Thermodynamic Analysis And Optimization Of Multi-phase Flow Evaporation Seawater Desalination System With High Concentration Rate

Scaling is one of the main problems in thermal desalination technology. In this paperthe technology of multi-phase flow for the preventing and removing scaling is used in theseawater desalination, the introduced solid particles not only can strengthen heat transfer,anti-scaling, but also improve the desalination concentration factor. Increasing the seawaterdesalination concentration factor can improve the performance ratio and decrease thepre-treatment amount of the seawater. The high brine concentration can effectively decreasethe post-treatment cost of the reusing concentrated seawater. At the same time, the highconcentrated seawater can also be used as raw material for the salt production. Therefore,the concentrated seawater resource is fully utilized, and provides an effective way to solvethe environmental problem.The mathematic model of multi-phase flow evaporation seawater desalination systemwith high concentration rate is established, including the evaporator, pre-heater, flash tank,condenser and steam ejector. The temperature losses caused by the heat and pressure loss,as well as the variation of brine concentration are taken into consideration. Moreover, theoptimization model is also established which is aims at minimum unit product cost. Allthese models are solved by using the tool of MATLAB.For the forward feed multi-phase flow evaporation seawater desalination system withhigh concentration rate, the effects of the solid particle volume fraction, the amount ofpre-heating, the first effect of heating steam temperature, and the concentration ratio on thesystem thermodynamic properties are analyzed. The results show that, the solid particlevolume fraction has significantly influence on the heat transfer area of each effect; theintroduced solid particles can strengthen heat transfer, as a result, the heat transfer area ofeach effect and the unit product cost decreased. At the calculation conditions of this paper,for the four effect desalination system that considering condensed water flashing andwithout preheating, when the solid particle volume fraction is12%, the evaporator heattransfer coefficient of each effect can be increased by8.74%,9.37%,9.02%,9.77%,respectively. The evaporator heat transfer area of each effect can be reduced by8.19%, andthe unit product cost can be reduced by1.21%, compared with the system without solidparticles; the results also indicate that the first effect heating steam temperature hassignificantly influence on the heat transfer area of each effect; the amount of pre-heatingheat can obviously influence the dawdling steam consumption and performance ratio; theunit product cost is obviously influenced by the concentration ratio. For the parallel feedmulti-phase flow evaporation seawater desalination system with high concentration rate andwithout TVC, the effects of the seawater and the evaporator temperature of the last effecton the system thermodynamic properties are compared with those of the forward feed system.For the non-preheated forward feed without TVC system and the non-preheatedparallel feed without TVC system, the obtained optimal results were compared with thoseof the non-optimal system. Different flow patterns were also analyzed, including thenon-preheating forward feed desalination system that ignoring condensed water flashing,the non-preheating forward feed desalination system that considering condensed waterflashing, the preheating forward feed desalination system that considering condensed waterflashing, the non-preheating parallel feed desalination system that ignoring condensedwater flashing, the non-preheating parallel feed desalination system that consideringcondensed water flashing, and the preheating parallel feed desalination system thatconsidering condensed water flashing. Results show that the preheating parallel feeddesalination system that considering condensed water flashing consumes the least amountof steam, which has the highest performance ratio, and minimum unit product cost.However, this process is easy to scaling.For the forward feed multi-phase flow evaporation seawater desalination system withthermal vapor compress(TVC), the effects of working steam pressure, suction steamtemperature and the TVC extracting position on thermodynamic performance and unitproduct cost of the seawater desalination system are analyzed. Meanwhile, comparisonsbetween the system mentioned above and the traditional multi-effect desalination (MED)system are also given. Calculation results show that thermodynamic performance ofTVC-MED desalination system is superior to that of MED desalination system; when theTVC is located in the second evaporator, the thermodynamic performance is best and theunit product cost is minimum with3.25￥·t-1.