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The Chemistry and Technology of Petroleum
in polar forms (pyridines, pyrroles, phenols, carboxylic acids, amides, etc.) than in nonpolar forms
(such as ethers). The distribution and characteristics of these molecular species account for the rich
variety of crude oils.
Feedstock behavior during refining is better addressed through consideration of the molecular
makeup of the feedstock (perhaps, by analogy, just as genetic makeup dictates human behavior).
The occurrence of amphoteric species (i.e., compounds having a mixed acid/base nature) is not
addressed nor is the phenomenon of molecular size or the occurrence of specific functional types,
which can play a major role in the interactions between the constituents of a feedstock. All of these
items are important in determining feedstock behavior during refining operations.
An understanding of the chemical types (or composition) of any feedstock can lead to an understanding of the chemical aspects of processing the feedstock. Processability is not only a matter of
knowing the elemental composition of a feedstock, but it is also a matter of understanding the bulk
properties as they relate to the chemical or physical composition of the material. For example, it
is difficult to understand, a priori, the process chemistry of various feedstocks from the elemental
composition alone. From such data, it might be surmised that the major difference between a heavy
crude oil and a more conventional material is the H/C atomic ratio alone. This property indicates
that a heavy crude oil (having a lower H/C atomic ratio and being more aromatic in character) would
require more hydrogen for upgrading to liquid fuels. This is, indeed, true but much more information is necessary to understand the processability of the feedstock.
The hydrocarbon content of petroleum may be as high as 97% by weight (e.g., in the lighter
paraffinic crude oils) or as low as 50% by weight or less as illustrated by the heavy asphaltic crude
oils. Nevertheless, crude oils with as little as 50% hydrocarbon components are still assumed to
retain most of the essential characteristics of the hydrocarbons. It is, nevertheless, the nonhydrocarbon (sulfur, oxygen, nitrogen, and metal) constituents that play a large part in determining the
processability of the crude oil (Speight, 2011). But there is more to the composition of petroleum
than the hydrocarbon content. The inclusion of organic compounds of sulfur, nitrogen, and oxygen
serves only to present crude oils as even more complex mixtures, and the appearance of appreciable amounts of these nonhydrocarbon compounds causes some concern in the refining of crude
oils. Even though the concentration of nonhydrocarbon constituents (i.e., those organic compounds
containing one or more sulfur, oxygen, or nitrogen atoms) in certain fractions may be quite small,
they tend to concentrate in the higher boiling fractions of petroleum. Indeed, their influence on the
processability of the petroleum is important irrespective of their molecular size and the fraction in
which they occur.
The presence of traces of nonhydrocarbon compounds can impart objectionable characteristics
to finished products, leading to discoloration and/or lack of stability during storage. On the other
hand, catalyst poisoning and corrosion are the most noticeable effects during refining sequences
when these compounds are present. It is therefore not surprising that considerable attention must be
given to the nonhydrocarbon constituents of petroleum as the trend in the refining industry, of late,
has been to process more heavy crude oil as well as residua that contain substantial proportions of
these nonhydrocarbon materials.
8.3.1 HydroCArBon ComPonents
The isolation of pure compounds from petroleum is an exceedingly difficult task, and the overwhelming complexity of the hydrocarbon constituents of the higher molecular weight fractions and
the presence of compounds of sulfur, oxygen, and nitrogen are the main causes for the difficulties encountered. It is difficult on the basis of the data obtained from synthesized hydrocarbons to
determine the identity or even the similarity of the synthetic hydrocarbons to those that constitute
many of the higher boiling fractions of petroleum. Nevertheless, it has been well established that
the hydrocarbon components of petroleum are composed of paraffinic, naphthenic, and aromatic
groups (Table 8.1). Olefin groups are not usually found in crude oils, and acetylenic hydrocarbons
The Chemistry and Technology of Petroleum
in polar forms (pyridines, pyrroles, phenols, carboxylic acids, amides, etc.) than in nonpolar forms
(such as ethers). The distribution and characteristics of these molecular species account for the rich
variety of crude oils.
Feedstock behavior during refining is better addressed through consideration of the molecular
makeup of the feedstock (perhaps, by analogy, just as genetic makeup dictates human behavior).
The occurrence of amphoteric species (i.e., compounds having a mixed acid/base nature) is not
addressed nor is the phenomenon of molecular size or the occurrence of specific functional types,
which can play a major role in the interactions between the constituents of a feedstock. All of these
items are important in determining feedstock behavior during refining operations.
An understanding of the chemical types (or composition) of any feedstock can lead to an understanding of the chemical aspects of processing the feedstock. Processability is not only a matter of
knowing the elemental composition of a feedstock, but it is also a matter of understanding the bulk
properties as they relate to the chemical or physical composition of the material. For example, it
is difficult to understand, a priori, the process chemistry of various feedstocks from the elemental
composition alone. From such data, it might be surmised that the major difference between a heavy
crude oil and a more conventional material is the H/C atomic ratio alone. This property indicates
that a heavy crude oil (having a lower H/C atomic ratio and being more aromatic in character) would
require more hydrogen for upgrading to liquid fuels. This is, indeed, true but much more information is necessary to understand the processability of the feedstock.
The hydrocarbon content of petroleum may be as high as 97% by weight (e.g., in the lighter
paraffinic crude oils) or as low as 50% by weight or less as illustrated by the heavy asphaltic crude
oils. Nevertheless, crude oils with as little as 50% hydrocarbon components are still assumed to
retain most of the essential characteristics of the hydrocarbons. It is, nevertheless, the nonhydrocarbon (sulfur, oxygen, nitrogen, and metal) constituents that play a large part in determining the
processability of the crude oil (Speight, 2011). But there is more to the composition of petroleum
than the hydrocarbon content. The inclusion of organic compounds of sulfur, nitrogen, and oxygen
serves only to present crude oils as even more complex mixtures, and the appearance of appreciable amounts of these nonhydrocarbon compounds causes some concern in the refining of crude
oils. Even though the concentration of nonhydrocarbon constituents (i.e., those organic compounds
containing one or more sulfur, oxygen, or nitrogen atoms) in certain fractions may be quite small,
they tend to concentrate in the higher boiling fractions of petroleum. Indeed, their influence on the
processability of the petroleum is important irrespective of their molecular size and the fraction in
which they occur.
The presence of traces of nonhydrocarbon compounds can impart objectionable characteristics
to finished products, leading to discoloration and/or lack of stability during storage. On the other
hand, catalyst poisoning and corrosion are the most noticeable effects during refining sequences
when these compounds are present. It is therefore not surprising that considerable attention must be
given to the nonhydrocarbon constituents of petroleum as the trend in the refining industry, of late,
has been to process more heavy crude oil as well as residua that contain substantial proportions of
these nonhydrocarbon materials.
8.3.1 HydroCArBon ComPonents
The isolation of pure compounds from petroleum is an exceedingly difficult task, and the overwhelming complexity of the hydrocarbon constituents of the higher molecular weight fractions and
the presence of compounds of sulfur, oxygen, and nitrogen are the main causes for the difficulties encountered. It is difficult on the basis of the data obtained from synthesized hydrocarbons to
determine the identity or even the similarity of the synthetic hydrocarbons to those that constitute
many of the higher boiling fractions of petroleum. Nevertheless, it has been well established that
the hydrocarbon components of petroleum are composed of paraffinic, naphthenic, and aromatic
groups (Table 8.1). Olefin groups are not usually found in crude oils, and acetylenic hydrocarbons