EP1423182A1

EP1423182A1 - A method and a system for dissolving gas in a liquid

Info

Publication number: EP1423182A1
Application number: EP02756000A
Authority: EP
Inventors: Morten Emilsen; Roger Abrahamsen
Original assignee: Norsk Hydro ASA
Current assignee: Yara International ASA
Priority date: 2001-09-07
Filing date: 2002-08-30
Publication date: 2004-06-02
Anticipated expiration: 2022-08-30
Also published as: MY128943A; DK1423182T3; ATE293008T1; PT1423182E; NO20014375D0; EP1423182B1; NO20014375L; DE60203722T2; WO2003022413A1; NO315029B1; DE60203722D1; ES2240783T3

Abstract

The present invention concerns a method for dissolving gas in a liquid and a system for doing the same. More specifically, the present invention concerns a flowing liquid into which gas is injected. The liquid flows in a pipe (10, 110) that has an injector (12, 112) attached to it for the supply for gas to the flow of liquid. The present invention may be used for oxygen enrichment of water and may be used in connection with fish farming or farming of other aquatic creatures. In particular, the invention may be used in connection with land-based fish farming. In connection with the dissolution of gas in liquid, the mixture is caused to flow via one or more turbulence-initiating elements (15, 115), thus achieving an approximately saturated molecular solution of gas in theliquid.

Description

A Method and a System for Dissolving Gas in a Liquid

The present invention concerns a method for dissolving gas in a liquid and a system for doing the same. More specifically, the present invention concerns a flowing liquid into which gas is injected. The liquid flows in a pipe to the top of a mixer. An injector is arranged for the supply of gas to the flow of liquid in the pipe. The gas and liquid flow through the mixer, where the liquid and gas are mixed. The present invention may be used for oxygen enrichment of water and may be used in connection with fish farming or farming of other aquatic creatures. For example, the invention may be used in connection with land-based fish farming.

US 4,171,681 shows a system for fish farming in which water from a lake and a well may be mixed and pumped through a device for supply of oxygen to the water mixture. The oxygen-enriched water then proceeds to a land-based farming tank. At its base, the farming tank has an outlet for draining off water. This solution relates to a system for farming as such and does not describe how the oxygen is actually introduced into and dissolved in the water.

FR 2 750 889 describes an injection device for the introduction of gas into a liquid for use in connection with oxygenation of water for fish farming. The injection device comprises a perforated elastic diffusion membrane made of rubber. The gas flows in a spiral around the membrane so that it is brought into contact with a flow of liquid. The device is designed to allow small bubbles to be introduced into a flow of liquid without disturbing it greatly.

One disadvantage of this solution is that any foreign bodies in the flow of gas or deposits of organic material may block the opening in the diffusion membrane. This may entail problems introducing the correct quantity of gas into the liquid and consequent unstable operation. Another disadvantage is that gas bubbles are introduced directly in the area where, for example, fish are located. This may have a disturbing, stress-inducing effect on the fish with the result that growth and quality may be reduced and there may be less biomass per volume unit.

A general disadvantage of prior art solutions is that the effectiveness or utilisation of gas may be relatively low, in particular with large gas quantities (when large gas quantities are to be dissolved in water, for example a part flow). This may entail increased costs for gas and/or the need for larger units to be able to treat more water, which, in turn, results in high investment costs.

The above disadvantages can be avoided with the present invention. With the present invention, it is possible for the oxygen to be mixed well into the water. The present invention is also simple, robust and reliable in operation. The present invention allows large quantities of gas to be dissolved with good utilisation of the gas.

The present invention will be described in further detail in the following with examples and figures, where:

Figure 1 shows a system for oxygen enrichment of water which is added to a tank for fish farming in which a separator is placed outside the mixer.

Figure 2 shows a system for oxygen enrichment of water which is added to a tank for fish farming in which a separator is integrated in the base of the mixer.

Figure 3 shows an axial section through a mixer equivalent to that shown in Figure 2.

Figure 4 shows details of the mixer' s inlet in an enlarged scale.

Figure 1 shows a tank 1 for fish farming in which there is an oxygen-enriched quantity of water 2. At one side of the tank 1, there is a jet pipe 3 for the supply of oxygen-enriched water via one or more outlet openings 4, 4' , 4" , 4' " , 4" " , 4" " ' . In the example, the jet pipe 3 is designed so that it extends down into the tank 1 in a mainly vertical position. The equipment for oxygen enrichment of water comprises, among other things, a supply pipe 5 for water, which may consist of a mixture of fresh water and salt water which is introduced at an upstream end of the supply pipe 5 via the inlet pipes 6 and 7 respectively. Between the supply pipe 5 and the inlet pipes 6 and 7, there may be valves 8 and 9 to regulate the quantity of water which flows into the inlet pipes 6 and 7. The flow of water may also be shut off completely using the valves. In this example, the supply pipe 5 is arranged in a vertical position and, at its upper end, it becomes a horizontal part 10 which communicates with an inlet 14 on the top of a mixer 11. In the horizontal part 10 there is an injector 12 for injection of oxygen gas from an oxygen supply pipe 13. A regulation/shutoff valve 30 may be fitted in the pipe 13 to control the flow of oxygen to the injector 12.

Water which flows past the injector 12 will carry with it oxygen gas in the form of bubbles of varying size to the inlet 14 of the mixer. The mixer 11 may be described as a downward-facing pipe, which may preferably have a greater cross-sectional area than the supply pipe 5. The cross-sectional shape may be round or circular. However, other viable cross-sectional shapes may also be used. One or more turbulence-initiating elements 15, 15' , 15" , 15" ' , 15" " , 15" ' " are fitted inside the mixer. They cause the flow through the mixer 11 to be turbulent in such a way that optimal dissolution of oxygen gas in the water is achieved. The turbulence-initiating elements used in the example may be designed like a first disc 16 with one or more through holes, where the outer periphery of the disc 16 is in contact with the internal wall surface of the mixer 11. Figure 1 shows a disc 16 with one circular, centrally located hole 17. At a short distance from the hole, downstream (below) it, is a second disc 18 of a smaller size than the first disc. It is fitted in such a way that a gap is formed between the internal wall of the mixer 11 and the outer periphery of the disc. The discs 16 and 18 may be fitted in a fixed position in the mixer or they may be arranged so that the distance between them can vary (not shown).

The function of such a turbulence-initiating element 15 is that, when a mixture of liquid and gas flows vertically downwards through the mixer 11 and into such an element 15, the flow will change from being axial to being radial in the element. Depending on the choice of dimensions such as the flow cross-section in the mixer 11 , the size of the hole 17 in the first disc and the size of the second disc 18, as well as the distance between the discs, it will be possible to initiate a change in speed in the flowing medium over the element 15 in addition to the relatively extensive change in direction of flow which occurs here.

With the element 15, it has been shown to be possible to initiate an effective turbulent flow which contributes to a good mixture of gas and liquid. In the example shown, six turbulence-initiating elements are used. However, the number may, of course, be higher or lower, depending on what is necessary to meet the specifications of the system in question.

At the lower end of the mixer 11 is an outlet 19 connected to a pipe 20, which may run horizontally and also be connected to an inlet 21 in the base of a separator 22. The purpose of the separator 22 is to separate surplus gas/oxygen from the mixture that comes from the mixer 11. The mixture of water and oxygen gas which comes from the mixer 11 in this example may be supersaturated with oxygen, i.e. it contains more oxygen than can be dissolved in water. As shown in Figure 1 , the separator 22 will comprise both a liquid phase 23 and a gaseous phase 24. The gaseous phase, which will generally consist of oxygen, is taken out of the top of the separator 22 via a return pipe 25, which may be fitted with a shutoff/regulation valve 26. The return pipe 25 conducts surplus gas/return gas back to the injector 12 so that it will follow the flow of water into the mixer 11 again together with a larger or smaller quantity of oxygen from the pipe 13. Alternatively, the return gas may be conducted back to the mixer 11 via a return pipe connected to its own injector, i.e. without being connected to the pipe 13 and injector 12.

The liquid phase from the separator 22 is conducted from an outlet 28 on to the farming tank 1 via a pipe 27. The pipe 27 is fitted with a manual or automatic regulation valve 29 which maintains the desired operating pressure in the mixer and meters the correct quantity of water. The expression liquid phase is used here for practical reasons even though the liquid is saturated with oxygen gas. With the present invention, the gas dissolved in the liquid after the separator 22 will be dissolved as molecular oxygen without significant content of visible gas bubbles. Figure 2 shows a system for oxygen enrichment of water which is added to a tank 101 for fish farming in which a gas/liquid separator 122 is integrated in the base of a mixer 111. As in Figure 1 , at one side of the tank 101 , there is a jet pipe 103 for the supply of oxygen-enriched water via one or more outlet openings 104, 104' , 104" , 104' " , 104" " and 104

As for the system shown in Figure 1 , the system comprises a supply pipe 105 for water, which may consist of a mixture of fresh water and salt water which is introduced at an upstream end of the supply pipe 105 via the inlet pipes 106 and 107 respectively. Between the supply pipe 105 and the inlet pipes 106 and 107, there may be valves 108 and 109 to regulate the quantity of water which flows into the inlet pipes 106 and 107 or to shut off the flow of water completely. The supply pipe 105 is, as shown, arranged in a vertical position and, at its upper end, it becomes a horizontal part 110 which communicates with an inlet 114 on the top of the mixer 111. In the horizontal part 110 there is an injector 112 for injection of oxygen gas from an oxygen supply pipe 113. A regulation/shutoff valve 130 may be fitted in the pipe 113 to control the flow of oxygen to the injector 112.

Water which flows past the injector 112 will carry with it oxygen gas in the form of bubbles of varying size to the inlet 114 of the mixer. The mixer 111 with the integrated separator 122 is described in further detail below with reference to Figures 2 and 3.

Figure 3 shows an axial section through a mixer 111 with an integrated separator 122, as shown in Figure 2. It can be seen that the design of the separator' s outlet differs somewhat from that shown in Figure 2. One or more turbulence-initiating elements 115, 115' , 115" , 115' " , 115" " are fitted inside the mixer. They cause the flow through the mixer 111 to be turbulent in such a way that optimal dissolution of oxygen gas in the water is achieved. The turbulence-initiating elements used in the example may be designed like a first disc 116" with one or more through holes, where the outer periphery of the disc 116" is in contact with the internal wall surface of the mixer 111. The figure shows a disc 116" with one circular hole which, together with a return pipe 125, forms a centrally located annulus 117" . The pipe 125 may be coaxially mounted in relation to the hole in the disc 116" and will be described in further detail later. At a short distance from the annulus 117" , downstream from (below) it, is a second disc 118" of a smaller size than the first disc. It is fitted in such a way that a gap is formed between the internal wall of the mixer 111 and the outer periphery of the disc. The disc 118" , as shown in the figure, is designed with a central hole through which the pipe 125 passes, preferably forming a tight seal against the surface of the hole. The discs 116" and 118" may be fixed or variable, equivalent to that described for the mixer 11 shown in Figure 1.

With the element 115, it has been shown to be possible to initiate an effective turbulent flow which contributes to a good mixture of gas and liquid. In the example shown, five turbulence-initiating elements are used. However, the number may, of course, be higher or lower, depending on what is necessary to meet the specifications of the system in question.

The separator 122, which is located in the base of the mixer 111 , may have the same diameter as or a larger diameter than the rest of the mixer 111. The size is dimensioned on the basis of requirements so that the speed of the water downwards in a vertical direction is low enough to allow the gas bubbles to rise. The separated gas is returned to the top of the mixer 111 via the vertical return pipe 125 inside the mixer 111 and out through a dissolution element 131. At its lower end (inlet end), the return pipe 125 expediently has a collection element 126 to collect gas bubbles in towards the inlet. The collection element may expediently be created by mounting an annular flange 127 on the lower side of the lowest disc 118" " in the turbulence-initiating element 115' " ' . The dissolution element 131 is designed so that the water which flows into the mixer is distributed radially and out over the lowest disc in the element 115. The dissolution element 131 is designed with holes on the side (all around it) through which the gas escapes. In addition to distributing the water and return gas in the mixer 111 , the dissolution element 131 has the function that it prevents incoming water from running down through the inlet where the return gas comes up. Surplus gas/return gas flows up through the return pipe 125, among other things because the gas bubbles have buoyancy (bubble buoyancy) and also because of the difference in pressure (height) between the top and bottom of the pipe. The liquid which flows past the opening in the dissolution element will cause a certain ejector effect. The liquid phase from the separator 122 is conducted from an outlet 119 (which may be in the base or on the side, etc., of the separator, depending on the conditions) and on to the farming tank 101 via a pipe 120. The outlet may consist of a pipe 132 which is placed across the mixer 111 and runs through it. One end of the pipe 132 may be blanked off and the other connected to the farming tank 101. The pipe 132 has a downward-facing recess 133 in its wall, which allows water to flow into the pipe from the separator 122. The pipe 120 is fitted with a manual or automatic regulation valve 129 which maintains the desired operating pressure in the mixer and meters the correct quantity of water. The expression liquid phase is used here for practical reasons even though the liquid may be saturated with oxygen gas. With the present invention, the gas dissolved in the liquid after the separator 122 will be dissolved as molecular oxygen without significant content of visible gas bubbles.

Figure 4 shows details of the mixer' s inlet 114 in an enlarged scale. The turbulence- initiating element 115 has a first disc 116 and a second disc 118. The dissolution element 131 is mounted centrally in relation to the disc 118 and has an inlet opening 135 which communicates with the return pipe 125 (Figure 3). The dissolution element 131 may be designed with one or more outlet openings 134, 134' in a mainly radial direction and have a rotationally symmetrical form so that the flow of liquid is distributed evenly in a radial direction over the disc 118.

Although the examples deal with the dissolution of oxygen gas in a liquid, the system may be used in connection with different liquids/liquid mixtures to dissolve different gases/gas mixtures. In connection with fish farming, etc., gases other than oxygen, such as CO₂ or air, may be dissolved.

For instance, a mixture of C0₂ and 0₂ can be dissolved in water for the stunning of fish before slaughter. Another example related to fish farming is to dissolve a mixture of CO and an inert gas in water to be used in a bleeding off tank for slaughtered fish to avoid browning of the fish' s gills.

Further applications of the invention can be summarized in the following;

-pH-adjustment of hard drinking water and alkalic waste water, -Separation of oil residues in process water by dissolution of inert gas followed by expansion,

-Stripping of oxygen or other gases from process water by dissolution of nitrogen gas,

-Dissolution of oxygen into water in biological purification steps,

-Dissolution of CO₂ in mineral water, lemonade or the similar. High dissolution yield by high pressure,

-Dissolution of CO₂ in process fluids in industrial processes for pH control, such as pulp

& paper and effluents (wastewater).

Claims

1. A method for dissolving gas in a liquid in which the liquid flows in a pipe (10, 110) in which an injector (12, 112) is mounted for the supply of gas to the flow of liquid, characterised in that the mixture of gas and liquid is caused to flow via one or more turbulence- initiating elements (15, 115), thus achieving an approximately saturated molecular solution of gas in the liquid.

2. A method in accordance with claim 1 , characterised in that the liquid with dissolved gas and any surplus gas flows on into a separator (22, 122) for separation of the surplus gas from the saturated mixture.

3. A method in accordance with claim 2, characterised in that surplus gas is returned via a return pipe (25, 125) to the mixer' s inlet (14, 114) to be reinjected into the flow of liquid.

4. A method in accordance with claims 1 -2, in which the liquid is fresh water and/or salt water and the gas is oxygen, characterised in that the liquid with dissolved gas is conducted via a jet pipe (3, 103) into a farming tank (1 , 101 ) for farming fish or other aquatic creatures.

5. A system for dissolving gas in a liquid, comprising a pipe (10, 110) with a flowing liquid and an injector (12, 112) attached to the pipe (10, 110) for supply of gas to the flow of liquid, characterised in that the system also comprises a mixer (11, 111) which is downstream from the injector (12, 112) and comprises one or more turbulence-initiating elements (15,

115) and that an approximately saturated molecular solution of gas in the liquid is achieved.

6. A system in accordance with claim 5, characterised in that the turbulence-initiating element (15, 115) comprises a first disc (16, 116), which comprises one or more through holes (17, 117), and a second disc (18, 118) placed at a distance from the first disc and with a size smaller than the internal dimension of the mixer, and that the resulting flow over the element is mainly in a radial direction.

7. A system in accordance with claim 6, characterised in that the first disc (16, 116) is circular and has a central circular hole (17) or annulus (117), while the second disc (18, 118) is circular and is also slightly larger in size than the size of the hole (17) or annulus (117) in the first disc (16, 116).

8. A system in accordance with claims 5-7, characterised in that it comprises a separator (122) with an outlet (119) in the base of the mixer (111) and that surplus gas is conducted from the separator (122) up through a vertical return pipe (125) inside the mixer (111) and out through a dissolution element (131 ) placed at the mixer' s inlet (114).

9. A system in accordance with claim 8, characterised in that the dissolution element (131) is rotationally symmetrical and has one or more outlet openings (134) arranged in a mainly radial direction.

10. A system in accordance with claims 5-7, characterised in that it comprises a separator (22) arranged downstream from the mixer (11) with an outlet for removal of surplus gas, which is conducted via a return pipe (25) to the mixer' s inlet (14), and an outlet (28) for removal of liquid with dissolved gas.

11. A system in accordance with claims 5-10, in which the liquid is fresh water and/or salt water, characterised in that the downstream system is connected to a jet pipe (3, 103) arranged in a farming tank (1 , 101) for the supply of gas-enriched water for farming fish or other aquatic creatures.

12. A system in accordance with claim 11 , characterised in that the gas is oxygen.

13. A system in accordance with claim 11 , characterised in that the gas is C0₂.