Cooking

The impregnation step passes into the cooking stage as the free liquor is displaced

from the chip column by hot cooking liquor. Note that the liquor in the digester

can be divided into a free portion moving between the chips and a bound portion

trapped within the chip volume. Only the free liquor is available to displacement,

whereas the bound liquor is accessible by diffusion and thermal conduction.

The cooking liquor is distributed by the central pipe discharge and flows radially

towards the cooking circulation screens. It is essential that the circulation flow

rate is sufficiently high to ensure uniform chemical and temperature profiles

across the digester cross-section. This is of critical importance also for the other

circulation operations in the digester. Otherwise, different radial temperature

and/or chemical levels prevail that lead to inhomogeneously cooked fibers.

Cooking usually occurs in more than one zone. As the chip column moves

down, the alkali, temperature and solid levels in the digester can be adjusted by

introducing white liquor or wash filtrate into circulation loops, by indirect heating

of the liquor in those loops, or by extracting spent liquor for chemical recovery.