Title page for ETD etd-10152004-113239

Document Type

Master's Dissertation

Author

Sandrock, Carl

URN

etd-10152004-113239

Document Title

Implementation and performance analysis of a model-based controller on a batch pulp digester

Degree

MEng (Control)

Department

Chemical Engineering

Supervisor

Advisor Name	Title
Prof PL de Vaal

Keywords

model based control
Monte Carlo
performance analysis
batch control
pulping
sulphite process

Date

2003-10-31

Availability

unrestricted

Abstract

The control of batch pulp digesters is hampered by insufficient measurements as well as nonlinearity and weak correlation between consecutive cooks. This makes a model-based approach to control attractive. Due to the age of the industry, many legacy controllers are in place on digesters around the world.
The theoretical variance obtained by Monte Carlo modelling of a new controller is used as a benchmark for performance comparison between an old control system (S-factor) and a new model based controller developed by the University of Pretoria (the UP controller). This study covers the development of the controller, Monte Carlo modelling of the old and new controllers and in-situ testing of the UP controller on an operating digester.
During Monte Carlo simulation, the UP controller outperformed the legacy controller, obtaining a theoretical overall variance of 3,07 (which will be used as the baseline for performance measurement) while also showing larger responses to tuning factors. The S-factor performed at 6,8 times the theoretical optimum variance during in situ testing, while the UP controller performed at 3,9 times the theoretical optimum (43% better than the S-factor controller). An average error 90% lower than that of the S-factor controller was obtained when using the UP controller.
Additional benefits of the new controller include easy inclusion of new measurements and clear relations between the tuning parameters used and the conditions in the digester.

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