Molecular Weight and Density Distributions of Asphaltenes from Crude Oils

Abstract

Asphaltenes self-associate, and the molecular weight and density distributions are a factor in asphaltene precipitiation. To determine these distributions, heptane-extracted asphaltenes from four crude oils were fractionated into solubility cuts. The asphaltenes were dissolved in toluene and then partially precipitated at specified ratios of heptane/toluene to generate sets of light (soluble) and heavy (insoluble) cuts. The molecular weight and density were measured for each cut. The asphaltenes were found to include both associating and non-associating asphaltenes. The content of non-associating components was up to 15 wt % of the asphaltenes. The density distributions were determined directly from the data. The molecular weight data were fitted with a self-association model to predict the distributions at any given concentration. Then, a guideline was developed to represent the molecular weight distribution of non-associated and associated asphaltenes with a Γ distribution function. Finally, the density of asphaltene cuts was correlated to their molecular weight. This correlation fit the data with an average absolute deviation of 11 kg/m 3 .

Determining the Kinetics of Asphaltene Adsorption from Toluene: A New Reaction–Diffusion Model

“…Both mechanisms reproduce the correct effect of aging on the initial adsorption rate and adsorbed mass 41 prior to rinsing, Figs. 8 and 10.…”

“…Rudrake and Rudrake et al reasoned from QCM and XPS data 25 that, given an apparent film thickness of 6-8 nm, the likely arrangement was edge-on or 'vertical' [40,20]. 26 Based on reported literature values [41,42,43,44,45], we adopt an asphaltene density of 1.2 gcm⁻³ and 27 estimate that the thickness of the adsorbed asphaltene film in our study varies with concentration from 4-28 7 nm, see Table 7. This assumes that the density of asphaltene in the thin adsorbed film is similar to its 29 value in the bulk.…”

“…Neither are all nanoaggregates. 41 The chemical make-up or composition of the monomer and nanoaggregate fractions is a function of 42 concentration as are their respective numbers. The log-log plot of initial rate vs. predicted monomer 43 concentration gives straight lines for both the young (non-equilibrated) and aged (equilibrated) solutions, 44 insert of Fig.…”

“…obtained. Originally developed for measuring adsorption from the gaseous phase, modifications for use in 41 liquid environments made QCM suitable for a wide range of applications. The high sensitivity (< 1 ng 42 cm⁻²) of QCM means that it can monitor sub-monolayer films in situ.…”

Role of Aqueous Phase Chemistry, Interfacial Film Properties, and Surface Coverage in Stabilizing Water-in-Bitumen Emulsions

“…The average nanoaggregate molecular weight in solution with toluene appears to range between 2-12 monomers per aggregate (Yarranton et al, 2000;McKenna et al, 2013).The association increased with concentration reaching a limiting value at asphaltene concentrations of 10 -20 g/L. Barrera et al (2013) observed average molecular weights of the asphaltenes ranging from 1,000 to 30,000 g/mol, equivalent to approximately 1−30 molecules per nanoaggregate, using vapor pressure osmometry (VPO). However, SANS and SAXS data suggest much wider molecular weight distribution with average molecular weights in the order of 30,000 g/mol (Yarranton et al, 2013).…”