Page 126 - Synthetic Fuels Handbook

112 CHAPTER FOUR

There are wide variations both in the bitumen saturation of tar sand (0–18 percent by
weight of bitumen) even within a particular deposit, and the viscosity is particularly high.
Of particular note is the variation of the density of Athabasca bitumen with temperature
and the maximum density difference between bitumen and water occurs at 70 to 80°C
(158–176°F), hence the choice of the operating temperature of the hot water bitumen-
extraction process.
The character of bitumen can be assessed in terms of API gravity, viscosity, and sulfur
content (Table 4.4). Properties such as these help the refinery operator to gain an under-
standing of the nature of the material that is to be processed. Thus, initial inspection of the
feedstock (conventional examination of the physical properties) is necessary. From this, it
is possible to make deductions about the most logical means of refining.

TABLE 4.4 Properties of Synthetic Crude Oil from Athabasca Bitumen
Property Bitumen Synthetic crude oil
Gravity, °API 8 32
Sulfur, wt % 4.8 0.2
Nitrogen, wt % 0.4 0.1
Viscosity, cP at 100°F 500,000 10
Distillation profile, by wt %
°C °F
0 32 0 5
30 86 0 30
220 428 1 60
345 653 17 90
550 1022 45 100

In the context of the Athabasca deposit, compositional inconsistencies arise 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 for-
mation. Furthermore, the proportion destructive distillate from the bitumen also decreases
with depth within the formation that leads to reduced yields of distillate from the bitumen
obtained from deeper parts of the formation. Variations (horizontal and vertical) in bitu-
men 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. However,
the bitumen is generally of poorer quality than the bitumen obtained from near the top of
the deposit insofar as the proportions of nonvolatile coke-forming constituents (asphaltenes
plus resins) are higher (with increased proportions of nitrogen, sulfur, and metals) near the
base of the formation.

4.3.1 Composition
Elemental (Ultimate) Composition. The elemental analysis of tar sand bitumen has been
widely reported. However, the data suffer from the disadvantage that identification of the
source is very general (i.e., Athabasca bitumen) or analysis is quoted for separated bitumen
that may have been obtained by, say, the hot water separation or solvent extraction and may
not therefore represent the total bitumen on the sand.
However, of the available data, the elemental composition of tar sand bitumen is gener-
ally constant and falls into the same narrow range as for petroleum. In addition, the ultimate