Asphaltene Aggregation:  A Molecular Thermodynamic Approach

Abstract

The aggregation behavior of asphaltenes in apolar solvents is studied using a molecular thermodynamic approach. The theory is based on a molecular model for asphaltene aggregates that describes them as aromatic cores, composed of stacked aromatic sheets, surrounded by aliphatic chains. Using this simple molecular model, an analytical expression is developed for the free energy of aggregation that incorporates five contributions due to (1) transfer of the polyaromatic rings from the solvent into the aromatic core, (2) mixing of the aliphatic chains with the solvent, (3) deformation of the aliphatic chains, (4) steric repulsion among the aliphatic chains, and (5) aggregate core−solvent interactions. The proposed approach provides a qualitative description of the main experimental trends observed for asphaltene aggregation. Specifically, the experimentally observed variation of cmc values and aggregate size with (1) asphaltene molecular characteristics, (2) asphaltene concentration, (3) solvent composition, and (4) temperature has been qualitatively reproduced by the theory. In addition, the thermodynamic molecular model developed does not utilize any information derived from experiments on asphaltene solutions, and therefore, it is strictly predictive.

Solid Petroleum Asphaltenes Seem Surrounded by Alkyl Layers

“…Despite controversial considerations on the existence of asphaltene micelles in organic media (DaSilva Ramos et al, 2001), the term c.m.c (critical micelle concentration) has been adopted for description of aggregation phenomena. The micellar model reportedly predicts many experimental data obtained with asphaltenes of diverse nature (Rogel, 2002). The developed model successfully predicts the appearance of an experimental c.m.c at low concentration values for two of the studied asphaltenes, which were those built with the less benzene soluble aromatic nuclei.…”

“…Recently, a thermodynamic molecular aggregation approach was developed for asphaltenes (Rogel, 2002). The theory is based on the molecular model for asphaltene aggregates that describes them as aromatic cores composed of stacked aromatic sheets, surrounded by aliphatic chains.…”

“…Their calculated critical aggregation values in benzene at 25 C are included. Predicted appearance of c.m.c's in toluene as a function of aromatic nuclei solubility for asphaltene molecules with C7 appendages so that fa ¼ 0.60 (calculation details were published by Rogel, 2002). Even though values for c.m.c have been reported in the literature for asphaltenes using different techniques (Andersen and Birdi, 1991;Rogacheva et al, 1980;Rogel et al, 2000;Sheu et al, 1992) there are some doubts about the existence of a c.m.c similar to the one exhibited by surfactants in aqueous solution (Andersen et al, 2001;Yarranton et al, 2000).…”

Methodology for Predicting the Phase Envelope of a Heavy Crude Oil and Its Asphaltene Deposition Onset

“…Specifically, P * corresponds to the cohesive energy density at 0 K, and ρ * corresponds to density at 0 K (Rogel, 2002). The characteristic parameters P * , ρ * , and T * depend on the microscopic parameters (ε * , ν * , r) which are the energy interaction per mer ε * , the average closed-packed volume of a mer ν * , and the number of cells or r-mers of a polymer (one mol of asphaltenes), in addition to the molecular weight (M w ) and can be expressed as:…”

Molecular Dynamics Simulation of Asphaltene Deposition During CO2Miscible Flooding

“…Many average model structures for asphaltenes have been constructed on the basis of structural data, and computer modeling of asphaltenes has shown that aggregated structures are more stable (Rogel, 1995;Murgich et al, 1996;Takanohashi et al, 2003). The structures have been used to simulate asphaltenes in different situations (Murgich et al, 1996;Rogel, 2002;Li et al, 2007).…”