Patent ID: 7112256

Claim:
A method for the continuous cooking of chemical pulp to improve heat economy in the digester system, comprising: providing an impregnation vessel having an inlet defined therein, the impregnation vessel being in fluid communication with a digester via a transfer system, the digester having an outlet defined therein; feeding a mixture of cellulose chips and process fluid into the inlet of the impregnation vessel; impregnating the cellulose chips in the impregnation vessel at a predetermined impregnation temperature T imp ; feeding the impregnated cellulose chips to the digester through the transfer system; cooking the impregnated cellulose chips at a predetermined temperature T cook ; feeding pulp dissolved in the digester out through the outlet of the digester: withdrawing black liquor from the digester via a cooking withdrawal after partial or complete cooking of the cellulose chips; leading the withdrawn black liquor to the transfer system at a bottom of the impregnation vessel; mixing the withdrawn black liquor with the impregnated cellulose chips in the transfer system for transport onwards to a top of the digester; adding a part of the black liquor from the black liquor withdrawal to an addition point of the impregnation vessel without cooling the withdrawn black liquor, the black liquor having a withdrawal temperature T av , to a beginning of the transfer system, the black liquor having maintained essentially the temperature T av , to raise a temperature of a chips mixture in the transfer system; a transport fluid being continuously withdrawn from the chips mixture at an end of the transfer system, the transport fluid maintaining a transport temperature T transp ; and returning the transport fluid to the impregnation vessel without heating at a location that is upstream of the transfer system and the addition point of the black liquor wherein the black liquor from the black liquor withdrawal maintains a temperature of Tav that exceeds the temperature Ttransp of the transport fluid that is withdrawn from the top of the digester by at least 50°C., such that T av −T transp ≧5° C.