Asphaltenes are the most polar molecules in crude oil and are responsible for a large number of deposition and fouling problems in the oil industry. The mechanisms by which asphaltenes deposit have not been fully elucidated, and the goal of this investigation is to reveal the underlying physics of the asphaltene deposition process. A new deposition apparatus was designed and constructed to investigate asphaltene deposition. The apparatus consists of a packed bed of stainless steel beads over which a mixture of oil and heptane is passed at a specified flow rate and run-time. The asphaltene deposition rate and the mass of deposit can be obtained along the packed bed. The dependency of the asphaltene deposition rate on concentration of unstable asphaltenes and on fluid flow velocity was studied. Experimental results show that a mass-transfer limited deposition model can explain the asphaltene deposition of nanometer-sized unstable asphaltenes in the viscous flow regime. This investigation sheds light on the asphaltene deposition process and provides a new tool that can be used to study asphaltene deposition.

To further develop an understanding of the wax deposition mechanism needed to model its occurrence in subsea oil pipelines, deposition experiments using a pure n-alkane and a binary n-alkane mixture were performed in a cold finger apparatus modified to allow for reliable and accurate visualization of the wax deposit thickness as a function of time. The deposit thickness is found to grow initially but to shrink at long times as the deposit composition enriches in the wax content and the wax concentration in the surrounding oil decreases. This non-monotonic time evolution of the deposit thickness and increase in wax composition are explained and modeled using transient heat and mass transfer kinetics. The ultimate shrinkage of the deposit thickness is traced to depletion of soluble wax in the bulk oil because soluble wax continuously diffuses from the bulk oil into the deposit and precipitates there by fast precipitation kinetics.

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The kinetics of asphaltene precipitation was investigated for five crude oils diluted with n-heptane at 21 °C in air and nitrogen atmospheres. The onset of precipitation, defined as the precipitant (n-heptane) content at which detectable asphaltene particles first appear, was measured in air at different contact times using optical microscopy and a gravimetric method. Asphaltene yields (mass asphaltene/mass oil) were measured in air over time gravimetrically. The data were compared with yields and "yield onsets" previously measured for the same mixtures in a nitrogen atmosphere. 1 In a nitrogen atmosphere, the yields increased and the onsets decreased over approximately 50 h but then reached plateau values, indicating that an equilibrium condition existed. In an air atmosphere, the yields and onsets were the same as in nitrogen for the first 50 h, but then the yields gradually increased for the duration of the experiments. The onsets shifted to lower values over time, and there was no equilibrium onset condition. Hence, precipitation data collected in air below 50 h can be used for kinetic modeling as is; however, data collected in air over longer times overstates asphaltene yields under anaerobic conditions and requires correction. It is hypothesized that the oxygen in the air catalyzes or participates in reactions that alter the asphaltenes and other crude oil components over time so that they become less soluble. The oxidation rate appears to correlate approximately with the asphaltene content of the oil. The population balance first developed by Maqbool et al., Modeling the Aggregation of Asphaltene Nanoaggregates in Crude Oil-Precipitant Systems. Energy Fuels, 25 (4), 2011, 1585-1596, and later modified by Duran et al., Kinetics of Asphaltene Precipitation/Aggregation from Diluted Crude Oil. Fuel, 255, 2019, 115859, was further adapted to account for the increase in yield over time due to oxygen by introducing a term for the generation of unstable asphaltene primary particles. The proposed model matched the precipitation yield data from this study and from the literature with an average absolute deviation of less than 2 wt %.