Delayed Aggregation of Asphaltenes in the Presence of Alcohols by Dynamic Light Scattering

Abstract

Structure of an n-heptane asphaltene obtained from atmospheric residue of crude oil has been analyzed using ultraviolet, infrared, and nuclear magnetic resonance (¹H and ¹³C) spectroscopy and elemental analysis. The molecular weight of asphaltene was deduced by GPC (M _n 3372 g · mole⁻¹). Asphaltene exhibited H/C ratio of 0.905 and high sulfur content, being of thioether, thiophene rings or other C-S-C structures. Detailed ultravioletspectra of asphaltene recognized its construction as sets of fused aromatic rings, with each set having an average of three rings, thus, deducing a model structure. The onset point and aggregation behavior of asphaltene have been studied in the presence of alcohols(n-C₆OH, n-C₇OH, n-C₈OH and n-C₁₀OH) versus n-heptane, n-hexane and n-pentane as precipitating agents. The aggregation study has been carried out using dynamic light scattering. N-heptanol and n-hexanol showed the most delaying influence for the asphaltene aggregation versus n-heptane, n-hexane and n-pentane, respectively. Similarly, the aggregation behavior explored by dynamic light scattering showed that the use of n-hexanol and n-heptanol results in aggregate sizes that were smaller than those obtained when using n-octanol and n-decanol.

Keywords:

• Asphaltene characterization
• light scattering
• onset point

Acknowledgments

The authors would like to acknowledge KNPC for providing atmospheric residue samples. The authors express their gratitude to Kuwait University Research Administration for supporting this study under General Facility project numbers GE03/08 and GE01/07. The authors also thank General Facility-Science, Project-GS01/01 for NMR, FI/FD MS and GPC measurements.

Notes

X = mass fraction of asphaltene, sp. V = specific volume (1/density), d²⁰ = density at 20°C.

n = average number of carbons per alkyl side chain, MW = molar mass by GPC and FD&FI MS.

DBE = double bond equivalents = 101−(50 × H/C).

fa (aromatic carbon fraction); As (percent substitution of aromatic rings); DBE: Double bonds equivalents); A_rDB (aromatic double bonds); A_lDB (aliphatic double bonds).

S: Strong band; m: Medium band; w: Weak band; As-ABD (S)*: Measured asphaltene using FTIR as KBr Disc, As-ABD (S)**: Measured asphaltene using FTIR as liquid in Toluene as blank.

*Samples are composed of 35 vol% alcohol (if any), toluene, and asphaltene, and 65% n-alkane. If alcohol is present in solution, its percentage as given in the table is in relation to the alcohol-toluene-asphaltene mixture.