Modelling of Black Liquor Evaporator Cleaning - A case study of Skärblacka pulp mill

Examensarbete för masterexamen
Master Thesis
To maintain good operation of a chemical pulp mill, efficient removal of sodium scales attached to the heating surfaces in the black liquor evaporators is necessary. Increased knowledge about the cleaning process for the scales can lead to overall improved performance for the mill, with reduced production losses and increased heat economy as main advantages. Making the black liquor evaporation process more efficient will not only reduce the running cost of the plant, it will also influence the global environment in a positive way since resources are used in a more efficient way. Within this thesis, an existing modelling tool that simulates the dissolution of sodium scales in black liquor evaporators have been further developed and modified for industrial operation. The updated modelling tool enables the scale distributions to be determined from the boiling point rise, and the probability with each scale distribution to be verified via analysis of the heat transfer coefficient. A case study was done at BillerudKorsnäs’ mill in Skärblacka and a full scale test was performed where liquor samples and online process data were collected during one wash sequence. The aim was to utilize the updated cleaning model, together with data obtained from the case study, to gain fundamental understanding of the dissolution process of the scales in an effort to improve evaporator cleaning. Furthermore the work also aimed on being able to predict how the scales were distributed within the evaporator. It was found that the most reliable parameters to monitor were boiling point rise from online process data and dry solids content from laboratory analyses. The previous showed high sensitivity towards changes in scale thickness and was concluded to be the most effective parameter to monitor trends during the wash and to follow to determine it the wash is finished or not. Dry solids content from lab analyses on the other hand showed better performance to predict absolute values. Another finding made was that if it is desirable to have a short cleaning time, the evaporation rate should be low.
Energi, Hållbar utveckling, Annan kemiteknik, Energy, Sustainable Development, Other Chemical Engineering
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