Experimental Study Of Low Temperature Multi-effect Distillation Seawater Desalination System

Seawater desalination appeared to be the best way to solve water crisis. With advantages of easy pretreatment requirements, low energy consumption, simple equipment, large operation elasticity and many other features, low temperature multi-effect distillation (LT-MED) technique becomes one of the widely used desalination technologies.This thesis summarized the development and characteristics of seawater desalination technologies, and some key technologies and process characteristics were analyzed for LT-MED. Experimental process and main equipment were designed and analyzed on the base of a small-scale LT-MED model of lt/d fresh water, which includes a multi-media filter, an ultrafiltration and a degassing tower for pretreatment, the equal temperature difference design method was used firstly in design and calculation of vertical tube evaporator, then equal-area design method was adopted in computer program to determine the final process parameters and the size of equipment, reasonable selections and designs of intereffect devices, liquid distributor, entrainment trap, pumps were optimized.Tap water was used to commission and completed the over-all start-up rehearsal on the small-scale (lt/d) LT-MED seawater desalination platform, and then seawater was introduced after insuring system can run steady. The relative performance parameters of the apparatus such as heat transfer coefficient, gained output ratio(GOR), concentration ration(CR),quality of fresh water were obtained at different inlet flow rates and various top brine temperature. The experimental results indicated that these performance parameters agree with design expectations under the design operation conditions. On this basis, key technologies, key equipments optimization and control technique were studied experimentally.The key process characteristics and mechanism during condensation process were studied in accordance with the low pressure operating condition in LT-MED. With infrared thermograph in the low pressure condensing environment, the surface temperature distribution of condensate droplet during dropwise condensation process was observed. The result showed that the droplet would roll and the surface film would be tracked in the dynamic process of droplet coalescence or falling off may be the main reason for the variation of the surface of droplet, the direct condensation on large droplet surface can be promoted by the dynamic process, which provide a new approach for condensation heat transfer enhancement.