Dynamic Modeling of the D0 Stage of a DEopD Bleach Sequence, 2002 Fall Technical Conference

Profitable operation of a bleach plant requires that the delignification stages of the bleach sequence be operated in a stable and optimal manner. Optimal performance of these first stages is often difficult to achieve in practice because of sensitivity to disturbances, long residence times which make control difficult, and complex reaction kinetics coupled with non-ideal pulp and liquor flows.

Optimization and control of the delignification portion of a bleach sequence may be facilitated by development of an accurate and robust bleaching model. This model must correctly represent the reactions that occur within the bleaching stages and must simulate the non-ideal pulp and liquor flows within the towers. In addition, these phenomena must be coupled in the model to achieve an accurate representation of the reactor’s dynamics.

A first principals dynamic simulation of the D0 stage of a commercial DEOPD bleach plant has been developed at the University of Washington. The model was developed using published bleaching kinetic data and data taken from mill trials. The flow characteristics of the bleach sequence are modeled using coupled systems of continuous stirred tank reactors (CSTRs). The D0 stage kinetics are based on a scheme that divides the lignin into fast and slow reacting lignin species with an unreactive floor lignin. Using the chemical charges, pulp flow rates, lignin carryover, and process temperatures the model predicts the outlet kappa and residual chlorine dioxide as a function of time. The model predictions are compared against mill data with good results.

The bleaching model that has been developed is well suited to assess the performance of various control strategies and to optimize the bleaching process.