FLOCCULATION OF ASPHALTENES IN HEAVY OIL AT ELEVATED-TEMPERATURES

It has been observed that there is a dispersed phase of particles in v acuum residue that is associated with the heptane insoluble asphaltene s. The particles are in the size range of 100 Angstrom. In this paper we present rheological and small angle X-ray scattering(SAXS) data for the vacuum residue of Arabian Medium/Heavy crude oil that suggests th e dispersed particles flocculate at elevated temperatures. We find the asphaltenic particles are stabilized below 200 degrees C against the attractive dispersion forces between asphaltenic particles by an adsor bed layer of non-asphaltenic molecules. However the strength of the in teraction holding the adsorbed layer to the particles is only about 1. 8 kca/mole(3kT), and so this layer is dissipated as the temperature is increased. We estimate the particles are unprotected at 200 degrees C . The strength of the attraction due to the dispersion force is about 7kT in this state, and so the dispersion becomes thermodynamically uns table with respect to flocculation. Additionally, the driving force fo r flocculation is augmented by the formation of particles in the non-a sphaltenic phase at elevated temperatures. The appearance of these par ticles increases the osmotic pressure, and forces the asphaltenic part icles together. We suggest the asphaltenic phase formed by flocculatio n, well below the temperatures at which chemical reactions occur, is t he precursor to the coke-producing phase in the reacting residue discu ssed by Wiehe.