Spent Sulfite Liquor Evaporator Fouling and Nitric Acid Cleaning, 2001 Pulping Conference Proceedings

L. A. Livingstone--Acid sulfite, bisulfite and neutral sulfite cooking of wood produce volatile fractions that include familiar compounds such as acetic acid, methanol, furfural, p-cymene and other terpenes. These volatile fractions are primarily released during digester blowing and spent sulfite liquor evaporation. Large quantities of steam, water vapour and, in the case of the acid sulfite cook and bisulfite cook, sulfur dioxide are simultaneously released. Latent heat is recovered in digester overhead condensers or in the evaporator train by downstream effects. In the condensing step, some of the volatile organics and SO2 are concentrated in the gas phase creating a condition for the formation of super-acids. The super-acids, in turn, catalyze the formation of reactive enols from ketones and aldehydes present in the volatiles. Aldehyde and enol groups can further undergo aldol-type condensation reactions with the elimination of water to form ethylene cross- links between carbon chains to ultimately form carbon-rich char deposits. Char deposits are difficult to remove chemically as they resist almost all solvents. However, they can be readily oxidized and removed with nitric acid. This can be done in a safe and controllable manner if procedures are followed to prevent nitric acid and concentrated SSL(or electron-rich char deposits) from mixing and reacting explosively.