Investigations of Asphaltene-Metal Interactions
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Asphaltenes are the most polar fraction of petroleum that can precipitate due to changes in pressure, temperature and composition which may be encountered during petroleum production and transportation. The precipitated asphaltenes can subsequently deposit on the surfaces of production/transportation pipelines and clog the pipeline partially or completely. Why asphaltenes deposit on a metal surface remains a poorly understood topic. This thesis investigates asphaltene-metal interactions through physical and chemical characterization of the adsorption behaviour of asphaltenes on metal surfaces. A relatively novel technique called quartz crystal microbalance (QCM) was employed to study the kinetics of asphaltene adsorption and the data analyzed to estimate the adsorption isotherm. The X-ray photoelectron spectroscopy (XPS) technique was employed to determine the functional groups in bulk asphaltene and adsorbed asphaltene. The asphaltene adsorption experiments on metal surfaces were carried out in toluene medium at dilute concentration ranging 10-1500 ppm. A mathematical model was developed to analyze the XPS data, which was subsequently assessed to estimate the fractional coverage and thickness of adsorbed asphaltene. The isotherms generated from QCM and XPS experiments were compared and analyzed to estimate the free energy. Three different asphaltene samples were investigated – one sample was derived from Cold Lake bitumen and two others were supplied by DBR-Oilphase (a Schlumberger company) and termed as MD and HO2 asphaltenes. Most extensive experimentations and analyses were carried out for Cold Lake asphaltenes. The same procedure was applied for the other asphaltene samples. The kinetic analyses of Cold Lake and MD asphaltenes indicated that the adsorption process is diffusion controlled at initial times but follows a first-order kinetic rate law at longer times. MD asphaltenes exhibited higher equilibrium adsorbed amounts than Cold Lake asphaltenes. The XPS spectral analysis for MD and Cold Lake asphaltene revealed presence of carboxylic, thiophenic, thiol, pyridinic and pyrrolic type species. All adsorption isotherms could be described by follows Langmuir (type-I) isotherm. The estimated thickness of adsorbed asphaltene varied between 8 and 12 nm.