GB2079167A

GB2079167A - Gasifying a liquid

Info

Publication number: GB2079167A
Application number: GB8020876A
Authority: GB
Original assignee: Individual
Current assignee: Individual
Priority date: 1980-06-26
Filing date: 1980-06-26
Publication date: 1982-01-20
Also published as: GB2079167B

Abstract

Liquid to be gasified is passed, in the presence of the gas, down a surface of a sufficient length in the direction of flow to cause the flowing liquid to achieve a terminal i.e. saturated, condition in which there is no further change in its velocity or in its thickness as measured normal to the surface; the surface being such that turbulence is induced in the flowing liquid at least when it achieves the terminal condition. Liquid gasified in one run down the surface may be further gasified by a second or subsequent run; and the total time the feedstock is maintained in its terminal condition will depend on the amount of gasification to be effected; the maximum time being that at which the liquid is incapable of taking up more gas. The invention is particularly useful for aerating sewage sludge. <IMAGE>

Description

SPECIFICATION Method and apparatus for gasifying a liquid The present invention relates to method and apparatus for gasifying a liquid.

Installations are known for gasifying a liquid, in particular, for aeration of, for instance, a sewage sludge.

It is an object of the present invention to provide an efficacious method and apparatus for gasifying a liquid.

In an embodiment of the present invention, the liquid feedstock to be gasified is passed, in the presence of the gas, down a surface of a sufficient length in the direction of flow to cause the flowing liquid to achieve a terminal condition in which there is no further change in its velocity or in its thickness as measured normal to the surface; the surface being such that turbulence is induced in the flowing liquid at least when it achieves the terminal condition. The time the flowing liquid is maintained in its terminal condition will depend on the amount of gasification to be effected; the maximum time being that at which the liquid is incapable of taking up more gas.

Conveniently, the surface may be constituted by a tube; the feedstock being introduced into the top of the tube to pass down the interior surface thereof. The gas is passed into the tube at or adjacent the top of the tube, and it may be introduced under pressure.

The gasified feedstock may be collected in a vessel receiving the feedstock from the bottom end of the tube.

Such apparatus may readily be used for aerating a sewage sludge. Thus, the vessel is used to hold the sludge to be aerated. Sludge is drawn off from the vessel, fed into the tube along with air or oxygen and then passed back into the vessel with the bottom end of the tube being immersed in the body of the sludge in the vessel.

The present invention will now be described, by way of example, with reference to the accompanying drawings, in which:~ Figure 1 is a diagrammatic sketch of an apparatus according to the present invention; and Figure 2 is a graph showing a general characteristic by which can be calculated the percent of air which will be entrained in water in apparatus as illustrated in Figure 1.

Referring to the drawings, the apparatus of Figure 1 comprises a tube 1 of circular crosssection. In this instance the tube was 51 mm internal diameter and was made of perspex. The bottom end of the tube was immersed in water 3 to lie near the bottom wall 5' of a vessel 5 used to contain the water. The vessel 5 itself, in this instance, was constituted by a perspex tube of 350 mm internal diameter and some 750 mm high.

Water was passed down the internal surface of the tube 1 and formed a standing head 7 therein of aerated water; aerated water accordingly passing from the bottom end of the tube into the body of water in the vessel 5. In practice, the water to be aerated is withdrawn from the vessel 5 by a pump and delivered to the top end of the tube. In this instance, the pumped water was introduced into the tube via a closed vortex chamber although any convenient means may be used to feed the liquid onto the interior surface of the tube. The tube was also formed with an air inlet 9, through which air either at atmospheric or superatmospheric may be passed.

The geometrical form of the entrained air within the water varied with flow rate. With low flow rates, and a short drop to the standing water surface, a small amount of air was entrained, only to be rejected, with no air being carried down.

With increased water flow some entrained air was carried down in the form of a long continuous core of varicose appearance, the whole mass of water and air in the shaft rotating rapidly. This was accompanied by small air bubbles about 1 mm diameter which were presumably too small to be displaced inwards to the air core by the centripetal acceleration of the water at their particular location. A further increase in the water flow rate produced bubbles of apparently uniform diameter of about 3 mm, uniformly distributed across the flow, and moving down the tube 1.

For practical reasons, the minimum flow rate employed was one giving rise to bubbles in the water of a uniform diameter of at least 3 mm; and the height of the tube was chosen to be sufficient to allow the water at a given maximum flow rate to achieve the terminal condition for a time to permit maximum entrainment of air.

Referring now to the graph, the gasiliquid ratio is plotted against the parameter defined by the expression 02/D263 wherein Q is the flow rate in litres/second, D is the tube internal diameter in metres and b is the thickness in millimetres of the liquid stream on the interior surface of the tube. In one trial a flow rate of 1.96 litres/second was established. From this, the thickness of the liquid stream at the terminal condition may be calculated for a given tube, and for the described tube, was 2.76 mm. Thus, for the given flow rate and tube, the above expression is equal to 30.97, which gives a gas/liquid ratio of 0.14 or 1 4tDxO entrainment i.e. 0.27 litres/second.

The calculation was borne out by the results produced in the trial using air at atmospheric pressure.

The surface of the tube must be such as to cause turbulence in the liquid stream at the terminal condition. This entails that the surface be "rough". However, virtually all surfaces are sufficiently rough for this purpose. When the liquid reaches its terminal condition, it has been observed that no further changes in the surface roughness of the liquid take place.

At the stated flow rate, bubbles of between 3 mm to 5 mm were produced and if the vertical velocity of the water exiting from the tube 1 is sufficiently great the bubbles are carried down into the body of water in vessel 5. In the above described trial, the vertical velocity of the water exiting from the tube 1 was 0.96 m/s althou .

lower rates of entrainment are possible and vertical velocities down to some 0.3 mXs may be used.

The present invention is not dependent on the plan area of the vessel 7 for its efficacy of aeration, nor does it require any mechanical device to be immersed in the stock to be treated such as stirrers.

Normally, unless gas under pressure is to be used, only a conventional pump is required in addition to the tube 1. The invention thus provides equipment which is simple and is easy to maintain.

Claims

1. A method of gasifying a liquid feedstock, comprising passing the liquid feedstock to be gasified, in the presence of the gas, down a surface of a sufficient length in the direction of flow to cause the flowing liquid to achieve a terminal condition in which there is no further change in its velocity or in its thickness as measured normal to the surface; the surface being such that turbulence is induced in the flowing liquid at least when it achieves the terminal condition.

2. A method according to claim 1 wherein the feedstock is passed down said surface for a time when in the terminal condition to cause it to saturate with the gas.

3. A method according to claim 1 or 2, wherein the feedstock is withdrawn from a vessel in which said surface is positioned so that gasified feedstock flows from said surface back into the vessel; circulation of the feedstock in this manner being effected until the entirety of the feedstock in the vessel has been gasified to the extent required.

4. A method according to any of the preceding claims, wherein the gas is employed under superatmospheric pressure.

5. A method according to any of the preceding claims, wherein the feedstock is passed down said surface to achieve a flow rate such as to produce in the feedstock bubbles of a uniform diameter of not less than 3 mm.

6. A method according to any of the preceding claims, wherein the feedstock is passed down said surface so that its vertical velocity on leaving said surface is not less than 0.3 m/s.

7. A method according to claim 6, wherein said vertical velocity is not less than 0.96 m/s.

8. A method according to any of the preceding claims, wherein said surface is constituted by the internal surface of a tube.

9. A method according to claim 8, wherein said tube provides means for admitting the feed-stock into the tube at or adjacent the top end thereof, and separate means for admitting gas thereinto.

10. A method according to claim 9, wherein said separate means admit gas into the tube at adjacent the top thereof.

1 A method according to claim 9 or 10, wherein the tube is supported vertically erect and the feedstock is introduced into the tube at the top thereof through a vortex chamber.

12. A method according to any of the preceding claims, wherein the feedstock is a sewage sludge and the gas is air or oxygen.

13. A method of gasifying a liquid, substantially as hereinbefore described with reference to the accompanying drawings.

14. Apparatus for gasifying a liquid feedstock, comprising a surface arranged so that liquid feedstock can be deposited thereon to cause the feedstock to flow theredown in the presence of the gas with which the feedstock is to be gasified; the surface having a sufficient length in the direction of flow to cause the flowing feedstock to achieve a terminal condition in which there is no further change in its velocity or in its thickness as measured normal to the surface, and the surface being such that turbulence is induced in the feedstock at least when it achieves the terminal condition; the apparatus further comprising means for conveying the feedstock from a source thereof to the place on said surface at which the feedstock is to be deposited, and means for collecting the gasified feedstock effluent from said surface.

15. Apparatus according to claim 14, wherein said collecting means is a vessel in which said surface is positioned so that gasified effluent runs from said surface directly into the vessel.

16. Apparatus according to claim 12, wherein said vessel also constitutes said source of feedstock said conveying means conveying feedstock from the vessel to said surface, and the vessel receiving the gasified feedstock from said surface, whereby a circulation of the feedstock is effected.

17.-Apparatus for aerating sewage, being apparatus according to claim 16, wherein said vessel is constituted by a sewage tank.

18. Apparatus as claimed in any of preceding claims 14 to 17, wherein said conveying means comprises a pump.

19. Apparatus according to any of preceding claims 14 to 18, wherein said surface is constituted by the internal surface of a tube.

20. Apparatus according to claim 19, wherein said tube provides means for admitting feedstock into the tube at or adjacent the top thereof; and separate means for admitting gas thereinto.

21. Apparatus according to claim 20, wherein said separate means admit gas into the tube adjacent the top thereof.

22. Apparatus according to any of claims 19 to 21, wherein the tube is supported vertically erect.

23. Apparatus according to claim 22, wherein said tube is provided with a vortex chamber at the top thereof to admit feedstock into the tube.

24. Apparatus for gasifying a liquid, substantially as hereinbefore described with reference to the accompanying drawings.