| The development of pulp and paper industry in China is affected by resources and environment.To achieve sustainable development of the industry,it is imperative to eliminate the traditional batch digester with high energy consumption,heavy pollution and low efficiency.Displacement Digester System(DDS)is an advanced pulping technology with energy-saving and environment-friendly.Now,DDS has been used as the preferred technical solution for new construction or technical transformation projects in China's pulp and paper factories.Therefore,DDS displacement cooking technology has been studied in applied technology filed,which is of great significance to upgrade pulping equipment technology and improve the automation level of pulping and papermaking industry in China.This subject is based on the technical innovation project of chemical pulp in a pulp mill undertaken by Shaanxi XiWei Process Automation Engineering Co.,Ltd.The DDS cooking process is presented in detail.The logical relationship is analyzed among subsystems.And characteristics of liquor temperature control subsystem are analyzed.Therefore,the study of DDS displacement cooking control system is carried out for produced chemical pulp.In this thesis,the main research contents are as follows:(1)Design of main interlock logic for displacement cooking system and cooking process modelingAccording to the demands of cooking process,many important process interlocks are discussed in each section of cooking process phase.And these interlock logic are designed for each subsystem.According to characteristics of displacement cooking process and cooking process schedule,the process sequence constraint equation of replacement cooking is deduced,which paves the way for reasonable planning of the interval period of liquor charge and subsequent process optimization.(2)Analysis of influence factors for liquor temperature rise process and determination of liquor temperature control schemeAccording to the mechanism of liquid temperature rising process,the influencing factors of liquid heating-up are analyzed.Then,it is concluded that there are some problems in the process of liquor temperature rising,such as slow heat transfer,many interference factors and strong correlation between channel flows.(3)Study on fuzzy fractional order PID cascade decoupling control liquor temperature rising processIn this project,a simple feed-forward decoupling scheme is introduced to solve the coupling relationship between upper and bottom channel's flow in liquor temperature rising process.The feasibility of the scheme is verified by simulation.And fuzzy fractional order PID controller is designed to overcome the difficulties that the time-delay and more interference in the liquor temperature control system.Finally,the proposed fuzzy fractional order PID cascade decoupling control strategy is simulated,and the simulation results indicate that the strategy can provide dynamic characteristics and robustness of liquor temperature control system.The feasibility of the control strategy is proved theoretically,and it can meet the control demands of liquid temperature rising process.(4)Implication of displacement cooking control systemPCS7 V8.0 as software development platform and S7-400 H PLC as control hardware core,a DCS control system is designed to meet the control requirements of displacement cooking process.Many works has been completed in this project such as the hardware configuration of Step7,interlocking logic design and process screen design.And implementation of advanced algorithm control scheme based on OPC technology is also completed.According to the running situation on the site,the system can work smoothly and the cooking processes are executed orderly.So the system can meet the requirements of pulping process production.The designed control strategies and technology schemes in this project can provide reference for the optimization of subsequent displacement cooking system,and have certain guiding significance for production practice. |
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