Molecular weight of asphaltene

The increase in viscosity of the product oil during a run is due primarily to an increase in asphaltenes and possibly pyridine sols content and not to a change in the molecular weight of the asphaltenes or to an increase in the viscosity of the heavy oil. This may be seen by comparing batches 7 and 88 of run FB-40 in tables VII and VIII. The viscosity of the centrifuged liquid product increases from 59 to 1,433 with an increase in asphaltene content from 22 to 40 and an increase in pyridine sols content from 2 to 5. In contrast, the viscosity of the heavy oil increases only from 12 to 17. There is no increase but rather a slight decrease in the molecular weight of asphaltenes from 459 to 416. 

Product oils from SYNTHOIL runs carried out at 415° and 450° C and 2,000 and 4,000 psi H2 pressures were analyzed with respect to asphaltene and oil content, elementary compositions (C, E, S, N), ash and physical properties (specific gravity and viscosity). Asphaltenes exert a large effect on the viscosity of the product oil, the viscosity increasing exponentially with asphaltene content. Viscosity of product oil is not only dependent on the amount but also on the molecular weight of asphaltenes present. At 415° C, asphaltenes with a molecular weight of 670 are formed and at 450° C asphaltenes with a molecular weight of 460. 

Asphaltene association factor the number of individual asphaltene species which associate in nonpolar solvents as measured by molecular weight methods the molecular weight of asphaltenes in toluene divided by the molecular weight in a polar nonassociating solvent, such as dichlorobenzene, pyridine, or nitrobenzene. 

Associated molecular weight the molecular weight of asphaltenes in an associating (nonpolar) solvent, such as toluene. 

Determining the molecular weights of asphaltenes is a problem because they have a low solubility in the liquids often used for determination. Also, adsorbed resins lead to discrepancies in molecular-weight determination, and precipitated asphaltenes should be reprecipitated several times prior to the determination (12). Thus, careful precipitation and careful choice of the determination method are both very important for obtaining meaningful results. 

Many questions concerning the nature of petroleum asphaltenes remain unresolved (1) What is the chemical composition of petroleum asphaltenes (2) What are the molecular weights of asphaltene components (3) Why are asphaltenes precipitated from solution in petroleum by the addition of a hydrocarbon solvent such as n-pentane In this chapter we attempt to answer these questions. In addition, we suggest that asphaltene formation is a general phenomenon that is pertinent to the chemistry of coals, tar sand bitumens, shale oil, and other complex solutions of organic compounds. 

All the indirect methods that are based on the determination of the colloidal properties of crude oil measure macroscopic properties such as density, viscosity, and molecular weight. An example is that the molecular weight of asphaltene... 

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]. 

Fig. 9.1 Molecular weight of asphaltenes versus residence time during co-processing.

In contrast to the results at 400°C, no reduction in asphaltene molecular weight was observed for residence times up to 40 minutes for reactions conducted at 425°C (see Fig. 9.2). This means that at higher temperatures, polycondensation reactions proceed faster than decomposition reactions. At any temperature, the determined molecular weight of asphaltenes shows that it reaches equilibrium as the reaction proceeds. This implies that at a longer residence time, the molecular weight of the asphaltene fraction will not increase any further because after achieving equilibrium molecular weight, they become less soluble in the maltenes. This leads to their flocculation from the maltenes fraction (Fig. 9.2) and, finally, to coke formation. 

This means that cracking of the paraffinic periphery (reaction (9.1)) is the only reaction that can cause the reduction of the molecular weight of asphaltenes at this temperature level. In other words, during polycondensation of asphaltenes at 425°C, it is not only the aromatic cores of native asphaltenes that react. Asphaltenes with paraffinic chains also undergo a reaction (reaction (9.3)). 

Molecular Weight. Asphaltene molecular weights (vapor pressure osmometry) are dependent not only on the nature of the solvent but also on the solution temperature at which the determinations were performed (78). However, data from molecular weight determinations by the cryoscopic method (79) indicate that the absolute molecular weight of asphaltenes cannot be determined by any one method. 

Coal asphaltene constituents are quite different in nature from petroleum asphaltene constituents (Table 18.7). The molecular weight of asphaltene constituents from coal liquids may be some 8-10 times lower than the observed molecular weight of petroleum asphaltene constituents, although this latter can be revised to lower values for a variety of reasons (Steedman, 1985). 

Also a thermodynamic model based on the coupled Equation of State model and Flory-Huggins theory for polymer solutions was developed. The model parameters such as solubility-parameter of asphaltenes, molecular weight of asphaltenes, and molar volume of asphaltenes were obtained by fitting the model to experimental data. 

The model consists of three parameters namely, (V ) the molar volume of asphaltenes, (6 ) the solubility parameter of asphaltenes and (M ), the molecular weight of asphaltenes. With the knowledge of these three parameters it is... 

Asphaltene and resin molecular parameters. The molecular weights of asphaltenes and resins are = 1000 = g/gmole (Storm and Sheu, 1995) and = 850 g/mole, based on the work of Lian, Lin, and Yen (1994). The contact area of a resin on the surface of a micellar core is 21 A (assuming that the polar head of the resin being the OH... 

FIGURE 1.5 Mass and mol average molecular weight of asphaltenes. 

By this means, yields of coke, gas, naphtha, and gasoil can be calculated from asphaltene content of the feed, VPO (vapor pressure osmometry) molecular weight of asphaltenes in toluene, and heteroatom content of asphaltenes. The problem with these correlations is that they require characterization of asphaltenes. 

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