The influence of temperature and pressure on sugar yields from sweet sorghum via subcritical water hydrolysis

This study investigates the effects of temperature and pressure on the conversion rate and efficiency of sweet sorghum bagasse during subcritical water hydrolysis. Batch experiments were carried out at 200, 240, and 280 °C under autogenous pressure (liquid–vapor equilibrium) and induced pressure (100 bar), allowing direct kinetic comparison under identical thermal conditions. At 200 °C, induced pressure significantly accelerated biomass conversion, resulting in faster depolymerization of cellulose and hemicellulose, a 25% higher peak cellulose conversion, and attainment of maximum biomass accessibility approximately 50 min earlier than under autogenous pressure. Under autogenous pressure at this temperature, conversion rates were lower and secondary products such as organic acids were favored. At 240 °C, biomass conversion proceeded rapidly and similarly under both pressure conditions, producing the highest fermentable sugar yields and indicating efficient carbohydrate conversion with limited degradation. At 280 °C, conversion rates were dominated by temperature, with overall biomass conversion efficiency reaching approximately 80% within 10–15 min. However, prolonged reaction times led to a decrease in measured conversion due to thermal degradation of sugars and formation of secondary products. Across the studied conditions, pressure influenced conversion rates only at the lowest temperature, while temperature was the dominant parameter governing reaction kinetics at 240 and 280 °C. These results demonstrate that pressurization above water saturation is unnecessary at elevated temperatures, enabling simplified operation with reduced energy demand and capital costs. • 240 °C maximizes fermentable sugar release. • Pressure affects hydrolysis only at 200 °C, not at 240/280 °C. • 280 °C subcritical hydrolysis degrades HMF and furfural, lowering yield. • Hydronium ions catalyze hydrolysis, peaking at 240 °C. • Saturated pressure cuts costs while maintaining high-temperature efficiency.