Oct-2011

Assessment of crude oil blends

A refiner’s assessment of the compatibility of opportunity crudes in blends aims to avoid the processing problems introduced by lower-quality feedstocks

Vivek Rathore, Rajiv Brahma, Tushar S Thorat, P V C Rao and N V Choudary
Bharat Petroleum Corporation

Article Summary

Opportunity crude oils and their blends play an important role in increasing refinery profitability, but the risks  are high because they are usually laden with contaminants such as destabilised asphaltenes, waxes and a high metals content. These contaminants can cause stable oil-water emulsion problems, heat exchanger fouling and catastrophic coking in furnace tubes, leading to high maintenance costs and equipment losses. Furthermore, incompatible crude oil blends can result in the flocculation and deposition of asphaltenes. The determination of insolubility number (IN) and solubility blending number (SBN) are key parameters in the prediction of flocculation when dealing with incompatible crude oil blends.

This article discusses the determination of IN and SBN for crude oil and the prediction of a compatibility scale for various blends. The results are compared with the colloidal instability index (CII) and a good correlation is observed. The criteria to mitigate problems during the processing of incompatible crude oil blends are also discussed.

Opportunity crudes: challenges and opportunities
Opportunity crude oils have attracted the attention of oil companies looking to increase their gross refinery margin. Although blends of various crude oils are often used in refining processes, this practice has several constraints in terms of logistics, such as the non- availability of sufficient numbers of storage tanks and vessels. It also has some unwanted consequences in terms of fouling in the preheat trains and heat exchangers, and coking in the pipe still furnace tubes. These problems may be caused by the precipitation of asphaltenes, oxidative polymerisation and the components of coke formation in the oil. Salts, sediment and corrosion products can arise from the impurities. The problems associated with the flocculation and deposition of asphaltenes can further increase the cost of oil recovery processes. Therefore, a detailed knowledge of the factors that affect the composition and physico-chemical structure of crude oils is necessary.

Structure of crude oil
The components of crude oil are broadly classified into four chemical classes based on differences in solubility and polarity. These components are called saturates (S), aromatics (A), resins (R) and asphaltenes (A). Asphaltenes are the major cause of fouling in crude oils and their blends during refining processes. They represent a wide variety of hydrocarbon molecules that are typically polyaromatic in nature, with some degree of alkyl substitution, and usually contain heteroatoms such as oxygen, nitrogen, sulphur and metal atoms in their structures. Asphaltenes are dispersed in oil with resins, and this dispersion is dissolved into petroleum oils with aromatics (solvents) but opposed by saturates (non-solvents). Thus, any variation in the original composition takes place during the blending of different crude oils. Hence, asphaltenes are held in petroleum oils in a delicate balance, and this balance can easily be disturbed by adding saturates or removing resins and aromatics. Therefore, the blending of oils can significantly  change the overall concentrations of SARA at a molecular level, subsequently disturbing the delicate balance and precipitating asphaltenes.  

Considering the hypothesis that resins and asphaltenes are always associated with each other, an oil’s behaviour is also guided by solubility and its aromatics-saturates balance. Therefore, asphaltenes are one of the major contributors to fouling problems during refining processes. Hence, fouling problems can be counteracted by preventing the precipitation of asphaltenes.

The prediction of fouling requires two dimensionless solubility parameters: IN and SBN. These are determined by mixing individual crude oils with non-polar solvent (toluene) and polar solvent (n-heptane). The point of incipient  (the point at which the separation of asphaltenes from a crude oil becomes apparent) locates the asphaltene precipitation. Crude oil blends are compatible when the volumetric SBN is greater than IN for any oil in the blend. The region where the SBN of any component of the feed stream is equal to or less than the IN of any component of the stream predicts incompatibility behaviour. The work reported here measured compatibility parameters such as IN and SBN for asphaltenic crude oils. The results were compared with theoretical models reported in the literature. The compatible blends obtained were also compared with the CII to predict the stable blend region.

Analysis of crudes
Asphaltenic crude oil samples, called Crudes A, B, C and D here, were selected for the experiments. Solvents such as n-heptane and toluene were used without further purification.

The crude oils and their blends were examined using the spot test method and optical microscopy. In the spot test method, a drop of the blend of test solvent mixture and oil is put on a piece of filter paper and dried completely. If the asphaltenes are insoluble, a dark ring or circle is seen around the centre of the yellow-brown spot made by the oil. If the asphaltenes are soluble, the colour of the spot made by the oil is relatively uniform in colour. Alternatively, samples were also analysed using an optical microscope with a magnification of 10 x 100 for evidence of asphaltene precipitation. The physical properties of the crude oils used in the study are summarised in Table 1.
 
Assessment
The IN and SBN for a petroleum oil containing asphaltenes was determined by establishing the solubility of the oil in a test solvent mixture, with a minimum of two volume ratios of oil to solvent mixture. The test mixtures were prepared by mixing two solvents in various proportions. One polar solvent (toluene) and one non-polar non-solvent (n-heptane) were used. These tests are generally classified on the basis of the selected solvents, such as “toluene equivalence test” and “heptane dilution test”.

For the toluene equivalence test, convenient volume ratios of oil to test solvent mixture were selected, such as 0.1 and 0.2. Various mixtures of the solvent were prepared by blending toluene and n-heptane in known proportions, then 1 ml or 2 ml of oil was added to 10 ml of solvent mixture and mixed well. After waiting for five minutes at room temperature, solubility or insolubility were determined for each of these samples using the spot test method or optical microscopy, as described earlier.