460
The Chemistry and Technology of Petroleum
important and primary function in the refinery is its use for the separation of crude oil into component fractions (Gruse and Stevens, 1960; Bland and Davidson, 1967; Hobson and Pohl, 1973;
Speight and Ozum, 2002; Hsu and Robinson, 2006; Gary et al., 2007; Speight, 2011). Thus, it is possible to obtain products ranging from gaseous materials taken off the top of the distillation column
to a nonvolatile atmospheric residuum (bottoms, reduced crude) with correspondingly lower boiling
materials (gas, gasoline, naphtha, kerosene, and gas oil) taken off at intermediate points.
The reduced crude may then be processed by vacuum or steam distillation to separate the highboiling lubricating oil fractions without the danger of decomposition, which occurs at high temperatures (>350°C, 660°F) (Chapters 16 and 18). Indeed, atmospheric distillation may be terminated
with a lower boiling fraction (boiling cut), if it is thought that vacuum or steam distillation will yield
a better quality product or if the process appears to be economically more favorable.
It should be noted at this point that not all crude oils yield the same distillation products. In fact,
the nature of the crude oil dictates the processes that may be required for refining. Petroleum can
be classified according to the nature of the distillation residue, which in turn depends on the relative
content of hydrocarbon types: paraffins, naphthenes, and aromatics. The majority of crude oils fall
into one of the following classifications (Chapter 2):
1. Asphalt-base crude oil contains very little paraffin wax and a residue primarily asphaltic
(predominantly condensed aromatics); sulfur, oxygen, and nitrogen contents are often relatively high. Light and intermediate fractions have high percentages of naphthenes. These
crude oils are particularly suitable for making high-quality gasoline, machine lubricating
oils, and asphalt.
2. Paraffin-base crude oil contains very little asphaltic materials and is a good source of
paraffin wax, quality motor lubricating oils, and high-grade kerosene. Paraffin-base crude
oil usually has a lower heteroatom content than asphalt-base crude oil.
3. Mixed-base crude oil contains considerable amounts of both wax and asphalt. Virtually all
products can be obtained, although at lower yields than from the other two classes.
For example, paraffin-base crude oil produces distillation cuts with higher proportions of paraffins
than asphalt-base crude. The converse is also true; that is, an asphalt-base crude oil produces materials with higher proportions of cyclic compounds. A paraffin-base crude oil yields wax distillates
rather than the lubricating distillates produced by the naphthenic-base crude oils. The residuum
from a paraffin-base petroleum is referred to as cylinder stock rather than asphaltic bottoms which
is the name often given to the residuum from the distillation of naphthenic crude oil. It is emphasized that, in these cases, it is not a matter of the use of archaic terminology but a reflection of the
nature of the product and the petroleum from which it is derived.
Petroleum refining, as we know it, is a very recent science and for the purposes of this chapter
will be acknowledged as such. Many innovations have evolved during the twentieth century, and it is
the purpose of this chapter to illustrate the evolution of petroleum refining from the early processes
to those in use at the present day.
17.2 PRETREATMENT
Even though distillation is, to all appearances, the first step in crude oil refining, it should be
recognized that crude oil that is contaminated by saltwater either from the well or during transportation to the refinery must be treated to remove the emulsion. If saltwater is not removed, the
materials of construction of the heater tubes and column intervals are exposed to chloride ion
attack and the corrosive action of hydrogen chloride, which may be formed at the temperature of
the column feed.
Various methods of pretreatment are open to the petroleum refiner, but three general approaches
have been taken to the desalting of crude petroleum (Figure 17.1). Numerous variations of each
The Chemistry and Technology of Petroleum
important and primary function in the refinery is its use for the separation of crude oil into component fractions (Gruse and Stevens, 1960; Bland and Davidson, 1967; Hobson and Pohl, 1973;
Speight and Ozum, 2002; Hsu and Robinson, 2006; Gary et al., 2007; Speight, 2011). Thus, it is possible to obtain products ranging from gaseous materials taken off the top of the distillation column
to a nonvolatile atmospheric residuum (bottoms, reduced crude) with correspondingly lower boiling
materials (gas, gasoline, naphtha, kerosene, and gas oil) taken off at intermediate points.
The reduced crude may then be processed by vacuum or steam distillation to separate the highboiling lubricating oil fractions without the danger of decomposition, which occurs at high temperatures (>350°C, 660°F) (Chapters 16 and 18). Indeed, atmospheric distillation may be terminated
with a lower boiling fraction (boiling cut), if it is thought that vacuum or steam distillation will yield
a better quality product or if the process appears to be economically more favorable.
It should be noted at this point that not all crude oils yield the same distillation products. In fact,
the nature of the crude oil dictates the processes that may be required for refining. Petroleum can
be classified according to the nature of the distillation residue, which in turn depends on the relative
content of hydrocarbon types: paraffins, naphthenes, and aromatics. The majority of crude oils fall
into one of the following classifications (Chapter 2):
1. Asphalt-base crude oil contains very little paraffin wax and a residue primarily asphaltic
(predominantly condensed aromatics); sulfur, oxygen, and nitrogen contents are often relatively high. Light and intermediate fractions have high percentages of naphthenes. These
crude oils are particularly suitable for making high-quality gasoline, machine lubricating
oils, and asphalt.
2. Paraffin-base crude oil contains very little asphaltic materials and is a good source of
paraffin wax, quality motor lubricating oils, and high-grade kerosene. Paraffin-base crude
oil usually has a lower heteroatom content than asphalt-base crude oil.
3. Mixed-base crude oil contains considerable amounts of both wax and asphalt. Virtually all
products can be obtained, although at lower yields than from the other two classes.
For example, paraffin-base crude oil produces distillation cuts with higher proportions of paraffins
than asphalt-base crude. The converse is also true; that is, an asphalt-base crude oil produces materials with higher proportions of cyclic compounds. A paraffin-base crude oil yields wax distillates
rather than the lubricating distillates produced by the naphthenic-base crude oils. The residuum
from a paraffin-base petroleum is referred to as cylinder stock rather than asphaltic bottoms which
is the name often given to the residuum from the distillation of naphthenic crude oil. It is emphasized that, in these cases, it is not a matter of the use of archaic terminology but a reflection of the
nature of the product and the petroleum from which it is derived.
Petroleum refining, as we know it, is a very recent science and for the purposes of this chapter
will be acknowledged as such. Many innovations have evolved during the twentieth century, and it is
the purpose of this chapter to illustrate the evolution of petroleum refining from the early processes
to those in use at the present day.
17.2 PRETREATMENT
Even though distillation is, to all appearances, the first step in crude oil refining, it should be
recognized that crude oil that is contaminated by saltwater either from the well or during transportation to the refinery must be treated to remove the emulsion. If saltwater is not removed, the
materials of construction of the heater tubes and column intervals are exposed to chloride ion
attack and the corrosive action of hydrogen chloride, which may be formed at the temperature of
the column feed.
Various methods of pretreatment are open to the petroleum refiner, but three general approaches
have been taken to the desalting of crude petroleum (Figure 17.1). Numerous variations of each