Asphaltenes molecular weight

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. 

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. 

Thus, asphaltene molecular weights are truly variable. Data produced from the determinations by using highly polar solvents, however, indicate that the molecular weights, in solvents that prevent association, usually fall into the range of 1500-2500. 

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. 

Table 8. Optimized Values of Asphaltene Solubility Parameter, Asphaltene Molar Volume and Asphaltene Molecular Weight For Various Solvents (West Sak Tank Oil)... 

Asphaltene molecular weight distribution and heteroatom content are both of great importance to design catalysts with suitable properties to process these heavy materials, i.e., optimal pore size distribution to allow the complex molecules to access the catalytic sites with minimal diffusion limitations and to enhance metal retention capacity. 

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