Structure physical, petroleum

Coke produced from delayed coking is described as delayed sponge, shot, or needle coke depending on its physical structure. Shot coke is the most common when running the unit under severe conditions with sour crude residues. Needle coke is produced from selected aromatic feedstocks. Sponge coke is more porous and has a high surface area. The properties and markets for petroleum cokes have been reviewed by Dymond. Table 3-4 shows the types of petroleum cokes and their uses. ... 

An early hypothesis of the physical structure of petroleum (52) indicated that asphaltenes are the centers of micelles formed by adsorption, or even by absorption of part of the maltenes, that is, resin material, onto the surfaces or into the interiors of the asphaltene particles. Thus, most of those substances with greater molecular weight and with the most pronounced aromatic nature are situated closest to the nucleus and are surrounded by lighter constituents of less aromatic nature. The transition of the intermicellular (dispersed or oil) phase is gradual and almost continuous. Continued attention to this aspect of asphaltene chemistry has led to the assumption that asphaltenes exist as clusters within the micelle. This arises mainly because of the tendency for asphaltenes to associate in dilute solution in solvents of low polarity and from possible misinterpretation of viscosity data (58, 64). The presence of asphaltene stacks in the solid phase, as deduced from x-ray diffraction patterns (68), also seemed to support the concept of the widespread existence of asphaltene clusters in the micelle. 

Petroleum coke is used principally as a fuel or, after calcining, for carbon electrodes. The feedstock from which the coke is produced controls the coke properties, especially sulfur, nitrogen, and metal content. A concentration effect tends to deposit the majority of the sulfur, nitrogen, and metals in the coke. Cokes exceeding about 2.5% sulfur content and 200 ppm vanadium are mainly used, environmental regulations permitting, for fuel or fuel additives. The properties of coke for non-fuel use include a low sulfur, metal, and ash content as well as a definable physical structure. 

Historical Aspects. An early hypothesis of the physical structure of petroleum (Figure 10) (94) suggested that asphaltenes are the centers of micelles formed by adsorption or even by absorption of part of the maltene fraction, that is, resin material, onto the surfaces or into the... 

Figure 10. An early model of the physical structure of petroleum showed the asphaltenes dispersed and peptized by resin species.

Speight, J. G. (1999 b). The Chemical and physical structure of petroleum effects on recovery operations. Journal of Petroleum Science and Engineering, 22, 3-15 ... 

Naphthenes or cycloparaffins are formed by joining the carbon atoms in ring-type structures, the most common molecular structures in petroleum. These hydrocarbons are also referred to as saturated hydrocarbons since all the available carbon atoms are saturated with hydrogen. Typical naphthenes and their respective physical properties are listed in Table 4.2 and shown in Figure 4.3. 

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. 

It seems imreasonable to assume that any one of these theories alone accoimts for the toxic effect of oils on stages other than the egg. It is more likely that several operate simultaneously and that other modes of action are also involved. In generalizing, it would appear that the chemically active unsaturated oils might exert a toxic effect by virtue of their chemical structure. However, the highly refined saturated petroleum oils being less reactive would appear to exert their toxic effect chiefly through their physical characteristics. 

The foregoing pages have shown a number of ways in which physical constants vary with molecular structure. Petroleum chemists and automotive engineers have come to recognize that the performance of a motor fuel in an internal combustion engine is also dependent upon the structure of the hydrocarbon molecules which the fuel contains. This does not mean that engine performance is a function of the physical constants, but rather that the features of molecular structure which determine the one also determine the other. 

TMS catalysts fell into a special category due to their exceptional resistance to poisons. In fact, the presence of sulfur compounds, the most common poison of metallic and oxide catalysts, does not decrease their catalytic activity, but is needed to maintain high activity. Sulfide catalysts are also very resistant to carbon deposition, which is illustrated by their use for converting residual oils. Arsenic, as well as nickel and vanadium contained in heavy petroleum fractions, are some of the few substances that cause significant deactivation, and this only occurs by physical blockage of pore structure in supported catalysts. 

Previous work hod shown that low temperature coke is formed from cools hooted to between 450° and 500° C. by a process of nudeation and growth of spherical bodies in the plastic vitrinite. An essentially similar process has now been found to occur with coke-oven and petroleum pitches, with polyvinyl chloride, and with some polynuclear hydrocarbons, all of which yield carbons which grophitize readily at high temperatures. The process is probably general for the initial stages of formation of such carbons from the liquid phase. Some control of the solidification process has been achieved on the laboratory scale, and the physical and chemical structure of the spherulites has been investigated. 

The cycloalkanes also are known as naphthenes, cycloparaffins, or alicyclic hydrocarbons. In the petroleum industry, this class of hydrocarbons is known as naphthenes. Naphthenes have saturated rings. The general formula for the ring without substituents is CnH2n. This is the same as the general formula for the alkene series however, the structural configurations differ completely and, thus, the physical and chemical properties are not at all similar. 

As a result of structural studies, it is evident that petroleum asphaltenes are agglomerations of compounds of a condensed aromatic type (I, 2). Thus, it is not surprising that asphaltenes undergo a wide range of chemical and physical interactions based not only on their condensed aromatic structure but also on the attending alkyl and naphthenic moieties. In the following discussion, we again rely heavily on the evidence... 

Comprehensive references on the structure, physical properties, and chemistry of porphyrins and metalloporphyrins include (1) Porphyrins and Metalloporphyrins, edited by K. M. Smith (1975). (2) The Role of Trace Metals in Petroleum edited by T. F. Yen (1975), and (3) The Porphyrins, Vols. I-VIII, edited by D. Dolphin (1978). Methods for isolation and identification of petroporphyrins have been discussed by Sugihara et al. (1970), Barwise and Whitehead (1980), Baker and Palmer (1978), and Sundararaman (1985). 

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