Optimization of wet oxidation pretreatment of wheat straw

The wet oxidation process (water, oxygen and elevated temperature) was investigated under alkaline conditions for fractionation of hemicellulose, cellulose, and lignin from wheat straw. At higher temperature and longer reaction time, a purified cellulose fraction (69% w/w) was produced with high enzymatic convertibility to glucose (66% w/w). At 185°C, nearly three times more hemicellulose was solubilized than at 150°C. Optimum conditions for hemicellulose solubilization and cellulose convertibility were for 60 g l−1 straw: 185°C, 6.5 g l−1 Na2CO3, and 12 bar O2 with a 15-min reaction time. Under these conditions, 55% of the lignin and 80% of the hemicellulose were solubilized, while 95% of the cellulose remained in the solid fraction. At 185°C, the reaction kinetics was of pseudo first-order. The rate constant for hemicellulose solubilization was higher than that for lignin, whereas the rate for cellulose was very low. The cellulose recovery (95–100%) was significantly higher than that for hemicellulose (60%). At temperatures above 185°C, recoveries decreased due to increased degradation. Only half of the COD-content could be accounted for by saccharides and carboxylic acids; hence, a significant proportion of reaction products remained unidentified.