Normally, the aim is to keep the production as stable as possible in all refining processes. Any disturbance in production is captured by the temperature profile measurements and thanks to the temperature profile control (TCtrl) and the consistency profile control (CCtrl) concepts, it become natural to describe the process in its simplest form

where y is the output vector comprising the maximum temperatures (T) and consistencies (C) out from the refining zones. The input vector in this example comprises the hydraulic pressure (h) and the dilution water feed rates (D). The dynamics are described by the diagonal elements gii in the transfer unction matrix (G).

Hence, in the simplest form, the maximum temperature and the consistency profile can be used for process control in a SISO-system structure for each refiner. This also opens for a number of simplifications when defining structures for quality control (QCtrl).

Using TCtrl and CCtrl for process stabilization in an inner control loop, it possible to formulate a basic cascade controller where the outer loop considers pulp and handsheet properties to be controlled. Which pulp property to control can differ dependent on mill tradition and most often three candidates are mentioned, MFL (mean fiber length), CSF (Canadian standard freeness) and shives content. Handsheet properties can be controlled as well if proper estimates have been derived, see further discussion in references given below.

To illustrate the idea, we focus on a system where the maximum temperature and the consistency are controlled in inner control loops for a primary and secondary full-scale Twin refiners. The mean fiber length (MFL) is controlled in an outer control loop.

The reason why the cascade controller is so attractive can be referred to its rudimentary structure. Some features are not so

obvious, like the possibility to split the energy input to the primary and secondary refiners and how to distribute the dilution water to each refining zone, but in general the system is designed to maximize the accessibility and minimize the maintenance of the control system. However, a structure based on model predictive control concept (MPC) can be used as well.

As the process disturbances can be handled by TCtrl and CCtrl it is natural to specify a narrower operating window for the pulp quality compared with traditional refining operation. As shown in Figure 1, the temperature can be control within ± 1 C. When the controller strives towards increased MFL the maximum temperature levels, via the inner loop set-points, are adjusted about 4 C, see Figure 2, without violating the shives content. It is also interesting to relate the adjusted maximum temperature to the reduction in the refiner motor loads as well, see Figure 3.

For further reading, please download "Refiner Optimization and Control Part IV: Long term".