The Conversion Characteristics Of Inferior Heavy Oil-derived Asphaltenes In Different Solvents

The asphaltenes are the heaviest fractions in crude oil. The association is much strongamong their molecules, giving rise to coagulation, aggregation, flocculation, and evendeposition during crude oil exploration, transportation and the down stream processing. Theparaffic solvents can promote the sedimentation of asphatenes from crude oils, and somearomatic solvents can disassociate the asphaltenes. The behaviors of asphaltene molecules indifferent media could be different during thermal treatment with respect to association ordiassociation. Therefore, studies on the thermal treatment of asphaltenes in defferent mediacould gain insight into the mechanism of disassociation and association of asphaltenes andcoke formation.In this paper, the differences in the chemical structures of the asphaltene fractions presentin three residue feedstocks, Tahe atmospheric residue (THAR), Liaohe Vacuum residue(LHVR) and Venezuela vacuum residue (VNVR), and in product oils after mild thermaltreatment in defferent media was studied using measurements of molecular weight, elementalanalysis, nuclear magnetic resonance (NMR) spectroscopy, and X-ray diffraction (XRD) etc..The results obtained characterized the association condition and the conversion of asphaltenesin media under different treatment temperature, which showed the effects of thermal treatmenttemperature, media and the origins of the asphaltenes on disassociation and association ofasphaltenes during mild thermal treatment.The results indicated that asphaltenes molecules werenot thermally stable, for not onlythe association/disassociation behavior of asphaltenes unit sheets, but also the starting crackof groups (involved with heteroatoms) and unstable side chains located in peripheral sites ofasphaltene structures under relative low temperature condition (240~280℃). Compensatingreaction also occurred to some extent at the same temperature condition. The associated asphaltenes further formed small amounts of toluene insoluble. The condensation wouldincrease with rising temperature, which meant more toluene insoluble at the end of the test.Increasing temperature could provoke the phase separation caused by the aggregates ofasphaltenes.Toluene, dimethyl benzene, and1-Methylnaphthalene all had peptization effect on theasphaltenes. The conversion degree of asphaltenes with well-dispersion in1-Methylnaphthalene is relatively low. The dispersion effect of three solvents reflected asphysical factor, which varied as the polarity and the structure of solvents. The stronger thepolarity, the more similar the structure of solvent was with condensed aromatic structure inasphaltenes, the better dispersion effect could be achieved. Solvent and temperature both hadimportant influence on the behavior of asphaltenes in the test. The dissolution effect ofsolvent would make the structure of asphaltenes loose, resulting in smaller structural unitnumber. Thermal treatment with long time would otherwise cause subtle reactions ofasphaltenes, which was characterized by the scission of alkyl groups in the asphaltenesstructure.The temperature had more influence on the conversion of asphaltenes than solvent. Theelemental analysis, the structural parameters, and the calculation results of association factorall showed that VNAs would break the peripheral sites of asphaltene structure more easilythan THAs and LHAs. The UV-vis analysis showed that aromatic structure of three-four ringswas the basic aromatic structure in asphaltenes unit sheet, with a few amount of aromaticstructure of more than five rings. The aromatic structure of LHAs after mild thermal treatmentmainly presented as linear arrangements. Furthermore, plane arrangements of more thanfour aromatic rings are more common in THAs and VNAs than in LHAs.