Aggregation and Solubility Behavior of Asphaltenes and their Subfractions

摘要

Asphaltenes (n-heptane insolubles) from Arab Heavy, B6, Canadon Seco and Hondo crude oils were fractionated in mixtures of heptane and toluene. The aggregation of these asphaltene fractions in heptane-toluene (heptol) mixtures and in methylnaph-thalene were studied by vapor pressure osmometry (VPO) and small angle neutron scattering (SANS). Solubility profiles of these asphaltenes in heptol were determined gravimetrically Solubility profiles of the more soluble and less soluble fractions isolated preparatively in heptane-toluene mixtures indicated strong cooperative asphaltene interactions. The less soluble asphaltene fractions had lower H/C ratios, higher N, V, Ni, and Fe contents than the more soluble or unfractionated asphaltenes. Number average molar masses by VPO in toluene indicated that the less soluble fraction of B6 asphaltenes was highly aggregated (>10,000 g/mol) while the unfractionated and more soluble fractions of asphaltenes approached monomer dimensions (～2,000 g/mol) at high dilution. Canadon Seco asphaltenes and their less soluble fraction had similar number average molar masses at high dilution (～3,000 g/mol). Neutron scattering studies at 25 and 80°C indicated Canadon Seco formed the largest aggregates in mixtures of heptane-toluene followed by B6, Hondo and Arab Heavy. Canadon Seco asphaltenes were the most sensitive to temperature, disaggregating appreciably as temperature increased from 25 to 80°C. In contrast, B6 asphaltenes responded more weakly to temperature, suggesting these asphaltenes are more strongly associated. Less soluble fractions formed aggregates considerably larger than the unfractionated asphaltenes, while soluble asphaltenes formed the smallest aggregates. Highly aggregated samples at 25°C were the most inclined to disaggregate upon heating and addition of 1-methylnaphthalene intensified this effect. The soluble fraction was near its minimum aggregate size at room temperature and was only marginally affected by the application of heat. Enhanced aromatic π-π bonding, dispersion forces and hydrogen bond interactions within the insoluble fraction likely caused large aggregate formation and low solubility.