Asphaltenes are a solubility class and are defined as the part of a crude oil that is soluble in toluene and insoluble in n-heptane. Asphaltene precipitation, and subsequent fouling, is a potential issue in refining when feedstock and/or process streams are blended. While asphaltene precipitation from native crude oils can be predicted from a small set of measurements using regular solution based models, these precipitation models have not been applied to reacted crude oils. This study is part of a larger project to extend a previously developed regular solution based precipitation model to reacted crude oils. The three properties required for this model are density, molecular weight, and solubility parameter. The objectives of the study are: 1) to determine the distributions of these properties for self-associated asphaltene nanoaggregates; 2) model asphaltene precipitation from solutions of n-heptane and toluene (heptol) using regular solution theory.
To determine these distributions, n-heptane extracted asphaltenes from hydrocracked and thermocracked samples were fractionated into solubility cuts. The asphaltenes were dissolved in toluene and then partially precipitated at specified ratios of heptane-to-toluene to generate sets of light (soluble) and heavy (insoluble) cuts. The molecular weight and density were measured for each cut. The refractive index and elemental analysis were also measured for potential use as correlating parameters. The density distributions were determined directly from the data. The molecular weight data were fitted with a self-association model in order to predict the distributions at any given concentration. Asphaltene solubility parameters were determined by fitting the regular solution model to asphaltene precipitation yield data.
The asphaltenes were found to include both associating and non-associating asphaltenes. The amount of non-associating material and the density of the asphaltenes increased as the extent of reaction increased. Thermal cracking appeared to have little effect on asphaltene average monomer molecular weight or the distribution of nanoaggregate molecular weights. Hydrocracking significantly decreased both the average monomer and nanoaggregate molecular weights. It was found that both hydrocracking and thermal cracking made asphaltenes denser and significantly less soluble. The onset point of precipitation for both cases moved to zero concentration of n-heptane in heptol solutions.
A previously developed regular solution model was adapted to calculate solubility parameter distribution reacted asphaltenes. The model was modified as follows: density was correlated to the cumulative mass percent of asphaltenes; 2) the correlation of the asphaltene solubility parameter to molecular weight was retuned. Two methods were used to represent the asphaltene molecular weight distributions: the gamma distribution and the distribution from an association model. Since the solubility model predictions are affected by the shape of the molecular weight distributions, different sets of solubility parameter were calculated for each. In general, the gamma distribution adequately represented the molecular weight distributions for both native and reacted asphaltenes and better fit asphaltene yield data.