Kraft continuous digester effective alkali control

Abstract

Optimal control of a continuous pulp digester is a difficult problem. The objective is to achieve maximum pulp production at a specified Kappa number with a minimum of chemicals and energy input. Existing Kappa number control strategies focus on H factor control. These strategies attempt to model the relationship between Kappa number, H factor, sulphidity, and effective alkali. Reducing the variability of effective alkali through closed loop control contributes to smoother digester operation and reduced Kappa number variation, However, closed loop control of effective alkali is a significant control problem due to the long dead time that exists between changes in white liquor flow and measured residual effective alkali. This paper describes the application of a predictive adaptive process controller that was able to provide closed loop control of effective alkali and resulted in reduced Kappa number variability. The controller used was based on dynamic modeling technology (DMT) and an introduction to DMT is provided. Following this the test results and the justification to purchase are presented.