Membranes and Membrane Technologies最新文献

In this paper, a new method of layer-by-layer formation of monopolymer membranes based on polyacrylonitrile (PAN) is proposed. The proposed approach allows independent adjustment of the structure and characteristics of individual layers of the membrane to achieve high performance characteristics. IR radiation has been used to modify PAN, the effect of which has allowed to convert the polymer into an insoluble form for the application of subsequent layers. An important feature of IR modification is that the pore size and permeability of the membranes remain unchanged. This makes it possible to form the individual membrane layers under different conditions. The obtained membranes have a well-defined spongy layer on the surface and finger-like pores in the other part of the membrane volume. The presence of the spongy layer on the surface reduces the probability of formation of undesirable defects, which reduce the membrane retention. As a result, defect-free membranes that combine a low molecular weight of MWCO cut-off equal to 1800 g/mol and a fairly good for such a dense membrane permeability of 38.7 L/m² h atm have been obtained. The pore size of the obtained membranes is 3.7 nm.

For the task of triethylene glycol (TEG) dehydration by thermopervaporation with a porous condenser (TPV-PC), the transport and separation characteristics of known commercial composite membranes with hydrophobic and hydrophilic properties are experimentally studied with respect to both the individual components (water, triethylene glycol) and TEG–water binary mixtures with various compositions. It is found that the most effective membrane for the TPV-PC dehydration of TEG is a PolyAn hydrophobic membrane (PolyAn GmbH, Germany) which demonstrates the maximum values of the permeate flux and pervaporation separation index. During the thermopervaporation separation of a TEG–water mixture (the water content in TEG of 30 wt %), a PolyAn membrane demonstrates the values of the water/TEG separation factor of 74 000 and permeate flux of 0.95 kg m⁻² h⁻¹. A long-term experiment on drying of 5 kg of a TEG–water solution is for the first time carried out using a PolyAn membrane. It turns out to be possible to reduce the water content from 30 down to 5 wt % over 113 hours of the thermopervaporation experiment on the dehydration of TEG.

The influence of the parameters of unipolar corona discharge (treatment time, voltage) on the productivity and selectivity of the separation of model Devonian oil-based oil-in-water emulsions using cellulose acetate (CA) membranes with a pore size of 0.2 μm has been studied. The concentrations of oil products in the model emulsion, actual formation water, and the filtrates were determined. The values of treatment time and membrane coronation voltage of 5 min and 15 kV, respectively, at which the highest flux of 20 dm³/(m² h) and a 99.3% efficiency of removal of hydrocarbons from the oil–water emulsion are achieved, have been determined. The increase in flux is due to an increase in the porosity of the filters from 65.8 to 83.6%, and greater efficiency is achieved as a result of chemical restructuring of the membrane surface. By the sessile drop method, enhancement of the surface wettability of a CA filter treated with a corona discharge (U = 15 kV and τ = 5 min), viz., a decrease in the contact angle from 72.5° to 64.6°, has been revealed According to electrophoretic light scattering data, the filtrate of the modified membrane has a lower aggregative stability and contains smaller particles compared to the untreated filter. The efficiency of salinity reduction in the formation water using a membrane treated with a corona discharge at U = 15 kV and τ = 5 min was 99.2%.

The recovery of lower aliphatic alcohols from dilute aqueous solutions presents a demanding task for solving a number of problems including the generation of energy from renewable raw materials. One of the promising processes for concentrating alcohols is the membrane vapor separation method using water-selective membranes. In this work, the transport and separation characteristics of the industrial nanofiltration membrane NaRM, manufactured by JSC RM Nanotech, in the separation of vapor mixtures of water and aliphatic C₁–C₄ alcohols at a temperature of 60°C have been studied. It has been shown that the transport and separation characteristics of the membrane used in the vapor phase process undergo a significant change, which is associated with the gradual removal of glycerol used as a preserving agent. The obtained stable values demonstrate high water vapor permeability, at the level of commercial pervaporation membranes, and the water/alcohol selectivity in the range of 25–45. The results of the study can help to expand the scope of practical application of the domestic membrane in the processes of recovery of alcohols from dilute aqueous solutions using the membrane vapor separation method.

It has been shown that supercritical CO₂ (sc-CO₂) treatment leads to a significant increase in free volume and gas permeability and a certain decrease in selectivity for films of amorphous polyimides based on a mixture of diethyl toluene diamine isomers and 6FDA, BPDA and BPADA dianhydrides. At the same time, higher permeability coefficients are attributed to the growth of both diffusion coefficients and solubility. The sc-CO₂ treatment leads to a certain increase in the selectivity of diffusion for PIs based on 6FDA and BPADA dianhydrides, which is interpreted as ordering of the chains packing in the polymer matrix, and the selectivity of diffusion decreases for PI based on BPDA dianhydride. Mechanical tests of PI films demonstrate an increase in the modulus of elasticity after the sc-CO₂ treatment, which indicates the ordering of the chains packing. However, an increase in the free volume due to the sc-CO₂ treatment leads to an increase in the brittleness of the films for PIs based on 6FDA and BPADA dianhydrides. Such drastic changes in mechanical characteristics are not observed for PIs based on BPDA dianhydride. In general, the analysis of mechanical characteristics allows one to supplement the analysis of gas separation data and confirm the connection between ordering of chains packing and gas separation characteristics of polymers. Comparison of data on the treatment of sc-CO₂ and uniform biaxial deformation shows that the swelling of polymer films in sc-CO₂ is equivalent to three-axial uniform stretching complicated by significant defect formation.

For more than a century, the theoretical concepts proposed in the works of Sand have been used by scientists in the field of chronopotentiometry of electrode and membrane systems. The development of these representations makes it possible to more accurately determine the parameters of objects under study and to identify significant factors that seemed insignificant. In this article, using a nonstationary two-dimensional galvanostatic convective-diffusion model of salt ion transport, a theoretical analysis is made of the influence of the dimensions of the electrodialyzer flow-through desalting compartment on the transition time, τ, of chronopotentiograms. It is shown that the local density of electric current is distributed unevenly along the entire length of the desalting channel, and its value at the entrance is more than an order of magnitude higher than the average and is 1–13% (depending on the length of the channel) lower in the rest of the region. The Sand theory, in turn, assumes a uniform distribution of current density. It has been established that the τ values obtained using the two-dimensional model are greater than those calculated using the Sand equation, τ_s. As the desalting channel length L decreases, the difference between τ and τ_s increases from 3% (at L = 30 mm) to 14% (at L = 1 mm). The results obtained are in good agreement with the experimental data presented in the previous paper for the homogeneous Neosepta CMX membrane. The found dependence will reduce the error in measuring the properties of flow-through electrodialyzers using the chronopotentiometic transition time.

The possibility of obtaining a composite track membrane (TM) is investigated. The TM surface was modified by the method of planar magnetron deposition of titanium. The parameters of the magnetron installation operation, such as the deposition rate, the working pressure in the chamber and the magnetron current, have been optimized. The features of the structure and morphology of the 80 nm thick titanium layer have been studied using a combination of methods such as atomic force microscopy, scanning and transmission electron microscopy. X-ray photoelectron spectroscopy revealed that the titanium nanosheet has a complex composition including titanium, titanium oxide, titanium nitride and titanium carbide. The Scratch test showed high adhesion of Ti to TM, which is associated with the formation of an interfacial layer of titanium carbide. It is established that magnetron deposition of Ti does not worsen the operational parameters of TM and reduces the marginal angle of water wetting to a value of about 33° ± 2°. Study of survival and growth rate of fibroblasts of Chinese hamster (V79 line) on PET TM and PET TM with Ti, a slight decrease in the survival rate of fibroblasts on metallized membranes was shown. Titanium sputtering suppresses autofluorescence of the TM surface, which makes it possible to use PET TM with Ti as a substrate for microscopic examination of fluorescent biological objects both in vivo and in vitro. The resulting PET TM with Ti can be used as the basis of skin prostheses and membrane-sorption materials of a new generation. The conducted studies show that magnetron sputtering is a promising approach to the manufacture of metal polymer membrane material.

Samples of two-layer ceramics based on polydisperse powder of the pearlite mineral and foam silicates possessing high compressive strength up to 50 MPa, thermal stability up to 1150°C, and water permeability of 272 m³/h m² bar have been obtained. According to the X-ray powder diffraction analysis, the supporting substrate material is X-ray amorphous. The average pore size of the supporting substrate is 40 µm, while the average pore size of the modifying layer is 17 µm according to the bubble method and electron microscopy. The obtained materials are promising for use as substrates of microfiltration, ultrafiltration, and nanofiltration membranes.

Mobility of alkaline metal cations Li⁺, Na⁺, Cs⁺ in membranes based on polyethylene and sulfonated grafted polystyrene has been investigated by NMR relaxation technique. The kinetic curves of longitude recovery and transverse decay magnetizations of ⁷Li, ²³Na, ¹³³Cs nuclei were recorded. It was indicated that spin relaxation is due to interaction of nuclear quadrupole moment with electric field gradient generated by cation hydrated water molecules and sulfonate groups. The correlation times and activation energies of translational cation mobility have been calculated from spin-lattice (T₁) and spin-spin (Т₂) relaxation temperature dependences. Cation mobility increases in the next sequence Li⁺ < Na⁺ < Cs⁺. Diffusion coefficients calculated from NMR relaxation have been compared with macroscopic diffusion coefficients measured by pulsed field gradient NMR and impedance spectroscopy techniques. On the basis of this comparison, a model of heterogeneous membrane ionic transfer is discussed. It has been shown that membrane conductivity is restricted by ionic transfer in narrow pores with low functional group concentration.

A procedure has been developed for determining the water transport numbers in an ion-exchange membrane by the gravimetric method. Based on a comparative study of this characteristic by the volumetric and gravimetric methods, the experimental conditions (current density, duration of the experiment, and the concentration range of the electrolyte solution) have been found under which the water transport numbers differ by no more than 5%. The electroosmotic permeability, water content, and electrical conductivity of a heterogeneous cation-exchange membrane MK-40 have been studied in a wide range of concentrations of sodium chloride and sulfate solutions. The influence of the nature of the coion on the equilibrium and dynamic hydration characteristics of a heterogeneous membrane has been evaluated. Using the representation of the membrane as a two-phase system, the structure of the hydrated fixed ion–counterion complex has been quantitatively characterized and the hydration numbers of the sulfo group, the sodium counterion, and the sulfate ion in solution have been calculated.