Research & Implementation Of Displacement Digester Process Control Strategy Of Dissolving Pulp

The dissolving pulp is high purity chemical pulp. It kept cellulose only in the material when the wood or bamboo was cooked out of lignin deeply. The market of the dissolving pulp was big. Displacement digester technology was energy saving and environment protective in pulp cooking process. Also, it could get out of the lignin deeply. So the Displacement Digester System(DDS) had become the first choice when the paper mill built new pulp production line or reconstructing the old line. But the control system of the DDS had not been got location. The different kinds and transportation of the cooking liquid in the pipe increased the control difficulties. The temperature difference had characters of time-delaying, time-varying and strong decoupling relationship. So the DMC-PID cascade decoupling control system was designed in this paper. And control system of the steam flow was designed because of the volatility for the steam consuming.The paper was funded by the Collaborative Research on the Key Technologies and Equipments of Displacement Pulp Cooking Process With Energy- Saving and Environment-Protective(2010DFB43660). The main works were as follows.(1) The control solutions of every cooking process were designed.The control solutions were put forward aiming at the difficulties and keystones of the system by researching all the cooking process. The control schemes of the cooking liquid delivery and relief condensing system. And constant liquid level control strategy in the warm tank was analyzed based on material balance. The results showed that the level of the liquid always stayed between 10% and 90%, and it could met the production requirements.(2) The DMC-PID cascade decoupling control system was designed.The liquid pumped out by the middle and flowed back from the top and bottom of the digester when liquid was heated by steam. The different velocity of backflow would cause different temperature in top and bottom of the digester. Then the pulp would be cooked uniformly. So the DMC-PID cascade decoupling control system was designed in the paper aiming at the characters of time-delaying, time-varying and strong decoupling relationship. It was simulated by the Matlab, and the results showed it got better performance and higher robustness.(3) The control system of the steam consuming was designed.The steam consumption would fluctuate when the direct steaming and heating process happened in different digesters on the same cooking production line. And it demanded to track the curves of the P or H factor. So the control system with limiters and priorities was designed. It used only one flow-meter. The program for temperature generating was written to track the set curve in the PLC.(4) The DCS for the dissolving pulp DDS was designed based on SIMATIC PCS7.The hardware based on S7-400 PLC and software based on PCS7 were designed to meet the control schemes requirements. The programs and the operation pictures were designed in the PCS7. The actual operation situations showed that the designed control system could meet the demand of the process.