Advanced Control and Sensor Technology in Mechanical Pulping

The use of thermo-mechanical pulp (TMP) for paper making provides significant economical benefits over chemical pulps. However, the TMP pulp must be of uniform and high quality to ensure high paper machine efficiency and good final paper quality. For example, uniform pulp freeness improves the drainage properties of the paper machine and hence has a direct effect on paper machine runnability. Furthermore, pulp properties such as brightness have a significant impact on the final paper quality. In the first half of this paper we discuss an advanced control strategy for the main-line refining process. The advanced control strategy is based on the model-based predictive control (MPC) paradigm that is able to provide tight control of the refining process. The control strategy is capable of handling multiple refiner lines that empty into a common latency chest. The final pulp quality is controlled within an operator-defined window. As a result, the mill has been able to improve paper machine runnability and reduce kraft consumption. In the second half of this paper we discuss the implementation of an on-line sensor for measuring the pulp brightness in a TMP bleaching plant. The sensor technology is based on optical principles that utilize fibre optics to measure the reflectance properties of the pulp. The sensor is installed directly in the process stream, hence eliminating the need for a pulp sampling system. The sensor is able to measure the pulp brightness (% ISO) over a wide range. The ability to measure pulp brightness has allowed the second mill to reduce pulp brightness variability and chemical consumption.