| Paper industry is an important basic raw material industry in China.Its development has been restricted by the high energy consumption,high pollution and unreasonable raw material structure.As an important part of the paper industry,pulping is still using the conventional batch cooking process.It pollutes the environment greatly due to the high energy consumption,low yield,and the subsequent bleaching process can only using CEH or CEHH bleaching.By setting several tanks,the displacement digester system(DDS)makes the cooking process divided into multi-steps,realizes the recycling of chemicals and energy from the black liquor,follow-up pulp can be adopted without chlorine and other totally chlorine free bleaching technology developed currently.The DDS is an energy-saving batch pulping technology,it has already been developed maturely and used widely in the US,Canada and other papermaking technology developed countries.Although China has successfully introduced DDS,there is still not a integrated control system of fully functional DDS in the country.DDS control system applicated in China is provided by ABB,Honeywell and SIEMENS,the cost is expensive,only the money-abundant pulp plant could purchases.Realizing the DDS control system localization is beneficial to the popularization and application of DDS technology in China.Based on the DDS project of Guangxi Mashan and Faqiang Co.Ltd and with funding support of ISTCP,this thesis analyzes the process control of GL&V DDS,expands application of technical study form the control difficulties study and solution strategies,control program and the host computer program design.The specific contents are as follows:(1)Study and solution strategies of control difficultiesThe process control of DDS is analyzed in detail based on the GL&V technical data introduced by ISTCP,and then strategies for three difficulties are proposed.The neural network cascade decoupling PID control algorithm is designed to solve the time delay and nonlinearity problems of digester temperature difference control.In view of the drift problem,the channel detection and destruction are designed to ensure the cooking liquid flow in the way of the plug flow.For the discharge pump cavitation problem,by controlling the discharge concentration,combined with variable speed discharge and compensation for NPSHa to ensure NPSHa larger than NPSHr during discharge process.(2)Study and design of DDS control schemeThis thesis proposed the control scheme of main sequence combied with subsequence according to the DDS process control,realized the automation of the process.The tanks zone level control strategy is obtained by modeling and analysis based on the material balance,the tanks level is always kept between 10% ~90%,meet the DDS production requirements well.The redundancy,interlock and diagnosis are designed to ensure the safe and effective operation of the control system.MATLAB and PLC are combined to facilitate the application of advanced control algorithm in DDS by using OPC interface technology.(3)Design and implementation of control system based on S7-400 PLCDistributed control system(DCS)is designed and applied to actual production according to the proposed strategies and control schemes,by using SIEMENS S7-400 PLC as the main control hardware and PCS7 as the software.This paper proposed strategies for three difficult problems of DDS and designed control schemes of process automation,based on analyzing the DDS process control of GL&V.Then hardware and software design of DCS is made by proposed strategies and control schemes.Actual application at Guangxi Mashan and Faqiang Paper Co.Ltd shows that the designed DCS could meets the demand of DDS production well. |
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