Alkali Consumption in Kraft Pulping of Softwoods, 1991 Pulping Conference Proceedings

One technique suitable for closed loop control of kraft digesters involves monitoring the residual alkali content of black liquor in real time. This is done commercially using continuous conductivity measurements or by conductometric titration on liquid samples. This paper discusses the relationship between alkali concentration and pulp composition and yield during kraft pulping. The residual alkali [OH-] is shown to be related to the lignin content (xl) and yield (Y) of the pulp by an equation of the form:

[OH-] = (e+fxlY+gY)/(d-Y)

where the parameters d, e, f, and g are related to the initial composition of the pulp, stoichiometric reaction coefficients, and the operating variables for the digester. The model is applied to literature data for pulping Red Spruce, Scotch Pine and Eastern White Cedar. Estimates are made for values of the stochiometric coefficients for reaction of hydroxyl ion [OH-] with the extractive, carbohydrate and lignin fractions of these species. Controlling kraft digesters by simultaneously monitoring residual alkali and total organic carbon is also discussed.