Kinetics of polysaccharide degradation during ethanol-alkali delignification of giant reed-Part 1. Cellulose and xylan

The degradation kinetics of the principal polysaccharides (cellulose and xylan) of the agro-fibre crop Arundo donax L. (giant reed) during ethanol-alkali delignification is reported. Based on the properties of a multi-component reaction system, the degradation kinetics of both polysaccharides was accurately described in terms of two simultaneous irreversible first-order reactions corresponding to removal of two kinetically homogeneous fractions. The moderate cellulose losses during pulping (about 4.5%) result mainly from the removal of the more reactive cellulose fraction, that accounted for 4% of initial cellulose. The bulk of the cellulose (96%) degrades slowly with three orders lower rate with pulping progress. The apparent activation energy of cellulose fractions degradation was estimated as 105.2 and 106.5 kJ mol−1, respectively. Substantial loss of xylan during pulping (about 55%, as a homoxylan) is caused by fast removal of the first very reactive fraction, covering about 48% of total xylan. The degradation rate of the second xylan fraction is only one order higher of the bulk cellulose degradation. The activation energy of xylan fractions degradation was found as 74.4 and 140.9 kJ mol−1, respectively.