Chromatographic separations asphaltenes

Without going into details of the chromatographic method, a SAR separation (asphaltenes having been eliminated) can be performed in a mixed column of silica followed by alumina. The saturated hydrocarbons are eluted by heptane, the aromatics by a 2 1 volume mixture of heptane and toluene, and the resins by a 1 1 1 mixture of dichloromethane, toluene and methanol. 

The crude oil produced from the Main Zone of the Torrance Field has an API gravity of 18° and contains 5.3 weight percent asphaltenes. The solubility of the asphaltene molecules in Main Zone oil was measured by the Oliensis Test(35). In this test, the solubility parameter Qf ie oil was lowered by adding to the oil successively larger volumes of hexadecane, a poor solvent for asphaltene molecules. The minimum volume (in milliliters) of hexadecane, which when added to 5 g of crude oil, will cause the chromatographic separation of the asphaltene fraction is termed the Oliensis Number. The Oliensis Number for the Main Zone crude oil is 3, indicating that the asphaltene molecules are not well-solubilized in the oil. Small changes in the solubility parameter of the Main Zone oil can cause the asphaltenes to precipitate. 

Liquid Chromatographic Analyses. Analyses of about half the samples by the standardized method of liquid chromatographic separation are presented in Table IX. lii addition to asphaltenes and minerals, all contained large amounts of resins and small amounts of oils that are mostly hydrocarbons. Similar results were obtained whether the samples were analyzed as bitumens or as asphalts. In general, however, the bitumens gave somewhat more resins and hydrocarbons, whereas asphalts gave more asphaltenes. With Farukhabad 404, which was analyzed both as bitumen and as asphalt, the difference in asphaltenes was 8%. 

Structure-Related Properties of Athabasca Asphaltenes and Resins as Indicated by Chromatographic Separation... 

It was concluded that definition of asphaltenes based only on solubility is not a satisfactory criterion and that the behavior of asphaltenes in chromatographic separations is incompatible with such structures where the polymer units are interconnected predominantly by a-bonds. The asphaltenes are a complex state of aggregation best represented by the stacked cluster structure (micelle), which, however, cannot explain some of the GPC behavior of very dilute asphaltene solutions. 

Some time ago work was initiated on chromatographic separations of asphaltenes and resins in various chromatographic systems (GPC, ion exchangers, adsorption) to compare their behavior in these analytical systems. Part of the results have been reported previously (10), but they are used here to provide a better overview of the results obtained from separating the same materials by various methods. Literature data on the various procedures were... 

Important analyses for the whole crude are as follows, including a liquid chromatographic separation adapted from the published SARA procedure (Saturates-Aromatics-Resins-Asphaltenes) for isolation of seven classes of compounds from mid-distillate (9). 

An atmospheric residue from heavy crude oil was used in order to obtain different fractions by means of chromatographic separation. Table 5.1 shows the properties of the heavy crude and its atmospheric residue. Asphaltenes were obtained by precipitation with -heptane in a pressurized system (25kg/cm ), with nitrogen as inert at temperature of 60°C and constant stirring. SARA fractionation was carried out in two chromatographic columns packed with clay and alumina. More details about the fractionation procedure are reported elsewhere (Alvarez et al., 2011). 

The oils and bitumen were separated chromatographically on silica gel into saturates, aromatics, polar aromatics, and asphaltenes fractions by the SAPA method of Barbour et al. (8). 

SRC, a detailed examination of the composition of these coal liquids is of fundamental importance. Numerous procedures have been published previously for investigating the composition of liquids derived from coal. In general, these procedures combine separation techniques with a variety of spectroscopic methods to provide the desired quantity of structural information. The separation techniques used include methods based on solubility fractionation (4,5), methods combining solubility fractionation and adsorption chromatography (6), and liquid chromatographic procedures for chemical fractionation (7,8). Chemical reactions also have been used to separate coal liquid asphaltenes into acidic and basic fractions (9). 

However, fractional separation has been the basis for most asphalt composition analysis (Fig. 15.5). The separation methods that have been used divide asphalt into operationally defined fractions. Three types of asphalt separation procedures are now in use (a) chemical precipitation in which n-pentane separation of asphaltenes is followed by chemical precipitation of other fractions with sulfuric acid of increasing concentration (ASTM D-2006) (b) adsorption chromatography with a clay-gel procedure in which, after removal of the asphaltenes, the remaining constituents are separated by selective adsorption/desorption on an adsorbent (ASTM D-2007 and ASTM D-4124) and (c) size exclusion chromatography in which gel permeation chromatographic (GPC) separation of asphalt constituents occurs based on their associated sizes in dilute solutions (ASTM D-3593). 

Zn, Hg, Co, Cr and Ni. Examination of the absorption spectra of the methanol-soluble, resin, and asphaltene fractions of the oil showed the presence of porphyrin complexes in all fractions. Liquid chromatography on silica and alumina was used to separate the metalloporphyrins in the low-molecular fractions of the three oil components. Only Ni porphyrins and very small amounts of V porphyrins were identified in the chromatographic fractions. The absorption spectrum of the separated Ni porphyrins corresponds to Ni deoxophyl-... 

The analysis presented in this work suggests that the chromatographic method used for separation of acids from extraveavy crude oil is efficient, effective and relatively fast. Table 1 shows the yields of the different isolated fractions. AcJ acids represent 1.6% of the crude oil, i.e. half of the acids present in the Carabobo crude oil (Acevedo et al., 2005). However, these crude oil components showed a good interfacial activity, as will be shown later. Table 2 shows the H/C values. AcJ acids, which are amber colored resins, are a highly aliphatic fraction according to this ratio (H/C =1.6), contrasting with AJ and ASA, which are blackish brown and show an H/C value of about 1.12, which was expected for asphaltenes. 

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