466
The Chemistry and Technology of Petroleum
through weirs and is actually prevented from returning to the tray below through the holes by the
velocity of the rising vapors. Needless to say, a minimum vapor velocity is required to prevent
return of the liquid through the perforations.
In simple refineries, cut points can be changed slightly to vary yields and balance products, but
the more common practice is to produce relatively narrow fractions and then process (or blend) to
meet product demand. Since all these primary fractions are equilibrium mixtures, they all contain
some proportion of the lighter constituents characteristic of a lower boiling fraction and so are
stripped of these constituents, or stabilized, before further processing or storage. Thus, gasoline is
stabilized to a controlled butanes–pentanes content, and the overhead may be passed to superfractionators, towers with a large number of plates that can produce nearly pure C 1 –C 4 hydrocarbons
(methane to butanes, CH 4 to C 4 H 10 ), the successive columns termed deethanizers, depropanizers,
debutanizers, and so on.
Kerosene and gas oil fractions are obtained as sidestream products from the atmospheric tower
(primary tower), and these are treated in stripping columns (i.e., vessels of a few bubble trays) into
which steam is injected and the volatile overhead from the stripper is returned to the primary tower.
Steam is usually introduced by the stripping section of the primary column to lower the temperature
at which fractionation of the heavier ends of the crude can occur.
The specifications for most petroleum products make it extremely difficult to obtain marketable material by distillation only. In fact, the purpose of atmospheric distillation is considered the
provision of fractions that serve as feedstocks for intermediate refining operations and for blending.
Generally, this is carried out at atmospheric pressure, although light crude oils may be topped at an
elevated pressure and the residue then distilled at atmospheric pressure.
The topping operation differs from normal distillation procedures insofar as the majority of the
heat is directed to the feed stream rather than by reboiling the material in the base of the tower.
In addition, products of volatility intermediate between that of the overhead fractions and bottoms
(residua) are withdrawn as sidestream products. Furthermore, steam is injected into the base of
the column and the sidestream strippers to adjust and control the initial boiling range (or point)
of the fractions.
Topped crude oil must always be stripped with steam to elevate the flash point or to recover the
final portions of gas oil. The composition of the topped crude oil is a function of the temperature of
the vaporizer (or flasher). In addition, the properties of the residuum are very dependent upon the
extent of volatiles removal either by atmospheric distillation or by vacuum distillation (Table 17.1).
17.3.2 reduCed Pressure dIstIllAtIon
The boiling range of the highest boiling fraction that can be produced at atmospheric pressure is
limited by the temperature at which the residue starts to decompose or crack. If the stock is required
for the manufacture of lubricating oils, further fractionation without cracking may be desirable, and
this may be achieved by distillation under vacuum.
Vacuum distillation evolved because of the need to separate the less-volatile products, such as
lubricating oils, from the petroleum without subjecting these high-boiling products to cracking
conditions. The boiling range of the highest boiling fraction obtainable at atmospheric pressure is
limited by the temperature (∼350°C; ∼660°F) at which the residue starts to decompose or crack,
unless cracking distillation is preferred. When the feedstock is required for the manufacture of
lubricating oils, further fractionation without cracking is desirable, and this can be achieved by
distillation under vacuum (reduced pressure) conditions.
The distillation of high-boiling lubricating oil stocks may require pressures as low as 15–30 mm Hg
(0.29–0.58 psi), but operating conditions are more usually 50–100 mm Hg (0.97–1.93 psi). Volumes
of vapor at these pressures are large and pressure drops must be small to maintain control, so
vacuum columns are necessarily of large diameter. Differences in vapor pressure of different fractions are relatively larger than for lower boiling fractions, and relatively few plates are required.
The Chemistry and Technology of Petroleum
through weirs and is actually prevented from returning to the tray below through the holes by the
velocity of the rising vapors. Needless to say, a minimum vapor velocity is required to prevent
return of the liquid through the perforations.
In simple refineries, cut points can be changed slightly to vary yields and balance products, but
the more common practice is to produce relatively narrow fractions and then process (or blend) to
meet product demand. Since all these primary fractions are equilibrium mixtures, they all contain
some proportion of the lighter constituents characteristic of a lower boiling fraction and so are
stripped of these constituents, or stabilized, before further processing or storage. Thus, gasoline is
stabilized to a controlled butanes–pentanes content, and the overhead may be passed to superfractionators, towers with a large number of plates that can produce nearly pure C 1 –C 4 hydrocarbons
(methane to butanes, CH 4 to C 4 H 10 ), the successive columns termed deethanizers, depropanizers,
debutanizers, and so on.
Kerosene and gas oil fractions are obtained as sidestream products from the atmospheric tower
(primary tower), and these are treated in stripping columns (i.e., vessels of a few bubble trays) into
which steam is injected and the volatile overhead from the stripper is returned to the primary tower.
Steam is usually introduced by the stripping section of the primary column to lower the temperature
at which fractionation of the heavier ends of the crude can occur.
The specifications for most petroleum products make it extremely difficult to obtain marketable material by distillation only. In fact, the purpose of atmospheric distillation is considered the
provision of fractions that serve as feedstocks for intermediate refining operations and for blending.
Generally, this is carried out at atmospheric pressure, although light crude oils may be topped at an
elevated pressure and the residue then distilled at atmospheric pressure.
The topping operation differs from normal distillation procedures insofar as the majority of the
heat is directed to the feed stream rather than by reboiling the material in the base of the tower.
In addition, products of volatility intermediate between that of the overhead fractions and bottoms
(residua) are withdrawn as sidestream products. Furthermore, steam is injected into the base of
the column and the sidestream strippers to adjust and control the initial boiling range (or point)
of the fractions.
Topped crude oil must always be stripped with steam to elevate the flash point or to recover the
final portions of gas oil. The composition of the topped crude oil is a function of the temperature of
the vaporizer (or flasher). In addition, the properties of the residuum are very dependent upon the
extent of volatiles removal either by atmospheric distillation or by vacuum distillation (Table 17.1).
17.3.2 reduCed Pressure dIstIllAtIon
The boiling range of the highest boiling fraction that can be produced at atmospheric pressure is
limited by the temperature at which the residue starts to decompose or crack. If the stock is required
for the manufacture of lubricating oils, further fractionation without cracking may be desirable, and
this may be achieved by distillation under vacuum.
Vacuum distillation evolved because of the need to separate the less-volatile products, such as
lubricating oils, from the petroleum without subjecting these high-boiling products to cracking
conditions. The boiling range of the highest boiling fraction obtainable at atmospheric pressure is
limited by the temperature (∼350°C; ∼660°F) at which the residue starts to decompose or crack,
unless cracking distillation is preferred. When the feedstock is required for the manufacture of
lubricating oils, further fractionation without cracking is desirable, and this can be achieved by
distillation under vacuum (reduced pressure) conditions.
The distillation of high-boiling lubricating oil stocks may require pressures as low as 15–30 mm Hg
(0.29–0.58 psi), but operating conditions are more usually 50–100 mm Hg (0.97–1.93 psi). Volumes
of vapor at these pressures are large and pressure drops must be small to maintain control, so
vacuum columns are necessarily of large diameter. Differences in vapor pressure of different fractions are relatively larger than for lower boiling fractions, and relatively few plates are required.