Adapting an existing batch pulp digester model for use in continuous digesters

Abstract

A mathematical model of a continuous Kraft wood digester was developed and tested. The model relies heavily on the work done previously by Christensen, Albright & Williams (1982). The batch Kraft digester model developed by Christensen et al (1982) was adapted to model a continuous Kraft wood digester at Ngodwana, South Africa. This adaptation centres around utilizing the method of lines to account for changes in both time and height of the digester simultaneously. The model was able to simulate the Kappa number of the digester accurately to an average absolute error of 7.88 that was reduced to 2.87 after certain process parameters were optimized for. A moving horizon state estimator was introduced into the model in an effort to keep internal state prediction accurate. This addition brought the average absolute error down further to 2.75. Adaptive control was also implemented into the model. The plant data the model was compared against to determine its accuracy was filtered with the use of a rolling median filter to reduce the influence introduced by noisy and infrequent measurements.