Asphaltenes heteroatom structures

From the investigation into asphaltene chemistry, it is obvious that paraffin side chains can be readily cracked during the thermal treatment of pure bitumen at a relatively low temperature. Aromatic and heteroatom structures are inclined to polycondensation reactions and finally to coke formation. Especially interesting are bridge-ring structures since these structural elements will only be cracked either at high temperature or by the addition of plastics to the feedstock. 

This concept of asphaltenes is useful in the interpretation of the present data, and conversely, the data support the concept. First, 13C NMR data show that the saturated hydrocarbon structure, which constitutes the majority of the carbon in the fractions, is virtually identical between the asphaltenes and the maltenes, within the limited sensitivity of 13C NMR. This factor is consistent with the argument that there is a partitioning between fractions and that the appearance of a particular species predominantly in the asphaltene fraction results because of a relatively higher aromaticity or the presence of polar heteroatoms for a specified molecular weight. It is important to recognize, from a processing standpoint, that only a minor weight percent of the fraction (or molecule) may be responsible for its classification as an asphaltene. 

Resins and asphaltenes are also of interest as a result of the fact that most of the heteroatoms reside in these fractions [7,16], Beaton and Bertolacini [5] report that about equal amounts of V and Ni reside in the resins as in the asphaltenes, while sulphur is concentrated in resins. The chemical associations between the heteroatoms and organic molecules are far from certain, although general descriptions in terms of porphyrin like structures have been advanced [5]. Sulphur appears to be easily accessible and thiophenic sulphur has been identified [8]. Vanadium is more accessible than nickel. 

Asphaltenes and resins are dark substances (from dark red to brown). They are soluble in aromatic solvents but insoluble in paraffin solvents [3]. Asphaltenes have various types of heteroatoms present in crude oil in their structure. Asphaltenes are the most complicated known compounds in crude oil. 

Figure 5. Total structures have been given to asphaltenes on the basis of NMR investigations these structures involve the use of large poynulcear aromatic systems, and heteroatoms are usually absent.

The insolubility of asphaltenes eauses deposits in pipes, wells, and valves, and in the formations. Dubbed as the colloids of crudes (47, 48), the asphaltene chemistry has received considerable attention. Asphaltenes are considered as major polar species with high aromaticity and are known as the major building blocks of the mechanical barriers or interfa-cial films formed at the W/0 interface. Increased MWts is consistent with high aromaticity and greater numbers of incorporated heterocyclic structures containing the heteroatoms. Their structures have received considerable attention of late. 

Infrared spectroscopy, NMR spectroscopy, X-ray methods such as X-ray absorption near-edge structure spectroscopy and ESR spectroscopy have been used primarily to probe the detailed chemistry of heteroatom speciation, polar functional group determination, and hydrogen and carbon types in asphaltenes. The consensus seems to indicate that most asphaltene molecules have one to three heteroatoms (S, N, and O) per molecule. Sulfur exists predominantly as thio-phenic heterocycles (typically 65—85%) with the remainder as sulfidic groups (46,47). Thiophenic moieties are not... 

Table 2 (63) shows additional stmctural data estimated for the fractions. These results are all dependent on the composition of the source crude oil, particularly heteroatom content and metals. Both Ni and V are found primarily in the heptane-precipitated asphaltenes and are evenly distributed without regard to molecular size. They seem to be interchangeable in structure in that in fractions of a given asphalt the ratio of V to Ni is constant over wide ranges of composition. These metals often exist in porphyrin structures and have been implicated in higher rates of asphalt oxidation. 

Much effort has been directed at asphaltenes flocculation and precipitation in petroleum crude mixtures that is a significant problem in oil production, transmission and processing facilities. Pressure, temperature, the chemical composition of the oil and the amount of dissolved gases affect this undesired phenomenon. In general, the prediction of asphaltenes precipitation is very difficult and suffers from the definition of an asphaltene. Usually, asphaltenes are defined as the part of the crude oil that is soluble in methylbenzene and benzene but insoluble in pentane or heptane. Asphaltenes consist of many thousands of species, differing in size and chemical structure. The aromatic character of the asphaltenes and their content of heteroatoms influence their solubility in different solvents and the tendency to flocculate. In most of the calculations of the phase equilibria the pseudo-component method has been... 

To Merdrignac and Espinat (2007), asphaltenes contain aromatic molecules variables and with different amounts of heteroatoms, metals and fimctional groups. Such structures can not be represented by a single model of the molecule. Several models are proposed in the literature to describe them, among the main continental and archipelago. Figure 09 presents some of these models. 

Heavy oils and residua contain hydrocarbons that are characterized by large amounts of heteroatoms and asphaltenes. The nature and chemical structure of these complex components are also other factors that strongly affect the extent of thermal reactions. For instance, it is more feasible to convert asphaltenes with small content of aromatic rings and high number of alkyl side chains are more feasible to convert (Ancheyta et al., 2009). 

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