Design And Multi-objective Optimization Of Mechanical Vapor Recompression System

The rapid development of the industrial economy has led to an increasing emission of industrial waste water,among which salty waste water in high concentration emitted from chemical,food,pharmaceutical and other industries accounts for a large part.Direct discharge of salty waste water is bound to cause water pollution.In addition,some inorganic salts contained in salty waste water have great recycling value.So the treatment of salty waste water in high concentration and the recycling of inorganic salts can bring both environmental and economic benefits.It is quite necessary to develop treating method of salty waste water with high operational and energy efficiency under low operating costs.Thus,based on the mechanical vapor recompression(MVR)technology,a parallel-connected double-effect mechanical vapor recompression evaporation crystallization system was put forward in this paper.Firstly,by analyzing the impacting mechanism of the properties of inorganic salt solution on the system,the design idea of a parallel-connected double-effect MVR evaporation crystallization system was put forward.Then the circulation process of the system was designed and thermodynamic principle in the circulation was studied.With the purpose of being operational and energy efficient,the types of main devices in the system were determined.Based on the necessary simplifications and assumptions,the mathematical models of main devices and the whole system were established,both of which were validated by comparing the calculation results with the experimental results of a reported MVR system.Following above mathematical models,the exergy analysis model of the system was established taking the effects of salt in the solution into account.Exergy analysis was combined with traditional energy analysis to evaluate the system performance.The treating process of sodium sulfate solution at an initial concentration of 5%under normal pressure was used as an example to simulate the system circulation process.After simulation,the temperature,flow rate,enthalpy and exergy of the working medium in each section were obtained.Meanwhile the important parameters including heat transfer area,exergy loss and exergy efficiency of main devices were also obtained.Then a traditional three-effect evaporation crystallization system was introduced as a contrast,which was simulated using the same design variables as the proposed system.The comparative analysis of the two systems shows that the proposed system has better thermal performance and is much more energy-efficient than the reference system.Under same working condition,the Coefficient of Performance(COP)of proposed system is 82.2%higher than that of the reference system and the unit energy consumption of proposed system is only 17.6%of the reference system.From the perspective of exergy analysis,the proposed system has a higher degree of thermodynamic perfection.Its exergy efficiency is 51.5%higher than that of the reference system,and exergy loss is 24.7%lower than that of the reference system.After the overall performance analysis,variable-controlling method was adopted to conduct theoretical study on the design of parallel-connected double-effect MVR evaporation crystallization system.The influence factors in the design process were analyzed,especially the effects of evaporation temperature,saturation temperature rise of compressor and feed concentration on energy consumption during operating and initial investment of the system.The results show that feed concentration has a relatively small effect on heat transfer area and total power consumption of the system.Higher evaporation temperature will reduce the total power consumption but increase the heat transfer area,while larger saturation temperature rise of compressor will increase the total power consumption and significantly reduce the heat transfer area.Under the condition of determined design variables,total power consumption and heat transfer area of the system have opposite trends with the variation of evaporation temperature and compressor saturation temperature rise.Therefore,the optimal evaporation temperature and compressor saturation temperature rise lie in a trade-off between total power consumption and heat transfer area.This is a further direction of the system parametric optimization.Finally,the mathematical model of multi-objective optimization was established,in which minimization of total power consumption and heat transfer area were taken as optimization goals.The optimization variables consist of evaporation temperature and compressor saturation temperature rise.All other factors were considered as constraints,including the mass flow rate,temperature and concentration of the feed.The optimization model was calculated by Strength Pareto Evolutionary Algorithm 2(SPEA2).After that the optimal combination of evaporation temperature and compressor saturation temperature rise were obtained based on the fuzzy set theory.Comparing the performance parameters of the system before and after optimization,the total power consumption of the system was reduced by 22.1 kW and the heat transfer area of that was reduced by 31.2 m2.Under the optimized operating condition,the COP and exergy efficiency of the system are increased by 7.94%and 5.91%respectively.In addition,exergy loss of the system is reduced by 38.4 kW.It means that the adopting the condition after optimization can improve the energy utilization and thermodynamic perfection degree of the proposed system.