Research&Application Of The Automatic Control System For DDS Displacement Digester Process

With the rapid development of China’s pulp and paper industry, facing thegrowing energy and raw material costs, and more critical environmentalprotection, the paper industry has cleaned out more and more convetional highenergy consumption, high pollution batch ball or digester cooking technology.International well-developed environmentally friendly displacement cookingtechnology (Rapid Displacement Heating、Displacement Digester System、SuperBatch et al) is more and more widely applied in China’s pulp mills. Batchdisplacement cooking is an energy efficient pulping technology developed in the1980s. Due to significant energy-saving, environmental and economic benefits, ithas become one important development direction for the pulp mills andreseachers in China.A set of DCS control system to design according to DDS displacementdigester system production technology and demand of process control in thispaper is used to implement of the production process automation control, whichis based on project of DDS displacement digester pulping with100,000tons peryear in SICHUAN YINGGE BAMBOO PULP&PAPER Co., Ltd. This paper issupported by international cooperation in science and technology project ofChina and Shaanxi Science&Technology Plan Project. It achieves the ends ofautomated control in the process of DDS displacement digester pulpingtechnology.The main results are as follows:(1) Research on the pulping technology of DDS displacement digester andcontrol scheme designThe function and control emphasis are presented for every subsystemthrough the analysis of DDS displacement digester pulping technology, so the corresponding sequence control logic and control method is also designed. Thecontrol of displacement digester subsystem is the emphasis of the whole pulpingprocess control, then an independent main sequence control of each displacementdigester is designed in this paper, which includes sequential control, differentsequence control from all destinations of the digester, liquid temperature anddigester pressure control, countercurrent flusching of top and middle screen,fuzzy predictive control for cold discharge and protection system.(2) Design of displacement digester temperature difference control systembased on fuzzy self-adjusting PID cascade controllerThe digester temperature control system has a character of serious delaying,time-varying and nonlinear in the displacement pulping process. During cookingprocess liquid in the digester was always circulated and displaced, and it is easyto produce the temperature gradient in the pulping digester. In order to ensurethat the liquid temperature keep uniform between the top and bottom of digester,the conventional liquid-flow PID cascade control method is difficulty to obtainsatisfactory control effects. By analysis the art of cooking circulation liquid toreturn to the top and bottom of digester, a method of PID-control with fuzzyself-adjusting parameter is proposed based on the cascade control, fuzzy controland PID control. It shows in the result of MATLAB simulation that the fuzzyself-adjusting PID controller has good dynamic response curve, small overshootand the strong capability of anti-disturbance. To decrease the temperaturegradient and ensure liquid temperature, Kappa number and pulp yield to beuniform.(3) Implementation of DCS control system for DDS displacement digestercooking processA conmlete DCS systen is designed based on the analysis of currentsituation of automated pulping and the characteristics of DCS control systemsfor DDS displacement digester in this paper. To adopt Siemens SIMATIC PCS7process control system, the control program and host computer interface aredesigned by using PCS7V7.0software system. the field device running state isreflected, so that the automatic control of DDS displacement digester productionprocess is achieved.