The challenges of crude blending - Part 2
The second part of a two-part article details the importance of crude selection and blending strategies to prosper in a low cost yet volatile market.
THOMAS GARRETT, PATRICK CHRISTENSEN, WILLIAM VUKOVICH and THOMAS YEUNG
Hydrocarbon Publishing Company
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The first part of this article (PTQ, Q1 2016) discussed how declining crude prices encourage more spot purchases that save feedstock cost, but mixing different grades to create look-alike crudes has created headaches for refiners. Increased spot purchases put an emphasis on traders and operational personnel being in communication to ensure that new crudes being sourced on the spot market are not creating operational problems once introduced into a refining system. So, what novel strategies and advanced technologies can refiners deploy to thrive in this low cost and yet volatile market?
The price is right
Crude blending is not new as refiners have been buying and blending inexpensive grades for years. However, a recent surge in fuel demand, particularly for motor gasoline, due to falling retail prices has prompted refiners to boost throughput and at the same time source cheaper crudes at home and abroad to increase profit margins. As a result, mixing activity has drastically increased. For instance, Italian refiner Saras reportedly processed 30-35 different grades in 2015, doubling the number of crudes its Sarroch plant handled in 2014.
Meanwhile, declining crude cost has prompted more frequent crude changes. Approximately between 2010 and 2013, US refiners had increased the use of light tight oil (LTO) due to the shale oil boom and at the same time imported more Canadian oil sands bitumen (for example, Western Canadian Select) due to a widening WTI-Brent spread. During this period, the crude slate was changed from a conventional mix of imports from abroad to newer domestic crudes. Then starting in 2014, the spread began to narrow and the costs of moving domestic crudes to both the US East and West Coasts were relatively high. As a result, refiners in these regions are returning to import and blend in more Brent priced foreign oils at a rate comparable to before the shale boom started in 2008-2009. The crude changing phenomenon is further magnified by the recent repeal of the US oil export ban in late 2015 and return of Iranian crude in early 2016.
Asian and European operators, on the other hand, are taking in inexpensive heavy grades from Latin America and in some cases Canada to process along with the conventional light oil. Oil producers in Brazil, Colombia, Ecuador, Kuwait, and Saudi Arabia have also equipped their refineries to process higher volumes of domestic heavy oil. That is to say more crude changes and increased blending are forthcoming in the foreseeable future on a worldwide basis.
There are eight major or primary criteria refiners and traders need to consider in crude selection and blending. In the previous article, we elaborated the challenges of the first four primary criteria, which include API gravity, paraffin content, total acid number (TAN), and contaminants. In this article, challenges of the other four criteria – asphaltene content, crude compatibility, desirable cuts, and carbon footprint – are addressed.
When heavy asphaltenic crude oil is blended with paraffinic light tight oil, the resulting blend can lead to asphaltene instability, thereby creating sludge deposits. Common problems are: formation of stable emulsion in the desalter, rendering poor separation; build-up of sludge in crude storage tank, reducing capacity; and fouling of downstream process equipment.
Asphaltenes, which have the highest molecular weights and are the most polar and most aromatic among the fractions of crude oil, have been blamed for a range of processing problems including extensive fouling and poor desalter performance. Asphaltene composition is crude specific. Figure 1 shows the elemental composition of the asphaltene fraction for seven different crudes. The asphaltene fraction was obtained using n-heptane as the solvent. One can see that the elemental composition varies significantly among these crudes. The ranges for the elements on a wt% basis are: carbon (78.4 to 87.2), hydrogen (7.0 to 7.6), oxygen (0.6 to 4.6), nitrogen (0.9 to 1.6), and sulphur (3.5 to 9.8). While these molecules can be quite problematic for refiners, the structures and chemistry of asphaltenes are still controversial and remain an active area of research. Specific recent research activities cover the composition and structure of asphaltenes, reversibility of asphaltene flocculation, flocculation of asphaltenes in the presence of naphthenic acids, modelling the inhibition of asphaltene aggregation, and kinetics of asphaltene coagulation. The goal of understanding the chemistry of asphaltenes is to identify methods by which to counter asphaltene fouling (for instance, in heat exchangers and CDU preheat trains) and emulsion stabilisation problems in desalters.
Finally, in addition to the stability of asphaltenes, the total quantity of asphaltenes in the crude is another contributing factor to the severity of asphaltene fouling. For example, crude with low asphaltene stability is less of a problem if the total quantity of asphaltenes in the crude is low since there is less asphaltene to agglomerate.
Crude compatibility, sometimes referred to as miscibility, is a very important property to consider when making decisions regarding which crude to purchase. Blending of incompatible crudes, a common occurrence when creating dumbbell crudes, can lead to extensive fouling and processing difficulties due to unstable asphaltenes. These problems can quickly reduce the benefits of purchasing the inexpensive crudes in the first place. Extensive fouling is particularly a burden on the crude preheat train, where fouling greatly decreases the efficiency of the heat exchangers, leading to an increase in furnace duty, which in turn raises fuel and energy costs as well as CO2 emissions. Furnaces are the largest contributor to greenhouse gases, accounting for over 34% of the total CO2 emissions from all refineries. Fouling across any region of a refinery will also raise maintenance costs and increase the frequency of shutdowns. In a worst-case scenario, crude throughput may be reduced, leading to significant financial losses.
There are three compatibility issues to be aware of when selecting and blending crudes: self-incompatible crudes, incompatible crude blends, and nearly incompatible crudes. Self-incompatible crudes are those that are prone to asphaltene destabilisation and fouling even when not blended with other crudes. These crudes are more likely to cause fouling and processing problems than compatible crudes, and more than 30 such crudes have been identified, including Isthmus (MX).2 It has been recommended that refiners generally avoid buying self-incompatible crudes. However, this advice may be hard to follow in view of a recent increase in the frequency of self-incompatibility observed for West Texas Intermediate (WTI) in the US. The cause has been attributed to contamination during transportation. Irv Wiehe of Soluble Solutions has recommended both increased testing for compatibility as well as transporting crudes that are compatible with both light oils and heavy oils in between shipments of the latter two.
Incompatible crudes are those that are stable individually, but become unstable when blended. In stable crudes, asphaltenes are generally dispersed in the crude by resins that are then dissolved in the oil by small ring aromatics. In general, mixing asphaltenic crudes with crudes that are more paraffinic, such as LTOs, causes the asphaltenes to become destabilised. The oils in the blend may be incompatible over some range of composition, or they may become incompatible if combined in the wrong order or, for certain blends, at high temperatures. The asphaltene precipitation that stems from asphaltene/paraffin immiscibility can lead to a lowering of capacity in crude tanks, emulsion formation in the desalter, and fouling in the preheat exchangers and furnace tubes.7 In order to avoid asphaltene precipitation problems when blending tight oils and heavy crudes, it is recommended that tight crudes be blended with super heavy crudes (API gravity of 13-16°API) in order to yield a crude blend with an API gravity range of 21-23°API.
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