Thermal characterization of crude oils in the presence of limestone matrix by TGA-DTG-FTIR

Abstract

In present work oxidation of two heavy oils in limestone matrix was studied using simultaneous thermogravimetry (TGA), derivative thermogravimetry (DTG) and FTIR-spectroscopy techniques in the temperature range from 25 to 900°C. Before the measurements, the composition and properties of crude oils and limestone were evaluated. Obtained TG and DTG curves shows four different reaction regions: low temperature oxidation (LTO), fuel deposition (FD), high temperature oxidation (HTO) and decomposition of limestone. LTO reactions were accompanied by evaporation of light hydrocarbons, which was confirmed by appearance of stretching vibration bands of C-H groups in FTIR-spectra of evolved gases. Formation of carbon dioxide was observed for all oxidation reaction regions according to spectroscopic data. At the same time, CO was formed only in HTO region for both studied crude oils. Despite the different composition two crude oils have practically the same reactions intervals and peak temperatures. However, crude oil with higher API-gravity has a greater mass loss at the LTO and evaporation regions. The conversion of heavier oil with higher content of asphaltenes is larger during the high-temperature oxidation step.Three different kinetic models (Arrhenius, Coats & Redfern and Ingraham & Marrier) were used for analysis of TGA-DTG curves in LTO and HTO regions. Activation energy values of the crude oil samples were varied between 6.9–10.6kJ/mol in low temperature oxidation and 91.8-.0–181.9kJ/mol in high temperature oxidation regions. For two crude oils activation energies are similar in low temperature oxidation region. In high temperature oxidation region, crude oil with higher content of asphaltenes has larger activation energy.