Investigation of Liquor Flow in a Model Kraft Batch Digester

Q. F. Lee and C. P. J. Bennington

Department of Chemical and Biological Engineering, University of British Columbia 2360 East Mall, Vancouver, BC V6T 1Z3 Canada, Email: cpjb@chml.ubc.ca

ABSTRACT

Batch digesters are solid-liquid reactors used to produce chemical pulp from wood chips. The literature shows that the extent of reaction (delignification) varies as a function of chip location in the vessel, with the extent of non-uniformity commonly attributed to poor and/or non-uniform liquor flow through the digester (which causes poor chemical and heat distribution through the chip mass during the cook). Electrical resistance tomography (ERT) was used to evaluate the uniformity of liquor flow in a laboratory scale-model digester (a 1:15 geometrically-scaled vessel) with model particles used in place of wood chips (the vessel to particle diameter ratio was 93:1 to minimize wall effects), and close approximation of the liquor superficial velocity and pore Reynolds number. Local flow velocities were also measured for one common flow condition using ERT data and pixel-pixel cross correlation techniques, with the results compared with computational simulations made using a commercial CFD code.

The tomographic data shows that it is possible to create uniform zones in the digester, although a stagnation point was observed in the centre of the vessel at the screen level. This point coincides with the location of highest kappa numbers (the lowest extent of reaction) reported in industrial tests. Within the resolution of the tomographic technique, the local axial flow velocities were found to agree with calculated and computational results. The flow velocity data were used as input to a two-dimensional, mathematical model of reaction within the digester, with the variability of lignin distribution compared to available industrial data.

Keywords kraft pulping, batch digesters, electrical resistance tomography, liquor flow uniformity, computational modelling