Refiner optimization and control Part II: Test procedures for describing dynamics in TMP refining processes

The present study used a system identification ARMAX modeling approach to analyze how the pulp properties can be predicted from the traditional input variables such as hydraulic pressures and dilution water feed rates at stable production. As a complement internal states from the refining zones are used as inputs in a commercial production line comprising two serially linked Twin refiners. It is shown that neither the hydraulic pressures and dilution water feed rates nor the motor loads from primary and secondary refiners are sufficient as inputs to the ARMAX model to describe the dynamics in the pulp properties. Instead, the temperature profile measurements from the refining zones outperform these traditional inputs and capture the major dynamics in the pulp property studied. It is also shown that the process should be controlled in a way that reduces as much variations inside the refining zones as possible. It is not only necessary to include the maximum temperatures in the refining zones as internal states when estimating pulp properties but also the periphery temperature since temperature profiles are affected differently depending on whether the changes are made in the hydraulic pressure, production or the dilution water feed rates. From a control perspective, refining zone temperature control is preferable to any concepts based on specific energy for minimization of pulp quality variations.