Implementation of a Model Based Controller on a Batch Pulp Digester for Improved Control

摘要

Effective control of the sulphite pulp digestion process in the production of dissolved pulp in a batch digester is limited by three important restrictions namely, the inability to measure the degree of polymerisation (DP) of the cellulose in the wood pulp, which is the controlled variable, the fact that flowrate of steam to an external heat-exchanger, through which the cooking liquor circulates, is the only manipulated variable available and that, due to scheduling requirements, cook time per digester is fixed. The traditional S-factor prediction of cook-time to control DP is inadequate. Use of a simplified model-based inferential technique to estimate DP offers an improved methodology to control the process. A simplified fundamental model with adjustable parameters was developed and its accuracy to predict DP based on given operating conditions was tested using available plant data. This model was built into a control structure for implementation on an operational batch digester. Use of this model enables adaptive response to changing conditions on the plant by optimal adjustment of the model parameters to fit the measured characteristics of the digester. Because the parameters form part of fundamental relationships in the model, realistic bounds can be placed on them during the optimisation process, enabling a better understanding of the behaviour of the model. By automating the optimisation process, the controller becomes independent, requiring no human intervention for day-to-day operation. The plant model is periodically adapted by adjusting the parameters in the model, based on historical data representing the performance of preceding cooks.