DYNAMIC MODELING AND SIMULATION OF THE RECAUSTICIZING PLANT IN A KRAFT PULP-MILL

The recausticizing plant has always been recognized as a very important part of a kraft pulp mill as it determines the quality of the white liquor (effective alkali, total titratable alkali, causticity, and sulfidity). White liquor sulfidity is one factor directly affecting the strength properties and kappa number of the produced pulp. Sulfidity is determined by the Na2S and the NaOH concentration. As the Na2S content is dictated by the recovery boiler operation the main objective of causticizing plant and lime kiln operation is to control the NaOH content. A dynamic model of a recausticizing plant was developed to study the impact of process changes on the white liquor characteristics. In addition, a model that includes the effect of lime specific surface area on the causticizing efficiency and reaction kinetics has been proposed. Dynamic simulations of the entire plant have been performed using the SIMNON simulation package to study the impact of a change in the smelt mass flow rate and in the lime mud filter mass flow rate on the total titrable alkali and active alkali.