Mathematical modelling and experimental study of the kinetics of the acid sulphite pulping of eucalyptus wood.

Abstract

The chemistry of the batch cooking process at Sappi Saiccor, relating to both the pulp and liquor, was investigated with the aim of using kinetic expressions to develop an improved process control model. The mill produces dissolving pulps using the acid sulphite method. Three process reactions were identified as important: cellulose hydrolysis, delignification and hemicellulose dissolution. Of these, cellulose hydrolysis is the most important since the primary aim is to achieve a targeted cellulose degree of polymerisation (DP) or viscosity (DP is commonly expressed in terms of this measurement). This is directly determined by the rate of this reaction during the cook, and the acidity of the cooking liquor was found to be the key factor. As existing equipment was not suitable for obtaining the data required to perform a kinetic analysis, a pilot plant was constructed. A commercially available probe was used for the first time to measure pH directly. The measured acidity is not directly equivalent to hydrogen ion activity at these temperatures and pressures; however, since the conditions of each cook are similar the errors incurred were found to be constant from cook to cook. The probe was found to be prone to drift due to ageing and this was accounted for by using an 'on line' calibration based on a liquor analysis. The kinetics of the cellulose hydrolysis reaction were determined using the on-line measurement of acidity and the concept of degradation increase (DI) which relates the reduction in DP value to the rate at which the polymeric chains are split. Delignification and hemicellulose dissolution were examined, since it is beneficial to maximise these reactions to reduce the quantities of chemicals consumed during the bleaching process. A model for controlling cooks to a set target cellulose DP value within a set time was developed based on the reaction kinetics. This was capable of predicting cooking conditions required with sufficient accuracy to control the cellulose DP value to within ±6 cp SNIA on the viscosity scale.