(108c) Equivalent Residence Time (ERT) As a Potential Correlation Parameter for Bitumen Visbreaking

The viscosity and density of bitumen must be reduced prior to transport in order to meet pipeline specifications. Typically, a diluent is added to do so, but this method reduces the capacity of the pipeline for bitumen. One approach to reduce the required amount of diluent is visbreaking. Visbreaking is a mild thermal cracking process that reduces the oil viscosity significantly and moderately reduces its density. The higher the severity (temperature and residence time), the greater the viscosity reduction. However, the practical extent of viscosity reduction is limited by the final product stability; that is, its tendency to precipitate asphaltenes or form coke.

To design a visbreaking process, it is desirable to predict the product properties and stability as a function of the reaction temperature and residence time. Visbreaking conversion increases as the temperature and residence time increase and both must be considered when attempting to predict the product properties. ERT is a measure of severity that combines the contributions of temperature and residence time into a single expression, given by (Yan, 1990):

where is the residence time in s, is the activation energy (209,500 J/mol), is the ideal gas constant in J/mol∙K, is the reaction temperature in K, and is the reaction temperature reference (711.15 K). The objective of this study is to determine if conversion, product properties and stability follow consistent trends with ERT and, if so, to establish correlations of properties to ERT.

An Athabasca bitumen was visbroken in a continuous induction-heated reactor at constant pressure and over a range of severities. The following measurements were performed on visbroken products at different combinations of temperature and residence time: SimDist, C₇-asphaltene content, toluene insoluble (TI) content, gas yield, viscosity, and density. The distillate content and conversion were determined from the SimDist. The product stability was assessed in terms of the wt% of n-heptane required to cause asphaltene precipitation (onset).

As ERT increased, conversion was found to increase monotonically but in three distinct zones. In the first zone (ERT up to 1400 s), conversion increased significantly and almost linearly to 38%. The distillable fraction (Tb<367) increased from 17 to 36 wt%, the asphaltene content increased from 12.6 to 15.0 wt%, and there was no change in the TI content. The composition changes are consistent with scission reactions where the removed side chains contributed to the distillate content, and the partially stripped polynuclear aromatic cores added to the asphaltene content. In the second zone (1400 s < ERT < 3000 s), the conversion increased gradually from 36 to 42%. The distillable fraction increased from 36 to 39 wt%, the asphaltene content increased from 15.0 to 15.9 wt%, and the TI content increased to 0.06%. The small increase in distillates suggests that the scission reactions almost ceased while the increase in the TI content suggests that condensation reactions began. In the third zone (ERT > 3000 s), the conversion increased more steeply reaching 46.1% as the ERT increased to 3,200 s. The distillable fraction increased only slightly to 40.9%, the asphaltene content increased to 16.1% and the TI content increased to 0.08%. It appears that condensation reactions dominated in this zone. The gas yield was below 1.5% in all cases.

The changes in properties also follow distinct trends in each ERT zone. In the first zone the viscosity decreased from 816,000 to 1500 mPa∙s, the density decreased from 1,012 to 993 kg/m³, and the onset decreased from 59 to 20 wt% n-heptane. These changes are consistent with the significant increase in distillate content. Changes in the asphaltene properties have also been shown to contribute to the decrease in density and viscosity (Marquez et al., 2020). In the second zone the viscosity decreased to 850 mPa∙s, the density decreased to 989 kg/m³, and the onset decreased to 19%, consistent with the small generation of distillables, asphaltenes, and toluene insolubles. In the third zone, the viscosity decreased to 460 mPa∙s, the density increased with respect to the previous zone to 989.5 kg/m³, and the onset decreased to 7%. These changes are consistent with condensation reactions dominating.

Similar property changes were obtained at same ERT even with different combinations of time and temperature. Hence, ERT is a sufficient parameter to correlate conversion, product composition, and properties, at least for one bitumen and a specific reactor. Preliminary correlations are proposed for conversion and product density, viscosity, and stability as a function of ERT. It remains to test the correlations on other feeds and reactors.

References

Marquez, A., Schoeggl, F. F., Taylor, S. D., Hay, G., and Yarranton, H. W. (2020). Viscosity of characterized visbroken heavy oils. Fuel, 271(January), 117606. https://doi.org/10.1016/j.fuel.2020.117606

Yan, T. Y. (1990). Characterization of visbreaker feeds. Fuel, 69(8), 1062–1064. https://doi.org/10.1016/0016-2361(90)90021-H