Optimization And Performance Analysis Of Low Temperature Multi-effect Evaporation Distillation With Mixed Feed Flow

Low temperature multi-effect evaporation distillation(LT-MED)technology is received extensive attention of the academics and become one of the mainstream of seawater distillation technology,due to its small scale of low risk,low power consumption,high purity water production,high thermal efficiency,and utilization of low grade steam heat source from thermal power plant.Researches on the LT-MED system reported were mainly focused on the backward and parallel feed flow pattern,further researches for the mixed feed flow configuration are needed.From the literature reviewed,most of the relactived works are only consider the LT-MED system with one mixed points.Many mixed flow points can be selected according to the requirement of the system efficiency and number of effects,to get a lower water cost and a better system performance.LT-MED optimization models with single and multiple mixed feed flow points are established.Both the heat loss and the variation of several physical parameters are considered,as the functions of temperature and concentration.The mathematical model of steam condensation inside the tube and water evaporation,as well as with steam ejector(TVC)are developed.Seawater is treated as a mixture of salt and water,and its enthalpy is expressed as a function of temperature,which could reduce the non-linearity of the optimization model,and could be converged easily.General Algebraic Modeling System(GAMS)is used to solve the optimization problems.Heat transfer area(s A)is used as the optimization objective.The thermodynamics preferences for the single mixed feed point and multiple mixed feed points was investigated.The optimum system configuration and the proper position of the TVC ejector named as 5+2+2&(3-1)process is obtained.Results show that under the same operating conditions,the gained output ratio(GOR)of the LT-MED system with multiple mixed feed points is 12.1%higher than that of the system with single-mixed flow point,while the specific heat transfer area is 8.4%lower than that of the system with single-mixed flow point system.Compared with the process without TVC,the GOR of the LT-MED-TVC system with multiple mixed feed points is increased by 43.77%,and the specific heat transfer area is decreased by 8.07%.Economic optimization model of LT-MED-TVC system is established for the system configuration with the largest GOR and lowest heat transfer area.The optimum water cost is 6.71 yuan/m~3.The annual investment cost is 7.103×10~7yuan,and annual operating cost is 1.367×10~7 yuan.Steam cost occupied the most share of the annual operating costs.The simulation results of the current model are in good agreement with relevant data in literature and working conditions of the actual plant,which can provide theoretical guidance and technical support for the design of low temperature multi-effect evaporate distillation system.