DEVICE AND METHOD FOR GENERATING BUBBLES, USE OF THE DEVICE AND A FUEL CELL COMPRISING THE DEVICE

Abstract

A device for generating bubbles, comprising a porous material having at least one hydrophilic surface (1), arranged such that a liquid (7) in which the bubbles (6) are intended to be formed may contact the hydrophilic surface (1) and at least one hydrophobic surface (2), arranged such that a gas (5) used to generate the bubbles (6) may flow past the hydrophobic surface (2) before it flows past the hydrophilic surface (1). The device may be used for creating fine bubbles in numerous applications, such as wastewater treatment, plant cultivation, aquafarming, aeration systems, bioreactors, fermeters, oil extraction or fuel cells.

Claims

1. A device for generating bubbles, comprising: a porous material having at least one hydrophilic surface, arranged such that a liquid in which the bubbles are intended to be formed may contact the hydrophilic surface; and at least one hydrophobic surface, arranged such that a gas used to generate the bubbles may flow past the hydrophobic surface before it flows past the hydrophilic surface.

2. The device according to claim 1, wherein the hydrophilic surface extends along a face of the porous material, with the pores of the material extending away from the hydrophilic surface.

3. The device according to claim 1, wherein the hydrophobic surface at least partially extends inside the pores of the porous material.

4. The device according to claim 1, wherein the hydrophobic surface is provided by a second hydrophobic material

5. The device according to claim 4, wherein the second hydrophobic material comprises a hydrophobic coating on at least part of the wall of the pores of the porous material.

6. The device according to claim 4, wherein the second hydrophobic material is porous,

7. The device according to claim 6, wherein tile second hydrophobic material is laminated to at least a portion of the porous material.

8. The device according to claim 6, wherein the second hydrophobic material is inserted at least partially into the pores of the porous material.

9. The device according to claim 6, wherein the second hydrophobic material comprises a membrane.

10. The device according to claim 9, wherein the membrane comprises PTFE, polypropylene or polyethylene.

11. The device according to claim 4, wherein the porous material has a higher gas permeability than the second hydrophobic material.

12. The device according to claim 11, wherein the second hydrophobic material has pores of a smaller diameter than those of the porous material.

13. The device according to claim 6, wherein the pores of the second hydrophobic material are tortuous.

14. The device according to claim 6, wherein the pores of the second hydrophobic material are randomly orientated.

15. The device according to claim 1, wherein the porous material has a uniform structure.

16. The device of claim 1, wherein the porous material comprises pores positioned sufficiently far away from each other that the bubbles formed from each pore do not interact on the surface of the device.

17. The device according to claim 1, wherein the porous material comprises a foil, optionally wherein the foil comprises stainless steel.

18. The device according to claim 1, wherein the hydrophilic surface comprises a coating, such as silicon dioxide, on the porous material.

19. A method of generating bubbles, comprising: providing a gas supply to the device of any preceding claim, such that the gas supply flows past the hydrophobic surface before it flows past the hydrophilic surface; and providing a liquid in contact with the hydrophilic surface of the device.

20. (canceled)

21. A fuel cell comprising the device of claim 1.

Description

[0051] One or more embodiments of the present invention will now be described, by way of example only, with reference to the following figures, in which:

[0052] FIG. 1 illustrates a cross sectional view of a device of the present invention, showing a single pore of the porous material;

[0053] FIG. 2 illustrates a cross sectional view of a device of the present invention including a second hydrophobic material inserted into a pore of the porous material;

[0054] FIG. 3 illustrates a cross sectional view of a device of the present invention including a hydrophobic coating on the inside of a pore of the porous material;

[0055] FIG. 4a illustrates an ESM image of a top view of a hydrophobic membrane for use in the present invention;

[0056] FIG. 4b illustrates an ESM image of a cross-sectional view of the hydrophobic membrane of FIG. 4a;

[0057] FIG. 5 illustrates a comparison of fuel cell performances under like conditions using i) backed foil according to the present invention, ii) unbacked foil and iii) MOTT porous sinter devices to oxidise a catholyte; and

[0058] FIG. 6 illustrates a comparison of bubble size within a fuel cell under like conditions using i) backed foil according to the present invention, ii) unbacked foil and iii) MOTT porous sinter devices.

[0059] FIG. 1 illustrates a single pore 3 of the porous material 1. The second hydrophobic material 2 is laminated to the porous material 1. The porous material 1 comprises a stainless steel foil while the second hydrophobic material 2 comprises a PTFE membrane. The stainless steel material of the porous material 1 is hydrophilic and so forms the hydrophilic surface of the present invention.

[0060] The pore 3 of the porous material 1 is 90 μm diameter, while the pores 4 of the second hydrophobic material 2 are 2 μm diameter. The second hydrophobic material 2 is laminated around its edges to the porous material 1 and the pressure applied during use by the gas flow 5 holds the second hydrophobic material 2 against the porous material 1. The second hydrophobic material 2 covers multiple pores 3 of the porous material 1, though only one is shown.

[0061] Gas flow 5 reaches the second hydrophobic material 2 and flows through the pores 4 therein. As the pores 4 are narrow, this restricts the flow of gas through the second hydrophobic material 2. The pores 4 of the second hydrophobic material 2 are illustrated as being uniform and straight, though this is not necessarily the case and is more for diagrammatic purposes. In actuality, it is likely that the pores of a PTFE membrane will be randomly orientated and tortuous.

[0062] The gas flow 5 then reaches the pore 3 in the porous material 1. This results in the formation of a bubble 6 on the surface of the porous material 1, in the liquid 7. The liquid 7 flows past the porous material 1, which acts to shear the bubble 6 from the surface of the porous material 1, thereby helping to reduce the size of the bubble 6 being created. Further, the is hydrophilicity of the porous material 1 helps to release the bubbles 6 from the surface more easily.

[0063] The hydrophobicity of the second hydrophobic material 2 prevents the liquid 7 from entering the pores 4 of the second hydrophobic material 2. It also acts to partially prevent the liquid 7 from entering the pore 3 of the porous material 1. If the liquid 7 enters the pores 3, 4, it may dry and/or crystallise, resulting in a deposit of the dried or insoluble components of the liquid 7 in the pores 3, 4. This can block the pores 3, 4, resulting in non-uniform bubble formation.

[0064] As the porous material 1 is uniform in structure, pores 3, such as that illustrated in FIG. 1, are evenly distributed throughout the portion. This therefore results in the formation of uniform small bubbles 6 in the liquid 7.

[0065] FIG. 2 illustrates a single pore 13 of the porous material 11, into which the second hydrophobic material 12 has been inserted. The porous material 11 comprises a stainless steel foil while the second hydrophobic material 12 comprises a PTFE membrane. The stainless steel material of the porous material 11 is hydrophilic and so forms the hydrophilic surface of the present invention.

[0066] The pore 13 of the porous material 11 is 90 μm diameter, while the pores 14 of the second hydrophobic material 12 are 2 μm diameter. The second hydrophobic material 12 is glued to the pore of the porous material 11.

[0067] Gas flow 15 reaches the pore 13 of the porous material 11 and must flow through the pores 14 of the second hydrophobic material 12 in order to pass through the pore 13. As the pores 14 are narrow, this restricts the flow of gas through the second hydrophobic material 12 and thereby also through the porous material 11. The pores 14 of the second hydrophobic material 12 are illustrated as being uniform and straight, though this is not necessarily the case and is more for diagrammatic purposes. In actuality, it is likely that the pores of a PTFE membrane will be randomly orientated and tortuous.

[0068] The second hydrophobic material 12 fills the entire volume of the pore 13 of the porous material 11. This makes it harder for the liquid 17 in which the bubbles 13 are to be formed to enter the pore 13, which improves durability as the liquid 17 cannot dry or crystallise and thereby block the pores 13 of the porous material 11. Further, this arrangement promotes the production of even smaller bubbles 16 by reducing the gas volume within the pore 13 of the porous material 11. It is this volume that is used as a compressed gas reservoir as the bubble 16 expands, and so reducing its volume will reduce the size of the bubbles 16 formed.

[0069] The gas flow 15 then forms a bubble 16 on the surface of the porous material 11, in the liquid 17. As in FIG. 1, the liquid 17 flows past the porous material 11, which acts to shear the bubble 16 from the surface of the porous material 11, resulting in smaller bubbles being created than would otherwise be formed. Further, the hydrophilicity of the porous material 11 acts to release the bubbles 16 from the surface more easily.

[0070] The hydrophobicity of the second hydrophobic material 12 prevents the liquid 17 from entering the pores 14 of the second hydrophobic material 12. In this embodiment, it therefore also prevents the liquid 17 from entering the pore 13 of the porous material 11. If the liquid 17 enters the pores 13, 14, it may dry and/or crystallise, resulting in a deposit of the dried or insoluble components of the liquid 17 in the pores 13, 14. This can block the pores 13, 14, resulting in non-uniform bubble formation.

[0071] As the porous material 11 is uniform in structure, pores 13, such as that illustrated in FIG. 2, are evenly distributed throughout the portion. This therefore results in the formation of uniform small bubbles 16 in the liquid 17.

[0072] FIG. 3 illustrates a single pore 23 of the porous material 21. The porous material 21 is laminated to the second hydrophobic material 22. The porous material 21 comprises a stainless steel foil while the second hydrophobic material 22 comprises a PTFE membrane. The stainless steel material of the porous material 21 is hydrophilic and so forms the hydrophilic surface of the present invention.

[0073] The pore 23 of the porous material 21 is 90 μm diameter, while the pores 24 of the second hydrophobic material 22 are 2 μm diameter. The second hydrophobic material 22 is laminated around its edges to the porous material 21 and the pressure applied during use by the gas flow holds the second hydrophobic material 22 against the porous material 21.

[0074] The functioning of the device of FIG. 3 is the same as that shown in FIG. 1. As with FIG. 1, the pores 24 of the second hydrophobic material 22 are illustrated as being uniform and straight, though this is not necessarily the case and is more for diagrammatic purposes. In actuality, it is likely that the pores of a PTFE membrane will be randomly orientated and tortuous.

[0075] The hydrophobicity of the second hydrophobic material 22 prevents the liquid from entering the pores 24 of the second hydrophobic material 22. It may also act to partially prevent the liquid from entering the pore 23 of the porous material 21. If the liquid enters the pores 23, 24, it may dry and/or crystallise, resulting in a deposit of the dried or insoluble components of the liquid in the pores 23, 24. This can block the pores 23, 24, resulting in non-uniform bubble formation.

[0076] Also shown in FIG. 3 is a hydrophobic coating 28 on the walls of the pore 23 of the porous material 21. This further acts to prevent the liquid in which the bubbles are formed from entering the pores 23 of the porous material 21. Despite the presence of the second hydrophobic material 22, the liquid may still be drawn into the pores 23 of the porous material 21 (which in this embodiment are hydrophilic) by capillary action and so the pores 23 of the porous material 21 could still become blocked with dried and/or insoluble components of the liquid. The application of a hydrophobic layer 28 to the inside of the pores 23 of the porous material 21 changes this capillary pressure to a negative one, thereby preventing the movement of liquid into the pores 23.

[0077] As the porous material 21 is uniform in structure, pores 23, such as that illustrated in FIG. 3, are evenly distributed throughout the portion. This therefore results in uniform small bubble formation in the liquid.

[0078] Example 1 is carried out in a fuel cell. The bubble formation devices tested were used to provide an oxidant to a catholyte that had been reduced at the cathode, in order to regenerate the catholyte. The device according to the present invention consisted of a semi permeable PTFE membrane (Zitex G-108, Saint-Gobain Performance Plastics Ltd, UK) (see Table 1 for general properties) and a 50 μm thick stainless steel foil, etched over a 150×40 mm area with around 42,000 90 μm holes.

[0079] The stainless steel foil is hydrophilic, while the PTFE membrane is hydrophobic. The PTFE membrane also has a lower gas permeability than the etched steel foil; due to the smaller pore size. Also tested under identical operating conditions was i) a microporous stainless steel foil with no membrane backing layer and ii) a sintered stainless steel bubble formation device (MOTT Grade 2, 150×40 mm panel).

[0080] FIGS. 4a and 4b show ESM images of a top view and a cross-sectional view respectively of the PTFE membrane used. The top view of FIG. 4a is shown at 50× magnification and the cross-sectional view of FIG. 4b is shown at 100× magnification. As demonstrated by these figures, the PTFE membrane comprises numerous; small, tortuous passages throughout, some of which are dead-ended. This means that gas will flow through the membrane non-uniformly, with some areas of the membrane having greater gas permeability than others. Further, the membrane will have a lower gas permeability than the foil layer,

TABLE-US-00001 TABLE 1 Table of Zitex G-108 properties Property Unit Zitex G-108 Functional Pore Size microns 3-4 Air Flow, 100 cc/1.0 in. 2/20 oz. seconds 4-5 (Gurley Densitometer Test) Bubble Point (Ethanol) psi 1.0 ± 0.2 Water Flow Rate Through 1 ft. GPM 40 ± 10 2 @ 13.5 psi Water Initiation Pressure psi   4 ± 0.5 Breaking Strength lbs./inch width (avg)   9.6 Elongation % 75 Pore Volume % 45 Thickness inches 0.008 ± 0.002

[0081] FIGS. 5 and 6 compare the results of these three tests in terms of regeneration current (i.e. a reflection of reaction rate) and bubble size. As shown in these figures, the addition of the membrane afforded significant benefits, such as faster reaction times and smaller bubble size. In addition, the membrane backed foil according to the present invention was suggested to outperform the sintered stainless steel of the prior art.

[0082] The backed foil of the present invention was also suggested to be more durable than the other arrangements. After several hours of operation, the visual performance of the unbacked foil was observed to deteriorate significantly (i.e. uneven gas distribution and the formation of large bubbles). Removal of the unbacked foil revealed many of the pores to have become blocked with dried and insoluble material. The sintered steel device was also observed to behave similarly. By contrast, the membrane backed foil showed a relatively small performance loss after approximately 50 hours cumulative operation.

[0083] The three arrangements were also operated at high gas:liquid ratios. Both the unbacked foil and sintered stainless steel devices were unable to generate stable foams beyond about 4:1. However, the backed foil was seen to support stable foams up to about 10:1.