Modified Continuous Cooking (MCC)

Modified Continuous Cooking [9,10]was the first in a string of alterations

imposed on the conventional continuous pulping process. A typical configuration

of an MCC single-vessel hydraulic digester is shown in Fig. 4.141. The chips enter

the top of the digester together with the top circulation liquor, and are fed to the

top separator, which is a screw conveyor surrounded by a cylindrical screen. The

vertical screw transports the chips downwards and also keeps the slots of the

screen clean. Circulation liquor is extracted through the screen and returned to

the chip feeding system, where the largest portion of the white liquor is added.

The excess liquor from the top circulation travels downwards concurrently with

the chips and enters the impregnation zone (see also Fig. 4.142).

Impregnation is typically performed at a temperature between 115 and 125 °C

and a pressure above 10 bar(g) for 45–60 min. As the chips approach the first

screen section, liquor is displaced horizontally from the central pipe discharge

through the chip column to the strainers, and is then circulated back to the central

pipe via the concurrent cooking heater. A small portion of white liquor is added to

the cooking circulation loop. The heater is operated with indirect steam and the

hot liquor introduced into the digester brings the temperature of the chip column

up to the cooking temperature of 150–170 °C.

Steam

Steam