Study On Separation Processes Of Concentrated Brine By Nanofiltration Technology

Nanofiltration is a new type of membrane technique which characteristics are between ultrafiltration and reverse osmosis. It has permselectivity and can remove divalent ions but monovalent ions could be allowed to permeate. Nanofiltration has very wide range of applications, such as water treatment, petrochemical, food processing, etc.In this work, the concentrated seawater after desalination of Beijiang power plant was chose to be feed. And some contrast experiments of different nanofiltration membranes have be done according to characteristics of nanofiltration in concentrated seawater separation and rejection rate of salt. The nanofiltration membrane HNF90-4040made by Hydranautics company of American was employed in our study. The dates and laws of mass transfer apply to industrial application have been got by studying the effects of pressure and temperature to rejection performance of divalent ions and monovalent ions. The results showed that inflow pressure and operating temperature had different influences on membrane flux. And the ions rejection rate&water production rate increased with increasing inflow pressure, but the rejection rate decreased while water production rate increased when operating temperature was raised. In addition, the rejection rate and water yield flux increased but operating pressure decreased with decreasing concentration of feed, and vice versa.According to the experiments, optimal operation condition in first-nanofiltration of nanofiltration membrane HNF90-4040was inflow pressure at1.4MPa. operating temperatu-re at28℃. When the concentrated solution produced in first-nanofiltration was employed as feed in second-nanofiltration, the optimal operation conditions were inflow pressures at1.4MPa, operating temperatures at25℃. In these optimal operation conditions, the rejection rate of monovalent ions (Na+, Cl-, K+) was less than15%and the rejection rate of divalent ions (Ca2+, Mg2+, SO42-) was more than40%. Moreover, the rejection rate of SO42-can reach more than97%, which could ensure the rejection of SO42-and the permeation of NaCl.The effect of membrane fouling to membrane flux has also been studied. The results showed that cleaning membrane by water was helpful to the recovery of flux in a short time, but this effect was not sustainable as the operating time increased. However, the membrane flux almost can be completely recovered when cleaned by chemical reagents.According to the results of experiment, the process route of concentrated seawater separation using nanofiltration membrane HNF90-4040has been designed, the optimal operation conditions have been found and the cycle of membrane fouling has been ensured. Besides, this work is helpful for the determination of feasible parameters in further desalination, extraction of some elements (Mg, K, Br, etc.) and refined salt production.