Evaluation of sodium salt scaling in black liquor evaporators using existing process data

Erik Karlsson

doi:10.1515/npprj-2020-0038

Abstract

Abstract Scaling caused by sodium carbonate and sodium sulphate salts is a common problem during black liquor evaporation and mills currently have no proper tools to monitor or follow it up. The method proposed enables automatic evaluation of scaling rates and behaviours, together with washing performance, by using measurements that are already available at most mills, such as the boiling point elevation and the overall heat transfer coefficient. The method identified the scaling correctly in most cases, normally >90 %, although fast and sudden scaling proved to be challenging; its performance was also dependent on the quality of the data used. Historical data from three mills was evaluated and it was found that the scaling rate generally increased at higher concentrations of black liquor: one of the mills had almost no scaling below 70 % dry solids content. Another mill, however, deviated from this trend and had significant problems with fast and sudden scaling in one effect at around 60 % dry solids content. The correlation of process parameters revealed that this scaling was triggered, to a large extent, by the absence of tall oil brine, which is a side stream from the production of tall oil that is normally fed to the evaporators.

Highlights

Black liquor is an important process stream in a pulp mill
The following conclusions can be drawn regarding the evaluation method: – The evaluation method proposed makes it possible to quantify both the scaling rate and washing performance automatically from process data that is already available in most mills
It provides new possibilities for the monitoring and follow-up of the evaporator operation, allows scaling rates to be correlated with other process parameters and enables comparisons to be made with other mills. – The scaling rate can be calculated from the trends in the overall heat transfer coefficient with an accuracy above 90 % for most cases: the change point detection is better at detecting fast and sudden scaling, whilst fitting a five-parameter logistic model gives good filtering for determining the rate of scaling in general

Summary

Introduction

Black liquor is an important process stream in a pulp mill It is generated during the kraft pulping of wood and contains spent cooking chemicals and dissolved wood components. The effects are designed with larger heat transfer surfaces to allow operation even under scaled conditions and have an automated washing sequence installed. Even with these measures, scale formation can be so fast that it creates severe process disturbances leading to lost production capacity for the whole pulp mill (Verrill and DeMartini 2006, Karlsson 2017). The solubility is important for when the crystallization starts (Bialik et al 2008)

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Conclusion