MODELLING OF PEROXIDE-BLEACHING OF PULP USING GAUSSIAN PROCESSES Henning Lenz 1 , Joachim Horn 2 , Thomas Runkler 1 , Markus Dinkel 3 , Thomas Schmidt 3 , Albrecht Sieber 4 , Volkmar Mickal 4 1 Siemens AG, Corporate Technology, Department Information and Communications, Otto-Hahn-Ring 6, 81730 Munich, Germany 2 Helmut-Schmidt-University of the Federal Armed Forces, Dept. of El. Eng., Inst. of Automatic Control, Holstenhofweg 85, 22043 Hamburg, Germany 3 Gebr. Lang GmbH Papierfabrik, Department of Technology, Fabrikstraße 4, 86833 Ettringen, Germany 4 Siemens AG, Industrial Solutions and Services, Industrial Plants, Pulp & Paper Technologies, Schuhstraße 60, 91052 Erlangen, Germany Abstract: Recycling of paper is typically performed in a chain of processes. An important process is the peroxide bleaching implementing the means to achieve a high brightness quality. As this process step requires consumption of various chemicals, it is quite cost intensive. This paper presents means to optimize the bleaching stage to achieve an (adaptable) trade-off between cost and brightness: First, the time-varying deadtime of the bleaching stage is estimated using a Kalman filter. Second, the bleaching stage is modelled by Gaussian processes aiming to be used for optimization of the bleaching stage by application of a model predictive controller. Copyright © 2005 IFAC Keywords: Paper Industry, Process Models, Nonlinear Models, Gaussian Processes, Optimization, Quadratic Programming. 1. INTRODUCTION Recycling of paper to provide pulp for papermaking is performed in a continuous process including different process steps. In the beginning recovered paper is disintegrated to an aqueous suspension typically within a pulper or a drum. Followed by cleaning stages including sorters and centrifugal units, coarse impurities (plastics, CDs...) are removed from the pulp suspension. The next process steps – two-stage flotation, disperger, oxidative and reductive bleaching - are aiming to remove the ink particles from the fibers and to increase the brightness of the pulp to be used for papermaking. In the flotation process air and chemicals are introduced to enable the removal of ink from the fibers. In principle the ink particles are prepared to stick to the air bubbles such that the rising bubbles move ink particles to the surface. The resulting fluffy dirty foam at the surface is removed and forwarded to the reject handling system. The brighter pulp suspension from the 1 st flotation is thickened and fed into the disperger (figure 1). Within this stage remaining ink particles and stickies are dispersed to a size below visibility and fibers are treated mechanically. As a result the brightness of the pulp suspension is decreasing. In most cases the bleaching agents for subsequent oxidative bleaching stage are introduced and mixed already in the disperger. Within the bleaching tower the fibers react with the chemicals resulting in an increase of pulp brightness. The following process stages – 2 nd flotation and reductive bleaching stage – are increasing pulp brightness to the desired level by the above described mechanisms. The final pulp (DIP - de-inked pulp) is typically fed into a storage tower and from that