Kinetics of Strong Acid Hydrolysis of a Bleached Kraft Pulp for Producing Cellulose Nanocrystals (CNCs)

Cellulose nanocrytals (CNCs) are predominantly produced using the traditional strong acid hydrolysis process. In most reported studies, the typical CNC yield is low (approximately 30%) despite process optimization. This study investigated the hydrolysis of a bleached kraft eucalyptus pulp using sulfuric acid between 50 and 64 wt % at temperatures of 35-80 degrees C over time periods of up to 240 mm for the production of CNCs. The experimental design captured the feature of the coexistence of a variety of reaction products, such as CNC, cellulosic solid residue (CSR), glucose, and xylose, in the product stream for accurate kinetic modeling to improve the CNC production yield. The kinetic model describing the solubilization of cellulose fibers used three phenomenological reactions, namely, hydrolysis of xylan to form xylose, depolymerization of cellulose to CNCs, and hydrolysis of cellulose to form glucose, each of which can be described by pseudohomogenous first-order kinetics. The concept of "degrees of hydrolyzable xylan or cellulose" to reflect the inhomogeneity of xylan or cellulose in hydrolysis was incorporated into the kinetic modeling to improve model accuracy. The developed model showed excellent predictability for CNC production. Both the experimental data and the model clearly indicate that CNC production was limited by cellulose depolymerization at low acid concentrations of below 58 wt %, but controlled by CNC degradation when the acid concentration was higher than 58 wt %. This work for the first time provides the most plausible description of CNC production kinetics, which is significant for the commercial production of CNCs.