DEVICE AND METHOD FOR GENERATING NANOBUBBLES

Abstract

 A device (1) for generating nanobubbles, comprising:
- an inlet (10) for supplying a liquid to the device,
- a plurality of mixing chambers (11), distributed around a longitudinal axis (X) of the device, and in communication with the inlet (10), each chamber (11) comprising protrusions (92) configured for increasing shear forces on the flow,
- at least one gas injector (16) for injecting a gas into the flow before it leaves the chambers (11), the gas injector (16) preferably being a porous element and preferably being positioned upstream the mixing chambers (11) and downstream the inlet (10).
- a flow collector (19) for collecting the flows leaving the chambers (11), the chambers (11) preferably having outlets (30) oriented radially inwardly through which the chambers (11) open out into the flow collector (19).

Description

[0001] The present invention relates to a device for generating nanobubbles of a gas in a liquid.

[0002] The general nature of bubbles is that they are unstable. This changes in the nanoscale. Gaseous nanobubbles are very small gas bubbles in liquids that typically have a diameter of less than 2000 nanometers.

[0003] Contrary to microbubbles, often referred to as fine bubbles and of a diameter of less than 100µm but larger than 2 µm, which rise and burst at the water surface and for which the buoyancy is essential for floating suspended solids to the surface, nanobubbles have remarkable stabilities, ascribed to different theories related to dissolved gas, unusual high surface tension and surface charges (DOI: 10.1021/acs.langmuir.6b02489 Langmuir 2016, 32, 11086-11100). Nanobubbles do not float and can remain stable in liquid for a relatively long period of time.

[0004] Nanobubbles have known applications such as wastewater-treatment, flotation, aeration, hydroponics, drip irrigation, cleaning, disinfecting, drinking water treatment, environmental remediation and decontamination as well as uses in mining and chemical industry where reactions between gas and liquid are vital.

[0005] Nanobubbles can be generated by exerting shear stress in static mixers or in motor-driven generators on larger size bubbles until they become nanosized.

[0006] In prior-art different categories of nanobubble generators can be identified:

• Engine based forced turbulence, for example as disclosed by US 2016/236158, WO2017/096444, CN 10792226A, or JP2013-107060;
• High shear flow-path using venturi pipe, injectors, ejectors, swirling and generally hydrocavitation and hypercavitation systems, such as disclosed in WO 2014/184585, US 2019/1241837, EP 2 671 631, EP 2 722 102, CA3029715, JP2017-176924, JP2014-231046A, JP2013-166143A, JP2011-156526 or JP2009-254984; among which:
  CA 3 029 715 discloses a nanobubble generating nozzle comprising, between an introduction part and a jetting part, a nanobubble generating structure. The latter comprises a plurality of flow paths having different cross-sectional areas through which the mixed fluid of the liquid and the gas is passed. The flow paths are divided and disposed in a plurality of stages in the axial direction of the nanobubble generating nozzle.

[0007] JP 2017/176924 discloses a micro-nanobubble generator mixing a gas into water supplied from a water supply port and outputting water containing gaseous micro-nanobubbles from a water output port. The micro-nanobubble generator includes a first mixing chamber and a second mixing chamber disposed along a flow direction from the water supply port to the water output port. The cross section of each chamber decreases towards their respective inlet and outlet ends.

[0008] JP 2014231046 discloses a method of generating micro-nanobubbles comprising: a step of generating a gas-liquid two-phase swirl flow in a two-phase flow swirl-type micro-nanobubble generator; a step of releasing the gas-liquid two-phase swirl flow into an external liquid from a releasing hole of the micro-nanobubble generator; and a step of moving the discharged microbubbles in the release gas-liquid two-phase swirl flow along the outer wall surface of the micro-nanobubble generator.

• Generation based on porous element in combination with a high-pressure gas, such as disclosed in US2017/0259219, WO2018/081868A1, KR20130002977, JP2014-226616, JP2018-176148 and WO 2017/130680, among which;

[0009] WO 2018/081868 and WO 2017/130680 disclose generating nanobubbles with a device comprising a plurality of inner tubes which are formed each in a tubular shape extending in the longitudinal direction, and in which at least a section of each tube comprises pores, air coming into contact with the liquid in the porous section and generating air bubbles in the liquid. In these devices no shear-force is generated by the pores within the fluid flowing through the different tubes. Also the tubes themselves do not contain flow-resisting elements that create further nanobubbles.

• Simple static mixers at the end of a pumped circuit such as disclosed in CN 105347519, which describes a nanobubble generator comprising an air feeding pipe, micropore aerators disposed at an air discharge end of the air feeding pipe, and highspeed rotor impellers disposed on the air feeding pipe; and
• A combination of several of the above techniques such as in WO 2014/184585.

[0010] Although all these approaches can make some nanobubbles under certain conditions, they are not ideal for the wide application of nanobubble technology across different industrial applications.

[0011] Typically, the applications of nanobubbles aim to reduce energy consumption and/or increase process efficiencies. In other words, the consideration of energy consumption per amount of gas transformed to nanobubbles is of the utmost importance. Nanobubble generating has been gaining attention in research, but field results have been focused on small scale applications with relatively clean water (fish and fruit cleaning, pond purification, small agricultural uses) - large scale industrial applications are still very limited. The lack of industrial scale applications is reflected in today's offer of nanobubble generating devices.

[0012] The break-through of nanobubbles in a large-scale industrial level hinges on the ability to bring nanobubble to 1) more robust high-yield solutions that are 2) scale-able and 3) controllable.

[0013] In the prior art, there are no general designs for nanobubble generators that can solve the following issues:

• High Flow, i.e. High Yield: the creation of large volumes of nanobubble enriched water with limited increase of power consumption. In other words, nanobubble generators have limited yield at larger scale.
• Scalability of design: designs that can be increased in size or flow-throughput whilst retaining the nanobubble generating capacity without increasing pressure-drop across the system so much that they need more powerful pumps.
• Durability, i.e. designs that do not clog, are easy to maintain or service without fragile and expensive parts.
• Robustness and Versatility of yield, i.e. designs that can retain their nanobubble generating capacity in changing environments and/or applications by a) monitoring the conditions and b) providing the ability to act with changes in the stream conditions, including but not limited to: solids content; micro-biological conditions; salt concentration; temperature; pressure; flow-rate; and other external factors that can have profound influence on the nanobubble generation.

[0014] In single chamber approach to the fluid dynamics, the yield is dependent on specific circumstances that must be controlled, while commercial applications require adaptability to 1) scale to the application to their needs, and 2) be robust in performance over long periods across various circumstances of time without service. In the field, traditional solutions are limited by:

• Engine based generators requiring exponential increase in power use when scale is increased and are therefore unsuitable for large applications;
• Traditional shear-path generators having too high a pressure-drop when the scale or flowrate is increased;
• Porous element containing generators not resisting to industrial settings where the water is not clean and require a lot of maintenance, get clogged, increase pressure-drop, reduce nanobubble yield.

[0015] There is a need for further improving devices for generating nanobubbles, in particular to provide devices for creating nanobubbles with reduced bubble size, for example with a maximum bubble size of between 20 and 2000 nm. There is also need to generate nanobubbles with minimum energy and improved flexibility with regard to conditions of formation of the nanobubbles.

[0016] According to a first of its aspect, the present invention provides a device for generating nanobubbles, comprising:

• an inlet for supplying a liquid to the device,
• a plurality of mixing chambers preferably distributed around a longitudinal axis of the device, and in communication with the inlet, each chamber comprising protrusions configured for increasing shear forces on the flow,
• at least one gas injector for injecting a gas into the flow before it leaves the chambers,
• a flow collector for collecting the flows leaving the chambers.

[0017] The presence of protrusions creates shear stress and helps improve the generation of nanobubbles in the fluid.

[0018] The protrusions of the different chambers may be the same, so as to allow fluid flowing through the different chambers to be subject to similar shear stress. In a variant, protrusions of at least two chambers are different and, in this way, fluid flowing through these chambers are subject to different shear stress.

[0019] The protrusions may be situated at a central portion of the chambers when observed along a longitudinal axis of the chamber. The protrusions may be present along at least half of the length of the chamber, more preferably between 50% and 80% of the length of the chamber. The protrusions may present circular symmetry around the longitudinal axis of the chamber.

[0020] The protrusions preferably form solid surfaces projecting inwardly towards the longitudinal axis of the chamber. Preferably, the shear stress induced by the protrusions on the fluid is between 30% and 80% more than the shear stress without protrusions as noted by increased pressure drop and increased dissolved oxygen measurement when using air or oxygen as a gas and water as a liquid.

[0021] The protrusions may have a length, when observed along a longitudinal axis of the chamber, that is preferably between 6 mm and 10 mm. The protrusions preferably have a height, when measured perpendicularly to a longitudinal axis of the chamber, that is greater than 2 mm, preferably between 2 mm and 5 mm.

[0022] The protrusions may be in the form of reliefs such as serrations present on the internal surface of the chambers. When observed in an axial section parallel to the longitudinal axis of the chamber, each serration may comprise an oblique side converging towards a main direction of the flow, followed by an opposite side perpendicular to the longitudinal axis of the flow. The oblique side preferably forms an angle of 50° with the main direction of the flow. Vortex may be formed locally at the junction between two adjacent serrations. The chamber may comprise between 15 to 25 serrations.

[0023] The protrusions may be formed integrally with the internal surface of the chamber. For example, the protrusions may be machined on the internal surface of the chamber or formed by molding.

[0024] In a variant, the protrusions may be attached to the internal surface of the chamber. The protrusions may form a monolithic element or may comprise individual protrusions arranged side by side along the longitudinal axis of the chamber. The protrusions may be metallic or plastic.

[0025] Each chamber may be selectively closed, so that the fluid only passes through the other chambers. For example, at least some chambers are equipped with their own valve so that a variable number of chambers may be put into service. For example, at least one or each chamber is equipped with a respective flow control valve for selective control of the flow within the chamber, said valve being controlled either manually or electronically. In this way, the flow may be split, after gas injection, into a variable number of chambers depending the on/off state of the flow control valves of the chambers. Controllable flow through one, several or all of these flow-chambers allows controlling the pressure-drop, the amount of nanobubbles generated and the flow rate through the device. This makes it possible to scale to high flow rates with limited yield loss.

[0026] The valves are preferably proportional valves.

[0027] The chambers are preferably positioned such that flows collected at the flow collector comes together for a swirling turbulence.

[0028] The chambers may have respective longitudinal axes that diverge from the longitudinal axis of the device when distance from the inlet increases. For example, the chambers have an elongated part of tubular shape. The elongated part of the chambers is preferably oriented obliquely with regard to a longitudinal direction of the device. The chambers preferably have outlets oriented radially inwardly through which the chambers open out into the flow collector. The flows are re-united after leaving the chambers, with swirling turbulence in the flow collector, instead of exiting the chambers with swirling turbulence.

[0029] In other words, turbulence is not only created within the chambers, but most importantly subsequently in the flow collector. This is advantageous because 1) it provides the opportunity to gain efficiency from multiple chamber's turbulence culmination as input at a single point 2) variable amount of chambers / shear paths / pressure configurations can be used to feed into the flow collector so as to make the generator adaptable to different circumstances 3) it is possible to vary the size of the flow collector so as to obtain different results for scaling the appliance.

[0030] The flow collector may have a collecting chamber into which the mixing chambers open out and an outlet channel, the collecting chamber diverging towards the outlet channel, the outlet channel converging and then optionally diverging on approaching an end of the outlet channel. In a variant, the outlet channel is of constant diameter.

[0031] The device may comprise downstream of the outlet channel of the flow collector an end nozzle.

[0032] The end nozzle may comprise a vortex forming element for creating a swirl flow, the vortex forming element preferably comprising at least one swirling fin. The end nozzle may comprise downstream of the vortex forming element a flow breaker for creating turbulence to the swirl flow. The flow breaker may comprise swirling interceptors in the form of projections on an internal surface thereof. The projections may be saw-shaped.

[0033] In a variant, the end nozzle may comprise a vortex generator. The vortex generator may comprise a main body having an upper part into which an inlet of the vortex generator radially opens. The main body may further comprise a conical lower part following the upper part in a longitudinal direction of the main body. An internal surface of the main body may comprise helicoidal grooves extending around the longitudinal direction of the main body so that turbulence is created in the main body.

[0034] A length of the mixing chambers may be greater than 20 mm, and an internal diameter of the inlet may be smaller than 60mm.

[0035] The number of mixing chambers may be range from 3 to 16, being preferably 4, the mixing chambers preferably being delimited at least partially by a monolithic insert having as many recesses as mixing chambers.

[0036] The gas injector preferably comprises a porous element positioned upstream from the mixing chambers and downstream from the inlet. This allows the gas to be transformed into micro-bubbles when being injected. The gas injector may be removable. The removable porous element can be removed for cleaning or replaced with minimum off-line time. This allows reduced service intervention time. It also allows reducing probability of replacing the entire device. The gas injector preferably has a frustoconical shape converging opposite the flow direction for facilitating the injection of gas.

[0037] The gas is preferably air, oxygen, ozone, carbon dioxide, nitrogen, chlorine or a mixture of thereof. The inlet of the device may be connected to a source of compressed gas, for example as an external device for gas production.

[0038] Exemplary embodiments of the invention also relate to a system for generating nanobubbles, comprising:

• a device according to the invention, as defined above,
• a recirculation loop for recirculating part of the flow having left the mixing chambers toward the inlet of the device, the recirculating loop preferably comprising a pump and a cavitation chamber for generating turbulence, this chamber preferably being upstream of the pump.

[0039] The recirculation loop recirculates flow taken up at the outlet channel of the flow collector.

[0040] Further aspects of the invention relate to a method for generating nanobubbles, comprising flowing a liquid through a device or a system according to the invention, as defined above, while injecting gas into said device so that it mixes with the liquid and forms nanobubbles.

[0041] The flow of liquid may range from 1 to 1000 m³/h, preferably from 10 m³/h to 200 m³/h.

[0042] The flow of gas, typically set at about 15% of the liquid flow, may range from 0.1-200 m³/h and preferably between 1.5 and to 30 m³/h.

[0043] The method may comprise recirculating part of the flow between downstream of the mixing chamber and upstream of the gas injector.

[0044] The liquid may be an effluent of a plant for processing wastewater or flow that needs to be mixed with gas including wastewater containing solids.

[0045] Specific embodiments of the invention will now be described in some further detail with reference to and as illustrated in the accompanying figures. These embodiments are illustrative only, and not meant to be restrictive of the scope of the invention.

• Fig.1 shows, in perspective view, an embodiment of a device according to the invention;
• Fig.2 is a side view of a device according to the invention;
• Fig.3 is an axial sectional view of the device of Fig.2 along III-III;
• Fig.4A is a partial view of an exemplary embodiment of a device of the invention, and Fig.4B to Fig.4D are respectively cross-sectional views, along B-B, C-C, and D-D of Fig.4A;
• Fig. 5A and Fig.5B are front views of the inlet of the device of Fig.3, respectively with and without the gas injector;
• Fig.6 illustrates schematically a device according to the invention in a cross-sectional similar to that of Fig. 3;
• Fig.7 is a cross sectional view of the device of Fig. 6, along VII-VII, connected to a gas flow meter;
• Fig.8 shows an exemplary embodiment of the invention with presence of water control unit;
• Fig. 9A and 9B illustrate dimensions of an exemplary embodiment of the invention;
• Fig.10B to 10F show in detail the zone A of the device of Fig.10A;
• Fig.11B and 11C show in detail the zone B of the device of Fig.11A;
• Fig.12B and 12C show in detail the zone C of the device of Fig.12A;
• Fig.13B shows in detail the zone D of the device of Fig.13A;
• Fig.14B shows in detail the zone E of the device of Fig.14A;
• Fig.15B shows in detail the zone F of the device of Fig.15A;
• Fig.16B shows in detail the zone G of the device of Fig.16A;
• Fig. 17B shows in detail the zone H of the device of Fig.17A;
• Fig.18A to 18C illustrate the replacing of the gas injector;
• Fig.19 is a perspective view of an end nozzle;
• Fig.20 illustrate a variant of the end nozzle;
• Fig.21A to 21C illustrate an automatic unclogging procedure.

[0046] An embodiment of a device 1 according to the present invention illustrated in Fig.1 to Fig 3 comprises an inlet 10 for supplying a liquid to the device 1 and a casing 20.

[0047] The casing 20 comprises, along a longitudinal axis X of the device 1, a first substantially cylindrical section 21 followed by a second substantially frustoconical section 22. The cross-section of the second section 22 first decreases and then increases towards an outlet 14 of the device. The inlet 10 and outlet 14 of the device 1 may have both a cylindrical shape of axis X.

[0048] The first section 21 comprises for example a monolithic insert 28 delimiting at least partially a plurality of mixing chambers 11. The insert 28 has as many recesses 48 as mixing chambers 11, as illustrated in Fig.4B, 4C and 4D. In the illustrated embodiment, the device comprises four mixing chambers 11. However, the invention is not limited to a particular number of mixing chambers, and the number of chambers may range from 3 to 16.

[0049] As illustrated in Fig.6 and 7, the recesses 48 are closed, on a radially outward side thereof, by removable parts 29 inserted into the recesses 48 in an airtight manner. The removable parts 29 delimit, together with the insert 28, the mixing chambers 11. The chambers 11 preferably have a circular cross-section along a major part of its length.

[0050] The insert 28 may be metallic. In a variant, the insert may comprise a plastic material. The removable parts 29 may comprise metallic or plastic material. The removable parts may comprise a window through which flow of the fluid inside a respective chamber 11 can be observed.

[0051] The insert 28 comprises, between a front flange 52 and a rear flange 53, a substantially frustoconical central portion 54 which diverges towards the rear flange 53. The plurality of chambers 11 are arranged around the central portion 54. The chambers have respective longitudinal axes Y that diverge from the longitudinal axis X of the device when distance from the inlet 10 increases. The mixing chambers 11 preferably have a constant cross-section, along a major part of their length.

[0052] The casing 20 and the removable parts 29 further define a flow collector 19 for collecting the flows leaving the chambers 11. The flow collector 19 has a collecting chamber 12 followed an outlet channel 13, arranged respectively in the first portion 21 and the second portion 22 of the casing 20.

[0053] The mixing chambers 11 open out into the collecting chamber 12 via outlets 30 which are oriented radially inwardly with regard to the longitudinal direction of the device 1. The flow collector 19 diverges along the length of the collecting chamber 12 and then converges along a major length of the outlet channel 13. The outlet channel 13 may slightly diverge on approaching the outlet 14 of the device 1.

[0054] In a variant as illustrated in Fig.6, the outlet channel 13 may have a constant diameter along its entire length.

[0055] As illustrated in Figs.2 and 3, the device also comprises a gas inlet 15, for example being connected to a source of compressed gas such as an external device for gas production 36 as illustrated in Fig.7. The device 1 comprises a gas injector 16 for injecting the gas introduced from the gas inlet 15 into the flow before it leaves the chambers 11.

[0056] The gas injector 16 comprises a porous injection section 61 and an inlet section 62. The injection section 61 protrudes, at least partially, into the inlet 10 of the device 1. The injection section 61 has a frustoconical shape so as to help separation of the fluid into the different mixing chambers 11. The liquid injected through the inlet 10 is then split into a plurality of flows flowing through respective mixing chambers 11 of the device 1. The inlet section 62 is received at least partially in a central hole 67 of the central portion 54. The outer diameter of the inlet section 62 is substantially equal to the inner diameter of the central hole 67. The inlet section 62 may not extend until a bottom 80 of the hole 67.

[0057] The gas injector 16 may be removable, as illustrated in Fig.10B to 10F. An intermediate element 63 is disposed between the injection section 61 and the central portion 54 of the casing 20 when the inlet section 62 of the gas injector 16 is inserted into the central hole 67. The intermediate element 63 has a through hole 65 which receives therein the part of the inlet section 62 not inserted into the central hole 67. The through hole 65 has a diameter that is slightly larger than the outer diameter of the inlet portion 62, as illustrated in Fig.10C. In this embodiment, the intermediate element 63 is sleeved onto the inlet portion 62 and can be removed from the inlet portion 62 after the latter is retreated from the central hole 67.

[0058] In a variant, the intermediate element 63 can be fixed to the central portion 54 of the casing 20, as illustrated in Fig.18A to 18C which illustrate a process for removing and installing removable gas injector 16. As shown in these figures, in order to remove the gas injector 16, the inlet 10 of the device is disconnected from the front flange 52. The recirculating loop 2 is disconnected at the level of the no-return valve 25. When a new gas injector 16 (or the same gas injector 16 after having been cleaned) is in place, the inlet 10 of the device is reconnected to the front flange 52, and the recirculating loop 2 is reconnected.

[0059] As illustrated in Fig.7, the bottom 80 of the central hole 67 is connected to the external device for gas production 36 via a central duct 44 formed in the casing 20. The external device for gas production 36 may also comprise a gas flowmeter and control 42.

[0060] The outlet 14 of the device may be connected to an end nozzle 3, for example through a flexible tube 5, as illustrated in Fig.1. The flexible tube 5 may be sleeved onto the outlet 14. In a variant, as illustrated in Fig.6, the end nozzle 3 may be connected directly to the outlet 14.

[0061] The end nozzle 3, as illustrated in Fig.19, may comprise a vortex forming element 31 comprising swirling fins for increasing the nanomizing of the nanobubbles at the outlet 14 of the device and a flow breaker 32 for creating turbulence to the swirling flow. The flow breaker may comprise swirling interceptors, for example in the forms of projections 93 projecting from the internal surface 83 of the flow breaker.

[0062] In a variant, the end nozzle 3 may comprise a vortex generator as illustrated in Fig.20. The outlet 14 of the device is connected to an inlet 70 of the vortex generator, which opens radially into a main body 71 of the vortex generator. The main body 71 comprises an upper part 78, followed in a longitudinal direction Z thereof, by a lower part 79 of conical shape. An outlet 90 is arranged at the end of the lower part 79. An internal face 73 of the main body 71 comprises grooves 73 of helicoidal shape extending around the longitudinal direction Z. Swirl is formed in the main body 73, which allows further decrease of the bubble size in the flow passing through the vortex generator. An opening 76 is arranged opposite the outlet 90. The opening 76 can be blocked by a simple screw. In a variant, the opening 76 can be used as a further gas inlet, for example of pressurized gas.

[0063] In the embodiment of Fig.6, the inlet 10 of the device 1 is connected to a water supply 33. A water flowmeter 34 may be arranged between the inlet 10 and the water supply 33 to measure the flow rate into the inlet 10.

[0064] Each mixing chamber 11 may be provided with a water pressure sensor 35. The device may further include a water pressure control unit 39 as illustrated in Fig.8. The device 1 comprises, for each mixing chamber 11, a flow control valve 17 for selective control of the flow within the chamber 11. In this way, the number of mixing chambers 11 in operation can be adjusted by controlling the valves 17. The valves 17 can be controlled manually or electronically.

[0065] As illustrated in Fig.6, the device 1 may be connected to a recirculation loop 2 for recirculating part of the flow taken up at the outlet channel 13 of the flow collector 19 towards the inlet 10 of the device 1. The recirculating loop 2 comprises a pump 24 with a cavitation chamber 23 for generating turbulence. The recirculating loop 2 may comprise a non-return valve 25 for preventing backward flow of recycled water 26.

[0066] As illustrated in Fig.9A and 9B, a length L1 of the mixing chambers 11 along the longitudinal direction of the device is preferably greater than 20 mm. A length L2 of the collecting chamber 12 is preferably greater than 5 mm. A length L3 of the outlet channel 13 is preferably greater than 15 mm. The length L4 of the outlet channel 13 along which it converges is preferably greater than 12 mm. Preferably, the ratio between L2 and L3 is between 0.25 to 0.35. Preferably, the ratio between L3 and L4 is between 1.18 to 1.30.

[0067] An internal diameter r0 of the inlet 10 is preferably smaller than 60 mm. An internal diameter r3 of the outlet 14 is preferably smaller than 60 mm. The collection chamber 12 preferably has a dimeter r2 smaller than 90 mm at its inlet and a diameter r3 preferably smaller than 80 mm at its outlet.

[0068] The angle α between the longitudinal axis X of the device 1 and the longitudinal direction Y of the chambers 11 is preferably between 4° and 8°, for example around 6°.

[0069] Fig.11B shows details of the chamber 11 observed from the direction S of Fig.11A, through a window 56 on the removable part 29. Fig.11C shows details of the chamber 11 between the cross-sections A-A and B-B of Fig.11B. The internal surface 91 of the chamber 11 comprises protrusions in the form of serrations 92 present on the internal surface 91 of the chambers. These serrations 92 induce shear forces on the flow, which travels through a shear-stress path. The serrations 92 have a length l, measured in a direction of flow U, which is preferably between 7 mm and 9 mm. The serrations 92 have a height h, measured perpendicularly to the direction of flow U, which is preferably between 3 mm and 4 mm. Each serration 92 has an oblique surface 93 that converges in the direction of flow U and an opposite surface 94 perpendicular to the direction of flow U.

[0070] In this way, the size of the bubbles generated by the gas passing the chamber 11 decreases in the direction of the flow U.

[0071] Fig.12B and 12C show details of the flow control valve 17. As can be seen in Fig.12B, when the flow control valve 17 is open, no turbulence is created and the bubbles travelling through the valve 17 does not change in size. When the valve 17 is semi-open as illustrated in Fig.12C, turbulence T is created locally and the bubbles travelling though the valve 17 further decrease in size after passing though the valve 17. When the valve 17 is closed (not illustrated), no flow passes in the corresponding chamber 11.

[0072] Fig.13B show details of the chamber 11 at its outlet 30 where the chamber 11 opens into the collecting chamber 12. Turbulence T is created locally at the outlet 30 of the chamber 11. Thus, the size of the bubbles further decreases in the flow which turns radially inward into the collecting chamber 12.

[0073] Fig.14B shows details of the collecting chamber 12 and the outlet channel 13. Turbulence is created inside the collecting chamber 12, which results in the size of the bubbles to further decrease in the flow leaving the collecting chamber 12 and entering the outlet channel 13.

[0074] Fig.15B shows details of the vortex forming element 31. The vortex forming element comprises two swirling fins 81. As shown, a swirl flow SF is generated in the flow passing though the vortex forming element 31, further reducing the size of the bubbles in the flow.

[0075] Fig.16B shows details of the flow breaker 32. The flow breaker 32 comprises swirling interceptors in the form of parallel lines of projection 93 on an internal surface 83 of the flow breaker 32. The lines of projections 93 may be saw-shaped. The swirling interceptors create turbulence T to the swirl flow SF, further reducing the size of the bubbles in the flow.

[0076] Fig.17B shows details of the pump 24 with cavitation chamber 23 of the recirculating loop. The cavitation chamber 23 is configured for creating turbulence in the flow passing through the valve 24, in order to further reduce the size of the bubbles in the flow.

[0077] The device may allow self-cleaning of the system through regulated increase of the pressure drop as well as control over the yield of the nanobubbles. Fig.21A to 21C illustrate a process of automatic unclogging of the device by monitoring clogging related pressure, and timely activating an automatic unclogging mechanism.

[0078] As shown in Fig.21A, when the flow control valve 17 is in semi-open mode, solids S carried into the device by the fluid tends to clog in the chamber 11 at the level of the control valve 17. The cumulation of the solids S in the chamber 11 changes the dynamic of the fluid and results in the generation of turbulence in the chamber 11 before the valve 17. The turbulence therefore induces a pressure drop in the chamber 11, which can be detected by the water pressure sensor 35.

[0079] As shown in Fig.21B, the water pressure sensor 35 is configured, upon detection of pressure drop, to send signals to control units of the valves 17, in order to open completely the valve 17 of the clogged chamber and close completely the valves 17 of the other chambers 11. In this way, the solids will be carried by the flow through the flow collector 19 to the outlet of the device. If several chambers are simultaneously clogged, they can be unclogged one by one or simultaneously.

[0080] After evacuation of the solids S, as shown in Fig.21C, the pressure with the unclogged chamber 17 comes back to the normal value. Therefore, upon detection of the return of the pressure, the water pressure sensor 35 is configured to send signals to control units of the valve 17, in order to set the valves 17 back into the semi-open mode.

[0081] The invention is not limited to the above described embodiments.

[0082] The valves 17 may be controlled manually.

[0083] Sensors may be linked to a data-collection and control system allowing on-line monitoring of all the conditions.

[0084] The sensors may be configured to detect changing conditions of the flow including but not limited to solids content, salt concentration, temperature, pressure, flowrate and other external factors that can influence the nanobubble generation. Data may be collected and treated in such a way that nanobubble yield and generation is controlled in changing conditions. This allows manual intervention or telecontrol and algorithms to optimize nanobubble yield.

Claims

1. A device (1) for generating nanobubbles, comprising:

- an inlet (10) for supplying a liquid to the device,

- a plurality of mixing chambers (11), distributed around a longitudinal axis (X) of the device, and in communication with the inlet (10), each chamber (11) comprising protrusions (92) configured for increasing shear forces on the flow,

- at least one gas injector (16) for injecting a gas into the flow before it leaves the chambers (11), the gas injector (16) preferably being a porous element and preferably being positioned upstream the mixing chambers (11) and downstream the inlet (10).

- a flow collector (19) for collecting the flows leaving the chambers (11), the chambers (11) preferably having outlets (30) oriented radially inwardly through which the chambers (11) open out into the flow collector (19).

2. The device of claim 1, each chamber (11) being equipped with a respective flow control valve (17) for selective control of the flow within the chamber (11).

3. The device of claim 1 or 2, the chambers (11) having respective longitudinal axes (Y) that diverge from the longitudinal axis (X) of the device when distance from the inlet increases, each chamber (11) preferably comprising serrations for generating shear stresses on the flow circulating along the chamber (11).

4. The device of any one of claims 1 to 3, the flow collector (19) having a collecting chamber (12) into which the mixing chambers (11) open out and an outlet channel (13), the collecting chamber (12) diverging towards the outlet channel (13), the outlet channel (13) converging and then diverging on approaching an end (18) of the outlet channel (13).

5. The device of any one of claims 1 to 4, comprising downstream of the outlet channel (13) of the flow collector(19) a vortex forming element (31) for creating a swirl flow (SF), the vortex forming element (31) preferably comprising at least one swirling fin (81), the device preferably comprising, downstream of the vortex forming element (31), a flow breaker (32) for creating turbulence (T) to the swirl flow (SF), the flow breaker (32) preferably comprising swirling interceptors in the form of projections (93) on an internal surface (83) thereof.

6. The device of any one of claims 1 to 4, comprising downstream of the outlet channel (13) of the flow collector(19) a vortex generator (3), the vortex generator (3) comprising an inlet (70) which opens radially into a main body (71) of the vortex generator (3), the main body (71) comprising an upper part (78), followed in a longitudinal direction (Z) thereof, by a lower part (79) of conical shape, an internal face (73) of the main body (71) preferably comprising grooves (73) of helicoidal shape extending around the longitudinal direction (Z).

7. The device of any one of claims 1 to 6, a length (L1) of the mixing chambers (11) being greater than 20 mm, an internal diameter (r0) of the inlet (10) being smaller than 60mm.

8. The device of any one of claims 1 to 7, the number of mixing chambers (11) ranging from 3 to 16, being preferably 4, the mixing chambers (11) preferably being delimited at least partially by a monolithic insert (28) having as many recesses (48) as mixing chambers (11).

9. A system for generating nanobubbles, comprising:

- The device (1) of anyone of claims 1 to 8,

- a recirculation loop (2) for recirculating part of the flow having left the mixing chambers (11) toward the inlet (10) of the device (1), the recirculating loop (2) preferably comprising a pump (24) with a cavitation chamber (23) for generating turbulence.

10. The system of claims 4 and 9, the recirculation loop (2) recirculating flow taken up at the outlet channel (13) of the flow collector (19).

11. A method for generating nanobubbles, comprising flowing a liquid through a device (1) as defined in any one of claims 1 to 8 or a system as defined in claims 9 or 10, while injecting gas into said device so that it mixes with the liquid and forms nanobubbles.

12. The method of claim 11, the flow of liquid ranging from 1-1000 m³/h and preferably from 10 m³/h to 200 m³/h.

13. The method of claim 11 or 12, the flow of gas ranging from 0.1-200 m³/h and preferably from 1.5 to 30 m³/h.

14. The method of any one of claims 11 to 13, comprising recirculating part of the flow between downstream of the mixing chamber (11) and upstream of the gas injector (16).

15. The method of anyone of claims 11 to 14, the liquid being an effluent of a plant for processing wastewater or flow that needs to be mixed with gas including wastewater containing solids.

Drawing