Deep Concentration Of Brine By Multiple-effect Membrane Distillation At High Temperature

Membrane distillation (MD) is a relatively new membrane separation process inwhich only vapor molecules from the feed stream are transported through poroushydrophobic membranes to the permeate stream, driven by the difference of vaporpressure across the membrane. However, membrane distillation did not yet attaincommercial status for limiting of membrane with the characteristics most suitable andthermal efficiency. Multiple-effect membrane distillation (MEMD) based on air-gapmembrane distillation (AGMD) module has distinct characteristics with latent heatrecovery and with a performance ratio (PR) obviously more than1.0.The traditional membrane distillation is usually operated at temperature of <90oC.However, permeation flux (J) and thermal efficiency are much lower when dealingwith concentrated salt solutions as a result of reduction of vapor pressure as well asconcentration and temperature polarization effects. In this dissertation, MEMD wasexplored in the range of85-105℃for deep concentrating both aqueous NaClsolution and excess mother liquor of combined-soda process. The effects of coldfeed-in temperature T1, heated feed temperature T3, feed-in concentration and feed-inflow rate on J, performance ratio (PR) and rejection rate were investigated. Theexperimental result indicated that J and PR were3.1L·m-2·h-1and15.2respectivelyfor a feed of5%NaCl when T3was100oC; both J and PR decreased with theincrease of feed concentration, however, their values could still achieve1.53L·m-2·h-1and5.8respectively for a feed of25%NaCl when T3was105oC, while the saltrejection rate could be>99.95%. Also, a long-term operational stability of the MEMDprocess was tested used a feed of15%NaCl solution for60days. The experimentalresults indicated that MEMD process was in good operation stability. Theexperimental result indicated that J was decreased from3.8L·m-2·h-1to2.4L·m-2·h-1and PR from8.6to4.3, respectively, when excess mother liquor of combined-sodaprocess was concentrated2-fold. Therefore, MEMD can be used to treat brineeffectively at high temperature.Membrane processes is limited because of scaling problem caused by calciumdeposition when feed stream containing calcium. In this dissertation, a novel approachwith adding sodium sulfite was investigated to selectively remove calcium ions fromseawater or concentrated seawater by use of special properties of calcium sulfite, such as lower solubility than calcium sulfate, less hydrolysis degree than carbonate, andstrong reducibility. The experimental results indicated that calcium removal efficiencywas higher than90%at high temperature (85℃) when brine of10°Béwas used as afeed, meanwhile, the loss of magnesium ions was less than2%. The decalcified brinecan be further used as a feed for desalination to further produce fresh water andrecovery mineral resource from downstream.In conclusion, this study showed that MEMD was feasible to deal with brine forthe advantage of high PR and thermal efficiency at high temperature. It has a greatpotential value for development and application.