Patent ID: 7828930

Claim:
A method for Kraft pulping employing a modified cooking process in conjunction with polysulfide pulping technologies in a cooking vessel to obtain higher pulping yields than previously obtained, wherein: a first quantity of cooking liquor is extracted from the cooking process at a first point in the modified cooking process and exchanged with a second quantity of cooking liquor with an effective alkali concentration lower than the first quantity of liquor extracted from the modified cooking process at another point, wherein: the total required cooking alkali charge, in the form of polysulfide liquor, is added to the beginning of the cook, and after essentially all polysulfide has reacted with lignocellulosic material at temperature around or below 135° C., at or below which no significant carbohydrate degradation occurs, said first quantity of the cooking liquor is removed from said first point in the cooking process and replaced with said second quantity of cooking liquor, the second quantity of cooking liquor being one of equal to, greater than, or smaller than the first quantity, and the removed first quantity of cooking liquor is then added elsewhere in the pulping process, where the effective alkali concentration is low, whereby the full yield benefit from polysulfide pulping is realized while at the same time a more even effective alkali concentration profile is achieved to retain benefits of modified cooking.