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