Transfer Process And Module Optimization Of Membrane Distillation

Membrane distillation (MD), which is being developed for the purification of water effluents and for removal of water from solution to effect concentration of thermally labile substances, is an innovative membrane separation process. Therefore to investigate the mechanism and to improve the transfer process of MD are important in practice. In this work, both theoretical and experimental investigations are carried out on direct contact membrane distillation (DCMD) and on air gap membrane distillation (AGMD). Based on the primary investigations, module optimization approach to enhance transfer process is proposed.Membrane distillation is investigated using a direct contact and an air-gap module. The permeate flux is monitored as the feed temperature, feed flow rate, feed concentration, cooling temperature, and cooling water flow rate are varied. A predictive mathematical model based on first principles of heat and mass transfer as well as vapor-liquid equilibrium is developed. This model takes into account polarization effect and non-isoenthalpy across the membrane or/and air gap. For AGMD, the transport processes including conduction and convection through the air-gap are both considered. The model's prediction in term of flux is validated with the experimental data, and agreement between experimental and predicted values is obtained.Different solutions were used to examine the effects that solute has on MD. Solute has an influence on transport properties and reduces flux through vapor pressure reduction. Untreated feed has a high potential for fouling and wetting MD membranes, and fouling layer will debase the quality and influence the quantity of permeate.Upon experimental results and theoretical analysis, the effect of a variety of factors on flux is discussed. These factors include structural parameters of membrane, temperatures of feed and cooling water, concentration and physical properties of feed, air partial pressure in membrane pores, thickness of air gap, etc. It appears that improvement of convective heat transfer is very important for enhancement of flux and heat efficiency. A correlation of convective heat transfer coefficient for DCMD module is obtained from the experimental results. Convective heat transfer coefficient for AGMD module is determined by vacuum membrane distillation with various conditions.A predictive model in term of flux for hollow fiber module is developed and verified by experimental data. Experimental data and predicted values agree well. Forhollow fiber module, the diameter, length and thickness of hollow fiber membranes have optimal values in order to get the largest flux or/and production rate, and each optimal value is related to other parameters. A conception of partial packing fraction is brought forward to illuminate the channeling phenomenon in axial hollow fiber modules. To alleviate the channeling effect on MD, hot feed should pass through the tube side. With helical wound module, channeling phenomenon can be relieved, and hollows fiber membranes can act as spacer in the shell. The transport process inside tube is also enhanced by Dean vortices. Helical wound modules are studied in oxygenation and MD operation, and results are compared with conventional module. It is shown that helical wound modules give better performances.An AGMD process with spiral membrane module is developed, in which exchanger is used to heat feed. There is an optimal value of flow rate to get the largest flux. The effect of channel spacers on transport process is studied. Results indicate that turbulance, which makes the flux to be enhanced, can be seen when spacers are used, and the coarse one gives better performance than does that of the thin one.