An Entropy-based Modeling Approach to Internally Interconnected TMP Refining Processes

Licentiate thesis, 2005

A successful control strategy for a thermomechanical pulp (TMP) production line must deal with continuous operation as well as uniform high-quality product aspects. The ability to formulate a control strategy of this kind can be considerably enhanced by the availability of relevant process models. In this work, modeling has been performed in two different time frames and a new refiner monitoring system, including sensor arrays for temperature measurements in the refining zones, has been used, in combination with traditional process variables for model validation. A dynamic first-principle model, describing the work carried out in the refiner, has been derived. The model derives both thermodynamic work and defibration work, i.e. both reversible and irreversible phenomena, and pin-points the entropy change in the system. The model shows that the total energy input is distributed over the refining plates, forming a characteristic profile along the radius with the highest energy input in the defibration zone. Although the total work profile was fairly uniform, significant changes in the division between defibration work and thermodynamic work may occur. Consequently, a constant motor load does not necessarily indicate constant pulp defibration. Moreover, the capability of the models to deliver residence time and consistency estimates was also demonstrated. An overall model structure for the TMP production line, including all the internal interconnections, has been derived as well. Simulation results show that the model structure reflects the characteristics of the interconnected process and that the nature of the disturbance pattern arises from the fact that so many internal interconnections are present. Moreover, it is shown that the temperature profiles in the refining zones provide a range of process outputs with individual responses to input changes depending on the spatial location. The internal interconnections of the TMP production line influence the production performance, as well as the system controllability, which demonstrates the importance of considering control aspects at the process design stage.