Optimal Design Of Slow Time-varying System For Multi-effect Desalination Based On Full Cycle Sustained Release Of Margin

Multi-effect distillation(MED)is one of the most widely used seawater desalination technologies.As a typical slow time-varying system,the heat transfer efficiency of its evaporator is often reduced due to fouling in its long-term operation,resulting in reduced production or even shut down for fouling cleaning.To avoid this problem,process designers usually increase the heat transfer area of each evaporator.This simple design may result in excessive heat exchange area,resulting in a waste of equipment investment.Although conventional optimization design method has carried out margin design,the design process only relies on engineering experience,which is not accurate.And the operation optimization of the system is only carried out at the beginning of the design,without considering the performance degradation of the system during its long-term operation.From the perspective of system optimization design,it still needs to be improved.In addition,general multi-effect distillation system optimization design does not consider the influence of the effect number on the design.For the multi-effect distillation system,the more the number of effects,the lower the operating cost.However,the increase in the number of effects will also lead to an increase in the total heat transfer area and the increase of equipment investment.Therefore,the choice of effect number is also very important for the optimal design of multi-effect distillation system.Aiming at the optimization design problem of multi-effect distillation system,a full-cycle optimization design method is firstly proposed in this paper.This method considers the influence of fouling,production capacity changes and control requirements on heat transfer area of each effect.The minimum of the total heat transfer area and the best operating conditions are obtained by optimizing the operating conditions of the multi-effect distillation system Taking the design of the eight-effect multi-effect distillation system as an example,by comparing several design methods,this paper confirms that the proposed full-cycle optimization design method is an effective optimization design method for the multi-effect distillation system which has good application prospects.On this basis,this paper conducts research on the effect number decision of multi-effect distillation system.First,a multi-objective optimization model involving equipment investment and operating costs is formulated for devices with different effects.Then full-cycle operation optimization is applied in the problem to deal with the fouling problem during the long-term operation of multi-effect distillation system.This not only reduces the full-cycle energy consumption of the system,but also significantly reduces the equipment investment.Based on this,the pareto solution sets of equipment investment and operating costs of the multi-effect distillation system under different effect numbers and water production ratios are obtained to guide the optimization design and effect number decision of the multi-effect distillation system under different design requirements.