Computer Control of M&D Continuous Digester; Basic Principles and an Application

Summary

The stabilization of the conventional control loops, the determination of optimal values for the setpoints of these and the optimal utilization of raw materials with the aid of a digital computer system in the M&D continuous digester process have been studied. The resulting control strategy reduces the variations in the quality ie. in kappa number of the produced pulp and it makes also possible to change the production rate or quality smoother than by conventional control. Also an effective reporting system has been included in the system.

The main ideas in this control system are the automatic calculation and correction of the demanded flowrates for the white liquor and for the black liquor. Here use is made of alkali-to-dry wood and liquid-to-dry wood relations. The calculation of cooking time is based on the constructive details of the digester and it will be controlled according to the desired production rate. The desired product quality will be achieved by controlling the target kappa with the aid of the H-factor simulation model. This strategy has been applied to the sawdust cooking process using Bauer M&D continuous digester in the Kaukas Company, Lappeenranta, Finland. The computer control system used here is the Nokia PP 6500. The first results and experiences of this project will be presented here.