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The Chemistry and Technology of Petroleum
naphtha cascades down the tower, the light ends separate and pass up the tower to leave as an overhead product. Since reflux is not used, considerable amounts of liquid hydrocarbons pass overhead
with the light ends.
Stabilization is usually a more precise operation than that just described. An example of more
precise stabilization can be seen in the handling of the mixture of hydrocarbons produced by cracking. The overhead from the atmospheric distillation tower that fractionates the cracked mixture
consists of light ends and cracked gasoline with light ends dissolved in it. If the latter is pumped
to the usual type of tank storage, the dissolved gases cause the gasoline to boil, with consequent
loss of the gases and some of the liquid components. To prevent this, the gasoline and the gases
dissolved in it are pumped to a stabilizer maintained under a pressure of approximately 100 psi
and operated with reflux. This fractionating tower makes a cut between the highest boiling gaseous
component (butane) and the lowest boiling liquid component (pentane). The bottom product is thus
a liquid free of all gaseous components, including butane; hence, the fractionating tower is known
as a debutanizer. The debutanizer bottoms (gasoline constituents) can be safely stored, whereas
the overhead from the debutanizer contains the butane, propane ethane, and methane fractions.
The butane fraction, which consists of all the hydrocarbons containing four carbon atoms, is particularly needed to give easy starting characteristics to motor gasoline. It must be separated from
the other gases and blended with motor gasoline in amounts that vary with the season: more in the
winter and less in the summer. Separation of the butane fraction is effected by another distillation
in a fractional distillation tower called a depropanizer, since its purpose is to separate propane and
the lighter gases from the butane fraction.
The depropanizer is very similar to the debutanizer, except that it is smaller in diameter because
of the smaller volume being distilled and is taller because of the larger number of trays required to
make a sharp cut between the butane and propane fractions. Since the normally gaseous propane
must exist as a liquid in the tower, a pressure of 200 psi is maintained. The bottom product, known
as the butane fraction, stabilizer bottoms, or refinery casinghead, is a high-vapor-pressure material
that must be stored in refrigerated tanks or pressure tanks. The depropanizer overhead, consisting of
propane and lighter gases, is used as a petrochemical feedstock or as a refinery fuel gas, depending
on the composition.
A depentanizer is a fractional distillation tower that removes the pentane fraction from a debutanized (butane-free) fraction. Depentanizers are similar to debutanizers and have been introduced recently to segregate the pentane fractions from cracked gasoline and reformate. The pentane
fraction when added to a premium gasoline makes this gasoline extraordinarily responsive to the
demands of an engine accelerator.
The gases produced as overhead products from crude distillation, stabilization, and depropanizer units may be delivered to a gas absorption plant for the recovery of small amounts of
butane and higher boiling hydrocarbons. The gas absorption plant consists essentially of two towers. One tower is the absorber where the butane and higher boiling hydrocarbons are removed from
the lighter gases.
This is done by spilling light oil (lean oil) down the absorber over trays similar to those in a
fractional distillation tower. The gas mixture enters at the bottom of the tower and rises to the top.
As it does this, it contacts the lean oil, which absorbs the butane and higher boiling hydrocarbons
but not the lower boiling hydrocarbons. The latter leave the top of the absorber as dry gas. The lean
oil that has become enriched with butane and higher boiling hydrocarbons is now termed fat oil.
This is pumped from the bottom of the absorber into the second tower, where fractional distillation separates the butane and higher boiling hydrocarbons as an overhead fraction and the oil, once
again lean oil, as the bottom product.
The condensed butane and higher boiling hydrocarbons are included with the refinery casinghead bottoms or stabilizer bottoms. The dry gas is frequently used as fuel gas for refinery furnaces.
It contains propane and propylene, however, which may be required for liquefied petroleum gas
for the manufacture of polymer gasoline or petrochemicals. Separation of the propane fraction
The Chemistry and Technology of Petroleum
naphtha cascades down the tower, the light ends separate and pass up the tower to leave as an overhead product. Since reflux is not used, considerable amounts of liquid hydrocarbons pass overhead
with the light ends.
Stabilization is usually a more precise operation than that just described. An example of more
precise stabilization can be seen in the handling of the mixture of hydrocarbons produced by cracking. The overhead from the atmospheric distillation tower that fractionates the cracked mixture
consists of light ends and cracked gasoline with light ends dissolved in it. If the latter is pumped
to the usual type of tank storage, the dissolved gases cause the gasoline to boil, with consequent
loss of the gases and some of the liquid components. To prevent this, the gasoline and the gases
dissolved in it are pumped to a stabilizer maintained under a pressure of approximately 100 psi
and operated with reflux. This fractionating tower makes a cut between the highest boiling gaseous
component (butane) and the lowest boiling liquid component (pentane). The bottom product is thus
a liquid free of all gaseous components, including butane; hence, the fractionating tower is known
as a debutanizer. The debutanizer bottoms (gasoline constituents) can be safely stored, whereas
the overhead from the debutanizer contains the butane, propane ethane, and methane fractions.
The butane fraction, which consists of all the hydrocarbons containing four carbon atoms, is particularly needed to give easy starting characteristics to motor gasoline. It must be separated from
the other gases and blended with motor gasoline in amounts that vary with the season: more in the
winter and less in the summer. Separation of the butane fraction is effected by another distillation
in a fractional distillation tower called a depropanizer, since its purpose is to separate propane and
the lighter gases from the butane fraction.
The depropanizer is very similar to the debutanizer, except that it is smaller in diameter because
of the smaller volume being distilled and is taller because of the larger number of trays required to
make a sharp cut between the butane and propane fractions. Since the normally gaseous propane
must exist as a liquid in the tower, a pressure of 200 psi is maintained. The bottom product, known
as the butane fraction, stabilizer bottoms, or refinery casinghead, is a high-vapor-pressure material
that must be stored in refrigerated tanks or pressure tanks. The depropanizer overhead, consisting of
propane and lighter gases, is used as a petrochemical feedstock or as a refinery fuel gas, depending
on the composition.
A depentanizer is a fractional distillation tower that removes the pentane fraction from a debutanized (butane-free) fraction. Depentanizers are similar to debutanizers and have been introduced recently to segregate the pentane fractions from cracked gasoline and reformate. The pentane
fraction when added to a premium gasoline makes this gasoline extraordinarily responsive to the
demands of an engine accelerator.
The gases produced as overhead products from crude distillation, stabilization, and depropanizer units may be delivered to a gas absorption plant for the recovery of small amounts of
butane and higher boiling hydrocarbons. The gas absorption plant consists essentially of two towers. One tower is the absorber where the butane and higher boiling hydrocarbons are removed from
the lighter gases.
This is done by spilling light oil (lean oil) down the absorber over trays similar to those in a
fractional distillation tower. The gas mixture enters at the bottom of the tower and rises to the top.
As it does this, it contacts the lean oil, which absorbs the butane and higher boiling hydrocarbons
but not the lower boiling hydrocarbons. The latter leave the top of the absorber as dry gas. The lean
oil that has become enriched with butane and higher boiling hydrocarbons is now termed fat oil.
This is pumped from the bottom of the absorber into the second tower, where fractional distillation separates the butane and higher boiling hydrocarbons as an overhead fraction and the oil, once
again lean oil, as the bottom product.
The condensed butane and higher boiling hydrocarbons are included with the refinery casinghead bottoms or stabilizer bottoms. The dry gas is frequently used as fuel gas for refinery furnaces.
It contains propane and propylene, however, which may be required for liquefied petroleum gas
for the manufacture of polymer gasoline or petrochemicals. Separation of the propane fraction