Press nip dewatering: From data collection to model validation using grey-box identification

This work treats the issue of developing physical dewatering models for a press nip in a paper machine. Emphasis is placed on the identification and validation as well as on the observability. The observability is treated from the viewpoint of the online measurement technique, focusing on the measurement of web and felt moisture content. The major difficulty is calibration since the measurement gauges turn out to be very sensitive to the operating conditions and to the distance between gauge and measurement object. The considered model structure assumes one-dimensional (transversal) one-phase (water) flow, which obeys Darcy's law. Simulations show that a similar external model behaviour does not necessarily imply similar internal behaviour. The approach for identification is the grey-box modelling technique where physically based models are combined with disturbance descriptions to obtain models with good predictive capabilities. Process data for identification was collected from the research paper machine (EuroFEX) at STFI in Sweden. It turns out that the model is capable of predicting the dewatering in a press nip for variations in the in-going web moisture content and in the press load. An improved felt description is, however, needed to predict the effect of variations in the felt moisture content.