Modelling of the Prehydrolysis Kraft Process for Process Control

Prehydrolysis kraft process adds a separate prehydrolysis stage prior to traditional kraft batch cooking cycle in order to selectively remove portion of the wood hemicelluloses. The removal of hemicellulose is unavoidably accompanied by adverse reactions, most importantly partial hydrolysis amorphous alpha-cellulose and release of condensation prone acid soluble lignin. On-line monitoring of the reactions is missing, and only available signals from the digester are the temperature and pressure profiles. This paper presents a mechanistic dynamic model for the prehydrolysis stage, which approximates the progression of the unmeasured reactions based on the energy and component balances determined for individual digester sub-sections. Energy balance is determined for solids, liquid fraction and the walls inside each section, while the reaction rates are modelled for liquid and solid fractions in each section using second order kinetics. The model can be used directly for the control and optimization of the process or for the development and testing of soft sensors and novel control strategies.