Storage Stability of Products from Visbreaking of Oilsands Bitumen

Abstract

Thermal conversion of bitumen by visbreaking is an important technology for partial upgrading of bitumen to pipeline transportable oil. The product from visbreaking of bitumen will likely spend a long time in storage or transport before being used, and due to cost considerations, postprocessing should be minimal. This study investigated the changes that take place over time during storage of visbroken bitumen (420 °C, 18–22 min) under nitrogen atmosphere in the dark. The study did not investigate phase instability, and it focused on chemical and physical changes taking place over a storage period of 20 weeks. With increasing storage time many properties of the visbroken liquid product changed; notably, refractive index, density, viscosity, microcarbon residue, and n-pentane insoluble content increased, while free radical content and aromatic hydrogen content decreased. The observed changes during storage are best explained by the formation of products with higher molecular mass. Although care was taken to limit the potential impact of oxidation, indirect evidence of the impact of oxidation due to dissolved oxygen was found during the initial 2 weeks of storage. Oxidation was not a prerequisite for free radical reaction during storage because the visbroken product already contained persistent free radicals, as is typical of high boiling petroleum fractions. Free radical termination reactions took place, but the extent of the changes in the properties with storage time was not consistent with simple radical–radical termination reactions as the only source of heavier products. Alkenes (olefins) formed during thermal conversion were likely involved in free radical propagation reactions by radical–alkene addition to form heavier products. The relationship between free radical content and storage instability, the nature of reactions taking place during storage, and the nature of the species in visbroken bitumen leading to storage instability were discussed. To reduce the formation of products with higher molecular mass during the storage of visbroken material, in addition to maintaining oxygen-free conditions, it is necessary to decrease the amount of free radicals and the amount of alkenes in the product.

Application of Ethylene Tar as an Additive in Visbreaking of Petroleum Vacuum Residue

“…Still, a high content of unsaturated hydrocarbons in the product can compromise its stability during storage. Alkenes quickly become involved in various reactions and contribute to an increase in the proportion of high-molecular-weight components. , On the other hand, in oil refineries, alkenes produced during pyrolysis or catalytic cracking of oil fractions are the target products …”

“…Asphaltenes in the liquid visbreaking product are generally less stable than asphaltenes in the feedstock because of their reduced solubility after thermolysis, and for that reason, they are prone to precipitate . Various methods are used to determine asphaltene stability in an oil system. ,− One is the colloidal instability index (CII), defined as the ratio of the mass of the total asphaltenes and flocculants (saturated hydrocarbons) to the mass of the total rubber peptizing agents (resins and aromatic hydrocarbons) in a colloidal oil system. The higher the CII value, the less stable the colloidal system. , The lowest value was observed in the VR + ET product (Table ), as cracking of resins, in this case, occurred to a greater extent and formed a greater quantity of aromatic hydrocarbons that increase the colloidal stability of asphaltenes in the visbreaking product.…”

“…Alkenes quickly become involved in various reactions and contribute to an increase in the proportion of high-molecular-weight components. 43,44 On the other hand, in oil refineries, alkenes produced during pyrolysis or catalytic cracking of oil fractions are the target products. 45 It is worth noting that for the VR + ET product, the content of alkenes is the lowest (1.2 wt %), although the ET additive contained more alkenes, mainly cycloalkenes (25.07 wt %) in Figure 2.…”

Processing Unconventional Oil: Partial Upgrading of Oilsands Bitumen

“…One obvious challenge for partial upgrading when H 2 is not used is dealing with the olefins that are produced during thermal conversion (Sections and ). In addition to the specification limits in Table , phase stability may become a problem, and the density and viscosity of thermally converted material may increase during storage …”

“…In addition to the specification limits in Table 4, phase stability may become a problem, 55 and the density and viscosity of thermally converted material may increase during storage. 71 These problems are not caused by the olefins per se but rather by having olefinic material present in a mixture with persistent free radicals. The presence of persistent free radicals in heavy oils cannot be avoided and are common in thermally treated and in straight run heavy oil fractions.…”