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Measuring uniformity in kraft digesters using flow-following sensors Albadvi, Elham

Abstract

Measurements of pulp variability and temperature distributions within kraft digesters have been the subject of intense interest for many years. An extensive survey carried out on the previous approaches to measure digester variability has shown that data directly taken from within the digester, during the kraft cook is scarce. Moreover, the increased size of modern digesters is believed to reduce the flow uniformity within them, increasing the risk of pulp variability and highlighting the need to collect data from within the digesters themselves. This has motivated the development of a new method for measuring digester variability: the “SmartChip”. This device is a flow-following sensor package that measures and records the temperature directly within the digester during the kraft cook. This research proposes a theoretical model to describe the heat transfer mechanisms occurring within the digester and then uses the data captured by the SmartChip to measure the digester variability. Preliminary test trials were conducted to ensure that the SmartChip could withstand the harsh environment of the kraft cook. Subsequently, multiple SmartChips were deployed in a single cook to provide insight on the temperature variability and heat transfer mechanisms occurring within the digester and these experimental results were then be compared with the predictions of the proposed model. Moreover, by taking pulp samples in the vicinity of the SmartChip sensors, the relation between nonuniformity and temperature gradients is established. The SmartChips have been tested in two laboratory batch digesters and as expected, temperature was already well-controlled within them and little variability was observed in these small-scale devices. Moreover, a good agreement is found between the model predictions and the experimental results and the model is experimentally validated, showing that the heat transfer through the digester contents is by pure advection with almost negligible thermal energy required for the heating of wood chips. These findings further suggest that the SmartChip works well under the harsh conditions of the kraft cook and future work is warranted to develop the instrument for its use at the industrial scale, where the temperature fluctuations along the digesters are more noticeable.

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