VALIDATION OF MODELS, SIMULATION AND OPTIMISATION OF PRESSURE SCREENING FOR RECOVERED PAPER PROCESSES

Abstract

Fine screening is one of the most effective methods of removing tacky sticky impurities from pulp suspension. The separation process is not a selective process and always contains a mixture of fibre, fibre fine and filler losses. Changes in machine parameters are normally made through investments on new design of screening baskets, rotors and basket profiles. Feed quality is controlled through defined consistency and composition. Normally, no dynamic control exists in the mills for fine screening and the screening runs according to a predetermined control strategy. Due to the above reasons, the total capacity can not be utilised properly. The optimisation criterion for screening system is mill specific and is based on available configuration in the mill and therefore it differs from mill to mill. No attempt has been made to optimize and control the screening system which is of general nature irrespective of the difference in configurations in the mill. Therefore there is need to analyse through a model to predict the behaviour of screening system to separate various components in the feed including stickies and further to optimize the process. The most important objective of the present work is therefore to optimize the sticky removal in the fine screening process for recovered paper grades through simulation. The screening efficiency is a function of reject rate and can be defined by several mathematical models. Most accepted models for describing a fine screening process are the plug flow model and the mixed flow model. In the current project samples are collected from various brown grade paper mills and analysed for balancing the screening systems based on the concentration of various components like fibres, fibre fine, stickies etc. The measurement data is used for validating the existing models of both plug flow and mixed flow. For this validation, the error sources are defined and analysed for improving the accuracy. Pilot plant trials are made to improve the accuracy and reproducibility of validation and to observe the sticky behaviour through screening. Also, these results are used to compare the responses with industrial results. A simulation tool is developed based on plug flow model to obtain the optimum situation. Profit calculations are made from the simulation based on a cost function for macro stickies. Finally, optimum strategy for a screening system is obtained from this model validation and simulation to control the screening result.