Study On Membrane Distillation Of NaCl Solution With Carbon Membranes

At present, the world is increasingly serious shortage of freshwater resources. The effective way to gain fresh water for human beings is desalination. Membrane Distillation process use hydrophobic membrane to separate two feed with different temperature, with the membrane vapor pressure which exist at the two side of the membrane, volatile components pass through the membrane hole in the form of vapor, and condense into liquid at the cold side, achieving mixture separation and purification purposes. There are many strength make the lower cost of membrane distillation, simple equipment, easy maintenance, high desalinization rate, under the low temperature and so on. The concentration of solution after desalination is high can be used to produce a by-product of salt, so, study on membrane distillation of NaCl solution become more and more important.This paper conducted a comprehensive study of membrane distillation experiment. Considering that the carbon membrane is a hydrophobic porous membrane, the pore size distributions are uniform, which suitable for membrane distillation experimental, so, this paper use carbon membrane as a membrane distillation unit for the first time, select the optimum carbon membrane and the membrane distillation process optimization of operating conditions. The results show that the contact angle of carbon membrane as prepared by bituminous coal is about at 101.08°, which is suitable for VMD(Vacuum Membrane Distillation), while the carbon membrane carbon membrane as prepared by small anthracite coal size which is suitable for DCMD(Direct Contact Membrane Distillation). The flux increases with the increase in feed temperature and flow rate, and decreases with the increase in NaCl concentration; the optimum operate techniques: temperature at 60°C, flow at 40 L·h^-1, concentration at 20g·L^-1, vacuum at 5MPaBased on experiment and pore size distribution, the separation theory on membrane distillation process of gas diffusion is controlled by both Knudsen diffusion and Poiseuille flow. A heat and mass transfer model for carbon membrane distillation of NaCl solution was established based on the dusty gas model. The average membrane pore size in the model was replaced by an experimentally fitted relationship of pore size distribution, achieving an effective combination of a consecutive pore size distribution with the membrane distillation model for the first time. The effects of operating conditions and distribution function parameters on the flux distillation were discussed. Results show comparison on flux variation with time between experimental data and simulation result shows that prediction of the established model has a better agreement than that of the average pore size model, which prove the model practicably. Simulated results show that the bigger average pore size, the bigger flux; at the same average pore size, the narrower pore size distribution, the smaller flux.