Modeling of chip bed packing in a continuous kraft cooking digester

This work focused on modeling of the chip bed packing phenomena in a continuous kraft cooking digester. A better understanding of chip bed packing would make it possible to optimize chip flow conditions in the digester, thereby ensuring uniform fiber quality and production efficiency. Chips are fed continuously into the digester, with the chip flow forming a solid bed. As the solid chip bed moves slowly downwards, cooking reactions proceed, which leads to softening of the chips. The softening of chips and the process conditions, flows and gravitational, are major variables affecting the chip bed packing. A new computer model, TKK Packing Simulator, was created to study the chip bed packing in a continuous digester. The simulator was verified against mill-scale conditions by conducting measurements in a continuous cooking plant. The packing simulator was found to give an accurate picture of the process flows inside the continuous digester. As digesters have grown bigger, controlling the liquor flows in the chip bed has become increasingly important. A tracer test was used to verify the liquor flow, which can be far from ideal in a modern digester. It is important to have optimal packing and flow conditions inside the digester, because too high packing and channeling of the liquor flow leads to non-uniform cooking, production disturbances and impaired fiber quality. The simulator developed in the present research can be used to determine optimal packing and flow conditions