Design And Development Of Control System Software For Digestion Process In Alumina Production

The digestion process is an important process in the production of alumina by bayer process.Its main function is to leach alumina from bauxite into sodium aluminate solution under high temperature,high pressure and strong alkali environment.The goal of the digestion process is to achieve the leaching rate as close as possible to the upper limit within the target range and the vapor consumption as close as possible to the lower limit within the target range,on the premise that the concentration of caustic soda in the solution is qualified.The digestion process has complicated processes,long processes,multiple equipment,and complicated material conversion processes,which makes it difficult to achieve automatic operation of the entire process.Key loops such as the cascade flashing process of the digestion process affect energy consumption.This process has the comprehensive complexity of large inertia,non-linearity,and strong interference.It is in the state of manual control by operators based on experience,leading to reduced production efficiency and increased energy consumption.Therefore,the realization of automatic control of the digestion process is the key to ensuring the goals of the digestion process.In view of the above problems,the thesis relies on a domestic alumina project to carry out research on the alumina production digestion process control system software.The main contributions in the thesis are as follows:(1)Aiming at the characteristics of strong coupling,non-linearity,and strong interference in the cascade flashing process of the digestion process,the mechanism model of cascade flashing process was established based on the principles of material balance,heat balance and phase balance;Based on the analysis of the characteristics of the model,the conventional single-loop PID control is difficult to achieve the automatic control of the level in the cascade flashing process.The idea of incorporating the interference from the upper flash tank to the lower flash tank and the high-order nonlinear modeling error caused by the coupling in the cascade flash process into the unmodeled dynamics,a PI control method based on the unmodeled dynamic compensation method for the level of the cascade flashing process is proposed.(2)According to the requirements of the digestion process,the DCS control system is combined with the digestion process,and the digestion process control system software is designed.The software consists of loop control software,logic control software,and process monitoring software.The loop control software realizes the control of the flow of circulating spent liquor,the temperature of the final outlet of the heating jacket,the level of the cascade flashing tank,and the pressure of self-evaporator.The logic control can realize the start and stop control of single equipment such as the electric valve in high pressure pump room,pre-desilication mixing motor,postdigestion mixing motor and safety interlock control of upper and lower equipment.The process monitoring software can monitor the data and equipment status of the digestion process,and automatically alarm when abnormalities are found.(3)Based on the DCS control system platform,the configuration software tool Studio 5000 and the modular programming thought were adopted to develop the digestion process logic control software module and loop control software module.The logic control software module uses standard pins to flexibly define the external interlocking conditions,and the loop control software module implements the level loop control algorithm and other conventional loop control algorithms proposed in the thesis.The monitoring configuration software FactoryTalk View SE was used to develop a digestion process monitoring software module based on the Client/Server architecture,realizing real-time monitoring of the digestion process.(4)Combined with the hardware-in-the-loop simulation system,the simulation software for the cascade flashing process model was developed,and the simulation experiments of cascade flashing control software were carried out.The software function test,system test and industrial field test were carried out for the logic control module and process monitoring module in the developed alumina production digestion process control system software.