IE45914B1

IE45914B1 - A method and apparatus for dissolving a gas in and removing a gas from a liquid

Info

Publication number: IE45914B1
Application number: IE2336/77A
Authority: IE
Original assignee: Portals Water Treatment Ltd
Priority date: 1976-11-17
Filing date: 1977-11-16
Publication date: 1982-12-29
Also published as: GB1559018A; IE45914L; AU3073077A; AU510998B2; ZA776790B

Description

The present invention relates to a method and apparatus for dissolving a gas in a liquid, and a method and apparatus for removing gas from a liquid.

It is known to dissolve, e.g. air, oxygen, carbon dioxide, ozone or any other slightly soluble gas in water in a situation where the water is passing through open vessels, channels etc. Such situations are encountered widely in water and waste water treatment for example in stabilisation after softening, flotation by the release of dissolved bubbles of gas, ozonisation and biological systems such as activated sludge and related processes The dissolution of gases in water in these situations usually requires the expenditure of power and not infrequently the use of additional pumps, motors, agitators or blowers which consume prodigious amounts of power.

It is common practice to use a pressure vessel with or with-24 5914 out an internal packinq in which water is led in at the upper end of the vessel through a distributor and allowed to fall freely through the vessel or down a packing contained in the vessel in order to dissolve gases in the water. This arrangement is used widely in dissolved air flotation in which case 5 to 50% or even more of the net flow through the plant is recycled through the pressure vessel.

An alternative method of dissolving gases in water, involves bubbling the gas up through the body of the water. The efficiency has been increased in some examples by injecting the air at a significant depth so that the hydrostatic pressure on the air bubbles and hence the solubility is increased. In one example of this approach the air was injected into the downgoing leg of an inverted syphon at a sufficient depth and at a sufficient fluid velocity that the air is carried on downwards to the bottom of the syphon. The gases occupy the full length of the upgoing leg of the syphon and the differential between the upgoing and down-going legs is sufficient to maintain circulation. Efficiency of dissolution can only be increased by increasing the length of the syphon.

It is also known to dissolve air in water using apparatus comprising a conical vessel (apex uppermost) with a downward flow of water which maintains a fluidised mass of air bubbles within itself. Such apparatus cannot be built on a larger scale, without involving a vessel of excessive height.

The invention provides in one aspect a method of dissolving in or removing gas from a liquid comprising passing a liquid downwardly through a column in which a gas space is defined in region of non-atmospheric hydrostatic pressure and introducing -34 5 914 gas into or removing released gas from the gas space being defined within the column by a distributor and the lower boundary of the gas space being defined within the volumn by a continuous 1 liquid phase.

The invention provides in another aspect apparatus for performing the above-mentioned method, comprising a column, a gas space in the column having a gas inlet thereto, and arranged so that in use liquid galling down the volumn passes through the gas space and forms a hydrostatic pressure in the gas space, and a riser downstream of the gas space so that in use the gas space is subjected solely to the hydrostatic pressure of liquid above the gas space so as to dissolve the gas in the liquid, the upper boundary of the gas space being defined within the column by a distributor and the lower boundary of the gas space being, in use, defined within the column by a continuous liquid phase.

The invention provides in yet another aspect apparatus for performing the above-mentioned method, comprising a column, a gas space in the column arranged so that in use liquid falling down the column passes through the gas space and forms a negative hydrostatic pressure in the gas space so as to release the gas, and means for removing the released gas from the column, the upper boundary of the gas space being defined within the column by a distributor and the lower boundary of the gas space being, in use, defined within the column by a continuous liquid phase.

Preferably, the gas space contains packing to provide a large surface area and efficient mass transfer. -4Embodiments of the invention will now be described with reference to the accompanying drawings, wherein: Figure 1 shows diagrammatically apparatus for dissolving gas in a liquid, Figure 2,3 and 4 show in section three alternative forms of diffuser plate, and Figure 5 shows diagrammatically apparatus for removing gas from a liquid.

In Figure 1, water to be aerated enters the apparatus through a pipe or channel 1, which normally is open or is subj ected to atmospheric pressure at its surface. The water flows downwards, either through a large-diameter shaft 2 or through a smallerdiameter downcoming tube 2', the choice between the large and narrow diameter downcomer depending upon the nature of the process involved. At a suitable depth sufficient to give hydrostatic pressure significantly greater than atmospheric pressure, a distributor plate 3 is provided which separates the water above from a gas space 4 below. The distributor plate 3 may be plain and perforated, or provided with nozzles, or indeed may be replaced by a diaphragm with a connection through to a distributor of the header/lateral type.

The water cascades through the gas space 4, which is advantageously filled with a packing 4' such as Raschig Rings, Pall rings or similar material. Air or the gas to be dissolved is fed into the gas space 4 via an inlet 5. An interface forms at a level which is dependent upon the mass transfer rate, the fluid flow rate and the gas flow rate. Normally the lower limit of -5the gas space 4 will be within the compartment, and packing where used will be partially within the gas chamber 4 and partially submerged.

Where a packing is used, support bars or a mesh 6 will be required.

The solution of the gas in water then passes forward fco a riser which may take the form of a narrow shaft as in certain forms of flotation, or the solution may flow to the base of a tank 7 (as shown) of sufficient volume to enable any subsequent desired reaction to proceed (as for example in an activated sludge process). The product flows out of the tank 7 via a channel or pipe 8. The surface level 9 in the tank 7 is of course lower than the surface level of the incoming Chanel 1 by the amount of the pressure loss through the distributor plate 3 and the free fall through the gas space.

Where a perforated plate is used it is necessary either to employ a relatively high headloss to prevent air breaking through in the reverse direction or to use short tubes or collars 10 (Figs. 2,3) inserted in the plate 3, such tubes having a length at least equal to the diameter and preferably twice the diameter. This latter arrangement allows the distributor to operate at a much lower headloss without break-through in the power lost through the system. As an .alternative to the tubes or collars, the distributor plate may be pressed to form a series of conical flares or bell mouth entries 11 at each orifice (Fig.4).

The perforated plate carrying collared orifices or flared -6orifices prevents packed penetration of air at a low headloss in a system where the downcoming velocity is also low, i.e. less than 0.3 metres/second, which is the normal rise rate of air bubbles through a mass of water.

Whilst in many cases the amoun t of gas fed in through inlet 5 will be chosen to be totally dissolved within the air chamber, there are cases where it is advantageous to have an excess gas flow to provide a continuous bleed through, for example where one wishes preferentially to dissolve oxygen and 1q not nitrogen. The undissoived nitroaen will accumulate and depress the solubility of oxygen. The excess gas may be allowed to escape either from the bottom or the top of the compartment so that flow of the gas is either co-current or counter-current as may be most appropriate.

There is no limit to the depth of submergence of the gas space. In many situations it will probably be more than 3m., less than this giving relatively little advantage. The maximum depth depends upon the construction, but at 1.0m. for example the pressure will be approximately twice that of atmospheric 2q pressure and the solubility of the gas twice that at the surface.

By way of example, an approach to equilibrium of 60-80% is achievable at rates of 100 m. per hour approach velocity in a packing depth of approximately 0.6m. The amount of gas dissolved per kilowatt hour of energy expended increases with the sub25 mergence but figures between 4.5 and 6 Kg. of oxygen per kW. hour are readily obtainable. In the absence of packing the approach velocity may be up to five times higher, although the approach towards the equilibrium solubility of the gas is less.

Nevertheless, the unpacked version is particularly suitable for -7certain biological systems where packing would tend to become fouled with slimes of biological growths and also where size is a more important factor than efficiency.

Where appropriate, part of the effluent flow from the channel or pipe 8 may be returned to the inlet channel or pipe 1. In situations such as an activated sludge precess for example any conventional means such as a screw pump or air lift may be provided. In the case of an air lift, excess air passing through the gas space 4 may be used to provide the motive force.

The embodiment shown in Figure 5 is for removing gas from a liquid, and is similar to the embodiment of Figure 1 except that the gas space is subjected to a negative hydrostatic pressure. The air space 4 is arranged at the top of the shaft 7 which acts as the down leg of a syphon, the up leg 12 of which is upstream of the air gap 4. The liquid passes through the distributor plate 3 (which may be as shown in Figures 2,3 or 4) into the air gap 4 and gives up its gas which may be removed by vents 13,14 leading to an interceptor 15. The interceptor 15 is provided with an exhauster 16 through which gas is exhausted, and a drain 17 for liquid drawn through the vents 13,14.

The shaft 7 is provided with a bypass weir 18, which allows water to flow round the syphon when the exhauster 16 is off or broken down. The weir 18 is also necessary to start up when water is required down stream to seal the base of the shaft 2, so that the exhauster 16 can start the syphon.

Claims

1. A method of dissolving gas in or removing gas from a liquid comprising passing the liquid downwardly through a column in which a gas space is defined in a region of nonatmospheric hydrostatic pressure and introducing gas into or 5 removing released gas from the gas space, the upper boundary of the gas space being defined within the column by a distributor and the lower boundary of the gas space being defined within the column by a continuous liquid phase.

2. A method as claimed in claim 1, wherein gas is dissolved 10 in the liquid by introducing gas into the gas space which is located at the base of the column, where it is subjected solely to the hydrostatic pressure of the liquid above the gas space.

3. A method as claimed in claim 1 or claim 2, wherein liquid in which gas has been dissolved passes to a riser downstream of 15 the gas space.

4. A method as claimed in claim 1, wherein gas is removed from the liquid by removing released gas from the gas space which is located at the top of the column where it is subjected to a negative hydrostatic pressure by the falling column.

5. A method as claimed in claim 1 or claim 4, wherein the liquid is syphoned by the falling column up a riser to the gas space.

6. A method as claimed in any one of the preceding claims, wherein the distributor comprises a perforated plate. -943914

7. A method as claimed in claim 6, wherein tubes are inserted into the perforations, the tubes having a length at least equal to the diameter.

8. A method as claimed in claim 7, wherein the length of the tubes is substantially equal to twice the diameter.

9. A method as claimed in claim 6, wherein each perforation is provided with a tapered flare pressed out from the plate.

10. A method as claimed in any one of the preceding claims, wherein the gas space is provided with a packing.

11. A method as claimed in any one of the preceding claims, wherein the liquid is water and the velocity of the vzater upstream of the gas space is not greater than 0.3 metres/second.

12. Apparatus for performing the method claimed in claim 1, comprising a column, a gas space in the column having a gas inlet thereto, and arranged so that in use liquid falling down the column passes through the gas space and forms a hydrostatic pressure in the gas space, and a riser downstream of the gas space so that in use the gas space is subjected solely to the hydrostatic pressure of liquid above the gas space so as to dissolve the gas in the liquid, the upper boundary of the gas space being defined within the column by a distributor and the lower boundary of the gas space being, in use, defined vzithin the column by a continuous liquid phase.

13. Apparatus as claimed in claim 12, wherein the gas space is arranged at the bottom of the column. -1014. Apparatus for performing the method claimed in claim 1, comprising a column, a gas space in the column arranged so that in use liquid falling down the column passes through the gas space and forms a negative hydrostatic pressure in the gas space 5 so as to release the gas, and means for removing the released gas from the column, the upper boundary of the gas space being defined within the column by a distributor and the lower boundary of the gas space being, in use, defined within the column by a continuous liquid phase. •jθ 15. Apparatus as claimed in claim

14. , wherein the gas space is formed at the top of the column. 16. Apparatus as claimed in claim 14 or 15 wherein the column is the downleg of a syphon, sm upleg being provided upstream of the gas space.

15. 17. Apparatus as claimed in anyone of claims 12-16, wherein the distributor comprises a perforated plate.

16. 18. Apparatus as claimed in claim 17, wherein tubes are inserted into the perforations, the tubes having a length at least equal to the diameter. 2o

17. 19. Apparatus as claimed in claim 18, wherein the length of the tubes is substantially equal to twice the diameter.

18. 20. Apparatus as claimed in claim 17, wherein each perforation is provided with a tapered flare pressed out from the plate. 25

19. 21. Apparatus as claimed in any one of claims 12 to 20 -11wherein the gas space is provided with a packing.

20. 22. A method of dissolving a gas in a liquid substantially as herein described with reference to Figures 1 to 4 of the accompanying drawings. 5

21. 23. A method of removing a gas from a liquid substantially as herein described with reference to Figure 5 of the accompanying drawings.

22. 24. Apparatus for dissolving a gas in a liquid substantially as herein described with reference to and as shown in Figures 1 10 to 4 of the accompanying drawings.

23. 25. Apparatus for removing a gas from a liquid substantially as herein described with reference to and as shown in Figure 5 of the accompanying drawings.