Vol 58, No 6 (2018)

The concentration dependences of the electrolyte sorption, specific electric conductivity, and diffusion permeability of MK-40 and Nafion-117 cation-Ion-exchange membranes in NaCl solutions have been obtained experimentally. Both the membranes contain macropores. The heterogeneous MK-40 membrane contains pores with a size of up to 1 μm between the particles of the ion Ion-exchange resin and polyethylene. The homogeneous Nafion-117 membrane has been subjected to a special thermal pretreatment which results in the formation of macropores. It has been shown for the first time that these experimentally found relationships can be quantitatively described using one set of structural and kinetic parameters of the microheterogeneous model.

A novel pervaporation technique has been proposed that makes it possible to selectively remove volatile chlorinated organics (VCO) from contaminated water; to localize them on activated carbon directly in the membrane module; to create a sufficient driving force of the process due to both high sorption capacity of activated carbon and its VCO adsorption selectivity; and to abandon the use of expensive vacuum equipment, thereby minimizing vacuum pump operating costs. The influence of VCO concentration in the solution and the feed flow rate on the efficiency of VCO removal from water has been studied. It has been found that the dominant resistance to VCO flux through the membrane is due to an additional flux resistance associated with diffusion constraints in the boundary liquid layer near the membrane surface. It has been shown that VCO flux values in the case of this pervaporation method (up to 0.47 kg/(m² h) at 30°C) are several times larger than those for conventional pervaporation approaches. The separation factor can be as high as 380–2400 for a chloroform/water mixture or 400–4800 for a trichloroethylene/water mixture.

Dependences of the transport rate of liquid water and saturated water vapor across commercial membranes (Nafion, MF-4SK) and proton-exchange membranes synthesized by the authors (PVDF, PP, UHMWPE, PTFE films modified with sulfonated polystyrene) on the membrane thickness have been studied. It has been found that at room temperature (17–25°C), the transport rate of liquid water and saturated water vapor across the membranes into an air stream hardly depends on the membrane type and thickness (60–240 μm), with the transport rate of saturated vapor being almost an order of magnitude below that of liquid water contacting one of the membrane surfaces. The fact that the flux of water and water vapor across the membrane does not depend on membrane thickness under conditions of maximum moistening suggests that the flow resistance is determined by the resistance at the feed and permeate interfaces. If one of the membrane surfaces is in contact with liquid water, the transport rate is equal to the rate of water removal from the permeate surface of the membrane; in the case of contact with saturated vapor, the transport rate is determined by the rate of water sorption from the vapor phase by the membrane. The results can be used to optimize the operation of fuel cells based on polymer proton-exchange membranes.

Performance of a three-module membrane column in high gas purification processes to remove highly permeable impurities has been experimentally evaluated using the separation of the CH₄/CO₂ binary gas mixture (CO₂ content of 1 vol %) as an example. The time to reach the steady-state regime in a total reflux operating mode has been determined for two configurations, the three-module membrane column (TMC) and the continuous membrane column (CMC) proposed by Hwang with colleagues. The influence of the efficiency of removal of the highly permeable impurity on the degree of purification of the low-permeability target component has been shown. The obtained product purity has appeared to be 99.997 or 99.93% at the recovery rate of 15 or 81%, respectively. It has been demonstrated that the three-module membrane column hold promise for use in high gas purification.