Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
,
DOCKET NO.: NS-580
FLUID COKING USING HIGH THRUST FEED NOZZLES
FIELD OF THE INVENTION
This invention relates to a fluid coking process for converting a heavy
hydrocarbonaceous feedstock to liquid products which uses high thrust feed
nozzles
for injecting feedstock into the circulating fluidized bed of heated coke
particles.
BACKGROUND OF THE INVENTION
Fluidized bed coking (fluid coking) is a petroleum refining process in which
heavy petroleum feeds, typically the non-distillable residue (resid) from
fractionation
or heavy oils are converted to lighter, more useful products by thermal
decomposition (coking) at elevated reaction temperatures, typically about 480
to 590
C., (about 900 to 1100 F) and in most cases from 500 to 550 C (about 930 to
1020 F). Heavy oils that may be processed by the fluid coking process include
heavy atmospheric resids, vacuum resids, aromatic extracts, asphalts, and
bitumen
from oil sands.
The process is carried out in a unit with a large reactor vessel containing
hot
coke particles that are maintained in the fluidized condition at the required
reaction
temperature with a fluidizing gas (e.g., steam) injected at the bottom of the
vessel.
The heavy oil feed is heated to a pumpable temperature, typically in the range
of
350 to 400 C (about 660 to 750 F), mixed with atomizing steam, and fed
through
multiple feed nozzles arranged at several successive levels in the reactor.
The
steam is injected into a stripper section at the bottom of the reactor and
passes
upwards through the coke particles in the stripper as they descend from the
main
part of the reactor above. The feed liquid coats the coke particles in the
fluidized
bed, which make up the emulsion phase of the fluidized bed. As the thermal
cracking reactions proceed, the liquid is transformed to vapour, which must
migrate
from the emulsion phase into the bubble phase in order to exit the system.
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. ,
Liquid yields in fluid coking can be increased by reducing the reaction
severity, or the time that molecules are exposed to process temperature. The
typical
approach taken to reduce reactor severity is to reduce reactor temperature.
However, the downside of reducing temperature is increased stripper and sore
thumb fouling, which can lead to reduced run lengths. Another approach to
reduce
reactor severity is to decrease the exposure time at high temperatures by
providing
short vapour phase residence times.
Long hydrocarbon vapour residence times are the most likely contributor to
higher than expected "gas make", defined as C4- components, in the fluid
coking
process. Vapour-liquid equilibrium suppression, coupled with less than
adequate
mass transfer between the emulsion and bubble phases, is the most probable
mechanism responsible for high "coke make", defined as the toluene insoluble
solid
by-product of the thermal cracking reaction. Both phenomena result in lower
liquid
yields, and preliminary estimates suggest that they can contribute to as much
as 11
wt% liquid yield loss. Optimizing the rate of removal of vapour from the
emulsion
phase should reduce the overall hydrocarbon vapour residence time of the
reactor,
increase liquid yields, and reduce gas make. It is estimated that a 3-5 wt%
liquid
yield increase can be achieved through maximizing vapour recovery from the
reactor
dense bed.
Technologies that increase mass transfer between the emulsion and bubble
phase and, thus, reduce the gas phase residence time and increase hydrocarbon
vapour stripping, are required.
SUMMARY OF THE INVENTION
It has been discovered that the reactor section of a fluid coker is comprised
of
a dilute, upward-flowing stream of gas in the central (core) region of the
reactor and
a dense, downward-flowing, outer (annular) region of particles. This is due to
the
vaporized hydrocarbons rising primarily in the core. Thus, the core region has
a
high vapour and low solids concentration (solids lean) and the annular region
has a
low vapour and high solids concentration (solids dense).
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The present invention is directed to the use of high thrust feed nozzles to
transport unreacted hydrocarbon and coke present in the annular region of the
fluidized bed to the high velocity core region of the fluidized bed to improve
hydrocarbon stripping, reduce the gas phase residence time, and increase
liquid
yields. Thrust is a mechanical force that is generated through the act of
accelerating
a mass of fluid. In other words, it is the reaction force created by the
ejection of fluid
from a nozzle at high velocity. The fluid pressure is related to the momentum
of the
fluid and acts perpendicular to any imposed boundary, which in this case is
the
fluidized solids in the reactor. The amount of thrust generated depends on the
mass
flow rate and the exit velocity of the fluid. High thrust can be achieved by
either
slightly accelerating a large mass of fluid, or greatly accelerating a small
mass of
fluid.
Prior art nozzles that are presently used in fluid cokers have a limited
ability to
transfer solids from the annular region of the fluidized bed to the upward
flowing core
of the fluidized bed. Thus, a process is provided herein for converting a
heavy
hydrocarbonaceous feedstock to liquid products, comprising:
= introducing the hydrocarbonaceous feedstock into a fluid coker comprised
in
part of a fluidized bed of heated coke particles, the fluidized bed having a
high
velocity core region of heated coke particles and a low velocity annular
region
of unreacted hydrocarbon and coke particles; and
= reacting the hydrocarbonaceous feedstock with the heated coke particles
in
the fluid coker to produce the liquid products;
the feedstock being introduced into the fluid coker using a plurality of high
thrust
nozzles, said nozzles designed to transport unreacted hydrocarbon and coke
from
the low velocity annular region to the high velocity core region to improve
hydrocarbon stripping, reduce gas phase residence time, and increase liquid
products yields.
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In one embodiment, the high thrust nozzles have a spray angle of about 3 -
1600 and a nozzle diameter between about 0.2 and 0.8".
BRIEF DESCRIPTION OF THE DRAWINGS
In the accompanying drawings:
FIG. 1 is a simplified diagram of the reactor of a fluid coking unit useful in
the
present invention.
FIG. 2 shows the local pressure profiles along the length of a GEN2 feed
nozzle when air and water are sprayed into ambient air, at air to liquid
rations (ALRs)
of 2.1 and 1.6 wt%.
FIG. 3 shows a drawing of a GEN3 feed nozzle having a diverging cloverleaf
disperser.
FIG. 4 shows a drawing of a GEN4 feed nozzle, which consists of the same
internal geometry as the GEN2 nozzle, but with slits at the nozzle tip.
FIG. 5 is a graph comparing measured axial thrust force (lb) with nozzle
pressure (psig) for a variety of nozzles when spraying water only.
FIG. 6 shows the local pressure profile along the length of a Diffuser nozzle
having a diverging/diffuser section when spraying air and water into ambient
air at
ALRs of 2.6 and 1.7 wt%.
FIGS. 7A and 7B show the differences in the jet plume for air-water mixtures
exiting the GEN2 nozzle and the Diffuser nozzle, respectively.
FIG. 8 shows a drawing of the GEN1 nozzle, which consists of a simple
constriction of 7 followed by a 3" long straight section.
FIG. 9 is a graph of the measured axial thrust force (lb) as a function of the
measured nozzle pressure for a variety of nozzles when spraying air and water.
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,
FIG. 10 shows the local axial pressure profile along the length of a GEN2
nozzle with an additional diverging section at the exit when spraying air and
water
into ambient air at an ALR of 2.2 wt%.
DETAILED DESCRIPTION
The detailed description set forth below in connection with the appended
drawings is intended as a description of various embodiments of the present
invention and is not intended to represent the only embodiments contemplated
by
the inventor. The detailed description includes specific details for the
purpose of
providing a comprehensive understanding of the present invention. However, it
will
be apparent to those skilled in the art that the present invention may be
practiced
without these specific details.
The present invention is directed to the use of high thrust feed nozzles in a
fluidized coking operation to push unreacted hydrocarbon and coke to the high
velocity core region of the fluidized bed to improve hydrocarbon stripping,
reduce the
gas phase residence time, and increase liquid yields.
Thrust is a mechanical force that is generated through the act of accelerating
a mass of fluid. In other words, it is the reaction force created by the
ejection of fluid
from a nozzle at high velocity (John and Keith, 2006). The fluid pressure is
related
to the momentum of the fluid and acts perpendicular to any imposed boundary,
which in this case is the fluidized solids in the reactor. The amount of
thrust
generated depends on the mass flow rate and the exit velocity of the fluid.
High
thrust can be achieved by either slightly accelerating a large mass of fluid,
or greatly
accelerating a small mass of fluid.
There are three main factors that affect thrust: friction effects, axial
momentum loss and thrust loss due to the pressure difference between the
nozzle
exit plane and the background. When friction is considered, it is best to have
nozzles with large exit angles. However, the axial momentum losses increase as
the angle increases since a higher percentage of the exiting flow will be non-
axial.
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Cruz (Cruz, N., "Interactions between Supersonic Gas Jets and Gas-Solid
Fluidized Beds", MSc Thesis, The University of Western Ontario, 2009) found
that
the thrust force of gas in a convergent-divergent nozzle used to attrition
particles in a
fluidized bed had a strong relationship with the particle grinding efficiency.
During
the attrition process, particles are entrained into the gas jet and are
accelerated to
high velocity where they collide with the particles in the dense phase of the
fluidized
bed at the tip of the jet plume and cause particle breakage to occur. This
concept
can be used to enhance the movement of solids from the annular section of the
reactor to the core region in the feed zone by using feed nozzles that produce
a high
thrust force.
FIG. 1 is a simplified diagram of the reactor of a fluid coking unit. The
reactor coking zone 10 contains a fluidized bed 11 of heated seed coke
particles,
heated to a temperature sufficient to initiate the coking (thermal cracking)
reactions,
into which the feedstock is added. The feedstock contacts the coke particles
and
reacts, and deposits a fresh coke layer on the hot fluidized coke particles
circulating
in the bed. The fluidized bed of coke comprises a dense bed surface 34, which
is
static, a dilute core region 32, which is upward flowing, and a dense annular
region
30, which is downward flowing.
The feed is injected through multiple high thrust nozzles located in feed
rings
12a to 12f, which are positioned so that the feed with atomizing steam enters
directly
into the fluidized bed of hot coke particles in coking zone 11. Each feed ring
consists of a set of high thrust nozzles (typically 10-20, not designated in
FIG. 1) that
are arranged around the circular periphery of the reactor wall at a given
elevation
with each nozzle in the ring connected to its own feed line which penetrates
the
vessel shell (i.e. 10-20 pipes extending into the fluid bed). These high
thrust feed
nozzles are typically arranged non-symmetrically around the reactor to
optimize flow
patterns in the reactor according to simulation studies although symmetrical
disposition of the nozzles is not precluded if the flow patterns in the
reactor can be
optimized in this way. There are typically 4-6 feed rings located at different
elevations although not all may be active at any one time while the unit is
working.
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Steam is admitted as fluidizing gas in the stripping section 13 at the base of
coker reactor 10, through spargers 14 directly under stripping sheds 15 as
well as
from lower inlets 16. The steam passes up into stripping zone 13 of the coking
reactor in an amount sufficient to obtain a superficial fluidizing velocity in
the coking
zone, typically in the range of about 0.15 to 1.5 m/sec (about 0.5 to 5
ft/sec). The
coking zone is typically maintained at temperatures in the range of 450 to
650° C. (about 840 to 1200° F.) and a pressure in the range of
about
0 to 1000 kPag (about 0 to 145 psig), preferably about 30 to 300 kPag (about 5
to 45
psig), resulting in the characteristic conversion products which include a
vapor
fraction and coke which is deposited on the surface of the seed coke
particles.
The vaporous products of the cracking reactions with entrained coke particles
pass upwards out of the reaction zone 11, through a phase transition zone in
the
upper portion 17 of the vessel and finally, a dilute phase reaction zone at
the inlets
of cyclones 20 (only two shown, one indicated). The coke particles separated
from
the vaporous coking products in the cyclones are returned to the fluidized bed
of
coke particles through cyclone dipleg(s) 21 while the vapors pass out through
the
gas outlet(s) 22 of the cyclones into the scrubbing section of the reactor
(not shown).
After passing through scrubbing section which is fitted with scrubbing sheds
in which
the ascending vapors are directly contacted with a flow of fresh feed to
condense
higher boiling hydrocarbons in the reactor effluent (typically 525 C+/975
F+) and
recycles these along with the fresh feed to the reactor. The vapors leaving
the
scrubber then pass to a product fractionator (not shown). In the product
fractionator,
the conversion products are fractionated into light streams such as naphtha,
intermediate boiling streams such as light gas oils and heavy streams
including
product bottoms.
The coke particles that pass downwards from the dense bed 11 to stripper
section 13 comprising sheds 15 are partially stripped of occluded hydrocarbons
in
the stripper by use of a stripping gas, usually steam, which enters via
spargers 14.
The stripped coke particles are passed via line 25 to a heater (not shown)
which is
operated a temperature from about 40 to 200 C, preferably about 65 to 175
C., and
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more preferably about 65 to 120 C in excess of the actual operating
temperature of
the coking zone and recycled back to the fluid coking unit.
Example 1Current commercial fluid coking feed nozzles are designed to
atomize the bitumen at the nozzle exit through shear from the high velocity
and rapid
decompression of the atomization steam upon exiting the nozzle. This
decompression happens both axially and radially.
One such coker nozzle is described in detail in Canadian Patent No.
2,224,615, and is referred to herein as TEBM-2b with circular exit, or GEN2
nozzle.
The GEN2 nozzle consists of a series of converging, diverging, and converging
sections. The pressure drop across the exit of the GEN2 coker feed nozzle is
on the
order of 70 psi. The flow exiting the nozzle consists of bubbles dispersed in
the
liquid phase and the large decompression from the resultant pressure drop at
the
exit causes an explosive expansion of the bubbles, resulting in a phase
inversion
where the flow changes from liquid continuous in the nozzle to gas continuous
in the
jet, with liquid droplets and ligaments distributed in the gas stream.
FIG. 2 shows the local pressure profiles along the length of a GEN2 feed
nozzle when air and water are sprayed into ambient air, at air to liquid
ratios (ALRs)
of 2.1 and 1.6 wt%. The increase in pressure along the diverging section of
the
nozzle is consistent with subsonic flow. FIG. 2 shows a significant pressure
drop at
the exit of the nozzle, which causes the gas to expand rapidly in both the
axial and
radial direction.
In this example, the GEN2 nozzle and the 1.25GEN2, which is the same as
the GEN2 nozzle except all of the dimensions are scaled up so that the throat
area
is 25% larger than the GEN2 nozzle, were tested in order to measured axial
thrust
force (lb) as a function of the nozzle pressure. In addition to the GEN2
nozzles,
three commercially available fan spray nozzles, referred to herein as Nozzle
B,
Nozzle C and Nozzle D, and a curved throat fan nozzle used in the FCC process,
described in detail in U.S. Patent No. 6,199,768, referred to herein as CTF,
were
tested in this example. A GEN3 nozzle, which consists of the same internal
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=
geometry as the GEN2 nozzle but contains a diverging cloverleaf disperser at
the tip
of the nozzle, was also tested. A drawing of a GEN3 nozzle is shown in FIG. 3
and
described in more detail in U.S. Patent No. 8,999,246. 1.1GEN3 and 1.25GEN3
nozzles are the same as the GEN3 nozzle, but all of the dimensions are scaled
up
so that the throat area is 10% and 25% larger than the GEN3 nozzle,
respectively.
Finally, FIG. 4 shows a drawing of a GEN4 nozzle, which consists of the same
internal geometry as the GEN2 nozzle but with slits at the nozzle tip. The GEN
4
nozzle is described in more detail in U.S. Patent No. 9,889,420. The 1.1GEN4
is the
same as the GEN4 nozzle, but all of the dimensions are scaled up so that the
throat
area is 10% larger than the GEN4 nozzle.
Experiments were conducted with the aforementioned feed nozzles having
different equivalent throat diameters and exit angles by spraying water into
open air
over a range of liquid flow rates and nozzle pressures. Table 1 shows a
summary of
the nozzles that were tested and their specifications. The nozzles were
mounted on
a stand that allowed them to move freely in the axial direction. The reaction
thrust
force was measured using a 3000 lb thru-hole compression load cell, which was
mounted on the nozzle conduit and was compressed between two plates while the
nozzle was spraying.
Table 1: Summary of Nozzle Specifications Tested with Water
Nozzle Description Throat Spray
Diameter Angle
(inches)
(0)
Nozzle B Fan spray nozzle 0.344 50
Nozzle C Fan spray nozzle 0.297 65
Nozzle D Fan spray nozzle 0.297
120
GEN2 TEBm-2b* with circular exit 0.512 11
GEN3 TEBm-2b* with cloverleaf 0.512 23
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=
disperser
1.1GEN3 TEBm-2b* with cloverleaf 0.537 23
disperser
1.25GEN3 TEBm-2b* with cloverleaf 0.572 23
disperser
CTF Curved throat fan nozzle 0.27 40
1.1GEN4 TEBm-2b* with four slits in the 0.557 50
exit
*Base et al. (1999)
FIG. 5 shows a plot of the measured axial thrust force (lb) as a function of
the
nozzle pressure for the various nozzle geometries. The axial thrust force was
highly
correlated with the nozzle pressure. FIG. 5 shows that nozzles that provide
the
same nozzle pressure, with the same exit area, operating at the same liquid
flow
rate, result in different axial thrust measurements due to the difference in
nozzle
geometry. For example, Nozzle C and Nozzle D both have an orifice diameter of
0.297", however, Nozzle C produces a spray with an angle of 65 , whereas
Nozzle D
produces a spray with an angle of 120 and when operating at a nozzle pressure
of
400 psig. The measured axial thrust force of Nozzle C was approximately 10Ib
greater than the measured axial thrust force of Nozzle D. The larger spray
angle
resulted in a smaller axial thrust force as more of the thrust force was
directed in the
radial direction. Nozzles B-D are BETE nozzles designed to be operated with
liquid
only and, therefore, produced higher thrust forces than the GEN2, GEN3, GEN4
and
CTF nozzles, which are designed to be operated with both gas and liquid.
However,
even when operating with only liquid, noticeable differences in the thrust
force were
observed with changes in nozzle geometry. For example, the GEN2 and GEN3
nozzles have the same exit orifice diameter, and when operating at a fluid
pressure
of 157 psig the GEN3 nozzle, which has a diverging nozzle exit, resulted in an
axial
thrust force that was approximately 7 lbs greater than the axial thrust force
of the
GEN2 nozzle that has a converging nozzle exit.
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In summary, the results in FIG. 5 show that the thrust force is highly
correlated with the fluid pressure upstream of the nozzle. In addition, nozzle
spray
angles and exit geometries can be optimized to produce higher thrust forces.
Example 2
In order to maximize the axial thrust force and reduce the expansion of the
jet
in the radial direction, a supersonic nozzle with a diverging/diffuser section
was
designed to accelerate the fluid axially in the nozzle exit, prior to
injection into the
fluidized bed (hereinafter referred to as the "Diffuser nozzle"). The Diffuser
nozzle
consists of the same internal geometry as the GEN2 nozzle but without the
final
constriction at the nozzle tip. The diffuser section now resulted in a much
narrower
jet plume. The phase inversion occurs within the nozzle, and the fluid
acceleration
through the nozzle will increase in the axial direction. In addition, a
supersonic
nozzle maximizes the velocity of the jet at a much larger cross sectional exit
area
compared to a subsonic nozzle.
FIG. 6 shows the local pressure profile along the length of a Diffuser nozzle
when spraying air and water into ambient air at ALRs of 2.6 and 1.7 wt%. The
pressure decrease along the diverging section of the nozzle indicates that the
flow is
supersonic. It can be seen that the fluid exits the nozzle at the same
pressure as
the ambient air, which reduces thrust loss and results in minimum radial
expansion.
FIGS. 7A and 7B are photographs showing the differences in the jet plume for
air-water mixtures exiting the GEN2 nozzle (FIG. 7A) and the Diffuser nozzle
(FIG.
7B). The images clearly show that the spray plume is much narrower with the
Diffuser nozzle, since all of the driving pressure has been used to accelerate
the
flow in the axial direction.
Experiments were conducted using feed nozzles with different equivalent
throat diameters and exit geometries by spraying air and water into open air
over a
range of liquid flow rates and nozzle pressures. Table 2 shows a summary of
the
nozzles that were tested and their specifications. The nozzles were mounted on
a
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stand that allowed them to move freely in the axial direction. The reaction
thrust
force was measured using a 3000 lb thru-hole compression load cell, which was
mounted on the nozzle conduit and was compressed between two plates while the
nozzle was spraying.
Table 2: Summary of Nozzle Specifications for Nozzles Tested with Air and
Water
Nozzle Description Throat Exit
Geometry
Diameter
(inches)
GEN1 Simple constriction with 0.512 Converging
with
long circular exit circular
exit
GEN2 TEBm-2b with circular exit 0.512 Converging
with
circular exit
GEN3 TEBm-2b with cloverleaf 0.512 Diverging
with
disperser cloverleaf
shaped
exit
1.25GEN2 TEBm-2b with circular exit 0.572 Converging
with
circular exit
1.25GEN3 TEBm-2b with cloverleaf 0.572 Diverging
with
disperser cloverleaf
shaped
exit
Diffuser Constriction followed by 0.516 Diverging
with
long diffuser circular
exit
1.2Diffuser Constriction followed by a 0.562 Diverging
with
long diffuser circular
exit
1.5Diffuser Constriction followed by a 0.628 Diverging
with
long diffuser circular
exit
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A drawing of a GEN1 nozzle is shown in FIG. 8 and consists of a simple
constriction of 7 followed by a 3" long straight section. The 1.2Diffuser
nozzle and
1.5Diffuser nozzle are the same as the Diffuser nozzle shown in FIG. 6, but
all of the
dimensions are scaled up so that the throat area is 20% and 50% larger than
the
Diffuser nozzle, respectively.
FIG. 9 shows a plot of the measured axial thrust force (lb) as a function of
the
measured fluid pressure. The axial thrust force was highly correlated with the
fluid
pressure. FIG. 9 shows that nozzles that provide the same fluid pressure, with
the
same throat diameter result in different axial thrust measurements due to the
differences in nozzle geometry. For example, the GEN1, GEN2 and GEN3 all have
the same throat diameter, however, at a nozzle pressure of 275 psig, the
measured
axial thrust force of the GEN3 nozzle was approximately 11 lb greater than the
GEN2 nozzle and 14 lb greater than the GEN1 nozzle. The diverging cloverleaf
disperser located on the tip of the GEN3 nozzle reduces the radial expansion
of the
fluid and results in a higher axial thrust force. The nozzles that provide the
largest
axial thrust force are the 1.5Diffuser and the1.2Diffuser. These nozzles have
a
larger throat and have a long diffuser section located at the end of the
nozzle, which
results in a decrease in pressure along the nozzle outlet, which means that
supersonic velocities are achieved. The axial thrust through these nozzles has
been
maximized since the pressure at the nozzle exit is equal to the ambient
pressure and
therefore, all of the driving pressure has been used to accelerate the flow in
the axial
direction.
Example 3
Another nozzle geometry that would maximize the axial thrust force would be
to add a diverging section to the GEN2 nozzle in order to accelerate the fluid
to
supersonic velocities before exiting the nozzle. FIG. 10 shows the local axial
pressure profile along the length of a GEN2 nozzle with an additional
diverging
section at the exit when spraying air and water into ambient air at an ALR of
2.2
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wt%. The pressure along the outlet diffusing section decreases, indicating
that
supersonic velocities were achieved. The axial thrust through this nozzle has
been
maximized since the pressure at the nozzle exit is equal to the ambient
pressure and
therefore all of the driving pressure has been used to accelerate the flow in
the axial
direction.
From the foregoing description, one skilled in the art can easily ascertain
the
essential characteristics of this invention, and without departing from the
spirit and
scope thereof, can make various changes and modifications of the invention to
adapt it to various usages and conditions. Thus, the present invention is not
intended to be limited to the embodiments shown herein, but is to be accorded
the
full scope consistent with the claims, wherein reference to an element in the
singular, such as by use of the article "a" or "an" is not intended to mean
"one and
only one" unless specifically so stated, but rather "one or more". All
structural and
functional equivalents to the elements of the various embodiments described
throughout the disclosure that are known or later come to be known to those of
ordinary skill in the art are intended to be encompassed by the elements of
the
claims. Moreover, nothing disclosed herein is intended to be dedicated to the
public
regardless of whether such disclosure is explicitly recited in the claims.
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