IE45487B1

IE45487B1 - Aeration nozzle

Info

Publication number: IE45487B1
Application number: IE1870/77A
Authority: IE
Original assignee: Portals Water Treatment Ltd
Priority date: 1976-09-09
Filing date: 1977-09-09
Publication date: 1982-09-08
Also published as: ZA775259B; GB1545559A; AU506966B2; AU2857577A; IE45487L

Description

The present invention relates generally to aeration in a flotation process and Blare particularly to a method of and a nozzle device for introducing a jet of liquid supersaturated with gas into a raw liquid.

In a dissolved air flotation process for the separation of solids from a liquid : by Bubble attachment a bypass or recycle flow is saturated with air at a high pressure and then blended back with a main stream. ; i ί In this system therefore, air is dissolved in water at a pressure above atmospheric, for example by passing the water down a suitable packing contained within a pressure vessel, to which a supply of compressed air is connected. The resultant solution of air in water is led to a manifold carrying orifices and submerged in the solid/liquid suspension to be treated. This may for example comprise a flocculated alumini urn hydroxide.

Ideally, air in tjhe recycled stream should remain in solution until the air has mixed completely with the raw water. This is not possible unless the entire system is pressurised but it is advantageous to delay the nucleation of air bubbles until just before mixing, thereby inhibiting growth of large bubbles by coalescence.

Large bubbles are unproductive because their high rise rate and consequent high surface shear prevent adhesion of floe particles and also disrupt existing floe/ bubble agglomerates. Thus, not only are large.bubbles wasteful of air but positively harmful to the process.

In the above-mentioned arrangement, the orifices on the manifold maintain the necessary back pressure through the recycle system to ensure solution of the air -24 Β <1 b ‘7 ..-,,, aSSS-ϊ . ............- ,™. - ............ - .into the water In the pressure vessel. It is unwise to use a pressure reducing valve in the delivery line because air will come out of solution prematurely and coalesce. The orifice must therefore be fixed at a point where the recycle or bypass stream mixes back into the main flow. Air comes out of solution therefore only on the downstream side of the orifice.

In many applications on the suspension of sdlids is delicate and may be broken down by local zones of high shear which will occur around a plain orifice. The above mentioned aluminium hydroxide and also iron hydroxide floes are vulnerable in this respect.

An object of the invention is to provide a method of and a nozzle device for introducing a jet of liquid supersaturated with gas into a raw liquid, which will not produce unduly high velocities on discharge and will produce the necessary back pressure and furthermore will allow mixing of the super-saturated water of the recycle stream with a bulk flow during or shortly after nucleation of the microbubbles and minimise the formation of large bubbles.

The invention provides in one aspect a method of introducing a jet of liquid a super-saturated with a gas into/raw liquid comprising passing the supersaturated liquid through a shroud in the raw liquid, the supersaturated liquid passing through at least one orifice arranged within the shroud, which extends past the orifice, -° and admitting a restricted flow of raw liquid into the shroud through a vent downstream of said at least one orifice so as to form a film of raw liquid over the inside surface of the shroud downstream of the vent.

The invention provides in a further aspect a nozzle device for performing the method hereinbefore described, comprising a shroud having at least one orifice 25 arranged within the shroud which extends past the orifice, and having a vent downstream of the at least one orifice, the diameter of the shroud lying within the range 5 - 15 D and the length of the shroud, as herein defined, being from 20 - 100 D, where D is the orifice diameter or equivalent diameter of a cluster of orifices.

The term length of the shroud as herein used is defined as the distance from the -3orifice to the downstream end of the shroud and the term equivalent diameter of a cluster of orifices as herein used is defined as the diameter of the single orifice which would have the same open area as the cluster.

Thus, the film of raw liquid separates the jet of supersaturated liquid from 5 ; the internal surface of the shroud and so minimises bubble nucleation. The emerging jet therefore contains a microbubble population of a uniform size and the formation of large bubbles is inhibited.

Preferably, the orifice or orifices are formed in a plate arranged in the shroud The vent may be formed by a plurality of circumferentially spaced apertures, e.g circular apertures or slots, or may be formed by the gap between two axially-spaced portions of the shroud.

Preferably, the vent is a ring at a distance of 10 - 30 0 from the orifice plate and has a total open area corresponding to 5 - 50% of the cross section of the shroud. Orifice sizes may lie within the range of 0.1 - 8 nan. A typical working pressure may lie within the range 3 - 7 bar (gauge).

The invention further provides liquid clarification apparatus having at least one of the above-described nozzle devices.

The invention will be further described with reference to three embodiments shown diagransnatically in section in the accompanying drawing.

In Fig. 1 a nozzle device comprises a shroud 1 having a partition 2 with a central orifice 3 of diameter D. The diameter of the shroud is 5 - 15 D. Water supersaturated with air flows through the orifice 3 in the direction of arrow 4.

The shroud 1 extends downstream of the orifice 3 and terminates in an outlet 5 at a distance of 20 - 100 D from the orifice 3.

The discharge from the orifice 3 expands as a 14°cone. The surrounding shroud enables this expansion to be controlled, and at the same time it prevents entrainment of the raw water into the high velocity base of the jet.

A vent comprising a ring of circular aperture 6 is formed in the shroud 1 downstream of the orifice 3. The total area of the apertures 6 is 5-50% of the cross.4. 4348'? sectional area of the shroud 1 and the apertures 6 are a distance of 10 - 30 D away from the orifice 3.

The flow of supersaturated water through the orifice 3 towards the end 5 draws in through the apertures 6 a film of raw water over the internal surface of the shroud. This film of raw water separates the jet of supersaturated water from the internal surface 7, and so minimises bubble nucleation.

Instead of circular apertures 6, the vent may comprise a plurality of circumferentially-extending slots 6' as shown in Fig. 2.

Alternatively, as shown in Fig. 3, the shroud may comprise two axially-spaced 10 portions Τ, I, and the vent is formed by a ring orifice 8 between the two portions having the same areas as the apertures 6 or the slots 6'. The two portions T, 1, are connected by means of external spacer ribs 9.

Claims

1. A method of introducing a jet of liquid supersaturated with a gas into a raw liquid comprising passing the supersaturated liquid through a shroud in the raw liquid, the supersaturated liquid passing through at least one orifice arranged within the shroud, which extends past the orifice, and admitting a restricted flow of raw 5 liquid into the shroud through a vent downstream of said at least one orifice so as to form a film of raw liquid over the inside surface of the shroud downstream of tha vent.

2. A nozzle device for performing the method claimed in claim 1, comprising a shroud having at least one orifice arranged within the shroud, which extends past 13 the orifice, and having a vent downstream of the at least one orifice, the diameter of the shroud lying within the range 5 - 15 0 and the length of the shroud, as herein defined, being from 20 - 100 D, where D is the orifice diameter or equivalent diameter of a cluster of orifices.

3. A nozzle device as claimed in claim 2, wherein the vent comprises a plurality 15 of circumferentially spaced apertures. ’ I

4. A nozzle device as claimed in claim 3, wherein the apertures are circumferentially extending slots.

5. A nozzle device as claimed in claim 2, wherein the shroud comprises two axiallyspaced portions and the vent is formed by the gap between the two portions. 20

6. A nozzle device as claimed in any one of claims 2 to 5, wherein the orifice or orifices are formed in a plate arranged in the shroud.

7. A nozzle device as claimed in claims 6 and 2, wherein the vent is a ring at a distance of 10-30 D from the orifice plate and has a total open area corresponding to 5 - 50% of the cross section of the shroud. 25

8. A nozzle device substantially as herein described with reference to and as shown in Fig. 1, Fig. 2 and Fig. 3 of the accompanying drawing. -5ύι β 1 b '7

9. Liquid clarification apparatus having at least one nozzle device as claimed in any one of claims 2-8.

10. A method of introducing a jet of liquid supersaturated with a gas into a raw liquid substantially as herein described.