Investigation of the mechanism of sediment formation in residual oil hydrocracking process through characterization of sediment deposits

In deep conversion of petroleum residues by hydrocracking, the refiners often face the problem of sediment or sludge formation, which causes equipment fouling and catalyst deactivation and leads to enormous financial burden in terms of increased costs of operation, maintenance and shutdown. The primary objective of the present study was to obtain information on the composition and structure of the sediments and to understand the mechanism of their formation. Two samples of coke-like sediments, one collected from an industrial vacuum residue hydrocracking unit, and the other produced in pilot plant experiments with the same industrially used feedstock (VR from Kuwait export crude), were analyzed by chemical analysis as well as by various other techniques, such as TGA, NMR, and FT-IR. Prior to analyses, the sediments were fractionated by sequential extraction with heptane, toluene, and tetrahydrofuran. Structural characterization of the sediment material showed the presence of a variety of polyaromatic structures with different degrees of condensation and alkyl substitution. Presentations of average molecular structures were constructed for a number of fractions, such as heptane soluble (HS, 38 wt.%), heptane insoluble-toluene soluble (HIS-TS, 31 wt.%) and tetrahydrofuran insoluble (THFIS, 28 wt.%), separated from the pilot plant sediment. The HIS-TS fraction contained a modified asphaltene-like material. It was more aromatic and contained shorter aliphatic chains than the asphaltenes obtained from VR. The presence of a large amount of HIS-TS modified asphaltenes in the pilot plant sediment suggests that asphaltenes play a key role in the initial stage of sediment formation. The industrial sediment, which underwent severe cracking and condensation reactions on prolonged exposure to high temperature conditions, was highly carbonized (THFIS, 82 wt.%).