| Description |
Asphaltenes derived from tar sand bitumen are similar to petroleum derived asphaltenes in that the geochemical history is similar for these sources. It has been shown that tar sand bitumen possesses an unusually high average molecular weight when compared with petroleum residues of similar initial boiling points (1). Molecular weight is, perhaps, the dominant factor in determining whether a particular structural type appears as an asphaltene or a maltene in bitumen. Certainly, aromaticity and polar functionality are contributing factors as well. It has been observed that asphaltenes from the relatively hydrogen rich Uinta Basin bitumen contribute significantly to liquid product yields during catalytic cracking and, conversely, that coke production can only be reduced from 12% to 8% by prior removal of 10% of the bitumen as asphaltenes (2). This evidence indicates that the species comprising the asphaltene fraction are not entirely or even largely responsible for the formation of coke, and that major portions of the asphaltenes species are convertible to valuable, albeit heavy liquids. The concept that molecules comprising the asphaltene fractions are not chemically unique, but merely an extension of the lower molecular species is quite plausible in view of the geochemical origins of petroleum and tar sands. Certainly to the extent that multifunctionality, viz. , polynuclear aromatics and polar heteroatoms, appears in a molecule, the opportunity to associate in solution and appear in the asphaltene concentrate is enhanced. Thus, aromatic carbon and polar functionality are typically concentrated to some extent in the asphaltenes. However, in terms of gross hydrocarbon structure, the molecules found in the asphaltenes fraction from bitumen contain large quantities of saturated carbon which should be theoretically convertible to lower molecular weight distillate products. In concept, the objective to conversion of bitumen asphaltenes is to r e duce the molecular weight through cracking of saturates portions while inhibiting the reactions of the aromatics to form coke. One process which has shown particular promise for achieving this objective is hydropyrolysis (3-6). This process appears to be particularly appealing for tar sand bitumen (and petroleum residue) processing because the economics of bitumen recovery are highly sensitive to large losses of bitumen to coke. Given the adverse effects that asphaltenes are commonly purported to have on processing of heavy ends it appeared important to us to attempt to quantify the effects of asphaltenes on conversion of bitumen by hydropyrolysis. The approach of this study was to characterize the bitumen, maltenes, and asphaltenes according to gross hydrocarbon structure and properties, to subject feedstocks of varying asphaltene content to hydropyrolysis, to assess the effect of asphaltenes on the results and to interpret these results in the context of chemical structure. The processing of the asphaltene fraction itself is probably not a fruitful approach and was not pursued in this study. Feedstocks consisting of deasphaltened bitumen (maltenes), virgin bitumen and an asphaltene enhanced (virgin bitumen spiked with asphaltenes) bitumen from the Sunnyside, Utah tar sand deposit were used for the processing studies. |
