AU2002100039A4 - Method and apparatus for gas dispersion - Google Patents

Description

AUSTRALIA

Patents Act 1990 COMPLETE SPECIFICATION INNOVATION PATENT Applicants: TONY FARRUGIA DARYL ANDERSON Invention Title: METHOD AND APPARATUS FOR GAS DISPERSION The following statement is a full description of this invention, including the best method of performing it known to

US:

METHOD AND APPARATUS FOR GAS DISPERSION Field of the Invention The present invention relates to a method and apparatus for the dispersion of a gas in a liquid. The process and apparatus can be applied to enhance the separation by gas of a substance from the liquid in a flotation step and will primarily be described with reference to this context.

Background Art Known methods of mechanically introducing gas into a flotation cell involve rotating an impeller to induce a suction of gas, via a passage in the impeller shaft or via a sleeve surrounding the shaft, directly into a fluid in the cell, the fluid usually containing a suspended or particulate substance. Gas induced by suction is broken up into smaller bubbles in the turbulent and high shear fluid regime near the impeller. The intimate mixing of bubbles, particulates and fluid in the cell can give a highly efficient gas/solid contact mixing through the bubble lamellar film so that improved recovery of a desired species occurs when the mixture is then allowed to separate by flotation.

In the flotation separation of finely dispersed substances such as suspended solids, oil and grease contained in industrial and municipal waste water and effluents, a fine bubble size distribution can increase the efficiency of gas/solid contact mixing through the bubble lamellar film. Very fine particles often can have difficulty contacting the surface of the coarse size bubbles routinely found in flotation processes.

It is to be understood that any prior art information referred to herein does not constitute an admission that the information forms a part of the common general knowledge in the art, in Australia or any other country.

3 Summary of the Invention In a first aspect the present invention provides an apparatus for the dispersion of gas in a liquid including a disc adapted for rotation by a shaft and having one or more protruding members at in use upper and lower opposing surfaces of the disc, with a lower number of members provided at the upper surface, wherein during rotation of the disc, a gas introduced at the upper surface is dispersed into the liquid at the lower surface.

Preferably the members are positioned in a radial direction and extend inwardly from a perimeter of the disc.

Preferably the members are bars rectangular in crosssection.

Preferably at least one hole is provided in the disc adjacent to one side of the or each member.

Preferably the apparatus further includes a stator positioned around the disc and in a close facing relationship with the or each protruding member at the upper surface. Preferably the stator also has a close facing relationship with the circumferential periphery of the disc.

In a second aspect the present invention provides an apparatus for the dispersion of gas in a liquid, the apparatus including: a disc adapted for rotation by a shaft; and a stator positioned around the disc and including an inlet for introducing a gas into a region between an upper surface of the disc and the stator.

In a third aspect the present invention provides an apparatus for the dispersion of gas in a liquid, the apparatus including: a disc adapted for rotation by a shaft; and (ii) means for introducing gas in a region adjacent to an upper surface of the disc and not via the shaft.

-4 Preferably in the third aspect the introduction means is part of a stator positioned around the disc.

Preferably, in addition to gas suction induced by rotation of the disc, gas is introduced by means of a blower or a compressor.

In a fourth aspect the present invention provides a method for the dispersion of gas in a liquid, the method including the steps of: rotating a disc in the liquid; and introducing a gas into a region adjacent to an upper surface of the disc, and not via a shaft (or any conduit attached thereto) used for supporting the disc.

Preferably the gas is introduced via an inlet in a stator that is positioned around the disc in use.

Preferably after introduction the gas flows from the region and is then dispersed into the liquid at a lower surface of the disc.

Preferably the gas flows from the upper surface of the disc to the lower surface thereof via a circumferential periphery of the disc and/or one or more holes provided in the disc.

Brief Description of the Drawings Notwithstanding any other forms which may fall within the scope of the present invention, preferred forms of the invention will now be described, by way of example only, with reference to the accompanying drawings in which: Figure 1 shows a cross-sectional side elevation of one embodiment of an apparatus for the dispersion of gas in a liquid in accordance with the invention.

Figure 2 shows a side elevation of an impeller disc used as part of an apparatus for the dispersion of gas in a liquid in accordance with the invention.

Figure 3 shows an underside elevation of the disc of Figure 2.

Figure 4 shows a plan view of the disc of Figure 2.

Figure 5 shows a graph depicting the gauge suction pressure of the apparatus as a function of the clearance distance between the inner surface of the stator and the upper surface of bars located on the impeller.

Modes for Carrying out the Invention Referring to the drawings, apparatus 10 is shown for the dispersion of gas in a liquid including an impeller disc 12 adapted for joining to and rotation by a shaft 14, the disc 12 having one or more protruding members in the form of radial bars 16. The bars extend at least partially inward from the circumferential periphery 28 of the disc 12 at in use upper and lower opposing surfaces, numbered and 22 respectively. In accordance with the invention, a lower number of bars 16 are provided at the upper surface compared with the lower surface 22. In rotation of the disc 12, a gas that is introduced at the upper surface is dispersed into the liquid at the lower surface 22.

The inventors have surprisingly shown that a greater number of bars 16 located at the lower surface 22 compared with the upper surface 20 can produce a finer bubble size distribution than that produced by conventional apparatus.

In the particular embodiment shown, the upper surface 20 of the disc 12 has three part-radial bars 16 attached thereto or formed therewith (typically at angles of 120 degrees to each other), while the lower surface 22 of the disc 12 has six part-radial bars 16 attached thereto (at angles of degrees to each other). This arrangement results in a particularly suitable gas dispersion regime for the chosen size (diameter) of disc.

In the embodiment shown, typically the bars 16 are rectangular in cross-section, although they may also be of another suitable shape, for example semicircular or triangular in cross-section, or blade shaped, for example.

In the drawings the bars extend inwardly from the 6circumferential periphery 28 although this is not mandatory. Typically the impeller disc 12 is 4mm in thickness and the bars 16 are 2mm in thickness at both the upper 20 and lower 22 opposing surfaces of the disc 12, although there is no requirement that the upper and lower bars be of equivalent thickness. Typically at least one hole 24 is provided in the disc 12 adjacent to one side of the or each bar 16 at the upper surface. In the embodiment shown the hole 24 is located adjacent the trailing edge of each bar 16 located on the upper surface 20 when the disc 12 is being rotated in use.

The impeller disc 12 also includes a centre hole for accommodating a shaft 14. The shaft 14 is affixed to the disc 12 by any one of a number of arrangements, such as a centre bolt 42 which is threadingly inserted into the base end 44 of the shaft 14 where the disc 12 is located between the bolt 42 and the base end 44 of the shaft 14.

In an alternative arrangement, as shown in Figures 2 to 4, four holes 46 are arranged around the centre of the impeller disc 12. A bolt can then be placed through each of these holes 46 and also inserted through aligned holes in a flange 48, located at the base end of the shaft 14, to enable fastening of the shaft to the disc 12 by nuts fitted to the four bolts. The latter fastening arrangement is suitable in situations where a large disc diameter is in service.

Typically the shaft 14 is located and is rotatable within a sleeve 50 and is mounted to the end of the sleeve by sealed bearing 52. The use of a sealed bearing eliminates the problems in prior art apparatus where a hollow sleeve or hollow shaft arrangement permits liquid to access the apparatus resulting in the ingress of solid material or deterioration of the apparatus.

The apparatus further includes a stator 26 seated at the lower end of the sleeve 50 such that the sealed bearing is now located between the sleeve 50 and the stator 26.

7 The stator 26 itself is a cylindrical structure open at a lower end and having sides comprising vertically extending slits or open channels where the interior walls of the slits are generally angled tangentially to the flow patterns of fluid which occur when the impeller disc is in use. The stator 26 is positioned around the disc 12 and is preferably in a close facing relationship with the or each bar 16 located at the upper surface 20 of the disc 12 and with the circumferential periphery 28 of the disc 12.

Typically gas is induced into a region 32 between the upper surface 20 of the disc 12 and the inner surface 34 of the stator 26. Typically for impeller diameters up to 200mm the distance between the inner surface 34 and the upper surface of each bar 16 is less than 2mm, although distances as small as 0.5mm are satisfactory for inducing sufficient quantities of gas into the region of the impeller. Clearance distances of less than 0.5mm can result in cavitation of the fluid and gas mixture and unsatisfactory gas suction. In the experiments to date, as illustrated in Figure 5 it has been shown that the gauge suction pressure is a near linear function of the clearance distance between the inner surface 34 of the stator and the upper surface of each bar 16. In the particular embodiment illustrated, using a 100mm disc, the optimal suction pressure is attained with a clearance distance of between and 2mm, which delivered a satisfactory gas flow into the apparatus at a liquid to gas volumetric ratio of approximately 1:1, although the preferred ratio will depend on the specific circumstances of the fluid and gas being used.

Typically the distance between the inner surface of the stator walls 56 and the circumferential periphery 28 of the disc is 5 millimetres, although this distance may be varied depending on the particular requirements of air flow for dispersion on the lower surface 22 of the disc 12 and the number of holes 24 located in the disc 12.

In some embodiments of the present apparatus, introduced gas can be passed into the region adjacent to the upper surface 20 of the disc 12 solely by way of the impeller shaft 14, or a sleeve surrounding the shaft 14, or an external tube attached to the shaft 14 as is the case in conventional apparatus. Gas introduced in such a manner can be dispersed in the liquid by the action of the disc 12 with a lower number of bars 16 at the upper surface compared with the lower surface 22 thereof. In accordance with the present invention, as shown in Figure 1, the stator 26 includes an inlet in the form of conduit 30 for introducing a gas G into the region 32 between the upper surface 20 of the disc 12 and the inner surface 34 of the stator 26. Gas may also be introduced through multiple conduits similar to conduit 30 in the stator 26. Where no stator is employed the gas may be delivered to region 32 by conduit(s) separate to those employed with or at the shaft.

In further embodiments of the invention, in addition to gas suction induced by rotation of the disc 12 and introduced via conduit 30, gas may also be introduced into the liquid from a blower or a compressor or other secondary gas source via the conduit 30 and/or by any of the listed known means. Thus the invention is not confined to an induced gas introduction method. In still further embodiments of the invention that use an inlet in the form of conduit 30 for introducing a gas into the region 32, the impeller used may not necessarily be of the type with a lower number of bars 16 at the upper surface 20 compared with the lower surface 22 thereof, but may have an identical number of bars on the opposing surfaces of the disc, or no bars on the upper surface at all.

When the disc 12 is rotated in use and a gas, typically air, is introduced into a region adjacent to an upper surface 20 of the disc 12, the rotating disc causes the gas to flow from the region 32 and be dispersed into the liquid at the lower surface 22 of the disc. It has 9been observed that the gas flows from the upper surface of the disc 12 to the lower surface 22 thereof via the circumferential periphery 28 of the disc and/or the hole or holes 24 provided in the disc 12. Any number of holes may be located in the disc to assist the transfer of induced gas to the lower surface 22 of the disc, and the holes can be in any location and are not restricted to the trailing edge of the bars 16. For example the holes can be in lines in an arc-like configuration extending between bars on the disc.

Surprisingly it has been found that the bubbles formed as a result of the induction of gas in the embodiment of the invention depicted in the drawings are typically less than 2 millimetres in diameter. The bubbles resulting from an induced gas flotation using conventional apparatus of this type without additional bars 16 on the lower surface of the disc impeller, and/or without a gas inlet passage in the stator) typically are only less than 6 millimetres in diameter.

The smaller bubble size distribution can produce several changes in a subsequent flotation operation which can give improved results. Firstly, the small size of the gas bubbles generated is not normally achievable by the shearing of induced gas by conventional apparatus. In some flotation treatment processes a similar reduction in bubble size can only be achieved by the addition of high reagent dosages of frothing agent, for example. In the present example, the reagent dosages required are considerably reduced. Furthermore, the flotation recovery of small particulates can be improved because of the increased likelihood of contact of these fine particles with a likesized bubble in a feed pulp.

The materials of construction of the apparatus can comprise any suitable materials which can be shaped, formed and fitted in the manner so described, such as metal (for example stainless steel) or hard plastics, to give a structurally sound apparatus that can withstand the rigours of an aqueous environment.

The embodiment of the invention shown can provide an improved efficiency of flotation recovery over the known techniques which typically are poor at recovering suspended solids, oil and grease, biomass or algae or a combination thereof, when finely dispersed in industrial or municipal waste water or effluents.

The invention has broad use in the separation of substances such as suspended solids, oil and grease and the like contained in industrial and municipal waste water, contaminated water and effluents in all manner of flotation separation apparatus. The invention also has use in fluid aeration applications where fine air bubble distribution is desirable in order to maximise oxygenation, for example.

Whilst the invention has been described with reference to a number of preferred embodiments it should be appreciated that the invention can be embodied in many other forms.

Claims (4)

1. An apparatus for the dispersion of gas in a liquid including a disc adapted for rotation by a shaft and having one or more protruding members at in use upper and lower opposing surfaces of the disc, with a lower number of members provided at the upper surface, wherein during rotation of the disc, a gas that is introduced at the upper surface is dispersed into the liquid at the lower surface.

2. An apparatus for the dispersion of gas in a liquid, the apparatus including: a disc adapted for rotation by a shaft; and a stator positioned around the disc and including an inlet for introducing a gas into a region between an upper surface of the disc and the stator.

3. An apparatus for the dispersion of gas in a liquid, the apparatus including: a disc adapted for rotation by a shaft; and (ii) means for introducing gas in a region adjacent to an upper surface of the disc and not via the shaft.

4. A method for the dispersion of gas in a liquid, the method including the steps of: rotating a disc in the liquid; and introducing a gas into a region adjacent to an upper surface of the disc, and not via a shaft (or any conduit attached thereto) used for supporting the disc. An apparatus for the dispersion of gas in a liquid, the apparatus being substantially as herein described with reference to the accompanying drawings. Dated this 18th day of January 2002 DARYL ANDERSON and ANTHONY FARRUGIA by their Patent Attorneys GRIFFITH HACK