Hydrothermal Liquefaction of Concentrated Acid Hydrolysis Lignin in a Bench-Scale Continuous Stirred Tank Reactor

Abstract

Although numerous studies on the liquefaction of lignin for the production of high-yield and high-quality bio-oil have been performed in a batch reactor, studies using a continuous flow reactor are very rare. Herein, a bench-scale continuous stirred tank reactor (CSTR) was employed for the liquefaction of lignin for the first time. Lignin obtained using a two-step concentrated acid hydrolysis process from oil palm empty fruit bunch (EFB) was used as a feedstock. The batch reactor experiment was initially conducted to select the best solvent for lignin liquefaction and investigate the effect of a formic acid (FA) additive. A bench-scale experiment then was conducted to determine how the continuous process conditions can affect the yield and composition of bio-oil. Results showed that more effective depolymerization of lignin to bio-oil is possible in the CSTR, because of the fast heating rate. The water/ethanol mixture medium at 350 degrees C and 28 min of space time was found to be the optimum reaction conditions to obtain a relatively high yield (51.5 wt %) and low molecular weight (597 g/mol) of bio-oil. Syringol was the most abundant monomer in bio-oils, regardless of process conditions, and higher temperature (e.g., 350 degrees C) was found to promote demethoxylation and alkylation reactions to produce guaiacol and alkyl guaiacol. The addition of FA to the reaction mixture not only increased the bio-oil yield from 51.S% to 60% but also reduced the O/C molar ratio of bio-oil from 0.36 to 0.30, increasing its calorific value to 27.85 MJ kg(-1).