399
Introduction to Refining Processes
petroleum is refined. Indeed, the distillation section of a modern refinery (Figures 15.3 and 15.4) is
the most flexible section in the refinery since conditions can be adjusted to process a wide range of
refinery feedstocks from the lighter crude oils to the heavier more viscous crude oils. However, the
maximum permissible temperature (in the vaporizing furnace or heater) to which the feedstock can
be subjected is 350°C (660°F). Thermal decomposition occurs above this temperature which, if it
occurs within a distillation unit, can lead to coke deposition in the heater pipes or in the tower itself
with the resulting failure of the unit.
The contained use of atmospheric and vacuum distillation has been a major part of refinery
operations during this century and no doubt will continue to be employed throughout the remainder
of the century as the primary refining operation.
15.4.2.3 Azeotropic and Extractive Distillation
As the twentieth century evolved, distillation techniques in refineries became more sophisticated to
handle a wider variety of crude oils to produce marketable products or feedstocks for other refinery
units. However, it became apparent that the distillation units in the refineries were incapable of producing specific product fractions. In order to accommodate this type of product demand, refineries
have, in the latter half of this century, incorporated azeotropic distillation and extractive distillation
in their operations.
All compounds have definite boiling temperatures but a mixture of chemically dissimilar compounds will sometimes cause one or both of the components to boil at a temperature other than
that expected. A mixture that boils at a temperature lower than the boiling point of any of the
components is an azeotropic mixture. When it is desired to separate close-boiling components,
the addition of a nonindigenous component will form an azeotropic mixture with one of the components of the mixture, thereby lowering the boiling point by the formation of an azeotrope and
facilitate separation by distillation.
The separation of these components of similar volatility may become economical if an entrainer
can be found that effectively changes the relative volatility. It is also desirable that the entrainer be
reasonably cheap, stable, nontoxic, and readily recoverable from the components. In practice, it is
probably this last-named criterion that limits severely the application of extractive and azeotropic
distillation. The majority of successful processes are those in which the entrainer and one of the
components separate into two liquid phases on cooling if direct recovery by distillation is not feasible. A further restriction in the selection of an azeotropic entrainer is that the boiling point of the
entrainer be in the range of 10°C–40°C (18°F–72°F) below that of the components.
15.5 THERMAL METHODS
15.5.1 HIstorICAl develoPment
Cracking was used commercially in the production of oils from coal and shales before the petroleum industry began, and the discovery that the heavier products could be decomposed to lighter
oils was used to increase the production of kerosene and was called cracking distillation.
The precise origins of cracking distillation 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).
Cracking distillation (thermal decomposition with simultaneous removal of distillate) was recognized as a means of producing the valuable lighter product (kerosene) from heavier nonvolatile
materials. In the early days of the process (1870–1900), the technique was very simple—a batch of
crude oil was heated until most of the kerosene had been distilled from it and the overhead material
Introduction to Refining Processes
petroleum is refined. Indeed, the distillation section of a modern refinery (Figures 15.3 and 15.4) is
the most flexible section in the refinery since conditions can be adjusted to process a wide range of
refinery feedstocks from the lighter crude oils to the heavier more viscous crude oils. However, the
maximum permissible temperature (in the vaporizing furnace or heater) to which the feedstock can
be subjected is 350°C (660°F). Thermal decomposition occurs above this temperature which, if it
occurs within a distillation unit, can lead to coke deposition in the heater pipes or in the tower itself
with the resulting failure of the unit.
The contained use of atmospheric and vacuum distillation has been a major part of refinery
operations during this century and no doubt will continue to be employed throughout the remainder
of the century as the primary refining operation.
15.4.2.3 Azeotropic and Extractive Distillation
As the twentieth century evolved, distillation techniques in refineries became more sophisticated to
handle a wider variety of crude oils to produce marketable products or feedstocks for other refinery
units. However, it became apparent that the distillation units in the refineries were incapable of producing specific product fractions. In order to accommodate this type of product demand, refineries
have, in the latter half of this century, incorporated azeotropic distillation and extractive distillation
in their operations.
All compounds have definite boiling temperatures but a mixture of chemically dissimilar compounds will sometimes cause one or both of the components to boil at a temperature other than
that expected. A mixture that boils at a temperature lower than the boiling point of any of the
components is an azeotropic mixture. When it is desired to separate close-boiling components,
the addition of a nonindigenous component will form an azeotropic mixture with one of the components of the mixture, thereby lowering the boiling point by the formation of an azeotrope and
facilitate separation by distillation.
The separation of these components of similar volatility may become economical if an entrainer
can be found that effectively changes the relative volatility. It is also desirable that the entrainer be
reasonably cheap, stable, nontoxic, and readily recoverable from the components. In practice, it is
probably this last-named criterion that limits severely the application of extractive and azeotropic
distillation. The majority of successful processes are those in which the entrainer and one of the
components separate into two liquid phases on cooling if direct recovery by distillation is not feasible. A further restriction in the selection of an azeotropic entrainer is that the boiling point of the
entrainer be in the range of 10°C–40°C (18°F–72°F) below that of the components.
15.5 THERMAL METHODS
15.5.1 HIstorICAl develoPment
Cracking was used commercially in the production of oils from coal and shales before the petroleum industry began, and the discovery that the heavier products could be decomposed to lighter
oils was used to increase the production of kerosene and was called cracking distillation.
The precise origins of cracking distillation 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).
Cracking distillation (thermal decomposition with simultaneous removal of distillate) was recognized as a means of producing the valuable lighter product (kerosene) from heavier nonvolatile
materials. In the early days of the process (1870–1900), the technique was very simple—a batch of
crude oil was heated until most of the kerosene had been distilled from it and the overhead material