Modeling a continuous digester extraction screen zone with an approximated flow model

Abstract

Abstract Phenomenological continuous digester models are normally limited to axial flow. However, under industrial conditions, liquor also flows radially in extraction zones. While multidimensional expansion using computation fluid dynamics is possible, the increase in mathematically complexity and computation burden is intractable for online control and optimization. In this work, an approximate algebraic flow model is proposed which removes the need to evaluate momentum continuity. The model, based on streamlines, superimposes radial and axial flow profiles. The dynamic response, when compared against a single continuous stirred tank model, was found to better predict the process response during industrial trials. Simulation studies suggest enhanced lignin removal and variability reductions are possible by increasing recirculated liquor flow rates in the zone, a result of flattening the radial chemical and temperature profiles in the chip phase. However, a minimum occurs as chemical and energy losses in the extracted flow leaving the zone take precedence.