Interactions Between Athabasca Pentane Asphaltenes andn-Alkanes at Low Concentrations

Abstract

The mass fraction and the properties of asphaltenes vary significantly with the n-alkane used to separate them from their parent oil and with the details of the separation procedure, such as washing steps. Measurement repeatability is challenging, with different error bounds reported within the American Society for Testing and Materials (ASTM) standards for a single operator using the same equipment and procedure vis-à-vis measurements performed by different operators in different laboratories. In this work, reversible interactions between Athabasca pentane asphaltenes and n-alkanes from pentane to hexadecane were observed using cross-polarized and visible light microscopy and were quantified using high-precision density measurements for mixtures ranging from 1000 to 8000 ppmw (from 0.8 to 6.5 g/L) and enthalpy of solution measurements for mixtures comprising from 1000 to 3500 ppmw (from 0.8 to 3 g/L) asphaltenes. The partial specific volumes of Athabasca pentane asphaltenes and the enthalpy of solution values, including a change of sign, were found to vary systematically with n-alkane carbon number. The microscopic observations revealed the formation of liquid crystals followed by isotropic liquid on the surface of the asphaltene particles. The interactions at low concentrations are consistent with n-alkane sorption by asphaltene particles, asphaltene particle swelling, and dissolution of a fraction of the asphaltenes in n-alkanes. The partial specific volume and enthalpy of solution results, simulated using a phenomenological model that includes these effects, explain the sensitivity of the repeatability of asphaltene mass fraction determinations to the details of the washing procedure applied during their preparation. Preparation techniques without washing appear to be preferred because the mass fractions of asphaltenes recovered are expected to be more repeatable and their properties are likely to be more consistent.

Asphaltene Aggregation and Solubility

“…5 It is well-known that asphaltenes swell in various solvents. 84,85 For example, Nikooyeh et al 85 recently showed that the projected area of a pentane asphaltene observed under a microscope increased ∼17% in octane. Assuming a value of δ solv = 15.5 MPa 0.5 and referring to Figure 11, this would suggest that asphaltene components with solubility parameters ∼21 MPa 0.5 will show a degree of swelling consistent with this value (i.e., volume fraction asphaltene ∼ 0.83).…”

Fickian and Non-Fickian Diffusion in Heavy Oil + Light Hydrocarbon Mixtures

“…At the single file limit, n w = 0. 25. This limit arises in highly structured fluids where molecules or particles cannot pass one another freely but must move through a medium or a constriction t x / = λ 5 sequentially.…”

“…Finally, addition of light hydrocarbons to resource samples impacts their phase behavior both in time 24,25 and with respect to the number and nature of phases [26][27][28] . While the exact structure and spatial distribution of phase domains in hydrocarbon resources such as Maya crude oil, Safaniya vacuum residue and Athabasca bitumen are unknown, free diffusion between a light hydrocarbon and structured resource samples may be viewed from the perspective of a simple molecular fluid penetrating into a structured fluid comprising a broad range of nanoscopic and microscopic fluid- 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 filled channels, as illustrated in Figure 1.…”

Application of taylor dispersion technique to measure mutual diffusion coefficient in hexane + bitumen system

“…In our case, this value for bitumen solute was approximately 512.5 cm 3 /mol. Nikooyeh et al recently reported the partial volume of Athabasca n ‐pentane asphaltenes in hexane as 0.84 cm 3 /g. However, due to the wide range of asphaltene molecular weight reported in the literature and also large uncertainty involved in relating the partial volume of Athabasca asphaltene to the partial molar volume of the MacKay bitumen, Fedors' recommendation was considered to be more reasonable.…”