GB2279648A - Flexible and hollow members for dispersing bubbles of gas into a liquid medium - Google Patents

Abstract

A flexible restraining member or cable 26 is separably connected to the exterior of a tube 21 and extends along the entire length of the tube 21, which consists of a membrane 4 having perforations distributed along it and over at least part of its circumference. Compressed air supplied to the tube 21 bubbles out into a liquid medium through the perforations, which have been formed by non-ablative piercing and which provide a passage cross-section decreasing towards the outside 4a of the membrane 4. The membrane 4 is supported by a cylindrical roller during piercing, so as to accurately control the penetration of the membrane 4 by needles carried by another roller. <IMAGE>

Description

Manufacture of flexible and hollow members This invention relates to flexible and/or hollow members having perforations for dispersing bubbles of gas into a liquid medium.

There is a growing demand within the water/sewage industry for effective means of addressing the problems of pollution. In particular, there is need for equipment that will more efficiently administer oxygen into aqueous solution, whether this be found in the treatment of active sewage, trade (e.g.

agricultural) effluent, or stagnant water-courses.

A basic means of introducing gas into a fluid is via a perforated membrane or porous rigid diffuser. However, the available systems fail to address the improvements in efficiency that are available through bubble control technology.

The present invention is based on the following theoretical analysis by the inventor.

The volume of a gas bubble is proportional to the cube of its diameter, whereas its surface area is only proportional to the square of its diameter. The surface area available per unit volume (of gas) is therefore inversely proportional to the bubble diameter, so that if the bubble size can be halved, the relative surface area may be doubled, thus dramatically increasing the potential rate of dissolution of the gas into the surrounding liquid. If the bubble size can be sufficiently reduced and there is sufficient time of contact between the bubble and the liquid, then the soluble gas may be totally dissolved, providing of course that the liquid never becomes fully saturated. There is a secondary benefit from the increased fluid drag of the gas bubble. This is also related to the size and surface area of the bubble, whereas the buoyancy force or upthrust of the bubble is proportional to its volume. Thus, reductions in bubble size will dramatically slow down the passage (rise) of the bubble through the medium.

Maximum efficiency may therefore be achieved by improving control of the bubbles in the following ways : (a) minimising the size of the apertures or pores through which the gas passes; (b) providing a minimum separation of the pores sufficient to prevent coalescence of the bubbles; (c) regulating the gas flow to control the bubble dimensions; (d) maximising the depth at which the bubbles are introduced into the medium; (e) optimising the bubble dispersion relative to the body of liquid.

The fundamental mechanism of bubble formation is controlled by surface tension, which is a force per unit width of surface effect. As the gas escapes through a given pore 1, a bubble 2 will be generated at the emission surface 3 (i.e. membrane/liquid interface), growing like a button mushroom, until such time as the buoyancy force of the bubble 2 is sufficient to cause it to break away and rise, as illustrated in Figure 1 of the accompanying drawings. The exit dimensions of the pore 1 are therefore critical to the size of bubble. This may be further demonstrated by theoretical analysis as follows: Assume pore diameter - d and bubble diameter = then buoyancy force = (413) < (D/2)3 p where p is the relative density of the gas, and restraining force = zit where t is the surface force/unit length.

But in the limit as the bubble breaks away these two forces must be equivalent; hence D = (6dot/ )3 Since t and p are constants for a given situation (i.e. types of fluids, temperature, and pressure), then it may be inferred, for example, that if it were possible to reduce the pore diameter from say 500 fm to say 5 cm, then the bubble size could be reduced by a factor of about 5, with a corresponding increase in its surface to volume ratio.

It may also be reasoned that instead of the pores being disposed vertically (as assumed in the above analysis), there will be a benefit in terms of bubble size from the pores being arranged horizontally, since the restraining forces at the root of the bubble are less effective (due to acting in shear rather than in tension) and are possibly halved, thus reducing the bubble size by about 20%. In practice, however, this benefit may be somewhat offset by the increased risk of bubble collision (and coalescence) in this mode. For similar reasons, it may be argued that the pore exits should be smooth and circular, so that the perimeter of the hole (and hence the surface tension at the neck or stem of the bubble) is minimised.

This invention recognises the foregoing philosphy and provides a practical means of generating and dispersing very fine bubbles of gas (e.g. air) into a liquid (e.g. aqueous) medium.

In a first aspect, the invention provides a method of manufacturing a membrane with very fine pores for efficient dissolution of the gas.

In a second aspect, the invention provides a preferred arrangement of the membrane in an elongate (tubular) form suitable for the effective dispersion of bubbles throughout the medium.

In a third aspect, the invention provides a preferred means of locating and holding the elongate membrane in an optimum position within the medium.

In a fourth aspect, the invention provides a preferred means for providing a controlled supply of gas to the said membrane.

The invention will be described further, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a fragmentary cross-section through a perforated tubular membrane containing gas under pressure and being surrounded by a liquid medium; Figure 2 diagramatically shows a device for perforating a membrane; Figure 3 diagramatically shows the pattern of perforations produced by the device; Figure 4 shows a roller adjustment mechanism for use in the device; Figure 5 shows a first embodiment of a tubular membrane; Figure 6 is a second embodiment of a tubular membrane; Figure 7 shows tubular members mounted on a frame; and Figure 8 schematically shows apparatus for delivering gas toan array of tubular members.

The preferred means of generating a membrane 4 with fine pores 1 is by non-ablative piercing with sharp fine needles 6.

The principle control variables are the sharpness/fineness of the needles 6 and the depth of penetration. The first variable is largely predetermined by practical considerations of needle strength, durability, and manufacturing limitations, so the depth of penetration is of greatest concern here. This may be best controlled by using a device such as that illustrated in Figure 2 comprising two (cylindrical) rollers 7,8. One roller 7 has an array of sharp needles 6 whose tips (towards which the needles taper) are positioned at a uniform height relative to the roller surface (or more particularly at a uniform radial distance relative to the axis of rotation of the roller), and dispersed around the roller 7 in a predetermined pattern and spacing. The pattern of the perforations 1 may be advantageously arranged slightly oblique to the roller axis 9 as indicated in Figure 3, to improve the bubble dispersion.

The spacing between adjacent needles 6 should be greater than the largest expected bubble diameter to avoid coalescence of the bubbles. The needles 6 may also be beneficially inclined in the direction of rotation at an acute angle to the radius of the roller (when viewed along the rotational axis 9 of the roller) as shown in Figure 2. The needle angle cm should be less than 250 and preferably be less than 200 and more preferably be within 5-15 of the radial position. This roller 7 is provided with a drive means (not shown). The other (non-driven) roller 8 is also cylindrical and of uniform radius, and is provided with a resilient outer layer 11, capable of being penetrated by the tips of the needles 6 without suffering significant damage or wear.

Preferred forms of resilient layer 11 are either a soft plastics or elastomeric tire (e.g. heat shrink material), or alfabric reinforced tube or wrapping. Alternatively a bristle roller may be used. The needles 6 will preferably be inserted into the membrane 4 in the direction of the intended gas flow (i.e. from the inside outwards), so that the membrane 4 is just penetrated by the tip of the needle 6, thereby giving the finest of holes on the (outer) emission surface 4a of the membrane.

It is important to control to depth of emergence of the tapering meedles 6 through the membrane 4, rather than the depth of penetration from the (needle) entry surface, since this will obviate the effects of any deliberate or incidental variations in material thickness on the pore size. This control is provided by adjusting the depth of penetration of the needle points into the resilient outer layer 11 of the non-driven roller 8, using arrangement shown in Figure 4. On each side of the roller 7, a trunnion 12 is mounted in a bearing block 13 which is vertically slidable in a support 14. The block 13 rests on a compression spring 16, and its vertical position is adjustable by means of a screw-threaded tappet 17. A means of interrupiting the coercing operation (for example at the ends of the membrane) is also provided in the form of toggle adjusters, also shown in Figure 4.

Each adjuster comprises a lever 18 which is pivotally mounted on the support 14 and is linked to a member 19 carrying the adjustable tappet 17.

The material of the membrane 4 should be sufficient resilient/elastic to allow through-penetration of the needles 6, preferably without tearing, and to ensure virtually instantaneous closing of the pore 1 when the needles 6 is withdrawn. (Many plastic materials will only recover slowly and/or partially.) Suitably resilient materials are, for example, natural rubber, synthetic elastomers (such as Du Pont's HYTREL - Trade Mark), and plasticised PVC.

The material of the membrane 4 must also be sufficiently strong and tough to withstand the fluid pressure and other forces exerted upon it without undue strain or yielding. This will constrain the type and thickness of material that can be used.

For large diffusers especially, this constraint may be in conflict with the requirements already expressed from the piercing operation point of view. These conflicting demands may be satisfied by introducing reinforcing means into the membrane 4, for example an open-weave fabric reinforcement of metal or natural/synthetic fibres. The amount of reinforcing material should be greater in the hoop direction than the axial direction, since for a tubular membrane the stresses in the hoop direction are twice as large as the axial stresses. Alternatively, the reinforcement may be external to the membrane, for example in the form of a woven or braided jacket or sleeve (not shown).

The membrane 4 will preferably be formed into a tubular member 21 of substantially constant diameter (e.g. 25 mm), for example by welding as shown in Figure 5. The welded seam 22 or seams will preferably be at the bottom of the tube and away from the perforated regions of the membrane. Provision may also be made, in the formation of the tubular member, for the attachment of positioning/securing means, e.g. in the form of punched holes and eyelets 23, or by provision of a co-extensive enclosure 24 (Figure 6) to accommodate securing means. The securing means may comprise a support/tension member 26 such as metal or composite (e.g. GRP) beam, or a cable of metal or synthetic material that can be coiled to assist in transportation and handling of the member in long lengths. The tubular member 21 is preferably separable from the securing means, to allow for the repair/replacement of components. The co-extensive enclosure 24 may therefore only loosely accommodate the securing means to facilitate removal, or may be formed by a reversible fastening, such as a Zip or Velcro (Trade Marks) fastening or lacing. The enclosure 24 has drainage holes 27.

For ease of installation it may be desirable to connect several lengths of the securing means (26) to an orthogonal support frame or structure 28 (Figure 7) so that a multiplicity of tubular members 21 can be fixed in a parallel array, providing a dispersed distribution means over the area of a tank or water-course. The tubular members 21 are preferably arranged to lie at least partially in a direction transverse to any stream flow, to maximise the dispersion of gas (air) into the body of liquid (water). An unperforated distribution tube 29 is connected to one end of each tubular member 21.

Where the body of liquid is too large for a holding frame 28 of the type illustrated in Figure 7 to be practical, the securing means may be directly anchored to the bottom, sides, or ends of the enclosure, e.g. to the banks or walls. This will preferably be effected via a system of arms, weights, or pulleys, which can be removably positioned from the surface, without the need for draining down the body of fluid or under-water working. The system will be designed to resist the upward thrust of buoyancy forces on the tubular member 21 as well as any fluid drag from the flow stream, by tensioning of the securing means (but not the tubular membrane).

In selecting the best arrangement of membrane for economic and efficient dispersion of gas, consideration must be given not only to the holding down means, but also the internal fluid flow.

The cross-sectional size, e.g. diameter, of the tubular member 21 must be sufficiently large to accommodate the required flow rate of gas without undue fall-off in pressure along the length, having regard to the loss of gas through the pores 1 of the membrane 4. If the pressure gradient is too large then the flux of bubbles along the length of the emitting surface 4a will be very non-uniform. Economic and mechanical design considerations, however, encourage the use of as small a tubular member as possible, since the hoop stresses increase in propotion to the tube diameter. A tubular member of 50 mm diameter for example, will need to be twice as strong as a tubular member of 25 mm diameter, if other factors such as fluid pressure remain unchanged.

To minimise the loss in pressure along the tubular member, the internal surface should be as smooth as possible to reduce fluid drag effects, and the fluid velocity within the tubular member shall preferably be less than 100 m/s. and more preferably be less than 50 m/s. Also the pores of the membrane may be beneficially inclined towards the direction of flow, i.e. so that the change in direction of the gas entering the pores is less than 900.

To avoid excessive pressure fluctuations that might damage the membrane and to provide good control of the fluid flow through the membrane, it is recommended that a pulse-free, low pressure compressor of fixed displacement type, such as a rotary vane compressor, be used. If the size of the compressor is correctly selected to suit the optimum flow of the membrane array, then this type of compressor may be used without the need for pressure regulators or flow or control valves, since the flow characteristic is typically less sensitive to pressure than that of the membrane (Figure 9) and effectively the system becomes volume controlled rather than pressure regulated. Such supply means will also accommodate variations in hydraulic head (e.g.

due to tidal flow) without undue variations in fluid flow through the membrane.

A system for delivering gas (air) to an array of tubular membranes is illustrated schematically in Figure 8. Each tubular membrane 21 is connected to a manifold or distribution pipe 29 which may contain nozzles or flow restriction devices, to ensure that the gas flow is uniformly distributed to each of the members, and also to allow a higher pressure to be used in the delivery and distribution pipes, to reduce their sizing. These restrictors will also protect the membranes against an accidental pressure overload.

A compressor 31 is connected to the distribution pipe 29 by a delivery pipe 32 incorporating a safety valve 33 and a combined filter and moisture trap 34. The compressor 31 receives air through a throttle valve 36 and is driven by an electric motor 37 whose power supply 38 includes a control switch 39 responsive to the output of an oxygen sensor 41 in the liquid medium in a tank 42.

Control of the supply of gas (air) into the medium (water) may be achieved either by periodically interrupting the operation of the compressor, for example under the control of the oxygen sensor in the aqueous medium, or by modulating the gas flow.

This will preferably be effected by adjusting the speed of the compressor or throttling the inlet to the compressor.

Claims (12)

Claims:

1. A flexible elongate assembly for dispersing bubbles of gas into a liquid medium, the assembly comprising a tube consisting of a membrane having perforations distributed along the tube and over at least part of the circumference of the tube, and a flexible restraining member which is separably connected to the exterior of the tube and which extends along the entire length of the tube.

2. An assembly as claimed in claim 1, in which the flexible restraining member is connected to a longitudinal formation integrally formed on the exterior of the tube, by connecting means spaced along the tube.

3. An assembly as claimed in claim 1 or 2, in which, when gas passes through the perforations from the inside to the outside of the tube, the passage cross-section of each perforation decreases towards the outside.

4. A method of making a flexible elongate assembly according to claim 1, comprising: (a) perforating a membrane at positions distributed along the membrane and over at least part of its width; (b) forming the perforated membrane into a tube; and (c) separably connecting a flexible restraining member to the exterior of the tube, the restraining member extending along the entire length of the tube.

5. A method as claimed in claim 4, in which the membrane is perforated by non-ablative piercing with needles, and the extent to which the tips of the needles emerge from the membrane is controlled.

6. A method as claimed in claim 5, in which the needles are carried by a first rotary member having a first axis, and the membrane is supported by a second rotary member having a second axis, the first and second axes being mutually parallel, the tips of the needles lying on an imaginary cylindrical surface whcse axis is the first axis, the secondary rotary member having 2 cylindrical supporting surface whose axis is the second axis and which is penetrated by the needles.

7. A method as claimed in claim 6, in which the distance between the first and second axes is adjustable in order to adjust the extent to which the tips of the piercing needles emerge from the membrane and penetrate the supporting surface.

8. A method as claimed in claim 6 or 7, in which the needles are inclined in the direction of rotation of the first rotary member with respect to a radius.

9. A method as claimed in any of claims 5 to 8, in which the needles penetrate the membrane from the side of the membrane which is to constitute the inside of the tube.

10. A method as claimed in any of claims 4 to 9, in which the tube is formed by welding the membrane along a seam so as to provide an external longitudinal integral formation to which the restraining member is subsequently connected.

11. A flexible elongate assembly substantially as described with reference to, and as shown in, Figure 5 or Figure 6 of the accompanying drawings.

12. A method of making a flexible elongate assembly for dispersing bubbles of gas in a liquid medium, substantially as described with reference to Figures 1 to 6 of the accompanying drawings.