Effect of pH, temperature, and solids content on rheological properties of wheat straw black liquor

Soda black liquor lignin is extracted from wood chips during the soda pulping process and ends up in the residual pulping liquor after brownstock washing. This liquor is commonly concentrated from approximately 15–20% to 70–80% solids which can be used as fuel to replace fossil fuel. In this work, the effect of temperature, pH, and solids content as process variables on the viscosity is analyzed. The viscosity of black liquor generated from soda pulping of wheat straw was optimized using Box-Behnken design (BBD) response surface method with numerical optimization technique. The results show that they have significant influence on the viscosity of black liquor. The experimentally observed data was analyzed through ANOVA. It is observed that the best optimum conditions for minimum viscosity (0.055 mPas) of black liquor were temperature (343 K), pH (7), and solids content (37%) respectively. The multiple regression analysis was used to develop second-order polynomial model equation, which fitted well and predicted the experimentally observed data within acceptable limits. The value of regression coefficient (R²) was high as near to unity. The viscosity is sensitive function of temperature, pH, and solids content. The black liquor viscosity plays major role on effectiveness of all operations used to process the black liquor through storage tank, multiple effect evaporators, concentrator, and its firing in the chemical recovery furnace. This study may be contributed for process equipment design and energy efficient operations of paper mill.