Countercurrent

COOKING

CONCURRENT

IMPREGNATION

CONCURRENT

COOKING

COUNTERCURRENT

IMPREGNATION

AND HEATING

EXTENDED

COOKING

Extraction

Liquor

White liquor

DILUTION /

DISCHARGE

STEAMING

Energy,

White liquor,

Wash filtrate

CHIPS

LIQUOR LIQUOR LIQUOR

Extraction

Liquor

LIQUOR

Energy,

White liquor,

Wash filtrate

Energy,

White liquor,

Wash filtrate

Extraction

Liquor

LIQU.

Fig. 4.147 Typical Lo-Solids process steps and flow regime [14].

4.2 Kraft Pulping Processes 389

390 4 Chemical Pulping Processes

The application of Lo-Solids pulping has led to reduced white liquor consumption

and improved washing efficiency in the digester. The latter can be attributed

mostly to higher extraction liquor flow rates. When running on a single extraction,

overloaded systems often experience screen limitations and countercurrent

flow restrictions. In such cases, when the digester cannot deal with the full quantity

of wash filtrate, the excess filtrate must by-pass the digester to the evaporation

plant. Such filtrate is lost for washing. Multiple extractions allow a larger total

flow of extracted liquor without excessive load on screens, and multiple wash filtrate

addition reduces the relative velocity of liquor and chip column in countercurrent

zones. It has been found that the improved chip movement, when

coupled with the increased extraction capacity, has boosted not only the washing

efficiency but also the digester capacity.

Further information regarding the general technological aspects of continuous

cooking is provided in Sections 4.2.8.3.4 and 4.2.8.3.5.

The Lo-Solids concept is undergoing continuing refinement in terms of accommodation

to cooking chemistry and reduced installation efforts. Someof the related technologies

are specifically addressing the requirements of a particular cooking application.

EnhancedAlkali Profile Cooking (EAPC) uses black liquor fromthe lower extraction,

together with white liquor, for impregnation [16,17]. The widely simplified process

configuration of the Downflow Lo-Solids concept is shown in Fig. 4.148.

Steam

Wash filtrate

Circulation transfer

White liquor

FILTRATE

PREHEATER

COOKING

HEATER

Pulp

Extraction liquor

Steam

Extraction liquor

Fig. 4.148 Typical Downflow Lo-Solids single-vessel hydraulic digester [8].

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