Organosolv Fractionation of Sorghum Stubble for Integrated Biopolymer Recovery: Structural Characterization and Preliminary Mass–Energy Assessment

Agricultural residues represent an abundant renewable resource for sustainable lignocellulosic biorefineries, offering opportunities to produce value-added biopolymers while mitigating the environmental impacts associated with conventional disposal. This study evaluated the technical feasibility of using sorghum stubble as a feedstock for the recovery of cellulose, hemicellulose, and lignin through an ethanol–water organosolv process. Specifically, the study aimed to separate the main structural polymers of sorghum stubble and evaluate the performance of the proposed fractionation system. The biomass was treated at 180 °C and approximately 30 bar, followed by chlorine-free peroxide bleaching. The hemicellulose fraction was obtained by extracting the holocellulose (Wise method) and precipitating the dissolved hemicellulose using a 3:1 (v/v) ethanol-to-liquor ratio. The lignin fraction was isolated from the black liquor through vacuum distillation and drying. This approach yielded a cellulose fraction (23.66 wt%), a hemicellulose fraction (36.22 wt%), and a lignin fraction (30.35 wt%), corresponding to an overall polymer recovery of approximately 90 wt%. Structural and physicochemical characterization (FTIR, SEM, XRD, and TGA) supported the fractionation of the lignocellulosic matrix and identified characteristic structural and physicochemical features of the recovered cellulose, hemicellulose, and lignin fractions. SEM revealed substantial structural breakdown following the organosolv treatment, while TGA demonstrated distinct degradation behaviors among the components. Notably, the crystallinity index increased from 38% in the raw material to 71% in the cellulose fraction, indicating effective removal of amorphous components. A preliminary mass–energy analysis showed that approximately 96% of the initial energetic content of the biomass was retained in the polymers. Furthermore, approximately 70% of the ethanol used was recycled, highlighting the potential for solvent reintegration under the evaluated laboratory conditions. Overall, these findings demonstrate the potential of sorghum stubble as a renewable feedstock for the recovery of lignocellulosic polymers and provide a technical basis for the future optimization of organosolv-based biorefineries.