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Application of a H-Factor control on the continuous digester: Case Study

Kozana Mokebe and Cephas Mutize Mondi Packaging South Africa (Pty) Ltd. Springs Mill .

Batch and continuous digester cooking degree is controlled by using the actual pulp residual lignin content measured by a hypo number. This means that there is a time delay equivalent to the time taken for the chip to traverse the whole digester. There is a need for real time and reproducible control of the digester cooking process, which is cannot be achieved by the present hypo number.

The literature suggests the H-factor as a possible solution. To get to the solution of using H-factor to control the cooking, the H-factor equation on the Distributive Control System (DCS) is verified using the original Vroom's definition of H-factor as a standard [1]. The comparison between the Hypo number and H-factor then follows. Correlation between the H-factor and Hypo number means that H-factor can be used for digester control and vice versa.

The two point Gauss Quadrature rule is used to estimate the indeterminate value of the Vroom's H-factor equation in the heating zone of the digester. It is found that there is an error in the DCS H-factor expression compared with the derived but similar to a certain extend. It was also found that there is no distinct relationship between the Hypo number and the H-factor, making the control of the continuous digester to use the operating window. This was attributed to the unsystematic tweaking of variables during operation to achieve a set point. The data used in this investigation is only in the operator set region, clustering of points in this region is so expected. The use of a model of the digester to get the unavailable data is recommended. The use of neural networks is recommended to make this predictive model.

Hypo number; H-factor; Gauss Quadrature Rule and Semi-Alkaline Sulphite with Anthraquinone (SAS-AQ)

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