Optimization and Control of a Two-Stage Oxygen Deliginfication System, 1996 International Pulp Bleaching Conference Proceedings

Howe Sound Pulp and Paper Ltd. (HSPP) started up an Ahlstrom Kamyr Modified Continuous Cook (MCC) digester and two stage medium consistency oxygen delignification system in 1990. This paper compiles the results of lab and mill tests which have been done to optimize strength retention in O2 delig. It also describes an O2 delig kappa control strategy which aims for maximum strength. Mill to lab strength comparison testing was carried out which pointed to substantial strength loss across the O2 delig stage. The true magnitude and location of the problem only became apparent when beater testing of fully bleached pulp was performed. O2 delig optimization tests were performed which investigated the effect of the following variables: magnesium sulphate, oxygen, caustic, reactor temperature, chemical split between reactors and washing efficiency. The impact on kappa drop, brightness and wet zero span strength was monitored. The control algorithm uses on line kappa measurement into O2 delig for feedforward model based adjustment of temperature, oxygen and caustic. On line post oxygen kappa is used for feedback adjustment of the model. Control objectives, constraints and performance measurement are discussed.