Technical assessment of natural deep eutectic solvent (NADES) mediated biorefinery process: A case study

Abstract

Comprehensive evaluation of an integrated process for cellulosic ethanol production from lactic acid+choline chloride+water (LA-CC-H2O) mixture based natural deep eutectic solvent (NADES) pretreated rice straw; solvent recovery and its reusability; extraction of value added products viz., lignin and xylan; enzymatic saccharification and fermentation were studied. Four separate processes viz. pretreatment at 5% (w/v) and 10% (w/v) solids loading and enzymatic hydrolysis at 10% (w/v) and 25% (w/v) solids loading were investigated. No significant difference was observed in holocellulose recovery and lignin solubility in all the tested processes. Hydrolysis at 10% (w/v) and 25% (w/v) solids loading produced maximum glucose yields of 38.6±1.2g/L and 90.3±2.3g/L, respectively. Additional concentration step of sugars is needed from 10% (w/v) hydrolysis for direct ethanol fermentation using Clavispora NRRL Y-50464, while this could be voided with 25% (w/v) hydrolysis sugars. Maximum ethanol yield of 36.7g/L was obtained with 9% (w/v) glucose with a theoretical conversion efficiency of 79.9% which estimated to produce ~129L of ethanol per ton of raw biomass. Alongside, NADES pretreatment resulted in solubilization of 72-75% (w/v) of biomass bound lignin from which 86–90% (w/v) of lignin was recovered. Additionally, >98% (v/v) of the treated NADES reagent was recovered and reused in three consecutive biomass pretreatment cycles with no loss in pretreatment efficiency. This study clearly demonstrated that pretreatment at 10% (w/v) solids loading followed by enzymatic hydrolysis at 25% (w/v) solids loading is the most effective integrated biorefinery process using NADES pretreated rice straw.