Experimental Study On Ammonia Separation In Aqueous Solution By Vacuum Membrane Distillation

With the continuous development of industry, ammonia wastewater has caused more and more serious environmental pollution with its increasing emissions, the treatment technology on ammonia wastewater is always the hot spot in water treatment. Conventional ammonia removal methods include steam stripping, chemical precipitation and so on, these methods have the disadvantage of high operating costs, low ammonia removal efficiency and secondary pollution, etc., so it is urgent to develop a highly efficient, energy-saving and eco-friendly approach. As a new membrane separation technology, vacuum membrane distillation has the advantage of high flux penetration, less heat loss, suitable for removing these volatile substances, etc., therefore, vacuum membrane distillation has good prospects if it is used to treat ammonia wastewater.this paper studied the effect of temperature of feed solution, flow rate of feed solution, the degree of vacuum of permeate side, the number of hollow fiber on membrane flux of VMD process. It was found that the temperature of feed solution had a greater impact on flux, with the feed temperature increasing, the flux increased significantly; the flux increased linearly as the degree of vacuum increasing; high flow rate of feed solution was good for the improvement of flux; when the number of hollow fiber increased, the flux would decreased. When the temperature of feed solution was 60℃, the degree of vacuum was 80 kpa, flow rate of feed solution was 40L/h, the number of hollow fiber was 100, pH was 12, the maximum flux would obtain, which was 5.98kg·m-2·h-1.this paper studied the effect of temperature of feed solution, pH of feed solution, flow rate of feed solution, initial concentration of feed solution and the degree of vacuum of permeate side on mass transfer coefficient, ammonia removal rate and selectivity coefficients of vacuum membrane distillation(VMD) process. The results showed that with the increasing of temperature of feed solution, ammonia removal rate and mass transfer coefficient increased, while the selectivity coefficient reduced; when the pH of feed solution increased, mass transfer coefficient, selectivity coefficient and ammonia removal rate all would increased significantly; the improvement of the flow rate of feed solution was conducive to ammonia removal rate and mass transfer coefficient, while the variation of selectivity coefficient was small; when the initial concentration of feed solution increased, selectivity coefficient decreased, ammonia removal rate increased, and mass transfer coefficient almost had no change; with the increasing of the degree of vacuum, the ammonia removal rate and mass transfer coefficient increased, while the selectivity coefficient showed the opposite trend. During the experiment, the mass value of transfer coefficient, selectivity coefficient and ammonia removal rate respectively reached 7.7×10-5m·s-1, 7.7 and 96.8%.Finally, this paper studied the attenuation of flux and control methods of membrane fouling under the optimum operating conditions. It was found that the membrane fouling would aggravate with the extension of operating time. Two cleaning methods of membrane fouling was compared, which is named "deionized water + citric acid" and "de-ionized water + hydrochloric acid", it was found that the former was more efficient, it could make the influence of irreversible membrane fouling on flux drop to 3.3%, which was benefit to extend the life of membrane.