Changes in asphaltene surface topography with thermal treatment

Abstract

The impact of thermal cracking reaction on asphaltene structure and morphology has been investigated by using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The structural and morphological changes at a microscopic level were monitored by comparing the parent asphaltenes from different vacuum residues (VRs) to their corresponding thermally treated asphaltenes, obtained from the by-product pitch after thermal treatment. The SEM analysis indicated that the asphaltene aggregates extracted from atmospheric residues have smooth and rough surfaces with agglomerate particles and bright inclusions. The SEM images of asphaltene aggregates that are extracted from the pitch samples after mild cracking demonstrated cleavage fracture morphology with obvious reduction in inclusions sizes and intensities. The TEM analysis, on the other hand, indicated that the asphaltenes from residual oils have tangled structures, with edges similar to a cauliflower. The tangled structure is mainly credited to the alkyl side-chains that impede the aromatic sheets from stacking. At mild cracking (400 °C), the asphaltene began to exhibit well-ordered layer structures near the edges due to the rupture of the alkyl side-chains. However, the tangled structure has been preserved in the interior of the sample. As the reaction severity increases (415 °C), the stacking of aromatic sheets became more evident even in the sample interior. At the most severe cracking condition (430 °C), an obvious reduction in the cluster diameter has been observed, which mainly resulted from the reduction in the number of aromatic sheets per stack.