| Membrane distillation(MD) is a new membrane separation technology which employs a hydrophobic porous membrane. The driving force of mass transfer in MD is vapor pressure across the membrane. MD is widely used in concentration of aqueous solution and removal of volatile organic. However, due to transmembrane heat conduction, environmental heat loss and energy consumption of phase transition,MD do not achieve large-scale industrialization. Besides preparing membranes with excellent performances, appropriate operating conditions, feeding methods and packing density are also particularly important to improve flux and thermal efficiency of MD process.Nowadays, heat recovery is an effective way to improve thermal efficiency of MD process. Multi-effect MD and heat pump are employed in this study. Two connections in series of modules were compared via flux and thermal efficiency, which laid the foundation for the multi-effect MD. MD coupled with heat pump not only reduces heat consumption, but also help reducing refrigerant energy. Operationg conditions were optimized to improve membrane thermal efficiency, heat pump coefficient of performance(COP) and gained output ration(GOR) of coupled system.This thesis focuses mainly on the following four aspects:(1) Effect of operating conditionsThe experimental study of effects of opercating conditions(feeding temperature, hot and cold side flow) on flux and thermal efficiency was performed. Heat and mass transfer model of direct contact membrane distillation(DCMD) based on hollow fiber membrane was use to analyse the mechanism. The results show that the flux increased with the increase of feed temperature, hot and cold side flow, which greatly increased the vapor pressure difference across membrane and mass transfer driving force, but had limit effect on mass transfer coeffiecient. The increase of feed temperature and hot side flow and the decrease of cold side flow helped to improve the thermal efficiency for higher average membrane temperature.(2) Effect of packing density and feeding methodsWith the increase of packing density, the flux increased first and then decreased; the thermal efficiency continuously rised. The maldistribution of hollow fiber membrane results the channeling effect, which was weakened and mass transfer was enhanced with the increase of packing density. Four feeding methods were compared, and the module in horizontal with cold inlet upturning was the best for higher flux and thermal efficiency.(3) Comparison of connections in seriesTwo connections in series were compared via the flux and thermal efficiency. Hot sides of two modules connected in series in connection(1) and connection(2). The difference between two connections was, the cold sides of two modules connected in series and were cooled by a cooler in connection(1), but the two cold sides were separately cooled by two cooler in connection(2). From the highest to the lowest, the flux order was first stage of connection(2), first stage of connection(1), second stage of connection(1), second stage of connection(2); the thermal efficiency order was first stage of connection(1), first stage of connection(2), second stage of connection(2), second stage of connection(1).(4) Performance optimization of DCMD process coupled with heat pumpHeat in DCMD cold side was recovered to heat the feed via compression heat pump. Three modules with different membrane area and packing density were used. Effects of feed temperature, hot and cold side flows on DCMD thermal efficiency, heat pump COP and coupled system GOR were investigated. Feed temperature, hot and cold side flows all affected the inlet temperature of evaporator, temperature rise of heat pump and COP. Experimental results showed when the feed inlet temperature in DCMD maintained in the range of 35℃ and 60℃, COP was greater than 3.0, the thermal efficiency was 60%, GOR was 2.0. COP could even get to 4.0 when a bigger module was employed. |
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