The Paradox of Asphaltene Precipitation with Normal Paraffins

Abstract

For bitumens and crude oils, the volume of n-paraffin at the flocculation point, which is the point of incipient asphaltene precipitation, increases as the n-paraffin carbon number increases, reaching a maximum at a carbon number of 9 or 10, and then decreases. Thus, asphaltenes often can begin precipitating with a smaller volume of n-hexadecane than with n-pentane, even though large volumes of n-hexadecane precipitate much less (and more aromatic) asphaltenes than large volumes of n-pentane. How can n-hexadecane be both a better and a poorer solvent than n-pentane for asphaltenes? This paradox of solvent quality can be resolved by combining the entropy of mixing of molecules of different sizes with the heat of mixing from solubility parameters, as expressed by the regular Flory−Huggins model. With sufficient characterization data, the approximations and methods of Yarranton et al. can quantitatively describe asphaltene precipitation from the flocculation point to large excesses of n-paraffins from pentane to hexadecane. To describe only the flocculation point data of bitumens and crude oils, the oil compatibility model of Wiehe can be used. Although the oil compatibility model was derived on the basis that the solubility parameter is constant for a given oil at the flocculation point, using “effective” solubility parameters, flocculation points can be predicted with little characterization data.

Asphaltene solubility in common solvents: A molecular dynamics simulation study

“…An optimization procedure is used to select a small subset of molecules that best match the experimental data. More detailed information on evaluation of the asphaltene molecular structures can be obtained from Rogel, [16] Redelius, [20] Headen and Boek, [28] Murgich et al, [29] Johansson et al, [30] Wiehe et al, [31] and Sun. [37] In this work, all 28 known asphaltene structures were used in the MD simulation.…”

“…Johansson et al [30] found that intrinsic viscosity and aggregate size both sensitively reflected the state of asphaltenes in a homogeneous solution and used these parameters for determining the asphaltene solubility parameter. Wiehe et al [31] analyzed the effect of n-paraffin's size on the flocculation point of crude oils and bitumen, and obtained the maximum volume of n-paraffin as a function of the carbon number of the paraffin at flocculation point. Socrates et al [32] obtained solubility parameters for asphaltenes and fractions (A1, A2), resins, and free asphaltenes sampled from natural surfactants.…”

Characterisation of Asphaltenes and Modelling of Flocculation – State of the Art

“…According to Wiehe and Kennedy, the flocculation point occurs at a unique solubility parameter, irrespective of the fluid mixture. This invariant is only meaningful, however, if the solvents and flocculants have similar molar volumes (Wiehe et al, 2005).…”

Modeling and measurement of thermo-physical properties for Athabasca bitumen and n -heptane mixtures

“…Wen and Kantsaz [9] measured the viscosity of Cold Lake bitumen diluted with n-heptane at atmospheric pressure and ambient temperature (25°C) and correlated the data with Shu and Cragoe models. The onset of asphaltene precipitation and asphaltene yield have been evaluated experimentally by Akbarzadeh et al [10] and Wiehe et al [11]. Recently, Saryazdi et al [12] measured the density of n-heptane/bitumen mixtures at two different concentrations (15 and 30 weight percent) over temperature and pressure ranges, 20-175°C and 0.1-10 MPa, respectively.…”