Performance of asymmetric hollow fibre membranes in membrane distillation under various configurations and vacuum enhancement

Abstract

Hollow fibre membrane distillation (MD) modules have a more compact structure than flat sheet membrane modules, providing potentially greater advantage in commercial applications. In this paper, a high-flux asymmetrically-structured hollow fibre MD module was tested under various conditions. The results show that increasing velocity and temperature are positive for flux, and salt rejection was more than 99% over the entire experimental range. The hollow fibre module also showed great variation in flux when altering the hot feed flow from the lumen side to the shell side of the fibre, and this phenomenon was analysed based on the characterisation of the asymmetric structure of the hollow fibre. The largest mass transfer resistance was determined to be in the small pore size skin layer on the outer surface of the membrane, and having the hot feed closest to this surface provided the greatest vapour pressure difference across this high resistance mass transfer layer. The results also show that placing the suction pump on the permeate outlet increased the flux by lowering the pressure within the pore and hence increased the rate of vapour mass diffusion. A maximum flux of 19 Lm -2 h -1 was obtained at 85˚C when hot feed was entering the shell side, and the mass transfer coefficient was relatively constant across the entire temperature range when operated at high velocities. These outcomes suggest that asymmetric hollow fibre MD modules should be operated with hot brine feed closest to the high resistant skin layer, and that vacuum enhanced MD further increases vapour transport and flux.

Modelling heat and mass transfers in DCMD using compressible membranes

“…However, the calculated pressure in the pores at 60°C based on the volume change was greater than the pressure actually applied on the membrane, so the modelling program was not applicable at this temperature under pressure higher than 20 kPa. It is speculated that more water may have protruded into the membrane pores due to the reduced water surface tension and enlarged pore size (membrane expansion) at high temperature [29], leading to relative less thickness change. However, further study is needed to verify this assumption.…”

Economic analysis of desalination technologies in the context of carbon pricing, and opportunities for membrane distillation

“…Research intensity picked up in the 1980's [5] due to rising water, energy and environmental issues. We have previously explored polymer and ceramic membranes for desalination, and explored MD in dairy processing and industrial process integration [24][25][26][27][28]. While MD researchers have already focussed on relative technology costs, process optimisation, module design and fouling, this paper presents results on a niche operation of extended RO recovery, as well as the relative price of MD under a carbon tax and in a modified operational mode.…”

Performance enhancement and scaling control with gas bubbling in direct contact membrane distillation

“…the mass of the permeate, the effective membrane area and time interval taken). Also, J can be determined theoretically by the product of the overall mass transfer coefficient and the 'average' transmembrane vapor pressure difference [14].…”