Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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INFLUENCE OF ACOUSTIC ENERGY ON COKE
MORPHOLOGY AND FOAMING IN DELAYED COKING
FIELD OF THE INVENTION
[0001] This invention relates to a process for controlling coke morphology and
foaming in delayed coking. More particularly, acoustic energy is used to
control
coke morphology and foaming in a delayed coking process.
BACKGROUND OF THE INVENTION
[0002] Delayed coking involves thermal decomposition of petroleum residua
(resids) to produce gas, liquid streams of various boiling ranges, and coke.
Delayed
coking of resids from heavy and heavy sour (high sulfur) crude oils is carried
out
primarily as a means of disposing of these low value feedstocks by converting
part
of the resids to more valuable liquid and gaseous products. Although the
resulting
coke is generally thought of as a low value by-product, it may have some
value,
depending on its grade, as a fuel (fuel grade coke), electrodes for aluminum
manufacture (anode grade coke), etc.
[0003] In the delayed coking process, the feedstock is rapidly heated in a
fired
heater or tubular furnace. The heated feedstock is then passed to a coking
drum
that is maintained at conditions under which coking occurs, generally at
temperatures above 400 C under super-atmospheric pressures. One of the aspects
of coke formation involves foam formation. In order to control foam formation,
an
anti-foam agent is typically added to the coke drum. Foam-overs in a coke drum
are generally highly detrimental to the coking process.
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[00041 The heated residuum feed in the coker drum also forms volatile
components that are removed overhead and passed to a fractionator, leaving
coke
behind. When the coker drum is full of coke, the heated feed is switched to
another
drum and hydrocarbon vapors are purged from the coke drum with steam. The
drum is then quenched with water to lower the temperature to less than 100 C
after
which the water is drained. When the cooling and draining steps are completed,
the
drum is opened and the coke is removed after drilling and/or cutting using
high
velocity water jets.
[0005] For example, a hole is typically bored through the center of the coke
bed
using water jet nozzles located on a boring tool. Nozzles oriented
horizontally on
the head of a cutting tool then cut the coke from the drum. The coke removal
step
adds considerably to the throughput time of the overall process. Thus, it
would be
desirable to be able to produce a free-flowing coke, in a coker drum, that
would not
require the expense and time associated with conventional coke removal.
[0006] Even though the coker drum may appear to be completely cooled, areas
of the drum do not completely cool. This phenomenon, sometimes referred to as
"hot drum", may be the result of a combination of morphologies of coke being
present in the drum, which may contain a combination of more than one type of
solid coke product, i.e., needle coke, sponge coke and shot coke. Since
unagglomerated shot coke may cool faster than other coke morphologies, such as
large shot coke masses or sponge coke, it would be desirable to produce
predominantly substantially free-flowing shot coke in a delayed coker, in
order to
avoid or minimize hot drums.
[0007] Coke morphology is difficult to proactively control as coke formation
is
not an exact science. For example, crude selection may influence coke
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morphology. However, it is difficult to predict in advance exactly what
influence
the make-up of any give crude will have on the morphology of coke produced.
Other process variables may be adjusted, but it is still very difficult to
control the
coking process to make a certain type of coke while excluding other types of
coke.
There is a need to be able to proactively control coke morphology.
SUMMARY OF THE INVENTION
[0008] One embodiment of the invention relates to a method for controlling
coke morphology in a delayed coking process that comprises: (a) heating a
coker
feedstock in a heater to produce a heated feedstock, (b) conducting the heated
feedstock to a coker vessel, (c) maintaining the coker vessel at delayed
coking
temperatures at effective delayed coking conditions to produce vapor products
and
coke, (d) quenching the coker vessel, and (e) subjecting at least one of steps
(a), (b)
or (c) to acoustic energy at an energy level and for a time sufficient to
produce shot
coke.
[0009] Another embodiment relates to a method for controlling foam formation
in a delayed coking process that comprises: (a) heating a coker feedstock in a
heater
to produce a heated feedstock, (b) conducting the heated feedstock to a coker
vessel, (c) maintaining the coker vessel at delayed coking temperatures at
effective
delayed coking conditions to produce foam, vapor products and coke, and (d)
subjecting the coker vessel in step (c) to acoustic energy at an energy level
and for
a time sufficient to reduce the amount of foam.
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DETAILED DESCRIPTION OF THE INVENTION
[0010] Petroleum atmospheric or vacuum residua ("resid") feedstocks are
suitable for delayed coking. Such petroleum residua are frequently obtained
after
removal of distillates from crude feedstocks and are characterized as being
comprised of components of large molecular size and weight, generally
containing:
(a) asphaltenes and other high molecular weight aromatic structures that would
inhibit the rate of hydrotreating/hydrocracking and cause catalyst
deactivation; (b)
metal contaminants occurring naturally in the crude or resulting from prior
treatment of the crude, which contaminants would tend to deactivate
hydrotreating/hydrocracking catalysts and interfere with catalyst
regeneration; and
(c) a relatively high content of sulfur and nitrogen compounds that give rise
to
objectionable quantities of SOa, SO3, and NOX upon combustion of the petroleum
residuum. Nitrogen compounds present in the resid also have a tendency to
deactivate catalytic cracking catalysts.
[0011] Resid feedstocks include, but are not limited to, residues from the
atmospheric and vacuum distillation of petroleum crudes or the atmospheric or
vacuum distillation of heavy oils, visbroken resids, tars from deasphalting
units or
conlbinations of these materials. Atmospheric and vacuum-topped heavy bitumens
can also be employed. Typically, such feedstocks are high-boiling
hydrocarbonaceous materials having a nominal initial boiling point of 538 C or
higher, an API gravity of 20 or less, and a Conradson Carbon Residue content
of 0
to 40 weight percent.
[0012] The resid feed is subjected to delayed coking. Generally, in delayed
coking, a residue fraction, such as a petroleum residuum feedstock, is pumped
to a
heater at a pressure of 50 to 550 psig (446 to 3893 kPa), where it is heated
to a
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temperature from 480 C to 520 C. The heater comprises one or more furnaces
containing one or more furnace tubes.
[0013] The heated feedstock from the furnace is then conducted into a coking
zone containing one or more vessels through at least one transfer line. The
transfer
line may be heated if necessary. The coking vessel is typically a vertically-
oriented, insulated coker drum and heated feedstock is transferred into the
coker
drum through an inlet at or near the base of the drum. Coker drums may be run
in
tandem so that while one drum is in operation, the other may be in the process
of
having coke removed. Pressure in the drum is usually relatively low, such as
15 to
80 psig to allow volatiles to be removed overhead. Typical operating
temperatures
of the drum will be between 410 C and 475 C. The hot feedstock thermally
cracks
over a period of time (the "coking time") in the coker drum, liberating
volatiles
composed primarily of hydrocarbon products that continuously rise through the
coke mass and are collected overhead. The volatile products are sent to a
coker
fractionator for distillation and recovery of coker gases, gasoline, light gas
oil, and
heavy gas oil. In an embodiment, a portion of the heavy coker gas oil present
in the
product stream introduced into the coker fractionator can be captured for
recycle
and combined with the fresh feed (coker feed component), thereby forming the
coker heater or coker furnace charge. In addition to the volatile products,
delayed
coking also forms solid coke product.
[0014] There are generally three different types of solid delayed coker
products
that have different values, appearances and properties, i.e., needle coke,
sponge
coke, and shot coke. Needle coke is the highest quality of the three
varieties.
Needle coke, upon further thermal treatment, has high electrical conductivity
(and a
low coefficient of thermal expansion) and is used in electric arc steel
production. It
is relatively low in sulfur and metals and is frequently produced from some of
the
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higher quality coker feedstocks that include more aromatic feedstocks such as
slurry and decant oils from catalytic crackers and thermal cracking tars.
Typically,
it is not formed by delayed coking of resid feeds.
[0015] Sponge coke, a lower quality coke, is most often formed in refineries.
Lower quality refmery coker feedstocks having significant amounts of
asphaltenes,
heteroatoms and metals produce this lower quality coke. If the sulfur and
metals
content is low enough, sponge coke can be used for the manufacture of
electrodes
for the aluminum industry. If the sulfur and metals content is too high, then
the
coke can be used as fuel. The name "sponge coke" comes from its porous, sponge-
like appearance. Conventional delayed coking processes, using the preferred
vacuum resid feedstock of the present invention, will typically produce sponge
coke, which is produced as an agglomerated mass that needs an extensive
removal
process including drilling and water-jet technology. As discussed, this
considerably complicates the process by increasing the cycle time.
[00161 Shot coke is considered the lowest quality coke. The term "shot coke"
comes from its shape which is similar to that of BB-sized balls. Desirable
shots
may be in the range of 1 to 10 mm in diameter. Shot coke, like the other types
of
coke, has a tendency to agglomerate, especially in admixture with sponge coke,
into larger masses, sometimes larger than a foot in diameter. This can cause
refinery equipment and processing problems. Shot coke is usually made from the
lowest quality high resin-asphaltene feeds and makes a good high sulfur fuel
source, particularly for use in cement kilns and steel manufacture. There is
also
another coke, which is referred to as "transition coke" and refers to a coke
having a
morphology between that of sponge coke and shot coke. For example, coke that
has a mostly sponge-like physical appearance, but with evidence of small shot
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spheres beginning to form as discrete shapes. The term "transition coke" can
also
refer to mixtures of shot coke bonded together with sponge coke.
[0017] Foam is usually formed in the delayed coking process. Foam-over
results when the delayed coker drum contains excessive foam and can result in
numerous problems such as partial plugging of lines, coke lay-down, plugged
heater tubes and the like. Foam-over is typically controlled by operational
constraints on the coking process itself, by the addition of antifoam
additives such
as silicone based chemicals, or both. Thus coke drums are not utilized to
their full
capacity in order to leave room for foam formation. In addition to or in the
alternative, siloxanes are injected into the over head of the coke drum to
control
excess foam formation. Anti-foam agents may also be non-silicone based,
including, for example, organic sulfonates, phenates, salicylates, carbon
powders,
oils (animal and vegetable) and polymers such as polyolefins, e.g.,
polyisobutylenes.
[0018] The present invention addresses both control of coke morphology and
foam formation by using acoustic energy during the coking process. Acoustic
generators generate acoustic energy in the form of sound waves to control both
coke morphology and foam formation. To control coke morphology, the sound
waves may be oriented in the direction axially along the length of the coker
drum,
across the diameter of the drum, i.e., perpendicular to the axis of the drum
or some
angle in between. The acoustic energy may be applied to at least one of the
drum
itself, to furnace tubes, or transfer lines. To control foam formation, sound
waves
are preferably applied across the diameter of the coke drum. The sound waves
may
be applied in conjunction with chemical anti-foam additives to control foam
formation.
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[0019] For the present control of coke morphology and foam formation, the
sound (acoustic) waves are in the frequency range from 15 to 20,000 Hertz
(Hz),
preferably from 50 to 10,000 Hz. The sound intensity, which is a measure of
the
acoustic energy transmitted to the aerosol mist, is in the range from 90 to
200
decibels (dB), preferably 120 to 150 dB. The duration of the sound waves is
for a
time sufficient to cause the desired degree of control of coke morphology and
foam
formation. This is typically in the range of 1 to 10 seconds and depends on
the
operating conditions within the coker unit. It is preferred to adapt the sound
wave
frequency, acoustic energy and the geometry of the coker system to achieve a
standing wave condition. The sound generators may be oriented perpendicular to
the coker drum or may be oriented at an angle varying from perpendicular to
parallel with the axis of the coker drum. The type of acoustic generator may
be any
of a variety of commercially available sound generators such as transducers,
sirens,
air horns, electromagnetic sonic devices and the like. The duration of
application
of acoustic energy is preferably from the inception of filling of the coke
drum to
completion of the filling of the coke drum to the desired level. However, the
application of acoustic energy may be either intermittent or for some period
less
than the full filling cycle.
[0020] In an embodiment, shot coke formation may be enhanced by treating the
residuum feedstock with one or more metal-containing additives in addition to
the
application of acoustic energy. The additives are those that enhance the
production
of shot coke during delayed coking. A resid feed is subjected to treatment
with one
or more additives, at effective temperatures, i.e., at temperatures that will
encourage the additives' dispersal in the feed stock. Such temperatures will
typically be from 70 C to 500 C, preferably from 150 C to 370 C, more
preferably
from 185 C to 350 C. The additive suitable for use herein can be liquid or
solid
form, with liquid/solution form being preferred. Non-limiting examples of
metals-
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containing additives include metal hydroxides, naphthenates and/or
carboxylates,
metal acetylacetonates, Lewis acids, a metal sulfide, metal acetate, metal
carbonate,
high surface area metal-containing solids, inorganic oxides and salts of
oxides; salts
that are basic are preferred. Non-limiting examples of substantially metals-
free
additives that can be used in the practice of the present invention include
elemental
sulfur, high surface area substantially metals-free solids, such as rice
hulls, sugars,
cellulose, ground coals, ground auto tires; inorganic oxides such as fumed
silica
and alumina; salts of oxides, such as ammonium silicate and mineral acids such
as
sulfuric acid, phosphoric acid, and acid anhydrides. These additives are
disclosed
in WO 2004104139, which is incorporated herein by reference.
[0021] While not wishing to be bound to any particular theory, one explanation
for shot coke formation is that shots are formed in the coker furnace and
transfer
line when the heaviest and most polar components (highest solubility parameter
components) of the resid feedstock begin to come out of a primary lower
solubility
parameter liquid phase and start to form a second liquid phase. Depending on
nucleation sides, coalescence sites, and process shear and turbulence
conditions, the
second liquid phase can coalesce and grow into spherical particles of a heavy
tar
the subsequently dry into hard spheres. In the present invention, the
application of
acoustic energy facilitates the coalescence of the second liquid phase
coniponents
into uniform spheres, preferably having a diameter of from 0.5 to 5 mm. In
addition, the application of acoustic energy helps collapse the foam and, if
used in
conjunction with anti-foam agents, increases the effectiveness of the anti-
foam
agents.
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[0022] The invention is further illustrated in the following non-limiting
examples.
EXAMPLES
[0023] The following examples are based on modeling studies.
Example 1
[0024] A heavy Canadian vacuum resid blend produces a mixture of shot (15%)
and shot coke bonded to sponge coke in the drum of a commercial delayed coker.
Use of transducer devices to introduce standing sound waves into the last four
tubes
of the furnace and through the transfer line increases the amount of shot coke
to
80%. Introducing standing waves into the furnace tubes, transfer line and coke
drum increases shot coke to 95%.
Example 2
[0025] Use if the feed of Example 1 produces a foam height of 15 feet (4.6 m)
in
the drum midway through the fill cycle. Introduction of silicone antifoam
knocks
the foam height back to 5 to 10 feet (1.5 to 3 m). Application of standing
sound
wave to the drum helps to collapse the foam and also increases antifoam
effectiveness such that only 1/3 the amount of antifoam gives the same 5 to 10
feet
(1.5 to 3 m) foam height.
