Chromatography asphaltenes

Liquid chromatography is preceded by a precipitation of the asphaltenes, then the maltenes are subjected to chromatography. Although the separation between saturated hydrocarbons and aromatics presents very few problems, this is not the case with the separation between aromatics and resins. In fact, resins themselves are very aromatic and are distinguished more by their high heteroatom content (this justifies the terms, polar compounds or N, S, 0 compounds , also used to designate resins). 

Brunnock et al. [67] have also determined beach pollutants. They showed that weathered crude oil, crude oil sludge, and fuel oil can be differentiated by the n-paraffin profile as shown by gas chromatography, wax content, wax melting point, and asphaltene content. The effects of weathering at sea on crude oil were studied parameters unaffected by evaporation and exposure are the contents of vanadium, nickel, and n-paraffins. The scheme developed for the identification of certain weathered crude oils includes the determination of these constituents, together with the sulfur content of the sample. 

The separation of coal liquids by gel permeation chromatography using lOOA Styragel columns and solvents such as THF and toluene has been reported elsewhere (7.8.9.13.14). Coal liquids and petroleum crude are similar in their physical appearance as well as the complexity in composition. The major difference between the two is that petroleum crude does not contain oxygenated compounds, such as alkylated phenols, in substantial quantity. In addition, the average linear molecular size of petroleum derived asphaltenes (15.16) is much larger than that of coal derived asphaltenes (. ... 

Heavy hydrocarbons were obtained by solvent extraction (4) of sediments, deasphalting with pentane, and separation by liquid chromatography (5) into saturate, aromatic, NSO-eluted, and asphaltene fractions. Saturate fractions were analyzed by gas-liquid chromatography (6) on these chromatograms (Figures 4 and 6) n-paraffins stand up as peaks above the naphthenic background. 

Fractionation of petroleum in the refinery, to obtain streams with specific boiling ranges for various downstream processes, is performed by distillation in a crude unit. To determine how Ni and V compounds are distributed as a function of boiling point is, therefore, useful for evaluating their impact in the refinery. Petroleum may also be fractionated by solvent separation and chromatography to obtain more detailed information on the distribution of Ni and V compounds. This section will review the available literature on how metals are distributed in petroleum by boiling point and solubility class. It will also include some discussion of the structure of heavy oil in general and asphaltenes in particular. Vercier etal. (1981) have provided an excellent review of methods and procedures involved in petroleum fractionations. 

Petroleum can be fractionated into four generic types of materials representing general chemical properties. These include saturated hydrocarbons, aromatic hydrocarbons, resins, and asphaltenes. The standard ASTM separation procedure (D2007) for isolating the asphaltenes and the other components in petroleum is based on solubility behavior and chromatography, as shown in Fig. 5. Commerically, many refineries utilize solvent separations to produce a solvent deasphalted oil which has lower impurity levels. 

Boduszynski et al. (1980) also employed the more conventional separation procedure based on solubility properties (Corbett, 1969) to provide asphaltene and maltene samples from the 675°C+ residuum. Asphaltenes are isolated by precipitation in an alkane solvent, with further separation of maltenes by chromatography in solvents of increasing elution strength. The FIMS results in Fig. 9 illustrate, significantly, that asphaltenes are not necessarily the highest-molecular-weight components in residuum. Asphaltenes have, rather, a relatively low but broad distribution of molecular weights. 

Narrow cuts of asphaltenes from Kuwait atmospheric residuum with molecular radius ranging from 26 to 153 A (based on size exclusion chromatography) were examined by Baltus and Anderson (1983) in pores... 

However CCR reduction in SCT SRC occurs with significant alteration of GEC classes, primarily toward the formation of less polar compounds. The classes of W. Kentucky SCT SRC, separated by GEC (gradient elution chromatography), are given in Table 2 and show that 75% of the SRC is polar and noneluted polar asphaltenes. 

Felix, G., Bertrand, C., and Van Gastel, F., A new caffeine bonded phase for separation of polyaromatic hydrocarbons and petroleum asphaltenes by high-performance liquid chromatography, Chro-matographia, 20, 155, 1985. 

For the characterization of RCC feedstocks, it was determined that a more detailed molecular description of the feedstock was necessary. The more detailed molecular description of RCC feedstocks involves dividing the feedstock into six molecular types 1) saturates 2) monoaromatics 3) diaromatics 4) greater than diaromatics 5) polar aromatics and 6) asphaltenes. This separation of the RCC feedstock is accomplished by using high performance liquid chromatography. 

The extracts were fractionated by a Preparative Liquid Chromatography method - PLC-8 [2], in eight distinct chemical classes FI-saturated hydrocarbons (HC), F2-monoaromatics, F3-diaromatics, F4-triaromatics, F5-polynuclear aromatics, F6-resins, F7-asphaltenes and F8-asphaltols. This method, proposed by Karam et al. as an extension of SARA method [4], was especially developed for coal-derived liquids. It combines solubility and chromatographic fractionation, affording discrete, well-defined classes of compounds which are readable for direct chromatographic and spectroscopic analysis. 

Asphaltenes are, by definition, a solubility class (8, 9, 10) that is precipitated from petroleums and bitumens by the addition of a minimum of forty volumes of the liquid hydrocarbon. In spite of this, there are still reports of asphaltenes being isolated from crude oil by much lower proportions of the precipitating medium (II), which leads to errors not only in the determination of the amount of asphaltenes in the crude oil but also in the determination of the compound type. For example, when insufficient proportions of the precipitating medium are used, resins (a fraction isolated at a later stage of the separation procedure by adsorbtion chromatography) also may appear within the asphaltene fraction by adsorbtion onto the asphaltenes from the supernatant liquid and can be released by reprecipitation in the correct manner (12). Thus, questionable isolation techniques throw serious doubt on any conclusions drawn from subsequent work done on the isolated material. 

The agreement between the two analyses regarding the percentage saturates in the asphaltene neutrals is satisfactory as follows. The Oklahoma State University (OSU) value of 3.9% was obtained from silica gel chromatography of the pentane-soluble (88.2%) asphaltene neutrals and, hence, represents a lower limit for the percentage saturates in the asphaltene neutrals. The value of 5.2% results from a proprietary Exxon saturate/aromatic hydrocarbon EI/MS technique developed for low-boiling coal liquids. 

The first three solvent systems eluted from silica over 17% of the applied asphaltenes while the quantity of neutral asphaltenes from ion exchanger chromatography, that is, those components that passed both the anion and cation exchanger columns, amounted to 20% of the material. This is a reasonably close agreement between fractions of asphaltene separated by two different methods. At first sight, this suggests that these fractions are very similar however, comparison of MWs of these fractions does not support this conclusion. 

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