Kinetics of sulfur dioxide-alcohol-water (SAW) pulping of sugarcane straw (SCS)

Abstract

Pulping kinetics of sugarcane straw (SCS) using the sulfur dioxide (SO2)-alcohol-water (SAW) system were investigated with different solvents (methanol, ethanol, and isopropanol) in order to evaluate the effect of the solvents. The methanol, ethanol, and isopropanol SAW pulps are identified as SMW, SEW, and SPW, respectively. The pulping experiments were carried out at 135°C, 145°C, and 155°C with cooking liquor (L/F = 4 L/kg feedstock) composition; SO2/alcohol/water of 12/44/44 w%. The pulps were characterized in terms of yield, kappa number, viscosity, cellulose, and hemicellulose and lignin content. After a fast initial dissolution phase, two regimes of delignification were identified in all solvent systems: a bulk and a residual phase. Bulk delignification and xylan removal followed first order kinetics in residual lignin and xylan, respectively, while the cellulose hydrolysis rate was zeroth order in cellulose. Linear relationships between Klason lignin content and kappa number were developed for each solvent system after correcting the acid insoluble lignin for ash and non-oxidizable matter. The corrected lignin content on original SCS was used to develop the bulk delignification kinetics. Rate constants and activation energies for bulk delignification, xylan removal, and cellulose hydrolysis were evaluated for each pulping system. In terms of degree of delignification, the pulping systems followed the sequence of: SMW <SEW <SPW. Bleachable grade pulps with acceptable intrinsic viscosity (>800 ml/g) were produced in SEW and SPW systems, while the lower rate of delignification and higher rate of cellulose hydrolysis in SMW yielded much lower viscosity pulps of higher kappa number at the same pulping temperature and time. The results are explained in terms of the effective acidity and lignin/carbohydrate dissolution properties of the pulping liquors.