Statistical optimization of operating conditions for supercritical carbon dioxide-based pretreatment of guayule bagasse

A central composite design (CCD) was used to find the optimal temperature, pressure, moisture and duration for the supercritical CO2-based pretreatment of guayule, a desert shrub for commercial production of hypoallergenic latex. The pretreatment involved: adding water to the biomass, raising system temperature, pressurizing with supercritical CO2, holding the system for a period of time, and exploding the biomass by rapidly opening the vent valve. The pretreated biomass was then hydrolyzed at 30 degrees C for 72 h in an enzyme solution with 5% (w/v) solid loading. The yields of released glucose and pentose were determined and used as the CCD response variables. Statistical analysis of results led to the following recommended condition: 175 degrees C, 26.2 MPa (3800 psi), 60% moisture and 30 min. The corresponding glucose and pentose yields were 56% and 61%. X-ray diffraction analysis was done to get the crystallinity index of the bagasse before and after pretreatment. Scanning electron microscopy was also used to examine the structural changes caused by pretreatment. These characterizations indicated that at milder conditions, the pretreatment exposed cellulose and hemicellulose for subsequent enzyme attack; at harsher conditions, the pretreatment destroyed cellulose crystallinity and gave higher glucose yields. To isolate the explosion effect (caused by instantaneous pressure drop, Delta P) from the reaction (pressure) effect, a series of experiments was made with the bagasse pretreated at the same condition (27.6 MPa) but exploded against different back pressures. The explosion needed to be severe enough (with Delta P > 17 MPa) to give a sugar yield of over 50%. (C) 2012 Elsevier Ltd. All rights reserved.