STEADY SHEAR FLOW PROPERTIES OF HIGH SOLIDS SOFTWOOD KRAFT BLACK LIQUORS: EFFECTS OF TEMPERATURE, SOLIDS CONCENTRATIONS, LIGNIN MOLECULAR WEIGHT AND SHEAR RATE

The steady shear flow properties of several softwood kraft black liquors (slash pine) from a two level, four variable factorially designed pulping experiment were determined for solids concentrations from 50% to 85%, temperatures from 40°C to 140°C and shear rates up to 10,000 s-1 by using Instron capillary and Haake coaxial cylinder rheometers. It was shown that the slip velocity at the wall of the capillary is insignificant and that a two capillary method can be used to determine the viscosity of the samples. At high solids, black liquor can exhibit non-Newtonian behavior dependent upon temperature, solids concentrations, solids composition and shear rate. In general, the liquors behave as pseudoplastic fluids. The exact level of viscosity at any given condition is dependent upon the solids composition which will vary from liquor-to-liquor. The flow behavior of the liquors was described using power-law, Cross and Carreau-Yasuda models. Superposition principles developed for polymer melts and concentrated polymer solutions were applied to obtain reduced correlations for viscosity behavior of the liquors. By using a suitable reference temperature, related to the glass transition temperature of black liquors, a generalized WLF type shift factor was obtained for the liquors used in this study and can be used to obtain a reduced plot of viscosity behavior of other black liquors.