Characterization of asphaltene deposition in a stainless steel tube

• Investigated the surface chemistry of asphaltenes and steel using IGC and XPS.
• The surface interaction forces at the asphaltene–steel interface were quantified.
• Asphaltene deposition is characterized by transient cycles of deposition/erosion.
• Examined the effects of asphaltene concentration and temperature on their kinetics.
• UV–visible spectrophotometer was used to measure the concentration of asphaltenes.

In this study, the mechanism and kinetics of asphaltene deposition in stainless steel tubes have been studied experimentally using n-heptane as the precipitant. The effects of oil velocity, temperature, and oil–precipitant volumetric dilution ratio on the rate of asphaltene deposition are investigated. Since the adhesion of asphaltenes on to the metal surface is controlled by its transport through the laminar sub layer, our focus was limited to studying the phenomena in laminar flow regime. The concentration of asphaltenes was measured using UV–visible spectrophotometer at 650 nm wavelength. This enabled us to accurately measure the amount and extent of asphaltene deposits in the stainless steel tube. It is observed that the deposition process is often accompanied by significant erosion due to the subsequent dominance of shear forces on the asphaltene deposits. However, the erosion is not complete and a certain layer of asphaltene deposits remain before the deposition process becomes dominant. The initial rate of asphaltene deposition increases with the increase in oil velocity and temperature. The deposition rates increase with the decrease in the oil–precipitant volumetric dilution ratio. The asphaltene deposits were also subjected to X-ray photoelectron spectroscopy to characterize the surface composition of atoms responsible for the interfacial interaction forces due to the surface functional groups. The fundamental interaction forces between asphaltenes and a stainless steel surface was measured by using inverse gas chromatography. The surface energy of stainless steel indicated the amphoteric nature of the surface, with a slightly higher basic component. The surface energy of asphaltenes displayed a very high component of Lifshitz–van der Waals forces. However the polar component of surface energy of asphaltenes could not be measured due to its very strong interaction with the polar probes on account of its polarity.