Energy & Fuels, Vol.29, No.8, 4730-4737, 2015
Polydisperse Size Distribution of Monomers and Aggregates of Sulfur-Containing Compounds in Petroleum Residue Fractions
Dimension of sulfur-containing compounds in a residue is crucial to hydrodesulfurization catalyst design. Size distributions of sulfur compounds in fractions of Venezuela atmospheric residue were determined from the bulk-phase diffusion coefficients, which were measured at 298 K by a diaphragm cell by using 200 nm polycarbonate membranes. Sulfur compounds in all fractions show obvious size polydispersity. The size of four narrow SFEF (supercritical fluid extraction and fraction) fractions varies slightly with the concentrations of 1 g/L to 40 g/L. However, maltenes and asphaltenes from the end-cut showed significant variation in size over concentrations of 0.1 g/L to 40 g/L, which indicates a coexistence of various monomers and aggregates. The monomers are dominated in maltenes, whereas aggregates dominated in asphaltenes. The hydrodynamic diameter of sulfur-containing monomers of four SFEF fractions ranges from 0.74 to 1.45 nm at a concentration of 1 g/L. The size of maltene monomers spans a range of 1.87 to 2.29 nm at 0.1 g/L and presents a more significant polydispersity than SFEF fractions. The size variation of the SFEF fractions and maltenes to yields demonstrates a continuous distribution in size for the petroleum residue. However, asphaltene aggregates cover the span of diameters from about 4.29 to 5.54 nm at a concentration of 0.1 g/L, and the values are larger than those of 1-3 nm found in most literature for asphaltene molecules. The average diameters of asphaltene fractions decreased to 4.02 and 3.95 nm at concentrations of 0.05 g/L and 0.03 g/L, respectively. It reveals that aggregation of asphaltene molecules can occur at concentration lower than 0.1 g/L and a state of coexistence of asphaltene monomers and aggregates at 0.05 to 0.1 g/L.