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Crude compatibility is a very important property to consider when making decisions regarding which crude to purchase. Blending crudes that are incompatible can lead to extensive fouling and processing difficulties due to unstable asphaltenes. These problems can quickly reduce the benefits of purchasing the opportunity crude in the first place. For example, extensive fouling in the crude preheat train may occur resulting in decreased energy efficiency, increased CO2 emissions, and increased frequency at which heat exchangers need to be cleaned. 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: (1) self-incompatible crudes, (2) incompatible crude blends, and (3) nearly incompatible crudes. However, 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 are less asphaltenes to agglomerate.
Self-incompatible crudes are those that contain unstable asphaltenes even when not blended with other crudes. These crudes are more likely to cause fouling and processing problems than compatible crudes. Over 20 such crudes have been identified. These crudes are prone to asphaltene destabilization and fouling even when the crude is not blended with other crudes. It has been recommended that refiners generally avoid buying self-incompatible crudes.
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. Changing the ratio of the molecules or increasing the concentration of saturates (i.e., paraffins) may result in asphaltenes precipitating out of the crude. The asphaltene precipitation causes fouling and processing problems. In general, mixing asphaltenic crudes with crudes that are more paraffinic causes the asphaltenes to become destabilized. 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 third category is nearly incompatible crudes. According to the Oil Compatibility Model (one option for determining compatibility), nearly incompatible crudes are those with a solubility blending number (SBN) to insolubility number (IN) ratio in the range of 1.0-1.3. Nearly incompatible crudes are likely to cause moderate fouling and contribute to rag layers (emulsion layer) in the desalter. Due to the fact that nearly incompatible oils form smaller asphaltene particles than those formed by incompatible crudes, nearly incompatible oils cause more stable water-in-oil emulsions; the smaller asphaltenes are able to more tightly surround water drops.
The problem of asphaltene instability when mixing crudes can be assessed by several analytical procedures, including the ASTM spot test and others as below. Although these tests have shortcomings, representatives at a past NPRA Q&A session noted that refiners who do not run tests to investigate crude compatibility are much more likely to run into processing problems than those who do test for compatibility.
The Asphaltene Stability Index Test (ASITTM) is a proprietary test developed by Baker Petrolite that examines the stability of asphaltenes and determines fouling potential in a crude oil or crude blend.
Finnish Measurement Systems markets the Porla analyzer, which is an automated system for determining the solubility blending number (SBN) and insolubility number (IN) for crude.
Formulaction offers an instrument called Turbiscan that can be used to examine the compatibility of crudes.
A spot test, developed originally for blending residual fuel oil, was standardized as ASTM D4740.
Lawler's Automated Fuels Stability Analyzer examines the stability of asphaltenes in oils, including crude oils.
LT Industries offers ParaFuel crude oil and fuel analyzers. ParaFuel analyzers use near infrared spectroscopy to quickly determine characteristics of crude samples.
The Asphaltene Dispersant Test (ADT) is a test developed by Nalco Energy Services (formerly Ondeo Nalco) to examine the presence and stability of asphaltenes in a solution.
Process NMR Associates's Crude Oil Blending application uses nuclear magnetic resonance (NMR) to determine the API gravity, boiling point, carbon distribution, viscosity, pour point, water content, and sulfur content of crude samples.