Study On Thermal Performance Of Mechanical Vapor Compression Desalination Equipment

The shortage of fresh water resources seriously affects human life and development.Developing desalination industry is an effective way to solve this problem.Low temperature multi-effect desalination(LT-MEE)technology is widely used because of its low energy consumption and high purity of fresh water.Combining the vapor compression equipment with MEE system,only a small amount of high-grade energy is consumed and a large amount of low-grade energy can be recovered to achieve the purpose of energy saving.The mechanical vapor compression multi-effect evaporation(MVC-MEE)equipment uses the MVC as the power to compress and reuse the low-pressure vapor discharged from the final effect evaporator of the MEE system,eliminating the heat loss of the condenser at the end of the MEE equipment.MVC-MEE equipment has the advantages of high thermal efficiency,compact structure and low power consumption,which is widely used in chemical,pharmaceutical,desalination,sewage treatment and other fields.The accurate calculation of the thermal process of the equipment under different working parameters is the premise of the optimal design of the equipment.In this paper,the mathematical model of low temperature desalination MVC-S/MEE equipment thermal process calculation is established,and the change rules of the equipment water production,specific power consumption,heat transfer area,specific area and other important evaluation parameters are summarized under different working parameters.The heat transfer process of evaporator is modeled based on the laws of thermodynamics,energy conservation theorem,mass conservation theorem and salt conservation theorem.The model includes the heat transfer change caused by the rise of boiling point elevation of seawater.The calculation of thermal parameters of water and vapor is based on IAPWS-IF97.Comparing the calculation results of the model in this paper with the actual operation results,the results show that the overall error is small,and the calculation results in this paper have reference significance for the actual project.Through modeling and calculation,the influence law of total heat transfer temperature difference,secondary vapor temperature of final effect,isentropic efficiency,the effect number of evaporators and feed seawater salinity on gain out ratio,concentration ratio,specific power consumption,total heat transfer area and specific area are obtained.The results show that,under the range of calculation assumptions and parameters,the gain out ratio increases with the decrease of total heat transfer temperature difference,the increase of isentropic efficiency and the increase of the effect number of evaporators;specific power consumption decreases with the decrease of total heat transfer temperature difference,the increase of isentropic efficiency and the increase of the effect number of evaporators;the concentration ratio increases with the decrease of total heat transfer temperature difference,the increase of secondary vapor temperature of the last effect,the increase of the effect number of evaporators and the decrease of feed seawater salinity;the total heat transfer area and specific heat transfer area of the system decreases with the increase of total heat transfer temperature difference,the decrease of secondary vapor temperature of the last effect,the decrease of the effect number of evaporators and the decrease of feeding seawater salinity.Therefore,when the specific power consumption is reduced by adjusting the working equipment,the specific area of the equipment is often increased.In the actual engineering design,there is an optimal parameter design for MVC-MEE equipment.The thermal performance and the consumption of the evaporation area of the MVC-MEE equipment with three feeding processes of forward feed,parallel feed and back feed under different working parameters was compared.The results show that,in the calculation assumption and parameter range,the change rule of gain out ratio and specific power consumption of the three feed processes is similar.When the effect number of evaporators is small,the specific power consumption of parallel feed is the lowest,and when the the effect number of evaporators is large,the specific power consumption of back feed is the lowest.In addition,the feeding process has little effect on the total heat transfer area and specific area consumption of the equipment,the maximum total heat transfer area difference between different feed process is only 8.5%.Therefore,in the actual engineering design of the feed process,priority should be given to practical perspectives such as system sensitivity and difficulty in construction or maintenance.