| Membrane distillation, a process in which vapor from a liquid feed passes through the pores of a hydrophobic membrane, was investigated as a possible technique for desalination and for the removal of ethanol, acetone and benzene from water. Three different membrane modules were used in the experiments examining the impact of operating variables upon permeate flux and quality. A mathematical model was developed and validated for the membrane distillation process.;In the desalination study, two modules with built-in condensing surfaces were used for conducting experiments on flat membrane sheets. The quality of the permeate, quantified by conductivity measurements, and the permeate flux were monitored as feed concentration, feed temperature, feed flow rate, cooling temperature, and cooling water flow rate were varied. The effect of the distance between the membrane and condensing surface was investigated by adjusting the air gap within one of the modules. The feed streams tested included sodium chloride solutions and artificial seawater. The membrane materials used were PVDF and PTFE. At optimal conditions, fluxes as high as 26 kg/m;A flat-sheet cross-flow module was used to study the separation of dilute solutions of ethanol and acetone. The same operating parameters tested in the desalination experiments were varied for ethanol and acetone feeds. The upper feed concentrations tested were limited by the risk of membrane wetting to 10 wt% ethanol and 6 wt% acetone. Within the feed temperature range of ;A mathematical model, based on first principles of heat and mass transfer, was developed and validated on the desalination, ethanol, and acetone experimental data. Good agreement between experimental and predicted values was obtained with the model version incorporating temperature and concentration polarization.;The possibility of using vacuum membrane distillation for treating water contaminated with volatile organic compounds was investigated by using benzene as a model compound. An experimental unit with polypropylene membrane tubes fitted within a polypropylene shell was used in the tests. Dilute benzene solutions were pumped through the lumen of tubular membranes while vacuum was applied to the shell side. The data obtained by varying operating conditions was analyzed in terms of the overall mass transfer coefficient based on the film theory. A comparison between the effectiveness of membrane distillation and air-stripping was drawn based on a published case study. The experimental unit with tubular membranes failed to compete but extrapolations to higher membrane contact areas showed promise for hollow fiber modules. |
