The influence of cooking conditions on the degradation of hexenuronic acid, xylan, glucomannan and cellulose during kraft pulping of softwood

The influence of hydroxyl ion concentration, hydrogen sulfide ion concentra tion, ionic strength and temperature on the dissolution/degradation of hexe nuronic acid, xylan, glucomannan and cellulose in kraft pulping of Norweiga n spruce :vas studied. A mathematical model of the dissolution/degradation of hexenuronic acid was developed. Approximate rate constants for glucomann an and xylan were also developed. The rate of dissolution/degradation of he xenuronic acid, xylan and glucomannan increases with increasing [HO-], incr easing [HS-] and increasing temperature. A higher ionic strength, estimated as [Na+], leads to a slight decrease in the rate of degradation of xylan a nd an increase in the rate of degradation of hexenuronic acid. The lower he xenuronic acid content found when pulping with a high [HO-], a high ionic s trength, a low temperature and a low [HS-] is mainly a result of the prolon ged cooking time required to reach the same target kappa level, although th e prolonged cooking time seems not to be the only explanation for the lower hexenuronic acid content at a given kappa number.