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The Chemistry and Technology of Petroleum
The feed to a fractional distillation tower is heated by flow-through pipe arranged within a large
furnace. The heating unit is known as a pipestill heater or pipestill furnace, and the heating unit
and the fractional distillation tower make up the essential parts of a distillation unit or pipe still. The
pipestill furnace heats the feed to a predetermined temperature, usually a temperature at which a
calculated portion of the feed changes into vapor. The vapor is held under pressure in the pipe still
furnace until it discharges as a foaming stream into the fractional distillation tower. Here, the vapors
pass up the tower to be fractionated into gas oil, kerosene, and naphtha, while the nonvolatile or liquid portion of the feed descends to the bottom of the tower to be pumped away as a bottom product.
Pipestill furnaces vary greatly in size, shape, and interior arrangement and can accommodate
25,000 bbl or more of crude petroleum per day. The walls and ceiling are insulated with firebrick,
and gas or oil burners are inserted through one or more walls. The interior of the furnace is partially
divided into two sections: a smaller convection section where the oil first enters the furnace and a
larger section into which the burners discharge and where the oil reaches its highest temperature.
Heat exchangers are also used to preheat the feedstock before it enters the furnace. These
exchangers are bundles of tubes arranged within a shell so that a stream passes through the tubes in
the opposite direction of a stream passing through the shell. Thus, cold crude oil, by passing through
a series of heat exchangers where hot products from the distillation tower are cooled, before entering
the furnace and saving of heat in this manner, may be a major factor in the economical operation
of refineries.
Steam reboilers may take the form of a steam coil in the bottom of the fractional distillation
tower or in a separate vessel. In the latter case, the bottom product from the tower enters the reboiler
where part is vaporized by heat from the steam coil. The hot vapor is directed back to the bottom
of the tower and provides part of the heat needed to operate the tower. The nonvolatile product
leaves the reboiler and passes through a heat exchanger, where its heat is transferred to the feed to
the tower. Steam may also be injected into a fractional distillation tower not only to provide heat
but also to induce boiling to take place at lower temperatures. Reboilers generally increase the
efficiency of fractionation, but a satisfactory degree of separation can usually be achieved more
conveniently by the use of a stripping section.
The stripping operation (see Section 17.5.1) occurs in that part of the tower below the point at
which the feed is introduced. The more volatile components are stripped from the descending liquid. Above the feed point (the rectifying section), the concentration of the less-volatile component
in the vapor is reduced.
The tower is divided into a number of horizontal sections by metal trays or plates, and each is
the equivalent of a still. These force a rising vapor to pass through a pool of descending liquid.
Therefore, the more trays, the more redistillation, and hence the better is the fractionation or separation of the mixture fed into the tower. A tower for fractionating crude petroleum may be 13 ft in
diameter and 85 ft high, but a tower stripping unwanted volatile material from gas oil may be only
3 or 4 ft in diameter and 10 ft high. Towers concerned with the distillation of liquefied gases are only
a few feet in diameter but may be up to 200 ft in height. A tower used in the fractionation of crude
petroleum may have from 16 to 28 trays, but one used in the fractionation of liquefied gases may
have 30–100 trays. The feed to a typical tower enters the vaporizing or flash zone, an area without
trays. The majority of the trays are usually located above this area. The feed to a bubble tower, however, may be at any point from top to bottom with trays above and below the entry point, depending
on the kind of feedstock and the characteristics desired in the products.
Liquid collects on each tray to a depth of, say, several inches and the depth controlled by a dam
or weir. As the liquid level rises, excess liquid spills over the weir into a channel (downspout), which
carries the liquid to the tray below.
The temperature of the trays is progressively cooler from bottom to top (Figure 17.5). The bottom
tray is heated by the incoming heated feedstock, although in some instances, a steam coil (reboiler)
is used to supply additional heat. As the hot vapors pass upward in the tower, condensation occurs
onto the trays until refluxing (simultaneous boiling of a liquid and condensing of the vapor) occurs
The Chemistry and Technology of Petroleum
The feed to a fractional distillation tower is heated by flow-through pipe arranged within a large
furnace. The heating unit is known as a pipestill heater or pipestill furnace, and the heating unit
and the fractional distillation tower make up the essential parts of a distillation unit or pipe still. The
pipestill furnace heats the feed to a predetermined temperature, usually a temperature at which a
calculated portion of the feed changes into vapor. The vapor is held under pressure in the pipe still
furnace until it discharges as a foaming stream into the fractional distillation tower. Here, the vapors
pass up the tower to be fractionated into gas oil, kerosene, and naphtha, while the nonvolatile or liquid portion of the feed descends to the bottom of the tower to be pumped away as a bottom product.
Pipestill furnaces vary greatly in size, shape, and interior arrangement and can accommodate
25,000 bbl or more of crude petroleum per day. The walls and ceiling are insulated with firebrick,
and gas or oil burners are inserted through one or more walls. The interior of the furnace is partially
divided into two sections: a smaller convection section where the oil first enters the furnace and a
larger section into which the burners discharge and where the oil reaches its highest temperature.
Heat exchangers are also used to preheat the feedstock before it enters the furnace. These
exchangers are bundles of tubes arranged within a shell so that a stream passes through the tubes in
the opposite direction of a stream passing through the shell. Thus, cold crude oil, by passing through
a series of heat exchangers where hot products from the distillation tower are cooled, before entering
the furnace and saving of heat in this manner, may be a major factor in the economical operation
of refineries.
Steam reboilers may take the form of a steam coil in the bottom of the fractional distillation
tower or in a separate vessel. In the latter case, the bottom product from the tower enters the reboiler
where part is vaporized by heat from the steam coil. The hot vapor is directed back to the bottom
of the tower and provides part of the heat needed to operate the tower. The nonvolatile product
leaves the reboiler and passes through a heat exchanger, where its heat is transferred to the feed to
the tower. Steam may also be injected into a fractional distillation tower not only to provide heat
but also to induce boiling to take place at lower temperatures. Reboilers generally increase the
efficiency of fractionation, but a satisfactory degree of separation can usually be achieved more
conveniently by the use of a stripping section.
The stripping operation (see Section 17.5.1) occurs in that part of the tower below the point at
which the feed is introduced. The more volatile components are stripped from the descending liquid. Above the feed point (the rectifying section), the concentration of the less-volatile component
in the vapor is reduced.
The tower is divided into a number of horizontal sections by metal trays or plates, and each is
the equivalent of a still. These force a rising vapor to pass through a pool of descending liquid.
Therefore, the more trays, the more redistillation, and hence the better is the fractionation or separation of the mixture fed into the tower. A tower for fractionating crude petroleum may be 13 ft in
diameter and 85 ft high, but a tower stripping unwanted volatile material from gas oil may be only
3 or 4 ft in diameter and 10 ft high. Towers concerned with the distillation of liquefied gases are only
a few feet in diameter but may be up to 200 ft in height. A tower used in the fractionation of crude
petroleum may have from 16 to 28 trays, but one used in the fractionation of liquefied gases may
have 30–100 trays. The feed to a typical tower enters the vaporizing or flash zone, an area without
trays. The majority of the trays are usually located above this area. The feed to a bubble tower, however, may be at any point from top to bottom with trays above and below the entry point, depending
on the kind of feedstock and the characteristics desired in the products.
Liquid collects on each tray to a depth of, say, several inches and the depth controlled by a dam
or weir. As the liquid level rises, excess liquid spills over the weir into a channel (downspout), which
carries the liquid to the tray below.
The temperature of the trays is progressively cooler from bottom to top (Figure 17.5). The bottom
tray is heated by the incoming heated feedstock, although in some instances, a steam coil (reboiler)
is used to supply additional heat. As the hot vapors pass upward in the tower, condensation occurs
onto the trays until refluxing (simultaneous boiling of a liquid and condensing of the vapor) occurs