Study On The Module Performance And Dynamic Process Of Membrane Distillation

Membrane distillation (MD), which is being developed for desalination and concentration of thermally labile substances, is a relatively new separation technique. Study on the performance of membrane modules employed in MD has much academic and practical sense. This research work started from the mechanism of mass transfer across the membrane, and mainly focused on the performance of MD module of flat sheet and hollow fiber membrane, and the results of these works were applied in the study of dynamic MD process.On analyzing mass transfer mechanism in porous media, the effect of temperature within membrane on membrane distillation coefficient (MDC) was determined. The obvious increase of MDC with temperature in the membrane of large pore size indicates that Poiseuille flow plays a very important role in the mass transfer process of MD. This conclusion was verified by direct contact membrane distillation (DCMD) experiment, in which three kinds of flat-sheet membrane were employed. Based on these results, a three-parameter model was presented to describe mass transfer process across the membrane in MD. The model parameters were obtained by fitting experiment data, and the result seemed to be good. The KMPT model was applied in the following modeling of MD module.DCMD experiment was performed on another membrane module for empirical relation of heat transfer coefficient, and the KMPT parameters for another membrane were obtained. These works prepared the segment model for large module. Based on these works, the mathematical model of multi-layer flat sheet DCMD module was set up. The model was solved by numerical method, and some analysis and discussion were performed, by which the effect of various conditions on module flux, production rate and maximum heat recovery rate was determined. Here, the large module shows several different properties with the small one employed in our MD experiment.Experimental study on air gap membrane distillation (AGMD) showed that flow rate and temperature have less effect on module flux compared with DCMD, and air gap thickness can largely affect module flux. These results can be attributed to the existence of air gap in membrane module, which form the main resistance of transfer process. Forcomparison, mathematical model of this process was set up. The modeling ol multi-layer flat sheet AGMD module was performed. The simulation result shows that AGMD module is less sensitive to various operating conditions compared with DCMD, and its flux is also much less than DCMD. But AGMD has some properties, which make its flow-sheet much brief than DCMD.Starting at the modeling of ideal hollow fiber module, the effect of polydispersity of hollow fiber radius on module performance was studied, and it can be concluded that this polydispersity has little influence on module performance on condition that the variation of hollow fiber radius is not so large. For the problem of longitudinal fully developed laminar flow between cylinders arranged in regular array, the momentum equation was solved and the method to calculate friction factor-Reynolds numberfactor( f ?Re) was obtained. Based on this result, the probability of local packing densityof practical module was obtained by Voronoi tesselation technique, and then the rule for the distribution of flow in the shell side of a randomly packed hollow fiber module was decided. The calculating result shows that the ununiformity of fiber distribution on the shell side can lead to the serious maldistribution of flow on this side, and significant channeling effect can be observed, the flux of practical hollow fiber module is much less than that of the ideal one. Neglecting the polydispersity of fiber radius and with the regard of channeling, the module performance was studied by numerical simulation.For study dynamic MD process, two kinds of batch operation were carried out. namely, batch operation with constant feed temperature and batch operation with decreasing feed temperature. The former experiment showed that MD process could be normally performed under a...