Modeling and Control of the Paper Machine Drying Section
Abstract: The topic of this thesis is modeling and control of the last part of the paper machine - the drying section. Paper is dried by letting it pass through a series of steam heated cylinders and the evaporation is thus powered by the latent heat of vaporization of the steam. The moisture in the paper is controlled by adjusting the set point of the steam pressure controllers. There exist several commercial incentives to focus on the performance of the moisture control. The time to perform a grade change is often limited by the moisture and shorter grade change time is directly correlated to economic profit. Studies have shown that the drying section uses 2/3 of the total energy requirement in paper making. Reduced variations in moisture gives opportunity for target shifts (changed set point) which reduces the amount of raw material and steam requirement. It also creates opportunity for increased production rate. The thesis is divided in two parts. The first part deals with the control of the steam pressure inside the cylinders. Both a black-box model and a physical model are given for the steam pressure process. A tuning rule for both PI and PID control is derived and various other controller structures are investigated. Many of the results are verified by experiments on paper machines at different paper mills. The second part of the thesis treats the moisture controller. The physical model from the first part is expanded with a model for the paper. This gives a complete simulation model for the drying section that is implemented in the object-oriented modeling language Modelica. Two new approaches to control the moisture by feedback are evaluated. The first utilizes the air around the paper in combination with the drying cylinders to improve the controller performance. The second uses only the last part of the drying section to control the moisture, while the first part is put at an appropriate level. Finally, feedforward of a surface temperature signal is examined.
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