CTMP process optimization Part I: Internal and external variables impact on refiner conditions

In this paper, internal variables (e. g. temperature, consistency and fiber residence time) are introduced and defined as physical states obtained in different parts of the refining zones. In short, they differ from the traditional external variables (e. g. dilution water feed rate and specific energy), which are not available as distributed variables from refining zone measurements. The internal variables can be seen as the backbone of physical models and we illustrate that based on a model for a CD-82 refiner in a CTMP production line. Such a model can be used for on-line implementation of soft sensors for advanced process control and it is shown that the refining segment pattern affects the temperature profile. It is particularly interesting to study how the segment parameters in terms of the distributed width of the bars and grooves together with the segment taper affect the back-flowing steam, the cross-sectional area and thereby the fiber residence time. To illustrate the capability to use a modeling strategy it is shown how to reach a 40% reduction in specific energy without violating the required pulp properties.