Determination of activation energy and pre-exponential factor for individual compounds on release from kerogen by a laboratory heating experiment

Abstract

Kerogen recovered from Neogene Shinjo sediments was heated from room temperature to 800°C using a thermogravimetric analyzer (TG). Organic compounds released from kerogen during heating were trapped and analyzed by a gas chromatograph (GC)-mass spectrometer (MS). Among these compounds, benzene, hexane, toluene, phenol, heptane, and indene were selected to determine temperatures at which maximum release of these compounds occurred based on TG-MS peak profiles. This analysis was carried out using programmed heating rates (Hr) of 5, 10, 20, 30, and 60°C min^-1 and five temperatures at peak tops (T_max in absolute temperature) were determined for each compound. The plot of 1/T_max vs. ln(Hr/(T_max)²) for each compound showed a good straight line with a high correlation coefficiency. Accordingly, a mean activation energy (E₀) and pre-exponential factor (A) at E₀ for each compound on the release from kerogen were calculated, using the equation of the approximate method (or Gaussian E-distribution method) by Braun and Burnham (1987), ln(Hr/(T_max)²) = −E₀/RT_max + ln(AR/E₀) where R is the ideal gas constant. The activation energies and pre-exponential factors of these compounds were in a range of 43.1 to 60.9 kcal/mol and of 3.56 × 10¹⁰ to 1.21 × 10¹⁶ s^-1, respectively. The averages of the activation energies and pre-exponential factors obtained from the five kerogens had decomposition rates in order of indene > benzene, toluene > phenol > hexane, heptane, which indicated that hexane and heptane were more strongly bound to kerogen than the three aromatic hydrocarbons.