Multiple-effect Membrane Distillation Used To Concentrate Dilute Solution Of Sodium Hydroxide And Underground Brine

Membrane distillation (MD) is a novel membrane separation process whichcombines membrane and distillation technology. It has attracted much attention for itsadvantages such as simple equipment, operating at mild conditions, high retention rate(R) and utilizing low grade heat sources, however, the low performance ratio (PR)limited its industrial application. The multiple-effect membrane distillation (MEMD)process significantly improves the PR of the traditional MD process by introducingthe internal heat online-recovery function to the traditional air gap membranedistillation. Since a huge amount of alkaline wastewater and underground brine needto be concentrated deeply, it is studied to deeply concentrate aqueous solutions ofsodium hydroxide and underground brine by using MEMD process. In order to solvethe calcium scaling problem when dealing with underground brine, the selectivelydecalcification of the underground brine was also studied.In the process of concentrating dilute aqueous solution of sodium hydroxide byusing MEMD, the influences of various operating parameters on the MEMDperformance were tested. The experimental results showed that the dilute solutioncould be successfully concentrated to a final concentration of>250g L-1, the retentionrate R maintained>99.9%for all the tested feed concentration and operationconditions; the maximum value of permeate flux (J) and PR could reach6.74L m-2h-1and11.3, respectively, for the feed solution of50g L-1at the cold feed-intemperature T1=30℃and the heated temperature T3=95℃; although J and PRdecreased significantly as the feed concentration increased which lead to a drasticallyincrease of the viscosity and boiling point of the feed, the values of both J and PRcould still achieved3.05L m-2h-1and5.04respectively for the feed concentration of200g L-1, which provided a similar thermal efficiency of a conventional7-effectevaporator. During an operational stability test lasting for60days, the performance ofMEMD modules was kept in good condition. The experimental results demonstratedthat MEMD process with the advantage of energy saving can be efficiently used toconcentrate dilute NaOH solution.The experimental results of the selectively removal of calcium from undergroundbrine indicated that when Na2CO3was used a decalcifying agent, the removalefficiency of calcium (R(Ca2+,%)) increased slightly and the loss ratio of magnesium ions (R(Mg2+,%)) increased significantly with the dosing ratio greater than0.9, whilethe R(Ca2+,%) could exceed the dosing ratio and R(Mg2+,%) was very low with thedosing ratio no more than0.7; for NaHCO3-distillation method, it had a significantadvantage of high R(Ca2+,%) and low R(Mg2+,%), but the cost of this method washigher than using Na2CO3; for Na2CO3/NaHCO3-distillation method, it had a selectivedecalcification effect as good as NaHCO3method and the cost was relatively low, soit was more competitive. The underground brine of8.9°Béwith softening treatmentwas successfully concentrated to a final concentration of24°Béby using MEMDprocess, and high retention rate and PR were obtained during the concentratingprocess. By treating the underground brine with a combination of selectivedecalcification and MEMD, it is possible to obtain fresh water and a deep concentratewhich can be utilized as a chemical recourse.The study indicates that MEMD process exhibits broad application prospects onalkaline wastewater treatment and comprehensive utilization of underground brine.