Solubility of asphaltenes

Two additional problems occurred in these separations. Firstly, the asphaltene samples when applied from cyclohexane solutions as described by McKay (3) were leaving 3%-4% residuum at the column head. Secondly, the solvent series was the same as that used by McKay, differing from that used for resin separations only in that cyclohexane was used as the initial solvent instead of nC5, and benzene/methanol was employed instead of methanol. These modifications are necessary because of the lower solubility of asphaltenes. 

Sulphur and nitrogen are removed as gases. Metals are deposited on the surface of the catalysts. This may be as a result of decreasing solubility of asphaltenes, as discussed above, or of chemical attack on the organic species associated with the heteroatom. The production of a molecule in which the inorganic species forms a larger fraction also leads to reduced solubility. Either associated with the chemical attack or subsequent to the initial attack, metals are converted to metal sulphides [17,18]. 

These fouling phenomena are believed to be due to the precipitation of asphaltenes from hydrocracked, effluent streams. Fragmentation reactions decrease the solubility power of effluent maltenes and the solubility of asphaltene micelles, thus facilitating precipitation [1]. 

All these problems arise from the sedimentation of asphaltenes. Sedimentation begins when asphaltenes achieve a predetermined size of asphaltene molecules and/or asphaltene particles. The growing of asphaltene molecules is caused by polycondensation reactions. Lowering the solubility of asphaltenes in oil causes polycondensation. This implies that in the case of deep cracking of asphaltene, polycondensation reactions cannot take place. 

It is obvious that the solubility of asphaltenes in a given solvent increases as the temperature of measurement is increased. For example, the molecular weight of the asphaltenes from Bitumen 200 Elf is 1440 mol/g defined at 60°C and 10050 mol/g defined at 37 °C. It is clear that at 37°C, asphaltenes were not completely solved in the toluene and the defined molecular weight is probably the molecular weight of the micelles which are formed from 6 to 8 asphaltene molecules. The molecular weight of 1440 is very close to the molecular weight of asphaltenes reported in many references [1,9]. 

McLean and coworkers [19,56] have investigated the effects of crude oil solvency and resin-asphaltene interactions on the stability of water-in-crude oil emulsions. They showed that there were three main factors that control the solubility of asphaltenes in crude oil, their tendency to aggregate, and their tendency to adsorb at oil-water interfaces. These are... 

The solubility of asphaltenes is highly dependent on the medium in which they are placed (81). The presence of dissolved asphaltenes in crude oil is mediated by a combination of crude aromaticity and petroleum resins that act to solvate asphaltene aggregates. Adding an excess of aliphatic solvent, namely n-hep-tane, sufficiently reduces the solubility of asphaltenes in crude oil and causes precipitation. To perform subsequent film experiments these precipitated asphaltenes were then redissolved in toluene. As n-heptane was added to the asphaltene—toluene solutions. 

In the second set of experiments, effect of pressure on asphaltene precipitation from a fixed C02-oil mixture was studied. Table 6 gives results for mixture containing 30 mol% CO - 70 mol% oil. Table 7 gives results for mixture containing 50 mol% CO2 - 50 mol% oil. From the results, asphaltene precipitation seems to be highest at the bubble point pressure and decreases with increase in pressure above the bubble point pressure and with decrease in pressure below the bubble point pressure. This can be explained by the fact that the solubility of asphaltenes in the C02-oil mixture will be governed by the dependence of solubility parameters for asphaltene and asphaltene-free solvent-oil mixtures with pressure. 

The solubility of asphaltene in the solvent with different polarity has been demonstrated by other researchers. According to the polarity of asphaltene and that of various solvents, the degree of asphaltene dispersion is various. The asphaltene size distributions in the different solutions will vary from each other. Using the derived Kord law, the relationship between the size distribution of asphaltene particles and solvent characteristics are established. 

The solubility parameter has a clear physical meaning when the difference between the solubility parameters of two substances is small, one can dissolve the other appreciably. The difference in solubility parameters is a measure of the solubility power. As an example, the solubility parameter of asphaltenes, heptane, and toluene are, 9.5, 7.5, 8.9 (cal/cm ), respectively (see Problem 5.10, Chapter 5). The solubility of asphaltenes in heptane is very low, and toluene dissolves the asphaltenes. In Chapter 3, the expression for the solubility parameter from the equation of state, will be presented (see Problem 3.10, Chapter 3). 

Finally, we should draw attention to the prevalent use of air-degassed crude oil systems and foam generation by sparging at ambient temperatures and pressures. It is known that solubilities of asphaltenes, resins, PDMS, and PDMS derivatives are likely to be influenced by temperature and dissolution of natural gas. Moreover, sparging represents a poor model for foam generation in gas-oil separators, which involves depressurization and nucleation of bubbles. Use of apparatus designed to replicate the conditions in actual gas-oil separators for basic studies should therefore be encouraged. 

Boussingault (1837) and Marcusson (1931) did a remarkable job on asphaltenes of oil that was used to establish a procedure for the separation of these, developed by Nellensteyn (1933), based on the solubility of asphaltenes in carbon tetrachloride. This procedure converged to the method known today for the separation of asphaltenes using n-heptane or n-pentane as a flocculating agent. 

Matsushita et al. (2004) defined the following relationship that takes into account the H/C atomic ratio of asphaltenes and maltenes, which gives certain information about the solubility of asphaltenes and its influence on coke formation during processing of petroleum ... 

However, for the past 30 years fractional separation has been the basis for most asphalt composition analysis (Fig. 10). The separation methods that have been used divide asphalt into operationally defined fractions. Four types of asphalt separation procedures are now in use ( /) chemical precipitation in which / -pentane separation of asphaltenes is foUowed by chemical precipitation of other fractions with sulfuric acid of increasing concentration (ASTM D2006) (2) solvent fractionation separation of an "asphaltene" fraction by the use of 1-butanol foUowed by dissolution of the 1-butanol solubles in... 

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. 

In modern terms, asphaltene is conceptually defined as the normal-pentane-insoluble and benzene-soluble fraction whether it is derived from coal or from petroleum. The generalized concept has been extended to fractions derived from other carbonaceous sources, such as coal and oil shale (8,9). 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. It was demonstrated that the elemental compositions of asphaltene fractions precipitated by different solvents from various sources of petroleum vary considerably (see Table I). Figure 1 presents hypothetical structures for asphaltenes derived from oils produced in different regions of the world. Other investigators (10,11) based on a number of analytical methods, such as NMR, GPC, etc., have suggested the hypothetical structure shown in Figure 2. 

It has been shown (9) that asphaltenes contain a broad distribution of polarities and molecular weights. According to these studies, the concept of asphaltenes is based on the solubility behavior of high-boiling hydrocarbonaceous materials in benzene and low-molecular weight n-paraffin hydrocarbons. This solubility behavior is a result of physical effects that are caused by a spectrum of chemical properties. Long also... 

The crude oil produced from the Main Zone of the Torrance Field has an API gravity of 18° and contains 5.3 weight percent asphaltenes. The solubility of the asphaltene molecules in Main Zone oil was measured by the Oliensis Test(35). In this test, the solubility parameter Qf ie oil was lowered by adding to the oil successively larger volumes of hexadecane, a poor solvent for asphaltene molecules. The minimum volume (in milliliters) of hexadecane, which when added to 5 g of crude oil, will cause the chromatographic separation of the asphaltene fraction is termed the Oliensis Number. The Oliensis Number for the Main Zone crude oil is 3, indicating that the asphaltene molecules are not well-solubilized in the oil. Small changes in the solubility parameter of the Main Zone oil can cause the asphaltenes to precipitate. 

The high viscosity of heavy crude oils is essentially due to the high levels of asphaltene content. Asphaltene is the highest MW component of crude oil, is a friable, amorphous dark solid, which is colloidally dispersed, in the oily portion of the crude. Asphaltenes are considered to be heavily condensed aromatic molecules with aliphatic side chains and with high heteroatom content (S, N, and O) as well as high-metal content. The asphaltene fraction is physically defined as that fraction insoluble in n-alkanes, but soluble in toluene and is the most polar fraction of oil. 

The character of fuel oil generally renders the usual test methods for total petroleum hydrocarbons (Chapters 7 and 8) ineffective since high proportions of the fuel oil (specifically, residual fuel oil) are insoluble in the usual solvents employed for the test. In particular, the asphaltene constituents are insoluble in hydrocarbon solvents and are only soluble in aromatic solvents and chlorinated hydrocarbons (chloroform, methylene dichloride, and the like). Residua and asphalt (Chapter 10) have high proportions of asphaltene constituents, which render any test for total petroleum hydrocarbons meaningless unless a suitable solvent is employed in the test method. 

Hydrogenation of bituminous coal was found more difficult than that of brown coal, chiefly because the primary products of hydrogenation of bituminous coal have a higher percentage of asphaltenes (material soluble in benzene but insoluble in w-hexane). The slower rate of destructive hydrogenation of asphaltenes is reflected in the much lower throughput of bituminous coal per unit volume of reactor. 

The amount of asphaltene precipitate from one crude oil is dependent on the carbon number of the alkane solvent. A decrease in the solvent carbon number results in an increase in the asphaltene precipitate. This observation would suggest that asphaltenes and resins are not greatly different materials. Rather, a continuum exists in the solubility behavior. An increase in boiling point in heavy oil is generally accompanied by an increase in both aromaticity and in the concentration of heteroelements or polar aromatic molecules (Corbett, 1969). Similarly, there is a gradual... 

Although the residuum is a mixture too complex for isolating chemically pure components, asphaltene investigators in recent years have developed techniques that separate residuum molecules on the basis of compound class rather than solubility class. These studies, discussed next, have greatly modified the concepts of asphaltene structure. 

Product oil containing 24 percent asphaltenes of 670 molecular weight has a viscosity of 86 (SSF at 180° F, ASTM D88), while product oil containing almost the same amount of asphaltenes (22 percent) but with a molecular weight of 460 has a viscosity of only 14. Benzene insolubles, heretofore regarded as unreacted coal, were found to be soluble in pyridine and to exert a large effect on viscosity. 

Figure 3. Effect of pyridine solubles (toluene insolubles) and of asphaltenes on viscosity...

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