Prediction model and in-digester control of residual alkali content in black liquor during kraft pulping of eucalyptus.

The relationships between process parameters (including cooking temperature, reaction time, H factor, initial alkali charge and sulfidity) and residual alkali content in black liquor during kraft pulping of eucalyptus were systematically studied in this study. Based on these relationships, an empirical model used to quantitatively predict residual alkali content in black liquor was developed. After comparing the model predictions and 26 independent experimental data, a good correlation (R-2 = 0.951) was obtained through least square fitting, indicating the validity and reliability of this empirical model for predicting residual alkali content in black liquor during kraft pulping of eucalyptus. Furthermore, skillful integration between this model and pulp kappa number prediction model developed in our previous work results in a process control strategy for simultaneously controlling residual alkali content in black liquor and kappa number of corresponding kraft pulps. Finally, simulation results from constant residual alkali content curves demonstrated that co-regulating the parameters including H factor, initial alkali charge and sulfidity, will be an effective method to select process parameters and address specific operation requirements during controlling process of residual alkali content in black liquor, which will be helpful for efficient recovery of black liquor and partially promoting the implement of the concept of transforming a chemical pulp mill into an Integrated Forest Biorefinery (IFBR) in near future.