Petroleum Elemental Analysis

Fractionation and Elemental Analysis of Crude Oils and Petroleum Cuts... 

Chaptar 2. FRACTIONATION AND ELEMENTAL ANALYSIS OF CRUDE OILS AND PETROLEUM CUTS... 

Chapter 2. FrACVONATION AND ELEMENTAL ANALYSIS OF CRUDE OlLS AND PETROLEUM CuTS... 

Although distillation and elemental analysis of the fractions provide a good evaluation of the qualities of a crude oil, they are nevertheless insufficient. Indeed, the numerous uses of petroleum demand a detailed molecular analysis. This is true for all distillation fractions, certain crude oils being valued essentially for their light fractions used in motor fuels, others because they make quality lubricating oils and still others because they make excellent base stocks for paving asphalt. 

The natural world is one of eomplex mixtures petroleum may eontain 10 -10 eomponents, while it has been estimated that there are at least 150 000 different proteins in the human body. The separation methods necessary to cope with complexity of this kind are based on chromatography and electrophoresis, and it could be said that separation has been the science of the 20th century (1, 2). Indeed, separation science spans the century almost exactly. In the early 1900s, organic and natural product chemistry was dominated by synthesis and by structure determination by degradation, chemical reactions and elemental analysis distillation, liquid extraction, and especially crystallization were the separation methods available to organic chemists. 

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. 

Non-destructive elemental analysis of solid or liquid samples for major and minor constituents. Used in routine analysis of metallurgical and mineral samples. Most suited to the determination of heavy elements in light matrices (e.g. Br or Pb in petroleum). Well suited for on-stream, routine analysis. Electron beam excitation methods valuable in surface studies in combination with electron microscopy. Detection limits generally in the range 10-100 ppm. Relative precision, 5-10%. 

There are two noncolumn cleanup methods, one of which uses acid partition (EPA SW-846 3650) to separate the base/neutral and acid components by adjusting pH. This method is often used before alumina column cleanup to remove acid components. The other method (EPA SW-846 3660) is used for sulfur removal and uses copper, mercury, and tetrabutylammonium sulfite as desulfurization compounds. Sulfur is a common interfering compound for petroleum hydrocarbon analysis, particularly for sediments. Sulfur-containing compounds are very common in crude oil and heavy fuel oil. Elemental sulfur is often present in anaerobically biodegraded fuels. Thus, abnormally high levels of sulfur may be... 

XPS spectra were collected for the same petroleum residua and asphaltene samples used in the XANES studies described above. For all samples the total amount of sulfur relative to carbon as measured by XPS was comparable to that determined by bulk elemental analysis. The spectra were deconvoluted by curve fitting, and the approximate quantifications thus derived are shown in Table 111. 

After sufficient information has been obtained as to the types of components which constitute a given fraction or boiling range of petroleum, subsequent examination of such material can be greatly simplified by the elimination of certain steps. In most cases, for example, it becomes unnecessary to make an elemental analysis or determine the molecular weight. 

Synthesis of the complex. In a 50-mL Schlenk flask, N,N-bis(2-mercapto-ethyl)2-methylthioethylamine (0.1 g, 0.47 mmole) is dissolved in methanol (10 mL) and subsequently cooled to 5°C in an ice bath. To the cooled solution, a solution of anhydrous nickel acetate (84 mg, 0.47 mmole) in methanol (10 mL) is added dropwise with rapid stirring. The product, a red-black, microcrystalline solid, forms immediately and is collected by filtration, washed several times with small amounts of methanol, and dried in vacuo. Yield = 160 mg (65%). The product obtained from purified ligand is pure by elemental analysis. If crude ligand is used, the complex may be purified by recrystallization from CH2C12 (minimum volume for dissolution) upon addition of petroleum ether or hexane (five times the volume of CH2C12 used). 

However, in order to define conventional petroleum, heavy oil, and bitumen, the use of a single physical parameter such as viscosity is not sufficient. Other properties such as API gravity, elemental analysis, composition, and, most of all, the properties of the bulk deposit must also be included in any definition of these materials. Only then will it be possible to classify petroleum and its derivatives (Speight, 1999). 

Nadkarni, R.A., Ed., Modern Instrumental Methods of Elemental Analysis of Petroleum Products and Lubricants, ASTM, STP 1109, Philadelphia, PA, 1991. 

Although the great majority of petroleum and coal-based pitch materials, as well as model compounds such as polyvinyl chloride, acenaphthylene, decacyclene and polynuclear aromatic hydrocarbons, form anisotropic graphitizable carbons, it is an almost impossible task to predict the type of optical texture of a coke from an elemental analysis of the pitch. The size, shape and reactivity of peri-condensed polynuclear aromatic molecules in the products of pyrolysis of a pitch play a more important role in determining optical texture. 

Problems of trace element analysis in the petroleum industry are many and varied in their nature. For this reason it is not possible to present here detailed methods for every possible combination of element of interest and sample type. Methods are presented for the most common analytical problems but many other determinations may be made with only minor changes to the methods given. 

Intensive research on elemental analysis has been carried out in this laboratory to overcome difficulties in determining nitrogen and sulfur in heavy fractions of petroleum or bitumens. With existing combustion techniques it was nearly impossible to obtain good overall balances for these elements when individual fractions coming from liquid chromatography were analyzed. Recoveries of 85%-95% of the quantity originally present in the... 

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