On the molecular basis of aggregation and stability of Colombian asphaltenes and their subfractions

Highlights

• Solubility subfractions have different compositions and molecular properties.

• Analytical centrifugation allowed to classify subfractions into unstable and stable.

• More polar subfractions exhibited low flocculation and high dispersion stability.

• Higher-aromaticity subfractions formed large fractals of small nanoaggregates.

Abstract

Vacuum residue (VR) is the heaviest fraction obtained by fractional distillation of the crude oil. Crude oil and its fractions, such as VR, can be separated in its components saturates, aromatics, resins, and asphaltenes (SARA fractions). Asphaltenes are of particular importance for up- and down-stream processes due to their molecular complexity, high heteroatom content, and strong tendency to self-aggregate. To evaluate the molecular characteristics of asphaltenes responsible for aggregation, we used a fractionation method based on mixtures of toluene and acetone to separate asphaltenes, isolated from a heavy Colombian residue, into different solubility subfractions. In this contribution, we show correlations between elemental composition and average molecular properties of subfractions and their solubility behavior. In addition, we were able to differentiate asphaltenes into ‘unstable’ and ‘stable’ subfractions in toluene solution by an analytical centrifugation method. From the stability analysis, we demonstrated that less soluble subfractions (consisting of more aromatic, heavier, and more condensed asphaltenes) are more prone to flocculation, and hence, they are less stable than more soluble subfractions. The results of colloidal characterization prove that less soluble subfractions form large fractal structures of small nanoaggregates, whereas the most soluble subfraction form highly stabilized dispersions of non-flocculating nanoaggregates. Results obtained in this work are expected to provide insights regarding the importance of solubility-based fractionation method as a basis for understanding asphaltene aggregation in heavy crude oils.

Introduction

In the last years, the analysis of vacuum residues has been fundamental in the petroleum field due to numerous refining processes are narrowly related to its chemical composition. The properties of a vacuum residue depend on the crude oil nature, the production process and the relative amounts of constituents present. Asphaltenes and resins may constitute up to 50% of the VR [1]. The asphaltenes are the most complex and polydisperse compounds of the SARA fractions, and they are responsible for the presence of colloidal structures in crude oils [2], [3]. These structures can flocculate and precipitate when changing the temperature, pressure, and composition of the petroleum during its production. The clog of wells, pipelines and refinery equipment due to the asphaltene deposition affects all steps of the petroleum industry chain increasing the operational costs [4], [5], [6]. Thus, it is clear that more knowledge is required about the main constituent of vacuum residues that cause specific complications during crude oil production and processing.

Asphaltenes are defined as the fraction of crude oil insoluble in n-alkanes of low molecular weight such as n-heptane or n-pentane, and soluble in aromatic solvents such as toluene. They comprise an extremely polydisperse mixture of molecules with a broad distribution of size and shapes that determine the overall physicochemical properties of crude oils. In general, asphaltenes comprise fused aromatic rings, aliphatic side chains, heteroatoms such as S, N and O, and some metals [7], [8], [9], [10], [11], [12], [13], [14]. The molecular interactions responsible for asphaltene aggregation involve the π-π stacking among polycondensed aromatic cores, hydrogen bonding promoted by heteroatom functional groups, metal-organic complexation through vanadium or nickel, and other charge transfer interactions [15], [16]. To get a better knowledge of the asphaltene properties, physical and chemical methods to separate these complex matrices into different fractions have been used [10], [17], [18], [19], [20], [21]. The conventional separation methods are based on asphaltenes solubility in a mixture of solvents (polar and nonpolar solvents). Gawrys et al. [22] fractionated the asphaltenes from four different crude oils in mixtures of heptane and toluene, finding that the most polar species were concentrated in the least soluble subfraction. Also, they observed this subfraction is responsible for the asphaltenes aggregation. On the other hand, Buenrostro-Gonzalez et al. [23] demonstrated that the asphaltenes fractionation with a polar precipitating solvent yields subfractions with more significant structural differences than those obtained from nonpolar media. They also noticed remarkable differences in the aggregation tendencies of the separated fractions.

Although analytical procedures for the asphaltenes fractionation have proven to be an alternative way to carry out an extensive characterization of these complex matrices, very few data have been reported about the fractionation of asphaltenes from vacuum residues, and the analysis of their colloidal properties such as aggregation. To understand the molecular basis of the aggregation phenomena, we employed a solubility-based fractionation method to separate the ‘whole’ asphaltenes from a Colombian vacuum residue into four subfractions. In this contribution, we present new data of the characterization of Colombian asphaltenes and their subfractions by elemental analysis, X-ray fluorescence (XRF), mass spectrometry (MS), nuclear magnetic resonance (NMR), analytical centrifugation, dynamic light scattering (DLS), and small angle X-ray scattering (SAXS). We also show how the compositional and molecular characteristics of asphaltenes play an essential role in its colloidal stability in toluene solutions.