Patent ID: 7217338

Claim:
A process for continuously cooking chemical pulp with improved pulp quality and increased yield, comprising: providing a digester system having at least one vessel for impregnating and cooking cellulose, the vessel having an inlet defined therein into which a mixture of chips and process liquid is fed, the vessel having an outlet defined therein, the vessel having an extraction position disposed between the inlet and the outlet, the chips and process liquid flowing in a flow direction, impregnating the chips at a predetermined impregnation temperature, T imp , cooking the chips at a predetermined cooking temperature, T cook , for at least 60 minutes, the chips passing through treatment zones in the digester system, continuously extracting process liquid from an extraction point, retaining the extracted process liquid in a buffer tank positioned outside the digester system during a dwell time (t) of at least 30 min, while in the outside buffer tank, maintaining the extracted process liquid at a temperature T return which is in an interval T min −T max , where T min =100° C. and T max =140° C., while maintaining the extracted process liquid in the buffer tank, cleaving hexenuronic acid (Hex-A) from xylan; returning the retained process liquid to the extraction point from which the extracted process liquid was extracted, or downstream thereof, mixing the returned process liquid with process liquid in the treatment zones of the digester system, re-precipitating xylan, received from the buffer tank, in the digester system, while the cleaved hexenuronic acid remaining in solution in the process liquid, and feeding out cooked pulp through the outlet.