Kinetics of polysaccharide degradation during ethanol-alkali delignification of giant reed-Part 2. Minor carbohydrates and uronic acids

The degradation kinetics of the minor neutral carbohydrates (composed of arabinosyl, galactosyl and mannosyl residuals) and uronic acid moieties of the agrofibre crop Arundo donax L. (giant reed) under conditions of ethanol-alkali delignification has been studied. Based on properties of multi-component reaction system, the degradation of arabinan, galactan and mannan (expressed as homopolymers) was accurately described in terms of two simultaneous irreversible first-order reactions, corresponding to two kinetically homogeneous polysaccharide fractions. The first and more reactive fraction accounted for about 30, 73 and 4% of total arabinan, galactan and mannan, respectively, and is rapidly lost during the first pulping period. The respective values of apparent activation energy were estimated as 65, 45 and 82 kJ mol−1. The second fraction slowly degraded during pulping with two-order lower rate (activation energy of 88-98 kJ mol−1). The degradation of uronic acids as well as the formation of hexenuronic acids was accurately described in terms of three simultaneous first-order reactions, corresponding to three kinetically homogeneous fractions. The overall rate of hexenuronic acid formation during ethanol-alkali pulping was found to be one-order lower of the overall rate of uronic acids degradation.