Stratification of Crude Oil Residues at Elevated Temperatures Detected Via in-Situ ESR
The prevention or reduction of fouling caused by asphaltene deposition in pipelines, heat exchangers, and furnaces is one of the most serious challenges that oil industry faces nowadays. The temperature and pressure conditions involved in crude oil and resid processing may also trigger or exacerbate the formation of asphaltene deposits. In order to select the optimum temperature and/or pressure, as well as the appropriate feedstock composition that does not lead to asphaltene instability, it is important to understand and evaluate the behavior of the heavy components present in crude oils at elevated temperatures.
There are limited number of physical methods and techniques that can provide in situ information about the heavy components in crude oils in molecular scale and at elevated temperatures and pressures. Recently, we have shown that ESR using the VO2+ as a natural spin probe can provide quantitative information about the size distribution of vanadyl-containing fragments (asphaltenes and their aggregates) in heavy oil fractions. It was also shown that the precise simulation of the fine structure (FS) of ESR spectra of the VO2+ions , measured in oils at temperatures when the partial averaging of the FS anisotropy is observed, allow us to separate the contributions of asphaltene molecules of different sizes that move with different characteristic rotational time.
The developed approach was used to register in situ the ESR spectra of atmospheric residues at temperatures of up to 420 °C. The simulated spectra fitted well with the experimental results obtained for all the evaluated residues at T < 350 °C using the smooth size distribution of vanadyl-containing fragments (VCF). Increase in temperature from 250 °C to 350 °C, decreased the viscosity of the fluid with concomitant decrease in the maximum size of the VCF; this response seems to be aligned with the disaggregation of the asphaltene agglomerates.
At 420 °C, the satisfactory agreement between experimental and simulated spectra could not be obtained using just plain size distribution of vanadyl-containing aggregates alone. The relatively good fitting between experimental and calculated spectra has been achieved using a two-step distribution function. Optimization of the parameters provided a satisfactory fitting of the simulated spectra with the experimental values. Similarly, we could obtain quantitative information about the size distribution of VCF present in the sample at elevated temperatures. The results also showed that some vanadyl-containing fragments remain mobile, while some "precipitate" or move to a sedentary state at high temperature.
Thus, the precise simulation of ESR spectra of vanadyl-containing fragments obtained in situ provides quantitative information on the dynamics and size changes of heavy components in crude oils at elevated temperatures.