New Thinking in Groundwood Process Control, 1999 International Mechanical Pulping Conference Proceedings

Jouko H.J. Lehto

Kaukas Research Center

Olli Tuovinen

Valmet

Tomas Björkqvist

Tampere University of

Technology

The control of the two-pocket grinders and the groundwood process is a challenging task because the grinder operation is discontinuous in nature and the production rate and pulp quality are dependent on the grinder stones, the condition of which is difficult to keep constant. The objective of this study was to investigate whether the basic grinder control could be improved by means of new control algorithms, and whether the stone condition could be characterized more precisely than at present by using process data, so that grinders could be loaded more efficiently and high, stable production rate and constant quality could be maintained.

The research program described in this paper consisted of three successive studies which were made in the pressure groundwood mill of the UPM-Kymmene Kaukas LWC paper mill in Lappeenranta, Finland. The first study showed that the pulp quality in the grinder pit varied considerably in the short term and that the production resources of the grinders were not fully utilized when running with constant shoe speed. In the second study, two new control modes, the constant SEC control mode and the true constant production rate control mode, were developed. The inter-dependence of SEC and freeness was shown to be valid in stone grinding in the short term as well. Both of the new control modes provided a more stable quality than the constant shoe speed control mode when grinding with one pocket. In normal two-pocket grinding the application of the SEC control mode proved to be difficult because of the unreliable power split between the two pockets. The constant production rate control mode worked better in two-pocket running and in a mill trial it increased production rate by 5 % due to the more efficient use of the grinder's resources. In the third study, an experimental information/decision making system for the monitoring of stone condition was developed. It was noticed that the traditionally calculated sharpness number was not sensitive enough to describe the stone condition when the stone was conditioned by means of high pressure water. The term 'saturation degree' was introduced. It is the time that the hydraulic pressure valve is open more than 90 %. It may be used for automatic triggering of stone conditioning by means of high-pressure water.

With efficient control of grinders and stones, it is possible to carry out the suggested control strategy, the 'optimum operating point strategy', where the idea is simply to maintain all the grinders in an operation point that provides the desired quality and maximum production rate at all times. The means for carrying out the strategy are well controlled stone conditioning and the careful monitoring of the grinders by utilizing the possibilities offered by modern DCSs and up-to-date control algorithms.