Patent ID: 7540937

Claim:
A method for the continuous cooking of cellulose pulp, comprising: providing an impregnation vessel in fluid communication with a digester, feeding a mixture of chips and process fluid to the impregnation vessel, impregnating the chips at a predetermined impregnation temperature T imp , feeding the impregnated chips together with a circulation fluid through a transfer line to a top separator on the digester, cooking the chips at a predetermined cooking temperature, T dig , where T imp is at least 20° C. lower than T dig , withdrawing a fraction of the circulation fluid at the top separator, returning the circulation fluid to a bottom of the impregnation vessel via a return line, withdrawing black liquor from the digester at a withdrawal location to a black liquor line that essentially maintains the cooking temperature, T dig , and essentially a full digester pressure with a first level of residual alkali, providing a heat exchanger in direct operative engagement with both the return line and the black liquor line for exchanging heat between the return line and the black liquor line, the heat exchanger cooling the black liquor in the black liquor line by 10-15 ° C. after passing through the heat exchanger, withdrawing more than 1 m 3 /ton wood and/or at least 25% of a total withdrawal for recovery (REC tot ) from the circulation fluid for recovery (REC 1 ), the heat exchanger heating the circulation fluid in the return line, leading the black liquor in the black liquor line to an upper section of the impregnation vessel after passing through the heat exchanger, the black liquor contributing to an establishment of a fluid-to-wood (f/w) ratio that exceeds 4 in the impregnation vessel, obtaining more than 40% of the f/w ratio in the impregnation vessel from the black liquor in the black liquor line, providing the circulation fluid in the return line with a second level of residual alkali, ensuring that the first level of residual alkali is at least 10 g/l higher that the second level of residual alkali, and consuming at least 60 kg NaOH per ton of wood of alkali in the impregnation vessel.