Vol 58, No 11 (2018)

A brief review of recent scientific publications concerning the steam reforming of methanol in membrane reactors for the production of pure hydrogen is presented. The use of membrane reactors makes it possible to lower the temperature of this process by 100°C, increase the selectivity of the process, and practically eliminate the effect of catalysts’ carbonization. A substantial advantage of the use of membrane reactors is the possibility for removing a stream of high-purity hydrogen from the permeate zone. First of all, this applies to CO impurities, whose presence is critical for the use of hydrogen in low-temperature fuel cells based on proton-conducting membranes. The use of metallic membranes based on Pd makes it possible to directly use the hydrogen produced in the fuel cells.

Disubstituted polyacetylene poly(4-methyl-2-pentyne) (PMP) exhibits one of the highest levels of gas/vapor permeability and selectivity of C3+ recovery from mixtures with permanent gases among known polymers. In this study, semi-interpenetrating networks based on compatible mixtures of PMP and thermally crosslinked polyethyleneimine (PEI) have been obtained to enhance the resistance of PMP to organic solvents. Investigation of the phase equilibrium of PMP and PEI mixtures by optical interferometry has revealed that PMP dissolves up to 30 vol % PEI at room temperature. The fact of thermal crosslinking of PEI is confirmed by IR data. The influence of the proportion of crosslinked PEI on the gas permeability, solubility, and swelling in organic solvents of the films prepared from PMP mixtures with PEI has been examined. Having the PEI content higher than 20 vol %, the films are resistant to organic solvents for at least 14 days. Moreover, with an increase in the proportion of PEI, the degree of swelling of the films is substantially reduced. The increase in stability can be explained by the retention of PMP macromolecules in the crosslinked PEI matrix, which probably reduces the swelling of the films and impedes the extraction of linear PMP macromolecules from the polymer network. The ideal O₂/N₂, CO₂/N₂, and CO₂/CH₄ selectivities increase with a growth in the PEI proportion.

The paper presents the characteristics of a laboratory-made water-selective composite membrane with a selective layer of hydrophobic polyvinyltrimethylsilane (PVTMS) for use in the processes of vapor-phase membrane recovery of alcohols from dilute aqueous alcohol mixtures of biogenic origin. The support is a polyacrylonitrile ultrafiltration membrane, and the PVTMS skin layer of the resulting composite membrane has a thickness of 3 to 4 microns. The vapor transport and separation characteristics of the membrane for water–ethanol and water–butanol mixtures in the temperature range of 60–80°C have been studied. It has been shown that the membrane selectivity varies in the range of 23–39 for the water/ethanol pair or 100–140 for the water/butanol pair. A specific feature of the hydrophobic water-selective membrane as applied to the vapor-phase process is the stability of its characteristics due to the absence of swelling, in contrast to hydrophilic water-selective membranes whose characteristics substantially depend on the activity of water vapor. The process of vapor-phase membrane recovery of ethanol and butanol from dilute aqueous alcohol mixtures of biogenic origin with their initial concentrations of 10 and 1 wt %, respectively, has been mathematically modeled on the basis of the experimental data. The calculation results show that the composite membrane obtained makes it possible to concentrate ethanol and butanol to 95 and 98 wt % with degrees of recovery of more than 0.8 and more than 0.9, respectively.

The effect of pH and concentration of the NaCl solution on the exchange capacity and transport properties of the MA-40 and MA-41 anion-exchange heterogeneous membranes with different nature of fixed groups has been studied: MA-40 has weakly basic groups, and MA-41 has strongly basic groups with a small proportion of weakly basic ones. The exchange capacity, thickness, water content, and electrical conductivity of the membranes equilibrated with NaCl solutions of various concentrations and pH were measured; for the same solutions, the diffusion permeability coefficients were found. The capacity was measured in the pH range from 1.5 to 12; the remaining properties, in the pH range from 3 to 9. Using the values of electrical conductivity and diffusion permeability, the ion transport numbers in the membranes were calculated. It was shown that at the external solution pH 9 the thickness of the membranes and their electric conductivity are minimal, and the transport numbers of co-ions are maximum. This is explained by the fact that in basic solutions weakly basic functional groups are largely deprotonated, and the effective capacity of the membrane is significantly reduced. The maximum effective capacity is achieved at pH \( \leqslant \) 3; in this case, transport numbers of co-ions in the MA-40 membrane are 5-fold, and in the MA-41 membrane, two-fold lower than corresponding values at pH 6 and 9. The changes in the transport properties of the membranes with increasing pH are due to a decrease in the degree of protonation of weakly basic functional groups, these changes are more pronounced for the MA-40 membrane than for MA-41.