Modeling And Optimal Control Of Reverse Osmosis Membranes In Seawater Desalination

In order to effectively reduce the cost of seawater desalination and meet the ever-increasing demand for fresh water,large-scale reverse osmosis membrane modules based on the series-parallel structure have increasingly become the focus of desalination research.However,the currently used reverse osmosis membrane system model is a performance analysis-oriented mechanism model,which with many problems,such as complex forms and hard to develop a research on control strategy,mostly aims to improve the performance of a single membrane.In addition,the optimization of reverse osmosis efficiency of the membrane unit is limited by the series-parallel structure.Thus,this paper studies the multi-membrane reverse osmosis membrane unit from several aspects of structure design,modeling,performance analysis and optimization control.System structure and modeling: First,in order to increase the water production of the Membrane group,this paper takes high recovery rate as the design goal,optimizes the selection of membrane type,membrane group structure and high-pressure water pump parameters.Aiming at the problem of insufficient control accuracy of traditional valves,solenoid valves based on BUCK converter control are used to achieve precise and flexible control of system pressure;Secondly,to solve the traditional mechanism model with complex form and large amount of calculation,the principle of electro-hydraulic similarity is adopted,that the rejection to permeation flux is unified abstracted as liquid resistance and reverse osmosis pressure is analogized to back electromotive force,to constructed a simple and easy-to-control reverse osmosis membrane group model.To complete the solution of the reverse osmosis membrane group model,the method of power flow analysis in the power system is adopted as well.Performance analysis: Based on the model of the reverse osmosis membrane group,the relationship between the performance of each section of the membrane and the pressure of the first membrane system is deeply analyzed,and the relationship between the performance of each section of the membrane is compared;Because of similarity between the reverse osmosis pressure and the back electromotive force in the motor control loop,a new concept of reverse osmosis efficiency is proposed.Through simulation experiments,the relationship between the reverse osmosis efficiency of the membrane group under variable working conditions and the pressure of the first-stage membrane system is analyzed,and the maximum reverse osmosis efficiency curve under variable working conditions is obtained.Optimization and control: Aimed at seawater salinity,membrane pollution and solenoid valve winding temperature rise and other factors that seriously affect the performance of the membrane group,also taking into account the non-linearity,strong coupling,large interference,and working conditions of the reverse osmosis membrane group,an optimization based control strategy for reverse osmosis membranes using RBF neural network is adopted,that is,the system pressure control under rated conditions is completed by state feedback control,and use the strong approximation ability of RBF neural network to effectively approach the uncertain parts in the system,so as to complete the fast tracking of the optimized pressure and ensure the stable operation of the system.A simulation experiment platform for optimal control of the reverse osmosis membrane seawater desalination system is built,and the performance of the control strategy proposed in this paper is compared with the traditional PID controller and the state feedback controller based on adaptive compensation.The simulation experiment results show that the control strategy proposed in this paper has a strong ability to track the optimized value of pressure.Under control,the pressure steady-state error is 0.045 MPa,the adjustment time is 1.2~1.7s,obviously,the precise control of the pressure is realized,and the reverse osmosis efficiency of the membrane group is improved.Since there is no overshoot while conditions changing,the impact of sudden change in pressure on the membrane group is greatly eased,the damage of the membrane is reduced,and the service life of the membrane is prolonged.