485
Thermal Cracking
The separation of the phases depends on the solvent characteristics of the liquid. Addition of
aromatic solvents will suppress phase separation (Chapters 14 and 15), while paraffins will enhance
separation. Microscopic examination of coke particles often shows evidence for the presence of a
mesophase; spherical domains that exhibit the anisotropic optical characteristics of liquid crystal.
This phenomenon is consistent with the formation of a second liquid phase; the mesophase liquid is
denser than the rest of the hydrocarbon, has a higher surface tension, and likely wets metal surfaces
better than the rest of the liquid phase. The mesophase characteristic of coke diminishes as the liquid phase becomes more compatible with the aromatic material.
From this mechanism, the following trends for coke yield production in thermal processes are
anticipated:
1. Higher molecular weight fractions should give more coke (Chapter 16).
2. Coke formation depends on phase incompatibility (Chapters 14 and 16).
3. Acidic contaminants (such as clay) in a feedstock may promote coking.
4. Higher asphaltene content in a feed will, in general, correlate with higher coke yield
(Chapter 16; Schabron and Speight, 1997).
5. Coke may not form immediately if the point of incipient flocculation of the coke precursor
is not exceeded, so that an induction time is observed (Magaril and Aksenova, 1968, 1970;
Magaril and Ramazaeva, 1969; Magaril et al., 1970, 1971; Speight, 1992, 1994).
6. Phase separation may be very sensitive to surface chemistry, hydrodynamics, and surface
to volume ratio, similar to other processes that require nucleation.
Putting this chemical information in perspective allows an understanding of the pathways by which
the various thermal processes proceed and also the chemical pathways by which excessive yields of
coke can be recorded (Chapter 16).
18.2 EARLY PROCESSES
As the demand for gasoline increased with the onset of World War I and the ensuing 1920s, more
crude oil had to be distilled not only to meet the demand for gasoline but also to reduce the overproduction of the heavier petroleum fractions, including kerosene. The problem of how to produce
more gasoline from less crude oil was solved in 1913 by the incorporation of cracking units into
refinery operations in which fractions heavier than gasoline were converted into gasoline by thermal
decomposition. The early (pre-1940) processes employed for gasoline manufacture were processes
in which the major variables involved were feedstock type, time, temperature, and pressure, which
need to be considered to achieve the cracking of the feedstock to lighter products with minimal
coke formation.
One of the earliest processes used in the petroleum industry, after distillation, is the non- catalytic
conversion of higher boiling petroleum stocks into lower boiling products, known as thermal
cracking.
The yields of kerosene products were usually markedly increased by means of cracking distillation, but the technique was not suitable for gasoline production. As the need for gasoline arose,
the necessity of prolonging the cracking process became apparent and a process known as pressure
cracking evolved. Pressure cracking was a batch operation in which some 200 bbls of gas oil was
heated to about 425°C (800°F) in stills (shell stills) especially reinforced to operate at pressures as
high as 95 psi. The gas oil was retained in the reactor under maximum pressure for 24 h. Distillation
was then started, and during the next 48 h, 70–100 bbls of a lighter distillate was obtained that contained the gasoline components. This distillate was treated with sulfuric acid to remove unstable
gum-forming components and then redistilled to produce a cracked gasoline (boiling range <205°C,
<400°F) and a residual fuel oil.
Thermal Cracking
The separation of the phases depends on the solvent characteristics of the liquid. Addition of
aromatic solvents will suppress phase separation (Chapters 14 and 15), while paraffins will enhance
separation. Microscopic examination of coke particles often shows evidence for the presence of a
mesophase; spherical domains that exhibit the anisotropic optical characteristics of liquid crystal.
This phenomenon is consistent with the formation of a second liquid phase; the mesophase liquid is
denser than the rest of the hydrocarbon, has a higher surface tension, and likely wets metal surfaces
better than the rest of the liquid phase. The mesophase characteristic of coke diminishes as the liquid phase becomes more compatible with the aromatic material.
From this mechanism, the following trends for coke yield production in thermal processes are
anticipated:
1. Higher molecular weight fractions should give more coke (Chapter 16).
2. Coke formation depends on phase incompatibility (Chapters 14 and 16).
3. Acidic contaminants (such as clay) in a feedstock may promote coking.
4. Higher asphaltene content in a feed will, in general, correlate with higher coke yield
(Chapter 16; Schabron and Speight, 1997).
5. Coke may not form immediately if the point of incipient flocculation of the coke precursor
is not exceeded, so that an induction time is observed (Magaril and Aksenova, 1968, 1970;
Magaril and Ramazaeva, 1969; Magaril et al., 1970, 1971; Speight, 1992, 1994).
6. Phase separation may be very sensitive to surface chemistry, hydrodynamics, and surface
to volume ratio, similar to other processes that require nucleation.
Putting this chemical information in perspective allows an understanding of the pathways by which
the various thermal processes proceed and also the chemical pathways by which excessive yields of
coke can be recorded (Chapter 16).
18.2 EARLY PROCESSES
As the demand for gasoline increased with the onset of World War I and the ensuing 1920s, more
crude oil had to be distilled not only to meet the demand for gasoline but also to reduce the overproduction of the heavier petroleum fractions, including kerosene. The problem of how to produce
more gasoline from less crude oil was solved in 1913 by the incorporation of cracking units into
refinery operations in which fractions heavier than gasoline were converted into gasoline by thermal
decomposition. The early (pre-1940) processes employed for gasoline manufacture were processes
in which the major variables involved were feedstock type, time, temperature, and pressure, which
need to be considered to achieve the cracking of the feedstock to lighter products with minimal
coke formation.
One of the earliest processes used in the petroleum industry, after distillation, is the non- catalytic
conversion of higher boiling petroleum stocks into lower boiling products, known as thermal
cracking.
The yields of kerosene products were usually markedly increased by means of cracking distillation, but the technique was not suitable for gasoline production. As the need for gasoline arose,
the necessity of prolonging the cracking process became apparent and a process known as pressure
cracking evolved. Pressure cracking was a batch operation in which some 200 bbls of gas oil was
heated to about 425°C (800°F) in stills (shell stills) especially reinforced to operate at pressures as
high as 95 psi. The gas oil was retained in the reactor under maximum pressure for 24 h. Distillation
was then started, and during the next 48 h, 70–100 bbls of a lighter distillate was obtained that contained the gasoline components. This distillate was treated with sulfuric acid to remove unstable
gum-forming components and then redistilled to produce a cracked gasoline (boiling range <205°C,
<400°F) and a residual fuel oil.