It was shown for the first time that the conditions of thermal treatment of the casting solution significantly affect the morphology and transport properties of porous, flat, and hollow fiber polysulfone (PSf) membranes. It is ascertained that the main solution components that are subjected to thermo-oxidative destruction are the pore-forming agent polyethylene glycol (PEG) and solvent N-methyl-2-pyrrolidone (NMP). It is proved that hydroxyl groups of PEG actively react in the process of the casting solution thermo-oxidative destruction. It is shown that despite the chemical conversion taking place in the casting solution, their stability towards coagulation virtually does not change. The differences in the membrane morphology associated with the increase of thermal treatment time at 120 °C are not connected to the thermodynamic properties of the casting solutions, but with the kinetics of the phase separation. It is revealed that the change of morphology and transport properties of membranes is connected with the increase of the casting solution viscosity. The rise of solution viscosity resulted in the slowdown of the phase separation and formation of a more densely packed membrane structure with less pronounced macropores. It is determined experimentally that with the increase of casting solution thermal treatment time, the membrane selective layer thickness increases. This leads to the decrease of gas permeance and the rise of the He/CO2 selectivity for flat and hollow fiber membranes. In the case of hollow fibers, the fall of gas permeance is also connected with the appearance of the sponge-like layer at the outer surface of membranes. The increase of selectivity and decline of permeance indicates the reduction of selective layer pore size and its densification, which agrees well with the calculation results of the average membrane density. The results obtained are relevant to any polymeric casting solution containing NMP and/or PEG and treated at temperatures above 60 °C.
The draw ratio (DR) is an important parameter in the process of spinning hollow fiber membranes, by which their separation and performance characteristics can be controlled over a wide range of values. For the first time, a method for determining the DR using a roller movable in the vertical direction is described, which makes it possible to establish the ratio of the dope extrusion linear flow rate to the take-up speed. In the course of the work, polysulfone hollow-fiber membranes were obtained at various DR values. The influence of the draw ratio on the morphology and geometric and gas transport properties of the resulting polysulfone hollow fiber membranes has been examined.
BACKGROUND: Fermentation broth is a complex mixture which contains target and other organic compounds which must be continuously recovered to maintain microorganism activity. To overcome this problem, a sorption-assisted thermopervaporation (SA-TPV) method has been proposed, which enables the removal of volatile organics by means of evaporation, and low and nonvolatile compounds by membrane adsorption. RESULTS:A dense membrane made of polymer of intrinsic microporosity in the SA-TPV process demonstrated stable performance (butanol flux 57.7 g m −2 h −1 ; separation factor butanol/water 30.4) within five months of operation with cell-free ABE fermentation broth. Scanning electron microscopy/energy dispersive X-ray spectroscopy analysis revealed that the proposed SA-TPV method reduced 'external' membrane fouling by a factor of three. Fourier transform infrared and gas chromatography/ mass spectrometry analysis confirmed that it was possible to extract 'external' and 'internal' foulants by membrane adsorption followed by in situ regeneration. CONCLUSION:The SA-TPV approach can be effective for recovery of both volatile and nonvolatile compounds from fermentation broth, whilst maintaining stable long-term membrane performance.
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