385630 Kinetic Modeling of Acid Hydrolysis of Biomass Via Conventional and Non-Conventional Techniques
Conversion of biomass to biofuels, in the recent past, has been one of the most interesting topics to research on. Cellulose, the major constituent of biomass, forms a complex matrix with hemicellulose and lignin, and hence, requires pretreatment before it can be converted to fuels like bio-ethanol. Conventionally, acid hydrolysis combined with thermal treatment is being used to pretreat various biomass feedstocks and also to hydrolyse cellulose to fermentable sugars. This technique has proved to be very efficient in reducing the hemicellulose content and degree of polymerization of cellulose. Kinetics of cellulose acid hydrolysis is reported to be very different under dilute acid−high temperature and concentrated acid−low temperature conditions due to the heterogeneous and homogeneous nature of the reactions, respectively. In the work to be presented, a unified kinetic model has been developed to capture cellulose deconstruction under extremely low (0.07%) to high (70%) acid concentrations and high (225 °C) to low (25 °C) temperatures. A continuous distribution kinetic model has been developed by including (a) random mid-chain and specific end-chain scissions of cellulose to form cellulo-oligomers and glucose; (b) specific scission of cellulo-oligomers to form glucose and other low molecular weight cellulo-oligomers; and (c) glucose degradation reactions.
The model very well predicts the experimental data of concentration of cellulose, glucose, and other oligomers obtained from different studies in a broad range of acid concentrations and temperatures. In this study, rate constants of hydrolysis reactions were assumed to depend on acid concentration and temperature according to modified Arrhenius equation. It was found that under dilute acid-high temperature conditions the rates of cellulose and cellulo-oligomer deconstruction increased by 2% with every 1% increase in acid concentration, while under concentrated acid-low temperature conditions the increase was 2.5%. Glucose degradation under dilute acid-high temperature conditions was found to increase by 3.3% with 1% increase in acid concentration, while that under concentrated acid-low temperature conditions it was approximately 2.4%. However, the effect of temperature remained similar under any condition with nearly 2.4% increase in glucose degradation per 1% increase in temperature. It was also found that the degree of polymerization of initial cellulose affects the cellulose conversion, and thus, the yield of glucose. The model also manifests its applicability for cellulose hydrolysis in lignocellulosic biomasses like walnut green skin and yellow poplar wood. Ultrasound-assisted treatment is an emerging non-conventional method in biomass conversion that is used to enhance the mass transfer rates in reaction mixture and also to reduce the total reaction time. The insights obtained from the kinetic model of acid hydrolysis were extended to non-conventional deconstruction technique like ultrasound treatment and the interesting results obtained on the change of reaction mechanism with ultrasound will also be presented.