Note: Descriptions are shown in the official language in which they were submitted.
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SEPARATING FINE SOLID PARTICULATES
FROM A GAS STREAM
BACKGROUND OF THE INVENTION
[0001] The present invention relates to a novel cyclone separator for removing
fine
solid particulates from a gas stream. The cyclone is especially applicable in
a third stage
separator apparatus, often used to purify the catalyst fines-laden flue gas
stream exiting a
refinery fluid catalytic cracking (FCC) catalyst regenerator.
[0002] The emission of particulates in industrial gas streams must be
carefully
controlled in light of federal, state, and local regulations designed to
curtail pollution. In
the area of oil refinery operations, a major concern regarding particulate
emissions lies in
the flue gas exiting the catalyst regenerator section of fluid catalytic
cracking (FCC)
units. Current United States federal regulations limit particulate levels to 1
kg of solids
per 1000 kgs of coke burned in the catalyst regenerator, or the equivalent of
a flue gas
particulate concentration of approximately 80-110 mg/Nm3. Corresponding
European
regulations currently vary considerably, from 80-500 mg/Nm3; however, this
value is
expected to decline potentially to 50 mg/Nm3.
[0003] FCC technology, now more than 50 years old, has undergone continuous
improvement and remains the predominant source of gasoline production in many
refineries. This gasoline, as well as lighter products, is formed as the
result of cracking
heavier (i.e. higher molecular weight), less valuable hydrocarbon feed stocks
such as gas
oil. Although FCC is a large and complex process involving many factors, a
general
outline of the technology is presented here in the context of its relation to
the present
invention.
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[0004] In its most general form, the FCC process comprises a reactor that is
closely
coupled with a catalyst regenerator, followed by downstream hydrocarbon
product
separation. A major distinguishing feature of the process is the continuous
fluidization
and circulation of large amounts of catalyst having an average particle
diameter of SO-
100 microns, equivalent in size and appearance to very fine sand. For every
ton of
cracked product made, approximately 5 tons of catalyst are needed, hence the
considerable circulation requirements. Coupled with this need for a large
inventory and
recycle of a small particle diameter catalyst is the ongoing challenge to
prevent this
catalyst from exiting the reactor/regenerator system into effluent streams.
[0005] Overall, the use of cyclone separators internal to both the reactor and
regenerator has provided over 99% separation efficiency of solid catalyst.
Typically, the
regenerator includes first and second (or primary and secondary) stage
separators for the
purpose of preventing catalyst contamination of the regenerator flue gas,
which is
essentially the resulting combustion product of catalyst coke in air. While
normal-sized
catalyst particles are effectively removed in the internal regenerator
cyclones, fines
material (generally catalyst fragments smaller than 50 microns resulting from
attrition
and erosion in the harsh, abrasive reactor/regenerator environment) is
substantially more
difficult to separate. As a result, the FCC flue gas will usually contain a
particulate
concentration in the range of 200-1000 mg/Nm3. This solids level can present
difficulties
related to either the applicable legal emissions standards or the desire to
recover power
from the flue gas stream. In the latter case, the solids content in the FCC
flue gas may be
sufficient to damage turbine blades of an air blower to the regenerator if
such a power
recovery scheme is indeed selected.
[0006] A further reduction in FCC flue gas fines loading is therefore often
warranted,
and may be obtained from a third stage separator (TSS) device containing a
manifold of
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cyclones. Electrostatic precipitators, are known to be effective for this
gas/solid
separation but are far more costly than a TSS, which relies on the induction
of centripetal
acceleration to a particle-laden gas stream, forcing the higher-density solids
to the outer
edges of a spinning vortex. To be efficient, a cyclone separator for an FCC
flue gas
effluent will normally contain many, perhaps 100, small individual cylindrical
cyclone
bodies installed within a single vessel acting as a manifold. Tube sheets
affixing the
upper and lower ends of the cyclones act to distribute contaminated gas to the
cyclone
inlets and also to divide the region within the vessel into sections for
collecting the
separated gas and solid phases.
[0007] In the area of cyclone design, significant emphasis has been placed on
so-
called "reverse flow" types where incoming gas is added around a gas outlet
tube
extending from the inlet side of a cylindrical cyclone body. Particle-rich gas
can be
withdrawn from openings in the sidewall of the cyclone body, while clean gas
essentially
reverses flow from its initial path toward the end of the cyclone body
opposite the gas
inlet, back toward the gas outlet. The gas outlet is a tube normally
concentric with, and
located within the cyclone body. These types of cyclones are described in US
5,514,271
B 1 and US 5,372,707 B 1, where the inventive subject matter is focused on the
shape and
distribution of the sidewall openings in order to minimize turbulent eddy
formation that
can re-entrain solids into the clean gas outlet. In US 5,643,537 B 1 and
parent US
5,538,696 B l, devices are contemplated for use with this fundamental cyclone
design to
further extend, or improve the uniformity of, the vortex flow pattern and
thereby increase
separation efficiency.
[0008] Unfortunately, the requirement by itself for a gas stream to reverse
direction
and exit the cyclone body on the same side as the gas inlet imposes flow
disturbances
that are not easily overcome. Cyclones of the type described in US 5,690,709 B
1, termed
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"uniflow", eliminate the re-entrair4ment of solids associated with the
reversal of gas
direction. In this case, clean gas moves continually downward and exits the
cyclone body
below a lower tube sheet, which serves as the physical boundary between the
separated
particles and purified gas. This design, however, also promotes non-uniform
flow
patterns, which are here associated with the discharge of particles at
essentially right
angles to the particle-laden gas vortex, through the open bottom in the
cylindrical
cyclone body. Again, the basic operation of the cyclone in this case involves
a change in
direction of gas flow that should ideally be avoided. Furthermore, the open
bottom
design provides a relatively large surface area for exiting "dirty" gas to
enter the bodies
of adjacent cyclones in an overall arrangement of cyclones, such as in a TSS.
This
communication of gas among cyclones reduces separation efficiency.
[0009] Aside from general considerations about cyclone design, such as the
induction
of centripetal acceleration and the maintenance of a uniform flow pattern,
further
improvements in efficiency associated with any particular cyclone
configuration must be
verified through actual testing. Indeed, some proposed designs that were
believed in
principle to mitigate uneven flow patterns and localized eddy formation
actually
performed quite poorly in laboratory experiments. Even sophisticated
computational
fluid dynamics computer software has been found in some cases to be a poor
predictor of
TSS separation efficiency. Therefore, through extensive trial and error,
coupled with the
overall objective of refining the cyclone internal flow pattern, a significant
improvement
in fine particle separation from gas streams has been achieved.
SUN~VIARY OF THE INVENTION
[0010] The present invention is an improved cyclone for the separation of
solid
particulates from a gas stream. Many of these cyclones can be combined in a
vessel for
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use as a third stage separator in the treatment of solid-contaminated gas
streams, and in
particular flue gas from a refinery fluid catalytic cracking unit or other
solid-
contaminated gas streams. The cyclone provides a high separation efficiency
because a
particulate-laden gas vortex is established and travels through the device
with minimal
flow pattern disturbances. The feed gas and exiting clean gas move in the same
direction
throughout the separation, and the clean gas, representing the bulk of the
feed gas on a
volume basis, is removed from the central portion of the vortex using a gas
outlet tube
extending with the cyclone body. Furthermore, solid particles are forced
through
openings in the sidewall of the cyclone body to prevent backflow and gas
communication
among adjacent cyclones, rather than discharged axially.
[0011] The use of a plate or other structure to close off the bottom of
cyclone body
means that particle-laden gas can exit only through openings on the cylinder
wall. Thus,
the pressure drop across the area through which the gas discharges is
generally higher
than that for open bottom designs. This increase in pressure drop and gas
velocity
induces a more forceful ejection of particulates through the cylinder sidewalk
thereby
preventing re-entry of solids into the cyclone body or any adjacent cyclones
operating
upon the same principal. In effect, the slots through which the particle-
contaminated gas
exits act as a "check valve" to prevent backflow and particle re-entrainment
into the
cyclone body.
[0012] The cyclone of the present invention is effective for separating even
fine dust
particles as small as 4-5 microns in diameter from the feed gas stream. These
solid
contaminants would otherwise render the contaminated gas non-compliant with
environmental regulations or possibly prove detrimental to the proper
functioning of
power recovery turbines.
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BRIEF DESCRIPTION OF THE DRAWINGS
[0013] FIG. 1 is a simplified schematic view of an FCC unit of the prior art.
[0014] FIG. 2 is a simplified schematic view of a third stage separator of the
prior
art.
[0015] FIG. 3 is a cross sectional view of the cyclone of the present
invention.
[0016] FIG. 4 is a sectional view of FIG. 3 taken along line AA.
[0017] FIG. 5 shows the improved separation performance efficiency of the
cyclone
of the present invention, compared to those of the prior art.
[0018] FIG. 6 shows the improvement associated with the present invention in
terms
of its d50 value, or measure of the particle diameter for which 50% removal
would be
obtained.
DETAILED DESCRIPTION OF THE INVENTION
[0019] The present invention applies to the purification of a broad range of
solid-
contaminated gas streams, and especially those containing dust particles in
the 1-10 ~m
range. A number of commercial gas purification operations meet this
description,
including the treatment of effluent streams of solid catalyst fluidized bed
processes, coal
fired heaters, and power plants. Several well-known refinery operations rely
on fluidized
bed technology, such as a preferred embodiment of the process for converting
methanol
to light olefins, as described in US 6,137,022 B1, using a solid zeolitic
catalyst
composition. Another area of particular interest lies in the purification of
fluid catalytic
cracking (FCC) effluent streams that contain entrained catalyst particles
resulting from
attrition, erosion, and/or abrasion under process conditions within the
reactor.
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[0020] As mentioned, fluid catalytic cracking (FCC) is a well-known oil
refinery
operation relied upon in most cases for gasoline production. Process variables
typically
include a cracking reaction temperature of 400-600°C and a catalyst
regeneration
temperature of 500-900°C. Both the cracking and regeneration occur at
an absolute
pressure below 5 atmospheres. FIG. 1 represents a typical FCC process unit of
the prior
art, where a heavy hydrocarbon feed or raw oil in line 12 is contacted with a
newly
regenerated catalyst entering from a regenerated catalyst standpipe 14. This
contacting
occurs along a narrow section extending from the bottom of the reactor 10,
known as the
reactor riser 16. Heat from the catalyst vaporizes the oil, and the oil is
thereafter cracked
in the presence of the catalyst as both are transferred up the reactor riser
into the reactor
10 itself, operating at a pressure somewhat lower than that of the riser 16.
The cracked
light hydrocarbon products are thereafter separated from the catalyst using
first stage 18
and second stage 20 internal reactor cyclones and exit the reactor 10 through
line 22 to
subsequent fractionation operations. At this point, some inevitable side
reactions
occurnng in the reactor riser 16 have left detrimental coke deposits on the
catalyst that
lower its activity. The catalyst is therefore referred to as being spent (or
at least partially
spent) and requires regeneration for further use. Spent catalyst, after
separation from the
hydrocarbon product, falls into a stripping section 24 where steam is injected
in line 26
to purge any residual hydrocarbon vapor. After the stripping operation, the
spent catalyst
is fed to the catalyst regenerator 30 using a spent catalyst standpipe 32.
[0021] In the catalyst regenerator 30, a stream of air from line 34 is
introduced
through an air distributor 28 to contact the spent catalyst, burn coke
deposited thereon,
and provide regenerated catalyst. The catalyst regeneration process adds a
substantial
amount of heat to the catalyst, providing energy to offset the endothermic
cracking
reactions occurnng in the reactor riser 16. Some fresh catalyst is added in
line 36 to the
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base of the regenerator 30 to replenish catalyst exiting the reactor as fines
material or
entrained particles. Catalyst and air flow upward together along the combustor
riser 38
located within the regenerator 30 and, after regeneration (i.e. coke burn),
are initially
separated by discharge through a "T" disengager 40, also within the
regenerator 30.
Finer separation of the regenerated catalyst and flue gas exiting the
disengager 40 is
achieved using first stage 44 and second stage 46 regenerator cyclone
separators within
the catalyst regenerator 30. Regenerated catalyst is recycled back to the
cracking reactor
through the regenerated catalyst standpipe 14. As a result of the coke
burning, the flue
gas vapors exiting at the top of the regenerator in line 42 contain COZ and
HzO, along
10 with smaller amounts of other species. While the first stage 44 and second
stage 46
regenerator cyclone separators can remove the vast majority of the regenerated
catalyst
from the flue gas in line 42, fine catalyst particles, resulting mostly from
attrition,
invariably contaminate this effluent stream. The fines-contaminated flue gas
therefore
typically contains 200-1000 mg/Nm3 of particulates, most of which are less
than 50
microns in diameter. In view of this contamination level, and considering both
environmental regulations as well as the option to recover power from the flue
gas, the
incentive to further purify the flue gas using a third stage separator (TSS)
is significant.
[0022] A typical TSS of the prior art, containing numerous individual
cyclones, is
shown in FIG. 2. The TSS vessel 50 is normally lined with refractory material
52 to
reduce erosion of the metal surfaces by the entrained catalyst particles. The
fines-
contaminated flue gas from the FCC regenerator enters the top of the TSS at
its inlet 54
above an upper tube sheet 56 that retains the top ends 58 of each cylindrical
cyclone
body 62. The contaminated gas stream is then distributed among cyclone feed
gas inlets
60 and contacted with one or more swirl vanes 64 proximate these inlets to
induce
centripetal acceleration of the particle-contaminated gas. The swirl vanes are
structures
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within the cyclone body that have the characteristic of restricting the
passageway through
which incoming gas can flow, thereby accelerating the flowing gas stream. The
swirl
vanes also change the direction of the contaminated gas stream to provide a
helical or
spiral formation of gas flow through the length of the cyclone body. This
spinning
motion imparted to the gas sends the higher-density solid phase toward the
wall of the
cyclone body 62.
[0023] The cyclone design shown in FIG. 2 represents the so-called "uniflow"
apparatus where a bottom end 66 of the cyclone body 62 is open, allowing solid
particles
that have been thrown near the wall of this cylinder to fall into the space 68
between the
upper and lower tube sheets. Clean gas, flowing along the centerline of the
cyclone body,
passes through an inlet 70 of a gas outlet tube 72 before reaching the bottom
end 66 of
the cyclone body 62. The clean gas is then discharged via the gas outlet tube
72 below a
lower tube sheet 74. The combined clean gas stream, representing the bulk of
the fines-
contaminated flue gas, then exits through a gas outlet 76 at the bottom of the
TSS vessel
50. The, separated particles and a minor amount (typically less than 10% of
the fines-
contaminated flue gas) of underflow gas are removed through a separate
particulate and
underflow gas outlet 78 at the bottom of the TSS 50.
[0024] In FIG. 3, an individual cyclone separator 100 of the present
invention, also
affixed between an upper tube sheet 102 and a lower tube sheet 104, is shown.
The
cyclone 100 comprises an essentially vertical cyclone body 106 having a closed
bottom
end 108 with the cyclone body fixed at its top end 110 to the upper tube sheet
102. The
closed bottom end 108 is preferably in the form of a horizontal plate. The
cyclone body
defines a feed gas inlet 112 at its top end 110 for receiving a particle-
contaminated gas
stream (e.g. a fines-contaminated flue gas stream) from above the upper tube
sheet 102.
Also, the cyclone body further defines a plurality of openings 114 for
discharging gas.
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These openings 114 are between the upper. tube sheet 102 and the lower tube
sheet 104,
and are generally located in the lower portion of the cyclone body 106.
Preferably, these
openings 114 are proximate the bottom end 108 and extend upward from it. These
openings allow for the discharge of particles along with a minor amount of an
underflow
gas, typically less than 10% of the particle-contaminated gas by volume,
between the
upper tube sheet 102 and the lower tube sheet 104. Closure of the bottom end
108
induces a high gas velocity and pressure drop through the discharge openings
114 by
providing relatively little surface over which the exiting gas can escape.
This leads to an
overall improved separation.
[0025] One or more swirl vanes 116 are located proximate the gas inlet at the
top of
the cyclone to induce centripetal acceleration of the particle-contaminated
gas stream. A
gas outlet tube 118 is located centrally within the cyclone body 106, extends
through the
closed bottom end 108, and further extends upward through the lower tube sheet
104.
The top and bottom ends of this gas outlet tube 118 define, respectively, a
clean gas inlet
120 for receiving a purified gas stream from within the cyclone body 106 and
near its
centerline, and a clean gas outlet 122 below the lower tube sheet 104 for
discharging the
purified gas stream. The clean gas inlet 120 is generally located above the
discharge
openings 114. The clean gas outlet 122 can be located anywhere below the
bottom end
108. As mentioned, the cyclone body 106 is oriented generally vertically, so
that
separation of the solid phase is assisted by gravity. Preferably, the cyclone
body is in the
form of a vertical cylinder, however, other shapes are certainly possible,
including, for
example, a cone shape.
[0026] As noted previously, the major advantage of this design is that it
provides a
very uniform vortex of swirling gas that is essentially undisturbed along its
downward
path through the cyclone body and gas outlet tube. A further advantage is
related to the
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increased pressure drop accompanying the.ejection of particulate-rich gas
through the
cylinder wall openings. These openings provide a relatively small surface area
for gas to
exit, compared to the larger bottom ring-shaped surface between the cyclone
body and
the gas outlet tube, used in the aforementioned uniflow cyclone designs. As a
result, each
opening provides a type of "check valve" through which backflow of discharged
gas, a
cause of reduced separation efficiency, is substantially eliminated.
[0027] The uniformity in gas flow is maintained in part through the use of a
plurality
of openings on the cyclone cylinder body for discharge of particles and a
small amount
of underflow gas. The openings may be of virtually any shape and located
anywhere on
the cyclone cylinder body, although it is preferred that at least some of
these openings are
near the closed bottom end of the cyclone to prevent an accumulation of solid
particles in
this region. The openings may also be of varying shapes, for example, slots
and holes,
and located at various elevations on the cyclone body. Preferably, at least
some of the
openings are in the form of rectangular slots with their major dimension
(length)
substantially parallel to the axis of the cyclone body, as depicted in FIG. 3.
These slots
are normally spaced uniformly about the circumference of the cyclone body.
Also, the
vertical slot lengths usually range from 5% to 25% of the length of the
cyclone body. In a
preferred embodiment, the lower ends of the rectangular slots are adjacent to
the closed
bottom of the cyclone body.
[0028] To further promote flow uniformity and thereby improve overall solid-
gas
separation efficiency, that the gas discharge openings are inclined from a
radial direction.
This allows gas to exit the cyclone body without a substantial change in its
swirling,
tangential flow direction, as established within the cyclone body. An example
of this
desired configuration is illustrated in FIG. 4, where the slots 114 also have
edges 200 that
are beveled (i.e. not normal to the line tangent to the circular cross section
of the cyclone
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body 106 where the slots 114 are located). This beveling with respect to the
curvature of
the cyclone body 106 has the desired effect of allowing gas to exit the
cyclone body 106
with a significant tangential velocity component and minimal change from the
direction
of gas flow within the cyclone body. Also, the leading edge along the
principal length of
each rectangular slot may be slightly raised from the general curvature of the
cyclone
body to divert the gas flow in the desired tangential direction. Alternatively
or
concurrently, the trailing edge of the slot may be sunk into the general
curvature for a
similar effect.
[0029] Furthermore, it has been determined that good solid/gas separation
efficiencies are obtained when the openings are located below the clean gas
inlet, which
is also represented in FIG. 3. The total open area through which spinning gas
may be
discharged is preferably from 0.05% to 5% of the surface area of the cyclone
body. This
parameter, of course, depends on several factors including solid contaminant
concentration, average particle size, gas flow rate, and pressure. When
multiple cyclones
of the present invention are used in the design of a third stage separator
(TSS) for an
FCC refinery unit, the separator performance efficiency preferably includes a
d50
particle size of below 5 microns. As understood in the art, the d50 value
represents the
diameter of a dust particle that is 50% removed in the underflow gas of the
TSS.
Accordingly, in a preferred embodiment, the purified gas stream has a
concentration of
particles of 5 microns or greater that is less than 50% of the concentration
of particles of
5 microns or greater in the catalyst fines-contaminated flue gas stream.
[0030] The performance benefit obtained using the cyclone of the present
invention
is further clarified in the following examples, which provide laboratory test
data from
experiments designed to simulate conditions found in FCC flue gas effluent
streams.
Although the following examples illustrate specific embodiments of the cyclone
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separator of the present invention, they are not intended to limit the overall
scope of the
invention as set forth in the claims.
COMPARATIVE EXAMPLES 1-7
[0031] The previously mentioned "uniflow" type cyclone separators of the prior
art
were compared in performance to various cyclone separators according to the
present
invention. Separation of particulate matter of 40 microns in diameter and
smaller from a
flowing gas stream was investigated. The cyclone separator in each test
included a 280
mm i.d. cylindrical body with a 130 mm gas outlet tube concentric with the
cylinder and
extending from 250 mm above, to well below, the bottom of the cylinder.
[0032] In the comparative tests, except for this gas outlet tube extension,
the bottom
of the cylinder was open, although a disk was mounted on the exterior of the
gas outlet
tube 130 mm below the cylinder bottom. Separated particulates, having been
discharged
at essentially right angles to the spinning feed gas flow, were collected,
along with a
minor amount of underflow gas, in a dust hopper surrounding the cylindrical
cyclone
body. Both this gas and the clean (overflow) gas exiting through the gas
outlet tube were
analyzed for solid contamination levels as well as the particle size
distribution of these
contaminants. Likewise, these analyses were performed on the feed gas.
[0033] In each separate experiment, the feed gas inlet flow rate to the
cyclone was
maintained at 0.45-0.50 Nm3/sec. This gas contained 300-400 mg/Nm3 of solids
with a
median particle diameter of 10-20 microns. After exiting the swirl vanes near
the gas
inlet, the gas velocity gas was accelerated due to the flow restriction
effected by these
vanes. The gas discharged with the bulk of separated solids, called the
underflow gas,
represented either 1% or 3% by volume of the feed gas, depending on the
specific test.
After each test, the efficiency of solid particulate removal was calculated as
a weight
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percentage of the feed solids that were removed in the underflow gas. The
percentage of
solid particles in this stream of less than 10 microns in diameter was also
determined,
along with the calculated estimate of the particle diameter for which 50%
removal would
be achieved (the d50 value).
[0034] Results for these comparative examples are summarized in Table 1.
TABLE 1
ComparativeUnderflowVane Exit Separationp~icles d 50
Example Gas Gas VelocityEfficiencyX10 m (%) Value (m)
(vol-%) (m/sec) (%)
1 1 24.1 76.3 78.5 7.5
2 1 39.3 82.8 84.8 5.7
3 3 40.5 82.1 93.9 6.2
4 3 40.3 83.7 93.3 5.5
5 3 24.1 80.7 76.8 6.7
6 3 38.9 84.9 87.5 5.5
7 3 39.5 85.0 87.4 5.4
EXAMPLES 8-12
[0035] The cyclone separator of the present invention was tested, by including
in the
cyclone design a horizontal base that was used to close the bottom of the
cylinder body.
In accordance with the description of the present invention, the solid
particulates were in
this case discharged from the spinning feed gas through openings in the
cyclone cylinder
sidewall. This was achieved by forming two rectangular slots of 90 mm in
length and 10
mm in width. The length was parallel to the axis of the cylindrical cyclone
body, and the
lower width dimension was adjacent to the horizontal base closing the bottom
of the
cyclone body. The conditions of the feed gas flow rate, particulate level, and
average
particulate diameter were maintained within the ranges given in the
Comparative
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Examples. Again, studies were performed using underflow values of 1% and 3% by
volume. Also, the same performance parameters were evaluated and are given in
Table 2.
TABLE 2
UnderflowVane Exit Separationpanicles d50 Value
Example Gas Gas VelocityEfficiency
(vol-%) (m/sec) (%) <10 m (%) (m)
8 1 39.7 88.7 91.6 5.1
9 1 24.1 85.0 89.5 6.2
3 40.2 89.9 71.5 4.4
11 3 40.2 90.1 87.7 5.0
12 3 24.6 87.7 87.8 5.7
[0036] From the above test results, it is evident that the cyclone of the
present
5 invention, when compared to the open-bottom "uniflow" cyclone of the prior
art,
provides greater efficiency of solid particulate removal at both the 1% and 3%
underflow
conditions. This is illustrated graphically in FIG. 5. Furthermore, the
cyclone separator of
the present invention is superior for removing particulates of 4-5 microns in
diameter,
which are relevant for the overall improvement of FCC third stage separator
designs. The
10 increased ability of the present invention cyclone separator to separate
small particulates,
based on its d50 performance parameter, is illustrated in FIG. 6. Lastly, in
contrast to the
results in the Comparative Examples for cyclone separators of the prior art,
the cyclone
separator of the present invention consistently achieved a clean (overflow)
gas solids
contamination level of less than 50 mg/Nm3, in compliance with current and
even
potential future legislation.
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