| Multi-effect mechanical vapor compression desalination system(MEE-MVC)is a kind of desalination system formed by the combination of multi-effect evaporation system(MEE)and mechanical vapor compression system(MVC).With the increasing pollution of water resources and the global energy crisis,the MEE-MVC system has been widely concerned in the current desalination,sewage treatment and other industries due to its advantages of using only electric energy,high energy efficiency,good economy and high water quality,becoming an effective way to solve the shortage of fresh water in the area with inadequate heat power.How to improve the energy efficiency of the system has become the focus of current research.In this paper,the multi-effect mechanical vapor compression desalination system is designed and calculated for the purpose of fresh water production of 10 kg/s,and the thermodynamic performance of the system is evaluated through the analysis of the results.Based on the first law of thermodynamics and the second law of thermodynamics,a complete mathematical model is established by considering the mass conservation equation,energy conservation equation and salt balance equation of each equipment in the system.In this model,the physical property parameters of seawater were calculated by referring to the compilation function of the existing empirical formula in the literature,and the physical property parameters of water and vapor were obtained by direct call from the database.The calculation program of the multi-effect mechanical vapor compression desalination system with forward-feed mode and parallel-feed mode was respectively compiled by C language.From the perspective of the first law of thermodynamics,this paper analyzes the variation rules of the gain output ratio,specific heat consumption,specific power consumption and heat transfer area under the influence of feeding mode,evaporator effects and constant entropy efficiency of the compressor.The results show that the gain output ratio of the parallel-feed is higher than that of the forward-feed,the specific power consumption and specific heat consumption are less than that of the forward-feed,and the heat transfer area is similar.Taking the constant entropy efficiency of 0.8 as an example,the number of evaporator effects increased from 2 to 6,the gain output ratio increased from 1.85 to 5.32 in the forward-feed,and from 1.9 to 5.67 in the parallel-feed mode.The specific heat consumption and specific power consumption both decreased with the increase of the number of evaporator effects,and the heat transfer area increased with the increase of the number of evaporator effects.The constant entropy efficiency of the compressor increases from 0.6 to 0.9,and the gain output ratio and specific heat consumption show a downward trend,but the reduction rate is within 3%,and the specific power consumption subsequently decreases by about 30%.The constant entropy efficiency of the compressor has almost no effect on the heat transfer area.From the perspective of the second law of thermodynamics,this paper analyzes the variation rules of the exergy efficiency and percentage exergy destruction in different componentss under the influence of feeding mode,evaporator effects and constant entropy efficiency of the compressor.The results show that the parallel-feed has higher exergy efficiency than the forward-feed,and the energy loss of the system is mainly concentrated in the evaporator and the intermediate water refill device.Taking the constant entropy efficiency of 0.8 as an example,the number of evaporator effects increased from 2 to 6,the exergy efficiency increased from 5.77% to 12.78% in the forward-feed,and from 6.05% to 13.78% in the parallel-feed mode,the energy loss of the evaporator decreased with the increase of the number of evaporator effects,but the reduction rate of the intermediate water refill device is within 2%.When the constant entropy efficiency of the compressor increases from 0.6 to 0.9,the exergy efficiency of the system subsequently increases,and the more the number of evaporator effects,the higher the growth rate of exergy efficiency. |
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