Tar sand bitumen asphaltenes

The classic definition of asphaltenes is based on the solution properties of petroleum residuum in various solvents. This generalized concept has been extended to fractions derived from other carbonaceous sources, such as coal and oil shale. With this extension there has been much effort to define asphaltenes in terms of chemical structure and elemental analysis as well as by the carbonaceous source. This effort is summarized by Speight and Moschope-dis (i) in their chapter in this volume along with a good summary of the current thinking. Thus, there are petroleum asphaltenes, coal tar asphaltenes, shale oil asphaltenes, tar sands bitumen asphaltenes, and so on. In this chapter I will attempt to show how these materials are special cases of an overall concept based directly on the physical chemistry of solutions and that the idea that they have a specific chemical composition and molecular weight is incorrect even for different crude oil sources. 

Characteristics of Tar Sand Bitumen Asphaltenes as Studied by Conversion of Bitumen by Hydropyrolysis... 

The structure and chemistry of tar sand bitumen asphaltenes were studied indirectly by inference from processing and characterization of virgin bitumen and deasphaltened bitumen (maltenes). Results differ significantly from those derived from characterization of asphaltenes as isolated... 

Vanadium in Tar Sands, Bitumen, Asphaltenes, and Crude Oils... 

Physical methods of fractionation of tar sand bitumen usually indicate high proportions of nonvolatile asphaltenes and resins, even in amounts up to 50% wt/wt (or higher) of the bitumen. In addition, the presence of ash-forming metallic constituents, including such organometaUic compounds as those of vanadium and nickel, is also a distinguishing feature of bitumen. 

Perhaps, where these fuels differ most is in the amount of resins, asphaltenes, and benzene insolubles. The shale oils contain a very large amount of resins from 12 to 69%, or approximately 50% by weight on a total shale oil basis. The tar sand bitumen contains only half as much resins, and the coal liquids contain much less. On the other hand, the asphaltenes are significant in the tar sand bitumen and in the Pitt Seam coal liquids. These latter materials are the only ones to contain a significant amount of benzene insolubles. 

The bitumen comes as a residue from the refining of conventional or heavy crude oil, or from natural deposits of oil (tar) sand. Bitumen, being a complex mixture of more than 1000 different molecules, is itself a colloidal suspension of asphaltenes in a continuous phase of saturated parrafins, aromatic oils and resins [774], Descriptions of different kinds of asphalts are given in Refs. [775,776], At low asphaltene concentration the suspension is Newtonian. Once the concentration increases above about 8 % v/v, however, the asphaltenes form a three-dimensional network and the suspension can become a viscoelastic gel [774]. The asphaltenes interact through van der Waals forces so that a bitumen containing 15% asphaltenes is solid at room temperature and liquid above about 60-100 °C. 

To test these hypotheses, a tar sand bitumen containing 20 wt % pentane asphaltenes was characterized and processed by hydropyrolysis before and after removal of asphaltenes. Product yields and structure were determined and the influence of asphaltenes on results was determined by inferrence. Feedstocks and products were characterized according to elemental analysis, physical properties, simulated distillation, and carbon-type analysis. Inferences made in this study are discussed in the context of the reported literature. 

Compound type analysis was not conducted in this work, but it is instructive to look at some literature results. With respect to heteroatoms, tar sand bitumen and petroleum asphaltenes have been variously reported as containing predominantly polar heteroatoms, principally oxygen types (18) or nonpolar heteroatoms, principally nitrogen types (14). The difference in these reported results apparently relates to the method of analysis in which the former is a direct determination, the latter an indirect determination. 

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. 

The available evidence is specific to the Athabasca deposit. For example, bitumen obtained from the northern locales of the Athabasca deposit (Bitumount, Mildred-Ruth Lakes) has a lower amount (by weight values approximately 16-20%) of the nonvolatile asphaltene fraction than the bitumen obtained from southern deposits (Abasand, Hangingstone River approximately 22-23 /o by weight asphaltenes). In addition, other data indicate that there is also a marked variation of asphaltene content in the tar sand bitumen with depth in the particular deposit. 

The bitumen comes as a residue from the refining of conventional or heavy crude oil, or from natural deposits of oil (tar) sand. Bitumen, being a complex mixture of more than 1000 different molecules, is itself a colloidal suspension of asphaltenes in a continuous phase of saturated paraffins, aromatic oils and resins... 

Extraction of tar sands with benzene or toluene yields a bitumen fraction and a mineral fraction. Further treatment of the bitumen with heptane or n-pentane yields asphaltenes and petrolene, which are the insoluble and soluble fractions, respectively. Depending on the source, these fractions contain varying amounts of vanadium (117,118). 

Table V shows metal distributions in two other domestic tar sand fractions. In the Battle Creek bitumen from Wyoming, bitumen metals and porphyrins are concentrated in the asphaltenes, as expected. Fractions obtained from the Edna, California outcrop have vanadium-to-...

In the more localized context of the Athabasca deposit, inconsistencies arise presumably because of the lack of mobility of the bitumen at formation temperature (approximately 4°C, 39°F). For example, the proportion of bitumen in the tar sand increases with depth within the formation. Furthermore, the proportion of the nonvolatile asphaltenes or the nonvolatile asphaltic fraction (asphaltenes plus resins) in the bitumen also increases with depth within the formation that leads to reduced yields of distillate from the bitumen obtained from deeper parts of the formation. In keeping with the concept of higher proportions of asphaltic fraction (asphaltenes plus resins), variations (horizontal and vertical) in bitumen properties have been noted previously, as have variations in sulfur content, nitrogen content, and metals content. Obviously, the richer tar sand deposits occur toward the base of the formation, but the bitumen is generally of poorer quality. 

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