂ selectivity by more than two times relative to the initial values. The results of modification of the composite membranes differ from those attained for the homogeneous films, and, what is more, for the composite membrane treated at the cathode the O₂/N₂ selectivity is higher by a factor of 2.5 than the initial value. The potential of using surface modification of polymer films and membranes by low-temperature plasma to improve their gas-separation properties is demonstrated.

Electrochemical characteristics of heterogeneous cation- and anion-exchange Ralex membranes and the mechanism of transfer of ions of salt, boric acid and its anions through membranes at different pH values have been investigated by the method of rotating membrane disk (RMD). It is shown that boric acid is transported mainly through an anion exchange membrane. At pH 9.5, the limiting stage of anion transfer through the anion exchange membrane is the reaction of tetrahydroxyborate ({text{B}}left( {{text{OH}}} right)_{4}^{ - }) anion formation. The study of the electrodialysis separation of a solution of sodium nitrate and boric acid has shown that the electrodialysis method makes it possible to effectively separate the components of the mixture, while the value of the specific selectivity coefficient ({{P}_{{{{text{B}} mathord{left/ {vphantom {{text{B}} {{text{NaN}}{{{text{O}}}_{{text{3}}}}}}} right. kern-0em} {{text{NaN}}{{{text{O}}}_{{text{3}}}}}}}}}) = (0.02–0.06), depending on the voltage on the electrodialyzer.

Electrodialysis is a water treatment and wastewater treatment technology that uses an electric field gradient and ion exchange membranes to separate ions in aqueous solutions. Studies conducted using this technology have shown the influence of the hydrodynamic regime on the efficiency of the process, the rate of mass transfer, and polarization concentration. The article presents experimental results, mathematical calculations, and numerical simulation in the universal software system of analysis by the Ansys finite element method. Theoretical calculated results correlate with the results of hydrodynamic processes in the apparatus obtained experimentally. The hydrodynamic regime in the channel of the electrodialyzer cell has been studied, the influence of the mesh geometry on the distribution of fluid flow over the membrane surface has been shown, and the change in flow rate and pressure depending on the structure of the mesh has been described.

The regularities of manufacturing hollow fiber membranes made of poly(2,6-dimethylphenylene-1,4-oxide) (PPO) for gas separation have been studied. The phase inversion method has been used to manufacture the membranes. The dependence of the separation characteristics of the membrane on such spinning parameters as the type of solvent, the exposure time of the polymer solution in the “air” gap, and the type of non-solvents (coagulants) has been studied. The characteristics of the membrane have been obtained by determining their gas permeability. It is shown that higher separation and gas transport characteristics of the PPO membrane are obtained using the wet spinning method. An intrinsic selectivity of 4.8 ± 0.4 has been obtained at a specific oxygen permeability (20°C) of (P/l) (790 ± 82) × 10^–9 [m³ (STP) m^–2 s^–1 kPa] for oxygen-nitrogen system. The developed membranes are promising for use in case for producing nitrogen and oxygen-enriched air.

The results of application of platinum bulk modified perfluorinated membranes in proton exchange membrane fuel cell (PEMFC) are presented. The change in physicochemical and transport characteristics of the membranes after modification with platinum and at different stages of their operation in the PEMFC are also discussed. The thickness, radius pore distribution obtained by the standard contact porosimetry method, the concentration dependences of the conductivity, and the current-voltage characteristic are studied. The influence of copper in bimetallic electrocatalyst on the characteristics of the perfluorinated membrane are considered. An increase in the efficiency of PEMFC with both commercial and bimetallic catalysts and platinum bulk modified membranes due to the formation of a self-humidifying structure is found.

One-sided modification of homogeneous polymer films of poly(vinyltrimethylsilane) (PVTMS), poly(2,6-dimethyl-1,4-phenylene oxide) (PPO), and polybenzodioxane (PIM-1) by liquid-phase fluorination with a fluorine–nitrogen mixture in perfluorodecalin is carried out in the work. The fluorination time is 10 up to 60 min. It is shown by X-ray diffraction analysis for the samples of PPO that the initial samples include a β nanocrystalline phase (48%) in addition to the amorphous phase and it is found that fluorination does not significantly affect the crystallinity index of the films under study. The effect of the fluorination time on the effective permeability, diffusion, and solubility coefficients of oxygen and nitrogen is studied. It is found that the modification leads to a decrease in both the effective diffusion coefficients and effective solubility coefficients of the gases; here, the resulting growth in the permeability selectivity ranges from 30% for PVTMS to a twofold increase in the case of PPO and PIM-1. It is found that such an improvement in the permeability selectivity is predominantly achieved due to the increase in the solubility selectivity. The values of the effective gas permeability coefficients are obtained for an O₂–N₂ mixture. It is found that the achieved values of separation factors for the modified samples are close to the ideal selectivity of the films. The obtained results demonstrate the possibility of effective application of this method not only for the modification of the homogeneous polymer films of the polymers under study but also for membranes with a selective nonporous layer based on them.