Sulfur balance of sulfur dioxide-ethanol-water fractionation of sugarcane straw

The sulfur balance of SO₂-ethanol-water (AVAP®) fractionation of sugarcane (SC) straw was investigated. Hydrogen sulfite and sulfite anions are nearly absent in the liquors, despite cations present in straw, whose effect is thus limited to neutralization of lignosulfonic acids decreasing the acidity. Higher degree of sulfonation was observed for dissolved straw lignin compared to wood lignin (0.8–0.9 vs. 0.25 S/C9). Sulfur dioxide amounts of 0.9–1.2, 4.1–4.3 and 44–49 g per o.d. kg straw were bound to pulp, precipitated lignin and lignosulfonic acid, respectively, while the rest of SO₂ was recovered by distillation from the spent fractionation suspensions.