Modelling heat and mass transfers in DCMD using compressible membranes

A model for predicting the flux and evaporation ratio in direct contact membrane distillation (DCMD) using a compressible membrane is presented. Polytetrafluoroethylene (PTFE) membranes, one of the most common types of membranes employed in MD, are characterised with high porosity (∼90%) and high hydrophobicity, and therefore have high water vapour permeability and high wetting resistance. However, the PTFE membrane is compressible due to its structure. Compression of the membrane will cause a change of its physical structure, such as porosity, thickness, and pore size. As a result, the thermal conductivity and vapour permeability of the membrane will be altered, causing a change in flux and energy efficiency. Such effects need to be accounted for when scaling up from laboratory data to full scale design, because there may be significant differences in the applied pressure. Therefore, in this paper, the influence of pressure on the flux of the compressible PTFE membrane was modelled. This paper also provides a mathematical method to correlate the applied pressures with physical structure changes based on the assumption of constant tortuosity. The modelling results were compared with experimental results over a range of variable process parameters, i.e., temperatures, velocities, membrane lengths, and pressure applied to the membrane. The errors between the model predictions and experimental results were less than 10% within the operating range used in this investigation

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

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 L m−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

CFD study of heat transfer enhanced membrane distillation using spacer-filled channels

[[abstract]]Membrane distillation (MD) can utilize low level thermal energy and holds high potential to replace conventional energetically intensive separation technologies. Direct contact membrane distillation (DCMD) is suitable for the applications of desalination and concentration of aqueous solutions. Employing spacer-filled channels can enhance the mass flux of the DCMD modules, which can further result in the increase of energy utilization efficiency of the separation. The trans-membrane mass flux is controlled by the boundary layer heat transfer of both fluid channels. The estimation of heat transfer coefficients is critical to the analysis and design of MD modules. This paper presents the results of a comprehensive 3-D computational fluid dynamics (CFD) simulation which covers the entire length of the module and takes into account the trans-membrane heat and mass transfer. The model was verified with experimental data in the literature. The contour maps show that spacers create high velocity regions in the vicinity of the membrane. The trans-membrane heat and mass fluxes both show fluctuating patterns corresponding to the repetitive structure of the spaces and the fluxes are much higher than that of the modules using empty channels. The heat transfer coefficient enhancement factors obtained from CFD simulation are significantly higher than the predictions from literature correlations. The model can serve as an effective tool for developing correlations of heat transfer coefficients and optimal design of spacer-filled MD modules.[[notice]]補正完