Evaluation of Sodium Salt Scaling in Falling Film Black Liquor Evaporators, 2000 Pulping / Process & Product Quality Conference Proceedings

J. Smith, J. S. Hsieh--Over the last several decades the pulp and paper industry has continued to improve its manufacturing operations. Despite these efforts inorganic salt scaling in black liquor evaporators continues to be one of the most persistent problems encountered in the pulp and paper industry. Black liquor, a viscous alkaline organic waste product from the wood digestion process, contains relatively high levels of sodium carbonate and sodium sulfate that will precipitate from the solution when the total solids concentration reaches approximately 50%. The precipitated salts, typically in the form of burkeite (2Na2SO4:Na2CO3), can adhere to the heat transfer surface and drastically reduce the overall efficiency of the evaporator.

This research utilized both a tube and plate falling film evaporator to quantify the effects of operational and compositional variables to aid in reducing scaling. Specifically it has been found that increasing the circulation rate of the evaporator can reduce scaling by as much as 42%. Decreasing the steam pressure, and therefore the evaporator temperature, demonstrated a 27% decrease in scaling. Finally, the degree of supersaturation in the liquor has been found to be a critical factor in sodium salt scaling from black liquor scaling. A 5% decrease in the solubility limit, and therefore an increase in the degree of supersaturation, has led to a four fold increase in scaling. Combining these results has provided insight into operational changes that may reduce the scaling problem in the pulp and paper industry.