The Role of Asphaltene Solubility and Chemistry on the Stability of Water-in-Oil Emulsions

摘要

Water-in-oil emulsions were prepared by homogenizing water and mixtures of heptane and toluene (heptol) containing asphaltenes (n-heptane insolubles) from four crude oils (Arab Heavy, B6, Canadon Seco, and Hondo). Emulsion stability was gauged by the volume percentage of water resolved after centrifugation at high speed (15,000 rpm). B6 and Hondo asphaltenes formed emulsions considerably more stable over a broad range of toluene volume fractions than Arab Heavy and Canadon Seco. Sauter mean droplet diameters determined from photomicrography ranged from 7.1 to 11.1μm immediately following homogenization. Preparative scale fractionations were performed on the four asphaltenes at toluene volume fractions between 0.3 and 0.4 via vacuum filtration. Solubility profiles of the more soluble and less soluble fractions isolated in heptol mixtures indicated strong cooperative asphaltene interactions. The less soluble asphaltene fractions had lower H/C ratios, higher nitrogen content, and formed much larger aggregates than the more soluble or unfractionated asphaltenes, suggesting that aggregation may be governed by the degree of aromatic π-π bonding and polar interactions between heteroatoms. The relative stabilities of emulsions indicated that asphaltenes fell into several classes. The defining characteristics of each category were asphaltene aggregate size, H/C ratio, and N, Ni, and V content. Class 1 emulsions were the weakest and formed by high aromaticity asphaltenes (H/C 1.12-1.17) with small aggregate sizes (34.5-138 A), containing lower concentrations of N (<1.32%) and heavy metals. Class 2 and 3 emulsions were very stable and formed by intermediate to large aggregates (53.2-262 A) with lower aromaticity but higher concentration of N, Ni and V. Class 4 emulsions were also quite weak and formed by highly aggregated asphaltenes (246-520 A) with higher aromaticity (H/C 1.09-1.11) and lower N, Ni, and V content. Asphaltenes aggregating due primarily to H-bonding and by π-π interactions were presumed to form a cohesive interfacial oil-water film and stable emulsions. Weaker emulsions were likely caused by less interfacially active n-n dominated asphaltene aggregates.