Modeling and control of an industrial pressure screen

Pressure screens are primarily used for contaminant removal. More recently, they have been used for fibre fractionation to produce pulp that is more uniform and of higher quality. Careful control is required for the safe operation of pressure screens, and for providing the required flows and level of cleanliness without failure. This is particularly important in fibre fractionation where optimal separation occurs when screens are operated as close as possible to the failure limit. Current control strategies are based on linear univariate control techniques. However, because the process is nonlinear and strongly coupled, univariate linear controllers are likely to perform very poorly when tight control is required. This kind of control requires an accurate model of the system. We describe a nonlinear dynamic model of pressure screens. Different methods were used to estimate the unknown parameters of the model, which was then validated against real data. Furthermore, we propose a simple nonlinear multivariate control strategy based on a static nonlinear model. For performance evaluation, we compare a conventional control strategy to the proposed strategy in simulation, and in pilot plant experiments