ESEL Paper Review_20141118
By Hong Guo
Phone: (+82) (0)10 82276568
1, Title and Author
Title: Membrane distillation crystallization of concentrated salts?flux and crystal formation
Journal: Journal of Membrane Science
Chan Mya Tuna,b, Anthony Gordon Fanea,b,*, Jose Thomas Matheickalb , Roya Sheikholeslamic
a UNESCO Centre for membrane science and technology, School of Chemical Engineering and Industrial Chemistry, University of New South Wales, Sydney 2052, Australia
b Institute of Environmental Science and Engineering, Block 2, Unit 237, Innovation Centre, 18 Nan yang Drive, Nan yang Technological University,
Singapore 637723, Singapore
c Desalination and Fouling Laboratory, School of Chemical Engineering and Industrial Chemistry, UNSW, Australia
2. Summary of Paper
? This paper discusses factors influencing flux at close to saturation and the formation of salt crystal. The flux behavior was investigated using two aqueous salt solution sulfate (Na2SO4) and sodium chloride which have different solubility-temperature coefficients.
? It was found that both concentration and temperature polarization influenced the performance of MD.
? When operated in batch concentration mode without the crystallizer the flux gradually declined due to vapor pressure suppression and concentration polarization, up to a critical degree of saturation. The flux data could be predicted by heat and mass transfer modeling or by a simple empirical relationship involving the overall vapor pressure driving force and the degree of saturation.
? Beyond the critical degree of saturation, rapid flux decline was observed due to crystal deposition and scale formation on the membrane which reduced the membrane permeability.
? Na2SO4 solution was able to operate at slightly higher degrees of saturation, which may be because the negative solubility ?temperature coefficient favors solubility in the polarization layers.
? At point A, the feed concentration reached a critical level of super saturation which resulted in a sharp flux decline. Beyond point A, the drastic flux decline was observed to coincide with a fouling by scaling, due to the rapid growth of crystal deposits on the membrane
surface. Eventually the membrane was completely covered with crystal deposits and the flux dropped to essentially zero.
? This degree of deposition did not contribute to the slight flux decline observed which could be attributed to vapour pressure lowering
? The growth direction of most of the sodium sulfate crystals found on the membrane was aligned with the flow direction of the feed solution. Moreover, larger crystal sizes were observed near the exit region compared to those near the entrance region of the feed channel
? Crystallites could develop or deposit below saturation possibly as a result of random small scale variations in flux due to local pore density distributions
? The directional growth may result from changes in surface shear caused by the deposited crystals and the larger features downstream could be due to the increased concentration polarization in the laminar boundary layer at the end of the channel.
? This oriented formation of nuclei parallel to the membrane surface also shows the influence of the polymeric membrane and is undesirable behavior from the point of MD performance.
? MDC system behaved as a batch-type evaporative crystallizer. As the MD process was terminated once a desired super saturation was reached and then followed by the crystallization, there was no further evaporation or solvent removal from the MD module.
? A decrease in density of the mother liquor revealed a desupersaturation profile inside the crystallizer. This would be due to nucleation and growth of crystals inside the crystallizer.
This paper found that MD can operate with aqueous salt solutions of extreme concentration at reasonably high fluxes. Concentration and temperature polarization as well as vapor pressure lowering associated with an increase in solute concentration cause a gradual flux decrease up to a critical super saturation. A drastic decline in flux beyond this critical super saturation is due to rapid growth of crystal deposition on the membrane and loss of membrane permeability. Controlling the operating temperature and concentration are the critical strategies for developing a steady state MDC process. It would be greatly helpful for the modeling simulation research.
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