Second-generation ethanol in Chile: optimisation of the autohydrolysis of Eucalyptus globulus

Abstract

A highly efficient pretreatment process to break down the recalcitrant structure of Eucalyptus globulus raw material to produce ethanol was investigated. A response surface methodology was used to optimise the pretreatment parameters. The inlet parameters were temperature and time, and the response parameters were the total glucose plus xylose yields and the glucose yield as a quantification of the susceptibility to enzymatic hydrolysis of the solid material. It was determined that the central point conditions (175 °C, 43 min) produced close to the optimum of the total glucose plus xylose yields, achieving 57 % (dry matter) xylose yield and 63 % glucose yield. However, the optimum of the glucose yield, 95 % (dry matter), in which almost all xylose was degraded, was achieved at 194 °C and 51 min, a condition close to a star point (N6). In addition, the maximum ethanol produced in this study was 262 L/ton of dry raw material at 194 °C and 51 min. The characterisation of the pretreated material enabled the correlation of glucose yield with physical, chemical and morphological properties of pretreated material. Enzymatic hydrolysis and the modified Simons’ stain technique indicated a similar increasing linear dependency with moderate severity factors. However, with high-severity factors, these findings indicated an inverse effect, suggesting that there is no dye retention in the modified material when no fibre-like structure remains. This behaviour was consistent with the physical and morphological appearances of the pretreated material, as demonstrated by scanning electron microscopy images, kink index and coarseness.