Transient and thermo-economic analysis of MED-MVC desalination system

Abstract

An exergo-economic model is used to assess the performance of a multi-effect desalination plant integrated to a mechanical vapor compressor unit (MED-MVC) with a water production capacity of 1500 m3/day. The results show that the second law efficiency (ηII) is 2.8%. The MVC and evaporator units are responsible for about 39 and 52% of the total exergy destruction, respectively. The total water price (TWP) is 1.70 $/m3 when calculated using a simple conventional economic model and 1.63 $/m3 when calculated using an exergy-based cost model. Increasing the number of effects from 1 to 6 results in a 39% reduction in the specific power consumption (SPC), a 70% increase in ηII and a 24% decrease in TWP. A dynamic model is developed to investigate the effect of fluctuations of compressor work (W˙c) and inlet seawater temperature (Tsw) on the plant behavior and performance. The dynamic model results show that the disturbance in W˙c has a significant effect on the plant transient behavior and may cause the plant to cease operation while a disturbance in Tsw has only a moderate impact. Increasing Tsw above a certain value of the steady-state condition without proper control on the plant response could lead to evaporator dry out. In term of performance, a reduction in W˙c causes a decrease in the plant production capacity and SPC, while it increases the plant performance ratio (PR). On the other hand, a reduction in the inlet Tsw causes a reduction in the plant production capacity and PR and an increase in SPC for the same compressor work. Furthermore, a comparison between a MED-MVC system and a MED integrated to a thermal vapor compressor system (MED-TVC) reveals that the latter system is rather sensitive to the reduction in Tsw due to the presence of the condenser unit in the MED-TVC. The response of the MED-MVC system is slower than the MED-TVC which is due to the high thermal capacity of the preheaters for the feed in the MED-MVC.