Refining zone temperature control: a good choice for pulp quality control?

Report, 2008

In control strategies for thermomechanical pulp refiners, the relation between energy consumption and production rate, often called specific energy, has been used as a key variable for decades. The importance of controlling the specific energy and thereby indirectly the pulp quality, has been an indisputable axiom for most engineers engaged in the pulp and paper industry. Recently, another competing concept based on refining zone temperature measurements has been presented as an alternative for improved pulp quality control, but so far no comparison between these two concepts has been made. In this study, the two concepts are compared based on a system identification procedure using “Auto Regressive Moving Average eXogenous” (ARMAX) models. The identification procedure adopted creates dynamic models that can provide predictions of the commonly used pulp quality variables Canadian Standard Freeness (CSF) and Mean Fiber Length (MFL). These predictions are all based on the traditional process variables, i.e. production rate, dilution water flow, and hydraulic pressure, in combination with information of either the specific energy and or the temperature profile. The results show that it is not motivated to use the specific energy for predictions of CSF and MFL, as it gives limited dynamic information of the refining process besides what already is given by the three traditional process variables. Whereas, using the refining zone temperature measurements as inputs to the ARMAX models results in a significant improvement of the ability to predict the pulp quality variables. From a control perspective, this implies that refining zone temperature control is preferable to any concepts based on specific energy when it comes to minimization of pulp quality variations.