Asphaltenes thermal decomposition

Indeed, the thermal decomposition of asphaltenes is a complex phenome-... 

Nevertheless, studies of the thermal decomposition of asphaltenes can provide relevant information about the mechanism by which asphaltenes are desulfurized. 

Figure 11. Molecular models can be used to show the possible changes during the thermal decomposition of asphaltenes both chemical and physical changes can be illustrated.

Table I. Various Aromatic Molecular Types Identified as Constituents of the Volatile Products from the Thermal Decomposition of Asphaltene Fractions...

If these results, with the data from thermal decomposition experiments (22,23, 24), can be projected to the particular asphaltenes in question, it would be indicative of sulfur existing predominantly in the heterocyclic form. 

The GPC size data also were used to monitor the effect of various processes on the asphaltene molecular size distributions. Only the asphaltenes were studied in these experiments since they are the hardest to process, and they cause the most diffusion and coking problems. One of the processes studied was visbreaking, that is, noncatalytic thermal processing. Previous workers had examined the thermal decomposition of an Athabasca asphaltene... 

The investigation into the influence of paraffinic plastics on asphaltene chemistry during thermal cracking showed that pure plastics affect only the equilibrium of alkylation reactions by the increase of the paraffinic radicals in the reaction zone (Figure 9.18). This means that asphaltene decomposition will be slowed down. As such, there will be no decomposition to form aromatic cores without paraffinic periphery. This decelerates polycondensation and coke formation during the thermal treatment of mixtures of vacuum residue and plastics. However, it does not promote the cracking of the asphaltenes. 

The thermal hydroconversion of Cold Lake asphaltenes was studied initially to provide a basis for evaluation of catalytic effectiveness in subsequent work. Series of thermal runs were made at 335°C, 365°C and 400°C and the reaction products were separated as described previously. Several kinetic models were tried, but after examining the variability of our data, we decided on the simple first-order asphaltene decomposition model shown below ... 

The thermal history of these particular residuum asphaltenes is much more severe in terms of heating time than would ordinarily be the case for a refinery product from a pipestill since, in the present instance, a pot distillation was used. It therefore seems likely that refinery asphaltenes should be even less different from their respective crude asphaltenes than in this investigation, assuming that pipestill temperatures would be kept below the decomposition temperatures for the asphaltenes,... 

An analogy to the deasphalting process occurs, except the paraffinic material is a product of the thermal decomposition of the asphaltene molecules and is formed in situ rather than being added from an external source. 

Figure 12. Simplified representation of the thermal decomposition of a neutral polar asphaltene.

In summary, it has been assumed, on the basis of the behavior of the thermal decomposition of polynuclear aromatic systems, that coal must also consist of large polynuclear aromatic systems (Chapter 10). Be that as it may, such assumptions are highly speculative and, to say the least, somewhat lacking in caution. As an example, similar lines of thought have been applied to structural assumptions about petroleum asphaltene constituents when it is known from other pyrolysis studies that smaller, but polar, systems can produce as much thermal coke as the larger nonpolar highly condensed systems (Speight, 2007). 

Kossiakoff A, Rice FO (1943) Thermal decomposition of hydrocarbons, resonance stabilization and isomerization of free radieals. J Am Chem Soc 65 590-594 Kowalewski I, Vandenbroucke M, Hue AY, Taylor MJ, Faulon JL (1996) Prehminary results on molecular modeling of asphaltenes using structure elucidation programs in conjunction with molecular simulation programs. Energy Fuels, 10 97-107... 

Decomposition of petroleum asphaltenes has received attention primarily because of its tendency toward coke formation under thermal conditions. For this reason a key parameter for understanding residue processing via coking is to study the chemistry of coke formation at different temperatures (Goncalves et al., 2001 Douda et al., 2004). Various reaction pathways have been proposed for asphaltene thermal decomposition and it has been reported that the main products are alkanes ranging from Cj to C40 and polynuclear aromatics (from 1 to 4 aromatic rings)... 

Once the saturate, aromatic, resin, and asphaltene fractions were recovered, TGA was carried out on each fraction. The atmospheric residue was also analyzed by TGA. The temperature ranged from ambient up to 800°C. Aromatics, resins, asphaltenes, and atmospheric residue were analyzed at three different heating rate temperatures, i.e., 8°C/min, 12°C/min, and 16°C/min to obtain kinetic parameters toward coke formation, while for saturates only one heating rate (8°C/min) was used due to its propensity to ease thermal decomposition. 

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