Simulation And Analysis Of The Multi-effect Distillation Seawater Desalination System

Low-Temperature Multi-effect Distillation (LT-MED) is one of the maindesalination methods. In LT-MED system, low-grade heat can be effectively utilized,requirements on the pretreatment of the feed water are easy to be satisfiyied, and theproduced freshwater is of good quality. Besides, the problems of scaling and foulingare greatly alleviated because of the low operating temperature, therefore, LT-MEDare being applied more and more widely. With a large number of LT-MED projectsbeing under construction, it is indispensable to develop approaches to design, simulate,and analyse the system. In this paper, with the parallel feed and counter-current feedLT-MED systems as the subject, the relevant relations of physical properties as well asvarious mathematical models were been established, and the means of analysis andsimulation were built, which laid a foundation for the engineering application ofLT-MED technology.In the paper, the corresponding relations of seawater physical properties wereerected, and according to the field datas, the regression equations of steam jet-ejectorwere confirmed. Meanwhile, as for the whole system and each subsystem, themathematical models, such as material balance equations, energy balance equations,heat transfer equations and so on, were founded, too. In order to solve so many highlynon-linear equations, newton’s iteration method was taken use of, and with the help ofMATLAB software, the source program to simulate and calculate was worked out andassured. Subsequently, the variation rhythms of various physical properties werefigured out by adjusting some operating parameters, such as the total number ofeffects, the steam temperature of the first effect as well as the salinity of intakeseawater. Then the erected models were further applied in two specific systems thecounter-current feed LT-MED system with the capacity of25000tons per day and theparallel feed LT-MED system with daily capacity of10000tons; the calculated resultscoincided well with the field datas.What’s more, with the beginning of exergy, the relevant exergy models, exergyloss models as well as energy quality models were respectively set up in this article.With the aid of these models, the exergy analyses of the above two specific systemswere conducted, and it demonstrated that their ratios of the exergy utilization were both less than20%; their biggest exergy losses both lied in the steam jet-ejector,respectively accounted for49%of the parallel-feed total loss and47%of thecounter-current total loss; the exergy losses of all effects in each system were nearlyconsistent with each other. Finally, the exergy loss distribution of every subsystemwas directly showed in the figures by taking advantage of Exergy Utilization Diagram(EUD), which could point the way to decrease the exergy loss and was beneficial tocut the desalination freshwater cost.