Assessing phase behavior of pyrolytic lignin in fast pyrolysis bio-oil via advanced distillation curve

Fast pyrolysis bio-oil (FPBO) holds promise as a renewable source for fuels and chemicals, yet its complex chemical composition poses significant challenges for efficient product recovery design and further downstream separation. In particular, the high molecular weight pyrolytic lignin fraction remains difficult to characterize, as its composition is largely unknown and its thermodynamic behavior poorly described. To improve understanding, the vapor-liquid equilibrium (VLE) behavior of FPBO was investigated, focusing on the pyrolytic lignin. A lignin-derived FPBO was selected for the study in order to minimize the influence of carbohydrate components of biomass. Advanced distillation curve (ADC) experiments were conducted to obtain thermodynamic data, which was then compared to simulations in which the FPBO was modeled as a surrogate mixture. To represent the pyrolytic lignin (PL) in the mixture, a range of surrogate molecules, from dimers to tetramers with varied inter-unit linkages and functional groups, were evaluated to find which structure best represents the PL in VLE calculations. Among the structures tested, dimers featuring biphenyl inter-unit linkages provided the best overall agreement to the experimental values. ... mehrIn general, dimers were more suitable for simulation, as some of the trimers and tetramers faced convergence issues and simulation errors. The effect of thermodynamic model was also taken into consideration, comparing the Ideal equilibrium model, the UNIFAC-Dortmund (DMD) activity coefficient method, and the Peng-Robinson Boston-Mathias (PR-BM) equation of state. The findings underscore the importance of surrogate and model selection and provide guidance for optimizing FPBO fractionation and upgrading.