Soap Skimming

The soaps which are generated during softwood pulping can cause foaming and

displacement problems if recycled back to the digester in a displacement cooking

plant. Skimming of soap is a tricky undertaking because the separation of soap

from the liquor occurs in narrow ranges of dry solids and pH. Usually, soap

removal functions best from a liquor returning to the atmospheric black liquor

tank. This liquor must then be allowed enough retention time for soap to come

afloat. A dedicated, separate soap separation tank has proven most effective. The

segregated soap is then pumped to the evaporation plant in a separate line or together

with the black liquor.

Gas Management

The gases vented from apparatus and equipment in the digester plant contain malodorous

compounds, and must be collected for reasons of emission control and maintaining

an acceptable workplace environment. Atmospheric tanks are connected to

the high-volume low-concentration (HVLC) gas collection system. Such tanks include

the atmospheric black liquor tank, the wash filtrate tank, and the pulp discharge

tank. The air evacuated during the chip fill goes also to the HVLC gas system.

4.2 Kraft Pulping Processes 371

Besides non-condensable constituents, the gases vented from the pressurized

tanks in the tank farm contain large amounts of moisture due to their elevated

temperature. They are therefore passing condensation before proceeding to the

low-volume high-concentration (LVHC) gas system. When the digester plant is

being used to process softwood, the condensate contains turpentine, which is separated

from the condensate by decanting.

Rapid Displacement Heating (RDH)

A typical RDH tank farm consists of three pressurized accumulators and two atmospheric

tanks (Fig. 4.130). The liquor accumulators A, B, and C are staged in

temperature. Fresh alkali is provided from the hot white liquor tank. Steam is

used for top-heating of white liquor in the tank farm and for bringing the digester

to cooking temperature by indirect condensation.

HBL

"C"

WBL DIG WF

"B"

BL

"A"

HWL

WHITE LIQUOR

CHIPS

PULP

BLACK

LIQUOR

WASH

FILTRATE

WW

HW

STEAM

STEAM

Fig. 4.130 Simplified Rapid Displacement Heating (RDH) process

flowsheet. BL = black liquor; WBL= warm black liquor;

HBL= hot black liquor; HWL= hot white liquor; DIG = digester;

WF = wash filtrate; WW = warm water; HW = hot water [1].

The RDH cooking cycle is shown schematically in Fig. 4.131. After chip filling,

warm black liquor (WBL) of up to 130 °C from the B accumulator is used for

impregnation. Then, hot white liquor (HWL) and hot black liquor (HBL) from the

C accumulator are charged to the digester during hot displacement. The displaced

liquor is first returned to the A tank and then to the B accumulator. After hot displacement,

the digester is brought to cooking temperature by indirect steam heating.

Circulation is sometimes stopped after the target temperature has been

reached. Subsequent to cooking, wash filtrate (WF) displaces the hot black liquor

first into the hot black liquor accumulator C and then to the warm black liquor

372 4 Chemical Pulping Processes

PULP

WF

STEAM

WF

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