Note: Descriptions are shown in the official language in which they were submitted.
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Carbon dioxide treatment of atmospheric
cooling water
The present invention relates to a novel
process for the treatment of atmospheric cooling
waters.
It is commonplace in industry to resort to
water circulation in order to cool a plant. The devices
to be cooled are extremely varied in nature. They can
be condensers, heat exchangers or chemical reactors.
The term "atmospheric cooling waters" is
understood to mean that the said waters are at a given
moment in contact with air, which brings about partial
evaporation and/or entrainment of this cooling water.
Cooling water circuits can be classified into
three categories:
- open circuits, in which the water which has
been used for a cooling operation (hot water) is
discharged to a river or to a drain,
- closed circuits, in which the hot water is
subsequently cooled by contact with a secondary fluid
(air or water) and subsequently returns to the devices
to be cooled without contact with air,
- so-called semi-open circuits, in which the
hot water is cooled by partial entrainment and/or
evaporation in an atmospheric cooler before returning
to the devices to be cooled.
Atmospheric cooling circuits or semi-open
circuits are therefore cooling water circuits in which,
at at least one point, a portion of the water to be
cooled is removed by evaporation and/or entrainment by
means of natural or forced convection of atmospheric
air.
An example of such circuits is composed of
cooling water circuits in which the water which has
been used to cool a plant (hot water) arrives at the
upper part of a cooling tower equipped with a packing
and air convection means and passes through this
cooling tower from the top downwards, the cold water
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arriving at the lower part of the cooling tower
subsequently being returned to the plant to be cooled.
Another semi-open circuit example is that of
water recirculation circuits in which the heat
exchanger which it is desired to cool acts as
atmospheric cooler.
It is generally well known that a number of
difficulties are inherent in cooling water circuits.
These difficulties arise from dirt, scale formation,
corrosion phenomena and biological growths.
The invention is targeted at solving the
specific problems posed by this third type of cooling
water circuit.
The dirt is composed of matter capable of being
deposited or of forming in a circuit. It can have
several sources: the make-up water, the atmospheric air
or the manufacturing operations in the plant.
Scale formation is related to the precipitation
on the surfaces of the pipes of sparingly soluble
calcium salts and possibly of silica. The main
parameters controlling the precipitation of scale are
the temperature, the rise in which generally decreases
the solubility of the salts concerned, the
concentration of the ions and the stirring.
The carbonate is the commonest cause of the
formation of scale which can be chemically redissolved
in operation.
Calcium sulphate, which has a maximum
solubility at 40 C, can precipitate under cold
conditions in the form of gypsum or under warm
conditions in the anhydrous or hemihydrate form.
In addition to bacterial corrosion, microbial
corrosion is a direct consequence of poor control of
the microbiology in a water circuit. This results
directly from biofilms, which decompose, resulting in a
size such that the region in contact with the material
is deprived of oxygen or encounters an acidic pH. As
corrosion is one of the major risks for industry, it is
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generally preferable to operate at a higher pH, often
of the order of 7.9 to 8.4, and at the same time to
counter the risks of formation of scale of calcium
sulphate, phosphate or carbonate type using sequester-
ing and/or dispersing agents.
Conditions which are favourable to the growth
of microorganisms and specifically of bacteria are
encountered in a cooling water circuit when the pH
values are greater than 7 and when the temperature is
greater than 25 C, which is often the case. The process
for the formation of a biofilm is always the same.
Conventionally encountered microbial strains have a
tendency to colonize the surfaces by secreting
attaching filaments which allow the bacteria to adhere
to the walls. The microorganisms, until then entrained
by the fluid, then become attached and grow into
colonies at all the points in a circuit. This is the
first stage in the creation of a biofilm. Once
attached, the bacteria have the distinguishing feature
of secreting a mucopolysaccharide, which can represent
a size several times that of the bacterium. This
bacterial gel or slime then acts as protection for the
colony of bacteria, which it will nourish by scavenging
the particles necessary for microbial growth present in
the waters. This phenomenon furthermore increases the
size of the layers. This layer, which grows,
furthermore results not only in progressive losses in
thermal efficiency of the plants but also in an oxygen
and pH gradient within the same biofilm. This gradient
will bring about, on contact with the material, an
anaerobiosis region representing conditions favourable
for the growth of sulphate-reducing bacteria which
produce hydrogen and are responsible for the sudden
microbial corrosion which is well known in the
industrial world. Control of the layers remains a
problem if the scale and corrosion phenomena are
present in the cooling circuit.
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Furthermore, closing a circuit has the
immediate consequence of increasing the pH, which is
one of the parameters favourable to bacterial growth
and therefore to the formation of biofilms.
It is seen that the problems which exist in all
cooling water recirculation circuits are found to be
further increased in the case of semi-open circuits due
to the recirculation in a loop of the water and the
increase in the concentration of salts related to the
partial evaporation of the water in this type of
circuit.
Various means for protecting against the risks
of scale formation and corrosion are already known.
Mention will be made, by way of example, of the
so-called natural equilibrium process, which consists
in adjusting the pH and the hardness of the circulating
water so that it is at equilibrium (monitoring of the
total alkalimetric assay (TAA), this assay reflecting
the content of OH-, C032- and HC03- ions in the water).
This adjusting is carried out by introducing acidic or
alkaline reagents and by limiting the level of
concentration. This process is attractive in its
simplicity but it has significant limits related mainly
to the concentration of dissolved salts, which requires
resorting to a significant extent to bleeding and thus
to high consumptions of make-up water.
Recourse is also had to chemical additives for
controlling scale which inhibit scale formation. They
are specific reagents, for example synthetic organic
polymers in the form of polyacrylates or of poly-
phosphonates. These chemical additives have
disadvantages. Thus, when these chemical additives are
of polyphosphonate type, they increase the hardness of
the water and thus the concentration of the salts.
Furthermore, these chemical additives are expensive.
Use is also made of agents intended to slow
down the precipitations. Such agents are generally
acids. However, when an acid is added, the hardness of
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the water is decreased, which results in an increase in
the risk of corrosion.
Another solution is to attempt to limit the
concentration of calcium and of magnesium and to resort
to a prior softening process on the make-up water.
However, the investment in and the maintenance of this
type of device are onerous.
In the case of semi-open circuits, it is well
known that all the problems mentioned above are found
to be accentuated by the phenomenon of concentration of
the salts. Furthermore, as the removal of the heat in
semi-open circuits takes place essentially by
evaporation of pure water without dissolved salt, this
results in:
- the need to resort to make-up water in order
to compensate for the evaporation,
- an increase in the salinity of the
circulating water. In order not to exceed_ the
solubility of some salts (for example, calcium
carbonate), it is then necessary to dilute the circuit
by bleeding.
Numerous documents are found in the literature
in which carbon dioxide has been used to treat the
water in circuits, either open circuits or closed
circuits.
Thus, Patent FR 2,697,827 provides a process
for descaling structures and for protecting structures
against scale formation, which structures are in
contact with scale-depositing water circulating in an
open circuit.
Application WO 85-03697 employs carbon dioxide
in the recovery of oils in flotation cells.
European Patent EP 0,380,299 discloses a
process for reducing the corrosion in a water
conveyance system which consists in using carbon
dioxide. However, this document in no way relates to
the specific problem of cooling water circuits and even
less to semi-open cooling water circuits.
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Patent FR 2,570,393 relates to a process
intended to remove the incrustations in a closed water
circuit by introducing pressurized carbon dioxide gas.
International Application WO 97/23414 discloses
a process for the treatment of water intended to remove
the volatile compounds which are found therein.
In the prior art, it was envisaged to use CO2
to treat the cooling water circulating in a semi-open
circuit but this use was always carried out under
highly specific conditions. This is because a person
skilled in the art expected complete desorption of the
dissolved COZ in the cooling tower and its removal into
the atmosphere at this point. This ought to have
rendered the use of CO2 uneconomic. This fear is
mentioned in Patent US-A-4,547,294 and it is indicated
therein that this problem of desorption of CO2 can only
be avoided if chemical additives for controlling scale
are introduced simultaneously with the CO2 into, the
cooling waters. These chemical additives can be
polymers or copolymers of maleic acid,
polyphosphonates, phosphonic acid derivatives,
aminophosphonic acid derivatives, polyacrylic acid and
polymethacrylic acid derivatives, polyesters or poly-
phosphates. The disadvantage of this implementation is
that these chemical additives are expensive and are
slowly removed in the cooling tower. It is therefore
necessary to regularly re-add them, which increases the
cost of their use.
The abstract of Japanese Patent JP-A-11028461
also discloses the injection of CO2 into the cooling
water of an open circuit. However, the water used has
to be softened beforehand by passing through an ion-
exchange resin. As indicated above, the investment in
and the maintenance of this type of device are onerous
and not suited to semi-open circuits.
The aim of the present invention is to provide
a simplified process for the treatment of the
atmospheric cooling waters circulating in a semi-open
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circuit recirculation loop which employs the injection
of C02r without the latter desorbing into the
atmosphere and without the addition of chemical
additives or the use of softened water.
The process according to the invention results
from the observation that, in contrast to all
expectation, it turns out that carbon dioxide, used
alone, i.e. in the absence of chemical additives for
controlling scale, in the treatment of water
circulating in a semi-open circuit, desorbs markedly
less than when the same circuit is treated solely with
inorganic acids and makes it possible simultaneously to
obtain better regulation of the pH, less fouling and a
decrease in bleeding. This is because it has been
demonstrated by the inventors of the present invention
that the introduction of carbon dioxide, in complete or
partial substitution for inorganic acids, makes it
possible to markedly reduce the scale-formation
phenomena.
Furthermore, the dispersing agents and
inorganic acids conventionally employed generate
inorganic sludges as a result of the salinity which
they bring about. Such an effect is in no way observed
with carbon dioxide, which releases only soluble
bicarbonate ions. The present invention thus provides a
process which makes it possible to reduce, indeed even
eliminate, the various disadvantages of the processes
of the prior art. This is because the inventors have
shown that, by virtue of the addition of CO2 at a point
in a water cooling circuit of semi-open type, it is
possible to decrease the amounts of salt generated by
the addition of scale inhibitor and of inorganic acids,
which makes it possible to decrease the amount of make-
up water while retaining the same level of
concentration of salts. This is particularly
advantageous since it is well known that, in order to
save water, it is advantageous to look for the highest
possible level of concentration of dissolved salts.
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This level depends essentially on the risks of
precipitation of the salts due to the lime/carbonic
acid equilibrium in water. In point of fact, the
currently existing solutions did not make possible a
maximum level of concentration without a significant
increase in the scale produced, limiting the heat
exchange. The invention has made it possible, by virtue
of the introduction of C02, to double the concentration
of dissolved salt. Thus, the invention has made it
possible to decrease the amount of make-up water by
increasing the level of concentration of salts and
without the appearance of scale, indeed even with a
decrease in the thickness of scale existing before the
injection of C02, up to its complete disappearance.
The introduction of CO2 makes it possible to
decrease the pH by contributing carbonic acid and the
soluble bicarbonates scavenge the calcium and magnesium
ions, which no longer precipitate and no longer act as
nutrient for the bacteria. The COz scavenges the
Ca2+ ions of the scale dissolved by the acid and
prevents it from precipitating or from being absorbed
by the bacterial films.
It is possible, by virtue of the use of C02, to
increase the concentration in the bleed by pushing back
the solubility limit for the carbonates, it being known
that there will in point of fact be doping by the COZ
less free CO2 desorbed after passage of the water
through the atmospheric exchanger. Furthermore, the CO2
does not result in dissolved salts of sulphate type and
therefore the solubility limit of the gypsum is pushed
back.
The inventors have been able to demonstrate
that this injection of CO2 can be carried out in a
general way directly at any point in a preexisting
semi-open circuit or in the water feed circuit for this
circuit but that it can also be carried out on a bypass
specifically provided on one or other of these circuits
for the purpose of this introduction, it being possible
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for the aim of such a bypass to be, for example, better
control of the amounts of COZ injected.
Thus, according to one of its essential
characteristics, the invention relates to a process for
the treatment of atmospheric cooling waters circulating
in a semi-open circuit recirculation loop comprising an
atmospheric cooling device equipped with means for the
natural or forced convection of atmospheric air, a
bleeding device and a make-up water feed, in which
process carbon dioxide is introduced at at least one
point in the recirculation loop or in the make-up water
feed or in a bypass circuit provided on the loop or the
feed for the purpose of the introduction and in which
process no chemical additive for controlling scale is
introduced into the waters.
The CO2 can be injected without distinction on
the recirculation loop for the cooling water or on the
inlet for the make-up water or on a bypass water
circuit provided specifically in order to facilitate
this introduction.
According to a particularly advantageous alter-
native form of the invention, the carbon dioxide is
introduced at the outlet of a water circulation pump,
such as the pump of the recirculation loop or of the
make-up water feed or of the bypass circuit. Good
results were obtained if the carbon dioxide is
introduced at a point in the process where the water
exhibits a pressure of at least 1 bar.
'The carbon dioxide can be introduced in the
liquid form or in the gaseous form. Recourse may also
be had to mixtures of carbon dioxide and an inert gas,
for example nitrogen. The combustion flue gases
resulting from a boiler constitute an example of such a
mixture.
As set out above, the carbon dioxide is
advantageously used in substitution for inorganic acids
conventionally used in the treatment of such a semi-
open circuit, in particular in complete substitution
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for H2SO4 or HC1. Such a substitution makes it possible
to decrease the pH of the water, releasing calcium ions
originating from the dissolution of the scale. The
addition of COz can thus be regulated by measuring the
calcium hardness of the water treated. The CO2 can also
be used in partial substitution for or in doping an
inorganic acid, in particular in doping H2SO4 or HC1.
The CO2 is advantageously injected at doses
such that as far as possible conditions of
lime/carbonic acid equilibrium in the water are
approached, that is to say conditions such that the
precipitation of the carbonates is avoided. To this
end, a person skilled in the art can either subject the
amounts of CO2 injected to an operation of quantitative
determination of the water, establishing a
lime/carbonic acid balance in the water, or to a pH
measurement on the water, this pH being regulated at a
set value close to the lime/carbonic acid equilibrium
value, or to a measurement of the dissolved calcium
ions.
The process of the invention makes it possible
to completely dispense with the use of the scale-
inhibiting chemical additives conventionally used.
The treatment of the invention applies to all
cooling circuits of semi-open type. As emerges from the
detailed description which will follow, the treatment
of the invention applies to all types of semi-open
circuits comprising an atmospheric cooling system. It
also applies to circuits in which the cooling of the
waters within the cooling loop is provided by an air
heat exchanger.
Other characteristics and advantages of the
invention will become apparent in the light of the
description and example which follow, which are
illustrated by Figures 1, 2 and 3.
Figure 1 diagrammatically represents a cooling
water semi-open circuit of a heat exchanger.
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Figure 2 diagrammatically represents a cooling
water semi-open circuit of a heat exchanger which acts
as atmospheric cooler.
Figure 3, given with reference to the example,
shows the change in the pH measured at the inlet and at
the outlet of the cooling tower in a process according
to the invention, in comparison with a control process
which does not resort to the use of CO2.
Figures 1 and 2 diagrammatically represent two
types of cooling water semi-open circuit to which the
invention in particular applies.
A person skilled in the art will have no
difficulty in devising other alternative forms of semi-
open circuits to which the invention also applies.
Figure 1 thus illustrates an example of a
cooling water semi-open circuit of a heat exchanger 1,
into which enters a hot fluid (arrow marked A) which it
is desired to cool in order to exit a cold fluid
(arrow B). The cooling of this heat exchanger is
carried out by a semi-open circuit, in which the water
which has been used to cool the fluid circulating in
the heat exchanger (hot water) arrives via the pipe
represented by the arrow C at the top of a cooling
tower 2 comprising a packing 3. This cooling tower is
equipped in its lower part with conventional air
convection means, not represented, allowing circulation
of air from the lower part to the upper part of the
tower. During its circulation from the top downwards in
the cooling tower, the water is subjected to a
phenomenon of partial entrainment and/or evaporation
related to the natural or forced convection of air in
the tower. The part which is not evaporated or
entrained during the circulation in the cooling tower
is recovered at the lower outlet of the tower in a
recovery tank 4. This tank is equipped with a bleeding
device, represented by the arrow D, and is connected to
a feed for making up with water represented by the
arrow E. A pump 5 makes it possible to recirculate the
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water collected in the tank 4 (cold water) to the heat
exchanger.
In an alternative form, the cold water can also
be subjected to an intermediate washing stage in a
bypass circuit, represented as dashes (--), comprising
a washing device 6 intended to remove the impurities
due to the air, the water returning to the heat
exchanger via a pump 7.
In such a process, it is possible to introduce
the CO2 at different points. The CO2 can be injected
without distinction on the loop at any point in the
water loop or on the inlet for the make-up water. It is
also possible to inject the CO2 at a point in a bypass
circuit specifically provided for this purpose. An
example of such a circuit is represented by a
discontinuous line composed of a series of dots and
dashes. The bypass is provided in this case on the
recovery tank 4 situated at the base of the cooling
tower 2. The circulation in this bypass circuit is
provided by the pump B. Possible injection position
examples are represented in the figure by the
indication "CO2-->". However, the CO2 is preferably
injected at the outlet of at least one of the pumps (5,
6 and/or 8) and in particular at the outlet of the pump
for returning the waters from the tank to the heat
exchanger.
Figure 2 represents an alternative form of the
invention in which the heat exchanger to be cooled acts
as an atmospheric cooler. In such a device, the heat
gradient in the water is reversed with respect to the
system represented in Figure 1.
More specifically, in the device represented in
Figure 2, the heat exchanger 1, equipped with its inlet
for hot fluid A and with its outlet for cold fluid B,
is inside the cooling tower 1. The cold water arrives
according to arrow C' at the upper part of the cooling
tower 2 and cools the heat exchanger by descending to
the base of the said tower in order to be collected in
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the tank 4. Here again, the water, during its
circulation inside the tower 2 in which air circulates
by natural or forced convection, is partially
evaporated or entrained via the upper part of the
cooling tower. The part which is not removed by
evaporation and/or entrainment is recovered in the
tank 4 equipped, as in Figure 1, with means for making
up with water E and with bleeding means D. A pump 5
subsequently recirculates the water recovered in the
tank 4 to the cooling tower.
In this case, as in the preceding one, the COZ
can be injected at any point in the loop for
recirculation of the water or on the inlet for the
make-up water or optionally on a bypass circuit
provided specifically on one of these circuits. Here
again, the injection can be carried out at various
points on the semi-open circuit. Examples of positions
for these injections are given material form in
Figure 2 by arrows (C02-+). According to the preferred
alternative form, the injection will take place at the
outlet of the pump 5.
It transpired that, in all the cases where COZ
is injected into a cooling water semi-open circuit, the
same advantages are observed. A person skilled in the
art can, of course, inject the CO2 both at a point in a
cooling water circuit or at a point in the circuit for
injection of the make-up water but he can also, in an
entirely equivalent way, provide an additional bypass
circuit on one of these circuits, in order to result in
better control of the amounts of CO2 injected and
complete dissolution of this CO2.
Thus, it was noticed that the injection of CO2
at any point in the water recirculation loop or on the
make-up water inlet or in a bypass circuit provided on
one of these circuits makes it possible to greatly
decrease scale formation and that very little CO2
dissolved in the loop desorbs in the aeration system,
in contrast to what could be feared. In fact, on the
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contrary, the C02 added makes it possible to prevent
the desorption of a large amount of C02 in the heat
exchanger. It therefore proves to be advantageous in
all the cases to inject C02 into a semi-open circuit
since the desorption of the free C02, in view of the
results, is not proportionally related to the level of
dissolved C02. Thus, it can be demonstrated that the
desorption of free C02 in the condensers and air
coolers is greater during the use of inorganic acids
alone than when they are coupled with an injection of
CO2 or replaced completely by C0Z. This is because it
transpired that the water is in a less unstable state
during its passage in the atmospheric cooler, the
desorption of C02 no longer being excessive and the
precipitation decreasing. Furthermore, it turned out
that it is not necessary to maintain a C02 partial
pressure in the atmospheric air circulating in the
tower by further addition of C02, as might have been
expected.
The process according to the invention there-
fore makes it possible, without increasing the salinity
of the water circulating in the circuit, to decrease
the scale of the bleeding and, consequently, to
increase the level of concentration, which makes it
possible to achieve considerable savings and an
increase in productivity.
The invention applies to all types of cooling
water circuit of semi-open type.
It proves to be particularly advantageous in
the case of the treatment of the cooling waters of a
nuclear power station. Nuclear sites are generally
constructed close to water courses in order to provide
an additional supply of water which allows the losses
by evaporation in the air coolers and the bleeds needed
for the discharge of concentrated salts in the circuits
to be compensated for. The flow of make-up water should
generally be conditioned by chemical treatment, in
order to limit the risks of scale formation in the
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circuits. The type of treatment depends on the quality
of the water and on the nature itself of the water
course used. Generally, the use of sulphuric acid in
the make-up circuit is restricted by legislation in
force relating to the content of dissolved salts. It is
conventional to use dispersing agents of polyacrylate
type in such a case. Tests carried out by the inventors
of the present invention have shown that CO2 can be
used either in complete substitution for H2SO4 or in
addition to treatment by H2SO4. The advantage of the
complete substitution of H2SO4 by COZ is that the
injection of CO2 makes it possible to decrease the pH
without modifying the TAC by contributing equilibrating
COZ, it being possible for the concentration of
carbonate to be thus reduced, and the formation of
scale. The use of CO2 in addition to H2SO4 makes it
possible to dope the acidification of the waters in the
circuit while observing the discharge standards which
restrict the use of acid. The decrease in the
phenomenon of scale formation makes possible, in this
case, a significant increase in electricity production.
Other advantages result from the complete or partial
substitution of HZSO4 by CO2, in particular economic
advantages related in particular to the fall in the
delivery costs for acidifying agents. Another advantage
is the fact that the use of CO2 which is dissolved in
the bicarbonate form greatly decreases saline
pollution. Another advantage is the improvement in the
safety of personnel since the COZ can be stored in the
anhydrous form and since it is a noncorrosive neutral
gas, which consequently makes it possible to extend the
lifetime of the plants. Furthermore, another advantage
economically is that the injection is carried out
without recourse to rotary equipment of the acid
metering pump type.
The same advantages could be observed in
various types of plant, in particular on aqueous
ammonia/water units using exchangers.
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Furthermore, the example which follows is given
purely by way of illustration of the invention and
relates to the treatment of such a cooling water
circuit.
EXAMPLE
Use is made, in this example, of a plant of the
type of that described in Figure 2.
The evaporative condenser is composed of a
tower 2, inside which is installed the heat exchanger
to be cooled which acts as atmospheric cooler in the
said tower.
The power of the exchanger is 1,556,000 kcal/h,
i.e. 1,244,800 frigories/h.
100 kg/h of COZ are injected on the lift side
of the recirculation pump 5.
The make-up with water is 4.6 m3/d.
The water bleed is 11.3 m3/h.
A comparative test in every respect equivalent
but not comprising injection of CO2 has made it
possible to show that the injection of CO2 under the
conditions of this test makes it possible to triple the
maximum level of concentration of salts and to
eliminate scale formation in the exchanger.
The water bleed changes to 5 m3/h.
The make-up with water is decreased by 50%.
No redeposition of scale is observed.
The scale present before the injection is
dissolved and disappears (via the bleed).
Figure 3 gives the pH of the water at the
bottom and at the top of the cooling tower in the case
respectively of the test according to the invention in
the presence of CO2 injected after the pressure pump
and in the case of a control test.
The pH at the top is maintained at the
equilibrium value, whereas the pH at the bottom
approaches the initial pH of the make-up water. A
gradient is established during the exchange but the
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limitation of the desorption of COZ amazingly prevents
scale formation, even at the bottom of the column,
where the pH is highest.
