Kinetic modeling of the continuous hydrothermal base catalyzed depolymerization of pine wood based kraft lignin in pilot scale

In this work, the continuous hydrothermal base catalyzed depolymerization of kraft lignin to phenolic oligomers was investigated. Using response surface methodology, a model equation for the oligomer yield in dependence of the four parameters temperature (513-573 K), NaOH concentration (1.67 and 2.50 wt%), reactor volume (0.055 and 1.000 L) and mass flow (10-20 kg/h) was obtained. To determine rate constants of the depolymerization reaction, a three-lump kinetic model was proposed, which shows a good correlation between the predicted and the experimental values. In dependence on the sodium hydroxide concentration, this model resulted in activation energies of 37 and 22 kJ/mol for the depolymerization with 1.67 and 2.50 wt% NaOH, respectively. The model enables the determination of the reaction time, where the rate of depolymerization reaches its maximum over repolymerization, which has been shown strongly depending on the reaction temperature and sodium hydroxide concentration.