Asphaltenes size polydispersity reduction by nano- and ultrafiltration separation methods - Comparison with the flocculation method

Asphaltenes consist of a very complex material in which molecules and aggregates can have very different chemical composition and molecular weights. It is now well documented that asphaltenes show a very large size polydispersity. All these differences make their properties and behavior hard to describe. In order to gain insight into asphaltene properties, it could be useful to reduce this size polydispersity. Within this framework, two different approaches were used to reduce asphaltene size polydispersity: flocculation (using mixtures of solvents with anti-solvents) and membrane filtration. Various asphaltene fractions were obtained by both methods, and were further investigated using the following techniques: Size-Exclusion Chromatography (SEC), Elemental analysis, Nuclear Magnetic Resonance (C-13-NMR) and Small-Angle X-ray Scattering (SAXS). It has been shown that asphaltenic aggregates of different sizes can be fractionated by membrane filtration in a more selective way than using the conventional solvent flocculation method. The effects of temperature and concentration on membrane separation performance were studied. When compared with large aggregates, small asphaltenic aggregates present lower aromaticity and higher aliphatic composition. Their alkyl chains also appear to be shorter and more alkylated. Elemental analysis indicates that smaller asphaltenes contain a lower metal concentration and are preferentially enriched in vanadium than nickel when compared with bigger aggregates.