Reaction model for the thermal dissolution of Texas lignite

A kinetic model for the thermal dissolution of Texas lignite from the Yegua formation at San Miguel, Texas has been proposed. The lignite was liquefied in a 1-liter batch reactor at three different temperatures (350, 360 and 385ºC) using tetralin as a hydrogen-donor solvent. The reaction times varied from 15 minutes to 2 hours under autogeneous pressure. Additional runs using programmed heating were performed. This allowed the activation of a particular step in the reaction sequence by soaking the system at a lower temperature level (240 and 275*0 for 8 hours) and then heating up to the desired temperature. This selective activation helped clarify the overall mechanism of this complicated reaction scheme.

Analysis of the kinetic data has indicated no shifts in the reaction curves (conversion versus time), henceforth a sequential reaction model C → 0 where C = Lignite C = Oils seems to be the prevalent reaction path. The kinetic parameters calculated using a first order model as proposed by Hill et al. gives an activation energy of 21 kcal/mol and a frequency factor of 2.5 ^ 10 min. These values were obtained using a nonlinear search that gave the best least square fit for the rate curves.

The activation energy for the lower temperature step (soaking) does not vary, but the frequency factors increased by 50%, indicating more frequent molecular collisions. (This phenomenon could be explained by the release of the macromolecules from the lignite matrix due to the lower temperature preheating.)