Method and a digester for the continuous cooking of wood raw material to cellulose pulp

A method for the continuous cooking of wood raw material for the production of cellulose pulp, comprising:

• feeding a wood raw material and a cooking fluid to a top of a continuous digester;
• establishing a cooking temperature of 130-170° in the digester while the wood raw material experiencing a retention time of at least 90 minutes at the cooking temperature;
• the wood raw material sinking continuously through the digester from the top down to a bottom of the digester in order to finally expelling the wood raw material from the bottom of the digester;
• arranging a first withdrawal position in the digester for the cooking fluid and arranging a second withdrawal position in the digester for the cooking fluid, the second withdrawal position being above the first withdrawal position;
• withdrawing cooking fluid at the first and second withdrawal positions, the wood raw material having experienced a first retention time in the digester at the first withdrawal position and a second retention time in the digester at the second withdrawal position, the first retention time being at least 10 minutes different from the second retention time;
• establishing a zone of a countercurrent or a concurrent flow in the digester between the first and second withdrawal positions;
• providing an openable shunt line extending between the first withdrawal position and the second withdrawal position, the shunt line being in a closed position;
• a differential pressure (ΔP) between the second withdrawal position and the first withdrawal position falling below a first predetermined threshold value or rising above a second predetermined threshold value;
• and
• opening the shunt line extending between the first and second withdrawal positions.