Supra-Molecular Asphaltene Relaxation Technology

Takanohashi Toshimasa, Sato Shinya, Tanaka Ryuzo

doi:10.1627/jpi.56.61

The repression of coke precursor formation is key to heavy oil upgrading. Asphaltenes are known to form aggregates that may be responsible for coke precursor formation and the consequent deactivation of catalysts. A Supra-Molecular Asphaltene Relaxation Technology (SMART) is introduced herein and may be applied to asphaltene aggregates in order to reduce their detrimental effects. The nature of asphaltene aggregates was examined using molecular simulation methods. Molecular mechanics and molecular dynamics calculations on asphaltenes obtained from vacuum residues revealed that the most stable conformations of asphaltene aggregates were those held together by several noncovalent interactions. At 673 K, where decomposition reactions begin, aggregates formed of aromatic-aromatic stacking interactions were still stable. These stable aggregates likely comprised heavier oil fractions such as coke precursors. Changes induced in these aggregated structures by pretreatment with various solvents were investigated. Some stacking interactions could be disrupted in quinoline at 573 K but remained stable when the aggregates were pretreated in 1-methylnaphthalene. Autoclave experiments showed that the coke yield after pyrolysis at 713 K was significantly decreased when the asphaltene was presoaked in quinoline for 1 h. In contrast, pretreatment with 1-methylnaphthalene resulted in negligible changes in coke yield. The results of both simulations and autoclave experiments suggest that, when aggregates were presoaked in quinoline, some aromatic stacking interactions were disrupted and molecular mobility increased. This prevented the asphaltenes from polymerizing via condensation reactions between aromatic rings. Thus, coke yields after pretreatment in quinoline were relatively low. The contribution of each type of interaction (aromatic stacking, aliphatic ring entanglement, heteroatom interactions, and hydrogen bonding) to the overall aggregation energy of asphaltene was estimated using an imaginary simulation technique. Aromatic-aromatic interactions accounted for approximately 50 % of the total aggregate interactions. Contributions from aliphatic side-chain entanglement and heteroatom interactions were around 27 % and 20 %, respectively. The combination of these interactions stabilized the asphaltene aggregates.

Supra-Molecular Asphaltene Relaxation Technology

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