Effect of surfactants on asphaltene interfacial films and stability of water-in-oil emulsions
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AbstractUndesirable water-in-oil emulsions often form during oil processes. Chemical treatment is a common method for breaking down these emulsions; however, this technique is not always effective. In order to improve the chemical treatment of emulsions, it is useful to have an understanding of emulsion stability. The stability of water-in-oil emulsions depends in part on the surface properties. The surface is composed of natural material present in the produced oil, such as asphaltenes, resins, clays and surfactants, which adsorb on the water-oil interface. Asphaltenes play an important role in stabilizing the emulsion since they irreversibly adsorb at the interface of the water droplets and form a steric barrier or rigid skin that prevents coalescence. An effective demulsifier must disrupt this film in order to accelerate coalescence. However, the chemical treatment design is still done by trial and error partly because the effect of surfactants on interfacial films is poorly understood. This thesis focuses on emulsions stabilized by asphaltene films. It was previously found that the stability of these emulsions could be predicted from both the compressibility and crumpling film ratio of irreversibly adsorbed asphaltene films. In this study, the effect of surfactants on asphaltene interfacial films is analyzed through the change in film properties. Surface pressure isotherms were measured at 23 °C for model interfaces between aqueous surfactant solutions and asphaltenes dissolved in toluene and heptane:toluene mixtures. Compressibility, crumpling film ratio and surface pressure were determined from the surface pressure isothe1ms. The stability of water-in-oil emulsions was determined for the same systems based on the free water resolved after repeated treatment involving heating at 60°C and centrifugation. Experimental variables included concentration of asphaltenes (5 and 10 kg/m³), concentration and type of surfactant (Aerosol OT, nonylphenol ethoxylates, dodecylbenzene sulfonic acids, sodium naphthenate) and aging time (from 10 min to 4 h). The effect of surfactants on film properties and emulsion stability was found to divide in two distinct behaviours. 1) surfactants that fom1ed reversible films and destabilized emulsions and 2) surfactants that maintained the irreversible adsorption at the interface and could enhance emulsion stability.
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