COLLOIDAL STRUCTURE OF HEAVY CRUDES AND ASPHALTENE SOLUTIONS

Many industrial problems that arise during petroleum processing are re lated to the high concentration of asphaltenes. A good knowledge of th e chemical composition of these macromolecules and a detailed understa nding of the evolution of the colloidal structures present in oil and its derivatives can play a decisive role in improving processing facil ities. Asphaltenes are defined by their insolubility in n-heptane. Sol uble molecules are called maltenes which can be fractionated by liquid chromatography in so-called resins, aromatic and saturated fractions. The major part of the research carried out on these complex molecules concerned the chemical composition determination from powerful techni ques measurements as for instance IR or NMR methods. Nevertheless, ver y little information on the colloidal structure of asphaltenes or resi ns in pure solvent or in real systems is accessible. The molecular wei ght determination was the first objective; several techniques, as vapo ur pressure osmometry (VPO), were used. The main conclusion of these d eterminations was the huge variation of the molecular weight measured by different methods. We used X-ray and neutron small angle scattering techniques in order to deduce the size polydispersity and the weight average molecular weight. Different systems as (i) asphaltenes or resi ns in solution with different solvents, or (ii) asphaltene and resin m ixtures in suspension with good or bad solvents were investigated as a function of temperature increase. We have exhibited that the aggregat ion number, i.e. the number of smaller entities, can strongly vary wit h solvent composition and temperature. Resins appear as very good solv ent for asphaltene molecules. Scattering measurements often exhibit st rong scattered intensity at small scattering vector, showing the prese nce in the suspension of large heterogeneities in diluted solutions of asphaltenes and resins. We can suggest that these heterogeneities are due to concentration fluctuations. It exists in the diluted suspensio n regions more or less extended, where the asphaltene concentration is greater than the average asphaltene concentration. Cryo-scanning elec tron microscopy has confirmed the system inhomogeneity. A two phase sy stem was clearly highlighted. The flocculation process was followed du ring n-heptane addition by scattering measurements and microscopy obse rvations. Small angle X-ray scattering (SAXS) appears as a powerful to ol for non-fractionated systems, as crude oil or other heavy derivativ es. Density fluctuations were observed for a vacuum residue.