June 16-21, 2019
In this presentation a comprehensive microkinetic modelling framework and experimental tools are used to describe product yield and composition of direct lignin liquefaction processes with and without solvents (See Figure 1). With the framework proposed we aim to develop a unified theory and models capable of describing both dry (pyrolysis) and wet (hydrothermal and solvolysis) lignin liquefaction processes. An important phenomenon that has been shown to occur during lignin pyrolysis (as well as cellulose) is the formation of a liquid intermediate phase, and subsequent ejection of heavy products (>~250 Da) as aerosols from this intermediate. In our presentation we will focus on the nature of lignin pyrolysis liquid intermediate through analysis of phase change equilibria temperatures for relevant lignin fragments, using group contribution methods. Specifically, estimation of boiling (Tb) and melting (Tm) points of lignin fragments was done using ARTIST software (Dortmund Data Bank Software & Separation Technology, GmbH). In total, 50 different lignin fragments were drawn, and their boiling and melting temperatures were calculated. The 50 fragments include monomers, dimers, trimers and tetrameters, with a variety of H, G and S units and inter-unit linkages. Figure 2 shows the calculated phase-change equilibria temperatures plotted against the number of aromatic units in a given lignin fragment. The dotted line at 400 °C is included as the approximate temperature at which both rupture of aliphatic linkages and conversion of short aromatic ring substituents occurs, but is less than the temperature for rearrangement of polycyclic structures. The collection of lignin fragments, such that Tm < 400 < Tb, make up the set of molecules that can exist as a liquid intermediate during pyrolysis, and are therefore the ones that have potential to be ejected as aerosols. The average lignin fragment in this range has 2.50 (± 0.11, standard error) aromatic units, molecular weight of 414 (± 20) Da, melting point of 292 (± 13) °C, and boiling point of 573 (± 19) °C. Relying solely on this analysis, one would expect these to be characteristics of an average molecule ejected as a liquid-phase aerosol during pyrolysis of lignin. Based on the quantification of phase equilibria temperatures, this liquid state can contain dimers and trimers, but typically not tetrameters or larger (they will preferentially depolymerize), or monomers (they will vaporize). It is these dimer and trimer products that should make up the majority of the heavy liquid products collected as aerosols. In order to validate this model, comparison was made with previously published work from Pecha, et al. (Ind. Eng. Chem. Res., 56, 2017, 9079-9089) and Bai, et al. (Fuel, 128, 2014, 170-179), who analyzed lignin pyrolysis oil with FT-ICR-MS. There is good agreement between the weights of species detected experimentally in these studies and those determined in this work based on group contribution calculations.
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Manuel Garcia-Perez, Evan Terrell, and Linda Broadbelt, "Challenges and progresses made on the microkinetic description of lignin liquefaction: Application of group contribution methods" in "Pyroliq 2019: Pyrolysis and Liquefaction of Biomass and Wastes", Franco Berruti, ICFAR, Western University, Canada Anthony Dufour, CNRS Nancy, France Wolter Prins, University of Ghent, Belgium Manuel Garcia-Pérez, Washington State University, USA Eds, ECI Symposium Series, (2019). https://dc.engconfintl.org/pyroliq_2019/48