In-Digester Washing

Today, most continuous cooking systems, and also modern batch displacement

cooking processes, use a washing step before the digester discharge. At the end of

this step the temperature of the digester content is lowered to below 100 °C, thus

allowing on the one hand recovery of heat and on the other hand a gentle pulp

discharge.

The regime inside the digester is characterized by the moving (continuous) or

immobile (batch) pulp column and the wash liquor flowing from the point of feed

to the point of extraction. Because of the large dimensions of digesters it is critical

for in-digester washing to provide a homogeneous liquor distribution and to avoid

channeling.

Diffusion is the other critical factor to be observed. Even at the end of the cook,

the original structure of the wood chip column remains intact, with chip-sized

fiber conglomerates being surrounded by cooking liquor. As the displacement

front reaches a digester zone, wash liquor first displaces the free cooking liquor

between the fiber bundles. Then, as the surrounding concentrations change, the

5.6 Washing Equipment 557

cooking liquor compounds from inside the fiber bundles begin to diffuse into the

wash liquor. It is apparent that especially in such a situation, sufficient time must

be provided for diffusion to occur.

Laboratory investigations on batch digesters have shown that the standardized

Norden factor E10 for in-digester washing is about 2.5 for good displacement and

about 2.0 for channeling. These figures are valid for the total of dissolved solids

and for a dilution factor of 3. The E10 numbers for residual alkali are about 30%

higher than those of the dissolved solids. Within the range of typical dilution factors

for in-digester washing (1.5–3.0) the E10 numbers can be regarded as fairly

constant. The results from the laboratory investigations correlate quite well with

industrial experience. At good displacement, an E10 of 2.5 could be confirmed for

dissolved solids. Nevertheless, really bad mill-scale channeling can bring the E10

down to 1.5 and below [43].

The efficiency of Hi-Heat washing in continuous digesters depends largely on

the design of the cooking system. The borderline between cooking and washing is

frequently indistinct, as systems have been retrofitted over time for modified and

extended cooking. Due to the elevated temperature and long duration, original Hi-

Heat systems have featured E factors up to 9 and a special effectiveness related to

extractives removal.

558 5 Pulp Washing

Дата добавления: 2015-10-21