Modelling of kappa number in Downflow Lo-Solids cooking using Gustafson’s model

N AN earlier study by the current authors, Gustafson’s model [1] has been applied to the modelling of the kappa number profile of a traditional Kamyr process [2]. In Downflow Lo-Solids cooking [3, 4] the process and flow conditions differ from the ones of the traditional Kamyr. The cooking reactions are, however, the same in both processes and more measurements and control variables are available in the Downflow Lo-Solids process. In Gustafson’s model [1], the changes of the temperatures, and especially the alkali concentrations in different cooking zones (initial, bulk and residual), are taken into account. That is its main advantage when compared to the widely used Hatton’s model [5]. Many papers recently published discuss the applications of the wellknown Purdue model [6, 7, 8]. The Purdue model including chip-compaction modelling is used in the simulation and control purposes of Kamyr digesters in normal operation and in grade transitions. Important features of Gustafson’s model are the possibilities to predict the viscosity of pulp, the pulping uniformity and the amount of rejects. Gustafson’s model also takes into account chip dimensions. Because of the features mentioned above and because the model is quite easy to implement into the plant’s automation system, Gustafson’s kappa number model [1] was chosen. The chief aim has been to apply Gustafson’s model to the Downflow Lo-Solids cooking and to model the real-time kappa number profile of the complete cooking process. The second goal has been to improve the predictability of the blowline kappa number. The results will be used in the monitoring and control of the kraft cooking. The main interest has been in softwood cooking because the process conditions in softwood cooking affect the kappa number more than in hardwood cooking. The structure of the paper is as follows. First, the Downflow Lo-Solids process and the used variables are presented. Then Gustafson’s model and the optimizations carried out in both softwood and hardwood cooking and grade transitions are introduced. The results of the kappa number modelling are shown and discussed. In the last section the conclusions are presented. PROCESS DESCRIPTION The studied industrial scale system consists of an impregnation vessel and a steam/liquor phase digester running the Downflow Lo-Solids process, Fig. 1. The digester is examined in four separate zones according to the extractions. The chips are impregnated in the impregnation vessel and in the first zone (D1) in a digester down to the upper extraction screens. Between the upper extraction and cooking circulation there is a counter-current washing zone (D2). In this zone, chips are displaced with cooking circulation liquor, in which the temperature and alkali concentration are high. The lignin is mainly removed in the comparatively long co-current cooking zone (D3). At the bottom of the digester is a short washing zone (D4). A characteristic feature of this process is the grade transitions between softwood and hardwood. Softwood chips mainly consist of pine chips with small amounts of spruce chips. Hardwood chips consist mainly of birch chips with small additions of aspen chips. The effective alkali concentrations of the white liquor, of the digester feed circulation liquor, of the two black liquor extractions and of the cooking circulation are measured. The white liquor is added to the impregnation vessel’s feed circulation, to the digester’s feed circulation and to the cooking circulation. The sulphide ion concentration of the white liquor is measured and assumed to stay constant during the cooking process. Temperatures are measured from the liquor circulations and from the heated steam at the top of the digester. The temperature profile from the top of the digester to the cooking circulation is constructed by emphasizing measured temperatures suitably. The temperature profile from the cooking circulation to the blow-line is based on the temperature of the cooking circulation. All the data used in the study are measurements from the industrial continuous digester. All process measurements are mean values of one minute, or values of samples at sampling intervals. Calculation interval in the modelling is one minute. Data handling consisting of evaluation and filtering, model identification and simulation are carried out with the MATLAB-program. I Abstract: Gustafson’s kappa number model has been applied to the modelling of an industrial Downflow Lo-Solids cooking process. The model parameters have been optimized to be suitable in both softwood and hardwood cooking. The real-time kappa number profile of an impregnation vessel and digester has been calculated. Based on a realtime model and process measurements before cooking zone, the blow-line kappa number has been successfully predicted. The modelling results will be utilized in control purposes.