Quality Control of a Newsprint TMP Refining Process based on Refining Zone Temperature Measurements
Licentiate thesis, 2007
The ultimate goal of this study was to examine the possibility of using temperature measurements for control purposes in a TMP refining process. The study was carried out in a commercial refiner line at Holmen Paper’s Hallsta Mill, were the primary stage refiner was equipped with temperature sensors.
The results suggest that the position of the temperature peak is closely correlated with disturbances in quality. It is notable that the peak position appeared to be better than the load/specific energy parameter at predicting the quality disturbances, although the specific energy is usually the base on which refiner control strategies are built.
An experimental control strategy with the aim of stabilizing the peak position (by adjusting the inlet pressure) was tested and was found to give reduced quality variations. A further improved control system, “Tctrl”, which included feedback from the quality sensor, was then implemented and tested. Reductions were noted in the standard deviations of all three quality parameters that are measured on-line: freeness (-28%), mean fiber length (-15%) and shives content (-66%). This further supports the hypothesis that the information provided by the temperature profile is probably more useful than the commonly used load/specific energy for reducing faster quality disturbances.
During the study, it became evident that methods were needed to quantify the economic value of improved quality control. One positive effect of reduced quality variations that probably gives a notable economic effect is improved paper machine stability and thereby fewer web breaks. Such a reduction was indicated during a six-month full-scale trial. Other economic aspects were also modeled.
The design of a successful control system must also include more than the technical functionality: usability and human factors considerations must be heeded. A particular problem related to the set point handling in hierarchically organized control systems, such as Tctrl, has been investigated further from a usability point of view. The insights gained from this study were also incorporated in the implementation of the new control system.