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 concentration, ionic strength and temperature on the dissolution/degradation of hexenuronic acid, xylan, glucomannan and cellulose in kraft pulping of Norweigan spruce :vas studied. A mathematical model of the dissolution/degradation of hexenuronic acid was developed. Approximate rate constants for glucomannan and xylan were also developed. The rate of dissolution/degradation of hexenuronic acid, xylan and glucomannan increases with increasing [HO(-)], increasing [HS(-)] and increasing temperature. A higher ionic strength, estimated as [Na(+)], leads to a slight decrease in the rate of degradation of xylan and an increase in the rate of degradation of hexenuronic acid. The lower hexenuronic acid content found when pulping with a high [HO(-)], a high ionic strength, a low temperature and a low [HS(-)] is mainly a result of the prolonged cooking time required to reach the same target kappa level, although the prolonged cooking time seems not to be the only explanation for the lower hexenuronic acid content at a given kappa number.