396
The Chemistry and Technology of Petroleum
Petroleum refineries were originally designed and operated to run within a narrow range of crude
oil feedstock and to produce a relatively fixed slate of petroleum products. Since the 1970s, refiners
had to increase their flexibility in order to adapt to a more volatile environment. Several possible
paths may be used by refiners to increase their flexibility within existing refineries. Examples of
these paths are change in the severity of operating rules of some process units by varying the range
of inputs used, thus achieving a slight change in output. Alternatively, refiners can install new processes, and this alternate scenario offers the greatest flexibility but is limited by the constraint of
strict complementarily of the new units with the rest of the existing plant and involves a higher risk
than the previous ones. It is not surprising that many refiners decide to modify existing processes.
Whatever the choice, refinery practice continues to evolve and (as will be seen in the relevant
chapter) new processes are installed in live with older modified process. The purpose of this chapter
is to present to the reader a general overview of refining that, when taken into the context of the following chapters, will show some of the differences occurring in refineries.
15.4 DISTILLATION
In the early stages of refinery development, when illuminating and lubricating oils were the main
products, distillation was the major, and often only, refinery process. At that time, gasoline was a
minor product, but as the demand for gasoline increased, conversion processes were developed,
because distillation could no longer supply the necessary quantities.
It is possible to obtain products ranging from gaseous materials taken off at the top of the distillation column to a nonvolatile residue or reduced crude (bottoms), with correspondingly lighter
materials at intermediate points. The reduced crude may then be processed by vacuum, or steam,
distillation in order to separate the high-boiling lubricating oil fractions without the danger of
decomposition, which occurs at high (>350°C, >660°F) temperatures. Atmospheric distillation may
be terminated with a lower-boiling fraction (cut) if it is felt that vacuum or steam distillation will
yield a better-quality product, or if the process appears to be economically more favorable. Not all
crude oils yield the same distillation products (Table 15.1)—although there may be variations by
several degrees in the boiling ranges of the fractions as specified by different companies—and the
nature of the crude oil dictates the processes that may be required for refining.
15.4.1 HIstorICAl develoPment
Distillation was the first method by which petroleum was refined. The original technique involved
a batch operation in which the still was a cast-iron vessel mounted on brickwork over a fire and the
volatile materials were passed through a pipe or gooseneck that led from the top of the still to a
condenser. The latter was a coil of pipe (worm) immersed in a tank of running water.
Heating a batch of crude petroleum caused the more volatile, lower-boiling components to
vaporize and then condense in the worm to form naphtha. As the distillation progressed, the
higher- boiling components became vaporized and were condensed to produce kerosene: the
major petroleum product of the time. When all of the possible kerosene had been obtained,
the material remaining in the still was discarded. The still was then refilled with petroleum
and the operation repeated.
15.4.2 modern ProCesses
15.4.2.1 Atmospheric Distillation
The present-day petroleum distillation unit, like the battery of 1800s, is a collection of distillation units but, in contrast to the early battery units, a tower is used in the typical modern refinery
(Figure 15.3) that brings about a fairly efficient degree of fractionation (separation).
The Chemistry and Technology of Petroleum
Petroleum refineries were originally designed and operated to run within a narrow range of crude
oil feedstock and to produce a relatively fixed slate of petroleum products. Since the 1970s, refiners
had to increase their flexibility in order to adapt to a more volatile environment. Several possible
paths may be used by refiners to increase their flexibility within existing refineries. Examples of
these paths are change in the severity of operating rules of some process units by varying the range
of inputs used, thus achieving a slight change in output. Alternatively, refiners can install new processes, and this alternate scenario offers the greatest flexibility but is limited by the constraint of
strict complementarily of the new units with the rest of the existing plant and involves a higher risk
than the previous ones. It is not surprising that many refiners decide to modify existing processes.
Whatever the choice, refinery practice continues to evolve and (as will be seen in the relevant
chapter) new processes are installed in live with older modified process. The purpose of this chapter
is to present to the reader a general overview of refining that, when taken into the context of the following chapters, will show some of the differences occurring in refineries.
15.4 DISTILLATION
In the early stages of refinery development, when illuminating and lubricating oils were the main
products, distillation was the major, and often only, refinery process. At that time, gasoline was a
minor product, but as the demand for gasoline increased, conversion processes were developed,
because distillation could no longer supply the necessary quantities.
It is possible to obtain products ranging from gaseous materials taken off at the top of the distillation column to a nonvolatile residue or reduced crude (bottoms), with correspondingly lighter
materials at intermediate points. The reduced crude may then be processed by vacuum, or steam,
distillation in order to separate the high-boiling lubricating oil fractions without the danger of
decomposition, which occurs at high (>350°C, >660°F) temperatures. Atmospheric distillation may
be terminated with a lower-boiling fraction (cut) if it is felt that vacuum or steam distillation will
yield a better-quality product, or if the process appears to be economically more favorable. Not all
crude oils yield the same distillation products (Table 15.1)—although there may be variations by
several degrees in the boiling ranges of the fractions as specified by different companies—and the
nature of the crude oil dictates the processes that may be required for refining.
15.4.1 HIstorICAl develoPment
Distillation was the first method by which petroleum was refined. The original technique involved
a batch operation in which the still was a cast-iron vessel mounted on brickwork over a fire and the
volatile materials were passed through a pipe or gooseneck that led from the top of the still to a
condenser. The latter was a coil of pipe (worm) immersed in a tank of running water.
Heating a batch of crude petroleum caused the more volatile, lower-boiling components to
vaporize and then condense in the worm to form naphtha. As the distillation progressed, the
higher- boiling components became vaporized and were condensed to produce kerosene: the
major petroleum product of the time. When all of the possible kerosene had been obtained,
the material remaining in the still was discarded. The still was then refilled with petroleum
and the operation repeated.
15.4.2 modern ProCesses
15.4.2.1 Atmospheric Distillation
The present-day petroleum distillation unit, like the battery of 1800s, is a collection of distillation units but, in contrast to the early battery units, a tower is used in the typical modern refinery
(Figure 15.3) that brings about a fairly efficient degree of fractionation (separation).