Asphaltenes pyrolysis

Oil sand asphaltenes pyrolysis oil also contain n-alkane-derived cyclic sulfides along with /z-alkane-derived thiophenes, benzo- and dibenzothiophenes, showing that the precursor oil was of n-alkanoic origin. Approximately 25% of the sulfur in these asphaltenes is in the form of sulfides. 

The rest of the sulfur in oil sand asphaltenes, which is not in the form of thiane and thiolane, is present as thiophenes. Indeed, the asphaltene pyrolysis oil contains homologous series of a and a,a n-alkyl substituted thiophenes, 2-, 4- and 2,4-... 

Table I. Summary of global asphaltene pyrolysis experiments. The ultimate yields and final selectivities for a series of different asphaltene feedstocks which were pyrolyzed for two hours at 400°C (7)...

Neurock, M., A. Nigam, D.T. Trauth, and M.T. Klein, Asphaltene Pyrolysis Pathways and Kinetics Feedstock Dependence, in Tar Sand and Oil Upgrading Technology. S. Shih and M.C. Oballa, eds., AIChE Symposium Series, 72-79,1991. 

The gases obtained from volatilization and asphaltene pyrolysis comprise about 8% by weight of the bitumen coked, more than half of which consists of hydrogen and methane (Table 17.6). 

In the 340, 350 and 360 °C asphaltene pyrolysis experiments, values of SF(C3 C5) substantially greater than those of SF(C2- Q) reflect the observed excesses of propane (above the C2-nCg exponential progression), as discussed and illustrated below. Non-identical results in the two 340 °C cases are indicative of deficient experimental control. 

Hauser, A., Bahzad, D., Stanislaus, A., and Behbahani, M. Thermogravimetric analysis studies on the thermal stability of asphaltenes Pyrolysis behavior of heavy oil asphaltenes. Ener Fu 2008 22 449 54. 

Mansuy et al. [97] investigated the use of GC-C-IRMS as a complimentary correlation technique to GC and GC-MS, particularly for spilled crude oils and hydrocarbon samples that have undergone extensive weathering. In their study, a variety of oils and refined hydrocarbon products, weathered both artificially and naturally, were analyzed by GC, GC-MS, and GC-C-IRMS. The authors reported that in case of samples which have lost their more volatile n-alkanes as a result of weathering, the isotopic compositions of the individual compounds were not found to be extensively affected. Hence, GC-C-IRMS was shown to be useful for correlation of refined products dominated by n-alkanes in the C10-C20 region and containing none of the biomarkers more commonly used for source correlation purposes. For extensively weathered crude oils which have lost all of their n-alkanes,it has been demonstrated that isolation and pyrolysis of the asphaltenes followed by GC-C-IRMS of the individual pyrolysis products can be used for correlation purposes with their unaltered counterparts [97]. 

Pyrolysis of kerogens and asphaltenes has demonstrated the nature and logic of this conversion sequence (58.63-64). Relative to elemental composition, initial asphaltenes have a much lower atomic O/C ratio, a slightly lower atomic S/C ratio, and almost the same H/C and N/C ratios as their source kerogens (53). 

Oxidation studies on Athabasca and other oil sand asphaltenes have shown the presence of aliphatic sulfides in amounts of up to 25% of the total sulfur (25). The structure of these sulfides has been established using mild thermolysis to liberate them from the polymeric framework of the asphaltene molecules. The produced pyrolysis oil contains significant concentrations of the sulfides and can be readily subjected to analysis. SIR-GC/MS traces of the homologous series of sulfides identified are... 

The n-alkyl thianes and thiolanes are thermally interconvertible, with the equilibrium shifted toward the thiolane side. Also, the thiolane ring can move along the n-alkyl side chain hopping by three carbon atoms until it reaches a terminal position (11). For this reason the original thiane/thiolane distribution in the asphaltene molecule before its thermal breakup could have been somewhat different from that in the pyrolysis oil. 

Figure 18. Distribution by carbon number of the monocyclic sulfides in the pyrolysis oil of Athabasca asphaltene as determined by SIR-GC/MS. Peaks labelled 15 correspond to compounds having 15 carbon atoms. Each fragmentogram is normalized to the most abundant peak. The relative intensities of the m/z = 87, 101 and 115 fragmentograms are 9.1 3.1 1.0, respectively. (Reproduced with permission from Ref. 26. Copyright 1988, Alberta Oil Sands Technology and Research Authority.)...

Figure 5. Proposed structures of alkylthiophene moieties in kerogens and asphaltenes and their presumed flash pyrolysis products. Examples are give for alkylthiophene moieties with (a) linear, (b) isoprenoid, (c) branched and (d) steroidal side-chain carbon skeletons. Carbon skeletons are indicated with bold lines.

The different properties of sulfur-rich kerogen and asphaltenes, on the one hand, and sulfur-rich resins on the other hand (flash pyrolysis behaviour) may be explained only by differences in degree of (sulfur) cross-linking and thus by differences in molecular size and in degree of condensation. 

Asphaltene and resid pyrolysis provide two relevant examples of global pyrolysis models. The pyrolysis of an isolated asphaltene feedstock typically yields the type of data summarized in Figure 2, a plot of the temporal variation of weight based product fractions as a function of time (7). This figure illustrates the exponential disappearance of asphaltene accompanied by the formation of coke, maltene and gas product fractions. Consideration of the initial slopes for the formation of coke, maltene and gas fractions led to the type of reaction network shown in Figure 3. Since resid and its reaction products can likewise be defined in terms of the solubility and volatility-based product groups asphaltene,... 

Figure 2. An example of the global reaction product yields for the pyrolysis simulation of a Hondo-derived Asphaltene feedstock at 400°C [1],...

Figure 3. Global reaction network for asphaltene and resid pyrolysis systems [7].

Table I summarizes relevant data for the pyrolysis of a series of isolated asphaltene feedstocks. This table highlights the dependence of reaction product yields and selectivi-ties on the resid origin. Clearly the reactivity of each feed depends upon its source (7).

Returning to the asphaltene/resid pyrolysis example, the mechanistic models of these processes are based on free radical chemistry. That is, the elementary steps which describe the reaction chemistry are written in terms of free radical species. Thus the development of a mechanistic model requires the time-dependent solution of an extensive set of material balance equations for every component as well as for each of the active centers. 

Trauth, D.T., M. Yasar, M. Neurock, A. Nigam, and M.T. Klein, Asphaltene and Resid Pyrolysis Effect of Asphaltene Environment., Energy and Fuels, (accepted), 1992. 

---