Effect of Air on the Kinetics of Asphaltene Precipitation from Diluted Crude Oils

The kinetics of asphaltene precipitation was investigated for five crude oils diluted with n-heptane at 21 degrees 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 %.