PROCESS AND PLANT FOR THERMAL HYDROLYSIS OF SLUDGE

Abstract

Method and installation of thermal hydrolysis of sludges implementing a group of thermal hydrolysis reactors (71,72,73,74) characterized in that it comprises successions of cycles, each of these successions of cycles being dedicated to one of said thermal hydrolysis reactors, each cycle comprising: a step a) for conveying a batch of non-preheated sludges to be treated into a thermal hydrolysis reactor (71,72,73,74), said step for conveying comprising the continuous passage of the sludges of said batch of sludges into a dynamic mixer (3) into which recovery steam is injected; a step b) for injecting live steam into said thermal hydrolysis reactor (71,72,73,74) containing said batch of sludges so as to increase the temperature and the pressure prevailing in this reactor; a step c) of thermal hydrolysis of the batch of sludges in the thermal hydrolysis reactor; a step d) for emptying the content of the batch of hydrolyzed sludges of said thermal hydrolysis reactor towards a recovery vessel (13), and for concomitant de-pressurizing of said reactor prompting the emission of recovery steam from the recovery vessel (13); the cycle starting points of the successions of cycles being staggered in time so that the steps a) of a succession of cycles are concomitant with the steps d) of another succession of cycles, the recovery steam emitted during the steps d) of a succession of cycles constituting the recovery steam injected during the steps a) of another succession of cycles.

Claims

1-22. (canceled)

23. A method of thermally hydrolyzing sludge employing a series of thermal hydrolysis reactors and a steam recovery vessel that yields recovered steam as a result of the lowering of pressure in the respective thermal hydrolysis reactors, comprising: thermally hydrolyzing the sludge in each of the thermal hydrolysis reactors by a process comprising: (i) in the absence of pre-heating the sludge, continuously pumping the sludge into and through a dynamic mixer having a driven mixer therein; (ii) injecting recovered steam from the steam recovery vessel into the dynamic mixer; (iii) reducing the viscosity of the sludge and homogenizing the sludge and recovered steam by mixing the recovered steam with the sludge in the dynamic mixer; (iv) directing the sludge from the dynamic mixer to a first thermal hydrolysis reactor of the series of thermal hydrolysis reactors; (v) injecting live steam into said first thermal hydrolysis reactor and increasing the temperature and pressure in said first thermal hydrolysis reactor; (vi) thermally hydrolyzing the sludge in said first thermal hydrolysis reactor; (vii) after the sludge has been thermally hydrolyzed in said first thermal hydrolysis reactor, directing the sludge from said first thermal hydrolysis reactor to said steam recovery vessel and decreasing the pressure in said first thermal hydrolysis reactor causing the emission of the recovered steam in the steam recovery vessel which constitutes the recovered steam used in the dynamic mixer; and thermally hydrolyzing sludge in each of the thermal hydrolysis reactors by repeating the above process.

24. The method according to claim 23 including sufficiently mixing the sludge and recovered steam in the dynamic mixer to yield a single phase mixture of sludge and recovered steam.

25. The method according to claim 23 wherein the dynamic mixer includes motor driven blades for homogenizing the sludge and recovered steam and the method includes driving the blades at a speed of over 500 rpm and at an rpm speed sufficient to homogenize the sludge and recovered steam; and wherein the blades only stir the sludge and do not push the sludge through the dynamic mixer; and wherein the method includes continuously pumping the sludge through the dynamic mixer such that the sludge continuously moves through the dynamic mixer and is not retained therein.

26. The method according to claim 25 wherein the dynamic mixer is not a tank but includes a chamber having a relatively small volume through which the sludge is continuously moved.

27. The method according to claim 23 wherein the dynamic mixer and the treatment of sludge therein is capable of handling sludge having a relatively high percentage by weight of dry matter and wherein the sludge pumped into the dynamic mixer includes 15%-35% by weight of dry matter.

28. The method according to claim 23 further including maintaining the pressure in the steam recovery vessel lower than the pressure in said first thermal hydrolysis reactor.

29. The method according to claim 23 wherein the series of thermal hydrolysis reactors include the first thermal hydrolysis reactor and second, third and fourth thermal hydrolysis reactors; and wherein the process described in claim 23 is simultaneously carried out for all four thermal hydrolysis reactors but the respective processes are staggered.

30. The method according to claim 23 including producing non-condensable gases in said first thermal hydrolysis reactors and venting the non-condensable gases from a top portion of said first thermal hydrolysis reactor.

31. The method according to claim 23 including maintaining the pressure within said first thermal hydrolysis reactor at 3.5-10 bar (a) while the sludge is being thermally hydrolyzed therein; and maintaining the pressure inside the steam recovery vessel at 1.1-3 bar (a).

32. A method of thermally hydrolyzing sludge in a group of thermal hydrolysis reactors characterized in that the method comprises successions of cycles, each of these successions of cycles being dedicated to one of said thermal hydrolysis reactors, each cycle comprising: a step a) for conveying a batch of non-preheated sludge to be treated into a thermal hydrolysis reactor, said step for conveying comprising the continuous passage of said batch of sludge into a dynamic mixer into which recovery steam is injected; a step b) for injecting live steam into said thermal hydrolysis reactor containing said batch of sludge so as to increase the temperature and the pressure prevailing in this reactor; a step c) of thermal hydrolysis of said batch of sludge in said thermal hydrolysis reactor; a step d) for emptying the contents of said batch of hydrolyzed sludge of said thermal hydrolysis reactor into a recovery vessel and for concomitantly de-pressurizing said reactor prompting the emission of recovery steam from said recovery vessel; the cycle starting points of the successions of cycles being staggered in time so that the steps a) of a succession of cycles are concomitant with the steps d) of another succession of cycles, and the recovery steam emitted during the steps d) of a succession of cycles constituting the recovery steam injected during the steps a) of another succession of cycles.

33. The method according to claim 32 characterized in that said sludge to be treated has a dryness of 10% to 35% by weight of dry matter.

34. The method according to claim 32 characterized in that the method comprises a discharging of non-condensable gases from said thermal hydrolysis reactor during said steps a) and d).

35. The method according to claim 32 characterized in that the duration of the step a) is from 5 to 30 min, the duration of the step b) is from 5 to 30 min, the duration of the step d) is from 5 to 30 min, and the duration of the step c) is from 5 to 120 min.

36. The method according to claim 32 characterized in that, during the step c) of thermal hydrolysis, the temperature of said batch of sludge is from 120° C. to 200° C.

37. The method according to claim 32 characterized in that, during the step c) of thermal hydrolysis, the pressure inside the thermal hydrolysis reactor is from 2 to 16 bar(a).

38. The method according to claim 32 characterized in that the pressure inside the recovery vessel is maintained at 1.1 to 3 bar(a).

39. The method according to claim 32 characterized in that said step a) is implemented in such a way that, during the step c), the thermal hydrolysis reactor is filled to between 70% and 95% of its total capacity by volume.

40. A system for thermally hydrolyzing sludge and recovering steam used to thermally hydrolyzed the sludge and employing the recovered steam to treat the sludge before the sludge is subjected to thermal hydrolysis, the system comprising: (a) a plurality of thermal hydrolysis reactors for receiving the sludge and thermally hydrolyzing the sludge; (b) each thermal hydrolysis reactor including a sludge inlet, a sludge outlet and a live steam inlet; (c) a steam recovery vessel disposed downstream of the thermal hydrolysis reactors and operatively connected to each of the thermal hydrolysis reactors for receiving sludge from the thermal hydrolysis reactors and recovering steam used in the reactors to thermally hydrolyze the sludge; (d) the steam recovery vessel including a sludge inlet, a sludge outlet, and a recovered steam outlet; (e) a dynamic mixer located upstream of the thermal hydrolysis reactors for receiving the sludge and recovered steam and for reducing the viscosity of the sludge and for homogenizing the sludge and recovered steam; (f) the dynamic mixer including a sludge inlet, a sludge outlet, a mixer disposed interiorly of the dynamic mixer for mixing the sludge and the recovered steam, and a recovered steam inlet; (g) sludge lines operatively connected between the sludge outlet of the dynamic mixer and the sludge inlets of the thermal hydrolysis reactors for conveying sludge from the dynamic mixer to the thermal hydrolysis reactors; (h) a steam recovered line operatively connecting the recovered steam outlet of the steam recovery vessel and the recovered steam inlet of the dynamic mixer for conveying recovered steam from the steam recovery vessel to the dynamic mixer; and (i) means for pumping sludge into the sludge inlet of the dynamic mixer and continuously pumping the sludge through the dynamic mixer and out the sludge outlet thereof such that the sludge and recovered steam is homogenized as the sludge passes through the dynamic mixer.

41. The system of claim 40 wherein there is no pre-heater upstream of the dynamic mixer for pre-heating the sludge.

42. The system of claim 41 wherein the mixer in the dynamic mixer comprises a rotating blade that turns at an rpm of over 500.

Description

LIST OF FIGURES

[0075] The invention, as well as the different advantages that it presents, will be understood more easily from the following description of one embodiment of the plant and two variants of implementation of a method according to this embodiment, as non-exhaustive examples with reference to the figures, of which:

[0076] FIG. 1 is a schematic representation of one embodiment of a plant according to the present invention, including four thermal hydrolysis reactors;

[0077] FIG. 2 represents a block diagram for implementing the plant of FIG. 1 using an example of a method according to the invention;

[0078] FIG. 3 represents a block diagram for implementing the plant of FIG. 1 by means of another example of a method according to the invention;

[0079] FIG. 4 is a graph illustrating the results of comparative tests obtained by means of the invention on the one hand and by means of the prior art as described in FR2820735 on the other hand.

DESCRIPTION OF EMBODIMENTS OF THE INVENTION

[0080] Plant

[0081] Referring to FIG. 1, the embodiment of the plant according to the invention herein described comprises a group of four thermal hydrolysis reactors. It would be noted however that, in other embodiments, the number of thermal hydrolysis reactors, which will always be at least 2, could be different from 4. In practice the number of thermal hydrolysis reactors will preferably range from 2 to 6.

[0082] The plant represented comprises a pipe 1 for conveying sludge to be treated to a dynamic mixer 3. To this end, a pump 2 is provided on the pipe 1. These sludges are not preheated.

[0083] The dynamic mixer 3 comprises an essentially cylindrical chamber 31 within which there is provided a blade rotor 32. An inlet 4 of recovery steam is also fitted into this dynamic mixer.

[0084] The plant furthermore comprises 4 thermal hydrolysis reactors 71, 72, 73, 74. These thermal hydrolysis reactors are identical, and therefore each of them has the same capacity by volume. Each of them is provided in its upper part with discharging means 101, 102, 103, 104 intended for discharging the non-condensable gases coming from the hydrolyzed sludges that they treat.

[0085] The plant also comprises a vessel 13 for recovering hydrolyzed sludges coming from the reactors 71, 72, 73, 74. This vessel 13 is a closed vessel provided in its upper part with a pipe 14 connected to the inlet of recovery steam equipping the dynamic mixer 3.

[0086] Finally, the plant comprises means 15 for discharging hydrolyzed treated sludges from the vessel 13.

[0087] It will be noted that, in the plant described herein, a single dynamic mixer 3 is used to serve the four thermal hydrolysis reactors 71, 72, 73, 74. Specific means are included in the plant to enable the setting up of the fluidic communication of this common dynamic mixer 3 alternately with each of these thermal hydrolysis reactors 71, 72, 73, 74. These means include a pipe 5, one end of which is connected to the dynamic mixer 3 and the other end of which is connected to pipe arms 51, 52, 53, 54 respectively serving the thermal hydrolysis reactors 71, 72, 73, 74. On each of these pipe arms 51, 52, 53, 54 a valve 61, 62, 63, 64 respectively is provided.

[0088] The thermal hydrolysis reactors 71, 72, 73, 74 are furthermore provided with means 8 for conveying live steam into their lower part. This live steam is produced out of a boiler (not shown). These means for conveying live steam to the thermal hydrolysis reactors include a pipe 8, one end of which is connected to the boiler and the other end of which is connected to four pipe arms 81, 82, 83, 84 respectively serving the thermal hydrolysis reactors 71, 72, 73, 74. Each of these pipe arms 81, 82, 83, 84 is equipped with a valve 91, 92, 93, 94 respectively. These means are used to supply live steam alternately to each of the thermal hydrolysis reactors 71, 72, 73, 74. The pipe 8 comprises a bypass 8a to convey live steam to the dynamic mixer 3 when the plant is started, when no recovery steam is as yet available. The vessel 13 for its part is provided with a water inlet 13a to prevent the emission of flash steam when the plant is stopped.

[0089] The vessel 13 is connected to the thermal hydrolysis reactors 71, 72, 73, 74 by means for setting up the fluidic communication of this vessel 13 alternately with each of the thermal hydrolysis reactors 71, 72, 73, 74. These means for setting up fluidic communication include a pipe 11 which opens out at one of its ends into the upper part of the recovery vessel 13 and is also connected to the thermal hydrolysis reactors 71, 72, 73, 74 by pipe arms 111, 112, 113, 114 respectively situated in the lower part of each reactor. Each of these pipe arms 111, 112, 113, 114 is equipped with a valve 121, 122, 123, 124 respectively.

[0090] First Example of Implementation of the Method

[0091] The working of the plant represented in FIG. 1 for implementing this example of a method according to the invention shall now be described.

[0092] For the sake of the clarity of this description, this operation shall first of all be described through a description of a treatment cycle implementing one of the reactors of the plant.

[0093] According to such a cycle, a batch of sludges is conveyed during a step a) called a filling step, in the absence of any preheating implemented by a heat exchanger and/or a preheating vessel in the thermal hydrolysis reactor 71. To this end, the valves 62, 63, 64 equipping the pipe arms 52, 53, 54 that serve the thermal hydrolysis reactors 72, 73, 74 respectively are closed while the valve 61 equipping the pipe arm 51 serving the reactor 71 is open.

[0094] This batch of sludges is pumped through the pump 2 by the pipe 1. It travels through the dynamic mixer 3 where it is intimately mixed with recovery steam provided to the dynamic mixer 3 via the inlet 4 of recovery steam with which this mixer is provided. This passage of the sludges into the dynamic mixer de-structures them, lowering their viscosity and homogenizing them. Thus, the recovery of heat from the hydrolyzed sludges is promoted.

[0095] During this step a), called a filling step, the thermal hydrolysis reactor 71 is filled in such a way that, during the step c), the thermal hydrolysis reactor is filled between 70% and 90% of its total capacity by volume. The volume of the interior of the reactor not occupied by sludges is occupied by a gas cloud containing among other things non-condensable gases which are discharged by the pipe 101 provided in the upper part of the reactor during the step a).

[0096] It will be noted that, to facilitate the discharge of these non-condensable gases during the step a), it can be planned to provide the pipes 101 with suction means enabling the content of the reactor to be placed in a state of slight low pressure.

[0097] In the present example, this step a) lasts 20 minutes.

[0098] At the end of this step, the valve 61 which equips the pipe arm 51 conveying the mixture of sludges and steam to the reactor 71 is closed.

[0099] Through the invention, according to which the viscosity of the sludges has been reduced and their homogeneity improved through their passage into the dynamic mixer 3, the transfer of energy from the recovery steam into the sludges travelling in the dynamic mixer is optimized. The recovery of energy is therefore optimized.

[0100] During a step b), the live steam (water vapor) produced by the boiler is conveyed by the pipe 8 and distributed to the reactor 71 by the pipe arm 81. This injection of live steam is done at the base of the reactor in order to favor its diffusion and its condensation in the sludges. To this end, the valve 91 equipping this pipe arm 81 is open while the valves 92, 93, 94 equipping the pipe arms 82, 83, 84 respectively are closed. This injection of steam within the reactor 71 increases the temperature and the pressure prevailing in this reactor.

[0101] In the present embodiment, this step b) for injecting live steam lasts 20 minutes and takes the temperature of the batch of sludges present in the reactor 71 to a temperature of 120° C. to 200° C., preferably 140° C. to 180° C. This thermal hydrolysis temperature could be chosen especially according to the nature of the sludges and the end purpose of the method (hygienization, solubilization, etc). Besides, since the thermal hydrolysis reactor is closed, the injection of steam could also increase the pressure prevailing within this reactor. In practice, this pressure is raised between two and 16 bar(a).

[0102] Through the invention, according to which the viscosity of the sludges has been reduced and their homogeneity improved through their passage in the dynamic mixer 3, the transfer of energy from the live steam into the sludges to be hydrolyzed, carried out in the reactor, is also optimized. The sludges are therefore heated more easily and the consumption of live steam is optimized.

[0103] During this step b) for injecting live steam, the vessel 13 is not in fluidic communication with the interior of the reactor 71. The valve 121, equipping the pipe arm 111 extended by the pipe 11 leading into the vessel 13, is therefore closed.

[0104] During a step c), known as a thermal hydrolysis reaction step, which in this example also lasts 20 minutes, the thermal hydrolysis of the sludges takes place, and the valve 16 and the valve 121 remain closed.

[0105] Through the invention, in which the viscosity of the sludges has been diminished and their homogeneity improved through their passage into the dynamic mixer 3, the transfer of energy from the steam into the sludges is also optimized during this step.

[0106] At the end of this step c), the valve 121 is open. This gives rise, during a step d), to the emptying of the content of the batch of hydrolyzed sludges contained in the reactor 71 towards the recovery vessel 13 and the depressurizing of the reactor. This depressurizing is permitted by the fact that the vessel 13 has pressure that is far lower than the pressure prevailing within the hydrolysis reactor 71 during the step c) of thermal hydrolysis. In practice, this pressure prevailing inside the vessel 13 ranges from 1.1 bar(a) to 3 bar(a). During this step d) for emptying the content of the batch of hydrolyzed sludges from the reactor towards the vessel 13, the depressurizing of the sludges causes the emission of recovery steam. This recovery steam is discharged from the vessel 13 by the pipe 14 which is itself connected to the inlet of recovery steam from the dynamic mixer 3. At the end of this step d), the hydrolyzed sludges are discharged from the plant by the pipe 15. This step d) also lasts 20 minutes.

[0107] In the present embodiment, the steps a), b), c) and d) each have a duration of 20 minutes and constitute an 80-minute treatment cycle.

[0108] This cycle is immediately repeated for one and then for other batches of sludges to be treated in the reactor 71. The treatments of different batches of sludges succeed one another therefore during a succession of 80-minute cycles during which these different batches of sludges travel through the reactor 71.

[0109] This succession of cycles is symbolized in FIG. 2 by the upper line designated by the letter A of this line. In this figure, the steps a) for filling are represented in black; the steps b) for injecting live steam are represented in dark grey; the steps c) of thermal hydrolysis reaction are represented as blanks and the steps d) for emptying and de-pressurizing are represented in light grey.

[0110] Identical successions of treatment cycles are implemented for other batches of sludges through the reactors 72, 73, 74 (provided with means 102, 103, 104 for discharging non-condensable gases). The successions of cycles are symbolized in FIG. 2 by the lines B, C, D. The description of the cycles of these successions of cycles is identical to that made here above with reference to the reactor 71, except that it is the valves associated with the reactor 72, 73, 74 that are activated, namely:

[0111] the valve 62 provided on the pipe arm 52, the valve 92 provided on the pipe arm 82 and the valve 122 provided on the pipe arm 112 for the treatment cycle implemented through the reactor 72;

[0112] the valve 63 provided on the pipe arm 53, the valve 93 provided on the pipe arm 83 and the valve 123 provided on the pipe arm 113 for the treatment cycle implemented through the reactor 73;

[0113] the valve 64 provided on the pipe arm 54, the valve 94 provided on the pipe arm 84 and the valve 124 provided on the pipe arm 114 for the treatment cycle implemented through the reactor 74.

[0114] According to the method of the invention, the beginnings of the cycles of these different successions A, B, C, D of cycles are staggered in time so that the steps a) of a succession of cycles are concomitant with the steps d) of another succession of cycles, the recovery steam emitted during the steps d) of one succession of cycles constituting the recovery steam injected during the steps a) of another succession of cycles. This is symbolized in FIG. 2 by curved arrows. In the present example, the cycle starting points of each succession of cycles are staggered by 20 minutes.

[0115] Thus, referring to FIG. 2, the recovery steam emitted during the steps d) of the succession of cycles A, constitutes the recovery steam injected during the step a) of the succession of cycles D, the recovery steam emitted during the steps d) of the succession of cycles B constitutes the recovery steam injected during the steps a) of the succession of cycles A, the recovery steam emitted during the steps d) of the succession of cycles C constitutes the recovery steam injected during the steps a) of the succession of cycles B, the recovery steam emitted during the steps d) of the succession of cycles D constitutes the recovery steam injected during the steps a) of the succession of cycles C, etc.

[0116] The feeding of sludges into the plant, the feeding of live steam in alternation into each thermal hydrolysis reactor and the discharge of sludges from the plant are thus continuous.

[0117] Second Example of Implementation of the Method

[0118] In this example of an embodiment, the reactor 74 is stopped and only the thermal hydrolysis reactors 71, 72 and 73 of the plant are used to implement the method according to the invention.

[0119] Besides, the treatment cycle has been increased by integrating a ten-minute pause at the end of each step d) of emptying and depressurizing, the steps a), b), c) and d) lasting 20 minutes. The succession of such 90-minute cycles is symbolized in FIG. 3 by the lines A′, B′, C′.

[0120] Comparison with the Prior Art

[0121] Sludges were treated according to the first example of implementation of the method of the invention described here above on the one hand and by the technique described in FR2820735 on the other hand, the duration of the thermal hydrolysis step being 20 minutes in both cases.

[0122] The invention made it possible, using the results of these comparative tests, to notably shorten the processing cycles, in practice from 120 minutes (for the technique according to FR2820735) to 80 minutes (for the invention implemented according to the first example of implementation of the method according to this example described here above).

[0123] The volumes of reactors needed for the treatment of these sludges were compared. The graph of FIG. 4 which expresses the totalized volumes of the thermal hydrolysis reactors needed for the treatment of these sludges shows that these volumes are far less in the context of the invention.

[0124] As compared with the prior art, it will be noted that the invention does not entail the need to keep, in the thermal hydrolysis reactors, a quantity of hot sludges between each cycle, thus optimizing the quantities of sludges treated in each reactor and the filling of these reactors.