Process for mixing a fluid in a fluid treatment and / or storage tank and associated installation

Abstract

A method for mixing a fluid in a fluid treatment and / or storage tank and associated installation. The method comprises the following steps: - injecting gas into the fluid by means of a gas injection system in the tank (16) comprising a plurality of gas injection openings (54) in the tank (16), - controlling the gas injection by the gas injection system to generate a shear field (80) of the fluid over at least the bottom (21) of the tank (16), the shear field (80) being non-uniform as it moves angularly about a central axis of the tank (16). The method includes pivoting the non-uniform shear field (80) about the central axis. Figure for the abstract: Figure 5

Description

Title of the invention: Method for mixing a fluid in a fluid treatment and / or storage tank and associated installation

[0001] The present invention relates to a method for mixing a fluid in a fluid treatment and / or storage tank, the tank comprising a bottom and at least one side wall delimiting an internal volume containing the fluid, the method comprising the following step:

[0002] - injection of gas into the fluid by means of a gas injection system in the tank including a plurality of gas injection openings into the tank.

[0003] Such a tank is for example integrated into a sludge digester intended to generate biogas, containing in particular biomethane, by anaerobic methanogenic fermentation.

[0004] Methanogenic fermentation has a cellular processing capacity capable of eliminating a significant amount of organic matter, producing biogas. Biogas production is encouraged because the biogas can then be used in a biomethane production unit by separating the carbon dioxide and methane contained in the biogas (for example, by a membrane process), and / or as an energy source for generating electricity, heating buildings, and / or powering vehicles.

[0005] In this regard, it is known to introduce sludge from water treatment into a tank of a sludge digester to conduct a fermentation producing biogas.

[0006] Many parameters influence the yield of biogas production from sludge. In particular, the sludge must be stirred effectively in the digester, notably to reduce variations in temperature and organic matter concentration within the digester mass, and to increase the chances of contact between microorganisms and the materials to be degraded.

[0007] The mixing operation must also be efficient in order to guarantee a residence time in the digester that provides thorough mixing. It must break up the solid layer that could form at the sludge-gas interface in the upper part of the digester. Furthermore, it must ensure sufficient mixing at the bottom of the digester to minimize the accumulation of deposits, so as not to lose usable volume, and to agitate the fluid sufficiently to facilitate the continuous rise of biogas.

[0008] In known mixing devices, biogas is reinjected into vertical tubes distributed around the central axis of the digester tank. The biogas is injected continuously through downward-facing injection openings. and distributed in a circle around the central axis.

[0009] Thanks to the buoyancy of the gas in the mud, the gas rises to promote mixing in the digester.

[0010] Such a mixing device is very reliable and requires minimal maintenance, which is very useful for limiting interventions within the digester.

[0011] However, it does not give complete satisfaction. Indeed, even if the rise of the gas in the tank produces a movement of the mud and a resulting shear at the bottom of the tank, the shear generated by the movement of the mud at the bottom of the tank is limited and is localized in certain high shear zones, while dead zones may remain at the bottom of the tank.

[0012] To overcome this problem, it is necessary to increase the number of injection rods, which increases the manufacturing and operating cost, while limiting the volume available for storage / treatment in the tank.

[0013] An object of the invention is therefore to provide a mixing device intended to be placed in a fluid treatment tank, which allows very efficient mixing of the fluid, particularly at the bottom of the tank, while limiting the bulk within the internal volume of the tank.

[0014] To this end, the invention relates to a method of the aforementioned type, characterized by the following step:

[0015] - control of gas injection by the gas injection system to generate a fluid shear field over at least the bottom of the tank, the shear field being non-uniform as it moves angularly around a central axis of the tank,

[0016] the method comprising pivoting the non-uniform shear field around the central axis.

[0017] The method according to the invention may include one or more of the following features, taken individually or in any technically possible combinations;

[0018] - the non-uniform shear field includes at least one region of ci high protrusion, with an angular extent of less than 180° around the central axis, and at least one region of lower or locally zero shear, angularly adjacent to the region of high shear, the pivoting of the non-uniform shear field including the joint rotational driving of the region of high shear and the region of lower or locally zero shear around the central axis;

[0019] - the non-uniform shear field comprises two shear regions high diametrically opposed with respect to the central axis, the pivoting of the non-uniform shear field causes the high shear regions to rotate jointly around the central axis;

[0020] - the or each region of high shear has a butterfly wing shape, the pivoting of the non-uniform shear field causing the high shear regions to rotate jointly in a butterfly-wing shape around the central axis;

[0021] - the rotational speed of the non-uniform shear field is greater than 0.5 revolutions / day and is for example between 0.8 revolutions / day and 1.2 revolutions / day;

[0022] - the pivoting of the non-uniform shear field around the central axis is achieved by controlling the gas injection system;

[0023] - the gas injection system comprises a plurality of injection openings gas distributed around the central axis, the creation of the non-uniform shear field comprising the injection of gas through at least one injection opening at at least one first angular position around the central axis at a nominal flow rate, and the injection of a reduced flow rate of gas or the absence of gas injection through at least one other injection opening at a second angular position offset angularly from the first angular position;

[0024] - the pivoting of the non-uniform shear field comprises successive steps cessives of reducing the injection flow rate by at least one injection opening at a first angular position from a gas injection at the nominal flow rate to a gas injection at the reduced flow rate or to an absence of gas injection, and of increasing the injection flow rate by at least one injection opening at a second angular position from a gas injection at the reduced flow rate or from an absence of gas injection to an injection at the nominal flow rate;

[0025] - successive stages of decreasing gas injection occur on or successive injection openings angularly moving along a direction of rotation, the successive stages of increasing gas injection occurring on successive injection openings in the same direction of rotation;

[0026] - the injection openings are distributed over at least one circle around the axis central;

[0027] - at least one additional injection opening is located at the periphery of the tank, near the side wall;

[0028] - at least one movable deflector is disposed in the tank, the pivoting of the field of non-uniform shear comprising an actuation of the movable deflector between a first position and a second position;

[0029] - the treatment and / or storage tank is a digester tank, the fluid being a sludge, advantageously from a water treatment unit, the gas being biogas, produced by fermentation of the sludge.

[0030] The invention also relates to a fluid mixing installation, comprising:

[0031] - a fluid treatment and / or storage tank, the tank comprising a bottom and at least one side wall delimiting an internal volume containing the fluid;

[0032] - a gas injection system into the tank comprising a plurality of openings gas injection into the tank,

[0033] characterized in that the gas injection system comprises a controller configured to control the gas injection in order to generate a fluid shear field on at least the bottom of the tank, the shear field being non-uniform as it moves angularly around a central axis of the tank,

[0034] the installation being configured to control the injection of gas in order to pivot the non-uniform shear field around the central axis.

[0035] The installation according to the invention may comprise one or more of the following features, taken individually or in any technically possible combination:

[0036] - the gas injection system comprises a plurality of distributed injection rods around the central axis, each injection lance opening downwards through a gas injection opening of the plurality of gas injection openings, the controller being suitable for selectively controlling the gas injection into each of the injection lances and through each gas injection opening between a gas injection configuration at nominal flow and a gas injection configuration at reduced flow or a configuration of no gas injection.

[0037] The invention will be better understood upon reading the following description, given solely by way of example, and made with reference to the accompanying drawings, in which:

[0038] - [Fig. 1] Fig. 1 is a schematic view, taken in cross-section along a vertical plane, of a fluid treatment and / or storage installation comprising a gas injection system for implementing the process according to the invention;

[0039] - [Fig.2] [Fig.2] is a partial perspective view of the gas injection system of the installation of the [Fig.1];

[0040] - [Fig.3] [Fig.3] is a view of the non-uniform shear field produced by the gas injection system of the [Fig.2], during the implementation of the process according to the invention, the non-uniform shear field comprising high shear regions in the shape of butterfly wings;

[0041] - [Fig.4] [Fig.4] is a view analogous to [Fig.2], illustrating the configuration of the gas injection system to pivot the non-uniform shear field produced on the [Fig.3];

[0042] - [Fig.5] [Fig.5] is a view analogous to [Fig.3], showing the pivoting of the non-uniform shear field under the effect of gas injection controlled by the gas injection system.

[0043] A first mixing process according to the invention is intended to be implemented in a treatment and / or storage installation 12 for a fluid 14, in order to mix the fluid 14 within the installation 12.

[0044] The treatment and / or storage installation 12 is, for example, a sludge treatment installation from a water treatment unit, for example, for the treatment of industrial water, drinking water, or wastewater.

[0045] The treatment and / or storage installation 12 is intended in particular to carry out fermentation, preferably anaerobic, to remove organic matter present in the fluid, producing biogas. The biogas includes, in particular, biomethane, produced by methanogenic fermentation. The biogas is intended to be separated to produce biomethane and / or is intended to be used as a fuel or as a heating oil.

[0046] More generally, the mixing process is intended for mixing a fluid 14 in the form of a liquid with a viscosity greater than that of water. The viscosity of the fluid 14 is generally greater than 20 mPa.s, in particular between 10 mPa.s and 80 mPa.s, measured for a shear rate of 100 s⁻¹.

[0047] The fluid 14 is preferably non-Newtonian, in particular shear-thinning, its viscosity decreasing as a function of the shear to which it is subjected.

[0048] The treatment and / or storage installation 12 includes a tank 16 for the treatment and / or storage of the fluid 14, a unit 18 for the recovery and recirculation of gas produced in the tank 16, and a gas injection system 20, disposed in the tank 16.

[0049] The tank 16 comprises a bottom 21, a side wall 22 and a roof 24 defining a closed internal volume 26. The internal volume 26 is intended to contain fluid 14, and a gaseous headspace 28 above the surface 30 of the fluid.

[0050] The internal volume 26 of the tank 16 is for example greater than 400 m3 and is in particular between 400 m3 and 20000 m3.

[0051] The tank 16 is provided with an inlet 32 ​​for injecting the fluid to be treated into the internal volume 26 and an outlet 34 for evacuating the treated fluid, advantageously located opposite the injection inlet 32, or at the lowest point of the bottom 21. An upward drive of the fluid to be treated by a bubble column can be provided opposite the injection inlet 32.

[0052] The recovery and recirculation assembly 18 includes a gas recovery circuit 40 in the gaseous space 28, and a gas recirculation circuit 42 to the gas injection system 20 to supply the gas injection system 20 with gas.

[0053] The recovery circuit 40 includes at least one pipe suitable for collecting gas from the gaseous space 28, to lead it for example to a gas storage 44, in particular a gasometer.

[0054] The recirculation circuit 42 is connected to the recovery circuit 40, upstream of the gas storage 44. It includes at least one compressor 46, to compress the recirculated gas and supply the gas injection system 20 with gas compressed to a pressure greater than 1.2 bar absolute, and for example between 1.4 bar absolute and 3.5 bar absolute.

[0055] The compressor 46 typically draws between 1 NmVh and 2 NmVh of gas per m² of the horizontal surface area of ​​the tank 16. The compressor 46 is preferably a positive displacement compressor, which pumps a constant flow rate.

[0056] Alternatively, the recirculation circuit 42 is tapped directly into the gaseous space 28, separately from the tapping of the recovery circuit 40.

[0057] In the example shown in [Fig.1], the gas injection system 20 in the tank 16 comprises a plurality of gas injection rods 50, arranged around a central axis AA' of the tank 16, in the vicinity of the central axis A-A'.

[0058] The rods 50 are located at a distance from the central axis A-A' less than 50%, in particular less than 30%, of the minimum distance separating the central axis A-A' at the point closest to the lateral wall 22.

[0059] The gas injection system 20 further includes a gas distributor 52 connected upstream to the circulation conduit 42 and connected downstream to each of the rods 50, to control the quantity of gas injected by each rod 50.

[0060] In the example shown in [Fig.1], each rod 50 defines at its lower end an opening 54 for distributing gas into the tank 16, directed downwards.

[0061] Each rod 50 has at least one upper section 56, connected to the gas distributor 52, which extends vertically downwards from the roof 24, parallel to the central axis AA'.

[0062] With reference to [Fig. 2], a first group 55 of outer rods 50 further comprises a lower section 58 extending the upper section 56 downwards. The lower section diverges radially away from the central axis AA' by moving away from the axis A-A'. Each rod 50 of the first group 55 of rods 50 thus has a general J-shaped form.

[0063] In this example, the rods 50 of the first group 55 are distributed on an outer circle 60. They are distributed angularly in a regular manner around the axis A-A'.

[0064] Advantageously, the rods 50 of the gas distribution system 20 further comprise a second group 62 of rods 50 arranged inside the first group 55 of rods 50.

[0065] The rods 50 of the second group 62 have only one section 56 parallel to the axis A-A', which opens vertically downwards through a gas injection opening 54.

[0066] The rods 50 of the second group 62 are distributed on an inner circle 64 concentric with the outer circle 60 and arranged inside the outer circle 60. They are distributed angularly in a regular manner around the axis A-A'.

[0067] In the example shown in figures 1 to 5, the number of rods 50 of the first group 55 opening onto the outer circle 60 is greater than or equal to 4, and is in particular greater than 8, preferably between 10 and 20.

[0068] The number of rods 50 of the second group 62 opening onto the inner circle 64 is for example greater than or equal to 2 and is in particular between 5 and 20.

[0069] The injection opening 54 of each rod 50 of the second group 62 opens onto the inner circle 64 above the gas injection opening 54 of each rod 50 of the first group 55 on the outer circle 60.

[0070] The gas distributor 52 includes, for each gas injection opening 54 associated with a rod 50, a valve 70 for controlling the gas flow through the rod 50 and consequently, the gas flow injected into the fluid 14 through the gas injection opening 54.

[0071] The gas distributor 52 includes a controller 72 configured to pilot each valve 70 in order to selectively control the gas flow through each pipe 50 and each gas injection opening 54.

[0072] In this example, the valves 70 are arranged outside the internal volume 26, for example above the roof 24. Each valve 70 is connected upstream to the compressor 46.

[0073] The controller 72 is configured here to control each valve 70 between a nominal gas injection configuration through the corresponding gas injection opening 54, and a reduced gas injection configuration or a no gas injection configuration through the gas injection opening 54.

[0074] As will be seen below, the controller 72 is configured to selectively control at least a first group of valves 70 in the nominal injection configuration to place a first group 76A, 76B of injection openings 54 in a nominal gas injection configuration into the fluid 14.

[0075] The controller 72 is configured to selectively control at least a second group of valves 70 in the reduced injection configuration or in the no injection configuration, so that at least a second group 78A, 78B of injection openings 54 is placed in the reduced injection configuration or in the no injection configuration (which is symbolized by a cross at the opening 54 in Figures 2 and 4).

[0076] Thus, as can be seen in Figures 3 and 5, the controller 72 is configured to generate in the fluid 14, in the vicinity of the bottom 21, a shear field 80 of the fluid 16 on the bottom 21, which is non-uniform in moving angularly around the central axis A-A'.

[0077] This shear field is taken tangentially with respect to the bottom 21 by following the bottom 21.

[0078] The non-uniform shear field 80 includes, in particular, at least one high shear region 82A, 82B, with an angular extent of less than 180° around the central axis A-A', which in this example has a butterfly wing shape. The angular extent around the axis A-A' of the high shear region(s) 82A, 82B is greater than 20°, preferably greater than 90°.

[0079] The non-uniform shear field 80 comprises, on either side of each high shear region 82A, 82B, a region 84A, 84B of lower or locally zero shear.

[0080] The high shear regions 82A, 82B are generally characterized by a shear stress at the bottom 21, taken parallel to the bottom 21, greater than 1.2 Pa, in particular between 1.3 Pa and 1.7 Pa.

[0081] On the contrary, the regions of lower or locally zero shear 84A, 84B are characterized by a shear stress at the bottom 21, taken parallel to the bottom 21, strictly less than 1.2 Pa and in particular between 0.8 Pa and 1.1 Pa.

[0082] Shear stresses are for example determined by simulation, for example by means of a finite volume calculation code taking into account the fluid mechanics equations, and with a user interface, using "Computational Fluid Dynamics" techniques.

[0083] The controller 72 is also configured to successively change the configuration of the valves 70, to move an injection opening 54 of each second group 78A, 78B of injection openings 54, adjacent to an injection opening 54 located at a first end of a first group 76A, 76B of injection openings 54, from the reduced injection configuration or the no-injection configuration to the nominal injection configuration.

[0084] The controller 72 is also configured to successively change the configuration of the valves 70 to simultaneously switch an injection opening 54 of the first group 76A, 76B located at a second end of the first group 76A, 76B of injection openings opposite to the first end, from the nominal injection configuration to the reduced injection configuration or to the no injection configuration.

[0085] Each first group 76A, 76B of injection openings 54 in the nominal injection configuration therefore moves in rotation around the axis A-A' in a direction of rotation marked by an arrow on the [Fig.5].

[0086] Correspondingly, each second group 78A, 78B of injection openings 54 in the reduced injection configuration or in the no-injection configuration also moves in rotation about the axis A-A' in the same direction of rotation marked by an arrow on [Fig.5]

[0087] Thus, the controller 72 is configured to rotate the non-uniform shear field 80 around the axis A-A', causing the high shear regions 82A, 82B and the locally lower or zero shear regions 84A, 84B to rotate around the axis AA', as seen in [Fig.5].

[0088] A process for mixing a fluid 14 in the tank 16 of the treatment and / or storage installation 12 will now be described.

[0089] Fluid 14 is supplied discontinuously, for example periodically, or continuously into the internal volume 26 through the injection inlet 32.

[0090] Correspondingly, treated fluid is discharged through the discharge outlet 34. The fluid flow rate 14 is adjusted so that the surface of the fluid 30 in the internal volume 26 extends below the roof 24 to delimit a gaseous ceiling 28.

[0091] During its passage through the internal volume 26, the fluid 14 is treated. For example, if it is sludge from water treatment, the sludge undergoes fermentation, in particular anaerobic fermentation eliminating the organic matter contained in the sludge and producing biogas which accumulates in the gaseous head 28.

[0092] The gas produced by the fluid 14 is then evacuated by the recovery circuit 40 to be brought to the gas storage 44. Part of the gas circulating in the recovery circuit 40 is recycled through the recirculation circuit 42 to the compressor 46, where it is compressed to a pressure for example within the ranges defined above.

[0093] The compressed gas is then brought to the gas distributor 52. The controller 72 then operates the valves 70 to maintain at least a first group of valves 70 in their nominal injection configurations, so that a first group 76A, 76B of injection openings 54 injects gas into the tank 16 at a nominal injection flow rate, related to the surface of the tank 16, taken horizontally, for example between 0.3 NmWm2 and 0.7 NmWm2.

[0094] The controller 72 drives at least a second group of valves 70 in their reduced injection configurations or in their no-injection configurations so that a second group 78A, 78B of injection openings 54 injects gas into the tank 16 at a reduced injection rate or exhibits no gas injection, for example at least 20% lower than the injection rate through the injection openings 54 of the first group 76A, 76B.

[0095] In the example shown in Figures 2 and 3, all the gas injection openings 54 of the inner circle 64 advantageously inject gas at a nominal flow rate.

[0096] In the example of [Fig.2], the controller 72 also controls the injection openings 54 of the outer circle 62 so that two groups 76A, 76B of diametrically opposed injection openings 54 inject gas into the tank at the nominal flow rate, where two groups 78A, 78B of diametrically opposed injection openings 54 inject gas into the tank 16 at a reduced injection rate or exhibit no gas injection.

[0097] For example, when the number of injection openings is equal to 12, as in the example of [Fig.2], two groups 76A, 76B of three consecutive injection openings 54 inject gas at the nominal flow rate, while two groups 78A, 78B of three consecutive injection openings 54, arranged between groups 76A, 76B, inject gas at a reduced flow rate or exhibit no gas injection.

[0098] Under the effect of such a gas injection, as illustrated by [Fig.3], two high shear regions 82A, 82B, here in the shape of butterfly wings, are created on the bottom 21 of the tank 16. Each high shear region 82A, 82B has an angular extent of less than 180° around the central axis A-A' and generally greater than 45°.

[0099] Regions of lower or locally zero shear 84A, 84B are interposed between the regions of high shear 82A, 82B.

[0100] The shear field 80 thus obtained is non-uniform when moving angularly around the axis AA'

[0101] Then, as illustrated by [Fig.4], successive switching steps of the valves 70, as described above, are carried out by the controller 72 to progressively pivot around the axis A-A' the first group 76A, 76B of injection openings 54 injecting gas into the tank 16 at the nominal flow rate, and jointly the second group 78A, 78B of injection openings 54 injecting gas at a reduced flow rate or with no injection into the tank 16, in a first given direction of rotation.

[0102] As illustrated by [Fig.5], this translates into a corresponding angular displacement around the axis AA' of each high shear region 82A, 82B and a corresponding displacement of each less high or locally zero shear region 84A, 84B in the first given direction of rotation.

[0103] The angular rotation speed around the central axis A-A' of the non-uniform shear field 80, and consequently of each high shear region 82A, 82B is generally greater than 0.5 revolutions / day and is for example between 0.8 revolutions / day and 1.2 revolutions / day.

[0104] By creating a non-uniform shear field 80 using the gas distribution system 20, it is possible to create high shear regions 82A, 82B and drive them into rotation around the central axis A-A' to sweep the entire surface of the bottom 21 of the tank 16.

[0105] Thus, a high shear is obtained at the bottom 21 of the tank 16, which minimizes long-term deposits, covers almost the entire surface of the bottom 21, with a minimum amount of gas injected possible, while sequencing the gas injections over time.

[0106] This also reduces the variable costs of the installation 12, since the number of cleanings of the bottom 21 of the tank 16 by operators can be reduced and their frequency decreased. This very surprising result is achieved very simply, just by controlled gas injection, even in the absence of moving parts in the tank 16, which can remain completely closed.

[0107] In the preceding example, the flow rate injected through the injection openings of the second group 78A, 78B is preferably zero. Alternatively, the gas injection into the second group 78A, 78B of injection openings 54 produces a lower flow rate than that of the injection openings 54 of the first group 76A, 76B, but not zero. Thus, the controller 72 varies the flow rate circulating through each valve 70, without completely eliminating it.

[0108] In one variant, at least part of the gas injection openings 54 are located at the periphery of the tank 16, in the vicinity of the side wall 22 or through the side wall 22.

[0109] In another variant, the direction of rotation of the groups 76A, 76B; 78A, 78B of injection openings 54 is changed over time, to create an alternating rotational movement of the high shear regions 82A, 82B.

[0110] In yet another variant, a plurality of movable parts, for example deflectors movable between a first retracted position and a second deployed position, are used to generate the non-uniform shear field 82 in combination with the gas injection, and to drive it into rotation around the axis A-A'.

[0111] At least part of the deflectors is for example deployable through the side wall 22 of the tank 16.

[0112] In another embodiment, at least one gas injection device 20 comprises, in combination with the rods 50 or as a substitute for at least one rod 50, a geyser mixer (or "geyser pump" in English) comprising an injection tube opening upwards, and a device for injecting gas bubbles into the tube from the bottom of the tube.

[0113] In another variant, only the outer circle 60 is used for gas injection, the inner circle 64 remaining without gas injection.

Claims

Demands

1. A method for stirring a fluid (14) in a tank (16) for processing and / or storing the fluid (14), the tank (16) comprising a bottom (21) and at least one side wall (22) delimiting an internal volume (26) containing the fluid (14), the method comprising the following step: - injecting gas into the fluid (14) by means of a gas injection system (20) in the tank (16) comprising a plurality of gas injection openings (54) in the tank (16), the method being characterized by the following step: - controlling the gas injection by the gas injection system (20) to generate a shear field (80) of the fluid (14) on at least the bottom (21) of the tank (16), the shear field (80) being non-uniform as it moves angularly about a central axis (A-A') of the tank (16), the method comprising pivoting the non-uniform shear field (80) around the central axis (A-A').

2. A method according to claim 1, wherein the non-uniform shear field (80) comprises at least one high shear region (82A, 82B), with an angular extent of less than 180° about the central axis (A-A'), and at least one locally lower or zero shear region (84A, 84B), angularly adjacent to the high shear region (82A, 82B), the pivoting of the non-uniform shear field (80) comprising the joint rotational driving of the high shear region (82A, 82B) and the locally lower or zero shear region (84A, 84B) about the central axis (A-A').

3. A method according to claim 2, wherein the non-uniform shear field (80) comprises two high shear regions (82A, 82B) diametrically opposed with respect to the central axis (A-A'), the pivoting of the non-uniform shear field (82) causing the high shear regions (82A, 82B) to rotate jointly around the central axis (A-A').

4. A method according to any one of claims 2 or 3, wherein the high shear region or regions (82A, 82B) have a butterfly-wing shape, the pivoting of the non-uniform shear field (80) causing the butterfly-wing-shaped high shear regions (82A, 82B) to rotate jointly around the axis central (A-A').

5. A method according to any one of the preceding claims, wherein the rotational speed of the non-uniform shear field (80) is greater than 0.5 revolutions / day and is, for example, between 0.8 revolutions / day and 1.2 revolutions / day.

6. A method according to any one of the preceding claims, wherein the pivoting of the non-uniform shear field (80) around the central axis (A-A') is achieved by controlling the gas injection system (20).

7. A method according to any one of the preceding claims, wherein the gas injection system (20) comprises a plurality of gas injection openings (54) distributed around the central axis (A-A'), the creation of the non-uniform shear field (80) comprising the injection of gas through at least one injection opening (54) at at least a first angular position around the central axis (A-A') at a nominal flow rate, and the injection of a reduced flow rate of gas or the absence of gas injection through at least one other injection opening (54) at a second angular position offset angularly with respect to the first angular position.

8. A method according to claim 7, wherein the pivoting of the non-uniform shear field (80) comprises successive steps of reducing the injection flow rate by at least one injection opening (54) at a first angular position from a gas injection at the nominal flow rate to a gas injection at the reduced flow rate or to a lack of gas injection, and of increasing the injection flow rate by at least one injection opening (54) at a second angular position from a gas injection at the reduced flow rate or from a lack of gas injection to an injection at the nominal flow rate.

9. A method according to claim 8, wherein successive stages of decreasing gas injection occur on successive injection openings (54) by moving along a direction of rotation, successive stages of increasing gas injection occurring on successive injection openings (54) in the same direction of rotation.

10. A method according to any one of claims 7 to 9, wherein the injection openings (54) are distributed over at least one circle (60, 64) around the central axis (A-A').

11. A method according to claim 10, wherein at least one opening additional injection (54) is located at the periphery of the tank (16), in the vicinity of the side wall (22).

12. A method according to any one of claims 10 or 11, wherein at least one movable deflector is disposed in the tank (16), the pivoting of the non-uniform shear field (80) comprising an actuation of the movable deflector between a first position and a second position.

13. A method according to any one of the preceding claims, wherein the treatment and / or storage tank (16) is a tank (16) of a digester, the fluid (14) being a sludge, advantageously from a water treatment unit, the gas being biogas, produced by fermentation of the sludge.

14. Installation for mixing a fluid (14), comprising: - a tank (16) for treating and / or storing the fluid (14), the tank (16) comprising a bottom (21) and at least one side wall (22) delimiting an internal volume (26) containing the fluid (14); - a gas injection system (20) in the tank (16) comprising a plurality of gas injection openings (54) in the tank (16), characterized in that the gas injection system (22) comprises a controller (72) configured to control the gas injection in order to generate a shear field (80) of the fluid (14) on at least the bottom (21) of the tank (16), the shear field (80) being non-uniform by moving angularly around a central axis (A-A') of the tank (16), the installation (14) being configured to control the gas injection in order to pivot the non-uniform shear field (80) around the central axis (A-A').

15. Installation (14) according to claim 14, wherein the gas injection system (20) comprises a plurality of injection rods (50) distributed around the central axis (A-A'), each injection rod (50) opening downwards through a gas injection opening (54) of the plurality of gas injection openings (54), the controller (72) being adapted to selectively control the gas injection into each of the injection rods (50) and through each gas injection opening (54) between a gas injection configuration at nominal flow rate and a gas injection configuration at reduced flow rate or a configuration of no gas injection.

Images