Petroleum molecular types

This same increase in the number of isomers with molecular weight also applies to the other molecular types present. Since the molecular weights of the molecules found in petroleum can vary from that of methane (CH4 molecular weight = 16) to several thousand (Speight, 1999, and references cited therein), it is clear that the heavier nonvolatile fractions can contain virtually unlimited numbers of molecules. However, in reality the number of molecules in any specified fraction is limited by the nature of the precursors of petroleum, their chemical structures, and the physical conditions that are prevalent during the maturation (conversion of the precursors) processes. 

Of the many factors which influence product yields in a fluid catalytic cracker, the feed stock quality and the catalyst composition are of particular interest as they can be controlled only to a limited extent by the refiner. In the past decade there has been a trend towards using heavier feedstocks in the FCC-unit. This trend is expected to continue in the foreseeable future. It is therefore important to study how molecular types, characteristic not only of heavy petroleum oil but also of e.g. coal liquid, shale oil and biomass oil, respond to cracking over catalysts of different compositions. 

This concept has resulted in the consideration of petroleum as a continuum of molecular types and the nature of continuum is dictated by the proportions of the precursors that form the protopetroleum after which the prevalent conditions become operational in the formation of the final crude oil product. With this is mind, it might be anticipated that similar molecular types occur in heavy oil and bitumen as do occur in conventional petroleum. It then becomes a question of degree as well as molecular weight (Speight, 1999). 

In order to understand the composition of heavy oils and residua, it is necessary to present a very brief description of the constituents of the lower boiling fractions of petroleum. Acceptance that petroleum is a continuum of molecular types that continues from the low-boiling fractions to the nonvolatile fractions (Speight, 1999 and references cited therein) is an aid to understanding the chemical nature of the heavy feedstocks. 

Yet another application of solid-state catalysis occurs in the desulfurization of petroleum. Natural petroleum includes various molecules that contain sulfur atoms. Combustion of this petroleum produces S02, which must be removed from the exhaust to prevent air pollution. One way to prevent pollution by S02 is to remove the sulfur from the petroleum before it is used for fuel—the desulfurization of petroleum. One type of sulfur containing molecules found in petroleum are thiols, which can be written R—SH, where R represents a molecular fragment containing a long chain of carbon atoms. In desulfurization the goal is to remove the sulfur from this molecule to produce a hydrocarbon (R—H) ... 

Detailed analysis of residual products, such as residual fuel oil, is more complex than the analysis of lower-molecular-weight liquid products. As with other products, there are a variety of physical property measurements that are required to determine whether the residual fuel oil meets specification, but the range of molecular types present in petroleum products increases significantly with an increase in the molecular weight (i.e., an increase in the number of carbon atoms per molecule). Therefore, characterization measurements or studies cannot, and do not, focus on the identification of specific molecular structures. The focus tends to be on molecular classes (paraffins, naphthenes, aromatics, polycyclic compounds, and polar compounds). 

The boiling point distribution of paraffin wax provides an estimate of hydrocarbon molecular weight distribution that influences many of the physical and functional properties of petroleum wax. To a lesser extent, distillation characteristics also are influenced by the distribution of various molecular types that is, n-paraffins, branched, or cyclic structures. In the case of the paraffin waxes that are predominantly straight chain, the distillation curve reflects the molecular size distribution. 

Several studies have shown that certain categories of poorly or untreated petroleum base oils can cause cancer in humans. The principal molecular types believed to be responsible are the three- to seven-ring polycyclic aromatics. The IP 346 test method selectively extracts these materials from a sample of the base oil and enables their concentration to be estimated, fully described in a CONCAWE report [3]. Base oils are now classified according to this test method for their carcinogenic potential and the labelling of finished lubricant products must now comply with these rules. 

A more realistic aspect of petroleum composition is based on the distribution of molecular types in petroleum and shows that petroleum is a... 

Figure 2. Representation of petroleum as a map of the various molecular types. (Reproduced with permission from reference 10. Copyright 1978.)...

Figure 1 SCHEMATIC REPRESENTATION OF THE DISTRIBUTION OF MOLECULAR TYPES IN A TYPICAL PETROLEUM PITCH (RANKED BY SOLUBILITY PARAMETER)...

As an aside, but certainly worthy of note, the identification of many of the constituents of petroleum have been achieved as a result of the volatility of these constituents and subsequent application of methods such as gas-liquid chromatography and mass spectroscopy. But in the case of petroleum residua, which are the nonvolatile constitnents of petrolenm (boiling point 345°C [650°F]), identification of the individual constituents is much more difficult and heavy reliance has to be pnt on identification by molecular type (Speight, 2007). The same is essentially true for coal. 

Identification of the constituents of complex mixtures (such as petroleum residua and coal) by molecular type may proceed in a variety of ways but generally can be classified into three methods (1) chemical techniques, (2) spectroscopic techniques, and (3) physical property methods whereby various structural parameters are derived from a particular property by a sequence of mathematical manipulations. The end results of these methods are indications of the structural types present in the material (Stadelhofer et al, 1981). 

The fact that coals are heterogeneous as a group and, indeed, heterogeneous individually does not mean that there cannot be a concept of a macromolecular structure. But such a concept should include a variety of molecular types, perhaps in a manner analogous to the formulation of the structural types in petroleum asphaltene constituents (Figure 10.36) (Long, 1979 Speight, 2007). 

Riazi, M.R., Daubert, T.E. 1986. Prediction of molecular type analysis of petroleum fractions and coal liquids. Ind. Eng. Chem. Process Des. Dev. 25(4) 1009-1015. 

Ketones are more stable to oxidation than aldehydes and can be purified from oxidisable impurities by refluxing with potassium permanganate until the colour persists, followed by shaking with sodium carbonate (to remove acidic impurities) and distilling. Traces of water can be removed with type 4A Linde molecular sieves. Ketones which are solids can be purified by crystallisation from alcohol, toluene, or petroleum ether, and are usually sufficiently volatile for sublimation in vacuum. Ketones can be further purified via their bisulfite, semicarbazone or oxime derivatives (vide supra). The bisulfite addition compounds are formed only by aldehydes and methyl ketones but they are readily hydrolysed in dilute acid or alkali. 

Compilations of Reference Spectra There are several compilations of reference mass spectra available of which the oldest is the American Petroleum Institute (Ref 82) collection of spectra obtained mostiy on the older type instruments. Recent collections index spectra variously, eg, under reference number (Ref 19). molecular weight (Refs 12 19), molecular formula (Ref 19), fragment ion values (Ref 19), and base peak (Refs 12 19). A quarterly journal, Archives of Mass Spectral Data ... 

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