Towards Improved Control of TMP Refining Processes

Doktorsavhandling, 2009

The need for improved energy usage has become a key issue in all industrial sectors. In the energy demanding production of mechanical pulps many efforts have focused on design development, but regarding control of refiners, there is still a potential for improvements. The possibilities of improved control performance are enhanced by reliable process measurements and here, refining zone temperature measurements are considered. This thesis includes two parallel paths. One path considers first-principles modeling of TMP refining processes. An entropy approach is applied when deriving estimates of the irreversible work associated with refining. Results show that this irreversible work, as well as the reversible thermodynamic work related to steam generation, are distributed over the refining plates forming characteristic profiles. Moreover, the entropy model can estimate material flows and pulp concentration as functions of the refining zone radius and this is made possible by measuring of temperature profiles. The entropy model provides an extensive description of important process phenomena that can be useful in relation to process design as well as process control. The other path considers refiner control. Firstly, it shows how unfortunate process design can restrict operational performance in a TMP production process. It is shown how the presence of internal interconnections could complicate control tasks and produce intricate disturbance patterns. Secondly, traditional control strategies are challenged by classifying specific energy as an integrated measure unable to reflect local phenomena inside the refining zone. Models for pulp quality variables, derived using a system identification approach, shown that no significant dynamic contribution is given by the specific energy, besides what is already given by the inputs production rate, dilution water flow and hydraulic pressure. It is, however, concluded that the models were significantly improved by the introduction of refining zone temperature measurements. Finally, the two parallel paths of this thesis meet in a discussion of control strategies involving different decoupling schemes. Besides a more general work considering simplified decoupling, a decoupling-based approach that utilizes the nature of refining processes is presented. This approach, referred to as natural decoupling, is enabled by the availability of refining zone temperature measurements and the increased process knowledge obtained during this project.