| Membrane distillation (MD) is an emerging membrane separation process. MD process has many attractive features, such as low operating temperature and pressure, high specific evaporation rate, high rejection rate of non-volatiles and utilizing low-grade or alternative energy sources, which makes it more promising separation process for the treatment of industry wastewaters and deeply concentrating aqueous solutions. Multiple-effect membrane distillation (MEMD) process is based on hollow fiber air gap membrane distillation (AGMD) module with function of internal heat recovery. The thermal efficiency achieved in MEMD process is much higher than that in the traditional MD process and MEMD process has been applied for concentrating aqueous solution of salt, acid, sugar and fruit juice, and so on. By using self-made hollow fiber AGMD modules, the feasibility of using MEMD process for concentrating the fiber wastewater containing dimethyl sulfoxide (DMSO) produced from carbon fiber factories and brine produced from seawater desalination plants was studied respectively in this dissertation. The performance of MEMD process was evaluated by permeate flux (J), gained output ration (GOR), selectivity (α) and recovery rate of DMSO (R).The results of concentrating the fiber wastewater containing DMSO by MEMD process showed that the fiber wastewater could be successfully concentrated to a final DMSO concentration of200~300g/L；the maximum value of GOR and α could reach12.4and76respectively for the initial feed concentration; all the values of J, GOR and α decreased as the feed concentration increased, however their values could still achieve3.74L/(m2·h),7.1and32.1respectively for the feed-in concentration of200g/L; R basically unvaried with the variation of operating parameters, which maintained above99.6%. When the feed-in concentration was up to the range of150~300g/L, the content of DMSO in the distillate was not negligible, and it should be suggested to further recover DMSO from the distillate using MEMD process again. The performance of MEMD process went well during a long-term operational stability test lasted for30days.It was found that decalcification pretreatment was an essential step in order to obtain a high concentrating ratio and high water recovery rate when the brine was used as a feed. In the process of calcium removal from seawater using sodium carbonate method, the results indicated that the reaction temperature was an important factor affecting calcium removal efficiency and Ca2+/Mg2+selectivity. Optimum operation conditions were determined as the reaction temperature of85℃, the reaction time of40min, the stirring rate of200r/min, the dosage of Na2CO3of a stoichiometric value and the seawater concentration of>5°Bé. Under those conditions the calcium removal efficiency was up to85.4%at the loss of magnesium ions of <6.7%. The selectively decalcified brine using sodium carbonate method was successfully concentrated4-fold to a final concentration of20~24°Bé by using MEMD process, high rejection rate and high GOR were obtained within the concentrating process. Experiment results showed that MEMD process combining with decalcification pretreatment was suitable for deep-concentration of brine from seawater desalination processes, which would improve the water recovery of traditional desalination processes and increase comprehensive benefits in following utilization process of chemical resources in concentrated seawater.This study demonstrated that MEMD was a highly energy-effective process for concentration of both fiber wastewater containing DMSO and brine from seawater desalination processes. Therefore, MEMD process with high thermal efficiency and rejection rate has a promising application future in field of recovering useful components from industrial wastewater and deeply concentrating the brine. |
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