Membranes and Membrane Technologies最新文献

Abstract

Polynaphthoylenebenzimidazoles (PNBI) with keto (PNBI-СО) and sulfonic (PNBI-SO₂) bridging groups are prepared by the solid-phase polycyclization of films of corresponding polyaminoimides (PANI) synthesized by the polycondensation of 1,4,5,8-naphthalenetetracarboxylic dianhydride with 3,3',4,4'-tetraaminobenzophenone and 3,3',4,4'-tetraaminodiphenyl sulfone in N-methylpyrrolidone, respectively. The polycondensation process and the chemical structure of the resulting PANI and PNBI are controlled by ¹Н and ¹³С NMR and IR spectroscopy. It is shown that variation in the temperature of solid-state polycyclization allows for the synthesis of polymers with various cyclization degrees. The experimental values of gas permeability and diffusion coefficients for He, H₂, N₂, O₂, CO₂, and CH₄ are measured, and the solubility coefficients and ideal selectivity for various gas pairs are calculated. It is found that in terms of the permeability/selectivity ratio completely cyclized PNBI are advantageous over incompletely cyclized ones. This finding should be taken into account when choosing a polymer and a selective layer formation method to develop novel composite membranes. The gas transport characteristics achieved for completely cyclized PNBI-SO₂ and their good film-forming properties combined with a very high thermal stability of this class polymers are of great interest for further expanding the PNBI range and prospects for applying novel polymers of this class in various gas separation processes.

Abstract

A procedure has been proposed for producing ceramic substrates for filtration membranes based on a narrow fraction of fine fly ash microspheres using cold uniaxial pressing followed by high-temperature firing. It has been shown that increasing the sintering temperature from 1000 to 1150°C leads to a decrease in open porosity from 40 to 24%, a decrease in the average pore size from 1.60 to 0.34 μm, and an increase in the compressive strength from 9.5 to 159 MPa. The resulting substrates are characterized by water permeability values of 1210, 310, 240, 170 L m⁻² h⁻¹ bar⁻¹ at sintering temperatures of 1000, 1050, 1100 and 1150°C, respectively. Experiments on filtration of aqueous suspensions of fine microspheres (d_av = 2.5 µm) and microsilica (d_av = 1.9 μm) through a substrate produced at a sintering temperature of 1150°C have shown the rejection close to 100%. The proposed methodology for using ash waste in the production of membrane materials promotes the development of technologies for the integrated processing of thermal energy waste.

Abstract

Сrosslinked polymer memranes are obtained by the heat treatment of films prepared from a solution of a mixture of bromine-containing poly(1-trimethylsilyl-1-propyne) (PTMSP) and polyfunctional amine polyethyleneimine (PEI) as a crosslinking agent. Crosslinked products are identified from IR spectra, elemental analysis data, and stability of reaction products to a solvent (CCl₄), in which the original brominated PTMSP is soluble. According to the IR spectra, the crosslinking reaction occurs via reactive C–Br bond in bromine-containing PTMSP with the participation of PEI amino groups at a temperature above 90°С. The crosslinking of bromine-containing PTMSP makes it resistant to organic solvents. An increase in the content of PEI in the mixture correlates with the proportion of bromine atoms involved in the reaction. For brominated PTMSP films crosslinked by PEI transport parameters for individual gases and in the mixture n-butane/methane (98.4 mol % methane and 1.6 mol % n-butane) are studied. In the sequence PTMSP–brominated PTMSP-Br–PTMSP-Br/PEI (before crosslinking)–PTMSP-Br/PEI (after crosslinking) permeability for individual gases decreases. Crosslinked PTMSP in the mixture methane/n-butane demonstrates high permeability coefficients of n-butane (({{P}_{{n{text{-}}{{{text{C}}}_{{text{4}}}}{{{text{H}}}_{{10}}}}}}) = 12 000 Barrer) and selectivity for n-butane separation from a mixture with methane (({{alpha }_{{n{text{-}}{{{text{C}}}_{{text{4}}}}{{{text{H}}}_{{10}}}{text{/C}}{{{text{H}}}_{4}}}}}) = 13).

Abstract

In this work, mixed matrix membranes (MMMs) were obtained based on polyimides synthesized from a mixture of diethyltoluene diamine (DETDA) isomers and BPDA and 6FDA dianhydrides by introducing metal-organic framework compounds ZIF-8 and ZIF-67 in a concentration of up to 20 wt % in chloroform solution. The starting polyimides were synthesized by one-stage high-temperature catalytic polycondensation in a benzoic acid melt. The studied MMMs were treated with sc-CO₂ followed by decompression. The gas transport and gas selective properties of the original and modified membranes were studied. Experimental values of the effective coefficients of permeability and diffusion of gases for He, H₂, O₂, N₂, CO₂, CH₄ were obtained, and the effective solubility coefficients of these gases were calculated. It was found that treatment of the studied MMMs in sc-CO₂ can significantly increase the level of gas permeability of membranes with gas selectivity at the level of initial values, while the achieved effect of changing the permeability of membranes depends on the gas, the nature of the matrix, and the concentration of the introduced particles. It was found that the treatment effect persists over time with a slight decrease in permeability of gases, which remains at a level significantly higher than the initial values. The demonstrated effect of improving gas transport properties when treating MMMs based on polyimide matrices 6FDA-DETDA and BPDA-DETDA in sc-CO₂ can be used for further application of the proposed modification method in order to increase gas transport through MMMs based on other polymers, including highly permeable ones.

Abstract

Hollow fiber membranes originally developed in the 1960s for the reverse osmosis process have since then become widely used for diverse separation processes. The advantages of hollow fiber membranes include the low energy consumption, ease of operation and, among the most important ones, highly efficient operation in a small footprint (a large membrane area can be packed into a module unit). The production of hollow fiber membranes involves many spinning parameters to be controlled. The list of these parameters, in particular, includes the viscosity of the spinning solution, the design and geometric parameters of the spinneret, the extrusion speed of the polymer solution, the composition and temperature of the bore fluid, the type of external coagulant, the air gap distance, the draw ratio, etc. The effect of these parameters on the properties of hollow fiber membranes is reviewed. Research data pertaining to the modification of polymers, both commercially available and at the stage of their synthesis, are also presented in the context of membrane applications. In addition, the preparation of membranes using non-toxic or less toxic solvents is discussed.

Analytical expressions for the specific coefficients of electrical conductivity and electrodiffusion of a bilayer ion exchange membrane have been obtained in terms of thermodynamics of irreversible processes and the homogeneous model of a fine-pore membrane. The influence of the physicochemical parameters of the modifying layer and the electrolyte concentration on the obtained values of the coefficients at fixed physicochemical characteristics of the substrate has been explored using mathematical modeling. It has been shown that the conductivity and electrodiffusion of the modified membrane increase with increasing the space charge density of the modifying layer when the signs of the space charges of the membrane layers are identical and decrease when they differ or the thickness of the modifying layer increases. With increasing electrolyte concentration, these characteristics of the modified membrane increase regardless of the sign of charges of the membrane layers. The obtained analytical expressions can be used in modeling electromembrane processes and predicting the characteristics of new surface-modified ion exchange membranes.

The article presents an analysis of the kinetic data on dry reforming of methane (DRM) in reactors with traditional (TC) and membrane catalysts (MC). The kinetic experiment in reactors with the TC and MC is performed in the temperature range of 820–900°С at CH₄ : CO₂ = 1 : 1. The experiment reveals intensification of the reaction of methane cracking; its rate constant increases by an order of magnitude. This difference in the DRM data obtained for the studied catalysts is explained by the fact in the case of the MC mass transfer is intensified due to the thermal slip phenomenon. A mathematical description corresponding to the kinetic scheme of the DRM process is proposed, and the rate constants of direct and reverse reactions in both reactors are determined. The DRM process carried out on the TC yields water vapor, while in the case of the MC syngas is produced. On the TC the DRM process is accompanied by the accumulation of carbon deposits (CDs), while on the MC this accumulation is absent. On the TC the DRM process is characterized by three main reactions (methane cracking, gasification of CDs with carbon dioxide and/or water vapor, and reverse water gas shift) which are assumed to be reversible under the experimental conditions. It is found that the gasification of CDs on the TC occurs in the reverse reaction of methane cracking; on the MC, in the reactions of CDs gasification with water vapor (mostly) and carbon dioxide. In the case of the MC, the process is characterized by the irreversible reactions of methane cracking and CDs gasification with water vapor and carbon dioxide. The reverse water gas shift reaction on the MC remains reversible, and its rate constants of direct and reverse reactions are an order of magnitude lower than similar rate constants on the TC.

In this work, the dependence of the output characteristics of the gas separation membrane process determined during the simulation on the gas transport characteristics of the membrane as parameters of the membrane module model has been studied. The study has been performed using the example of a laboratory sample containing hollow fibers from polyphenylene oxide. As a result of this comprehensive study, including theoretical and experimental approaches, it has been determined that when using the gas transport characteristics obtained for pure gases for process simulation, the error expressed in the achievable concentration of the target component in the product stream is from 1.5 to 8.8% in comparison with the experimentally obtained values for the module of the same geometry and the same membrane area. This discrepancy can lead both to the setting of unattainable targets when creating a technological line and to an incorrect technical and economic assessment of the process. Thus, when designing technological lines using mathematical modeling tools, one should rely on the gas transport characteristics of a material and/or product obtained for components of real or simulating real gas mixtures.

This work is aimed at obtaining a membrane material that is resistant to the formation of a precipitate on the surface upon contact with an ABE fermentation mixture and possesses a good separating ability during the pervaporation isolation of n-butanol from a water–alcohol mixture. In this regard, this work for the first time proposes creating pervaporation membranes based on polymethyltrifluoroethylacrylatesiloxane (F3-Acr) as well as a copolymer of polydecylmethylsiloxane and polymethyltrifluoroethylacrylatesiloxane (C10–F3-Acr). The structure and sorption properties of the developed membrane materials for n-butanol, ethanol, and acetone are studied in comparison with polydecylmethylsiloxane (C10). It should be noted that the highest sorption of n-butanol is characteristic for C10–F3-Acr (0.46 g/g). The change in the surface properties is assessed by the value of the contact angle and elemental composition of the surface before and after exposure for 1 month in a fermentation medium. The transport and separation properties of the synthesized membrane materials are studied in the vacuum pervaporation mode during the separation of a model ABE fermentation mixture. It is shown that introducing a fluorine-containing substituent into the side chain of polysiloxane makes it possible to increase the hydrophilicity of the polymer: the water flow for F3-Acr is 0.7 × 10⁻⁶ kg m m⁻² h⁻¹, which is almost threefold higher when compared to C10. A positive effect of the combination of C10 and F3-Acr groups in polysiloxane is worth noting. Thus, with an increase in the total flow by 60% when compared to a C10 membrane, the values of the separation factor for n-butanol, acetone, and ethanol are 40.5, 32.7, and 4.3 and increase by 6, 15, and 12%, respectively, when compared to a C10 membrane. For a C10–F3-Acr membrane, the pervaporation separation indices for n-butanol, acetone, and ethanol are 136, 109, and 11, respectively. Therefore, this membrane is twice as efficient as C10. Taking into account the absence of detectable contamination of the surface of the membrane material with fermentation products, one can note a high potential of a C10–F3-Acr membrane for the task of isolating alcohols from an ABE fermentation mixture.

With the development of oil fields, the proportion of the highest-molecular-weight components, asphaltenes, increases in the composition of the extracted raw materials. The propensity of asphaltenes to aggregate causes a number of problems, which makes the task of oil deasphalting relevant. In this work, studies on separation of the asphaltene fraction from oil using PAN membranes are carried out. To decrease the pore size of membranes obtained by a phase inversion method, an additional component, acetone, is introduced into the casting solution. The permeability of the resulting membranes for water is 37.6 ± 1.7 L m⁻² h⁻¹ atm⁻¹ and for toluene, 25.3 ± 1.8 L m⁻² h⁻¹ atm⁻¹, and the pore size is 4.6 ± 0.5 nm. When filtering solutions of oil diluted with toluene (1 g/L), the retention of the membranes for asphaltenes is 73 ± 4%, while it exceeds 95% when the oil content in the solution is over 10 g/L. The parameters of membrane fouling during filtration of solutions of oil in toluene are studied. It is noted that, upon moving from toluene to solutions of oil, the permeability of the membranes decreases tenfold. At the same time, the decrease in permeability is reversible, and when the solution of oil is replaced by a pure solvent, the membrane restores up to 99% of its initial permeability.