Online Optimization And Control Of Multi-effect Distillation Seawater Desalination System

Low temperature multi-effect distillation seawater desalination technology has become one of the three major seawater desalination technologies due to its high thermal efficiency and good product water quality.However,the production process of obtaining fresh water by evaporating seawater makes the heat exchange equipment easy to deposit fouling.Therefore,when the multi-effect distillation unit runs for a long period of time,problems such as increased operating energy consumption and decreased fresh water production will occur due to high fouling resistance.The full-cycle operation of the multi-effect distillation system under fouling accumulation is investigated in this study.In this paper,a mathematical model of an eight-effect multi-effect distillation system with a thermal vapor compressor is first established,and then a fouling resistance growth model is added so that the multi-effect distillation model can simulate the effect of fouling accumulation on heat transfer efficiency of the system during its full-cycle operation.The full-cycle operating result shows that the fouling problem will lead to the decrease of the heat transfer rate of each effect,which in turn affects the fresh water production of the whole system.In order to reduce the operating energy consumption of the multi-effect distillation seawater desalination system,the general steady-state optimization method is used to adjust the operating conditions,but the results show that although the steady-state operation optimization can greatly reduce the motive steam consumption rate at the beginning of the full-cycle operation,it also leads to the increase of fouling.The greater fouling causes the rapid decrease of fresh water production,indicating that general steady-state optimization method cannot maintain the stable operation of the multi-effect distillation system in the whole cycle.In order to solve the operation optimization problem of multi-effect distillation system,the feasible region of this system is proposed in this paper.The analysis shows that the reason for the deterioration of the operating performance of the system caused by fouling is that the increase in fouling resistance let the feasible region of each effect shrinks,making the operating point of each effect beyond the feasible region,causing the production of secondary steam of each effect to decline,and then reducing the total fresh water production of the multi-effect distillation system.Subsequently,using the properties of the feasible region,a steady-state optimization method with operating margin constraints is proposed which avoids the operating point exceeding the feasible region when the feasible region shrinks by reserving a sufficient distance between the operating point and the feasible region boundary,so as to avoid the system operating performance be affected by fouling accumulation.Then in order to solve the difficulty in determining the optimal constraints for the steady-state optimization method,a two-layer optimization method is proposed,in which the outer layer optimizer is used to optimize the constraint range,and then the inner layer optimizer is used to optimize the operating conditions.The results show that the two-layer optimization method not only enables the multi-effect distillation system to meet the full-cycle fresh water production task,but also reduces its full-cycle energy consumption.Since the fouling accumulation changes the operating state of the multi-effect distillation system and its optimal operating conditions are constantly changing,the fixed operating conditions obtained by steady-state optimization methods cannot obtain the best full-cycle operating results of the multi-effect distillation system.To solve this problem,a full-cycle operation optimization method is proposed in this paper.This method divides its operating cycle into multiple intervals,and uses an optimization algorithm to solve the optimal operating conditions in each time interval to obtain the lowest full-cycle energy consumption.In order to avoid the problem that the full-cycle optimization only pursues the lowest overall energy consumption and ignores the operating benefits at each moment,this paper proposes a time-varying constrained full-cycle optimization method based on feasible region,which uses changing constraints to avoid operating points from leaving the feasible region.Results show that the full-cycle optimization method with time-varying constraints maintains the full-cycle efficient operation of the multi-effect distillation system,making it a very effective operation optimization method to deal with the full-cycle operation problem of the multi-effect distillation system under fouling accumulation.The multi-effect distillation system will deviate from its normal operating state due to various unpredictable disturbances during actual operation.To this end,this paper firstly proposes a method for real-time monitoring of multi-effect distillation system operating status,which uses operating margin(the distance from the operating point to the boundary of feasible region)as the monitoring indicator.By monitoring the actual value and the offline optimization trajectory of the operating margin,the deviation of the system caused by various disturbances can be detected in a timely manner.When the deviation occurs,the time-varying constrained full-cycle optimization method is used to complete the operation optimization of the multi-effect distillation system in the remaining cycle to obtain the optimal operation result of the system under disturbances.This online optimization method based on real-time monitoring of operating margin can respond to disturbances in time and reduce the impact of disturbances on the normal operation of the multi-effect distillation system.