Petroleum hydrocarbons asphaltenes

Petroleum is typically described in terms of its physical properties (such as density and pour point) and chemical composition (such as percent composition of various petroleum hydrocarbons, asphaltenes, and sulfur). Although very complex in makeup, crude can be broken down into four basic classes of petroleum hydrocarbons. Each class is distinguished on the basis of molecular composition. In addition, properties important for characterizing the behavior of petroleum and petroleum products when spilled into waterways or onto land and/or released into the air include flash point, density (read specific gravity and/or API gravity), viscosity, emulsion formation in waterways, and adhesion to soil. 

The character of fuel oil generally renders the usual test methods for total petroleum hydrocarbons (Chapters 7 and 8) ineffective since high proportions of the fuel oil (specifically, residual fuel oil) are insoluble in the usual solvents employed for the test. In particular, the asphaltene constituents are insoluble in hydrocarbon solvents and are only soluble in aromatic solvents and chlorinated hydrocarbons (chloroform, methylene dichloride, and the like). Residua and asphalt (Chapter 10) have high proportions of asphaltene constituents, which render any test for total petroleum hydrocarbons meaningless unless a suitable solvent is employed in the test method. 

There are, however, important differences between resins and asphaltenes. Asphaltenes do not dissolve in petroleum but are dispersed as colloids. Resins readily dissolve in petroleum. Pure asphaltenes are solid, dry, black powders and are nonvolatile. Pure resins are heavy liquids or sticky solids and are as volatile as hydrocarbons of the same size. The resins of high molecular weight are red the lighter resins are less colored. 

All caustobolites, however, contain carbon, hydrogen and oxygen even though in different proportions for different caustobolites. Specifically, petroleum is a complex mixture of hydrocarbons and other carbon compounds. At the elemental level, it consists of elements such as carbon (84-87%) and hydrogen (12-14%) as well as oxygen, nitrogen and sulfur (1-2%). The sulfur content can sometimes be up to 3-5%. Overall, petroleum consists of hydrocarbons, asphaltenes and resins, paraffins, sulfur and ash. There are three main groups of hydrocarbons in petroleum—namely, paraffinic, naphthenic and aromatic hydrocarbons [2]. 

Petroleum crude oil, gas condensate, and natural gas are generally complex mixtures of various hydrocarbons and nonhydrocarbons with diverse molecular weights. In order to analyze the contents of a petroleum fluid it is a general practice to separate it first into five basic fractions namely, volatiles, saturates, aromatics, resins, and asphaltenes [74, 77]. Volatiles consist of the low-boiling... 

Residua are the dark-colored nearly solid or solid products of petroleum refining that are produced by atmospheric and vacuum distillation (Figure 11.1 Chapter 3). Asphalt is usually produced from a residuum and is a dark brown to black cementitious material obtained from petroleum processing that contains very high-molecular-weight molecular polar species called asphaltenes that are soluble in carbon disulfide, pyridine, aromatic hydrocarbons, and chlorinated hydrocarbons (Chapter 3) (Gruse and Stevens, 1960 Guthrie, 1967 Broome and Wadelin, 1973 Weissermel and Arpe, 1978 Hoffman, 1983 Austin, 1984 Chenier, 1992 Hoffman and McKetta, 1993). 

Asphaltene (asphaltenes) the brown to black powdery material produced by treatment of petroleum, petroleum residua, or bituminous materials with a low-boiling liquid hydrocarbon (e.g., pentane or heptane) soluble in benzene (and other aromatic solvents), carbon disulfide, and chloroform (or other chlorinated hydrocarbon solvents). 

Deasphaltening removal of a solid powdery asphaltene fraction from petroleum by the addition of low-boiling liqnid hydrocarbons snch as n-pentane or n-heptane nnder ambient conditions. 

Deasphalting the removal of the asphaltene fraction from petrolenm by the addition of a low-boiling hydrocarbon liqnid snch as n-pentane or n-heptane more correctly, the removal asphalt (tacky, semisolid) from petroleum (as occurs in a refinery asphalt plant) by the addition of liqnid propane or liquid butane under pressnre. 

The GPC of a local crude (Bryan, Texas) sample spiked with a known mixture of n-alkanes and aromatics is shown in Figure 5 and the GPC of the crude is shown in Figure 6. The hydrocarbon mixture is used to calibrate the length of the species which separates as a function of retention volume. Ttie molecular length is expressed as n-alkane carboa units although n-alkanes represent only a fraction of the hydrocarbons in the crude. In addition to n-alkanes, petroleum crude is composed of major classes of hydrocarbons such as branched and cyclic alkanes, branched and cyclic olefins and various aromatics and nonvolatiles namely asphaltenes. Almost all of the known aromatics without side chains elute after n-hexane (Cg). If the aromatics have long side chains, the linear molecular size increases and the retention volume is reduced. Cyclic alkanes have retention volumes similar to those of aromatics. GPC separates crude on the basis of linear molecular size and the species are spread over 10 to 20 ml retention volume range and almost all of the species are smaller than the polystyrene standard (37A). In other words, the crude has very little asphaltenes. The linear... 

Fig. 3 Petroleum-type curves of different oU components from the North Sea showing a positive oU-oU correlation and a negative source rock - oU correlation (SAT saturated hydrocarbons, AROM aromatic hydrocarbons, NOSS heterocomponents, AS PH asphaltenes (Stahl, 1977)...

The chemicals in petroleum are classified as paraffins, naphthenes, aromatics, and resins-asphaltenes. We have discussed the first three classes, which are hydrocarbons. Now we turn to resins and asphaltenes. 

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. 

Distillation concentrates the metallic constituents in the residua (Table 3-5) some can appear in the higher-boiling distillates but the latter may, in part, be due to entrainment. Nevertheless, there is evidence that a portion of the metallic constituents may occur in the distillates by volatilization of the organometallic compounds present in the petroleum. In fact, as the percentage overhead obtained by vacuum distillation of reduced crude is increased, the amount of metallic constituents in the overhead oil is also increased. The majority of the vanadium, nickel, iron, and copper in residual stocks may be precipitated along with the asphaltenes by low-boiling alkane hydrocarbon solvents. Thus, removal of the asphaltenes with n-pentane reduces the vanadium content of the oil by up to 95% with substantial reductions in the amounts of iron and nickel. 

After removal of the asphaltene fraction, further fractionation of petroleum is also possible by variation of the hydrocarbon solvent. For example, liquehed gases, such as propane and butane, precipitate as much as 50% by weight of the residuum or bitumen. The precipitate is a black, tacky, semisolid material, in contrast to the pentane-precipitated asphaltenes, which are usually brown, amorphous solids. Treatment of the propane precipitate with pentane then yields the insoluble brown, amorphous asphaltenes and soluble, near-black, semisolid resins, which are, as near as can be determined, equivalent to the resins isolated by adsorption techniques. 

Briefly, the asphaltene fraction of crude oil is that fraction which is precipitated by the addition of a large excess of a low-boiling liquid hydrocarbon (usually n-heptane) (Chapter 3). On the other hand, resins are those materials which remain soluble in the pentane but will be adsorbed by a surface-active material such as fuller s earth, while the oils fraction is soluble in pentane but is not adsorbed from the pentane solution by any surface-active material. The asphaltic fraction of any petroleum, heavy oil, or residuum is usually a combination of the asphaltene and resin fraction and, in many instances, may constitute a large portion of a heavy oil or, especially, of a residuum. 

Deasphaltened oil the fraction of petroleum after the asphaltenes have been removed using liquid hydrocarbons such as n-pentane and n-heptane. 

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. 

We should caution that the above concept of the genetic relationship between kerogens and asphaltenes differs from the more historic view that asphaltenes are condensation and/or alteration products of hydrocarbons and resins. Certainly, in some petroleum processing treatments and probably at higher maturation levels in nature, various reactions do form new products with asphaltene solubility characteristics. These new condensation products may be regarded as altered asphaltenes and intermediates in the coke or pyrobitumen formation process (62-64)- Contamination of original asphaltenes by subsequently formed or altered products, of course, will result in a less definitive correlation between an asphaltene and its source kerogen. 

The utility of sulfur K-edge X-ray absorption spectroscopy for the determination and quantification of sulfur forms in nonvolatile hydrocarbons has been investigated. X-ray Absorption Near Edge Structure (XANES) spectra were obtained for a selected group of model compounds, for several petroleum asphaltene samples and for Rasa coal. For the model compounds the sulfur XANES was found to vary widely from compound to compound, and to provide a fingerprint for the form of sulfur involved. The use of third derivatives of the spectra enabled discrimination of mixtures of sulfide and thiophenic model compounds, and allowed approximate quantification of the amount of each component in the mixtures, in the asphaltene samples and the coal. These results represent the first demonstration that nonvolatile sulfide and thiophenic sulfur forms can be distinguished and approximately quantified by direct measurement. 

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