Pentane asphaltenes

Thus, although the use of both -pcntanc and w-heptanc has been widely advocated, and although n-heptane is becoming the deasphalting liquid of choice (ASTM, 1995), this is by no means a hard-and-fast rule. And it must be recognized that large volumes of solvent may be required to effect a qualitative and quantitative reproducible separation. In addition, whether -pentane or n-heptane is employed, the method effects a separation of the chemical components with the most complex structures from the mixture and this fraction should be correctly identified as n-pentane asphaltenes or as n-heptane asphaltenes. 

Asphaltite a variety of naturally occurring, dark brown to black, solid, nonvolatile bituminous material that is differentiated from bitumen primarily by a high content of material insoluble in n-pentane (asphaltene) or other liquid hydrocarbons. 

Asphalt Solubility in Normal Alkanes. The separation of an asphalt into two fractions—asphaltenes and maltenes—by precipitation with low molecular weight alkanes is a physical method of separation based on solubility. Figure 1 shows that pentane precipitates more asphalt components (17.0 wt %) than does heptane (10.6 wt %). It would be expected that when pentane asphaltenes are treated with heptane, the amount of material equal to 10.6 wt % (based on asphalt) would be precipitated. However, more—14.8 wt %—is precipitated. A similar, but even more pronounced, effect can be seen when heptane and decane treatments are compared (see Figure 1). 

PENTANE Asphaltenes (C5). No established method is available for the preparation of pentane asphaltenes. ASTM D-2006 was discontinued in 1976. Another ASTM method (for rubber extender and processing oils) (23) uses a 10-g sample and only 100 mL of pentane, which is insufficient for a correct dispersion of the sample into the solvent. A very precise method for these asphaltenes from coal-derived liquids has been described (24), but some might object to the use of benzene because of the solubilizing properties of this solvent towards asphaltenes. 

To test these hypotheses, a tar sand bitumen containing 20 wt % pentane asphaltenes was characterized and processed by hydropyrolysis before and after removal of asphaltenes. Product yields and structure were determined and the influence of asphaltenes on results was determined by inferrence. Feedstocks and products were characterized according to elemental analysis, physical properties, simulated distillation, and carbon-type analysis. Inferences made in this study are discussed in the context of the reported literature. 

Boduszynski et al. (70,71), using field ionization mass spectroscopy (FIMS), obtained average molecular weights from 873 to 1231 for the Corbett fractions, with asphaltenes actually the smallest molecules. The VPO value for asphaltenes was over 4000. The values obtained for polar aromatics was 1020 by FIMS and over 1400 by VPO. Results for naphthene aromatics and saturates were quite close by the two methods. It should be realized that the designation of asphaltenes is arbitrary, depending on the precipitating solvent (72,73). Propane precipitates most of the polar aromatics, and pentane asphaltenes can be nearly twice the heptane asphaltenes. 

According to the nature of the solvent employed, the yields and constitutions of the asphaltenes are different. In the United States, asphaltenes are obtained by precipitation from normal pentane. 

Tars can be hydrogenated to produce Hquid fuels. High hydrogen and low asphaltene, ie, benzene-soluble and pentane-iasoluble, contents are desirable. The central German brown coals are attractive for this reason. The tars from the eastern part of Germany require much lower pressures and less hydrogen per unit of product than do brown coals near Cologne, which can require pressures up to 71 MPa (700 atm) (see Petroleum). 

Fig. 11. Schematic of a residuum oil supercritical extraction (ROSE) process using compressed pentane to separate vacuum resids into asphaltenes (high...

Fractionation. Kett-McGee developed the ROSE process for separating the heavy components of cmde oil, eg, asphaltenes, resins, and oils, in the 1950s. This process was commercialized in the late 1970s, when cmde oil and utility costs were no longer inexpensive. In the ROSE process (Fig. 11), residuum and pentane ate mixed and the soluble resins and oils recovered in the supetctitical phase. By stepwise isobatic temperature increases, which decrease solvent density, the resin and oil fractions ate precipitated sequentially. 

However, for the past 30 years fractional separation has been the basis for most asphalt composition analysis (Fig. 10). The separation methods that have been used divide asphalt into operationally defined fractions. Four types of asphalt separation procedures are now in use ( /) chemical precipitation in which / -pentane separation of asphaltenes is foUowed by chemical precipitation of other fractions with sulfuric acid of increasing concentration (ASTM D2006) (2) solvent fractionation separation of an "asphaltene" fraction by the use of 1-butanol foUowed by dissolution of the 1-butanol solubles in... 

Asphaltenes seem to be relatively constant in composition in residual asphalts, despite the source, as deterrnined by elemental analysis (6). Deterrnination of asphaltenes is relatively standard, and the fractions are termed / -pentane, / -hexane, / -heptane, or naphtha-insoluble, depending upon the precipitant used (5,6,49). After the asphaltenes are removed, resinous fractions are removed from the maltenes-petrolenes usually by adsorption on activated gels or clays. Recovery of the resin fraction by desorbtion is usually nearly quantitative. 

Bitumen Insoluble in Paraffin Maphtha (AASHPO T46). This test designated by the American Association of State Highway and Transportation Officials (AASHTO) is used to indicate the content of naphtha-insoluble asphaltenes in an asphalt. Other solvents such as / -heptane (ASTM D3279), / -hexane, and / -pentane have been substituted for the naphtha solvent. 

Residues containing high levels of heavy metals are not suitable for catalytic cracking units. These feedstocks may be subjected to a demetallization process to reduce their metal contents. For example, the metal content of vacuum residues could be substantially reduced by using a selective organic solvent such as pentane or hexane, which separates the residue into an oil (with a low metal and asphaltene content) and asphalt (with high metal content). Demetallized oils could be processed by direct hydrocatalysis. 

Temperature-Controlled Residuiun Oil Supercritical Extraction (ROSE) The Kerr-McCee ROSE process has been used worldwide for over two decades to remove asphaltenes from oil. The extraction step uses a hquid solvent that is recovered at supercritical conditions to save energy as shown in Fig. 20-21. The residuum is contacted with butane or pentane to precipitate the heavy asphaltene fraction. The extract is then passed through a series of heaters, where it goes from the liquid state to a lower-density SCF state. Because the entire process is carried out at conditions near the critical point, a relatively small temperature change is required to produce a fairly large density change. After the light oils have been removed, the solvent is cooled back to the liquid state and recycled. 

In modern terms, asphaltene is conceptually defined as the normal-pentane-insoluble and benzene-soluble fraction whether it is derived from coal or from petroleum. The generalized concept has been extended to fractions derived from other carbonaceous sources, such as coal and oil shale (8,9). With this extension there has been much effort to define asphaltenes in terms of chemical structure and elemental analysis as well as by the carbonaceous source. It was demonstrated that the elemental compositions of asphaltene fractions precipitated by different solvents from various sources of petroleum vary considerably (see Table I). Figure 1 presents hypothetical structures for asphaltenes derived from oils produced in different regions of the world. Other investigators (10,11) based on a number of analytical methods, such as NMR, GPC, etc., have suggested the hypothetical structure shown in Figure 2. 

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. 

---