Note: Descriptions are shown in the official language in which they were submitted.
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PROCESS AND DE VICE FOR CONTINUOUS THERMAL HYDROLYSIS
1. Field of the invention
The present invention pertains to a method and a device for continuous
thermal hydrolysis of sludges containing organic matter. These sludges can be
derived for example from the treatment of domestic wastewater (sludges from
cleaning processes, fats derived from pre-treatment operations) or derived
from the
treatment of industrial wastewater or it can corne from drainage matter or
from grease
tanks. The term "sludges" shah l be used here below in the document. Such
sludges
have a dryness of 10% to 50% by weight.
2. Prior art
Sludges coming from the treatment of wastewater, whether of domestic or
industrial origin, can be treated biological means, especially by anaerobic
digestion.
The purpose of biological treatment is to degrade the organic matter contained
in these sludges. This degradation can be aimed at stabilizing the sludges,
enabling
the production of energy and/or again reducing the volume of sludges. However,
certain organic compounds are more difficult than others to degrade by
biological
means and it is known that pre-treatment by thermal hydrolysis accelerates the
process of biological degradation. This heat treatment is generally carried
out under
pressure at a temperature of over 100 C, and in practice capable of going up
to
220 C, for a predetermined period of time, in practice generally half an hour.
Through such thermal hydrolysis treatment, the poorly biodegradable organic
matter
is degraded into compounds that are then more easily biodegradable.
Classically, this subsequent biological degradation can be done by digestion
within an anaerobically operating closed reactor called a digester. Such
anaerobic
digesters cannot work properly unless they operate at an adequate and constant
temperature generally requiring a heating system and unless they are properly
stirred.
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This stirring is ail the easier as the sludges entering the digester are
fluids, i.e. they
have low viscosity.
There are various types of known methods of thermal hydrolysis in the prior
art. Certain of them are carried out by treating given quantities of sludges
to be
hydrolyzed, one by one, that is discontinuously (called batch operation) while
other
methods are designed to enable the continuous treatment, or at least semi-
continuous
treatment, of the sludges to be hydrolyzed.
In the prior art pertaining to these devices and methods of thermal
hydrolysis,
we can cite especially the patent documents W096/09882 and W02006/027062 both
of which pertain to batch-treatment methods.
Such batch-treatment methods have the drawback of necessitating the
management of the cycles of treatment of the different batches of sludges that
have to
be treated.
The techniques for the continuous or semi-continuous treatment of sludges by
thermal hydrolysis include the techniques described in the patent document
EP1198424 and those described in the patent document W02009/121873.
In the technique described in EP1198424, the sludges are conveyed into a
reactor where they travel in transit for a period of 5 to 60 min at a
temperature of
130 C to 180 C. The sludges hydrolyzed by such treatment are then cooled by
means
of a heat exchanger so as to make sure that their temperature is low enough to
prevent
the biomass of a digester from being destroyed. The energy thus recovered is
used to
heat the sludges before they enter the thermal hydrolysis reactor. This
technique
however implements steam recovery steps, the management of which, in practice,
proves to be difficult and is a constraint for the user. In addition, these
steam recovery
steps give the method in question a semi-continuous rather than a continuous
character.
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In the technique described in the patent document W02009/121873, the
sludges are treated continuously in a tube-shaped thermal hydrolysis reactor
into
which steam is directly injected.
This method has the advantage of being a truly continuous method. However,
although this method has considerably improved commercially existing methods
for
treating sludges by thermal hydrolysis, it nevertheless has certain drawbacks.
Firstly, if the viscosity of the sludges to be hydrolyzed introduced into the
reactor is excessively high, it can be difficult to inject steam into them. In
practice,
this method can treat sludges that have a high dryness ratio or "dry solids
content".
Beyond certain dryness ratios, the thermal hydrolysis could prove to be
incomplete or
non-optimal, which would limit the performance of the anaerobic digestion
placed
downstream from the thermal hydrolysis.
Secondly, the experiments conducted by the Applicant have shown that the
thermal and mechanical constraints observed within the thermal hydrolysis
reactor
used in the method described in W02009/121873 could necessitate special
constructional arrangements. It has been observed that the steam injected was
flot
totally condensed in the sludges beyond certain dryness ratios. In practice,
the steam
injected into the reactor can follow preferred paths. This problem is besides
identified
in the patent W02009/121873, especially the first paragraph of page 5 of this
document which specifies that when the reactor has a horizontal part, the
steam and
the sludges can tend to get separated into two layers, namely an upper layer
containing steam and a lower layer containing sludges.
Now, for ail the methods of thermal hydrolysis and especially for those that
work continuously, the critical phase of the method corresponds to the
transfer of the
steam into the sludge and its condensation therein. Indeed, if this phase is
not done
properly, the performance of the method of thermal hydrolysis can be
considerably
impaired whether it is in terms of chemical reaction or in economic terms
since the
quantity of steam to be used is then greater.
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The methods of thermal hydrolysis on dewatered sludges therefore corne
against the difficulty of injecting steam into the sludges in an efficient way
and, as a
corollary, against the difficulty of mixing them if they are too viscous.
Since the
sludges are viscous by nature, the greater their dryness, the more difficult
it is for the
steam to be injected into the sludge, to be mixed with it and to transfer its
energy to it
to bring about the thermal hydrolysis of the poorly biodegradable compounds.
In the batch methods, it is planned to carry out a stirring operation in the
treatment tanks to promote the intimate mixing of the steam with the sludges
to be
treated. Through such a stirred mixture made in the stirring tanks, the mixing
of
sludge and steam becomes intimate and the steam finally yields its energy in
getting
condensed in the sludges. However, in continuous-operation methods as well as
in
batch-operation methods, the problem constituted by the dryness of the sludges
is a
major one and, in practice, at least in the industrial-scale transposition of
the
techniques described and claimed in the patent documents described here above,
the
sludges must be limited to 25 % of dry matter by weight.
The patent document W02009/121873 recommends the use of static or
dynamic mixers in the reactor so as to improve the mixing of this steam with
the
sludges. This is explained in the last paragraph of page 5 of W02009/121873.
Such
mixers are particularly recommended when the steam is injected into a
horizontal part
of the reactor since such a horizontal part is identified, at indicated here
above, as a
zone in which steam has a special propensity for finding a preferred path of
discharge
and flot getting fully mixed with sludge, hence not properly yielding its
energy to the
sludge. This propensity therefore lowers the performance of the thermal
hydrolysis
reactor. It will be noted however that, to the Applicant' s knowledge, no
industrial-
scale embodiment implementing such dynamic or static mixers has effectively
been
implemented in commercial-scale installations until now.
Besides, in these installations, the hydrolysis reactors are of great length.
This
great length has a corresponding high retention time or retention time for the
sludges
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and steam in the reactor. Thus, the coefficient of transfer of energy from the
steam to
the sludge can be optimized. However, such great reactor lengths entail high
manufacturing costs.
3. Goals of the present invention
5 The
goal of the present invention is to propose a method, and a device
associated with the application of this method, that improves the technique
disclosed
in W02009/121873. This document is considered here to be the prior art closest
to
the invention, which shah l be described here below.
In particular, it is a goal of the present to describe such a method and such
a
device that enable the treatment of sludges intended for thermal hydrolysis
and
having a dry solids content greater than the maximum dry solids content that
can be
effectively implemented by the prior art, and to achieve this without
impairing the
performance of the digestion that classically follows the thermal hydrolysis
of the
sludges.
It is a also a goal of the present invention to propose a method of this kind
and
a device of this kind making it possible to obtain homogenous temperatures of
the
mixture of sludges and steam inside the reactor in order to attain high
performance
values of thermal treatment and thus do away with the mechanical constraints
on the
reactors related to inhomogenous temperatures.
It is yet another goal of the present invention to disclose such a method and
such a device that reduce the consumption of steam necessary for the
hydrolysis of
the sludges.
It is yet another goal of the invention to describe such a method and such a
device that can use reactors of smaller volumes, especially smaller lengths,
than in
the prior art while at the same time providing for optimized condensation of
the steam
in the sludges.
It is yet another goal of the invention to describe such a method and such a
device that enable the hygienization of the sludges.
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4. Summary of the Invention
Ail or part of these goals are achieved according to the invention which first
of ail relates to a method for the continuous thermal hydrolysis of sludges
containing
organic matter, said method comprising the steps of:
simultaneously injecting pressurized steam into said sludges and mixing said
sludges with said steam by means of a dynamic mixer-injector so as to obtain a
single-phase mixture,
conveying said single-phase mixture towards a tube reactor under pressure
and bringing about the plug flow of this mixture into said reactor for a
retention time
that is sufficient and at a temperature that is sufficient to enable the
thermal
hydrolysis of the organic matter present in said sludges,
cooling said single-phase mixture at its exit from said tube reactor to a
temperature enabling the subsequent digestion of the hydrolyzed organic matter
that
it contains,
depressurizing said cooled single-phase mixture.
It will be noted that, in the present description, the term "dynamic mixer-
injector" is understood to refer to any mixer constituted by a preferably
cylindrical
chamber that continuously receives said sludges, means for injecting steam
directly
into said chamber and means to prompt the vigorous stirring, through motor-
driven
mechanical means, of the different phases entering said chamber. The stirring
is
strong enough to enable a single-phase mixture of sludges and steam to be
obtained.
In practice, such means can advantageously be constituted by blades mounted on
a
rotation shaft moved by a rotor rotating at a speed of over 500 rpm,
preferably 1000
rpm to 2000 rpm. It will be noted that the purpose of such stirring means is
flot to
push the matter into the chamber but only to stir it. Thus, when they include
blades,
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these blades are shaped according to the knowledge of those skilled in the art
so that
putting them into motion does flot cause matter to move forward into the
chamber.
In the method of continuous thermal hydrolysis according to the invention, the
retention time of the matter in the dynamic mixer-injector is short. The
preferably
cylindrical chamber of the mixer-injector therefore advantageously has a small
volume. As a corollary, the load loss of this matter when it passes into the
chamber is
small. In practice, this load loss must be smaller than 10%.
The mixer-injector implemented in the framework of the invention is therefore
distinct from the simple mixers constituted by a tank provided with stirring
means in
which the time of retention of the matter is lengthy and enables the treatment
of only
a given limited quantity of this matter at a time.
This mixer-injector is also distinct from simple sludge-conveying devices, for
example devices using worm screws.
Thus, the invention proposes to carry out the mixing of pressurized steam with
the sludges to be hydrolyzed in order to obtain a perfect, single-phase
mixture of
heated sludges upstream to the thermal hydrolysis step subsequently performed
in a
tube reactor.
Thus, according to the invention, the phase for mixing sludge with pressurized
steam is clearly distinct from the phase of thermal hydrolysis. Besides, both
these
phases are conducted efficiently in distinct apparatuses.
The single-phase mixing done prior to the thermal hydrolysis enables the
steam to get condensed in the sludges at the dynamic mixer-injector This
homogeneous mixture is then conveyed to the reactor where it can flow in a
plug
flow. In the form of single-phase liquid phase, it enters the reactor at
uniform or
almost uniform temperature at which the poorly biodegradable compounds can
undergo thermally hydrolysis efficiently and in an optimized way.
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Classically, at the exit from the tube reactor, this single-phase mixture
which
contains hydrolyzed, organic matter is brought to a temperature and a
concentration,
by dilution if necessary, enabling its subsequent digestion
Thus, the invention is clearly distinct from the prior art and especially from
the patent document W020069/121873 by the characteristic according to which
the
mixing of sludges to be hydrolyzed with the steam is done upstream to the
thermal
hydrolysis reactor and flot within it.
Such an option marks a break with the teaching of this prior art which
indicated the possibility of using a static or dynamic mixer integrated with
the
reactor. This prior art does flot make it possible, however, to obtain a
mixture that is
homogeneous enough to optimize the thermal hydrolysis. The present invention
resolves this problem by making this mixture upstream to the reactor so that
the phase
that enters this reactor is completely homogenous and so that the energy
provided by
the steam within this mixture can be transferred to the sludge in such a way
that ail
the matter capable of being thermally hydrolyzed can be hydrolyzed by
providing for
a sufficient retention time, i.e. a sufficient length of reactor.
Through the homogeneity of the sludge and steam travelling through the
reactor, a homogeneity of temperature of this mixture can be obtained in it.
Such
homogeneity of temperature can remove the need for preferred paths to carry
steam
within the reactor and, as a corollary, remove the thermal and mechanical
constraints
inherent in the appearance of such preferred paths of flow.
In particular, the perfect mixing of steam and sludges uniformly reduces their
viscosity and therefore removes the mechanical effects related to sludge
shear.
The obtaining of a single-phase homogeneous mixture of sludges heated
upstream to the reactor, obtained from sludges to be hydrolyzed and steam,
within a
dynamic mixer-injector has the advantage of enabling the treatment of sludges
to be
hydrolyzed with high dry solids content, and especially a dry solids content
of over
20% by weight.
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According to a preferred variant of the invention, at its exit from said mixer-
injector, said single-phase mixture has a temperature of 100 C to 200 C (i.e.
the
temperature in the reactor enabling the thermal hydrolysis of the organic
matter
present in said sludges) and pressure of 1 bar a to 25 bar a. It will be noted
that, in the
present description, the unit of pressure in the absolute bar or bar a.
Advantageously, at its exit from said mixer-injector, said single-phase
mixture
has a temperature of 150 C to 170 C (i.e. the temperature in the reactor
enabling the
thermal hydrolysis of the organic matter present in said sludges) and pressure
of 5 bar
a to 20 bar a.
According to a preferred variant of the invention, the steam that will be used
to make the single-phase mixture will have a temperature of 100 C to 220 C
and a
pressure of 1 bar a to 23 bar a. Preferably, among ail the possible values, a
temperature of 180 C to 200 C and a pressure of 10 bar a to 16 bar a will be
chosen
.for this steam.
The quantity of steam thus provided to the sludges will depend on the dry
solids content of these sludges as well as on their concentration in organic
matter to
be hydrolyzed.
As indicated here above, the retention time of the single-phase mixture within
the reactor will be sufficient to enable the thermal hydrolysis of the organic
matter
but, in principle, will preferably range from 10 minutes to 2 hours and
preferably,
inter alia, from 20 to 40 minutes.
Advantageously, the retention time of said single-phase mixture in said
reactor will be at least 20 minutes and the temperature of said mixture in the
reactor
will be at least 100 C so that the method of the invention will also enable
the
hygienization of said sludges, the totality of these sludges then being
subjected to the
steam for a sufficiently lengthy period of time, and at a sufficiently high
temperature.
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A temperature of over 70 C for at least 20 minutes applied to the sludges is
necessary
to hygienize them.
According to a prefeffed variant of the invention, the step for cooling the
single-phase mixture at its exit from the tube reactor at a temperature
enabling the
5 subsequent digestion of the hydrolyzed organic matter that it contains
will be carried
out by addition of water and/or sludges and/or by the use of a heat exchanger.
This
will also enable the dilution of this single-phase mixture. Such dilution is
indeed
necessary to enable efficient subsequent digestion of these thermally
hydrolyzed
sludges. This mixture will then attain a sufficiently low temperature and will
be
10 diluted enough to comply with the biology of the digester.
Also preferably, the method according to the invention comprises preliminary
steps for dewatering and homogenizing the sludges in order to convey them to
the
dynamic mixer-injector, these preliminary steps leading to sludges having a
dry solids
content of 10% to 50% by weight, advantageously 20% to 35% by weight. It may
be
recalled that, in practice, the prior-art devices do flot enable the efficient
hydrolysis of
sludges having a dry solids content of over 25 by weight.
According to one advantageous variant of the method of the invention, it
comprises a step for adapting the conditions of implementation of the dynamic
mixture according to the dry solids content of the sludges. Thus, the dynamic
mixer-
injector will include a bladed rotor. The speed of rotation of these blades
could be
modified according to the dry solids content so that the single-phase mixture
can be
made even when this dry solids content is high
According to another aspect, the invention also covers any method for
implementing the above-described method comprising:
means for intake of sludges containing organic matter,
means for intake of pressurized steam,
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a tube reactor for thermal hydrolysis,
means for injecting dilution water and/or sludges provided downstream from
said tube reactor,
means for cooling provided downstream from said tube reactor,
characterized in that it comprises at least one dynamic mixer-injector
provided
upstream to said tube reactor for thermal hydrolysis and,
means for depressurizing provided downstream from said cooling means.
Such a device according to the present invention can be clearly distinguished
from the prior art disclosed W02009/121873 by the characteristic according to
which
a dynamic mixer-injector is planned upstream to the technical hydrolysis tube
reactor
and flot integrated with the thermal hydrolysis reactor. As specified here
above, the
use of instrumentation to mix the sludges to be thermally hydrolyzed and the
steam,
namely the use of the dynamic mixer, and a distinct instrumentation to carry
out the
thermal hydrolysis of the thermally hydrolysable compounds contained in these
sludges, namely a tube reactor, optimizes the working of this thermal
hydrolysis tube
reactor. This optimization leads to obtaining hydrolyzed sludges having a
higher
content in hydrolyzed compounds easily digestible within a digester and to the
possibility of being able to give this tube reactor a smaller volume.
Such a device according to the invention therefore enables the treatment of
the
sludges by thermal hydrolysis in a smaller reactor volume. This is a non-
negligible
economic advantage over the prior art.
As already specified, different types of dynamic mixers could be used in the
implementing of the present invention. However, the device according to the
invention will advantageously be provided with a dynamic mixer-injector that
has a
chamber provided with a bladed rotor, the speed of rotation of which could be
adapted to the dry solids content of the sludges as indicated here above and
in
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practice rotating at more than 500 rpm and preferably between 1000 rpm and
2000
rpm. It will be noted that the geometry of the blades could itself be adapted
to the dry
solids content and viscosity of the sludges.
The prior art according to the patent W02009/121873 provides, in its general
descriptive part, for approximately ail the possible shapes of tube reactor.
However,
the embodiments of this technique given in this patent document recommend that
this
reactor be made horizontally. One embodiment described in this patent document
W02009/121873 provides for an entry of sludges into one end of the tube
reactor,
with an injection of steam in proximity to this end, an exit of hydrolyzed
sludges
being provided at the other end of this tube reactor, means for injecting
cooling water
being provided at this second end. In another embodiment described in this
patent
document W02009/121873, the thermal hydrolysis tube reactor has a first
vertical
part extended by a second longer horizontal part. The reason why each of these
prefeffed embodiments has a relatively long horizontal part results from the
need to
put the sludge into contact with steam for a sufficiently lengthy retention
time so that
not only does the thermal hydrolysis occur but, prior to this hydrolysis,
within the
tube reactor, the steam injected at the start of the reactor can get condensed
in the
sludges in order to transfer to them the energy needed for their hydrolysis.
Through the invention, since the injection of steam has taken place upstream
to the reactor, through the use of the dynamic mixer-injector, it is a
perfectly mixed
single-phase mixture that arrives into the reactor so much so that the reactor
in
question no longer has to act as a condenser but only as a thermal hydrolysis
reactor.
Its volume can therefore be reduced as compared with the prior art. Indeed, in
the
prior art, the reactor must act both as a condenser and as a reactor, and this
gives it a
great volume and especially a great length.
According to the invention, the thermal hydrolysis reactors implemented
could have varied shapes. However, according to a preferred variant, the tube
reactor
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for thermal hydrolysis will be vertical and will have an inlet at its lower
end and an
outlet at its upper end.
According to another preferred variant, this tube reactor for thermal
hydrolysis will have a first vertical section directly extended by a second
vertical
section, the inlet of the reactor being provided at the foot of said first
vertical section
and the outlet of said reactor being provided at the foot of said second
vertical
section. It will be noted that, in the context of the present invention, the
expression
first vertical section "directly extended by a second vertical section" will
be
understood to cover the embodiments in which there is no straight horizontal
section
provided between the first vertical section and the second vertical section.
Indeed,
such a horizontal section is urmecessary inasmuch as the tube reactor of the
device
according to the invention is a thermal hydrolysis reactor and flot a reactor
also acting
as a condenser.
According to yet another variant, said tube reactor for thermal hydrolysis has
a first vertical section connected to a second vertical section, the inlet of
the reactor
being provided at the head of said vertical section and the outlet of said
reactor being
provided at the foot of said second vertical section.
According to an interesting variant, the device according to the invention
also
comprises a heat exchanger provided downstream from the reactor.
Also advantageously, the device comprises a pump and a valve, preferably a
progressing cavity pump, intended to maintain the pressure in the tube reactor
for
thermal hydrolysis.
5. List of figures
The invention as well as its different advantages will be understood more
easily from the description of embodiments given with reference to the
figures, of
which:
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Figure 1 is a schematic view of a device for the thermal hydrolysis of
sludges according to the invention (surrounded by a une of dots and
dashes) integrated into an installation including a digester provided
downstream from this digester;
Figure 2 represents a form of thermal hydrolysis tube reactor that can
be implemented in the present invention;
Figure 3 represents another form of a thermal hydrolysis tube reactor
that can be implemented in the present invention;
Figure 4 represents yet another form of thermal hydrolysis tube reactor
that can be implemented in the present invention;;
Figure 5 is a graph showing, on the one hand, the development of the
temperature within the tube reactor of a prior-art installation according
to the patent documents W02009/121873 that does not integrate a
dynamic mixer-injector but in which steam and sludge are conveyed to
the head of the reactor and, on the other hand, the development of the
temperature within the tube reactor of an installation corresponding to
the invention integrating a dynamic mixer-injector in which steam and
sludge are mixed and then conveyed in the form of a homogeneous
single-phase mixture to the head of the reactor.
6. Description of embodiments
Referring to figure 1, a device according to the invention is described
schematically. This device 1000 is integrated into an installation including a
digester
9 that is flot a part as such of the device according to the invention.
Such an installation can be used to implement a method of lysis-digestion
(LD) but it will be noted that it is also possible to integrate the method
according to
the invention into known prior-art configurations called digestion-lysis (DL)
or
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digestion-lysis-digestion (DLD), given that in the configuration called DL a
part of
the sludge is hydrolyzed and then returned to the digester.
Referring to figure 1, centrifuged sludges are conveyed by a pipe 1 to a
hopper 2 provided with two worm screws enabling these sludges to be
homogenized.
5 These
two worm screws also serve to cram a feeder pump 3 feeding sludges to
the dynamic mixer-injector 4. The dewatered and homogenized sludges coming
from
the hopper 2 are thus pumped by means of the pump 3 into a pipe using means
for
leading these sludges into the dynamic mixer-injector 4. This dynamic mixer-
injector
4 is also provided with means for injecting steam 100 generated by a steam
generator
10 flot
shown in figure 1. The mixer-injector comprises a cylindrical chamber provided
with stitTing means constituted by blades mounted on a rotation shaft moved by
a
rotor rotating at a speed of 1000 rpm to 2000 rpm, this speed being adjustable
according to the dry solids content of the sludges. The retention time of the
matter
travelling continuously through the mixer-injector is short and, in practice,
shorter
15 than
10 min. The blades do flot make the matter move forward in the chamber but
only stir it vigorously.
A wash water intake 200 is planned upstream to the dynamic mixer-injector 4.
Through such water inflow means 200, the dynamic mixer-injector could be
cleaned
if need be.
At the exit from the dynamic mixer, a pipe enables the single-phase mixture
made within this mixer to be conveyed towards a thermal hydrolysis reactor 5.
The treatment within this thermal hydrolysis reactor 5 is done at a
temperature
of 165 C to 180 C, the interior of the reactor being maintained at a pressure
of 8 bar
a to 10 bar a (in this respect, it will be noted that lower or higher
pressures could be
implemented, depending especially on the dry solids content of the sludges).
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A water inlet 101 situated at the entry to the reactor 5 is provided to enable
cleaning water to be led into the reactor during the cleaning phases that can
be carried
out when starting up the installation or during phases of maintenance of the
installation.
At the exit from the reactor 5, a drain 102 is, for its part, provided in
order to
remove non-condensable gases if any.
The hydrolyzed sludges in the reactor 5 are then conveyed by a pipe to a heat
exchanger 7. Before reaching this heat exchanger 7 cooling and dilution water
is led
into the hydrolyzed sludges by water injection means 201. If need be, this
dilution
could also be done after the exchanger 7.
At the exit from the exchanger 7, the diluted sludges are conveyed to the
digester 9. The depressurizing unit 8 which, by definition, causes a drop in
pressure,
is used to maintain the pressure prevailing in the thermal hydrolysis reactor
5. In the
present example, this unit is constituted by a progressing cavity pump
provided
between the heat exchanger and the digester. In other embodiments, it could be
constituted by a valve or any other unit used to carry out this function.
At the exit from the device according to the invention, the thermally
hydrolyzed sludges are sent to the digester 9 where they can be easily
digested
because they have undergone thermal hydrolysis.
It is clearly specified that the representation in figure 1 of an installation
integrating a device according to the invention is a schematic representation.
In
particular, the reactor 5 in which the thermal hydrolysis of the single-phase
mixture
of sludges and steam is carried out could have different shapes. Three of
these shapes
are given with reference to figures 2, 3 and 4.
As shown in figure 2, the reactor 5 has a vertical shape. The reactor is
provided, in its lower part, with an intake of single-phase mixture of sludges
heated
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with steam 501 and, in its upper part, with an outlet from the reactor 502. A
drain
503 is provided to remove any non-condensable gases and means for measuring
the
pressure prevailing inside the reactor are also provided in its upper part.
Referring to figure 3, the thermal hydrolysis reactor has a first vertical
section
provided at its base with an intake of single-phase mixture of sludges and
steam 401,
directly connected to a second vertical part provided at its foot with a
discharge
feature 402 for hydrolyzed sludges. A drain 403 is provided at the junction
between
these two vertical parts to remove the non-condensable gases if any. Means for
measuring the pressure and the temperature in the reactor are also planned. It
will be
noted that, in this configuration, the second vertical section part is
directly connected
to the first vertical section without a horizontal section between the two.
Referring to figure 4, the thermal hydrolysis reactor has a first vertical
section
provided at its head with an intake of single-phase mixture of sludges and
steam 601,
directly connected to a second vertical part provided at its foot with a
discharge 602
of the hydrolyzed sludges. A drain 603 is provided at the junction between
these two
vertical parts to remove the non-condensable gases if any. Means for measuring
the
pressure and the temperature in the reactor are also planned. It will be noted
that, in
this configuration, the second vertical section part is directly connected to
the first
vertical section without a horizontal section between the two.
Figure 5 shows the progress in time of the temperature prevailing within the
thermal hydrolysis reactor:
- on the one hand, in the invention, implementing a dynamic mixer-injector
provided upstream to the thermal hydrolysis reactor and;
- on the other hand, in a similar installation according to the prior art in
which
no dynamic mixer-injector is used, the steam being injected at the foot of the
reactor.
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Referring to this figure 5, it can be seen that, in the present invention, the
temperature prevailing within the reactor gradually rises until it reaches and
keeps the
set-value temperature enabling the optimized thermal lysis of the hydrolysable
organic compounds contained in the treated sludges.
In the prior art, the temperature observed in the reactor is, at the outset,
that of
the injected steam. The temperature then undergoes major variations. This
results
from the fact that, in the technique according to this prior art, there is no
systematic
occurrence of an intimate mixing of steam with the sludges. On the contrary,
the
temperature fluctuations observed within the reactor result from the existence
of
polyphase flows within it. In the example described here, since the steam is
injected
at a speed (in practice above 5 m/s) far greater than that of the sludges (in
practice
below 3m/s), it finds preferred passage through this sludge and does not
intimately
mix with this sludge, and does not efficiently yield its energy to the sludge.
Quite to the contrary, through the use of a dynamic mixer-injector according
to the invention upstream to the hydrolysis reactor, the mixture reaching this
reactor
is a perfectly liquid and homogenous single-phase mixture. It can therefore
flow in a
plug flow in this reactor. The set-value temperature is kept throughout the
retention
time in the reactor. The energy of the steam is therefore transferred in an
optimized
way to the sludges and the hydrolysis of the poorly biodegradable compounds
can be
donc efficiently.
It will also be noted that, through the invention, the theoretical quantity of
energy to hydrolyze a given quantity of sludges corresponds more or less to
the
quantity effectively implemented to obtain this hydrolysis. In this respect,
it will be
noted that it is easy to compute the energy needed to increase the temperature
of a
fluid from a temperature A to a temperature B. In the framework of trials made
by the
Applicant, the computed theoretical flow rate of steam was 25 kilograms of
steam at
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19
13 bar a per hour, and the trials have shown that it was exactly this flow
rate of steam
that was effectively necessary to efficiently hydrolyze the sludges.
In the prior-art installation, it was shown that the mixture between the
sludge
to be hydrolyzed and the steam was imperfect since the quantity of steam
effectively
injected to heat the sludge (15kg/h) was smaller than the theoretically
computed
quantity (25kg/h). A certain quantity of steam was therefore flot condensed in
the
sludge. The presently described trials confirm the interest of the pre sent
invention.
Finally-, it will be noted that the invention enables the use of reactors
having a
volume 20% to 25 % smaller than the volumes of the prior-art reactor.
