Thermal Performance Analysis And Optimization Of The Multi-effect Evaporation Desalination System

Multi-effect evaporation technology is one of the most effective desalination technologies.It has the advantages of durable equipment,high reliability,and high water quality.It has gradually become one of the mainstream technologies.However,the multi-effect evaporation process is a complex thermal system with a high cost and high energy consumption.Thermal performance analysis and optimization of the multi-effect evaporation desalination system,and further development of efficient and low-cost system process play a vital role to promote the continued development of multi-effect evaporation technology and solve the problem of the shortage of freshwater resources.At present,multi-effect evaporation technology still has some shortcomings,which are embodied in insufficient consideration of the thermodynamic model,insufficient optimization theory,insufficient system flow,low system efficiency,and weak development ability of core components.Therefore,this paper adopts the method of combining theoretical analysis and experimental research,focusing on the thermal performance analysis and optimization research of multi-effect evaporation system,to promote the formation of a richer and more complete theory,and promote the further development of multi-effect evaporation technology.The main research contents and conclusions of this paper are as follows:(1)A mathematical model was established to consider the spatial distribution of parameters in the horizontal tube falling film evaporator.The mathematical model can not only describe the spatial distribution of parameters but also guide the evaporator structural design.The results show that the calculated results of the model are in good agreement with the actual plant data in the literature,and the relative errors of fresh water production,GOR(Gained Output Ratio),and A_s(heat exchange area)are less than 5%,respectively.Compared with other thermodynamic models,the error of this model is smaller,the accuracy is higher.(2)The influence of feeding mode on the thermal performance of the system was studied.The thermal performance of three kinds of systems,namely,F-MEE,P-MEE,and B-MEE,were analyzed.Under the same parameters,the GOR of the system from high to low is F-MEE,B-MEE,and F-MEE respectively.The A_s from high to low is F-MEE,B-MEE,and F-MEE.The influence of preheating process on the thermal performance of the system(forward and parallel feeding)was studied given the problem that the system performance is affected by the lower brine temperature.Compared with the process without preheating,the GOR and A_sincrease after preheating.The GOR of the parallel feed system also increases,and the A_s is affected by the preheating position and the number of preheaters.When the preheating position is in the front and the number of preheaters is small,the A_s is lower than that of the process without preheating.The A_s will gradually exceeds that of the system without preheating as the preheating position moves backward and the number of preheating increases.With the increase in the number of preheaters,the GOR and A_s of the two systems show an increasing trend.When the number of preheaters is determined,different preheating positions have different effects on the thermal performance of the two feeding systems.As the preheating position moves backward,the GOR increases first and then decreases,and the A_s approximately presents a linear trend of increase.The GOR and A_s of the parallel feed system increase first and then decrease.(3)A complex multi-effect evaporation system optimization method combining the regression analysis approximation model and the multi-objective genetic algorithm was proposed.On the premise of guaranteeing enough precision,the complex thermodynamic model was simplified to a bilinear model by using regression analysis.The multi-objective optimization problem was solved with an elitist strategy(NSGA II)optimization solution.The thermal performance and economic performance of the multi-effect evaporation system are improved effectively.In the paper,compared with the initial scheme,the GOR of the forward feed with preheating system is increased by 3.6%,and A_s is decreased by 16%.The GOR of the the parallel feed with preheating system is increased by 7.8%,and the A_s is decreased by 19%.The results show that the optimization method proposed in this paper has the advantages of high calculation accuracy and a simple solution process,which provides a theoretical basis for the design optimization of a complex multi-effect evaporation system.(4)The state-space superstructure of the forward-backward-parallel feeding system was created,and a comprehensive forward-backward-parallel feeding system synthesis method was proposed.Using this method,more efficient and energy-saving mixed feed flow structures of a multi-effect evaporation system can be developed.According to the optimization target the best energy distribution network can be constructed by separating and mixing the flow,which can effectively improve the thermal performance.Within the scope of calculation in the paper,the GOR of the optimal scheme is about 37%higher than that of the forward feed with preheating(PF-MEE)system,and about 11%higher than that of the parallel feed with preheating(PP-MEE)system.Compared with the PF-MEE system and PP-MEE system,the A_s of the optimal scheme decreases by 22%and 20.4%respectively.The SCOW(Simplified Cost of Water)is approximately 13.5%lower than PF-MEE and 8.1%lower than PP-MEE.Compared with the optimization of GOR,A_s and SCOW,the optimization of SCOW method can optimize the GOR,A_s and SCOW at the same time,and the optimization goal is more comprehensive and the optimization effect is more significant.(5)The multi-effect evaporation experimental device was built by using the multi-objective optimization method,and the performance analysis and optimization research were carried out.Firstly,the steady-state performance of the design condition is analyzed.The results show that the experimental data are in good agreement with the theoretical values,and the GOR of the system is about 91%of the theoretical value,which proves the validity of the thermodynamic model and optimization method in the paper.Secondly,the original experimental system was optimized by the forward-backward-parallel feeding system synthesis method,and the feeding mode was adjusted from forward feeding to mixed feeding.Compared with conventional forward feeding,the thermal performance of the mixed feeding process is greatly improved,and the GOR of the experimental device increases by about 7.9%.The results prove the effectiveness of the synthesis method and the superiority of the mixed feeding process.Thirdly,the temperature change rule of system startup is found.The multi-effect evaporation system startup can be divided into the initial stage,change stage,and steady-state stage.In the initial stage,the temperature of the feeding brine increases,while the temperature in each evaporator remains unchanged.In the change stage,the temperature of each evaporator presents the change characteristics of"sequential,equal temperature difference,rapid first,slow afterward".In the steady-state stage,the feed temperature and the temperature of each effect evaporator do not change significantly.Finally,the thermal performance of the system under variable working conditions was studied.When the heating steam flow rate is constant,the fresh water production and the GOR of the system increase first and then decrease with the increase of feed flow.When the feed brine flow rate is constant,the fresh water production increases,and the GOR decreases with the increase of the steam flow rate.