Refining models for control purposes

Artikel i vetenskaplig tidskrift, 2008

Many studies have been carried out with the goal of improving pulp quality control in TMP refining processes without a deeper understanding about the physical conditions inside the refining zone. The refiner is thereby considered as a black box where the chips are defibrated to fibers. In this paper, a non-linear model for the estimation of thermodynamic and defibration/fibrillation work is presented. It is shown that the model requires temperature measurements to span the energy balances and obtain reliable pressure gradients inside the refining zone. Based on process data from a single disc refiner, it is shown that the defibration/fibrillation work varies substantially along the radius of the refining segments, even though the total work, i.e. the motor load, is almost the same over the specific time sequences. This is a consequence of variations in the concentration of the fiber pad built up inside the refining zone. Moreover, it is shown that the commonly used control concept, where the traditional specific energy is used as a control variable should be reconsidered. Besides a deeper analysis about the assumptions made in the refiner modeling procedure, obtained results regarding estimated fiber concentration along the radius, consistency and steam efficiency are discussed.