Membranes and Membrane Technologies最新文献

Significant increase of ion selectivity has been observed for Nafion membranes modified by poly(diallyldimethylammonium chloride) (PDADMAC) in the presence of lower aliphatic alcohols and NaCl. It has been found that addition of the salt into the alcohol-containing modification solutions results not only in decrease of diffusional permeability towards vanadyl ions P but also in simultaneous two- to threefold growth of proton conductivity σ in comparison with the membranes modified in the alcohol-water solutions in the absence of the salt. As a result, ion selectivity of the membranes calculated as σ/P ratio has been increased in ∼4 orders of magnitude as compared with pristine Nafion 112 membrane. Conceivable mechanism of the ion selectivity increasing is proposed for the membranes modified by PDADMAC in the presence of lower aliphatic alcohols and NaCl.

In this work, poly(4-methyl-2-pentyne) (PMP) with quaternary ammonium salts has been functionalized in order to increase CO₂ selectivity. Functional groups have been introduced by a two-stage method: (i) bromination of the initial polymer and (ii) addition of tertiary alkylamines, namely trimethylamine (TMA) and trimethylamine (TEA). It has been established that the optimal amount of introduced functional groups, while maintaining the mechanical properties of the polymer, is up to 5 mol %. The results of organoelemental analysis and IR spectroscopy confirm the PMP functionalization. X-ray diffraction patterns of the samples indicate an increase in the interchain distance in the series initial PMP–brominated PMP–functionalized PMP. TGA data confirm high thermal and thermal-oxidative stability. The coefficients of permeability, solubility, and diffusion of PMP samples containing TMA and TEA salts have been determined for individual gases. An increased ideal selectivity for the separation of gas pairs CO₂/N₂ by 2–3 times and CO₂/CH₄ by 1.5–2 times has been achieved while maintaining the permeability at a high level.

The paper considers the specific features of obtaining metal nanowires by matrix synthesis based on track membranes. The first part of the work considers the main ideas of the method and reviews the published sources devoted to producing nanowires of various types—single-component (from one metal) and multicomponent (from two or several metals). Variants of obtaining homogeneous structures (so-called alloyed nanowires) and heterogeneous structures (so-called layered nanowires) are considered for the latter case. A series of specific features of the electrodeposition method in the case of carrying out the process in a limited volume of membrane pores is considered. The second part of the work considers the experimental results obtained by the authors upon studying the electrodeposition of nanowires made of an iron–nickel alloy. The aim is to find a relationship between the conditions of the synthesis of nanowires and their structure and elemental composition. The features of the electrodeposition of nanowires are investigated and their topography is studied by electron microscopy (with elemental analysis); X-ray method is applied for studying the structure. So-called abnormal electrodeposition of iron is detected. The dependence of the integral elemental composition of the obtained nanowires on the pore diameter and growth voltage is discussed. Data on the nature of distribution of elements along the length of the nanowires are obtained; it is shown that the nonuniformity of the composition is determined by the conditions of production (in particular, different diffusion mobilities of ions in narrow pore channels) as well as depends on the voltage and diameter of the pore channels. Based on the X-ray diffraction data, the type of the lattice (FCC) is determined, and the nature of the change in the lattice parameter is shown which is presumably associated with the difference in the ionic radii of metals.

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.