Kraft lignin depolymerisation in sub- and supercritical water using ultrafast continuous reactors. Optimization and reaction kinetics

Abstract

Kraft lignin was rapidly depolymerised in a continuous reactor using sub- and supercritical water. The reaction yielded an aromatic oil rich in high value aromatic monomers such as guaiacol, vanillin, acetovanillone and homovanillic acid. Different temperatures between 300 and 400 °C and reaction times from 60 ms were studied. An increment in reaction time at every temperature studied promoted secondary reactions of repolymerisation of the lignin products. Those undesired reactions were more relevant as temperature increased. An optimal temperature of 386 °C at a reaction time of 170 ms was found, with a maximum yield in phenolic compounds obtained. At those conditions, a bio-oil containing low molecular weight compounds was obtained with a yield of 44.6 %. A selectivity to the four cited monomers of 9.9 % was achieved, with little formation of solid residue (3.5 %). A simple kinetic model was proposed to describe the yield for the different fractions and fitted to experimental data.