WO2023161101A1

WO2023161101A1 - System and method for dissolving gas into a fluid

Info

Publication number: WO2023161101A1
Application number: PCT/EP2023/053779
Authority: WO
Inventors: Ingar Kjøstolfsen; Bård-Inge Hansen; Stig Are Karlsen
Original assignee: Nippon Gases Norge As
Priority date: 2022-02-22
Filing date: 2023-02-15
Publication date: 2023-08-31
Also published as: NO347580B1; NO20220230A1

Abstract

The invention discloses a system and a method for operating such system for supplying gas bubbles into a fluid. The system comprises a longitudinal cylindrical fluid pipe for the fluid to pass therethrough, a restriction element arranged within the longitudinal cylindrical fluid pipe and a gas supplying device. The gas supplying device comprises an outer surface and an inner surface enclosing a chamber, a gas inlet configured to feed gas into the chamber and a membrane arranged at the inner surface, the membrane being configured to allow gas to pass from the chamber into the flow of fluid as gas bubbles. The restriction element defines an annular flow path between an outer surface of the restriction element and the membrane.

Description

System and method for dissolving gas into a fluid

Technical Field

The present invention relates to a system for supplying gas bubbles into a fluid as defined in the preamble of claim 1 and a method for using such a system.

Background and prior art

Without limiting the scope of the present invention, the background of the invention will be described with reference to aquaculture. Oxygen is a major contributing factor in aquaculture alongside water and feed. The oxygen content of the water should be above 80% saturation in relation to air to maintain adequate fish welfare and fish growth. In intensive fish farming, the oxygen supplied naturally from the water source is usually not sufficient to maintain the requirement for good fish welfare. Therefore, extra oxygen must be added and dissolved in the water.

The development in aquaculture is increasingly directed towards land-based installations and closed containment aquaculture installations. In land-based installations seawater is pumped into the installations, and a part of the water is often reused. In closed containment aquaculture installations, which are floating in the sea, seawater is pumped into the installation from great depths. In both cases an oxygen supply is needed. Today's traditional solutions are either diffuser hoses that provide low utilization of the added oxygen, typically 25% of the added amount, or dissolving oxygen with pressure in cones, columns and the like which typically have 90% utilization of the added oxygen, but have very large energy requirement because the water must be pressurized up to 3-5 bar overpressure.

The solubility of oxygen in water is low because the surface tension of the water must be broken for sufficient gas absorption. Different solutions for dissolving gas in water are known.

WO2021219345 discloses a system and method for supplying gas bubbles into a fluid. The system comprises a longitudinal cylindrical fluid pipe for the fluid to pass therethrough, a turbulence inducing device arranged within the longitudinal cylindrical fluid pipe for generating a turbulent flow of the fluid and a gas supplying device. The gas supplying device comprises an outer surface and an inner surface enclosing a toroid shaped chamber, a gas inlet configured to feed gas into the toroid shaped chamber and a membrane arranged at the inner surface, the membrane being configured to allow gas to pass from the toroid shaped chamber into the flow of fluid as gas bubbles. The gas supplying device is arranged dowstream relative to the turbulence inducing device such that the turbulent flow of fluid passes a cross-sectional area enclosed by the membrane when passing the turbulence inducing device or immediately after exiting the turbulence inducing device.

US2018/0050312 discloses a system and a method for dissolving water soluble gas such as oxygen into water, employing a microporous membrane. The specification discloses a device comprising a conduit having an inlet, an outlet, and a throat section sealed to the downstream end of the inlet and to the upstream end of the outlet, the throat section defining a flow passage through which liquid can flow from the inlet to the outlet, wherein a portion or all of the inner surface of the throat section comprises a smooth microporous discharge face of a solid membrane through which gas can pass from the interior of the membrane out of the membrane into the flow passage in the throat section, the membrane also having an exterior face through which gas can be passed into the membrane from outside the throat section without the gas being able to enter the throat section other than through the discharge face, the device further comprising an outer housing that defines a chamber surrounding the exterior face of the membrane, and at least one port in the outer housing through which gas can be passed into the chamber, wherein gas can be passed into the flow passage in the throat section only through the membrane.

In view of the prior art, it is an object of the present invention to provide a more efficient system for solubilizing gas in a fluid, for example by reducing the energy consumption.

Further, it is an object of the present invention to provide a gas supplying device having a high degree of gas utilization, preferably above 90%.

Another object of the present invention is to utilize existing conventional fluid pipes and instruments already present in the fluid pipes, providing an energy efficient gas supply to fluid.

Summary

The present invention is defined by the appended claims and in the following:

The present invention relates to a system and a method for supplying gas bubbles into a flow of fluid for dissolving the gas bubbles in the fluid.

The system comprises a longitudinal cylindrical fluid pipe for fluid to pass therethrough.

The system further comprises a gas supplying device for supplying gas to the flow of fluid. The gas supplying device comprises an outer surface and an inner surface, wherein the inner surface has a smaller cross-sectional area than the outer surface. The gas supplying device further comprises a chamber. The chamber may have a toroid shape, an annular shape or any other suitable shape.

The inner surface of the gas supplying device forms a part, or intermediate section, of the fluid pipe. The gas supplying device forms an intermediate section of the fluid pipe.

More specifically the chamber may be enclosed by parallel inner and outer axial surfaces oriented along an axial direction, and by two parallel radial surfaces oriented perpendicular to the inner and outer surfaces. In this exemplary design, the chamber is toroid shaped and has a rectangular cross-section circumventing an axial center axis. The axial direction is parallel to the longitudinal direction of the longitudinal cylindrical fluid pipe.

The gas supplying device further comprises a gas inlet allowing gas to be fed into the chamber and a membrane arranged at the inner surface. The membrane is configured to allow gas to pass from the chamber into a flow of fluid as gas bubbles. Hence, the gas entering the flow of fluid may only pass through the membrane.

The system further comprises a restriction element for controlling the speed of the flow of the fluid along the gas supplying device. This increase in flow of the fluid will increase the solubility of the gas in the fluid.

The restriction element is arranged within the fluid pipe, the restriction element defining an annular flow path between an outer surface of the restriction element and the fluid pipe, especially along the membrane. In other words, the restriction element is configured to define an annular flow path along which the fluid is forced/directed to flow when passing through the fluid pipe. At least part of this annular flow path is defined between the inner surface of the gas supplying device and the outer surface of the restriction element. Without being bound by theory, the increase of the flow of the fluid will cause the gas bubbles to detach from the membrane surface at an earlier point, therefore creating smaller gas bubbles, which will cause an improvement in the solubility of the gas.

The restriction element may have any shape and size, as long as it reduces the available space for the fluid to pass along the gas supplying device, thereby increasing the flow of fluid along the gas supplying device.

The restriction element may preferably be centered in the fluid pipe’s longitudinal direction.

In an embodiment the restriction element may be a cylindrically shaped elongate body.

In an embodiment the cylindrically shaped elongate body may be at least as long as the chamber, in a longitudinal direction of the pipe, i.e. perpendicular to a cross section of the pipe, to ensure constant flow speed along the gas supplying device.

In an embodiment, the restriction element may be a cylindrically shaped elongate body, and the restriction may be at least as long as the chamber, in a longitudinal direction of the pipe, i.e. perpendicular to a cross section of the pipe, to ensure constant flow speed along the gas supplying device, and the membrane is adjacent to the elongate body of the restriction element .

Here the person skilled in the art will understand that “adjacent” means that the inner surface of the gas supplying device, where the membrane is located faces the surface of the cylindrically shaped elongate body of the restriction element, i.e. the membrane is concomitant with the elongate body of the restriction element.

The cylindrically shaped elongate body may further comprise end part at each end of the cylindrically shaped elongate body.

These end parts may be independent in size and shape. In an embodiment, these end parts may be in the shape of cones, spheres, trapezoidal cylinders, truncated spheres, oblique circular cylinder, or a combination of any of these shapes.

Alternatively, the restriction element may only be composed of two end parts.

Preferably the end parts may have a shape and size that will favor a reduction in the Reynolds number of the flow (having an increased flow speed) along the gas supplying device, compared to a restriction element that does not have such end parts.

In an embodiment, the restriction element may comprise a cylindrically shaped elongate body, an upstream conical end, attached to one end of the cylindrically shaped elongate body, wherein the base of the cone (of the upstream conical end) is the cross section of the cylindrically shaped elongate body; and a downstream conical end, attached to the other end of a cylindrically shaped elongate body wherein the base of the cone (of the downstream conical end) is the cross section of the cylindrically shaped elongate body and wherein the downstream conical end is longer than the upstream conical end. The upstream conically shaped end ensures a steady increase in the water velocity towards the tubular part of the restriction element and the downstream conically shaped end ensures a steady reduction in the water velocity after the tubular part of the restriction element. When the fluid pipe has a constant cross section, the water velocity upstream from the restriction element is equal to the water velocity downstream from the restriction element.

In an embodiment the restriction element may be dimensioned to increase the Reynolds number of the flow of fluid along the gas supplying device by between 10% and 1000%, by between 50% and 500%, by between 100% and 300%, or by between 100% and 200%, compared to the flow of fluid upstream from the restriction device.

When the water passes past the membrane, the water velocity increases because the available cross-sectional area in the pipe is reduced. At increased water velocity the gas bubbles that penetrate the membrane are quickly carried away by the water in the form of small bubbles with a large surface to volume ratio and thus high diffusion and dissolution in the water. The bubble size will vary with membrane pore opening, water velocity and water quality such as salinity.

The capacity of the amount of gas supplied to the water depends on the area of the membrane, the pressure in the gas and in the water and the amount of water passing past the membrane.

Typically, the gas may be oxygen. Typically, the fluid may be water. The amount of oxygen gas added can be adjusted as needed and according to available membrane area and according to the static pressure in the pipe. A typical amount of oxygen gas added may be 2 to 50 mg oxygen per liters of water, preferably 10 to 20 mg oxygen per liters of water, more preferably 14 to 18 mg oxygen per liters of water. The microbubbles follow the water flow from the gas supplying device and over to the fish tank / consumption point where the water with oxygen is further distributed in the water masses.

The system may typically function with a fluid velocity in the fluid pipe in the range 0.5 to 5 meters per second, preferably 1 to 3 meters per second.

In system where the incoming fluid velocity varies or in system where it is desirable to optimize the gas dissolution in the fluid at any given time, the restriction element, and especially its tubular section may have a variable cross section. By varying ratio of the cross section of the restriction element to cross section of the longitudinal cylindrical fluid pipe, a constant optimum fluid velocity may be maintained past the membrane.

In an embodiment the fluid velocity may be increased by the restriction element along the membrane at least by 10%, 20%, 50%, 100%, 200%, 300%, 400%, 500%, 1000%, 2000%, or 5000%.

In an embodiment the fluid velocity may be increased by the restriction element along the membrane between 10% and 5000%, 20% and 2000%, 50% and 1000%, 100% and 1000%, 200% and 1000%, 300% and 1000%, 400% and 1000%, 1000% and 5000%, between 10% and 400%, or between 200% and 300%.

In an embodiment the fluid velocity may be increased by the restriction element along the membrane to between 0,55 and 25 meter per second (depending on the fluid velocity in the pipe), between 1 and 15 meter per second, between 3 and 10 meter per second, between 3 and 8 meter per second, between 5 and 6 meter per second.

In an embodiment, the diameter of the restriction element may be adjusted in real time, for example by varying the pressure internally in the central element, for example with air or gas, by a calibrated internal spring load that adjusts the diameter according to the applied pressure, by a material that is flexible and to some extent compressible at applied pressure and regains shape at reduced pressure again, or by any other suitable means.

The restriction element is attached to the longitudinal cylindrical fluid pipe by attaching means.

In an embodiment, the attaching means may be a series of rigid bars, wires, chains or fins attached on one end to the restriction element and on the other end to the longitudinal cylindrical fluid pipe. There can for example be 2 or more, 3 or more, 4 or more, 5 or more, 6 or more attaching means around the restriction element. They can be arranged in the same cross section, or on different planes, they can be regularly disposed around the restriction element, or at irregular intervals. The attaching means may be arranged upstream or downstream from the gas supplying device.

In an embodiment, the restriction element may be molded together with the attaching means.

In an embodiment, the restriction element may be attached to the longitudinal cylindrical fluid pipe by a central element, for example a bar going through the restriction element, the central element being attached to the longitudinal cylindrical fluid pipe by at least a bar or fin at at least an end of the central element The membrane may be made of a corrosion and wear resistant material. The material may be selected from at least one of a polymer-based material such as polyurethane or polyethylene; a ceramic-based material such as alumina (AI2O3) and zirconium oxide (ZrCh); a steel-based material such as stainless steel; and a nickel-based material such as nickel alloys comprising alloying elements of at least one of copper, molybdenum, chromium, iron and tungsten.

In a preferred embodiment, the membrane may be made of a steel-based material or nickel-based material, and in an even more preferred embodiment of a nickel-based material.

In an embodiment of the invention the membrane may comprise pores having a pore size of any one of 0.1 to 10 pm, or preferably 0.3 to 5 pm or even more preferably 0.4 to 1 pm, for example 0.5 pm.

The longitudinal cylindrical fluid pipe may comprise an inlet at a first end and an outlet at the opposite second end. The fluid passes inside the inner surface of the longitudinal cylindrical fluid pipe from the first end towards the second end. In a preferred embodiment, the longitudinal cylindrical fluid pipe may have constant or close to constant inner diameter through the whole length of longitudinal cylindrical fluid pipe comprising the restriction element and the gas supplying device.

The longitudinal cylindrical fluid pipe may be submerged in water during use.

During operation of the system the gas pressure within the chamber should be high enough to move the gas from the chamber through the membrane and into the flow of fluid running along the restriction element within the longitudinal cylindrical fluid pipe. Hence the gas pressure within the chamber should be higher than the counter pressure of the fluid passing the cross-sectional area enclosed by the membrane for the gas to pass through the membrane from the chamber.

The gas passes into the flow of fluid in the form of gas bubbles attaching themselves to an outer surface of the membrane which corresponds to the inner surface of the gas supplying device, hence facing the fluid. The flow of fluid pushes on the gas bubbles arranged at the outer surface of the membrane by shear forces. The gas bubbles are hence disengaged therefrom as the gas dissolves into the fluid. Hence, it is an important feature of the present invention that the flow of fluid passes the cross-sectional area enclosed by the membrane when passing the restriction element or immediately after exiting from the restriction element, as the increased velocity of the flow of fluid is required to disengage and dissolve the gas bubbles into the fluid.

If the gas bubbles are not disengaged from the outer surface of the membrane, the gas bubbles will grow in size until they self-disengage from the surface. After such self-disengagement, the gas bubbles will start moving along an inner wall/surface of the longitudinal cylindrical fluid pipe in an upstream direction until they exit the longitudinal cylindrical fluid pipe. These gas bubbles are hence not dissolved into the fluid. Such growth of gas bubbles should therefore be avoided to keep the gas loss at a minimum.

If the gas pressure within the chamber is lower than the counter pressure of the fluid passing the cross-sectional area enclosed by the membrane, the fluid may be allowed to enter into the chamber causing interruption to the system. Further, fluid passing through the membrane may comprise particles which may clog the pores of the membrane. In such undesired situations, maintenance of the membrane must be performed before the system can continue to operate.

To minimize the risk of fluid entering the chamber, the gas supplying device may in an advantageous embodiment comprise a plurality of gas inlets. In an embodiment the toroid shaped chamber may comprise at least a first and a second gas inlet allowing gas to be fed into the chamber, for example by connecting a gas container with pressurized gas in gas communication with the gas inlet(s). If the first gas inlet fails to allow the needed gas pressure to enter the chamber, the second gas inlet can be activated to take over the gas supply. The required gas pressure is thereby maintained.

In an embodiment, the gas supplying device may comprise a pressure sensor configured to detect the gas pressure inside said chamber. The pressure sensor may be configured to transmit via a transmitter the detected pressure to a controller configured to control the gas pressure in the chamber. The gas pressure within the chamber may consequently be adjusted to optimize operation efficiency.

Further, the pressure sensor may comprise another transmitter sending signals to said controller, optionally to a second controller configured to detect an anomalous pressure within the chamber. Upon such pressure readings, the pressure can be adjusted, manually or automatically, thereby ensuring that the pressure is always above a predetermined value prohibiting fluid within the longitudinal cylindrical fluid pipe from entering into the chamber.

In an embodiment any pressure loss sensed by the pressure sensor approaching below the counter pressure of the fluid may be registered by the pressure sensor which will transmit signals thereof to the control device. The control device will ensure that for example the feeding rate of gas into the gas supplying device is increased sufficiently to maintain the gas pressure within the chamber above the hydrostatic pressure of the fluid.

In a further embodiment of the system, the chamber may comprise a plurality of individual compartments comprising individual gas inlets wherein the compartments are not in gas communication with each other. Dividing the chamber into a plurality of compartments having their own gas inlet(s) and respective gas supply may for example be advantageous if the gas supply to one of the compartments fails. While fixing the broken gas supply, gas can still be fed into the fluid through the remaining compartments such that a complete shutdown of the system can be avoided.

Further, if the longitudinal cylindrical fluid pipe is arranged horizontally during use, the counter pressure of the fluid passing the cross-sectional area enclosed by the membrane will be different along the circumference of the membrane due to gravity forces acting on the fluid. The gravity forces influence the counter pressure of the fluid such that the counter pressure of the fluid is at its highest at a lowest gravity area of the membrane and is at its lowest at the opposite part arranged at an upper gravity area of the membrane. In such an embodiment it may be advantageous that the chamber is divided into a plurality of individual compartments, wherein each compartment comprises an individual gas pressure and an individual gas inlet. Consequently, each of the individual compartments can have an adjusted gas pressure such that each compartment comprises a gas pressure being equal to or above the hydrostatic pressure of the fluid passing the individual compartments.

According to an embodiment of the system, the gas supplying device may be designed as a pipe segment of the longitudinal cylindrical fluid pipe circumventing the radial diameter of the longitudinal cylindrical fluid pipe. The inner surface of the gas supplying device may be arranged flush with the inner surface of the longitudinal cylindrical fluid pipe. The inner surface of the gas supplying device may for example have a shape that corresponds to the shape of the inner diameter of the longitudinal cylindrical fluid pipe.

For example, if the longitudinal cylindrical fluid pipe is straight, meaning that the inner diameter of the longitudinal cylindrical fluid pipe is a constant throughout at least the part of the longitudinal cylindrical fluid pipe comprising the restriction element, then the diameter of the inner surface of the gas supplying device will be equal to the diameter of the inner surface of the longitudinal cylindrical fluid pipe.

If the circumference of the inner surface of the longitudinal cylindrical fluid pipe has a continuously increasing diameter, i.e. the diameter is increasing in the direction of the fluid flow, then the circumference of the inner surface of the chamber will have a corresponding continuous increasing diameter such that the inner surface of the chamber is flush with the inner surface of the longitudinal cylindrical fluid pipe. If the circumference of the inner surface of the longitudinal cylindrical fluid pipe has a continuous decreasing diameter, i.e. the diameter is decreasing in the direction of the fluid flow, then the circumference of the inner surface of the gas supplying device will have a corresponding continuous decreasing diameter such that the inner surface of the gas supplying device is flush with the inner surface of the longitudinal cylindrical fluid pipe.

Further, a radial extension of the gas supplying device may be larger than the radial extension of the longitudinal cylindrical fluid pipe, wherein the radial extension is relative to the longitudinal cylindrical fluid pipes longitudinal center axis. In this embodiment the gas inlet may be arranged at the radial extension of the gas supplying device extending beyond the radial extension of the longitudinal cylindrical fluid pipe.

The gas supplying device may be attached to the longitudinal cylindrical fluid pipe by welding or by bolts extending from the outer surface of the gas supplying device into the outer surface of the fluid pipe.

The fluid may be a liquid, preferably a water-based liquid such as seawater, brackish water, fresh water, potable water, tunnel water, wastewater, process water etc.

The gas may be oxygen, carbon dioxide or an inert gas.

In a first embodiment of the method, the gas may be oxygen and the fluid may be a water-based liquid such as sea water, brackish water, fresh water or wastewater. For example, the system of the present invention may be used to add oxygen to seawater before the water is supplied to a fish tank

In a second embodiment the gas is CO2 and the fluid may be water-based liquid, such as wastewater, potable water, process water or tunnel water. For example, the system of the present invention may be used to add CCh-gas to alkaline (or basic) water such as wastewater or tunnel water for pH-regulation.

In a third embodiment the gas may be an inert gas such as nitrogen or argon and the fluid is a water-based liquid such as sea water, potable water, wastewater, fresh water. For example, the system of the present invention may be used to add inert gas to a water-based liquid for Ch-stripping.

The system of the present invention may be used in systems requiring a high degree of fluid flow through the pipe, such as between 100 to 100 000 liters fluid per minute. Further, the inner diameter of the pipe may be at least 50 mm, at least 100 mm, at least 200 mm, at least 500 mm, at least 1000 mm, at least 2000 mm. The inner diameter of the pipe may be between 50 mm and 5000 mm, between 160 mm and 1000mm, or between 50 and 160 mm.

In the context of the application the term “circular” should be interpreted to include any cross-sectional shapes which are common for longitudinal cylindrical fluid pipes. Most longitudinal cylindrical fluid pipes comprise a merely circular cross- section. However, the term may also include cross sections having an oval shape, irregular circular shape etc. Further the cross-sectional shape may comprise an increasing or decreasing diameter in the direction of the fluid flow.

In the context of the application the term “pore size” should be interpreted as the average distance between two opposite walls of the pore, e.g. the diameter of cylindrical pores.

In the following description, numerous specific details are introduced to provide a thorough understanding of different embodiments of the system. One skilled in the relevant art, however, will recognize that these embodiments can be practiced without one or more of the specific details, or with other components, systems, etc.

In other instances, well-known structures or operations are not shown, or are not described in detail, to avoid obscuring aspects of the disclosed embodiments.

Brief description of the drawings

These details and other characteristics of the invention will be clear from the following description of embodiments, given as non-restrictive examples, with reference to the attached drawings wherein:

FIG. 1 is a perspective view of a fist example embodiment of the invention wherein the gas supplying device is designed as a pipe segment of the longitudinal cylindrical fluid pipe.

FIG. 2 is a cross-sectional open view of the first example embodiment shown in FIG. 1.

FIG. 3A is a perspective view of a first embodiment of the restriction element.

FIG. 3B is a perspective view of a second embodiment of the restriction element.

FIG. 3C is a perspective view of a third embodiment of the restriction element.

FIG. 3D is a perspective view of a fourth embodiment of the restriction element.

FIG. 3E is a cross-sectional view of the second embodiment of the restriction element.

FIG. 4A is a perspective view of a first embodiment of a restriction element attached to the longitudinal cylindrical fluid pipe using attaching means.

FIG. 4B is a perspective view of a second embodiment of a restriction element attached to the longitudinal cylindrical fluid pipe using attaching means.

FIG. 4C is a perspective view of a third embodiment of a restriction element attached to the longitudinal cylindrical fluid pipe using attaching means.

FIG. 5A is a cross-sectional view of a gas supplying device comprising one compartment.

FIG. 5B is a cross-sectional view of a gas supplying device comprising two compartments.

FIG. 5C is a cross-sectional view of a gas supplying device comprising four compartments.

FIG. 6 is a perspective view of a gas supplying device.

In the drawings, like reference numerals have been used to indicate like parts, elements or features unless otherwise explicitly stated or implicitly understood from the context. Detailed description of the invention

For reasons of convenience, the description below considers that the terms “upwards” or “upstream” correspond to an axial longitudinal direction (z) that is generally parallel to the central axis of the longitudinal cylindrical fluid pipe 200 and that extends from the first end towards the second end corresponding to the main direction of the fluid therein, whereas the terms “downwards” or “downstream” correspond to the opposite direction.

FIGS. 1 and 2 illustrate a first exemplary embodiment of system 1.

In FIG. 1 the system 1 comprises a longitudinal cylindrical fluid pipe 200 and a gas supplying device 100 integrated as a pipe segment of the fluid pipe 200. The gas supplying device 100 has a chamber 101. Gas is supplied into the chamber 101 via gas inlet 106 being connected to a gas cylinder or the like (not shown). The drawing illustrates how the radial extension of the gas supplying device 100 can be larger than the radial extension of the fluid pipe 200 and how the gas inlet 106 can be arranged at the radial extension 112 of the gas supplying device 100 since the radial extension 112 extends beyond the radial extension of the fluid pipe 200. The direction of the radial extension 112 can be seen as being perpendicular to the longitudinal direction of the fluid pipe 200. Here the chamber 101 is toroid shaped.

Even if only one gas inlet 106 is shown, there may be a plurality of gas inlets arranged for supplying gas into the chamber 101. The gas inlet(s) 106 may be positioned as needed, as shown in FIG.1, on an outside surface of the toroid chamber 101 or at any desirable position.

FIG. 2 shows the same first exemplary embodiment as FIG. 1. The open arrow within the fluid pipe 200 indicate a direction of the flow of fluid 300 upstream from the restriction element 400 while the small closed arrows indicate the flow of fluid 300 along the restriction element 400 before and/or after dissolving gas into the fluid 300.

A membrane 110 of the gas supplying device 100 is shown introducing gas bubbles 500 onto the inner surface of the gas supplying device 100. The gas supplying device 100 is arranged along the restriction element 400. Hence the flow of fluid 300 passes the cross-sectional area enclosed between the inner surface 104 of the gas supplying device 100/membrane 110 of the gas supplying device 100 and the outer surface of the restriction element 400. The membrane 110 can be seen as having an inner surface facing inside the chamber 101 and an outer surface facing the flow of fluid 300.

The inner surface 104 of the gas supplying device 100 with respect to the radial direction is shown being arranged flush with the inner surface 206 of the fluid pipe 200. The restriction element 400 is illustrated as a cylinder having a conical section at each end. However, this is only one of many possible embodiments for the restriction element 400. The person skilled in the art will understand that the restriction element 400 can be any element reducing the available cross section for the flow of fluid 300 to pass in the cross-sectional area 600 enclosed by the membrane 110, therefore increasing the velocity of the flow of fluid 300 and improving the dissolution of the gas bubbles 500 into the fluid.

Even if not illustrated in Fig. 2, gas is supplied into the chamber of the gas supplying device 100 via the at least one gas inlet (106) which is passed through a hole in the fluid pipe 200.

FIGS. 3A, 3B, 3C, 3D and 3E illustrate exemplary embodiments of the restriction element 400.

As shown in FIG 3A, the first embodiment of the restriction element 400 is a simple cylinder. This simple version of the restriction element 400 will increase the speed of flow of fluid 300 in the cross-sectional area 600 enclosed by the membrane 110. However, such a geometry will cause at least some turbulence and an increase in the Reynolds number of the flow of fluid which may in turn lead to gas bubbles 500 merging together, which would somewhat reduce the efficacy of the system.

As shown in FIG 3B, the second embodiment of the restriction element 400 is a combination of two cones joined at their base. Such a restriction element 400 will increase the speed of flow of fluid 300 in the cross-sectional area 600 enclosed by the membrane 110, and the flow will have a reduced Reynolds number compared to the first embodiment illustrated in FIG. 3A. However, such a geometry will not lead to a constant flow speed in the cross-sectional area 600 enclosed by the membrane 110.

FIGS. 3C and 3D illustrate a third and a fourth exemplary embodiments of the restriction element 400 which combine the advantages of the first and second embodiments. The restriction element 400 comprises a cylindrically shaped elongate body 402 and two end parts 401, 403, here cones of identical or different length. Such restriction elements 400 will increase the speed of flow of fluid 300 and keep it at a desired optimal speed in the cross-sectional area 600 enclosed by the membrane 110 and the flow will have a reduced Reynolds number compared to the first embodiment illustrated in FIG. 3 A.

In other words, the first end part and/or the second end part 401, 403 have an outer circumference tapering off towards an inlet or outlet, respectively, of the fluid pipe. The first end part 401 and/or the second end part 403 may taper off to a point. The end parts may have a maximum circumference equal to the circumference of the cylindrically shaped elongate body 402. The third embodiment of the restriction element 400 has an adjustable diameter/cross-section and is further illustrated in FIG. 3E, where the restriction element 400 further comprises an elastic outer housing 420 and is filled with a fluid 430.

In other embodiments the variation of the diameter/cross-section of may be adjusted in real time, for example by varying the pressure internally in the central element, for example with air or gas (an inlet/outlet to vary the pressure in the restriction element 400 may be present but was not illustrated in FIG.3C), by a calibrated internally spring load that adjusts the diameter according to the applied pressure, by a material that is flexible and to an extent compressible at applied pressure and regains shape at reduced pressure again, or by any other suitable means.

The third embodiment also comprises a central rod 410, which helps the restriction element 400 to keep its shape, while its volume may vary.

FIGS. 4 A, 4B and 4C illustrate three different ways that the restriction element 400 may be attached to the longitudinal cylindrical fluid pipe 200.

FIG. 4A shows a first embodiment where the restriction element 400 is attached to the longitudinal cylindrical fluid pipe 200 by three attaching means 450. Here the attaching means are illustrated as rods, but any suitable attaching means, such as fins, may be used. When a low Reynolds number is desirable, the attaching means should have a shape that creates as little turbulence as possible.

FIG. 4B shows a second embodiment where the restriction element 400 is attached to the longitudinal cylindrical fluid pipe 200 by two series of attaching means 450, placed near each ends of the cylindrically shaped elongate body 402. This embodiment further stabilizes the restriction element 400.

FIG. 4C shows a third embodiment where the restriction element 400 is attached to the longitudinal cylindrical fluid pipe 200 by two series of attaching means 450, placed at the end of a central rod 410 passing through the restriction element 400. This embodiment is preferable when the restriction element 400 has an adjustable diameter/cross-section.

FIGS 5 A, 5B and 5C are cross-sectional views of different configurations of the gas supplying device 100 taken in the plane perpendicular to the direction of flow of fluid 300 indicated in FIG. 2.

Fig 5 A shows a first example configuration of the gas supplying device 100 wherein the chamber 101 has one single compartment 120 having a gas pressure Pi. Further, the gas supplying device 100 has one gas inlet 106 allowing gas to be fed into the chamber 101. The gas pressure is equal to or higher than the counterpressure of the fluid passing the cross-sectional area 600, thereby preventing fluid from entering into the compartment 120.

Fig 5B shows a second example configuration of the gas supplying device 100 wherein the chamber comprises a first and a second compartment 120,121. The first compartment has a first gas inlet 106, while the second compartment has a second gas inlet 107. The first compartment 120 has a first gas pressure Pi which is controlled through the first gas inlet 106 and the second compartment 121 has a second gas pressure P2 controlled through the second gas inlet 107. The first gas pressure Pi and the second gas pressure P2 may be the same or different depending on the counterpressure of the fluid passing the first and second compartments 120,121 through the cross-sectional area 600 enclosed by the membrane 110. Both gas pressures Pi,P2 should be equal to or higher than the counterpressure of the fluid passing the cross-sectional area 600, thereby preventing fluid from entering into the compartments 120,121. If the fluid pipe is vertically arranged, both the first and second pressures Pi,P2 can be equal. If the fluid pipe is horizontally arranged, the first and the second gas pressures Pi,P2 may be different as the gravity force exerted on the fluid will result in a different counterpressure from the fluid passing the first and second compartment 120,121.

Fig 5C shows a third example configuration of the gas supplying device 100 wherein the chamber comprises a first, second, third, fourth, fifth and sixth compartment 120, 121,121 ’, 122, 122 ’,123. The first compartment 120 has a first gas pressure Pi, the second and third compartments 121,121 ’ have a second gas pressure P2, the fourth and fifth compartments 122,122’ have a third gas pressure P3 and the sixth compartment 123 has a fourth gas pressure P4. The first compartment 120 has a first gas inlet allowing control of the first gas pressure Pi, the second and third compartments have a second and third gas inlet 107,107’ respectively allowing control of the second gas pressure P2, the fourth and fifth compartments have a fourth and fifth gas inlet 108,108’ respectively allowing control of the third gas pressure P3, and the sixth compartment has a sixth gas inlet 109 allowing control of the fourth gas pressure P4.

Hence, the shown second gas pressure P2 is identical or near identical within the second and third chambers 121,121 ’, and the shown third gas pressure P3 is identical or near identical within the fourth and fifth chambers 122,122’, but may all be regulated to be different.

The first, second, third and fourth gas pressures Pi,P2,P3,P4 should be equal to or higher than the counterpressure of the fluid passing the cross-sectional area 600, thereby preventing fluid from entering into the compartments 120, 121, 121’, 122, 122’, 123. If the fluid pipe is vertically arranged, the compartments 120, 121, 121 ’, 122, 122’, 123 may have the same gas pressure Pi,P2,P3,P4. If the fluid pipe is horizontally arranged, the four gas pressures Pi,P2,P3,P4 may be different as the gravity force exerted on the fluid will result in a different counterpressure from the fluid passing the different compartments 120, 121,121’, 122, 122 ’,123, however being equal for the second and third chamber 121,121’, and equal for the fourth and fifth chamber 122,122’.

Further, all the compartments 120, 121, 121 ’, 122, 122’, 123 can be equipped with pressure sensors for controlling the pressure therewithin, even if not shown.

FIG. 6 is a perspective view of the gas supplying device 100 having a chamber 101 enclosed by the outer surface 102, the inner surface 104 and the two radial surfaces 112,114 being perpendicularly oriented to the inner and outer surfaces 102,104. Hence, the chamber 101 has a rectangular cross section taken in the plane indicated as B-B.

The chamber 101 comprises a first and a second gas inlet 106,107 for controlling the gas pressure within the chamber 101.

The chamber of the gas supplying device may comprise a pressure sensor (not shown) having a transmitter which sends signals to a controller. The controller detects any anomality in pressure within the chamber. Such anomalous pressures can be accompanied by pressure adjustments ensuring that the pressure is always equal to or higher than the counter pressure of the fluid within the fluid pipe. The fluid may consequently be prevented from entering the chamber.

Claims

1. A system (1) for supplying gas bubbles (500) into a fluid (300), wherein the system (1) comprises a longitudinal cylindrical fluid pipe (200) for the fluid (300) to pass through, a gas supplying device (100) comprising an outer surface (102), an inner surface (104), a chamber (101) between the outer and inner surfaces (102,104), a gas inlet (106) configured to feed gas into the chamber (101), and a membrane (110) arranged at the inner surface (104), the membrane (110) being configured to allow gas to pass from the chamber (101) into the flow of fluid (300) as gas bubbles (500), and a restriction element (400) arranged within the fluid pipe (200), the restriction element (400) defining an annular flow path between an outer surface of the restriction element (400) and the membrane (110).

2. The system (1) according to claim 1, wherein the restriction element (400) has a cylindrically shaped elongate body (402), wherein the elongate body (402) is at least as long as the membrane (110) and wherein the membrane (110) is adjacent to the elongate body (402) of the restriction element (400).

3. The system (1) according to claim 1 or 2, wherein the restriction element (400) is dimensioned to increase the Reynolds number of the flow of fluid (300) along the gas supplying device by between 10% and 1000%.

4. The system (1) according to any one of the preceding claims, wherein the restriction element (400) has an adjustable cross section.

5. The system (1) according to any one of the preceding claims, wherein the membrane (110) comprises a pore size of any one of 0.1 to 10 pm or 0.3 to 5 pm or 0.4 to 1 pm or 0.5 pm.

6. The system (1) according to any one of the preceding claims, wherein the membrane (110) comprises a material selected from at least one of a polymer-based material, a ceramic-based material, steel -based material and nickel -based material.

7. The system (1) according to any one of the preceding claims, wherein the gas supplying device (100) comprises at least two gas inlets (106,107,108,109) configured to allow gas into the chamber (101).

8. The system (1) according to claim 7, wherein the chamber (101) is divided into a plurality of compartments (120,121,122,123), wherein the at least two gas inlets (106,107,108,109) are configured to allow gas into each of the compartments (120,121,122,123).

9. The system (1) according to any one of the preceding claims, wherein the gas supplying device (100) comprises a pressure sensor configured to detect a pressure inside said chamber (101).

10. The system (1) according to claim 9, wherein the pressure sensor is configured to transmit the detected pressure to a control device, wherein the control device is configured to control the gas pressure within the chamber (101).

11. The system according to any one of the preceding claims, wherein the gas supplying device (100) is configured as a pipe segment of the longitudinal cylindrical fluid pipe (200).

12. The system according to claim 11, wherein the inner surface (104) of the gas supplying device (100) is arranged flush with the inner surface (202) of the longitudinal cylindrical fluid pipe (200).

13. A method for supplying gas bubbles (500) into a fluid (300) by use of a system (1) comprising a longitudinal cylindrical longitudinal cylindrical fluid pipe (200), a gas supplying device (100) comprising an outer surface (102), an inner surface (104), a chamber (101) between the outer and inner surfaces (102,104), a gas inlet (106) configured to allow gas communication into the chamber (101) and a membrane (110) arranged on the inner surface (104), the membrane (110) being configured to allow gas to pass from the chamber (101) into the fluid (300) as gas bubbles (500), and a restriction element (400), defining an annular flow path between an outer surface of the restriction element (400) and the membrane (110) arranged at the inner surface (104) of the gas supplying device; wherein the method involves the following steps: a) introducing a gas to the gas inlet (106) of the gas supplying device (100), b) introducing a flow of fluid (300) into the longitudinal cylindrical fluid pipe (200), c) adjusting the gas pressure within the chamber (101) generating an overpressure relative to a pressure within the flow of fluid (300), d) routing the flow of fluid (300) around the restriction element (400) increasing the speed of the flow of the fluid (300) e) routing the flow of fluid (300) through a cross-section (600) of the longitudinal cylindrical fluid pipe (200) enclosed by the membrane (HO), and e) allowing the gas entering from the gas inlet (106) to pass from the chamber (101) via the membrane (110) into the fluid (300) as gas bubbles (500).

14. The method according to claim 13, wherein the gas is oxygen and the fluid is a water-based liquid such as sea water, brackish water, fresh water or wastewater.

15. The method according to claim 13, wherein the gas is CO2 and the fluid is water-based liquid, such as wastewater, potable water, process water or tunnel water.

16. The method according to claim 13, wherein the gas is an inert gas such as nitrogen or argon and the fluid is a water-based liquid such as sea water, wastewater, potable water or fresh water.