Note: Descriptions are shown in the official language in which they were submitted.
NS-355
RECOVERY OF HYDROCARBON DILUENT FROM TAILINGS
FIELD OF THE INVENTION
The present invention relates to a method for recovery of a hydrocarbon
diluent from
a slurry or tailings such as froth treatment tailings produced in a bitumen
froth
treatment plant. More particularly, hydrocarbon diluent is removed from the
tailings
in a high pressure stripping vessel that is operated at above-atmospheric
pressure.
BACKGROUND OF THE INVENTION
Oil sand, as known in the Fort McMurray region of Alberta, Canada, comprises
water-wet sand grains having viscous bitumen flecks trapped between the
grains.
The oil sand lends itself to separating or dispersing the bitumen from the
sand grains
by slurrying the as-mined oil sand in water so that the bitumen flecks move
into the
aqueous phase.
=
For the past 25 years, the bitumen in McMurray oil sand has been commercially
recovered using a hot/warm water process. In general, the process involves
slurrying oil sand with heated water, optionally, a process aid such as
caustic
(NaOH) and naturally entrained air. The slurry is mixed, commonly in tumblers,
for a
prescribed retention time to initiate a preliminary separation or dispersal of
the
bitumen and the solids and to induce air bubbles to contact and aerate the
bitumen.
The conditioned slurry is then subjected to flotation to further separate the
bitumen
from the sand.
A recent development in the recovery of bitumen from oil sand involves a low
temperature process whereby the oil sand is mixed with heated water directly
at the
mine site to produce a pumpable, dense, low temperature slurry. The slurry is
then
pumped through a pipeline to condition the slurry for flotation. It is
understood,
however, that other bitumen extraction processes exist, each producing
conditioned
oil sand slurry.
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Conditioned oil sand slurry may be further diluted with flood water and
introduced
into a large, open-topped, conical-bottomed, cylindrical vessel (termed a
primary
separation vessel or "PSV"). The diluted slurry is retained in the PSV under
quiescent conditions for a prescribed retention period. During this period,
the
aerated bitumen rises and forms a froth layer, which overflows the top lip of
the
vessel and is conveyed away in a launder. The sand grains sink and are
concentrated in the conical bottom, They leave the bottom of the vessel as a
wet
tailings stream. Middlings, a watery mixture containing solids and bitumen,
extend
between the froth and sand layers.
The wet tailings and middlings are withdrawn and may be combined for further
processing in a secondary flotation process. This secondary flotation process
is
commonly carried out In a deep cone vessel wherein air is sparged into the
vessel to
assist with flotation. This vessel is referred to as the TOR vessel. It and
the process
conducted in it are disclosed in U.S. Pat, No. 4,545,892. The bitumen
recovered by
the TOR vessel is recycled to the PSV. The middlings from the deep cone vessel
are further processed in air flotation cells to recover contained bitumen.
The froths produced by these units are generally combined and subjected to
further
processing. More particularly, it is conventional to dilute the bitumen froth
with a
light hydrocarbon diluent, such as naphtha or a paraffinic diluent, to first
improve the
difference in specific gravity between the bitumen and water and to reduce the
bitumen viscosity, to aid in the separation of the water and solids from the
bitumen,
Separation of the bitumen from water and solids is commonly achieved by
treating
the diluent diluted froth in a sequence of inclined plate settlers, scroll and
disc
centrifuges, and the like. Other processes for separating solids and water
from
diluted bitumen froth are known in the art and include stationary froth
treatment
(SFT) as described in U.S. Patent No. 6,746,599,
The primarily water and solids fraction obtained after separation is commonly
referred to as froth treatment tailings. Paraffinic froth treatment tailings
typically
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comprise water, asphaltenes, fines solids, bitumen and about 5010 wt% residual
paraffinic solvent. Naphthenic froth tailings typically comprise water, fines
solids,
residual bitumen and about 2-4 wt% naphtha. It is desirable both economically
and
environmentally to recover the hydrocarbon diluent from the tailings prior to
disposal
of the tailings. However, the unique nature of the diluent-containing tailings
makes
diluent removal a challenge to the industry. In particular, it is believed
that some of
the diluent is intimately associated with the solids, making diluent removal
from the
solids more difficult.
Canadian Patent No. 1,027,501 discloses a process for treatment of centrifuge
tailings to recover naphtha. The process comprises introducing the tailings
into a
vacuum flash vessel maintained at vacuum conditions (e.g., about 35 kPa) in
order
to flash the naphtha present in the tailings. The vessel is also equipped with
a
plurality of shed decks so that any residual naphtha remaining in the tailings
stream
will be vaporized by the introduction of steam beneath these shed decks. In
practice, however, this process results in only 60 to 65% recovery of the
diluent, as
the vacuum at the tailings feed inlet of the vessel may have resulted in the
tailings
bypassing the shed decks and pooling near the bottom of the vessel. In the
alternative, or additionally, the reduction in pressure in the tower to below
atmospheric resulted in steam condensation and reduced heat transfer to the
slurry.
Thus, the pooled tailings at the bottom of the vessel still contained a
substantially
large amount of diluent. Canadian Patent No. 2,272,035 partially addressed
this
issue by introducing the steam into the tailings pool for vaporizing the
residual
diluent pooling near the bottom of the vessel.
Canadian Patent No. 2,272,045 discloses a method for recovery of hydrocarbon
diluent from tailings produced in a bitumen froth treatment plant comprising
introducing the tailings into a steam stripping vessel maintained at near
atmospheric
pressure (e.g. around 95 kPa) in an attempt to avoid the problem of the
tailings
bypassing the shed decks. Without a vacuum, vessel pressure increased to
atmospheric, or slightly above, and temperature increased to around 100 C.
This
resulted in increased steam to slurry heat transfer, greater steam flowrate to
the
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condenser and consequently increased naphtha recovery. The
operating
temperature of the vessel was preferably maintained at approximately 100 C.
However, while operating a steam stripping vessel for recovery of hydrocarbon
diluent from tailings produced in a bitumen froth treatment plant at about 100
C and
at near atmospheric pressure significantly improved diluent recovery over
previous
operations at below atmospheric pressure, there still was a substantial amount
of
diluent remaining in the tailings pool. As stated in Canadian Patent No.
2,272,045,
operating the vessel at near atmospheric pressure and at a steam to tailings
ratio of
approximately 9.0 wt. % increased the naphtha recovery to only about 80%.
SUMMARY OF THE INVENTION
In one aspect of the present invention, a method for recovering hydrocarbon
diluent
from tailings is provided comprising introducing the hydrocarbon diluent
containing
tailings (feed tailings) into a steam stripping vessel operating at above-
atmospheric
pressure (hereinafter referred to as a "high pressure stripping vessel"). In
one
embodiment, the high pressure stripping vessel is operated at a pressure of
between
100-200 kPa. Because of the high pressure conditions, the temperature in the
high
pressure stripping vessel is generally above 100 C, thereby producing high
temperature or hot tailings, which pool at the bottom of the high pressure
stripping
vessel. As used herein, "high temperature tailings" or "hot tailings" mean
tailings
produced in the tailings pool of a high pressure stripping vessel which have a
temperature that is higher than the temperature of the feed tailings. In
general, the
high temperature tailings will have a temperature of between about 100 C to
about
120 C. In one embodiment, the stripping gas used in the high pressure
stripping
vessel is steam. In one embodiment, a portion of the tailings pool formed in
the high
pressure stripping vessel is recycled back to the stripping vessel.
In another aspect of the present invention, both a high pressure stripping
vessel,
operating at a pressure of between 100-200 kPa, and a low pressure flash
vessel
operating at a pressure below 100 kPa, are used to recover hydrocarbon diluent
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from hydrocarbon diluent containing tailings. Thus,
a method for recovering
hydrocarbon diluent from tailings is provided, comprising:
= introducing the tailings into a high pressure stripping vessel operating
at a
pressure greater than 100 kPa such that a stripped tailings pool is formed at
the bottom of the high pressure stripping vessel and hydrocarbon diluent and
water vapors are formed and released from the top of the high pressure
stripping vessel;
= introducing steam into the high pressure stripping vessel either above
the
stripped tailings pool or into the stripped tailings pool or both; and
= removing a portion of high temperature stripped tailings from the stripped
tailings pool and introducing the portion of high temperature stripped
tailings
to a low pressure flash vessel operating at a pressure below 100 kPa to
remove additional hydrocarbon diluent from the portion of high temperature
stripped tailings.
In one embodiment, the high pressure stripping vessel comprises a stack of
internal,
vertically and laterally spaced shed decks and the tailings are introduced
into the
vessel such that the tailings are distributed over at least some of the shed
decks. In
this embodiment, the steam is introduced below the shed decks but above the
stripped tailings pool. In one embodiment, the steam is introduced into the
stripped
tailings pool. In one embodiment, steam is introduced both below the shed
decks
but above the stripped tailings pool and into the tailings pool. In another
embodiment
the shed decks are arranged in vertical sections with a portion of the feed
and/or
steam introduced in between sections.
In one embodiment, the tailings are introduced into the stripping vessel by
injecting
the tailings into the stripped tailings pool. In another embodiment, tailings
are
introduced into the stripped tailings pool, in between a stack of shed decks
and
directly above a stack of shed decks.
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In one embodiment, a portion of the stripped tailings pool is recycled back to
the
high pressure stripping vessel.
In one embodiment, the hydrocarbon diluent containing tailings are first
separated
into a fine solids tailings slurry and a coarse solids tailings slurry. As
used herein,
"fine solids" or "fines" refers generally to clays and silts having a particle
size
(diameter) of less than 44 microns. As used herein, "coarse solids" refers
generally
to sand having a particle size (diameter) greater than 44 microns.
BRIEF DESCRIPTION OF THE DRAWINGS
Referring to the drawings wherein like reference numerals indicate similar
parts
throughout the several views, several aspects of the present invention are
illustrated
by way of example, and not by way of limitation, in detail in the figures,
wherein:
FIGURE 1 is a schematic showing one embodiment of a hydrocarbon diluent
extraction circuit useful in the present invention.
FIGURES 2A and 2B are schematics showing another embodiment of a
hydrocarbon diluent extraction circuit useful in the present invention.
FIGURE 3 is a schematic of the continuous flash evaporation pilot used in the
experiments described below.
FIGURE 4 is a graph showing the wt.% naphtha concentration in tailings after
flashing as a function of flash temperature drop (AT) in C.
FIGURE 5 a graph showing the wt,% naphtha concentration in tailings after
flashing
as a function of temperature of the tailings before flashing (Ti) in C.
DESCRIPTION OF THE PREFERRED EMBODIMENT
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
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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.
One embodiment of the present method for hydrocarbon diluent recovery from
froth
treatment tailings can be best described with reference to FIG. 1. Froth
treatment
tailings 10 are initially housed in feed tank 12 where additional water 14 may
or may
not be added. Optionally, process additives 16a, 16b can be added to the
tailings
either into the feed tank 12 or to the stream of tailings 18 which is removed
from the
feed tank 12, respectively, for further processing. Additives may include
surfactants,
defoaming agents, emulsifiers and the like which are added to break up any
hydrocarbon/diluent lumps which may be present in the tailings. For example,
when
naphtha is used as the hydrocarbon diluent, the naphtha may mix with the
residual
hydrocarbon present in the tailings and form hydrocarbon/naphtha lumps.
Tailings stream 18 is then, optionally, pumped through a high shear pump 20,
where
further break-up of hydrocarbon/diluent lumps may occur. The sheared tailings
22
are then fed into a high pressure steam stripping vessel 30 at various
locations, i.e.,
as tailings streams 22a, 22b and/or 22c, which is discussed in more detail
below.
The high pressure stripping vessel 30 is maintained at a pressure above
atmospheric pressure, preferably, between about 100 kPa to about 200 kPa,
however, it is understood that higher than 200 kPa could be used.
In one embodiment, sheared tailings stream 22c is introduced into vessel 30
via a
feed box distributor 28 having a plurality of openings and which is located
near the
top of the vessel 30. Directly below the distributor 28 is a series of shed
decks 32.
The distributor 28 functions to evenly distribute the feed (i.e. tailings)
over the series
of shed decks 32. The shed decks 32 ensure that the tailings are spread over a
large surface area that can subsequently be exposed to steam. Shed decks are
inherently less efficient in mass transfer between the gas and liquid phases
than
other types of internals typically used in stripping vessels such as sieve
trays,
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packed beds, etc. However, due to the high concentration of coarse solids
present in
the feed slurry, the more open and, therefore, less fouling structure of shed
decks is
required.
Directly below the shed decks 32 is a steam ring 34 having a plurality of
openings for
the release of steam. The steam countercurrently contacts the tailings
distributed
over the shed decks 32 and provides heat for vaporizing the hydrocarbon
diluent
and a portion of the water contained in the tailings. In one embodiment, a
second
steam ring 35 can be positioned between the stacks of shed decks 32 to ensure
that
the tailings on the uppers are sufficiently contacted with steam as well. The
diluent-
stripped feed settles to the bottom of the vessel and forms a stripped
tailings pool
36.
The vaporized diluent and water is removed from vessel 30 as vapor stream 38
and
is then introduced into a knock-out pot 40, which is a vapor-liquid separator.
The
vapor product 52, which comprises primarily hydrocarbon diluent, can then be
passed through a condenser-cooler 50, where it is cooled and forms liquid
product
54. The liquid product 42 from knock-out pot 40, which comprises primarily
water,
and the liquid product 54 are combined and introduced into decanter 44, where
water 46 and diluent 48 are separated, for example, by using a weir 45 such
that the
naphtha overflows into a separate compartment. The liquid product from the
knockout pot 40 may contain entrained solids and may alternatively sent back
to the
feed section of the high pressure stripping vessel via conduit 42b or be sent
straight
to the decanter 44 via conduit 42a. Diluent 48 produced in decanter 44 can be
reused and water 46 can be recycled back to the feed tank 12.
In one embodiment, sheared tailings stream 22b can be introduced into vessel
30
via a second feed box distributor 56 having a plurality of openings and
located near
the middle of the vessel 30. Directly below the distributor 28 are a portion
of shed
decks 32. The distributor 56 also functions to evenly distribute the feed
(i.e. tailings)
over the portion of shed decks 32 below distributor 56. As previously
mentioned, the
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shed decks 32 ensure that the tailings are spread over a large surface area
that can
subsequently be exposed to steam.
In one embodiment, sheared tailings stream 22a can be introduced directly into
the
stripped tailings pool 36. In this embodiment, steam 58 is also injected
directly into
the stripped tailings pool 36. It is understood that sheared tailings 22 can
be
introduced in either one, two or three injection sites, i.e., as stream 22a,
22b, and/or
22c and steam can be injected either directly below the sheds 32 or into the
stripped
tailings pool 36 or both, and/or between shed deck sections.
In one embodiment, a portion of the stripped tailings pool 36 can be removed
as
tailings stream 62 and, optionally, sheared in high shear pump 64 to form
tailings
stream 66. Tailings stream 66 is then introduced into a second vessel which is
a low
pressure flash vessel 70, operating below atmospheric pressure, e.g., below
about
100 kPa. The portion of the stripped tailings pool, i.e., tailings stream 62,
may still
have a significant amount of hydrocarbon diluent associated therewith (in
addition to
water) and by introducing these hot tailings into a low pressure flash vessel,
additional hydrocarbon diluent and water can be removed (flashed) from the
tailings
as vaporized hydrocarbon diluent and water.
A throttling valve 68 (or an orifice) is needed at the low pressure flash
vessel inlet to
maintain elevated pressure in the feed pipe in order to prevent flash
evaporation in
the pipe and to control pressure drop at varying tailings feed rates.
The vaporized hydrocarbon diluent (and water) is removed from the top of low
pressure flash vessel 70 as stream 76. Stream 76 is passed through condenser-
cooler 50, where it is cooled and forms liquid product 84 and vapor product
85. The
vapor product 85 is then introduced into knock-out pot 80, where liquid
diluent/water
is separated to form liquid stream 82. Liquid stream 82 from knock-out pot 80
and
the liquid product 84 are combined and introduced into decanter 86, where
water 88
and diluent 90 are separated, i.e., the water settles to the bottom of
decanter 86 and
the diluent floats to the top of decanter 86. Diluent 90 can be reused and
water 88
can be recycled back to the feed tank 12. Gases from the knock-out pot 80 are
sent
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to a vacuum pump 83 which draws gases at a sufficient rate to keep vessel 70
under
the desired low pressure.
Cleaned tailings 74, which form a pool at the bottom of low pressure flash
vessel 70,
are removed from the bottom of low pressure flash vessel 70 as tailings stream
75
and, optionally, quenched with quench water 92 before being pumping via pump
94
to a designated disposal site 96.
In one embodiment, the portion of the stripped tailings pool 36 removed as
tailings
stream 62 and, optionally, sheared in high shear pump 64 can be recycled as
stream
60 and introduced in either one, two or three injection sites, i.e., as stream
22a, 22b,
and/or 22c for further stripping with steam. In a preferred embodiment,
tailings
stream 60 is returned to high pressure stripping vessel 30 via distributor 28
for
further contact with sheds 32 and subjected to further steam stripping.
It is understood that additional elements can be present in both high pressure
stripping vessel 30 and low pressure flash vessel 70, for example, each vessel
can
further comprise a demister 95, 97, respectively, located at or near the top
of the
vessels. Demisters 95, 97 will remove suspended slurry droplets from the
vaporized
hydrocarbon diluent and water.
Because the high pressure stripping vessel is operated at a pressure above
atmospheric pressure, the tailings in the stripped tailings pool have a much
higher
temperature than the feed tailings (referred to herein as "hot tailings",
i.e., tailings
having a temperature of greater than about 100 C). Thus, when hot tailings
(Ti
above 100 C) having an elevated pressure (P, greater than 100 kPa) are
delivered
to a low pressure flash vessel, which is operating at a lower temperature (T2
less
than 100 C) and a lower pressure (P2 less than 100 kPa), the residual
hydrocarbon
diluent in the tailings will vaporize (flash) in the low pressure flash
vessel. The rate
of flash evaporation was found to be directly related to the temperature drop,
AT,
where AT = Ii ¨ T2. It was further discovered that hydrocarbon
diluent
concentrations in the vapor phase in the low pressure flash vessel was related
to the
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equilibrium conditions at T2/P2 and the hydrocarbon diluent concentration in
feed
tailings.
Figures 2A and 2B show another embodiment of the present invention. In this
embodiment, the solvent containing tailings slurry from the Froth Treatment
Unit
(FTU) can be separated into two slurries; a coarse tailings slurry 210a and a
fine
solids slurry 210b. This separation can be achieved outside the FTU by means
of
additional equipment such as hydrocyclones or centrifuges or the coarse and
fine
slurry streams can be piped separately from the FTU, where they are typically
generated separately without mixing into each other. Each stream (210a and
210b)
is then individually treated in a high pressure stripping vessel as shown in
Figures
2A and 2B.
Because the slurry with fine solids, 210b, is inherently less fouling, the
internals 224
of the high pressure stripping vessel treating this stream, vessel 230b, can
be
chosen to maximize mass transfer. Thus, the high vessel pressure vessel 230a
treating the slurry with coarser particles, 210a, comprises internals 226
which have
more open area for slurry and vapors to flow and are less sensitive to
fouling, such
as shed decks. On the other hand, the high pressure stripping vessel 230b
treating
the slurry with finer or less concentration of solids 210b can use internals
224 which
have a more restricted flow area and, thus, the transfer of solvent between
slurry
and vapor phases is more efficient, such as trays or packings.
The pressure and temperature conditions will typically be kept the same in
both
vessels 230a and 230b by using the same amount of steam to feed slurry mass
flow
rate, however, it is understood that each vessel could be individually
maintained at
different temperature and pressure conditions, if so required. The hot, high
pressure
tailings 262a and 262b from the two columns may be combined and the combined
hot tailings 266 can be fed into a common low pressure vessel 270 through a
pressure reducing device such as a valve 268. As previously stated, the
pressure let
down will cause some or all of the solvent in the tailings stream 266 to
vaporize
(flash) thereby further reducing the solvent concentration in the final
treated tailings
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275 which are then disposed. In this embodiment, by treating part of the
slurry feed
in a more efficient manner the overall recovery of solvent may be increased.
Example 1
Continuous batch testing was used to investigate the kinetics of hydrocarbon
diluent
removal from tailings by flashing. A number of tailings were tested having a
broad
range of residual hydrocarbon diluent. In these experiments, the hydrocarbon
diluent was naphtha. A schematic of the continuous flash evaporation pilot
used is
shown in FIGURE 3.
The naphtha concentration in the tailings tested ranged from as low as ¨0.6-
0.9 wt.
% to as high as -6.74-8.02 wt. %. FIGURE 4 shows the naphtha concentrations in
the tailings after flash as a function of flash temperature drop (AT). As
shown, feed
naphtha concentration in the tailings greatly affects naphtha concentrations
in the
tailings after flashing. At any given AT value, the higher the naphtha
concentration
in the feed tailings, the higher the naphtha concentration in the final
tailings.
However, at AT values of 30 to 35 C, even with feed tailings naphtha
concentrations as high as 5.38 wt.%, the amount of naphtha in the flashed
tailings
was reduced to <0.2 % in a single-stage flash. For feed samples with naphtha
concentrations ranging from 1.23 to 1.65 wt.%, flash at AT values between 10
and
15 resulted in a final naphtha concentration in the flashed tailings of below
0.3 wt.%.
At AT values between 20 and 25, naphtha concentration after flash dropped to
<0.1
wt. 0/0. The tailings sample with a naphtha concentration of ¨0.3 wt. /0,
required only
a small AT value of <100 C to produce tailings with naphtha concentrations
below
0.1 wt. %.
FIGURE 5 shows naphtha concentrations in flashed tailings as a function of the
temperature of the feed tailings temperature (T1). As shown, in general, the
higher
the temperature of the feed tailings entering the flash vessel, the lower the
amount
of naphtha in the flashed tailings. However, the results in FIGURE 5 still
shows that
the naphtha concentration in the feed tailings still affects the naphtha
concentrations
in the flashed tailings.
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Based on the continuous pilot test results, it was discovered that flash
evaporation is
suitable for naphtha recovery from tailings having a wide range of residual
naphtha.
To reduce the final tailings naphtha concentrations below 0.1 wt. %, feed
tailings
with high naphtha concentrations require correspondingly higher AT values. For
feed tailings having an initial naphtha concentration of ¨0.3 wt. %, a
relatively low
flash AT of 10 C (e.g., from 106 C to 96 C) was sufficient to reduce
tailings
naphtha concentrations to <0.1 %. Thus, in one embodiment, feed tailings from
a
tailings pool of a high pressure stripping vessel can be pumped to a flash
tank,
where tailings pressure will be letdown from ¨105 C @ 1.2 bar absolute in the
feed
pipeline to 0.6 bar absolute in the flash tank resulting in a temperature of
about
85 C.
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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