Note: Descriptions are shown in the official language in which they were submitted.
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Method and installation for the thermal hydrolysis of sludge
1. Field of the invention
The invention pertains to the field of the treatment of sludges highly charged
with fermentable organic matter and especially that of:
- sludges derived from the processes of depollution of urban or
industrial
wastewater;
- organic wastes;
- mixtures of sludges derived from processes of depollution of urban
or
industrial wastewater and organic wastes.
2. Prior art
Currently, a part of the sludges produced by the cleansing stations is
recycled
into the agricultural sector while another part is generally either put into
landfills or
incinerated. Since the production of these sludges is becoming ever greater,
it is
necessary that they should not pre sent any danger for the environment and
human
health. Indeed, these sludges contain germs some of which are pathogenic
(coliform
bacteria, salmonella, helminth eggs etc). In addition, they are fermentable
and are the
source of gases (amines, hydrogen sulfide, mercaptans), which cause olfactory
nuisance. These considerations explain the need to implement at least one
step, in the
above-mentioned treatment systems for stabilizing these sludges, aimed at
obtaining
sludges that no longer evolve or at least evolve slowly both on the biological
plane
and as on the physical/chemical plane. Other preoccupations relate to the wish
to
reduce the volume of the sludges and recycle the sludges in the form of
biogas.
Various methods have been proposed in the prior art to treat these sludges,
among them:
- aerobic digestion;
- anaerobic digestion;
- chemical conditioning;
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- thermal conditioning;
- thermal hydrolysis.
It is to this last-named type of treatment that the invention pertains.
The thermal hydrolysis of sludges consists in treating these sludges at high
temperature and under pressure so as to sanitize them (i.e. very greatly
reduce their
microorganism content), solubilize a major part of the particulate matter and
convert
the organic matter that they contain into biodegradable, soluble COD
(alcohols,
aldehydes, volatile fatty acids).
One particularly efficient technique for the hydrolysis of sludges implements
at least two reactors working in parallel, in each of which the sludges
undergo a full
cycle of thermal hydrolysis.
Each of the cycles of thermal hydrolysis implemented in a reactor comprises
the steps for feeding sludges to be treated into the reactor, injecting live
steam therein
to bring the sludges to a pressure P and a temperature T enabling hydrolysis,
maintaining them at this pressure P and this temperature T for a certain
period of
time, suddenly bringing the sludges to a pressure close to atmospheric
pressure in
releasing flash steam which is recycled to pre-heat the sludges to be treated
of the
reactor working in parallel and empty the reactor of the sludges thus
hydrolyzed.
According to this technique, it is planned that the cycle will be staggered
time
from one reactor to another to use the flash steam produced from one reactor
to inject
it into the other reactor.
Such an implementation makes it possible to take advantage of the flash steam
produced in one of the reactors to feed steam into the other reactor.
This type of method, which removes the need to make the sludges travel from
one reactor to another to perform the different steps of thermal hydrolysis,
has several
advantages. In particular, it simplifies the plants needed to implement the
method,
reduces the speed of clogging of these plants, minimizes the odors that can be
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produced during the passage of the sludges from one reactor to another and
reduces
live steam requirements. However, it has some limitations.
3. Drawbacks of the prior art
This type of method for the thermal hydrolysis of sludges can be improved.
This is especially the case when it is implemented in small-sized or medium-
sized
plants, i.e. plants treating a daily volume of sludges smaller than 10 m3 for
about
10,000 equivalent inhabitants.
In thermal hydrolysis, the main item of cost/expenditure relates to the
quantity
of steam injected into the sludges. At the sizing level, this affects the size
of the
steam-producing plants (boilers, steam generators, steam recovery equipment,
piping
systems, etc) implemented for this purpose. At the exploitation level, this
affects the
consumption of fuel to generate steam. It is therefore important to reduce the
quantity
of steam used to treat sludges to the utmost.
The quantity of steam to be injected into a sludge in order to take it to the
temperature desired in order to carry out its thermal hydrolysis is linked to
its
concentration in dry matter. Sludges are indeed constituted by a mixture of
dry matter
and water. When the sludges are heated, therefore, the temperature of both the
dry
matter and of the water needs to be heated. The result of this is that the
lower the
concentration of the sludge, i.e. the lower is its viscosity or its dryness,
the greater is
the volume of sludge to be heated and therefore the greater is the quantity of
steam
needed to heat it. This increases the consumption of live steam and therefore
increases the consumption of fuel (biogas, ou, natural gas, etc) used to
produce this
live steam.
Furthermore, the risk of odors being released at ah l levels of the sludge
treatment system is all the greater as the volume of the hydrolyzed sludges is
great.
It is therefore necessary to treat sludges that are as concentrated as
possible,
i.e. that have high viscosity or dryness, in order to limit the consumption of
steam and
reduce the production of hydrolyzed sludges and therefore the emanation of
odors.
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The transfer of steam into highly concentrated sludge however causes
problems. Indeed, it has been noted especially in existing methods that the
transfer of
steam into highly concentrated sludges is flot optimal. This problem of
transfer of
steam is especially encountered when injecting flash steam into sludges to be
treated,
at the start of thermal hydrolysis. This can be explained by the fact that the
transfer of
steam into sludges is linked to their concentration, the transfer being ail
the smaller as
the concentration of the sludges is great. The concentration of sludges to be
treated
should flot be excessively high so as flot to hinder the transfer of steam.
Ultimately, the optimizing of the thermal hydrolysis of sludges in terms of
reduction of steam consumption assumes that the following two antagonistic
factors
are taken into consideration:
the greater the concentration of the sludge, the smaller is the volume to be
treated (and the smaller the risks of emanation of odors) and the smaller is
the
quantity of steam to be injected to heat the sludges,
- BUT the greater the concentration of the sludge, the more difficult it is
to
carry out this transfer of steam and therefore the more difficult it is to use
a
small quantity of steam: this is therefore a boundary observed in prior-art
methods whereby the sludges are not concentrated beyond a certain value
failing which there the risk of having a poor transfer and an excessively high
consumption of steam.
4. Goals of the invention
The invention is aimed especially at overcoming these drawbacks of the prior
art.
More specifically, it is a goal of the invention to provide a technique of
thermal hydrolysis of sludges that leads to the limiting, in at least one
embodiment, of
the consumption of steam.
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It is another goal of the invention, in at least one embodiment, to implement
a
technique of this kind that reduces steam heat losses, especially flash steam
heat
losses.
The invention again is aimed at procuring a technique of this kind that makes
5 it
possible, in at least one embodiment, to reduce emanations of odors outside
the
reactors.
It is another goal of the invention to procure a technique of this kind that
makes it possible, in at least one embodiment, to provide for an efficient
hydrolysis of
sludges having high dryness in ensuring efficient transfer of the steam with
said
sludge in a confined environment improving steam/sludge exchanges enabling
especially fast condensation of steam in sludge, thus limiting steam
consumption.
It is another goal of the invention, in at least one embodiment, to improve
the
viscosity of the sludge before the injection of steam in improving especially
the
mixture of hydrolyzed sludges and fresh sludges.
The invention is also aimed, in at least one embodiment, at procuring a
technique of this kind that is reliable and/or low cost and/or compact and/or
simple to
implement.
5. Summary of the invention
These goals as well as others that shall appear here below are achieved by
means of a method for thermal hydrolysis of sludges to be treated, said method
being
carried out in at least two reactors working in parallel, in each of which the
sludges
undergo a full cycle of thermal hydrolysis, said cycle comprising the steps
consisting
in feeding said sludges to be treated into one reactor, injecting therein
flash steam
coming from the other reactor to preheat the sludges, injecting therein live
steam to
bring them to a pressure P and to a temperature T enabling hydrolysis,
maintaining
them at said pressure P and at said temperature T for a certain time, bringing
said
sludges to a pressure close to atmospheric pressure in order to release flash
steam and
cool the sludges, and emptying said reactor of said sludges thus hydrolyzed,
said
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cycle being staggered in time from one reactor to another to use the flash
steam
produced from one reactor to inject it into the other reactor.
According to the invention, such a method comprises a step consisting in
extracting a part of the sludges present in each of the reactors and
reintroducing them
into the corresponding reactor.
Thus, the invention relies on a wholly original approach, which consists of
the
extraction of a part of the sludges contained in a thermal hydrolysis reactor
and then
in reintroducing them into this reactor. In other words, the invention
consists of the
circulation of a part of the content of a thermal hydrolysis reactor in
itself, i.e. the re-
introduction, into a thermal hydrolysis reactor, of sludges that have been
extracted
from this reactor.
The pressures mentioned are expressed in effective pressure values.
The at least partly hydrolyzed sludges contained in a reactor have dryness
below that of the sludges to be treated introduced into the reactor and a
temperature
above that of these sludges. Thus, their viscosity is lower than that of the
sludges to
be treated. Through this invention, the mixture of partially hydrolyzed
sludges with
sludges to be treated is improved, and the mixture is thus perfectly
homogenous, and
the viscosity of the mixture is thus optimized.
Furthermore, depending on the nature of the means implemented to recirculate
the sludges (a pump for example), the viscosity of the mixture is diminished
by
mechanical effect when it passes through these sludges.
The transfer of steam into the sludges increases inversely proportionally to
their viscosity. The implementation of the invention therefore improves the
transfer
of steam into the sludges. The heat tosses and the steam consumption can thus
be
reduced.
Given that the heat tosses, for example the leakages of steam out of the
reactors, are reduced, the technique of the invention also limits olfactory
nuisance
throughout the sludge treatment system.
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The inventors have thus been able to optimize the thermal hydrolysis of the
sludges by circumventing two limiting factors:
the first factor in which the greater the dryness of the sludges to be
treated, the
less efficient is the transfer of steam into the sludges and therefore the
lower is
the efficiency of the hydrolysis;
the second factor according to which the lower the dryness of the sludges to
be treated, the greater is the volume of sludges to be treated and the higher
is
the consumption of steam and therefore the lower is the efficiency of the
hydrolysis.
According to an advantageous characteristic, at least a part of said live
steam
and/or said flash steam is injected into the sludges extracted from the
reactors before
they are reintroduced into the corresponding reactor.
By injecting steam into the recirculation loop, which is a confined
environment, we obtain an improvement in the steam/sludge exchanges enabling
especially fast and efficient condensation of the steam in the sludge, thus
limiting the
consumption of steam.
The improvement of the transfer of steam into the sludges obtained by such an
injection of steam into the sludge recirculation loop is such that the
technique
according to the invention efficiently hydrolyzes sludges that are more
concentrated
than those usually treated in prior-art methods. The sludges to be treated by
the
technique of the invention have a dryness preferably ranging from 14% to 30%.
Said sludges to be treated could be directly introduced into the reactors.
According to an advantageous characteristic of the invention, said sludges to
be treated are mixed with said extracted sludges before feeding the reactors.
The sludges to be treated are thus mixed with partly hydrolyzed sludges,
having lower dryness and higher temperature, before they are introduced into
the
reactor. The dryness of the mixture of the sludges introduced into the reactor
is
therefore lower than that of the sludges to be treated and the temperature of
the
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mixture of sludges introduced into the reactor is therefore higher than that
of the
sludges to be treated. By these two phenomena, the transfer of steam into the
sludges
inside the reactor is thus improved as compared with the case where the
sludges to be
treated are directly introduced into the reactor. This implementation thus
improves
the performance of the hydrolysis of the sludges while limiting steam
consumption,
in increasing the dryness of the sludges to be treated and reducing olfactory
nuisance.
According to first advantageous characteristic, 0% to 100% of the flash steam
produced in one reactor, and preferably 25% to 75%, is introduced into the
sludges
extracted from another reactor before they are reintroduced into this reactor.
According to a second advantageous characteristic, 0 to 100 % of the live
steam needed to conduct a cycle in a reactor, and preferably 0% to 50%, is
introduced
into the sludges that are extracted from it before being reintroduced therein.
According to these two variants, the remainder of the flash steam and/or live
steam is then introduced directly into the reactor.
The direct introduction, into the sludges extracted from a reactor, of such a
proportion of the total quantity of flash steam and/or live steam needed to
carry out
hydrolysis in the reactor gives efficient results in terms of hydrolysis of
the sludges,
reduction of the steam consumption, reduction of olfactory nuisance and
reduction of
the dryness of the sludge to be treated.
According to a preferred characteristic of the invention, the injection of
flash
steam and the step consisting in feeding the sludges to be treated into the
reactor take
place simultaneously.
The time of a full cycle of sludge hydrolysis can thus be reduced,
contributing
to improving productivity.
According to an advantageous characteristic of the invention, the pressure P
ranges from 3 to 12 bars, and said temperature T ranges from 140 to 180 C.
According to an advantageous characteristic, the pressure of the flash steam
ranges from 1 to 12 bars.
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The invention also covers a plant for implementing the method according to
any one of the variants described here above. Such a plant comprises at least
two
thermal hydrolysis reactors mounted in parallel, means for conveying sludges
to be
treated into each of said reactors, means for discharging hydrolyzed sludges
from
each of said reactors, means for injecting enabling live steam to be injected
alternately into each of said reactors and means for conveying and injecting
flash
steam coming from one reactor to the other reactor.
A plant according to the invention also comprises means for extracting a part
of the sludges present in each of the reactors, and means for reintroducing,
into the
corresponding reactor, sludges that have been extracted from it.
Such a plant advantageously comprises means for injecting live steam and/or
flash steam leading out between said means for extracting and said means for
reintroducing.
According to one variant, said means for conveying sludges to be treated open
directly into each of the reactors.
According to another advantageous variant, said means for conveying sludges
to be treated open out between said means for extracting and said means for
reintroducing.
6. List of figures
Other features and advantages of the invention shall appear more clearly from
the following description of preferred embodiments, given by way of simple
illustratory and non-exhaustive examples, and from the appended drawings, of
which:
- Figure 1 illustrates an example of a plant comprising two reactors
for
implementing a method according to the invention, comprising means for
conveying sludges to be treated directly into the reactors;
- Figure 2 illustrates an example of a plant comprising two reactors
for
implementing a method according to the invention, which comprises means
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for conveying sludges to be treated that lead into loops for the recirculation
of
sludges in the reactors;
- Figure 3 illustrates a plant implemented during trials performed to
verify the
efficiency of a technique according to the invention;
5 - Figure 4 illustrates the curve representing the steam consumption
as a function
of the dryness of the sludges to be treated during the implementing of thermal
hydrolysis according to the prior art and according to the invention.
7. Description of one embodiment of the invention
7.1. Reminder of the principle of the invention
10 The general principle of the invention is based on the recirculation, in
a
hydrolysis reactor, of a part of the sludges that it contains. It consists, in
other words,
in extracting a part of the sludges contained in a thermal hydrolysis reactor
and then
reintroducing these sludges into this reactor.
Because the extracted sludges have lower dryness, the transfer of steam into
the sludges is improved.
The invention thus reduces heat losses and therefore reduces the consumption
of steam and, as the case may be, the consumption of biogas used to produce
this
steam as well as olfactory nuisance. It also makes it possible, so long as the
transfer
of steam into the sludges is promoted, to more efficiently hydrolyze sludges
to be
treated, the dryness of which is relatively high.
7.2. Treatment plants
7.2.1. Example of a first embodiment of a plant according to the
invention
Referring to figure 1, we present a first embodiment of a plant for
implementing a method for the thermal hydrolysis of sludges according to the
invention.
Thus, as shown in this figure 1, such a plant comprises a first reactor 10 and
a
second reactor 20.
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The first reactor 10 comprises a first sludge inlet 320, a steam inlet 102, an
outlet 103 for sludges to be recirculated, an outlet 104 for hydrolyzed
sludges, and a
flash steam outlet 105. It comprises a second sludge inlet 101.
The second reactor 20 comprises a first sludge inlet 360, a steam inlet 202,
an
outlet 203 for sludges to be recirculated, an outlet 204 for hydrolyzed
sludges and a
flash steam outlet 205. It comprises a second sludge inlet 201.
The outlet 105 is connected to a conduit 11 for extracting flash steam, of
which one outlet is connected to a valve 12 and the other outlet is connected
to a
valve 13. The outlet of the valve 12 is connected to a vent. The outlet of the
valve 13
is connected to a conduit 14. The conduit 14 is connected to a conduit 15. A
conduit
16 is connected at one of its ends to the conduit 14 and the other one of its
ends to a
valve 17, the outlet of which is connected to a conduit 37 for conveying flash
steam
and/or live steam.
The conduit 14 is connected to another valve 18. The valve 18 is connected to
another conduit 19 for extracting flash steam. The outlet 205 is connected to
the
conduit 19 for extracting flash steam. This conduit 19 is also connected to a
valve 21,
the outlet of which is connected to a vent. A conduit 22 is connected at one
of its ends
to the conduit 14 and the other one of its ends to a valve 23, the outlet of
which is
connected to the conduit for conveying flash steam and/or live steam 33.
The conduit 14 opens into the conduits 16 and 23. This conduit 14 is situated
on either side of the conduit 15.
The conduit 15 is connected to a valve 24 and to a valve 25 which are
respectively connected to a conduit 26 and a conduit 27.
The conduit 26 is connected to inlet 102 of the first reactor 10.
The conduit 27 is connected to the inlet 202 of the second reactor 20.
The conduit 15 is connected to a conduit for conveying live steam 28 via a
valve 29.
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A conduit 32 for conveying sludges to be treated and a conduit 36 for
conveying sludges to be treated open into the first reactor 10 and the second
reactor
20 respectively via inlets 320 and 360.
One recirculation loop comprises a conduit 30, the inlet of which is connected
to the outlet 103 of the first reactor 10 and the outlet of which opens into
the inlet 101
of the first reactor 10. A pump 31 is mounted on this conduit 30. The conduit
33 for
conveying flash steam and/or live steam opens into the conduit 30.
Another recirculation loop comprises a conduit 34, the inlet of which is
connected to the outlet 203 of the second reactor 20 and the outlet of which
opens
into the inlet 201 of the second reactor 20. A pump 35 is mounted on this
conduit 34.
The conduit for conveying flash steam and/or live steam 37 opens into the
conduit 34.
A conduit 38 for extracting hydrolyzed sludges is connected to the outlet 104
of the first reactor 10. A conduit 39 for extracting hydrolyzed sludges is
connected to
the outlet 204 of the second reactor 20.
The conduits 28, 33 and 37 are connected to means for producing live steam
such as a boiler that is flot shown.
Control means (flot shown) are used to control the valves, the injection of
steam and sludges into the reactors as well as the extraction of hydrolyzed
sludges
from the reactors.
7.2.2. Example of a second embodiment of a plant according to the
invention
Referring to figure 2, we present a second embodiment of a plant for
implementing a method for the thermal hydrolysis of sludges according to the
invention.
As shown in this figure 1, such a plant is distinguished from the plant
according to the first embodiment by the fact that the conduits 32, 36 for
conveying
sludges to be treated, which open respectively into the first reactor 10 and
second
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reactor 20, are eliminated here and replaced by conduits 32', 36' for
conveying
sludges to be treated that respectively open into the recirculation conduits
30, 34.
7.3. Methods of treatment
7.3.1. Example of a method implementing the plant according to the first
embodiment
When implementing a method according to the invention, cycles of thermal
hydrolysis are implemented successively in each of the first and second
reactors 20.
Each cycle of thermal hydrolysis comprises:
- a step for feeding sludges to be treated into one reactor,
- a step for injecting, into this reactor, flash steam coming from the
other reactor
to pre-heat the sludges that are located therein and cool this reactor. This
step
can be performed simultaneously with the step for feeding sludges or it can be
performed in succession,
- a step for injecting live steam to take the sludges to be treated to the
pressure
P ranging from 3 to 12 bars and to the temperature T ranging from 140 C to
180 C enabling hydrolysis;
a step for keeping the sludges at the pressure P and the temperature T for a
time Tps ranging from 10 to 60 minutes;
a step for bringing the sludges back to a pressure close to atmospheric
pressure by releasing flash steam. This step takes place simultaneously with
the step for injecting flash steam into the other reactor;
a step for emptying the reactor of the sludges thus hydrolyzed.
The cycles are implemented in each of these reactors in a manner that is
staggered in time in order to inject, into one reactor, the flash steam
produced in the
other reactor at the end of the cycle.
In a first stage, sludges to be treated are introduced via the conduit 32 into
the
first reactor 10.
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The pump 31 is put into operation in order to extract a part of the sludges
contained in the first reactor 10 through the outlet 103 and to reintroduce
them via the
conduit 30 and the inlet 101.
Flash steam coming from the reactor 20 is injected simultaneously with the
feeding of sludges to the reactor 10:
either in the conduit 30 via the conduits 19, 14, 22 and 33;
or at the inlet 102 of the reactor 10 via the conduits 19, 14, 15 and 26.
After the feeding of sludges to the first reactor 10 is completed, the arrivai
of
sludges to be treated via the conduit 32 is stopped.
When the steps for feeding sludges and injecting flash steam are completed,
live steam is injected:
either into the first reactor 10 via the conduits 28, 26 and the inlet 102;
or into the conduit 30 via the conduit 33.
The pump 31 continues to be operated so that a part of the sludges contained
in the first reactor 10 is extracted via the outlet 103 and recirculated in
the first
reactor 10. If necessary, the live steam is injected into the sludges via the
conduit 33.
At the same time, the injection of live steam continues until the sludges are
gradually carried to a pressure P and to a temperature T:
either in a first reactor 10 via the conduits 28, 26 and the inlet 102;
- or in the conduit 30 via the conduit 33.
When these conditions are achieved, the injections of live steam are stopped
and the pump 31 is stopped so that the recirculation of sludges via the
conduit 30 is
stopped. The sludges are then kept at the pressure P and the temperature T for
a time
Tps to enable thermal hydrolysis.
When the thermal hydrolysis is completed in the first reactor 10, the pressure
of the hydrolyzed sludges is rapidly released until pressure close to
atmospheric
pressure is achieved, thus producing flash steam.
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At the same time, the sludges to be treated are introduced into the second
reactor 20 via the conduit 36. The pump 35 is put into operation in order to
extract a
part of the sludges contained in second reactor 20 through the outlet 203 and
reintroduce them via the conduit 34 and the inlet 201.
5 The flash steam thus produced in the first reactor 10 is extracted from
the
outlet 105 and introduced simultaneously with the feeding of sludges to the
second
reactor 20:
either in the second reactor 20 via the conduits 11, 14, 15, 27 and the inlet
202;
10 - or in the conduit 34 via the conduits 11, 14, 16 and 37.
When ail the flash steam coming from the first reactor 10 is injected into the
second reactor 20, the hydrolyzed sludges are extracted from the first reactor
10 via
the outlet 104 and the conduit 38.
After the feeding of the second reactor 20 is completed, the arrivai of
sludges
15 to be treated via the conduit 36 is stopped.
When the steps for feeding and injecting flash steam are terminated, live
steam is injected:
either into the second reactor 20 via the conduits 28, 27 and the inlet 202;
or into the conduit 34 via the conduit 37.
The pump 35 continues to be operated so that a part of the sludges contained
in the second reactor 20 is extracted therefrom via the outlet 203 and then
recirculated. If necessary, live steam is injected into these sludges via the
conduit 37.
At the same time, the injection of live steam continues until the sludges are
gradually carried to a pressure P and a temperature T:
- either in the second reactor 20 via the conduits 28, 27 and the inlet
202;
or in the conduit 34 via the conduit 37.
When these conditions are attained, the injections of lives steam are stopped
and the pump 35 is stopped so that the recirculation of sludges via the
conduit 34 is
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stopped. The sludges are then maintained at the pressure P and at the
temperature T
for a period of time Tps to enable thermal hydrolysis.
Each cycle therefore comprises a step of recirculation in which a part of the
sludges present in the reactor fed with sludges to be treated is extracted and
reintroduced into this reactor. This step of recirculation is preferably
implemented
during the step for feeding sludges to be treated into the reactor and during
the step
for injecting flash steam into this reactor. It could also be implemented from
the
feeding step up to the start of the step in which the sludges are maintained
under the
pressure P and at the temperature T, also called a maintaining step. In
general, the
step for recirculation can be implemented at the start of the feeding step and
the
maintaining step.
When the thermal hydrolysis is completed in the first reactor 20, the pressure
of the hydrolyzed sludges is quickly relaxed until it reaches a pressure close
to
atmospheric pressure, thus producing flash steam.
At the same time, a new step for feeding the first reactor 10 is implemented.
Sludges to be treated are introduced via the conduit 12 into the first
reactor. The
pump 31 is implemented in order to make the sludges circulate inside the
conduit 30
and they are introduced into the first reactor 10 via the inlet 101. Flash
steam coming
from the second reactor is injected into the first reactor 10 or into the
conduit 30.
The cycle continues then in the first reactor 10.
A plurality of cycles can thus be implemented in a staggered manner in each
of the reactors.
In the case of a starting of the plant, and when there is no flash steam
available (since ah l the reactors are stopped), ail the steam injected into
the first
reactor is live steam.
7.3.2. Example of a method implementing a plant according to the
second embodiment of the invention
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A method implementing a plant according to the second embodiment of the
invention is identical to the one implementing a plant according to the first
embodiment except that the sludges to be treated are injected no longer
directly into
the reactors but into recirculation loops. This injection can take place
upstream or
downstream to the pumps 31, 35.
7.4. Variants
In one variant, it can be plarmed that the injection of flash steam will flot
take
place simultaneously with the feeding with sludges to be treated but
subsequently.
A plant implemented to carry out a method according to the invention could
include more than two reactors within each of which cycles of thermal
hydrolysis are
implemented in succession, the cycles being staggered from one reactor to
another to
inject the flash steam produced in one reactor into another reactor within
which the
cycle has reached this stage.
7.5. Trials
Comparative trials were conducted to verify the efficiency of the technique
according to the invention in terms of steam consumption.
A first series of trials consisted in treating sludges by thermal hydrolysis
in the
prior-art plant illustrated in figure 3, which is comparable to that
illustrated in figure
1 except that the means for recirculating sludges and injecting steam into
extracted
sludges which are then reintroduced into the reactor was not implemented.
The consumption of steam according to the dryness of the sludges during
these first trials is represented by the squares shown in figure 4.
The second series of trials consisted in treating sludges by thermal
hydrolysis
in a plant comparable to the one illustrated in figure 2.
The consumption of steam according to the dryness of the sludges during
these second trials is represented by the diamonds of figure 4.
Analysis of the curves of figure 4 shows that whatever the value of the
dryness of the sludges to be treated, implementing the technique according to
the
CA 02871854 2014-10-28
18
invention reduces the consumption of steam. It also reduces the consumption of
steam when the dryness of the sludges to be treated increases.
The technique of the invention therefore reduces the consumption of steam
and provides efficiently for the thermal hydrolysis of sludges to be treated
having a
relatively high dryness and, in any case, having a dryness higher than of the
sludges
treated with the methods of the prior art.
The technique of the invention also avoids the need to implement a pre-
heating reactor upstream to the reactors within which the cycles of thermal
hydrolysis
are implemented. The invention thus enables the sludges to be treated in more
compact, low-cost plants.
