Asphaltene definition

During coagulation analysis, the ratio of the coagulation agent to the substance is 40 to 1 based on recommendations by many authors that this ratio is the optimum for this kind of analysis [1, 22], The coagulation of the asphaltene fraction in the scheme is done according to the Golde method (DIN 51595, IP 143/57) recommended as the most exact method of asphaltene definition [1, 23, 24],... 

This technique is used to quantify one or more components in a mixture, i.e., extracting them from mixtures to facilitate their final analysis. An example is that for the asphaltenes, already described in the definition of these components in article 1,2.1. 

The classic definition of asphaltenes is based on the solution properties of petroleum residua in various solvents. The word asphaltene was coined in France by J.B. Boussingault in 1837. Boussingault described the constituents of some bitumens (asphalts) found at that time in eastern France and in Peru. He named the alcohol insoluble, essence of turpentine soluble solid obtained from the distillation residue "asphaltene", since it resembled the original asphalt. 

The resins and asphaltenes from tar sands and from the other synthetic fuels have not been analyzed beyond the extent shown by the tables. The resins can be fractionated and analyzed in more detail using methods developed for petroleum resins by Jewell (46) and McKay (47), but more extensive work is necessary to have definitive analysis methods for these materials. 

By definition, the asphaltene fraction is that portion of the feedstock that is precipitated when a large excess (40 volumes) of a low-boiling liquid hydrocarbon (e.g., w-pentane or w-heptane) is added to (1 volume) of the crude oil (Speight, 1994, 1999). w-Heptane is the preferred hydrocarbon with w-pentane still being... 

In addition, thermally cracked residua will also contain two other fractions the carbenes and the carboids (Figure 6-6) These fractions are also defined by solubility and are precursors to coke (Speight, 1999). Both fractions are, by definition, insoluble in benzene (or toluene) but the carbenes are soluble in carbon disulfide (or pyridine) whereas the carboids are insoluble in carbon disulfide (or pyridine). Only traces of these materials occur in conventional petroleum and heavy oil. Any such quantities are not sufficient to alter the character of the asphaltenes if the benzene-treatment of the feedstock is omitted. On the other hand, feedstocks that have received some thermal treatment (such as visbroken feedstocks and cracked residua) may have considerable quantities of these materials present as they are also considered to be precursors to coke. 

Hydrotreating processes have two definite roles (1) desulfurization to supply low-sulfur fuel oils and (2) pretreatment of feed residua for residuum fluid catalytic cracking processes. The main goal is to remove sulfur, metal, and asphaltene contents from residua and other heavy feedstocks to a desired level. 

We should caution that the above concept of the genetic relationship between kerogens and asphaltenes differs from the more historic view that asphaltenes are condensation and/or alteration products of hydrocarbons and resins. Certainly, in some petroleum processing treatments and probably at higher maturation levels in nature, various reactions do form new products with asphaltene solubility characteristics. These new condensation products may be regarded as altered asphaltenes and intermediates in the coke or pyrobitumen formation process (62-64)- Contamination of original asphaltenes by subsequently formed or altered products, of course, will result in a less definitive correlation between an asphaltene and its source kerogen. 

This is termed the a rent miscibility pressure because there is a considerable amount of evidence that asphaltenes, which are colloidally dispersed in most crude oils, will aggregate under appropriate dilution conditions and precipitate, so that the physicochemical definition of miscibility (single-phase for all proportions of the fluids in question) is not realized. There are also conditions, particularly found in oil reservoirs at temperatures below about 135 F, at which two liquid phases, or two liquid phases plus a gas phase, appear in addition to an asphaltene precipitate (11-13). In general, however, this does not prevent the attainment of 95 % oil recovery or more in slim-tube tests at the same pressures at which these multiple phases appear. Hence, the process is deemed "miscible," for all practical purposes. 

A survey of the methods used to determine asphaltene structure indicates that there are serious shortcomings in all of the methods because of the assumptions required to derive the molecular formulae. The continued insistence that a complex fraction such as asphaltenes, derived in a one-step process from petroleum as a consequence of its insolubility in nonpolar solvents, has a definitive molecular structure is of questionable value to petroleum technology, and it is certainly beyond the scope of the available methods to derive such formulae. Asphaltenes would best be described in terms of several structural types rather than definite molecular structures. 

Asphaltenes are dark brown to black friable solids that have no definite melting point, and when heated, usually intumesce, then decompose leaving a carbonaceous residue. They are obtained from petroleums and bitumens by addition of a nonpolar solvent (such as a hydrocarbon) with a surface tension lower than 25 dynes cm-1 at 25°C (such as liquefied petroleum gases, the low-boiling petroleum naphthas, petroleum ether, pentane, isopentane, and hexane) but are soluble in liquids having a surface tension above 25 dynes cm-1 (such as pyridine, carbon disulfide, carbon tetrachloride, and benzene) (6, 7). 

Asphaltenes are, by definition, a solubility class (8, 9, 10) that is precipitated from petroleums and bitumens by the addition of a minimum of forty volumes of the liquid hydrocarbon. In spite of this, there are still reports of asphaltenes being isolated from crude oil by much lower proportions of the precipitating medium (II), which leads to errors not only in the determination of the amount of asphaltenes in the crude oil but also in the determination of the compound type. For example, when insufficient proportions of the precipitating medium are used, resins (a fraction isolated at a later stage of the separation procedure by adsorbtion chromatography) also may appear within the asphaltene fraction by adsorbtion onto the asphaltenes from the supernatant liquid and can be released by reprecipitation in the correct manner (12). Thus, questionable isolation techniques throw serious doubt on any conclusions drawn from subsequent work done on the isolated material. 

The classic definition of asphaltenes is based on the solution properties of petroleum residuum in various solvents. This generalized concept has been extended to fractions derived from other carbonaceous sources, such as coal and oil shale. 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. This effort is summarized by Speight and Moschope-dis (i) in their chapter in this volume along with a good summary of the current thinking. Thus, there are petroleum asphaltenes, coal tar asphaltenes, shale oil asphaltenes, tar sands bitumen asphaltenes, and so on. In this chapter I will attempt to show how these materials are special cases of an overall concept based directly on the physical chemistry of solutions and that the idea that they have a specific chemical composition and molecular weight is incorrect even for different crude oil sources. 

In Figure 1, the classic definition of asphaltenes is illustrated. This definition is an operational one that is, asphaltenes are soluble in benzene and insoluble in pentane. Usually, for virgin petroleum samples, the residuum is completely soluble in benzene. However, with heat-soaked samples or coal-derived liquids, the benzene insolubles can be appreciable. Therefore, further... 

In modern terms, asphaltene is conceptually defined as the n-pentane-insoluble and benzene-soluble fraction whether it is derived from coal or from petroleum. There are a number of procedures used to isolate asphaltene (2-7), all of which appear to be reproducible (8) but do not necessarily provide equivalent end-products. The similarity between coal- and petroleum-derived asphaltenes begins and ends at the definition of the separation procedure. Puzinauskas and Corbett s (9) comments on asphalt may be paraphrased and applied to asphaltene. They state that the broad solvent classification is unfortunate it leads to misconceptions that petroleum and coal materials are alike, or at least similar. However, these two classes of materials differ not only in their origin, mode of manufacture and uses, but also in their chemical composition and physical behavior. 

It was concluded that definition of asphaltenes based only on solubility is not a satisfactory criterion and that the behavior of asphaltenes in chromatographic separations is incompatible with such structures where the polymer units are interconnected predominantly by a-bonds. The asphaltenes are a complex state of aggregation best represented by the stacked cluster structure (micelle), which, however, cannot explain some of the GPC behavior of very dilute asphaltene solutions. 

Asphaltenes have been the subject of considerable discussion and controversy in the literature. Controversy and ambiguity arise largely because of the lack of chemical definition of asphaltene mixtures for which composition is dependent upon the source material and method of isolation. While... 

Asphaltenes are generally defined as those components in petroleum and coal liquids that under certain conditions are soluble in benzene but insoluble in aliphatic solvents, such as n-pentane, n-heptane, or cyclohexane. This definition obviously includes a broad variety of components, as insolubility in the above solvents can be caused by high molecular weight, high polarity, hydrogen bonding, acid-base complexing, or combinations of these parameters. 

On the other hand, the results for demetallation, i.e., V and Ni removal, and asphaltenes reduction for both catalysts revealed definite improvements. The improvements for V and Ni removal are on the order of 8-10% and 15-20%, respectively. Similarly, the results presented in Figure 1 show that the presulfided catalyst is more active than the unsulfided catalyst by around 5-10% towards asphaltenes reduction. For hydrocracking, the NiMo/AI2O3 catalyst exhibited around 15% higher conversion for the presulfided catalyst over the untreated catalyst. 

The classic definition of asphaltenes is based on the solution properties of petroleum residuum in various solvents. Broadly speaking, asphaltenes are insoluble in paraffin solvents but soluble in aromatic solvents. Structurally, asphaltenes... 

Determination of the asphaltene fraction of petroleum has been investigated for most of this century (5-11), and therefore the art is not new. However, it is now generally accepted that asphaltenes are, by definition, a solubility class that is precipitated from petroleum, heavy oil, and bitumen by the addition of an excess of liquid hydrocarbon (11), The procedure not only dictates asphaltene yield but can also dictate the quality of the fraction (12-16), In fact, the very method of asphaltene separation is a prime example of the disturbance of the system by the addition of an external agent. Thus, during deasphalting, the dispersibility (or compatibility) of the asphaltenes in the system is changed. The... 

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