Membranes and Membrane Technologies最新文献

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.

The external steady flow of a viscous incompressible fluid and the convective-diffusion mass transfer of a solute in an ordered system of parallel hollow fiber membranes located perpendicular to the flow have been calculated in the ranges of Reynolds numbers (operatorname{Re} ) = 0.01–100 and Schmidt numbers ({text{Sc}}) = 1–1000. The Navier–Stokes equations and the convective diffusion equation have been solved using computational fluid dynamics methods with the no-slip boundary condition and the condition of a constant solute concentration on the outer surface of the streamlined fiber. Calculations have been performed for one row of fibers and for a system consisting of four and sixteen rows of fibers. The output concentrations and impurity absorption coefficients by the fiber (eta ) have been calculated depending on the packing density of the fibers (alpha ) and numbers (operatorname{Re} ) and ({text{Sc}}). The studies have shown that the absorption coefficient (eta ) by the fiber in an isolated row of fibers can be used to calculate the absorption efficiency of a thick fibrous bed.

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.