Modelling and Control of a Hypochlorite Bleaching Stage for Dissolving Pulp, 1995 Process Control, Electrical & Information Conference Proceedings

Marc Barrette*

McGill University

Michel Perrier

Ecole Polytechnique

Modelling and simulation are valuable tools that can be used effectively to achieve proper control of a bleach plant. In this paper, a dynamic model of a bleaching stage is developed to study the dynamics of the system and to evaluate different control strategies. The system under consideration in this study is a sodium hypochlorite (NaOCl) bleaching stage for softwood dissolving sulfite pulp at the Tembec Inc. mill in Temiscaming, Canada. For dissolving pulp producers, the most important quality parameter is the pulp viscosity which is closely related to the degree of polymerization of the cellulose. The pulp viscosity is controlled by reacting the pulp in an up-flow tower with hypochlorite under specific conditions. The tower was modelled as five (5) Continuously Stirred Tank Reactors (CSTR’s) in series in order to simulate plug flow conditions. The simulations were performed using the MATLABrM with SIMULINKTM software package. Actual mill operating conditions were used as inputs to the simulations. The results were compared to mill data and they agreed very well. The model was also used for system identification, controller tuning and evaluating control system performance. A cascade control strategy along with time-delay compensation was found to provide good control for this system. Although the equations presented in this paper are specific to this system, they can be easily modified to model and simulate other bleaching stages where the pulp quality parameter differs.