The Effect of Compositional Distribution on Asphaltene Precipitation

To understand the effect of composition on asphaltene precipitation phenomena, we consider it in terms of the molecules’ solubility parameter (δ) and molecular weight (MW) distribution. We summarize the observations reported in the literature and present additional data on the improved solubility of asphaltenes in the presence of resins or other asphaltene fractions. We discuss this in terms of both regular solution theory and a heuristic formalism extending Wiehe’s framework, which is extremely valuable in understanding the mechanisms involved and potentially simple to use. By using the Hildebrand–Scatchard equation, one can map the metrics of the compositional distribution to a single parameter (AN) representing the solvent power needed to solubilize a narrow, conceptually monodisperse fraction of the distribution, and propose a physically meaningful form to get the collective property IN from the AN distribution, which captures the observed trends. We also employ regular solution theory and show that a narrow binned continuous solubility parameter distribution of asphaltene pseudocomponents is necessary to describe the behavior of the system, and produces the observed phenomena regarding both fractionation and the solvent power required to dissolve fractions. The purpose of this work is to explain the mechanisms at work and directional dependencies due to the compositional distribution, rather than to quantitatively model a data set. However, the framework presented here could be carried forward to do the latter.