Improved Understanding of CO₂-Water Pretreatment of Guayule Biomass by High Solids Ratio Experiments, Rapid Physical Expansion, and Examination of Textural Properties

In this work, we provide a systematic study of CO₂-water pretreatment of guayule biomass to optimize the residual ground bagasse from natural rubber extraction for hydrolysis and fermentation. Guayule biomass is mixed with water then loaded into a 250 mL reactor with exposure to a biphasic environment consisting of a CO₂-rich vapor phase and water-rich liquid phase. The pressure is then rapidly released for a "physical expansion" effect. The pretreated biomass is enzymatically hydrolyzed, and the sugar concentration in hydrolysate is measured. Experimental runs are conducted in the temperature range of 145 to 210 °C and pressure range of 3.4 to 34 MPa. The solids ratio (dry solids mass/water mass) is between 0.17 and 1.7. The packing density is between 0.03 and 0.2 g of biomass per cm³ of the reactor, and the holding time ranges from 20 to 840 min. A 4-fold increase in the reactor volume is performed and optimized with a 1-L vessel. High-pressure carbon dioxide-water pretreatment increases surface area of guayule biomass, introduces ruptured morphological features, and improves enzymatic digestibility. We achieve a total sugar yield of 85% (of theoretical) at two different reactor sizes of 250 mL and 1 L. At the smaller reactor, the optimum operational condition is 180 °C, 26 MPa, and 0.5 solid ratio. At the larger reactor, the optimum operational condition is 200 °C, 12 MPa, and 0.33 solid ratio.