Pyrolysis behaviour of petroleum pitches prepared at different conditions

Abstract

Three petroleum pitches were prepared by applying three different treatments to the same petroleum feedstock: (i) distillation, and (ii) two different thermal treatments. Pitch composition was monitored by standard procedures (elemental analysis, solubility parameters, softening point (SP), carbon yield, etc.), and chromatographic (gas chromatography and high performance liquid chromatography), spectroscopic (Fourier transformed infrared spectroscopy) and rheometric techniques. Pitch pyrolysis behaviour was studied by thermogravimetric analysis (TG/DTG), differential scanning calorimetry (DSC) and optical microscopy of the pitch pyrolysis products obtained at 450 °C and pitch cokes. The results show that the preparation conditions affect the structure of pitch components considerably. Distillation gives rise to a high SP pitch enriched in alkyl- and aryl-substituted aromatic compounds. Pitches obtained by thermal treatments underwent a certain degree of polymerization and a substantial amount of the alkyl- and aryl-substituted aromatic compounds present in the petroleum feedstock were transformed into planar aromatic cata-condensed compounds. The pyrolysis behaviour of pitches clearly reflects the differences in chemical composition, and consequently, in the preparation conditions. In the pitch obtained by distillation, the weight loss and thermal phenomena associated with polymerization reactions are much more pronounced than in pitches obtained by thermal treatments. On the other hand, under the same experimental conditions, thermally treated derived pitches generate a higher amount and more developed mesophase than distilled derived pitch. Studies carried out by DSC in conjunction with optical microscopy revealed that the exothermal peaks observed on the DSC curves of the pitches between 400 and 500 °C correspond to phenomena that occurred prior to mesophase formation.