189
Chemical Composition
The major exception to these narrow limits is the oxygen content of bitumen, which can vary from
as little as 0.2% to as high as 4.5%. This is not surprising, since when oxygen is estimated by
difference, the analysis is subject to the accumulation of all of the errors in the other elemental
data. In addition, bitumen is susceptible to aerial oxygen and the oxygen content is very dependent
upon the sample history. In addition, the ultimate composition of the Alberta bitumen does not
appear to be influenced by the proportion of bitumen in the oil sand or by the particle size of the oil
sand minerals.
Bitumen from US tar sand has an ultimate composition similar to that of the Athabasca bitumen
(Speight, 1990 and references cited therein). As already noted, when the many localized or regional
variations in maturation conditions are assessed, it is perhaps surprising that the ultimate compositions are so similar. Although several generalities can be noted from the ultimate composition, these
can only give indications of how the material might behave during processing (Chapters 8 and 16).
The viscosity of bitumen is related to its hydrogen-to-carbon atomic ratio and hence the required
supplementary heat energy for thermal extraction processes. It also affects the bitumen’s distillation
curve or thermodynamic characteristics, its gravity, and its pour point. Atomic hydrogen-to-carbon
ratios as low as 1.3 have been observed for tar sand bitumen although an atomic hydrogen-tocarbon ratio of 1.5 is more typical. The higher the hydrogen–carbon ratio of bitumen, the higher is
its value as refinery feedstock because of the lower hydrogen requirements. Elements related to the
hydrogen–carbon ratio are distillation curve, bitumen gravity, pour point, and bitumen viscosity.
The occurrence of sulfur in bitumen as organic or elemental sulfur or in produced gas as compounds of oxygen and hydrogen is an expensive nuisance. It must be removed from the bitumen at
some point in the upgrading and refining process. Sulfur contents of some tar sand bitumen can
exceed 10% w/w. Elements related to sulfur content are hydrogen content, hydrogen–carbon ratio,
nitrogen content, distillation curve, and viscosity.
The nitrogen content of tar sand bitumen can be as high as 1.3% by weight and nitrogen-containing constituents complicate the refining process by poisoning the catalysts employed in the refining
process. Elements related to nitrogen content are sulfur content, hydrogen content, hydrogen– carbon
ratio, bitumen viscosity, distillation profile, and viscosity.
8.3 CHEMICAL COMPOSITION
Petroleum contains an extreme range of organic functionality and molecular size. In fact, the variety is so great that it is unlikely that a complete compound-by-compound description for even a
single crude oil would not be possible. As already noted, the composition of petroleum can vary
with the location and age of the field in addition to any variations that occur with the depth of the
individual well. Two adjacent wells are more than likely to produce petroleum with very different
characteristics.
In very general terms (and as observed from elemental analyses), petroleum, heavy oil, bitumen,
and residua are a complex composition of (1) hydrocarbons, (2) nitrogen compounds, (3) oxygen
compounds, (4) sulfur compounds, and (5) metallic constituents. However, this general definition
is not adequate to describe the composition of petroleum as it relates to the behavior of these feedstocks. Indeed, the consideration of hydrogen-to-carbon atomic ratio, sulfur content, and API gravity are no longer adequate to the task of determining refining behavior.
Furthermore, the molecular composition of petroleum can be described in terms of three classes of
compounds: saturates, aromatics, and compounds bearing heteroatoms (sulfur, oxygen, or nitrogen).
Within each class, there are several families of related compounds, namely, (1) saturated constituents
include normal alkanes, branched alkanes, and cycloalkanes (paraffins, iso-paraffins, and naphthenes,
in petroleum terms); (2) alkene constituents (olefins) are rare to the extent of being considered an
oddity; (3) monoaromatic constituents range from benzene to multiple fused ring analogs ( naphthalene,
phenanthrene, etc.); (4) thiol (mercaptan) constituents contain sulfur as do thioethers and thiophenes
forms; and (5) nitrogen-containing and oxygen-containing constituents are more likely to be found
Chemical Composition
The major exception to these narrow limits is the oxygen content of bitumen, which can vary from
as little as 0.2% to as high as 4.5%. This is not surprising, since when oxygen is estimated by
difference, the analysis is subject to the accumulation of all of the errors in the other elemental
data. In addition, bitumen is susceptible to aerial oxygen and the oxygen content is very dependent
upon the sample history. In addition, the ultimate composition of the Alberta bitumen does not
appear to be influenced by the proportion of bitumen in the oil sand or by the particle size of the oil
sand minerals.
Bitumen from US tar sand has an ultimate composition similar to that of the Athabasca bitumen
(Speight, 1990 and references cited therein). As already noted, when the many localized or regional
variations in maturation conditions are assessed, it is perhaps surprising that the ultimate compositions are so similar. Although several generalities can be noted from the ultimate composition, these
can only give indications of how the material might behave during processing (Chapters 8 and 16).
The viscosity of bitumen is related to its hydrogen-to-carbon atomic ratio and hence the required
supplementary heat energy for thermal extraction processes. It also affects the bitumen’s distillation
curve or thermodynamic characteristics, its gravity, and its pour point. Atomic hydrogen-to-carbon
ratios as low as 1.3 have been observed for tar sand bitumen although an atomic hydrogen-tocarbon ratio of 1.5 is more typical. The higher the hydrogen–carbon ratio of bitumen, the higher is
its value as refinery feedstock because of the lower hydrogen requirements. Elements related to the
hydrogen–carbon ratio are distillation curve, bitumen gravity, pour point, and bitumen viscosity.
The occurrence of sulfur in bitumen as organic or elemental sulfur or in produced gas as compounds of oxygen and hydrogen is an expensive nuisance. It must be removed from the bitumen at
some point in the upgrading and refining process. Sulfur contents of some tar sand bitumen can
exceed 10% w/w. Elements related to sulfur content are hydrogen content, hydrogen–carbon ratio,
nitrogen content, distillation curve, and viscosity.
The nitrogen content of tar sand bitumen can be as high as 1.3% by weight and nitrogen-containing constituents complicate the refining process by poisoning the catalysts employed in the refining
process. Elements related to nitrogen content are sulfur content, hydrogen content, hydrogen– carbon
ratio, bitumen viscosity, distillation profile, and viscosity.
8.3 CHEMICAL COMPOSITION
Petroleum contains an extreme range of organic functionality and molecular size. In fact, the variety is so great that it is unlikely that a complete compound-by-compound description for even a
single crude oil would not be possible. As already noted, the composition of petroleum can vary
with the location and age of the field in addition to any variations that occur with the depth of the
individual well. Two adjacent wells are more than likely to produce petroleum with very different
characteristics.
In very general terms (and as observed from elemental analyses), petroleum, heavy oil, bitumen,
and residua are a complex composition of (1) hydrocarbons, (2) nitrogen compounds, (3) oxygen
compounds, (4) sulfur compounds, and (5) metallic constituents. However, this general definition
is not adequate to describe the composition of petroleum as it relates to the behavior of these feedstocks. Indeed, the consideration of hydrogen-to-carbon atomic ratio, sulfur content, and API gravity are no longer adequate to the task of determining refining behavior.
Furthermore, the molecular composition of petroleum can be described in terms of three classes of
compounds: saturates, aromatics, and compounds bearing heteroatoms (sulfur, oxygen, or nitrogen).
Within each class, there are several families of related compounds, namely, (1) saturated constituents
include normal alkanes, branched alkanes, and cycloalkanes (paraffins, iso-paraffins, and naphthenes,
in petroleum terms); (2) alkene constituents (olefins) are rare to the extent of being considered an
oddity; (3) monoaromatic constituents range from benzene to multiple fused ring analogs ( naphthalene,
phenanthrene, etc.); (4) thiol (mercaptan) constituents contain sulfur as do thioethers and thiophenes
forms; and (5) nitrogen-containing and oxygen-containing constituents are more likely to be found