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The Chemistry and Technology of Petroleum
9.2.1 AtmosPHerIC Pressure
Distillation is a common method for the fractionation of petroleum that is used in the laboratory as
well as in refineries. The technique of distillation has been practiced for many centuries and it was
recognized in the early days of the refining industry that desirable product (kerosene as a lamp oil)
could be separated by distillation. Thus, it is not surprising that distillation became the process of
choice for petroleum refining and the process has evolved from the simple distillation units to the
complex multi-plate still used in the refining industry (Chapter 15).
Distillation has found wide applicability in petroleum science and technology but it is generally
recognized that the fractions separated by distillation are only rarely, if at all, suitable for designations as a petroleum product. Each usually requires some degree of refining, which of course varies
with the impurities in the fraction and the desired properties of the finished product (Chapter 24).
Nevertheless, distillation is the most important fractionating process for the separation of petroleum
hydrocarbons; it is an essential part of any refinery operation and will continue to be so for the next
several decades (Speight, 2011).
However, insofar as petroleum is a mixture of several thousand (or even more) individual chemical compounds, there is little commercial emphasis on the isolation of the individual components.
The aim of the distillation of petroleum is predominantly an assessment of the nature and volatility
of the material through separation into several fractions of substantially broad boiling ranges.
The initial fractionation of crude oil essentially involves distillation of the material into various
fractions, as illustrated by the distillation procedure used for the boiling range specifications of
petroleum (Chapter 9). The fractions into which crude oil is commonly separated (Chapter 17) do
vary depending on the nature and composition of the crude oil; indeed, as the terminology indicates,
there is considerable overlap between the various fractions.
The kerosene (stove oil) and light gas oil fractions are often referred to as middle distillates and
usually represent the last fractions to be separated by distillation at atmospheric pressure. This
leaves the fractions from the heavy gas oil and higher boiling material that are collectively called
reduced crude.
An atmospheric distillation unit (atmospheric pipe still) is a distillation unit that contains a
large number of theoretical plates, and the side streams are taken off at different heights up the
distillation tower. The petroleum is partially or totally vaporized in a furnace and then fed to the
bottom of the atmospheric still. The distillation tower separates the petroleum by boiling point, that
is, molecular weight, with the lower molecular weight more volatile constituents concentrating at
the top of the column and the higher molecular weight less volatile constituents concentrating at
the bottom of the column. Naphtha is the chemical precursor of gasoline and boils over the same
range, while the atmospheric residuum has an initial boiling point on the order of 350°C (660°F).
The atmospheric residuum can be distilled further in a vacuum distillation unit.
9.2.2 reduCed Pressures
Separation of the reduced crude into the constituent fractions requires that the next-stage distillation be carried out under reduced pressure. The higher boiling constituents undergo thermal decomposition at temperatures above 350°C (660°F). This will result in molecular fragmentation leading
to volatile products (that were not indigenous to the crude oil) and to coke.
To avoid these thermal reactions it is necessary to reduce the pressure at which distillation is
performed, and since the vapor pressure and temperature are related, the lowering of the pressure is accompanied by a corresponding decrease in the boiling points of the individual constituents. For example, a specific compound boiling near 350°C (660°F) at 1 atm (760 mmHg) may
boil over 100°C (180°F) lower (approximately 250°C, 480°F) at 25–30 mmHg, and the danger
of thermal decomposition, as with the other thermal interactions, is markedly reduced, if not
eliminated.
The Chemistry and Technology of Petroleum
9.2.1 AtmosPHerIC Pressure
Distillation is a common method for the fractionation of petroleum that is used in the laboratory as
well as in refineries. The technique of distillation has been practiced for many centuries and it was
recognized in the early days of the refining industry that desirable product (kerosene as a lamp oil)
could be separated by distillation. Thus, it is not surprising that distillation became the process of
choice for petroleum refining and the process has evolved from the simple distillation units to the
complex multi-plate still used in the refining industry (Chapter 15).
Distillation has found wide applicability in petroleum science and technology but it is generally
recognized that the fractions separated by distillation are only rarely, if at all, suitable for designations as a petroleum product. Each usually requires some degree of refining, which of course varies
with the impurities in the fraction and the desired properties of the finished product (Chapter 24).
Nevertheless, distillation is the most important fractionating process for the separation of petroleum
hydrocarbons; it is an essential part of any refinery operation and will continue to be so for the next
several decades (Speight, 2011).
However, insofar as petroleum is a mixture of several thousand (or even more) individual chemical compounds, there is little commercial emphasis on the isolation of the individual components.
The aim of the distillation of petroleum is predominantly an assessment of the nature and volatility
of the material through separation into several fractions of substantially broad boiling ranges.
The initial fractionation of crude oil essentially involves distillation of the material into various
fractions, as illustrated by the distillation procedure used for the boiling range specifications of
petroleum (Chapter 9). The fractions into which crude oil is commonly separated (Chapter 17) do
vary depending on the nature and composition of the crude oil; indeed, as the terminology indicates,
there is considerable overlap between the various fractions.
The kerosene (stove oil) and light gas oil fractions are often referred to as middle distillates and
usually represent the last fractions to be separated by distillation at atmospheric pressure. This
leaves the fractions from the heavy gas oil and higher boiling material that are collectively called
reduced crude.
An atmospheric distillation unit (atmospheric pipe still) is a distillation unit that contains a
large number of theoretical plates, and the side streams are taken off at different heights up the
distillation tower. The petroleum is partially or totally vaporized in a furnace and then fed to the
bottom of the atmospheric still. The distillation tower separates the petroleum by boiling point, that
is, molecular weight, with the lower molecular weight more volatile constituents concentrating at
the top of the column and the higher molecular weight less volatile constituents concentrating at
the bottom of the column. Naphtha is the chemical precursor of gasoline and boils over the same
range, while the atmospheric residuum has an initial boiling point on the order of 350°C (660°F).
The atmospheric residuum can be distilled further in a vacuum distillation unit.
9.2.2 reduCed Pressures
Separation of the reduced crude into the constituent fractions requires that the next-stage distillation be carried out under reduced pressure. The higher boiling constituents undergo thermal decomposition at temperatures above 350°C (660°F). This will result in molecular fragmentation leading
to volatile products (that were not indigenous to the crude oil) and to coke.
To avoid these thermal reactions it is necessary to reduce the pressure at which distillation is
performed, and since the vapor pressure and temperature are related, the lowering of the pressure is accompanied by a corresponding decrease in the boiling points of the individual constituents. For example, a specific compound boiling near 350°C (660°F) at 1 atm (760 mmHg) may
boil over 100°C (180°F) lower (approximately 250°C, 480°F) at 25–30 mmHg, and the danger
of thermal decomposition, as with the other thermal interactions, is markedly reduced, if not
eliminated.