Petroleum heteroatom concentration

Boduszynski, M.M. Composition of heavy petroleum. 1. Molecular weight, hydrogen deficiency, and heteroatom concentration as a function of atmospheric equivalent boihng point up to 100°F (160 C), Energy Fuels 1987, 7,2-11. 

In a mixture as complex as petroleum, the reaction processes can only be generalized because of difficulties in analyzing not only the products but also the feedstock as well as the intricate and complex nature of the molecules that make up the feedstock. The concentration of the heteroatoms in the higher molecular weight and polar constituents (Figure 4-13) is detrimental to process efficiency and to catalyst performance (Speight, 1987 Dolbear, 1997). 

Separating a whole sample of a coal liquid or shale oil into classes poses special problems since these materials contain high concentrations of heteroatomic species compared with natural petroleums. Many of these compounds are quite polar and can cause emulsification, precipitation, and may even react to produce artifactual compounds at some stage during a separation procedure. Many liquid chromatographic techniques have been useful in class separations and analyses of petroleums. More often, these have been applied to particular analytical scale operations with fossil-derived liquids. The most common applications are for aromatic-aliphatic and molecular weight types of separations. 

The most abundant heteroatom is invariably sulfur, appearing in concentrations from below 0.1 wt. % in North African or Indonesian light crudes to over 5 wt. % in Venezuelan heavy crudes (Boscan) or Canadian tar sands. A wide variety of sulfur containing compounds are present in petroleum and refinery fractions, ranging from thiols to thiophenes the most important classes of organosulfur compounds of interest for our purposes are represented in Fig. 1.1. 

However praiseworthy this may be, it is still difficult to portray the various maturation paths of coal (or, in other instances, petroleum) as occurring regularly so that, for example, the heteroatoms are scattered evenly throughout the model structure. Nature does not always behave in such regular fashion and there may be a tendency to concentrate, say, the heteroatoms only in certain parts of a molecule. Recognition of this fact alone detracts markedly from, and even nullifies, many of the structures that have been proposed for coal. For the present purposes, it is sufficient to note that spectroscopic methods of estimating the structure of coal also (as do chemical methods of structural determination) suffer from several, sometimes severe, limitations. Thus, while some useful information can be derived about the structure of coal, the concept that the data will eventually lead to an average or even to a representative structure can only be cautiously (if at all) accepted. 

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