GB2390984A

GB2390984A - Jet aeration system for dissolving gases into a liquid wherein the gases are introduced to the liquid before the liquid is pumped through a liquid nozzle

Info

Publication number: GB2390984A
Application number: GB0216941A
Authority: GB
Inventors: Jack Elliot Taylor
Original assignee: Individual
Current assignee: Individual
Priority date: 2002-07-20
Filing date: 2002-07-20
Publication date: 2004-01-28
Also published as: GB0216941D0

Abstract

A jet aeration system which dissolves a gas, for example oxygen into a liquid, for example water is disclosed. The system includes liquid from a first channel 9 of a sparge pipe (1,Fig.1) which is injected via a liquid jet nozzle 7 into gas being pumped from a second channel 10 of the sparge pipe through at least one mixing nozzle 8. Gas from the second channel is injected into a delivery duct 11 and introduced into the liquid before the liquid is pumped through the liquid nozzle. The system may be used to clean up dirty water such as sewage, where oxygen is dissloved in the water to help meet the oxygen demand from organic matter suspended in the water.

Description

\ 2390984

IMPROVEMENT S IN JET AERATION SYSTEMS

This applicationrelates to systems for dissolving gases into a liquid and in particular an improved jet aeration system utilising a high purity oxygen feed.

A conventional jet aeration system usually comprises of a pump, discharge pipe work and water nozzles, an air supply (aspirated, compressed or blower), distribution pipe work and air nozzles. Such a system is commonly used to dissolve oxygen into dirty water using air as the source gas. Dirty water is distinguishable from clean water (usually potable I O water) in that it can contain dissolved and/or suspended, inorganic and/or organic material.

Sewage is a typical example of dirty water. The dissolved and/or suspended organic material exerts a demand for oxygen which is usually equal or proportional to its concentration. The nature and composition of dirty water will vary and will directly affect the oxygen transfer of the system. In order to bench mark one aeration system with another 15 or to validate the performance of the selected system clean water is used as the standard.

Due to the high demand for oxygen, the volume of gas to volume of liquid ratio (the v/v ratio) may exceed the optimum for efficient transfer, and thus larger bubbles will be formed and transfer efficiency will reduce.

In certain oxidation/exothermic processes (e.g. biological and chemical treatment 20 where carbon dioxide and water may be formed) where more and more oxygen is required to satisfy the increasing biological and/or chemical demand, a problem with the capacity and efficiency of the system will soon be reached if air is used as the source gas.

This increase in demand can lead to an increase in temperature, which in turn reduces the solubility of the air. If the air volume is increased to satisfy demand this will 25 create larger bubbles, which ( as the volume and velocity of the liquid cannot be changed in respect to the increased air volume) will cause coarse bubble breakout and may indeed reduce the mass transfer.

Air is only a 21% pure source of oxygen, therefore theoretically at least, 79% of the air flow will be present in the liquid as bubbles (in practice this is over 90%), generating 30 foam, odour, aerosol and Volatile Organic Carbon (VOC) emissions. Increasing the oxygen concentration will reduce the severity and/or cost of control of these emissions. One obvious method of reducing these problems would be to switch to pure oxygen instead of air. However, this is not a feasible solution as air delivery systems (i.e. the pipework) will

b._ _ usually contain oil from the compressors and therefore this would present a major fire hazard with oxygen concentrations above 25%.

Systems were devised in the 1 970s, trying to address such problems. However these used a venturi instead of a nozzle and a high pressure/low volume pumped system 5 compared to a jet aeration system. These systems required a secondary method of removing carbon dioxide in a biological process a step not required in the proposed invention to be described below.

EP1002567A uses both air and oxygen but is more complex using discreet nozzle type eductors (some for oxygen and some for air due to hazard) and not the main delivery 10 water pipe.

In a first aspect of the invention there is provided a method of dissolving a gas into a liquid wherein a first source of said gas is injected via a first main conduit through at least one gas nozzle and introduced to said liquid, said liquid being pumped via a second main 15 conduit through at least one liquid nozzle and wherein at least one further source of said gas is injected into said second main conduit and thus introduced to said liquid before said liquid is pumped through said liquid nozzle.

In one embodiment, said liquid is water and said gas is oxygen. In this embodiment, preferably said first source of oxygen is an impure source such as air and said second source 20 of oxygen is a substantially pure source. Preferably said pure source is only introduced when required such as when the oxygen from the first source can no longer be dissolved economically or efficiently in the body of the liquid requiring the target gas.

Preferably said at least one liquid nozzle and said at least one gas nozzle form a nozzle arrangement wherein said liquid nozzle is located substantially concentrically inside 25 and feeds into said gas nozzle and said gas is introduced to said liquid in said gas nozzle.

Preferably there is an array of such nozzle arrangements each fed directly or indirectly by both the main conduits.

Preferably said nozzles ultimately feed into a tank or other body of liquid requiring the gas wherein further dissolution occurs.

30 Said liquid may be pumped by means of a water pump.

Preferably said second source of gas is injected at the pump discharge.

Preferably said second source of gas is injected via a sonic nozzle.

In a further aspect of the invention there is provided apparatus for dissolving a gas into a liquid comprising a first main conduit for supplying a first source of a gas to at least one gas nozzle, through which said gas is injected, and a second main conduit for supplying a liquid to at least one liquid nozzle through which said liquid is pumped, and wherein at 5 least one further source of such gas is injected into said second main conduit and introduced to said liquid before said liquid is pumped through said liquid nozzle.

BRIEF DF,SCRIPTION OF THE DRAWINGS

10 Embodiments of the invention will now be described, by way of example only, by reference to the accompanying drawings, in which: Figure 1 shows a conventional apparatus for the aeration of dirty water using a jet system. 15 Figure 2 shows a typical concentric dual nozzle arrangement, incorporating both gas and liquid nozzle.

Figure 3 shows apparatus for the aeration of dirty water using a jet system incorporating a second gas inlet.

Figure 4 shows the concentric dual nozzle arrangement of Figure 2 as fitted to the 20 apparatus of figure 3.

Figure 5 shows a common variation on the apparatus of Figure 3.

DETAILED DESCRIPTION OF THE EMBODIMENTS

25 Figure I shows conventional apparatus for the aeration of dirty water. There is a sparge pipe 1 with two channels along which are a number of nozzle arrangements 2 The nozzle arrangements 2 are of a concentric dual type with the primary liquid nozzle inside the secondary gas nozzle. There is a gas pipe 3 which feeds into the sparge pipe 1. A liquid inlet 4 feeds into a water pump 5. A pipe 6 connects the pump 5 to the sparge pipe 1.

30 In conventional jet systems the dissolution is achieved only at the nozzle arrangement 2 and in the tank, no use is made of the pipe work 1,6 between the pump 5 and primary nozzle 7 to dissolve gas. In such systems a liquid such as water is pumped by the pump 5 into a first channel of the sparge pipe 1 and is pumped through the centre (primary)

u nozzles of a number of concentric dual nozzle arrangements 2 into a tank or body of liquid requiring the gas (not shown). A gas, such as air, is then also injected into a second channel of the sparge pipe 1 and through the outer primary nozzle of the dual concentric nozzle arrangements 2.

5 In order to efficiently dissolve any gas into any liquid it is essential to form small bubbles (mass transfer) relative to the volume of liquid being recirculated. It is also important to ensure that the volume, pressure and velocity of liquid is equal at each nozzle along the sparge pipe.

This is achieved in a very effective way using a dual concentric nozzle arrangement 10 2 as shown in greater detail in figure 2. There is an inner primary (liquid jet) nozzle 7 fed by the first channel 9 of the sparge pipe 1, and an outer secondary (mixing) nozzle 8 fed by the second channel 10 of said sparge pipe 1 via an integral air delivery duct 11. With such a nozzle arrangement 2, much of the dissolution occurs in the secondary nozzle 8 itself.

However to prevent small bubbles coalescing and forming larger bubbles (reducing mass 15 transfer) it is important to maintain a minimum volume of gas and maximum volume and velocity of liquid in the recirculation system.

The sudden reduction in CSA (cross sectional area) as the liquid enters the primary nozzle creates a high pressure, high velocity jet which causes a shear effect forming small bubbles at the air / liquid interface within the secondary nozzle chamber. The properties of 20 the gas (e.g. purity, solubility, temperature etc.) and the liquid (e.g. temperature, dissolved and suspended material etc.) have to be accommodated within the design of the system and are defined in several formulae. e.g SOR= AOR

a x ((p x Cs x D) - C) x (4 - 20) 25 (C20 X D)

where: SOR = Standard Oxygen Requirement (kg/h) AOR = Actual Oxygen Requirement for process (kg/h) a = Ratio of oxygen transfer coefficient (BOA) of the waste water to clean water 30 = Ratio of the oxygen saturation of the wastewater to clean water = Temperature correction coefficient Cs = Surface dissolved oxygen saturation concentration in clean water at actual atmospheric temperature and pressure process liquid (mg/l)

64. _ D = depth correction factor C = Required residual dissolved oxygen concentration (mg/l) C20 = Surface dissolved oxygen saturation concentration in clean water at 20 C and 760mm Hg or 1013 mbar (mg/1) 5 T = Wastewater temperature ( C) Figure 3 shows a system suitable for carrying out the invention. The main difference of this system is that feeding into the pump outlet pipe 6 is a second gas inlet 12. This allows the air flow to be reduced to its optimum value based on the process conditions while additional oxygen of a higher purity is provided.

10 Oxygen (in this case the target gas) is injected at up to 100% purity (i.e. a far purer source than air). using a connection into the pump discharge 6 and utilising, where appropriate, a sonic nozzle (a constricting nozzle shaped so as to increase the velocity of the gas entering the liquid to its sonic velocity) to form small bubbles and dissolve the gas.

Some of the gas will dissolve immediately, being up to 5 times more soluble than the air is and with maximum driving force available due to the low background concentration. The

v/v ratio of any undissolved gas at this point will be of a value which will not adversely affect the dissolution efficiency of air at the nozzle arrangement 2. Provided liquid velocities and v/v ratios are maintained at the nozzle and that the target gas concentration does not exceed saturation there is no reason for any undissolved gas or air bubbles to 20 coalesce in the reactor or body of liquid requiring the target gas.

The concentric nozzle arrangement 2 for this situation is shown in figure 4, the difference being that the liquid stream through the primary nozzle is water in which oxygen has already been dissolved by the oxygen injection into the pump discharge.

This method can be used either to provide additional oxygen transfer capacity or as a 25 back up in the case of an air supply failure. The removal of unwanted gases (e.g. CO2 in a biological reactor) can be achieved by using lower purity oxygen. Additionally the ability to increase oxygen purity can also reduce foam, aerosol, odour and VOC (volatile organic compounds) emissions.

As a design basis at least 45g of pure oxygen can be dissolved at atmospheric 30 pressure for every lm3 of liquid (water) pumped.

In most cases the supplementary equipment and connections required can be installed without taking the operating process off line. Typical equipment configurations are shown in figs 6 and 7.

Figure 5 shows a slight but common variation on figure 4 wherein the main gas pipe 3 feeds into the centre of the sparge pipe 1 instead of one end. This ensures less pressure variation at each concentric nozzle arrangement 2.

It should be noted that the above is an example only and it is envisaged that the 5 invention can be used for the dissolution of any gas into any liquid and not just the aeration of water, whether it be dirty or clean. Also other variations on the apparatus design are envisaged.

Claims

64042GB 19/7iO2 CLAIMS

1. A method of dissolving a gas into a liquid wherein a first source of said gas is injected via a first main conduit through at least one gas nozzle and introduced to said 5 liquid, said liquid being pumped via a second main conduit through at least one liquid nozzle and wherein a second source of said gas is injected into said second conduit and thus introduced to said liquid before said liquid is pumped through said liquid nozzle.

2. A method as claimed in claim 1 wherein said liquid is water.

3. A method as claimed in claim 1 or 2 wherein said gas is oxygen.

10 4. A method as claimed in claim 3 wherein said first source of oxygen is an impure source and said second source of oxygen is a substantially pure source.

5. A method as claimed in claim 4 wherein said impure source of oxygen is air.

6. A method as claimed in any preceding claim wherein said at least one liquid nozzle and said at least one gas nozzle in use form a nozzle arrangement wherein said liquid 15 nozzle is located substantially concentrically inside and feeds into said gas nozzle and said gas is introduced to said liquid in said gas nozzle.

7. A method as claimed in claim 6 wherein there is an array of such nozzle arrangements each fed directly or indirectly by both the main conduits.

8. A method as claimed in any preceding claim wherein said nozzles ultimately 20 feed into a tank or body of liquid requiring the said gas in which further dissolution occurs.

9. A method as claimed in any preceding claim wherein said liquid is pumped by means of a water pump.

10. A method as claimed in any preceding claim wherein said second source of gas is injected at the pump discharge.

25 11. A method as claimed in any preceding claim wherein said second source of gas is injected via a sonic nozzle.

12. A method as claimed in any preceding claim wherein said second source of gas is only introduced when required.

13. Apparatus for dissolving a gas into a liquid comprising a first main conduit 30 for supplying a first source of a gas to at least one gas nozzle, through which said gas is injected, and a second main conduit for supplying a liquid to at least one liquid nozzle through which said liquid is pumped, and wherein a second source of such gas is injected

64042GB 19Oi02 into said second main conduit and introduced to said liquid before said liquid is pumped through said liquid nozzle.

14. Apparatus as claimed in claim 13 wherein said liquid is water.

15. Apparatus as claimed in claim 13 or 14 wherein said gas is oxygen.

5 16. Apparatus as claimed in claim 15 wherein said first source of oxygen is an impure source and said second source of oxygen is a substantially pure source.

17. Apparatus as claimed in claim 16 wherein said impure source of oxygen is air. 18. Apparatus as claimed in claims 13 to 17 wherein said at least one liquid 10 nozzle and said at least one gas nozzle form a nozzle arrangement wherein said liquid nozzle is located substantially concentrically inside and feeds into said gas nozzle and said gas is introduced to said liquid in said gas nozzle.

19. Apparatus as claimed in claim 18 wherein an array of such nozzle arrangements is provided each fed directly or indirectly by both the main conduits.

15 20. Apparatus as claimed in claims 13 to 19 wherein there is further provided a tank or body of liquid into which said nozzles ultimately feed and where further dissolution occurs. 21. Apparatus as claimed in claims 13 to 20 wherein there is further provided a water pump.

20 22. Apparatus as claimed in claims 13 to 21 wherein said second source of gas is injected at the pump discharge.

23. Apparatus as claimed in claims 13 to 22 wherein said gas nozzle is a sonic nozzle.