199
Chemical Composition
less than 2% w/w, although larger amounts have been reported, but when the oxygen content is
phenomenally high it may be that the oil has suffered prolonged exposure to the atmosphere either
during or after production. However, the oxygen content of petroleum increases with the boiling
point of the fractions examined; in fact, the nonvolatile residua may have oxygen contents up to 8%
w/w. Although these high-molecular-weight compounds contain most of the oxygen in petroleum,
little is known concerning their structure, but those of lower molecular weight have been investigated with considerably more success and have been shown to contain carboxylic acids and phenols.
The presence of acid substances in petroleum first appears to have been reported in 1874, and it
was established 9 years later that these substances contained carboxyl groups and were carboxylic
acids. These were termed naphthenic acids. Although alicyclic (naphthenic) acids appear to be the
more prevalent, it is now well known that aliphatic acids are also present. In addition to the carboxylic acids, alkaline extracts from petroleum contain phenols.
It has generally been concluded that the carboxylic acids in petroleum with fewer than eight
carbon atoms per molecule are almost entirely aliphatic in nature; monocyclic acids begin at C 6 and
predominate above C 14 . This indicates that the structures of the carboxylic acids correspond with
those of the hydrocarbons with which they are associated in the crude oil. In the range in which paraffins are the prevailing type of hydrocarbon, the aliphatic acids may be expected to predominate.
Similarly, in the ranges in which monocycloparaffins and dicycloparaffins prevail, one may expect
to find principally monocyclic and dicyclic acids, respectively.
In addition to the carboxylic acids and phenolic compounds, the presence of ketones, esters,
ethers, and anhydrides has been claimed for a variety of crude oils. However, the precise identification of these compounds is difficult because most of them occur in the higher molecular weight
nonvolatile residua. They are claimed to be products of the air blowing of the residua, and their
existence in virgin petroleum may yet need to be substantiated.
Although comparisons are frequently made between the sulfur and nitrogen contents and such
physical properties as the API gravity, it is not the same with the oxygen contents of crude oils. It is
possible to postulate, and show, that such relationships exist. However, the ease with which some of
the crude oil constituents can react with oxygen (aerial or dissolved) to incorporate oxygen functions
into their molecular structure often renders the exercise somewhat futile if meaningful deductions
are to be made.
8.3.2.3 Nitrogen Compounds
Nitrogen in petroleum may be classified arbitrarily as basic and nonbasic. The basic nitrogen compounds (Table 8.3), which are composed mainly of pyridine homologues and occur throughout the
boiling ranges, have a decided tendency to exist in the higher boiling fractions and residua. The
nonbasic nitrogen compounds, which are usually of the pyrrole, indole, and carbazole types, also
occur in the higher boiling fractions and residua.
In general, the nitrogen content of crude oil is low and generally falls within the range
0.1%–0.9%, although early work indicates that some crude oil may contain up to 2% nitrogen.
However, crude oils with no detectable nitrogen or even trace amounts are not uncommon, but
in general, the more asphaltic the oil, the higher its nitrogen content. Insofar as an approximate
correlation exists between the sulfur content and API gravity of crude oils (Speight, 2000), there
also exists a correlation between nitrogen content and the API gravity of crude oil. It also follows that there is an approximate correlation between the nitrogen content and the carbon residue: the higher the nitrogen content, the higher the carbon residue. The presence of nitrogen in
petroleum is of much greater significance in refinery operations than might be expected from the
small amounts present. Nitrogen compounds can be responsible for the poisoning of cracking
catalysts, and they also contribute to gum formation in such products as domestic fuel oil. The
trend in recent years toward cutting deeper into the crude to obtain stocks for catalytic cracking
has accentuated the harmful effects of the nitrogen compounds, which are concentrated largely
in the higher boiling portions.
Chemical Composition
less than 2% w/w, although larger amounts have been reported, but when the oxygen content is
phenomenally high it may be that the oil has suffered prolonged exposure to the atmosphere either
during or after production. However, the oxygen content of petroleum increases with the boiling
point of the fractions examined; in fact, the nonvolatile residua may have oxygen contents up to 8%
w/w. Although these high-molecular-weight compounds contain most of the oxygen in petroleum,
little is known concerning their structure, but those of lower molecular weight have been investigated with considerably more success and have been shown to contain carboxylic acids and phenols.
The presence of acid substances in petroleum first appears to have been reported in 1874, and it
was established 9 years later that these substances contained carboxyl groups and were carboxylic
acids. These were termed naphthenic acids. Although alicyclic (naphthenic) acids appear to be the
more prevalent, it is now well known that aliphatic acids are also present. In addition to the carboxylic acids, alkaline extracts from petroleum contain phenols.
It has generally been concluded that the carboxylic acids in petroleum with fewer than eight
carbon atoms per molecule are almost entirely aliphatic in nature; monocyclic acids begin at C 6 and
predominate above C 14 . This indicates that the structures of the carboxylic acids correspond with
those of the hydrocarbons with which they are associated in the crude oil. In the range in which paraffins are the prevailing type of hydrocarbon, the aliphatic acids may be expected to predominate.
Similarly, in the ranges in which monocycloparaffins and dicycloparaffins prevail, one may expect
to find principally monocyclic and dicyclic acids, respectively.
In addition to the carboxylic acids and phenolic compounds, the presence of ketones, esters,
ethers, and anhydrides has been claimed for a variety of crude oils. However, the precise identification of these compounds is difficult because most of them occur in the higher molecular weight
nonvolatile residua. They are claimed to be products of the air blowing of the residua, and their
existence in virgin petroleum may yet need to be substantiated.
Although comparisons are frequently made between the sulfur and nitrogen contents and such
physical properties as the API gravity, it is not the same with the oxygen contents of crude oils. It is
possible to postulate, and show, that such relationships exist. However, the ease with which some of
the crude oil constituents can react with oxygen (aerial or dissolved) to incorporate oxygen functions
into their molecular structure often renders the exercise somewhat futile if meaningful deductions
are to be made.
8.3.2.3 Nitrogen Compounds
Nitrogen in petroleum may be classified arbitrarily as basic and nonbasic. The basic nitrogen compounds (Table 8.3), which are composed mainly of pyridine homologues and occur throughout the
boiling ranges, have a decided tendency to exist in the higher boiling fractions and residua. The
nonbasic nitrogen compounds, which are usually of the pyrrole, indole, and carbazole types, also
occur in the higher boiling fractions and residua.
In general, the nitrogen content of crude oil is low and generally falls within the range
0.1%–0.9%, although early work indicates that some crude oil may contain up to 2% nitrogen.
However, crude oils with no detectable nitrogen or even trace amounts are not uncommon, but
in general, the more asphaltic the oil, the higher its nitrogen content. Insofar as an approximate
correlation exists between the sulfur content and API gravity of crude oils (Speight, 2000), there
also exists a correlation between nitrogen content and the API gravity of crude oil. It also follows that there is an approximate correlation between the nitrogen content and the carbon residue: the higher the nitrogen content, the higher the carbon residue. The presence of nitrogen in
petroleum is of much greater significance in refinery operations than might be expected from the
small amounts present. Nitrogen compounds can be responsible for the poisoning of cracking
catalysts, and they also contribute to gum formation in such products as domestic fuel oil. The
trend in recent years toward cutting deeper into the crude to obtain stocks for catalytic cracking
has accentuated the harmful effects of the nitrogen compounds, which are concentrated largely
in the higher boiling portions.