Membranes and Membrane Technologies最新文献

The results of the study of the oscillations in the chronopotentiograms during scaling on the surface of an MA-41P anion-exchange membrane facing the concentration chamber of an electrodialyzer during the processing of dilute stratal water similar in composition to natural brines incidentally produced in the gas and oil fields of the Angara–Lena basin are presented. If Mg²⁺ ions are present in the solution, porous Mg(OH)₂ is predominantly formed on the surface of an MA-41P membrane despite the excess Ca²⁺ content and sufficient amount of ({text{HCO}}_{3}^{ - }) in the solution. In the absence of Mg²⁺ ions in the solution, CaCO₃ precipitates on the surface of the membrane. During scaling, an increase in the amplitude and period of the oscillation of the potential drop ∆φ from 5 mV and on average 5–10 s (due to the development of equilibrium electroconvection) up to 50 mV and 430 s, respectively, is recorded in the chronopotentiograms of MA-41P membrane. It is established that the amplitude and period of oscillations of the potential drop as well as the pH values of the concentrate can serve as a signal for taking actions to flush and regenerate ion-exchange membranes during the electrodialysis processing of dilute solutions of stratal water.

Based on the analysis of the results of sorption experiments and the study of the influence of the oxidant nature at different stages of the preparation of composites, a method has been developed for modifying anion exchange membranes with sulfonated polyaniline by in situ oxidative polymerization of 2-aminobenzenesulfonic acid. The formation of the modifier in the membranes has been proven by analyzing the ATR IR spectra of the samples. The study of the physicochemical and transport properties of composite membranes has shown that the synthesis of the modifier leads to a significant decrease in the exchange capacity from 3.41 to 1.65 mmol/g_dry for the MA-40 membrane and from 0.87 to 0.21 mmol/g_dry for MA-41. In this case, the electrical conductivity of the composite based on the MA-40 membrane increases by 2 to 3 times in sodium chloride solutions and that of the composite based on the MA-41 membrane decreases by a factor of 1.5–2. The current–voltage characteristics of the composite membranes were found to have asymmetry, which confirms the gradient character of the modifier distribution.

Perfluorosulfonic acid membranes MF-4SC containing 0.5–1.0 wt % carbon nanotubes with carboxyl groups on the surface have been manufactured. On their basis, multisensory systems have been developed for the simultaneous determination of amino acids (the total concentration of their anionic and zwitterionic forms) and K⁺ cations in aqueous solutions at pH 8–10 in the concentration range from 1.0 × 10^–4 to 5.0 × 10^–2 M. The relative error in the determination of alanine (5–15%), valine (0.3–10%), and phenylalanine (0.7–5%) is comparable with that for K⁺ cations (1.4–11%). Differences in the cross sensitivity of sensors based on the pristine and modified membranes, necessary for their use in multisensory systems, are achieved due to changes in the microstructure of membranes and the appearance of new reaction centers that affect the conditions of non-exchange sorption of aliphatic and aromatic amino acids in different ways.

A study of the effect of temperature in a wide range (from 258.15 up to 293.15 K) on the gas transport characteristics of asymmetric polymeric gas separation fibers based on polysulfone for the components of a CO₂–N₂ binary gas mixture with various compositions is carried out. Within the framework of the work, the values of the permeability, diffusion, and solubility coefficients of pure gases in the temperature range of interest as well as the values of the permeability coefficient of the components of a CO₂–N₂ binary gas mixture are determined using gas chromatographic analysis of the permeate flow. It is demonstrated in this work that the process of separation the most effectively occurs at room temperature in the case of a low concentration of carbon dioxide (up to 15 mol %) in the binary mixture. It is experimentally shown that the separation process is the most effective at a low temperature (273.15 K) in the rest of the range of concentrations of CO₂. Further decrease in temperature does not make the process of separation of the mixture more effective throughout the entire considered range of concentrations of carbon dioxide.

Lithium metal batteries are a promising replacement for lithium-ion batteries due to their ability to achieve high energy densities. A significant issue of this type of battery is dendrite growth through the separator, which makes these batteries unsafe and limits their commercial application. In this work, a single-ion conducting polymer electrolyte based on a cation-exchange membrane Nafion solvated by a ternary mixture of ethylene carbonate/dimethyl carbonate/N,N-dimethylacetamide has been obtained. It has an ionic conductivity of 1.8 mS cm^–1 at 25°C and an electrochemical stability window of 4.1 V. It has been shown that the symmetrical Li/Li cell operates stably at a current density of 0.1 mA cm^–2 for >350 h.

The process of heterogeneous sulfonation of a radiation-grafted copolymer of polystyrene and polyvinylidene fluoride depending on the reaction time and the type of sulfonating agent (chlorosulfonic acid or its equimolar mixture with acetic acid) has been studied. The water uptake, ion-exchange capacity, and ionic conductivity of the prepared membranes have been characterized. In addition, composition and morphology of materials at different synthesis stages have been analyzed by FTIR, ¹Н NMR, and EPR spectroscopies, elemental analysis, and scanning electron microscopy combined with energy dispersive X-ray microanalysis. The ionic conductivity of the prepared materials exceeds that of Nafion®212 membranes. In addition, the mechanical properties and hydrogen gas permeability of the membrane with the highest ionic conductivity (52 mS/cm at 80°C in contact with water) are better than those of the Nafion®212 membrane.

In this paper, the process of film distillation with a porous condensing surface (FD-PCS) is studied as applied to the tasks of concentration of lithium-containing solutions. Concentration of solutions is a part of a three-stage lithium extraction cycle that includes softening of the brine by the precipitation of Ca²⁺/Mg²⁺ cations with sodium carbonate (calculated in PHREEQC) followed by an integrated system consisting of a membrane distillation unit and a crystallizer (deposition of NaCl) and membrane extraction (extraction of Li⁺). The productivity flows of the film distillation module are investigated (4.15–7.49 kg m⁻² h⁻¹ at the temperatures of heating of the evaporation surface of 60 and 80°C, respectively). The complex operation of the three-stage system is modeled in Simulink/MATLAB. The modeling of the process based on the experimental and published data shows a higher efficiency of film distillation with a porous condensing surface in comparison with membrane distillation with a porous condensing surface (4.2 kg of lithium versus 1.4 kg over two months of stationary operation of the system).

The effect of the main parameters of the spinning process of polyethersulfone (PES) hollow fiber (HF) membranes using the free-fall spinning technique has been studied. The influence of the flow rate of the polymer dope solution, the internal coagulant (bore fluid), the air gap distance, and the bore fluid temperature on the geometric dimensions, structure, and transport properties of the membranes has been analyzed. It has been shown that the variation of these spinning parameters makes it possible to control the transport properties of the obtained membranes in a wide range: the pure water flux (PWF) within 120–950 L/m² h and the rejection coefficient with respect to PVP K-30 in the range of 20–93%. It has been established that the PWF of PES hollow fiber membranes increases linearly with an increase in the fiber extrusion ratio (ER) regardless of the spinning parameter that is varied. As shown by SEM and AFM, an increase in ER leads to an increase in the proportion of interconnected pores on the selective surface of PES HF membranes and the transformation of the pore shape from round to slitlike. A comparative analysis of the structure and properties of hollow fiber and flat sheet membranes obtained from the same composition of the dope solution has shown that the flat sheet membranes are characterized by an anisotropic spongy structure and the hollow fiber membranes have the support layer filled with finger-shaped vacuoles (macrovoids). It is concluded that the formation of macrovoids, as well as an order of magnitude higher water flux of the PES hollow fiber membranes, is due to a violation of the integrity (microfractures) of the selective layer being formed as a result of fiber drawing in the air gap.

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.