Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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MITIGATION OF COKE DEPOSITS IN REFINERY REACTOR UNITS
FIELD OF THE INVENTION
A preferred embodiment of the invention is directed to mitigating the
formation of coke deposits in petroleum refinery reactor units, particularly
in the
cyclones of fluidized bed coking units (fluid cokers) and reactor overheads of
fluid
catalytic cracking units.
BACKGROUND OF THE INVENTION
Fluidized bed coking (fluid coking) is a petroleum refining process in
which mixtures of heavy petroleum fractions, typically the non-distillable
residue
(resid) from fractionation, are converted to lighter, more useful products by
thermal
decomposition (coking) at elevated reaction temperatures, typically about 900
to
1100 F (about 480 to 590 C). A large vessel of coke particles maintained at
the
reaction temperature is fluidized with steam. The feed is heated to a pumpable
temperature, mixed with atomizing steam, and fed through a plurality of feed
nozzles
to the fluidized bed reactor. The light hydrocarbon products of the coking
reaction
are vaporized, mixed with the fluidizing steam and pass upwardly through the
fluidized bed into a dilute phase zone above the dense fluidized bed of coke
particles.
The transition between the dense bed (dense phase zone) and dilute phase,
where
product vapor is substantially separated from solid particles, is hereinafter
referred to
as the phase transition zone. The remainder of the feed liquid coats the coke
particles
and subsequently decomposes into layers of solid coke and lighter products
which
evolve as gas or vaporized liquid. The solid coke consists mainly of carbon
with
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lesser amounts of hydrogen, sulfur, nitrogen, and traces of vanadium, nickel,
iron,
and other elements. The fluidized coke is circulated through a burner, where
part of
the coke is burned with air to raise its temperature from about 900 F to about
1300 F
(about 480 to 704 C), and back to the fluidized bed reaction zone.
The mixture of vaporized hydrocarbon products and steam continues to
flow upwardly through the dilute phase at superficial velocities of about 3 to
6 feet
per second (about 1 to 2 meters per second), entraining some fine solid
particles.
Most of the entrained solids are separated from the gas phase by centrifugal
force in
one or more cyclone separators, and are returned to the dense fluidized bed by
gravity. The gas phase undergoes pressure drop and cooling as it passes
through the
cyclone separators, primarily at the inlet and outlet passages where the
velocity is
increased. The cooling which accompanies the pressure decrease causes
condensation
of some liquid which deposits on surfaces of the cyclone inlet and outlet.
Because
the temperature of the liquid so condensed and deposited is higher than about
900 F
(about 480 C), coking reactions occur there, leaving solid deposits of coke.
Coke
deposits also form on the reactor stripper sheds, and other surfaces of the
fluid coker
reactor.
The mixture of steam and hydrocarbon vapor is subsequently discharged
from the cyclone outlet and quenched to about 750 F (about 400 C) by contact
with
downflowing liquid in a scrubber vessel section of the fluid coker equipped
with
internal sheds to facilitate contacting. A pumparound loop circulates
condensed
liquid to an external cooling means and back to the top row of scrubber sheds
to
provide cooling for the quench and condensation of the heaviest fraction of
the liquid
product. This heavy fraction is typically recycled to extinction by feeding
back to the
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fluidized bed reaction zone, but may be present for several hours in the pool
at the
bottom of the scrubber vessel and the pumparound loop, allowing time for coke
to
form and deposit on shed surfaces because of the elevated temperatures.
Feed is injected through nozzles with atomizing steam into the fluidized
bed reactor. The feed components not immediately vaporized coat the coke
particles
and are subsequently decomposed into layers of solid coke and lighter products
which evolve as gas or vaporized liquids. During this conversion process some
coke
particles may become unevenly or too heavily coated with feed and during
collision
with other coke particles stick together. These agglomerated, now heavier,
coke
particles may not be efficiently fluidized by the steam injected into the
bottom of
stripper section and are subsequently carried under from the reactor section
to the
stripper section where they adhere to and build up on the top rows of sheds in
the
stripper section. Build up of deposits on the stripper sheds can become so
severe due
to overlapping of the deposits on adjacent sheds as to restrict fluidization
of the coke
in the reactor section above and eventually shut the unit down.
Fouling of cyclone outlets and scrubber sheds in a Fluid Coker results
in decreased capacity and run length of the unit, culminating in costly
unplanned
shutdowns. The deposits are sometimes removed from the outlet of the cyclone
with
metal rods and water jets at high pressure to clear the cyclone outlet area
and to keep
the unit running. The effectiveness of this approach is temporary and
unpredictable.
Chunks of coke may fall back into the cyclone body and interfere with cyclone
operation. The coke deposits must similarly be removed from the reactor
scrubber
sheds, reactor walls and other areas of the fluid coker that become fouled. It
is well
known in the art that providing sufficient cooling of the pumparound loop will
help
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minimize fouling of scrubber sheds, but this technique does not affect the
cyclone
outlet area.
Fluid Catalytic Cracking (FCC) is another petroleum refining
conversion process in which heavy oil, typically the highest boiling
distillable
fraction, is converted to gasoline, diesel and jet fuel, heating oil,
liquefied petroleum
gas (LPG), chemical feedstocks, and refinery fuel gas by catalytic
decomposition at
similarly elevated temperatures of about 900 to 1100 F (about 480 to 590 C).
In a
Fluid Catalytic Cracking Unit (FCCU), the heavy oil feed is typically mixed
with
steam and sprayed into a rising stream of hot (1100 to 1400 F or about 590 to
760 C)
powdered silica-alumina catalyst. The feed is vaporized by contact with the
hot
catalyst, and the vapor decomposes catalytically into the desired products
within a
few seconds, whereupon the solid catalyst particles are separated
centrifugally from
the vapor by means of cyclone separators or equivalent means. The product
vapor
passes through the cyclone outlets into a plenum chamber at the top of the
reactor,
through a discharge nozzle into an overhead line, then to a fractionator where
the
vapor is quenched and condensed in a zone similar to the coker scrubber
described
above. The separated catalyst is introduced into a stripping zone in which it
is further
stripped with steam to recover entrained vapor. Because the stripping steam is
typically at a significantly lower temperature than the spent catalyst, the
catalyst is
cooled by the stripping steam to a temperature significantly below the
reaction
temperature.
Run length or capacity of an FCCU may likewise be limited by
deposition of coke in the stripper, reactor overhead, plenum, nozzle, transfer
line, or
inlet to the fractionator. Coke formation occurs where heat loss allows
condensation
of heavy hydrocarbons which decompose to form coke. Deposit formation is
further
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aggravated by entrapment of entrained catalyst particles in the condensate.
Deposits
are most likely to occur where flanges or other heat sinks provide surfaces
below the
dew point of the product vapor. Deposits may also form at the inlet to the
fractionator where expansion cooling of the hot product vapor causes
condensation
and subsequent coke formation. The coke buildup restricts flow and increases
pressure drop between the reactor overhead and the fractionator. In units
limited by
compressor capacity, the pressure drop may be sufficient to limit capacity
long
before the end of the run, and may ultimately require premature or unplanned
shutdown.
In both fluid coking (FCU) and fluid catalytic cracking units (FCCU),
there is a reaction zone in which the product vapor is in intimate contact
with
particulate solids, known as the dense phase zone, and a dilute phase zone in
which
the solids have been substantially separated from the product vapor or where
coke
and catalyst have disengaged from the dilute (vapor) phase. The mass of solid
particles in the reaction zone is many times the mass of the product vapor,
and in
both types of units the heaviest reaction products condense on the solid
particles to
form coke. The dew point of the product vapor emerging from the reaction zone
(dense phase zone) into the dilute phase zone is essentially the same
temperature as
the temperature at the transition from the reaction zone (dense phase zone) to
the
dilute phase zone known as the phase transition temperature. In many FCCU's
the
reaction zone is terminated by cyclone separators, and the dilute phase is the
zone
into which the cyclone outlets discharge, typically a plenum at the top of the
vessel
housing the cyclones.
What is needed in the art is an efficient, predictable, and effective way
to mitigate the formation of detrimental coke deposits in the dilute phase
overhead
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equipment such as fluid coker cyclones and accompanying surfaces and in the
overhead dilute phase zone, plenum, discharge nozzle and overhead lines of
fluid
catalytic cracking units to avoid loss of capacity and expensive shutdowns.
SUMMARY OF THE INVENTION
An embodiment of the present invention is a method for mitigating the
condensation of liquid hydrocarbons and subsequent coke formation at
temperatures
in the range of about 700 F to 1100 F (about 370 C to 590 C ) in the dilute
phase
zones of fluid cokers and FCCU's such as occurs in overhead equipment.
An embodiment of the invention is directed to a method for mitigating
the condensation of liquid hydrocarbons and subsequent coke deposition in
refinery
reactor units wherein during operation of said units a dense phase zone
comprising
hydrocarbon feed and a dilute phase zone comprising vaporized hydrocarbon
products produced from said hydrocarbon feed and a phase transition zone
between
said dense and dilute phase zones are present, said method comprising
injecting a
non-condensable medium wherein said medium is selected from the group
consisting
of non-condensable vapors, gases, and mixtures thereof into said dilute phase
zone,
to form an admixture with said vaporized hydrocarbon products wherein the dew
point of the dilute phase zone is suppressed while maintaining the temperature
above
the suppressed dew point and wherein said dew point of said dilute phase zone
will
be suppressed by at least about 5 F (about 3 C) below the temperature of said
dilute
phase zone.
Practice of the invention comprises introducing a stream of gas or
vapor, typically steam, which is non-condensable at temperatures above about
705 F
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(about 374 C) into the dilute phase zone to form an admixture with the product
hydrocarbon vapor. If another gas is used, it will be non-condensable at
temperatures
equal to or greater than the dilute phase zone.
BRIEF DESCRIPTION OF THE FIGURES
Figure 1 depicts a typical fluid coking unit. A are scrubber sheds, B the
cyclone outlet, C, D, and E are the dense phase reaction zone, phase
transition zone,
and dilute phase reaction zone, respectively. El, E2, E3, E4 are feed
injection ports,
and F are stripper sheds.
Figure 2 depicts a typical FCCU. M = flue gas outlet,
N= regenerator, P = air injection, Q = regenerated catalyst standpipe,
R = spent catalyst standpipe, S = feed inlet, G = stripper, H = cyclone
separators,
I= plenum, J = Product Vapor outlet, K = dilute phase zone, and
L = dense phase reaction zone.
DETAILED DESCRIPTION OF THE INVENTION
It has been found that formation of coke proceeds within a few hours in
the condensed liquid phase at temperatures above about 700 F (about 370 C),
but
very slowly in the gas or vapor phases even at significantly higher
temperatures than
are encounted in fluid coking or fluid catalytic cracking. It has further been
found
that the dew point of the mixture of fluidizing steam and hydrocarbon product
vapors
in the dilute phase of both fluid cokers and fluid catalytic cracking units is
the
temperature of the phase transition zone from the dense phase to the dilute
phase.
Applicants have discovered that by injecting a non-condensable vapor
and/or gas into the dilute phase zone, the dew point of the admixture so
produced by
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the medium and dilute phase will be below the pre-injection temperature of the
dilute
phase, and condensation and, hence, coke deposition, will thereby largely be
prevented. Thus, the non-condensable medium is a medium that is non-
condensable
at temperatures equal to or greater than the dilute phase zone. For water, it
is greater
than or equal to 705 F (374 C). The non-condensable medium will be injected
under
conditions sufficient to prevent cooling of the product vapor in the
transition phase
zone and to maintain or raise the temperature of the product vapors in the
phase
transition zone about I to about 20 F (about 0.6 C to about 11 C) above that
of the
phase transition zone. The non-condensable medium may itself serve to prevent
cooling of the product vapor by maintaining or raising its temperature, or
another
means may be utilized.
An embodiment of the invention is directed to a method to mitigate the
formation of coke deposits formed in refinery units such as fluid coker units
and fluid
catalytic cracking units during operation of said units, wherein said units
have a
reaction zone in which the product vapor is in intimate contact with a mass of
solid
particles such as coke or catalyst, said mass of solid particles being
substantially
greater than the mass of said product vapor, and a dilute phase zone in which
said
product vapor has been substantially separated from said solid particles in
the
reaction zone. The method comprises mitigating the formation of coke deposits
in
such refinery units, by injecting a gas or vapor such as steam into the dilute
phase
zone of the said reactor unit, said gas or vapor being at a temperature at
least about
equal to the temperature of said phase transition zone at the transition
between the
dense phase zone and the dilute phase zone, wherein said gas or vapor is non-
condensable at temperatures above about 705 F (about 374 C), and wherein said
gas
or vapor is injected in an amount and at a temperature sufficient to lower the
dew
point of said dilute phase zone while maintaining the temperature of said
dilute phase
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zone above said lowered dew point. Typically, it will be desirable for the dew
point
of the admixture of non-condensable medium and dilute phase to be at least
about
F less (about 6 C) than the temperature of the dilute phase post injection.
Any
temperature difference between the dew point and dilute phase post injection
of non-
condensable medium will suffice, preferably a difference of at least about 5 F
(difference of about 3 C), more preferably at least about 10 F (difference of
about
6 C) will be used. The above operation is carried out continuously during
normal
operation of the refinery units.
Preferably, the non-condensable medium will be injected into the dilute
phase zone of the reactor at a temperature higher than the temperature of the
phase
transition zone, hereinafter called the phase transition temperature.
Otherwise, there
is a risk that the cooling of the product vapor, already at its dew point,
will cause the
condensation of liquid which it is intended to prevent. Preferably, the
temperature
will be at least about 1 to about 100 F, (about 0.6 to about 55 C) above the
phase
transition temperature and, more preferably, about 10 to about 50 F (6 to
about 28 C)
higher than the phase transition temperature. Lower temperatures than the
preferred
ranges will be less effective against downstream cooling effects, and higher
temperatures will increase thermal decomposition of product components to less
valuable constituents. Preferably, the temperature will be such that the
temperature
will maintain the temperature at the cyclone outlet at least about 5 F (at
least about
3 C higher) higher than the phase transition temperature.
During operation of a fluid coker, for example, coke is laid down in
several areas of the cyclone and also in the scrubber section. Areas such as
the
cyclone outlet are of particular concern since the deposited coke can restrict
flow
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ultimately requiring system shutdown. Likewise, in the operation of a
fluidized
catalytic cracking unit, the product vapor at its dew point is introduced into
the larger
cross-sectional area of the fractionator, and the accompanying expansion
cooling can
be sufficient to condense droplets of liquid, some of which adhere to the
entrance
opening, where the liquid is held at elevated temperatures sufficient to cause
formation of coke deposits. The buildup of coke restricts flow and decreases
capacity.
An embodiment of the method offers a cost effective and efficient way
to prevent coke deposits that form in refinery units such as fluid cokers
(FCU) and
fluid catalytic cracking units (FCCU) to facilitate longer run times and to
maintain
throughput. Effective mitigation of coke deposits in areas of the units such
as the
cyclone and cyclone outlet of a FCU unit and the stripper, reactor overhead,
plenum,
nozzle transfer line or fractionator inlet of FCCU is achieved.
The coke deposit mitigation taught herein recognizes that coke deposits
form when products are condensed or are in the liquid phase. Coke deposits
form at
a substantially slower rate when the reactor products are in the gaseous
phase. Thus,
by maintaining the reaction products produced in refinery units in the gaseous
phase
in the dilute phase zone, prior to quenching, coke deposition can be easily
mitigated
thus preventing coke deposits from forming on reactor surfaces that can become
plugged necessitating unit shutdowns.
The invention involves suppressing the dew point of the dilute phase of
the reactor products while maintaining the temperature above the reduced dew
point.
To accomplish this, a non-condensable medium which may also be referred to as
a
diluent medium is injected into the dilute phase zone of the reactor unit.
Typically,
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the amount of non-condensable medium injected will be that amount necessary to
cause the dew point of the dilute phase zone to be at least about 5 F (about 3
) below
the temperature of the dilute phase zone. Typically, at least about 5 mole %
non-
condensable medium will be injected. Non-condensable mediums can be selected
from, for example, steam, recycled hydrocarbon, inert gases and mixtures
thereof.
The term non-condensable as used herein means that the medium will be a gas or
a
vapor at temperatures above about 705 F (about 374 C). Though the
non-condensable medium can be used to maintain or raise the temperature of the
product vapors in the phase transition zone other methods may also be
employed.
Any alternative means which can accomplish this will suffice and may be used
alone
or in combination with the non-condensable medium being employed. to raise the
temperature of the product vapor. For example, scouring coke can be injected
into
the unit to maintain or raise the temperature of the product vapors. Other
means
known to the skilled artisan could also be employed.
As used herein, the dilute phase, the transition phase and dense phase
occupy the dilute phase zone, phase transition reaction zone, and dense phase
reaction zone, respectively. Thus, injection of the non-condensable medium
into the
dilute phase zone necessarily means injecting the medium into the dilute
phase.
Those skilled in the art readily appreciate such terminology and its
interchangeability.
The preferred amount of non-condensable medium will normally range
from about 1% to about 50% by volume of the product vapor and, more preferably
from about 5% to about 20% by volume. Lesser amounts than the preferred ranges
will be less effective at reducing the dew point, and greater amounts than the
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preferred ranges will be more costly to provide and subsequently separate from
the
desired commercial products.
Preferably, in a fluid coker, the non-condensable medium will be
introduced into the dilute phase zone above the top of the dense bed and below
the
cyclone inlets.
Preferably, in an FCCU, the non-condensable medium will be
introduced into the plenum of the vessel housing the reactor cyclones, into
which
plenum the reaction products are discharged from the cyclone outlets.
Alternatively,
the non-condensable medium may be introduced anywhere downstream of the
primary cyclone outlet. If the unit configuration allows the stripping gas
from the
spent catalyst stripper to mix with the product vapor between the primary and
secondary cyclones, the non-condensable medium may be introduced into the
stripper as long as the spent catalyst is not cooled in the stripper as is
typical in the
current state of the art practice. If recycled hydrocarbon is to be utilized
as the non-
condensable medium introduced to the stripper, it is preferable to inject it
into the
dilute phase zone of the stripper to minimize hydrocarbon carryunder to the
regenerator.
In the course of performing the instant invention, one skilled in the art
can easily monitor with existing or installed thermocouples the temperature of
the
dense phase zone, the phase transition zone, or the dilute phase zone. It is
preferable
that the products, immediately prior to being quenched, are at a temperature
above
that of the phase transition zone. Preferably, they will be at a temperature
at least
about 1 to about 20 F (about 0.6 to about 11 C) and; more preferably, about 2
to
about 10 F (about 1 to 6 C) higher than the phase transition zone or the
phase
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transition temperature. Maintaining this higher temperature ensures that
product
vapor is above its dew point and minimizes the risk of liquid condensation and
subsequent formation of coke deposits. Preferably, the non-condensable medium,
when used to prevent the product vapor from cooling, will be of a temperature
when
injected such that it maintains the temperature of the reactor cyclone outlet
products
at a temperature of at least about 1 to about 20 F (0.6 to about 11 C) higher
than the
temperature of the phase transition zone.
In the case of an FCCU, it is preferable that the non-condensable
medium be injected at a temperature such that it maintains the temperature of
the
product vapor at the plenum outlet or the temperature of the fractionator
inlet at least
about 5 F (3 C) higher than the temperature of the riser outlet.
One skilled in the art will readily recognize that the non-condensable
medium can be injected into the dilute phase zone of the FCCU by injecting
into the
cyclone outlet plenum chamber, the product line upstream of the fractionator
inlet or
in some configurations the catalyst stripper where steam stripping occurs.
