Study On The Preparation And The Permeability Evaluation Of Membrane For Membrane Distillation

Membrane distillation(MD) is a relatively new membrane separation process. Essentially the process employs hydrophobic porous membrane which is permeable to vapor but not to the liquid phase to realize water purification and solution concentration. The advantages of MD are energy efficient, cost efficient and operation easy. At present, the wide commercial use of MD has not yet been achieved mainly due to the preparation of fine and cheap MD membrane.In this paper, the preparation of hydrophobic microporous polyvinylidene fluorid (PVDF) membrane for MD has been studied theoretically and experimentally. The effects of the concentrations of PVDF and addictive in the casting solution, the second addictive and its content, the composition and temperature of the coagulation bath on the morphology of the membrane were discussed in detail. The morphology of the membrane was examined by scanning electron microscope. The result showed that the casting solution composed of 15%PVDF, 75%N,N-dimethyl acetamide, 3%lithium chloride and 7%polyethylene glycol could perform the membrane with high porosity and pore size. Through analyzing the factors that affects gelatin rate and membrane structure of PVDF formed through phase inversion using aqueous alcohol solution as the gelatin medium, the 30℃ water was chosen to be the gelatin medium because of its fastest gelatin rate and being suitable to the large scale industrial production. In addition, the matching of membrane casting solution with non-woven cloth could improve the final membraneproperties and avoid the membrane detects.In this paper, the permeability of several kinds of microporous membrane, which were made of different materials, have also been testing by direct contact MD and air-gap MD. It was found by the experiment that the evaluation of membrane's permeability could base on the function of both the permeability coefficient and the temperature coefficient. Furthermore, the operation condition has great affection on the membrane's permeability. It is important to control the temperature, flow rate and the concentration of the solution. The analysis implied that the membrane fouling and wetting control could be achieved through the prefiltration of feed and measure should be taken to reduce the flux decay resulted from the viscosity increase during the concentration of solution.