A Comprehensive Approach to Modeling and Control of Thermomechanical Pulping Processes

Abstract: The central issue of the thesis is modeling and control of the Thermomechanical Pulp (TMP) process in which large refiners are used to produce pulp for newsprint. The TMP process is one of the most complex and versatile, electricity consuming industrial processes today. In this thesis, modeling has been addressed from many different perspectives, timescales and levels of detail, to enable a more thorough understanding of the process. The theories comprise mill-wide energy modeling, dynamic modeling and control of a refiner line and modeling of the refiner zone process. The modeling theories are verified using measurements from a full-scale refiner line and include measurements of temperatures and high-frequency pressure fluctuations inside the refining zones. In this thesis, a complete model describing the TMP process from an energy perspective is presented. As the steam produced in the refiners is used at the paper mill together with additional steam from a power plant, a reduction in energy consumption in the TMP-plant does not necessarily imply lower overall energy usage. With the derived mill-wide energy model, a complete energy optimization can be performed. The refiner line is a complex dynamic system where the internal interconnection, the dynamics of the unit operations, the production rate and possible stiction in control valves determine the major part of the plant's dynamic behavior. To illustrate this, a dynamic model of the refiner line has been included in the thesis and has also been implemented on-line parallel to the true refiner line. Moreover, a theory for control of the refiner has been derived as a 3 x 3 decoupling strategy using load, consistency and temperature at a certain radius in the refining zone as outputs. This relatively complex control structure appears to be a reasonable solution, but sensitive to model uncertainties. It is shown that a more robust choice for control would therefore be a 2 x 2 decoupling control, or exchange an output, preferably the load to another, possibly a soft sensor, output. The physical models of the defibration process presented in this thesis use temperatures and pressure measurements in the refining zone. The consistency estimation routine has proven to be faster than the online measurements and more accurate than a pure energy balance of the refiner. The principal contribution is, however, the increased understanding of the refining process as it gives plausible explanation to the refining mechanism, most notably of the refining work distribution in the refining zone and the transfer of macroscopic motion to inner energy via viscous dissipation. Finally, the measured pressure signals presented are shown to be the generated dynamic pressure as bars pass over the sensor. It is also concluded that the pressure measurements, and thereby the defibration, differ between the two refining zones in the specific refiner studied. The energy content in important frequency peaks are also found to vary with operation point.