482
The Chemistry and Technology of Petroleum
distillation of fractions directly from petroleum. Each of these processes may be carried out in a
number of ways, which differ in details of operation, or essential equipment, or both (Bland and
Davidson, 1967).
When kerosene was the major product, gasoline was the portion of crude petroleum too volatile
to be included in kerosene. The first refiners had no use for it and often dumped an accumulation
of gasoline into the stream or river that was always nearby. As the demand for gasoline increased,
more and more of the lighter kerosene components were included in gasoline, but the maximum
suitable portion depended on the kind of crude oil and rarely exceeded 20% of the crude oil. To
increase the supply of gasoline, more crude oil was run to the stills, resulting in overproduction of
the heavier petroleum fractions, including kerosene. The problem of how to get more gasoline from
less crude oil was solved in 1913 by the use of cracking in which fractions heavier than gasoline
were converted into gasoline (Purdy, 1958).
Thermal processes are essentially processes that decompose, rearrange, or combine hydrocarbon
molecules by the application of heat. The major variables involved are feedstock type, time, temperature, and pressure and, as such, are usually considered in promoting cracking (thermal decomposition) of the heavier molecules to lighter products and in minimizing coke formation.
The origins of cracking are unknown. There are records that illustrate the use of naphtha in
Greek fire almost 2000 years ago (Chapter 1), but whether the naphtha was produced naturally by
distillation or by cracking distillation is not clear. Cracking was used commercially in the production of oil from coal and oil shale before the beginning of the modern petroleum industry. The
ensuing discovery that the higher boiling materials could be decomposed to lower molecular weight
products was used to increase the production of kerosene and was called cracking distillation (Kobe
and McKetta, 1958). Thus, a batch of crude oil was heated until most of the kerosene was distilled
from it and the overhead material became dark in color. At this point, the still fires were lowered,
the rate of distillation decreased, and the heavy oils were held in the hot zone, during which time
some of the large hydrocarbons were decomposed to yield lower molecular weight (lower boiling)
products. After a suitable time, the still fires were increased and distillation continued in the normal
way. The overhead product, however, was light oil suitable for kerosene instead of the heavy oil that
would otherwise have been produced.
The precise origins of the modern version of cracking distillation, as applied in the modern petroleum industry, are unknown. It is rumored that, in 1861, a stillman had to leave his charge for a longer
time than he intended (the reason is not known), during which time the still overheated. When he
returned he noticed that the distillate in the collector was much more volatile than anticipated at that
particular stage of the distillation. Further investigation led to the development of cracking distillation (i.e., thermal degradation with the simultaneous production of distillate). However, before giving
too much credit to the absence of a stillman, it is essential to recognize that the production of volatile
product by the destructive distillation of wood and coal was known for many years, if not decades or
centuries, before the birth of the modern petroleum industry. Indeed, the production of spirits of fire
(i.e., naphtha, the flammable constituent of Greek fire) was known from early times. The occurrence
of bitumen at Hit (Mesopotamia) that was used as mastic by the Assyrians was further developed for
use in warfare though the production of naphtha by destructive distillation.
When petroleum fractions are heated to temperatures in excess of 350°C (660°F), the rates of
the thermal decomposition proceed at significantly higher rates (Chapter 16). Thermal decomposition does not require the addition of a catalyst. Therefore, this approach is the oldest technology
available for residue conversion. The severity of thermal processing determines the conversion and
the product characteristics. Thermal treatment of residues ranges from mild treatment for the reduction of viscosity to ultrapyrolysis (high-temperature cracking at very short residence time) for better conversion to overhead products (Hulet et al., 2005). A higher temperature requires a shorter
time to achieve a given conversion but, in many cases, there is a change in the chemistry of the
reaction. The severity of the process conditions is the combination of reaction time and temperature
to achieve a given conversion.
The Chemistry and Technology of Petroleum
distillation of fractions directly from petroleum. Each of these processes may be carried out in a
number of ways, which differ in details of operation, or essential equipment, or both (Bland and
Davidson, 1967).
When kerosene was the major product, gasoline was the portion of crude petroleum too volatile
to be included in kerosene. The first refiners had no use for it and often dumped an accumulation
of gasoline into the stream or river that was always nearby. As the demand for gasoline increased,
more and more of the lighter kerosene components were included in gasoline, but the maximum
suitable portion depended on the kind of crude oil and rarely exceeded 20% of the crude oil. To
increase the supply of gasoline, more crude oil was run to the stills, resulting in overproduction of
the heavier petroleum fractions, including kerosene. The problem of how to get more gasoline from
less crude oil was solved in 1913 by the use of cracking in which fractions heavier than gasoline
were converted into gasoline (Purdy, 1958).
Thermal processes are essentially processes that decompose, rearrange, or combine hydrocarbon
molecules by the application of heat. The major variables involved are feedstock type, time, temperature, and pressure and, as such, are usually considered in promoting cracking (thermal decomposition) of the heavier molecules to lighter products and in minimizing coke formation.
The origins of cracking are unknown. There are records that illustrate the use of naphtha in
Greek fire almost 2000 years ago (Chapter 1), but whether the naphtha was produced naturally by
distillation or by cracking distillation is not clear. Cracking was used commercially in the production of oil from coal and oil shale before the beginning of the modern petroleum industry. The
ensuing discovery that the higher boiling materials could be decomposed to lower molecular weight
products was used to increase the production of kerosene and was called cracking distillation (Kobe
and McKetta, 1958). Thus, a batch of crude oil was heated until most of the kerosene was distilled
from it and the overhead material became dark in color. At this point, the still fires were lowered,
the rate of distillation decreased, and the heavy oils were held in the hot zone, during which time
some of the large hydrocarbons were decomposed to yield lower molecular weight (lower boiling)
products. After a suitable time, the still fires were increased and distillation continued in the normal
way. The overhead product, however, was light oil suitable for kerosene instead of the heavy oil that
would otherwise have been produced.
The precise origins of the modern version of cracking distillation, as applied in the modern petroleum industry, are unknown. It is rumored that, in 1861, a stillman had to leave his charge for a longer
time than he intended (the reason is not known), during which time the still overheated. When he
returned he noticed that the distillate in the collector was much more volatile than anticipated at that
particular stage of the distillation. Further investigation led to the development of cracking distillation (i.e., thermal degradation with the simultaneous production of distillate). However, before giving
too much credit to the absence of a stillman, it is essential to recognize that the production of volatile
product by the destructive distillation of wood and coal was known for many years, if not decades or
centuries, before the birth of the modern petroleum industry. Indeed, the production of spirits of fire
(i.e., naphtha, the flammable constituent of Greek fire) was known from early times. The occurrence
of bitumen at Hit (Mesopotamia) that was used as mastic by the Assyrians was further developed for
use in warfare though the production of naphtha by destructive distillation.
When petroleum fractions are heated to temperatures in excess of 350°C (660°F), the rates of
the thermal decomposition proceed at significantly higher rates (Chapter 16). Thermal decomposition does not require the addition of a catalyst. Therefore, this approach is the oldest technology
available for residue conversion. The severity of thermal processing determines the conversion and
the product characteristics. Thermal treatment of residues ranges from mild treatment for the reduction of viscosity to ultrapyrolysis (high-temperature cracking at very short residence time) for better conversion to overhead products (Hulet et al., 2005). A higher temperature requires a shorter
time to achieve a given conversion but, in many cases, there is a change in the chemistry of the
reaction. The severity of the process conditions is the combination of reaction time and temperature
to achieve a given conversion.