Refiner quality control in a CTMP plant

The increasing requirements for better quality with smaller variations are often difficult to fulfill in pulping plants. In chemi-thermo mechanical pulping (CTMP), the non-uniformity in the chemical treatment and the changes in the refiner's energy consumption often cause variations in pulp properties. The CTMP process has strong interactions between process variables and therefore necessitates decoupling of interactions in the refining control system. Together with the high variability in the process dynamics, lack of direct and reliable measurements makes it difficult to control the process. Consistency of the blow-line pulp is not always measured using an on-line sensor. If high consistency sensors are used, also redundant measurements or soft sensors may need to be developed for feedback control purposes. Pulp quality variables such as freeness and shive content are often analyzed infrequently and/or there is a long dead time and time constant between the refiner's blow line and the on-line analyzer's sensor. In this paper, pulp properties of CTMP refining are modeled and a new two-level quality control system is proposed. The nonlinear quality models are verified by simulation and compared with measurements from an existing on-line pulp quality analyzer. The two-level quality control system uses the process models for stabilizing the refiner conditions (pulp consistency and specific energy consumption) and for optimizing the process operation and the pulp quality (freeness). The stabilizing controllers are designed using multivariable Hammerstein and cascade control structures. The optimizing controller calculates the setpoint for the stabilizing controller by using the inverse of the nonlinear multivariable quality model. A simple scheme for adapting the freeness model based on the on-line measurements is presented. The performance of the control system is illustrated in simulations.