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The Chemistry and Technology of Petroleum
a limiting value with conversion, although the total product viscosity can continue to decrease. The
minimum viscosity of the unconverted residue can lie outside the range of allowable conversion
if sediment begins to form. When shipment of the visbreaker product by pipeline is the process
objective, addition of a diluent such as gas condensate can be used to achieve a further reduction
in viscosity.
Other variations of visbreaking technology include the Tervahl-T and Tervahl-H processes (q.v.).
The Tervahl-T alternative includes only the thermal section to produce a synthetic crude oil with
better transportability by having reduced viscosity and greater stability. The Tervahl-H alternative
adds hydrogen that also increases the extent of the desulfurization and decreases the carbon residua.
18.3.2 CokIng ProCesses
Coking is a thermal process for the continuous conversion of residua into lower boiling products.
The feedstock can be reduced crude, straight-run residua, or cracked residua, and the products are
gases, naphtha, fuel oil, gas oil, and coke. The gas oil may be the major product of a coking operation and serves primarily as a feedstock for catalytic cracking units. The coke obtained is usually
used as fuel, but processing marketing for specialty uses, such as electrode manufacture, production of chemicals, and metallurgical coke, is also possible and increases the value of the coke. For
these uses, the coke may require treatment to remove sulfur and metal impurities. Furthermore, the
increasing attention paid to reducing atmospheric pollution has also served to direct some attention
to coking, since the process not only concentrates such pollutants as feedstock sulfur in the coke but
usually yields products that can be conveniently subjected to desulfurization processes.
Coking processes generally utilize longer reaction times than thermal cracking processes. To
accomplish this, drums or chambers (reaction vessels) are employed, but it is necessary to use two or
more such vessels so that coke removal can be accomplished in those vessels not on-stream without
interrupting the semi-continuous nature of the process.
Coking processes have the virtue of eliminating the residue fraction of the feed, at the cost of forming a solid carbonaceous product. The yield of coke in a given coking process tends to be proportional
to the carbon residue of the feed (measured as the Conradson carbon residue; please see Chapter 10).
The formation of large quantities of coke is a severe drawback unless the coke can be put to use.
Calcined petroleum coke can be used for making anodes for aluminum manufacture and a variety
of carbon or graphite products such as brushes for electrical equipment. These applications, however, require a coke that is low in mineral matter and sulfur.
If the feedstock produces a high sulfur, high ash, high vanadium coke, one option to use the coke
is combustion of the coke to produce process steam (and large quantities of sulfur dioxide unless the
coke is first gasified or the combustion gases are scrubbed). Another option is stockpiling.
For some feedstocks, particularly from heavy oil, the combination of poor coke properties for
anode use, limits on sulfur dioxide emissions, and loss of liquid product volume have tended to relegate coking processes to a strictly secondary role in any new upgrading facilities.
In spite of the various limitation outlined earlier, visbreaking has much potential and, in fact,
remains an important, relatively inexpensive bottom-of-the-barrel upgrading process in many areas
of the world. Most of the existing visbreakers are the soaker type, which utilize a soaker drum in
conjunction with a fired heater to achieve conversion and which reduces the temperature required
to achieve conversion while producing a stable residue product, thereby increasing the heater run
length and reducing the frequency of unit shut down for heater decoking.
However, a recurring issue with the soaker visbreaker is the need to periodically decoke the soaker
drum and the inability of the soaker process to easily adjust to changes in feedstock quality because
of the need to fine tune two process variables, temperature, and residence time. Recent combination of the visbreaking technology and the addition of new coil visbreaker design features have provided the coil process with a competitive advantage over the traditional soaker visbreaker process.
Limitations in heater run length are no longer a problem for the coil visbreaker. Advances in visbreaker
The Chemistry and Technology of Petroleum
a limiting value with conversion, although the total product viscosity can continue to decrease. The
minimum viscosity of the unconverted residue can lie outside the range of allowable conversion
if sediment begins to form. When shipment of the visbreaker product by pipeline is the process
objective, addition of a diluent such as gas condensate can be used to achieve a further reduction
in viscosity.
Other variations of visbreaking technology include the Tervahl-T and Tervahl-H processes (q.v.).
The Tervahl-T alternative includes only the thermal section to produce a synthetic crude oil with
better transportability by having reduced viscosity and greater stability. The Tervahl-H alternative
adds hydrogen that also increases the extent of the desulfurization and decreases the carbon residua.
18.3.2 CokIng ProCesses
Coking is a thermal process for the continuous conversion of residua into lower boiling products.
The feedstock can be reduced crude, straight-run residua, or cracked residua, and the products are
gases, naphtha, fuel oil, gas oil, and coke. The gas oil may be the major product of a coking operation and serves primarily as a feedstock for catalytic cracking units. The coke obtained is usually
used as fuel, but processing marketing for specialty uses, such as electrode manufacture, production of chemicals, and metallurgical coke, is also possible and increases the value of the coke. For
these uses, the coke may require treatment to remove sulfur and metal impurities. Furthermore, the
increasing attention paid to reducing atmospheric pollution has also served to direct some attention
to coking, since the process not only concentrates such pollutants as feedstock sulfur in the coke but
usually yields products that can be conveniently subjected to desulfurization processes.
Coking processes generally utilize longer reaction times than thermal cracking processes. To
accomplish this, drums or chambers (reaction vessels) are employed, but it is necessary to use two or
more such vessels so that coke removal can be accomplished in those vessels not on-stream without
interrupting the semi-continuous nature of the process.
Coking processes have the virtue of eliminating the residue fraction of the feed, at the cost of forming a solid carbonaceous product. The yield of coke in a given coking process tends to be proportional
to the carbon residue of the feed (measured as the Conradson carbon residue; please see Chapter 10).
The formation of large quantities of coke is a severe drawback unless the coke can be put to use.
Calcined petroleum coke can be used for making anodes for aluminum manufacture and a variety
of carbon or graphite products such as brushes for electrical equipment. These applications, however, require a coke that is low in mineral matter and sulfur.
If the feedstock produces a high sulfur, high ash, high vanadium coke, one option to use the coke
is combustion of the coke to produce process steam (and large quantities of sulfur dioxide unless the
coke is first gasified or the combustion gases are scrubbed). Another option is stockpiling.
For some feedstocks, particularly from heavy oil, the combination of poor coke properties for
anode use, limits on sulfur dioxide emissions, and loss of liquid product volume have tended to relegate coking processes to a strictly secondary role in any new upgrading facilities.
In spite of the various limitation outlined earlier, visbreaking has much potential and, in fact,
remains an important, relatively inexpensive bottom-of-the-barrel upgrading process in many areas
of the world. Most of the existing visbreakers are the soaker type, which utilize a soaker drum in
conjunction with a fired heater to achieve conversion and which reduces the temperature required
to achieve conversion while producing a stable residue product, thereby increasing the heater run
length and reducing the frequency of unit shut down for heater decoking.
However, a recurring issue with the soaker visbreaker is the need to periodically decoke the soaker
drum and the inability of the soaker process to easily adjust to changes in feedstock quality because
of the need to fine tune two process variables, temperature, and residence time. Recent combination of the visbreaking technology and the addition of new coil visbreaker design features have provided the coil process with a competitive advantage over the traditional soaker visbreaker process.
Limitations in heater run length are no longer a problem for the coil visbreaker. Advances in visbreaker