Modeling of the Acid Sulfite Pulping Process. – Problem Definition and Theoretical Approach for a Solution with the Main Focus on the Recovery of Cooking Chemicals

This study investigates the requirements for modeling of acid sulfite pulping with regard to the recovery of cooking chemicals, especially SO2. Such a model has to predict the temporal change of SO2 distribution on solid, liquid and gaseous phase in the digester throughout the whole pulping process, dependent on the pulping parameters: total SO2, free SO2, liquor-towood ratio, temporal temperature course, maximum pressure and time. The approach is based on the mass balance of SO2 to achieve data for determination of kinetics of relevant chemical reactions, such as sulfonation of lignin, dissolution of lignosulfonate, or formation of sulfate, strong acids and carbon dioxide. Results from capillary electrophoresis measurements in laboratory experiments were used to determine the temporal change of inorganic sulfur components in cooking acid. Some experimental problems remain (pHand gas measurements) for a comprehensive description acid sulfite pulping, considering the mass balance of sulfur dioxide.