473
Distillation
17.5 OTHER PROCESSES
Atmospheric distillation and vacuum distillation provide the primary fractions from crude oil to
use as feedstocks for other refinery processes for conversion convert into products. Many of these
subsequent processes involve fractional distillation and some of the procedures are so specialized
and used with such frequency that they are identified by name.
17.5.1 strIPPIng
Stripping is a fractional distillation operation carried out on each sidestream product immediately
after it leaves the main distillation tower. Since perfect separation is not accomplished in the main
tower, unwanted components are mixed with those of the sidestream product. The purpose of stripping is to remove the more volatile components and thus reduce the flash point of the sidestream
product. Thus, a sidestream product enters at the top tray of a stripper, and as it spills down the four
to six trays, steam injected into the bottom of the stripper removes the volatile components. The
steam and volatile components leave the top of the stripper to return to the main tower. The stripped
sidestream product leaves at the bottom and, after being cooled in a heat exchanger, goes to storage. Since strippers are short, they are arranged one above another in a single tower; each stripper,
however, operates as a separate unit.
17.5.2 rerunnIng
Rerunning is a general term covering the redistillation of any material and indicating, usually, that
a large part of the material is distilled overhead. Stripping, in contrast, removes only a relatively
small amount of material as an overhead product. A rerun tower may be associated with a crude
distillation unit that produces wide boiling range naphtha as an overhead product. By separating the
wide-cut fraction into light and heavy naphtha, the rerun tower acts in effect as an extension of the
crude distillation tower.
The product from chemical treating process of various fractions may be rerun to remove the
treating chemical or its reaction products. If the volume of material being processed is small, a shell
still may be used instead of a continuous fractional distillation unit. The same applies to gas oils and
other fractions from which the front end or tail must be removed for special purposes.
17.5.3 stABIlIzAtIon And lIgHt-end removAl
The gaseous and more volatile liquid hydrocarbons produced in a refinery are collectively known as
light hydrocarbons or light ends. Light ends are produced in relatively small quantities from crude
petroleum and in large quantities when gasoline is manufactured by cracking and re-forming. When
a naphtha or gasoline component at the time of its manufacture is passed through a condenser, most
of the light ends do not condense and are withdrawn and handled as a gas. A considerable part of
the light ends, however, can remain dissolved in the condensate, thus forming a liquid with a high
vapor pressure.
Liquids with high vapor pressures may be stored in refrigerated tanks or in tanks capable of
withstanding the pressures developed by the gases dissolved in the liquid. The more usual procedure, however, is to separate the light ends from the liquid by a distillation process generally known
as stabilization. Enough of the light ends are removed to make a stabilized liquid, that is, a liquid
with a low enough vapor pressure to permit its storage in ordinary tanks without loss of vapor. The
simplest stabilization process is a stripping process. Light naphtha from a crude tower, for example,
may be pumped into the top of a tall, small-diameter fractional distillation tower operated under
a pressure of 50–80 psi. Heat is introduced at the bottom of the tower by a steam reboiler. As the
Distillation
17.5 OTHER PROCESSES
Atmospheric distillation and vacuum distillation provide the primary fractions from crude oil to
use as feedstocks for other refinery processes for conversion convert into products. Many of these
subsequent processes involve fractional distillation and some of the procedures are so specialized
and used with such frequency that they are identified by name.
17.5.1 strIPPIng
Stripping is a fractional distillation operation carried out on each sidestream product immediately
after it leaves the main distillation tower. Since perfect separation is not accomplished in the main
tower, unwanted components are mixed with those of the sidestream product. The purpose of stripping is to remove the more volatile components and thus reduce the flash point of the sidestream
product. Thus, a sidestream product enters at the top tray of a stripper, and as it spills down the four
to six trays, steam injected into the bottom of the stripper removes the volatile components. The
steam and volatile components leave the top of the stripper to return to the main tower. The stripped
sidestream product leaves at the bottom and, after being cooled in a heat exchanger, goes to storage. Since strippers are short, they are arranged one above another in a single tower; each stripper,
however, operates as a separate unit.
17.5.2 rerunnIng
Rerunning is a general term covering the redistillation of any material and indicating, usually, that
a large part of the material is distilled overhead. Stripping, in contrast, removes only a relatively
small amount of material as an overhead product. A rerun tower may be associated with a crude
distillation unit that produces wide boiling range naphtha as an overhead product. By separating the
wide-cut fraction into light and heavy naphtha, the rerun tower acts in effect as an extension of the
crude distillation tower.
The product from chemical treating process of various fractions may be rerun to remove the
treating chemical or its reaction products. If the volume of material being processed is small, a shell
still may be used instead of a continuous fractional distillation unit. The same applies to gas oils and
other fractions from which the front end or tail must be removed for special purposes.
17.5.3 stABIlIzAtIon And lIgHt-end removAl
The gaseous and more volatile liquid hydrocarbons produced in a refinery are collectively known as
light hydrocarbons or light ends. Light ends are produced in relatively small quantities from crude
petroleum and in large quantities when gasoline is manufactured by cracking and re-forming. When
a naphtha or gasoline component at the time of its manufacture is passed through a condenser, most
of the light ends do not condense and are withdrawn and handled as a gas. A considerable part of
the light ends, however, can remain dissolved in the condensate, thus forming a liquid with a high
vapor pressure.
Liquids with high vapor pressures may be stored in refrigerated tanks or in tanks capable of
withstanding the pressures developed by the gases dissolved in the liquid. The more usual procedure, however, is to separate the light ends from the liquid by a distillation process generally known
as stabilization. Enough of the light ends are removed to make a stabilized liquid, that is, a liquid
with a low enough vapor pressure to permit its storage in ordinary tanks without loss of vapor. The
simplest stabilization process is a stripping process. Light naphtha from a crude tower, for example,
may be pumped into the top of a tall, small-diameter fractional distillation tower operated under
a pressure of 50–80 psi. Heat is introduced at the bottom of the tower by a steam reboiler. As the