Note: Descriptions are shown in the official language in which they were submitted.
WO 2022/018192
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1
DEVICE FOR COOLING FLUE GAS ORIGINATING FROM A PLANT FOR THE
PRODUCTION OF ALUMINUM BY FUSED-SALT ELECTROLYSIS AND PLANT
IMPLEMENTING SUCH A DEVICE
DOMAIN OF THE INVENTION
The invention pertains to the field of fused-salt electrolysis aluminum
production, and
more particularly aims at the treatment of the flue gas resulting from the
implementation of this
method.
BACKGROUND
The production of aluminum by the so-called fused-salt electrolysis method is
now widely
known and controlled. The electrolysis of aluminum in the presence of fused
cryolite generates
the production of flue gas, which particularly contains carbon dioxide,
fluorinated products, and
particularly hydrofluoric acid HF, and dust. Due to the increased severity of
anti-pollution
standards, the discharge of this flue gas cannot occur as such, and is thus
processed in a flue
gas processing center, precisely to comply with environmental standards.
Devices capable, on the one hand, of capturing the flue gas at the outlet of
the electrolysis
cells or pots, and then of processing it to comply with these environmental
standards have thus
been developed. Such flue gas processing centers conventionally comprise
filtering means,
most often in the form of sleeves made of polymer, and typically of polyester,
capable of
capturing dust, and, on the other hand, chemical treatment means intended to
neutralize the
fluorinated gases via a method of adsorption on alumina.
Although such a treatment of the flue gas by means of these processing centers
is
generally satisfactory, it is however insufficient in terms of quality, from
the moment that, as is
the trend for operators of such aluminum production plants, the production
capacity is desired
to be increased_ Indeed, such an increase results in an increase in the
intensity of the electrolysis
current and, accordingly, typically in the increase of the flue gas volume, in
addition to their
temperature increase.
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To fight this temperature increase, likely, in particular, to impact the
integrity of the
filtering means and to decrease the efficiency of the fluorinated gas
treatment, various technical
solutions have been provided.
They comprise the principle of diluting the flue gas with ambient air,
typically by means
of dilution doors located upstream of the flue gas processing center(s). Apart
from the fact that
the implemented principle requires a significant flow of added air to achieve
the desired
temperature decrease, it also generates a larger volume of gas to be treated
and thus an
accordingly oversized processing device, thereby affecting the general economy
of the
aluminum production plant. This is actually the reason why this principle is
generally only
implemented as a backing, in exceptional situations and when the outside
environment is itself
very warm.
Another technical solution comprising injecting water droplets into the flue
gas has also
been provided. The evaporation of said water droplets induces the cooling of
said gases (see for
example EP 1 172 326). This relatively inexpensive method is however little
implemented or
only in a limited way to ascertain a decrease in the flue gas temperature
below the maximum
temperature typically admissible by the filtering sleeves, in the case in
point 135 C.
It has also be provided to implement air-to-water heat exchangers, such as for
example
described in document EP 2 431 498. Typically, these exchangers are formed of
tubes having
the flue gas originating from the electrolysis pots flowing therethrough, and
having a coolant
fluid, particularly water, flowing outside thereof, advantageously in the
opposite direction, the
water playing the role of a heat-carrying fluid intended to cool the tubes and
accordingly the
gases.
Although, thermal speaking, these exchangers are efficient and particularly
enable to
achieve the desired temperature decrease of the flue gas originating from the
pots, they however
generate hot water resulting from the heat exchange at the pipe level, which
is forbidden to be
rejected at such a temperature by environmental standards, and which thereby
requires the
implementation of batteries of cooling towers in closed circuit. Thereby, the
cost of the plant is
significantly increased. Besides, in a number of countries where the water
resource is low, this
type of device may turn out being inappropriate, even though the water flows
in closed circuit.
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As a summary, there is currently no device enabling to sufficiently and
efficiently
decrease the temperature of the flue gas originating from the electrolysis
pots together with an
easy implementation, a bulk decrease, decreased implementation costs, and an
easy
maintenance.
Such is one of the objects of the present invention.
Further, plants for the collection of such flue gas conventionally comprise
collection tubes
or collectors connected to each electrolysis pot, called individual
collectors, and a general
collector at the level of which said individual collectors are connected, and
indented to collect
the gases collected by the tubes. To obtain a substantially constant flow rate
of the flue gas at
the level of the series of electrolysis pots all along the length of the
general collector, the
individual pipes or collectors are equipped with a differential pressure
device, of diaphragm or
butterfly valve type, intended to create a head loss. However, the head losses
thus caused require
oversizing the flue gas suction element(s).
The aim targeted by the implementation of these differential pressure devices
is to balance
the suction flow rates on the pots, the action of these differential pressure
devices being different
for each pot. the pots most distant from the inlet of the general collector
thus having a lower
additional head loss. However, such an additional head loss generates a higher
power
consumption.
To overcome this difficulty, it has been provided to implement flue gas bypass
loops,
connected between the general collector and the individual collectors, said
bypass loop typically
extending in ambient air, obviously below the flue gas temperature. However,
this technology,
although it induces little or no additional head loss, however generates a
particularly limited
decrease in the temperature of said flue gas, in any case insufficient for a
number of
applications.
The invention thus aims, according to a second aspect, at a plant of the type
in question,
enabling to sufficiently cool the flue gas originating from each of the
electrolysis pots or cells,
without generating a head loss, or by decreasing the latter with respect to
plants of the prior
state of the art.
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SUMMARY
According to a first aspect, the invention thus provides a device for cooling
flue gas
originating from a plant of aluminum production by fused-salt electrolysis,
intended to be
positioned between a collector of said flue gas, be it an individual collector
originating from
each of the electrolysis pots of said plant, or a general collector having
said individual collectors
emerging at the level thereof, and a center for processing said flue gas.
According to the invention, this device is formed of a plurality of hollow
tubes assembled
parallel to one another, having a diameter smaller than the diameter of said
individual collectors
or of said general collector, which tubes have said flue gas flowing
therethrough, said tubes
being in contact with the ambient air to form an air-to-air heat exchanger:
= one of the ends of said tubes being in communication with an upstream
plenum,
having said individual collector or said general collector emerging at the
level
thereof,the other end of said tubes being in communication with a downstream
plenum, itself in communication with a pipe directly or indirectly reaching
the gas
processing center.
In other words, conversely to prior art devices, the general principle
retained by the device
of the invention relies on an air-to-air heat exchanger, the heat exchange
occurring by
convection between the outer air transiting at the periphery of and in contact
with the tubes and
the flue gas transiting within the tubes. Of course, the number of these tubes
and their diameter
are determined to optimize this heat exchange according to the quantity of
flue gases and to
their temperature to be processed, but also to decrease risks of deposition of
material in the
tubes, risks of abrasion, risks of scale formation, in addition to minimize
head losses.
Thus, according to an advantageous feature of the invention, to optimize this
heat
exchange, all or part of the tubes forming the exchanger is provided with
external radial fins
originating from the periphery of the tubes, and capable of optimizing the
convection.
Advantageously, these fins have a thickness in the range from 0.5 to 3
millimeters, and
extend from the external peripheral wall of the tubes over a distance
typically in the range from
20 to 60 millimeters. These dimensions are determined to optimize the heat
exchange.
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Further, these fins typically result from one or a plurality of spirals,
defining between
each turn a pitch in the range from 10 to 100 millimeters, this pitch being
here again determined
to optimize the heat exchange. These spirals are typically rolled tight around
the tube to ensure
the best contact between the tube and the fin, to maximize the heat transfer.
Typically, the
spiral(s) forming the fins are only welded on the tube at their two ends.
According to another advantageous feature of the invention, and still to
improve and
optimize the heat transfer, these tubes, and if present, the fins, are made of
a material selected
from the group comprising aluminum, black steel, stainless steel, and
electroplated steel.
Typically, the tubes have a length in the range from 6 to 12 meters, and a
diameter in the
range from 50 to 300 millimeters. These values are purely indicative.
Advantageously, when the tube is finned, the spiral(s) are assembled on tubes
of standard
dimensions.
According to the invention, the tubes are aligned with one another. however,
and
according to an alternative feature of the invention, the tubes may be
assembled in quincunx
with respect to one another. Whatever the assembly of the tubes relative to
one another, the
desired aim is to optimize the flowing of outer air in contact with the tubes,
and thus to optimize
the exchange surface area between the outside and the peripheral surface of
said tubes.
Typically, the distance between tubes, that is, between two generating lines
of two
contiguous tubes in a same exchanger, is in the range from 5 to 100
millimeters if the tubes are
finless. However, if the tubes are provided with fins, the bulk that they
generate has to be taken
into account, and this distance may reach several hundreds of millimeters.
To optimize the installation of such an exchanger, the tubes are assembled on
a skid,
typically by group of from 30 to 200, it being specified that according to the
quantity of flue
gas to be treated, a plurality of these skids may be assembled side by side
and coupled to a same
general collector.
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Further, to decrease head losses inherent to the turbulences generated by the
flow at the
inlet of the tubes, and more precisely phenomena of separation of the gas
stream within said
tubes, the upstream plenum is provided with means configured to favor the
introduction of the
gases into the tubes. These means are typically formed at the level of one of
the walls forming
said plenum, said wall being pierced with through openings having a diameter
corresponding
to the diameter of said tubes positioned vertically in line with the inlet of
the tubes, and being
provided with deflectors or with equivalent systems.
As a variant, said wall is folded, to intrinsically define deflectors or pipes
capable of
decreasing the turbulences of the flue gas.
According to another feature of the invention, the downstream plenum is also
provided
with means, substantially of same design as those implemented at the level of
the downstream
plenum, and having the function of decreasing turbulences and overspeed areas
at the level of
the outlet pipe reaching the flue gas processing center.
According to another feature of the invention, the exchanger is added a source
of
additional air, that is, other than the ambient air surrounding the exchanger,
this source being
typically formed of a fan or the like, capable of increasing the air flow
intended to come into
contact with said tubes to optimize the heat exchange.
According to still another feature of the invention, one or a plurality of
fans or equivalent
devices are positioned within the outlet pipe reaching the flue gas processing
center, to
compensate for the previously-mentioned head losses.
According to another advantageous feature of the invention, thermoelectric
modules are
assembled on the fins associated with the tubes, to transform the heat
resulting from the heat
exchange into electric energy, and thus value a fraction of the heat thus
dissipated.
According to a second aspect, the invention also aims at a plant for the
collection of the
flue gas originating from electrolysis pots for the production of aluminum by
fused-salt
electrolysis. This plant comprises, for each pot, an individual collector of
said flue gas, each
connected to a general collector, said general collector conveying the flue
gas thus collected to
a flue gas processing center. According to the invention, for at least part of
said pots, a flue gas
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cooling device of the previously-described type is interposed between the
individual collector
of the considered pot and the general collector.
According to a first variant of the plant of the invention, the latter
comprises at least one
series of n consecutive pots (ni..., ni, having
N Ni, Ni-pi,..., Nz) individual
flue gas cooling devices of the type in question associated therewith, the
individual collector of
pot ni is connected to the upstream plenum of device Ni, having its downstream
plenum coupled
to the general collector in the vicinity of the area occupied by pot ni-pi,
and accordingly the
individual collector of pot ni-pi is connected to the upstream plenum of
cooling device Ni-q,
having its downstream plenum coupled to the general collector in the vicinity
of the area
occupied by said pot ni+2. In this configuration, said individual flue gas
cooling devices are
positioned in quincunx, to minimize the general bulk resulting from these
individual cooling
devices.
According to another variant of the plant of the invention, the individual
flue gas cooling
devices of two consecutive pots extend adjacently and parallel to each other.
In this
configuration, the individual collector of pot ni is connected to the upstream
plenum of
individual cooling device N, having its downstream plenum connected to the
general collector
in the vicinity of the area occupied by pot ni-pi, and accordingly, the
individual collector of pot
ni-pi is connected to the upstream plenum of individual cooling device NJ-pi,
having its
downstream plenum connected to the general collector vertically in line with
pot ni, typically
upstream of the place of connection of the downstream plenum of the individual
cooling device
Ni to the general collector. In this configuration, the flue gas originating
from individual cooling
device Ni-pi ends up in the general collector upstream of the flue gas
originating from cooling
device Ni. Further, the access to the plenums, respectively upstream and
downstream of each
of said cooling devices, and accordingly the maintenance operations likely to
occur at the level
of these devices, are facilitated. Further, the actual plant is simplified, by
suppressing the
support structures of said devices, typically for one pot out of two.
In still another variant of the plant of the invention, the individual flue
gas cooling devices
are also oriented parallel and two by two for two consecutive pots, these two
devices being
"coupled" to each other by a double plenum, each of said plenums being
provided with a valve
capable of closing, and thus of thus differentiating when needed, an upstream
plenum and a
downstream plenum for each of said devices, and where here again, the flue gas
originating
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from individual cooling device Ni-hi end up in the general collector upstream
of the flue gas
originating from cooling device N.
The notions of upstream and downstream are to be understood with respect to
the flue gas
flow direction within the general collector.
BRIEF DESCRIPTION OF THE DRAWINGS
The way in which the invention may be implemented and the resulting advantages
will
better appear from the following non-limiting embodiments, in relation with
the accompanying
drawings.
Figure 1 is a simplified representation of an electrolysis pot and of its
coupling to a general
collector.
Figure 2 is a simplified representation of the travel of the flue gas
according to a first
embodiment of the invention.
Figure 3 is a simplified representation similar to Figure 2 of a second
embodiment of the
invention, implementing the principle of forced convection.
Figure 4 is a simplified representation similar to Figure 2, of another
embodiment of the
invention, implementing a fan assembled on the outlet pipe reaching the flue
gas processing
center.
Figure 5 is a simplified perspective representation of an embodiment of the
device of the
invention, connected to the general collector.
Figure 6 is a simplified perspective representation of an embodiment of a tube
implemented in the device of the invention.
Figure 7 is a simplified perspective representation of a first embodiment of
the means
implemented within the upstream plenum to decrease the head loss
Figure 8 is a simplified perspective representation of a second embodiment of
the means
implemented within the upstream plenum to decrease the head loss
Figure 9 is a simplified perspective representation of a third embodiment of
the means
implemented within the upstream plenum to decrease the head loss
Figure 10 is a simplified cross-section and top view of the embodiment of the
means of
Figure 9
Figure 11 is a simplified cross-section and top view of a variant of the
embodiment of ht
means of Figure 9.
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Figure 12 is a simplified perspective representation of a fourth embodiment of
the means
implemented within the upstream plenum to decrease the head loss
Figure 13 is a simplified cross-section and top view of the embodiment of the
means of
Figure 12.
Figure 14 is a simplified perspective representation of a fifth embodiment of
the means
implemented within the upstream plenum to decrease the head loss
Figure 15 is a simplified cross-section and top view of the embodiment of the
means of
Figure 14.
Figure 16 is a simplified cross-section view of a sixth embodiment of the
means
implemented within the upstream plenum to decrease the head loss
Figure 17 is a simplified cross-section view of a seventh embodiment of the
means
implemented within the upstream plenum to decrease the head loss
Figure 18 is a simplified cross-section view of an eighth embodiment of the
means
implemented within the upstream plenum to decrease the head loss
Figure 19 shows a simplified representation of a fin associated with a tube of
the device
of the invention, equipped with thermoelectric modules.
Figure 20 is a simplified representation in transverse cross-section
illustrating a first mode
of arrangement of the tubes of the device of the invention
Figure 21 is a view similar to Figure 20, of another mode of arrangement, in
the case in
point in quincunx, of the tubes of the device of the invention.
Figure 22 is a simplified perspective representation of a portion of the flue
gas collection
plant according to the invention.
Figure 23 is a view similar to Figure 22 of another embodiment of the
invention
Figure 24 is a view similar to Figure 23 of an alternative embodiment of the
invention.
Figure 25 is a simplified top view of the plant of Figure 24,
Figure 26 is a lateral view of the plant of Figure 24.
Figure 27 is a profile view of the plant of Figure 24.
Figure 28 is a simplified view of the operation of still another embodiment.
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DETAILED DESCRIPTION
A simplified view of an electrolysis pot has thus been shown in Figure 1 Such
a pot (1)
is conventionally formed of a plurality of anodes (2) fastened by anode rods
(3) on an
electrically conductive frame, said anodes being partially immersed in a fused
cryolite and
alumina melt. Removable covers (4) enable to change the anodes (3). The pot
(1) is coupled by
an individual pipe or collector (5) to a general collector (6), to collect and
then conduct the flue
gas generated within said pot during the electrolysis operation at the level
of a flue gas
processing center (not shown).
Such an architecture is perfectly well known, so that there is no need to
describe it in
further detail herein.
According to the described specific embodiment of the invention, the general
collector(s)
(6) emerge into the device for cooling the flue gas thus collected,
schematically illustrated in
Figures 2 to 5. However, the invention also concerns such a device for cooling
said flue gas,
which is not assembled on the general collector(s) (6), but on the individual
collector(s) (5), the
principle however remaining identical.
More precisely, and in connection with Figure 5, the flue gas originating from
the general
collectors (6) is conveyed by a pipe (7) to an assembly (8) of hollow tubes
(9) assembled parallel
to one another.
The connection between the pipe (7) and the inlet of the tubes (9) is formed
at the level
of an upstream plenum (10), further detailed hereafter. The flue gas crosses
said tubes (9) and
is then collected at the level of a downstream plenum (11), in communication
with another pipe
(12), coupled in turn to a collector (13) intended to convey said flue gas
after its passage through
the tubes (9) and thus after the cooling at the level of a flue gas processing
center (not shown
in this Figure 5).
This assembly (8) of tubes (9) is positioned outside of the civil engineering
structures
housing the series of electrolysis pots (1) and particularly in free air, to
enable ambient air to
flow in contact with the outside of said tubes, and to enable, by convection,
the cool the flue
gas flowing within the tubes. It is however specified that in the
configuration according to which
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the assembly (8) is assembled, rather than on a general collector (6), on an
individual collector,
said assembly is then positioned inside of the civil engineering structure.
Whatever the
configuration, an air-to-air heat exchanger is thus formed, the cooling of the
flue gas transiting
within the tubes (9) resulting from the convection with outer air with respect
to said tubes.
Although, in Figure 5, only one assembly (8) of tubes has been illustrated, it
can be
envisaged to have a plurality thereof, assembled in series or in parallel, to
obtain the desired
decrease in the flue gas temperature before the latter end up in the flue gas
processing center,
for the reasons discussed as a preamble.
These tubes, having a typical length in the range from 6 to 12 meters, and a
diameter
typically in the range from 50 to 300 millimeters, are advantageously made of
aluminum, due
to the good thermal properties of this metal, in addition to its low density.
The assembly (8) may typically comprise between 100 and 200 of such tubes,
assembled
in the form of skids, thus giving the device a modular character, and further
favoring all the
associated logistics.
Within a same skid, all the tubes (8) are identical and positioned parallel to
one another.
They may advantageously be assembled in alignment with one another (Figure 20)
or in
quincunx (Figure 21), to optimize the displacement of ambient air (represented
by the arrows)
in contact with said tubes, and thus accordingly improve the heat exchange by
convection.
Additionally, and to further increase the heat exchange, each tube (9) is
provided with
radial fins (16) extending from the external wall (15) of said tubes (see
Figure 6). These fins
may in facts be made of one or a plurality of metal plates, advantageously
made of the same
metal as that forming the tube, having a thickness in the range from 0.5 to 3
millimeters, and
fastened to said external wall by welding to each end only. This or these
helical spirals, of same
axis as the axis of revolution of the considered tube, define at each rotation
a fin, separated from
the contiguous fins by a same pitch typically in the range from 10 to 100
millimeters.
Figures 2 to 4 show three possible configurations of the invention.
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Figure 2 illustrates the basic configuration, where the assembly (8) of the
tubes (9) is only
submitted to the action of ambient air. According to the number of assemblies
and/or of tubes
per assembly, in addition to the volume of flue gas to be treated or to the
temperature of said
flue gas at the outlet of the pots (1), such a configuration may turn out
being sufficient.
Downstream of the exchanger (8), the cooled gases are mixed in one or a
plurality of
reactors (21) with so-called "fresh" metallurgical-grade alumina, previously
stored in a silo
(19). In these reactors, the gaseous HF will adsorb on the alumina; this so-
called "fluorinated"
alumina is then separated from the HF-purified gas in one or a plurality of
filters (20). One or
a plurality of exhaust fans (23) ensure the depressurizing of the assembly and
the discharge of
the clean gases into one or a plurality of chimneys. The fluorinated alumina
is stored in a silo
(22) before being used as a raw material for the feeding of the pots (1).
Under the assumption where the configuration thus described is insufficient in
terms of
flue gas temperature decrease, another configured, such as illustrated in
Figure 3 provides the
implementation of pulsed air, typically by means of one or a plurality of fans
(26) or of
equivalent devices. Such fans are in this case sized to achieve the desired
flue gas temperature
decrease.
Further, to decrease as much as possible the head loss resulting from the
entering of the
flue gas into the tubes (8), one positions at the level of the upstream or
inlet plenum (10) means
described in further detail in relation with Figures 7 to 18.
Further, the inlet (10) and outlet (11) plenums of the exchanger typically
have a tapered
shape, as can be well observed in Figure 5. The widest portion or base of the
taper directly
communicates with the pipe (7) and is contiguous to the first tubes (9)
forming the assembly
(8), that is, at the level of the area where the flue gas speed is the
greatest. Then, the tapered
shape of the plenum towards the most distant tubes (9) enables to maintain the
speed of said
flue gas in the plenum within an acceptable range as the gas feeds the tubes
(9) (typically
between 14 m/s and 20 m/s) while ensuring a homogeneous distribution of said
gas between
the tubes (9), enabling to optimize the heat dissipation.
The tapered shape of the downstream plenum (11) contributes to a similar
result.
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Further, the means illustrated in Figures 7 to 15 contribute to reaching this
homogeneity.
The general principle underlying these different variants of said means relies
on the
implementation of a profile or deflector of appropriate shape, and
particularly curved, to
minimize the separation of the gas stream at the inlet of the tubes (9), as
concerns the
downstream plenum.
Thus, Figure 7 illustrates a first embodiment of such means, on which
reference (27)
materializes the plate of the plenum (10) which positions at the level of the
inlet of the
exchanger (8). This plate (27) is pierced with through openings (28), having a
diameter identical
to that of the tubes (9) and positioned opposite each of said tubes. The plate
(27) is provided
with deflectors (29), typically in sagittal cross-section in the shape of a
shark fin, fastened to
said plate, for example, between spacers (30), at the border of each of the
through openings,
and downstream with respect to the flue gas flow direction materialized by the
arrow. Due to
the curved profile of said deflectors, in addition to their positioning with
respect to the gas flow
direction, the head loss is significantly decreased.
Figure 8 illustrates a variant of Figure 7, where instead of the deflectors
(29), one welds
at the level of each of the through openings (28) of the plate (27) a hollow
elbow (31) oriented
towards the gas flow, and capable of ensuring a direction change of
approximately 45 of said
gas flow, but here again according to a curved profile.
Figures 9 and 10 illustrate another variant of these means, respectively in
perspective and
in cross-section view and in top view. In this variant, a half round piece
(32) is welded on the
plate (27) upstream of each of the lines of through openings (28) with respect
to the gas flow
incoming direction.
Figure 11 illustrates a variant of Figures 9 and 10, respectively in cross-
section and in top
view. Half-round portions (33) extending between the half-round pieces (32)
are added with
respect to this embodiment.
Figures 12 and 13 illustrate another embodiment of these means. In the case in
point, said
means are formed of circular half-round pieces (34), each welded to the
periphery of each
through opening (28) of the plate (27).
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Figures 14 and 15 illustrate still another embodiment of these means. The
latter are here
again formed of partial circular cross-sections of half-round pieces (35)
welded on the plate
(27) upstream of each of the lines of through openings (28) with respect to
the gas flow
incoming direction.
Still to decrease the head loss, the upstream plenum may have a configuration
of the type
of that illustrated in Figures 16 to 18.
Thus, in the embodiment of Figure 16, the plate (27), intended to be
positioned upstream
of the tubes, is folded, defining a broken line, having its surfaces
positioned in the vicinity of
the tubes pierced with through openings, at the level of which hollow elbows
with a curved
profile (36), having their other end welded to the tubes (9), are welded.
Figures 17 and 18 illustrate a principle similar to that of Figure 16.
In the same way, and still to minimize the head loss of the assembly, and more
particularly
to homogenize the speeds in the downstream plenum or outlet plenum (11) by
minimizing
turbulences and overspeed areas therein, the same type of means as those
previously described
are positioned at the level of said downstream plenum (11)
Still to overcome the issue due to the head loss, inherent to the passage of
the flue gas
within the exchanger (8), it may be envisaged (see Figure 4) to position one
or a plurality of
fan(s) (24) or equivalent devices downstream of the downstream plenum (11).
Such fans are
known to operate in the specific conditions characteristic of the flue gas
originating from the
pots (1). Thus, the material forming the blades of the fan (24) is selected to
resist dusty gases
likely to generate abrasion or scale formation phenomena. A specific steel
grade such as the
S690QL according to standard EN10025-6 is advantageously used.
Further, axial-type fans are typically used, which meet the specific needs
characterized
by a high flow rate (several tens of m3/s) and a relatively low pressure
differential (typically <
2,000 Pa).
A plurality of these fans (24) may be arranged in parallel, to ensure a
redundancy.
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In an advantageous embodiment of the invention, advantage is taken of the very
large
exchange surface area provided by the fins (16) equipping the tubes (9)
(several thousands of
m2 per exchanger), and of the fact that due to the environment into which the
exchanger is
plunged, a surface temperature of said fins greater by a quantity close to 40
C above the room
temperature is available to collect the thermal power thus generated and
transform it into
electric power. For this purpose, thermoelectric cells (25) are positioned on
said fins (16) to
achieve this, as illustrated in Figure 19.
In the context of a typical application, several thousands of kW of thermal
power are to
be dissipated for each processing center. Considering a power efficiency in
the order of a few
percents (<10%) for these thermoelectric modules, it may be envisaged to
recover a few tens of
kW.
The implementation of thermoelectric cells provides the advantage of
modularity; more
restrictedly, only a fraction of the fins might thus be equipped, to power a
number of electric
appliances located close to the exchanger (measurement instruments, lightings,
...).
Different variants of the positioning of individual flue gas cooling devices
in relation with
Figures 22 to 28 will be described hereafter. In the drawings, the flow
direction of said flue gas
in the general collector (6), enabling to define the notion of upstream and
downstream, has been
materialized by an arrow.
A simplified view of a first embodiment of the plant for the collection of the
flue gas
originating from a series of electrolysis pots has been shown in relation with
Figure 22.
In this drawing, the pots as such have not been shown. However, it shows the
individual
collectors (5) originating from each of the pots. According to the invention,
these individual
collectors (5) are thus not directly coupled to the general collector (6), but
emerge into an
individual flue gas cooling device (8) of the type of those described in
relation with Figures 1
to 21. It can thus be observed, in this Figure 22, that these devices (8) are
oriented along a same
direction parallel to the general collector (6) but however offset with
respect to one another and
typically positioned in quincunx. As a corollary, it can be observed that the
length of each of
these individual devices, provided with their respective upstream and
downstream plenums,
typically 6 meters, substantially corresponds to the distance between pots.
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In this embodiment, the upstream plenum of each of these individual cooling
devices (Si,
8,+1) is coupled to the individual collector (Si, 5,-q) of each of the pots,
and accordingly, the
downstream plenum of said devices is coupled at the level of the general
collector (6) via a
downstream pipe (141, 14,+i). However, and due to this quincunx positioning,
it can be observed
that the downstream plenum of the individual cooling device (8i) of the pot
(i) emerges at the
level of the general collector (6) by a downstream pipe (14i) substantially
vertically in line with
the area occupied by the consecutive pot (i+1). Accordingly, concerning said
consecutive pot
(i + 1), for which the individual flue gas cooling device (8,+,) is offset,
and in the case in point
a little more distant from the considered pot, and for bulk reasons, the
upstream plenum of said
individual cooling device (8,+,) of said pot (i + 1) is located vertically in
line with the concerned
pot, and the downstream plenum is also connected to the general collector (6)
by a downstream
pipe (14,+t) substantially vertically in line with the consecutive pot (i +
2).
Concerning the embodiment of the invention shown in Figures 23 and 24, the
individual
flue gas cooling devices (8) are always oriented along a same direction
parallel or substantially
parallel to the direction of the general collector (6). However, they are
positioned adjacently,
two by two for an assembly of two consecutive pots. The differences between
these two variants
essentially lie in the positioning of the downstream pipes (14) originating
from the individual
devices (5)
Thus, for a first assembly of two consecutive pots (i, i + 1), thus having two
parallel
individual flue gas cooling devices (Si, 8i + 1) parallel and adjacent to each
other, the flue gas
which has been cooled, that is, after having transited through the individual
device of the pot
(i + 1) systematically emerge into the general collector (6) upstream of this
same cooled flue
gas of the first one (i) of said two pots. Thereby, the plant is simplified
due to the
implementation of fewer support structures. Accordingly, the accessibility to
the plenums,
respectively upstream and downstream of each of the individual cooling
devices, and thus the
maintenance of these devices, are optimized.
According to still another embodiment of the invention very schematically
shown in
Figure 28, where here again for two consecutive pots, the individual flue gas
cooling devices
(Si, 8, +1) are positioned parallel to each other and adjacently to each
other, said pair of devices
thus implemented is coupled upstream and downstream by a respective
intermediate plenum
(37, 38). These intermediate plenums are likely to form, respectively, the
upstream plenum of
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one of the individual devices (5) and the downstream plenum of the adjacent
individual device,
and vice versa. For this purpose, these intermediate plenums are provided with
a valve or an
equivalent device (39, 40), likely to differentiate, at the level of each of
said intermediate
plenums, the downstream plenum and the upstream plenum of each of said
intermediate cooling
devices. Thereby, when the valves (39, 40) in question are closed, the
configuration is that
described in relation with the embodiment described in Figures 23 to 27.
However, if the flow rate on pot n is desired to increase, to maximize the
efficiency of
the collection of the flue gas emitted during the maintenance operations
carried out on said pot
n, when the corresponding cover (4) is removed, it becomes possible to bypass
the considered
individual device (5i) by opening the valve (39), whereby the flue gas
originating from said pot
n are then not cooled and are directly conveyed to the level of the general
collector (6) via the
downstream pipe (14i+1). The same operation can be envisaged with pot n + 1,
by opening the
valve (40).
Accordingly, said downstream pipes (14i, 14i+1) are also likely to receive a
valve-type
member (41, 42) to reach their total or partial closing to optimize this
bypass operation.
In this configuration, the maintenance operations on the individual flue gas
cooling
devices (8) may be carried out efficiently without altering the general
operation of the plant.
Further, and as discussed hereabove, the plant of the invention does not
require equipping
each of the pots with such an individual flue gas cooling device. Thereby, by
selecting the
number of these devices and their implantation, the generated head loss is
relatively small and
in any case minimizes the impact on the total head loss of the flue gas
collection and processing
circuit of the plant.
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