SPECIFICATIONGas injection apparatusThis invention relates to gas injection apparatus for introduction of gas into liquid for treatment purposes.
Atypical application is in absorber tanks forthe wet desulphurising of flue gas, wherein the flue gas is introduced into the absorber tank and sulphur dioxide is bound on lime or calcium hydroxide in water and then oxidised by the addition of air to form calcium sulphate (gypsum) via sulphite, which gypsum is maintained in suspension by agitators in the absorbertank.
In the course of this, the treatment with gas is generally effected via a so-called gas treatment cross which is disposed about 1-3 metres above the bottom of the absorber tank. The gas treatment cross consists of a plurality of tubes which are provided with appropriate holes through which the air supplied emerges in order to achieve a treatment with gas over the whole area of the absorber tank.
For example, three agitators, which are installed laterally, are installed in the tank, which has a diameter of 12-20 metres for example, and serve to suspend the gypsum formed, that is to say to prevent it from settling out.
The arrangement of the agitators, particularly the angular position of their axes in relation to the absorber tank, has a strong influence on the suspension but is not a subject of the invention.
For reasons of corrosion, the gas treatment crosses must consist of high-quality steel, for which reason the costs of such a gas treatment cross are very high (for example about 25,000.00).
Furthermore, blowers are necessary, which must deliver gas treatment rates of 8000 m3/h, as a result of which further relatively high costs are incurred.
The aim of the invention is to reduce such costs.
Accordingly this invention provides gas injection apparatus for introduction of gas into liquid for treatment purposes, said apparatus comprising one or more agitators in the liquid, characterised in that the apparatus comprises at least one tubular lance to introduce the gas into the liquid, the mouth of the tubular lance being in the region of an agitator.
One tubular lance may appropriately be associated with each agitator, the outlet of the tubular lance being situated at the pressure side of the agitator propeller.
Furthermore, the outlet of the tubular lance may advantageously be disposed above the centre axis of the agitator propeller.
As a result of the use of a simple gas treatment tube for each agitator, the expensive gas treatment cross can be dispensed with as a result of which the necessary investment costs can be reduced considerably. Distinct savings can also be achieved in the operating costs as will be explained with reference to a comparative example.
Oneform of embodiment of the invention is explained in more detail below, by way of example with reference to the drawing, Figs. 1 and 2 showing an absorber tank with a conventional gas treatment cross, in side view and in plan view.
Fig. 3 shows an absorber tank with an agitator and a tubular lance according to the invention, diagrammatically, in section.
Fig. 1 shows, diagrammatically, the lower portion of an absorber tank 10 such as is used for the wet desulphurising of flue gas.
During this desulphurising, the flue gas is introduced into the absorber tank and the sulphur dioxide is bound on lime, quicklime or calcium hydroxide with the addition of water. It is subsequently oxidised by the addition of air in the lower portion of the absorber tank to form calcium sulphate via sulphite.
In the course of this, a suspension 12 forms in the lower portion of the absorber tank 10, which suspension consists essentially of water and particles of gypsum, the proportion of solids being about 16% by weight. The suspension must be constantly agitated for which purpose agitators 16 are provided which are installed laterally in the absorber tank 10 in order to suspend the particles of gypsum and to prevent settling out.
The supply of air is effected through a so-called gas treatment cross 14, which is disposed at the bottom of the absorber tank 10 and which consists of a plurality of tubes which are connected to one another and provided with holes through which the air supplied to the gas treatment cross can emerge and enter the suspension 12 in order to oxidise the SO2 bound on lime to form gypsum which is then drawn off in a suitable manner, not illustrated.
Three agitators 16, for example, are used to agitate the suspension 12 and, as Fig. 2 shows, are disposed at angular distances apart of 120C and are installed close to the inner wall of the absorber tank 10. As Fig. 1 shows, the axes of the agitators 16 are inclined downwards at an acute angle and, as Fig. 2 shows, are also set at an acute angle in the circumferential direction, although the arrangement of the agitators is not a subject of the invention, as already mentioned.
The suspension 12 is thoroughly agitated by the agitators 16 while the above-mentioned oxidation is caused by the air bubbles 18 rising from the gas treatment cross 14.
The diameter of the absorber tank may, for example, be between 10 and 20 metres while it may be filled to a height of about 5 metres.
The form of embodiment shown in Fig. 3 shows an apparatus for treating with gas according to the invention.
Three agitators 22, for example, are installed in the absorber tank 20 and disposed as shown in Fig.
2. Only one agitator 22 with an agitator propeller 24 is illustrated in Fig. 3, however.
A tube or a tubular lance 26 is introduced into the absorber tank 20 from above with its mouth 28 in the region of the agitator 22.
The agitator propeller 24 delivers in the direction of the arrow P and the tubular lance 26 is preferably disposed at the pressure side of the propeller 24, as illustrated in Fig. 3.
The arrangement of the tubular lance 26 at the suction side of the agitator propeller 24, which is also possible in itself, may lead to flooding of the propeller 24 by the air bubbles 38 emerging from the mouth 28 of the tubular lance 26 which may have the result that the propeller24 is working more or less in an air bubble as a result of which its efficiency is impaired. It is therefore preferred to dispose the tubular lance 26 at the pressure side of the propeller 24.
The spacing of the centre axis 32 of the tubular lance 26 from the median plane of the propeller 24 is designated by h in Fig. 3 and the diameter of the propeller 24 is designated by d2.
The ratio h/d2 is preferably in the range from 0.4--0.5, particularly 0.46.
The mouth 28 of the tubular lance 26 may appropriately be situated somewhat above the centre axis 30 of the agitator 22.
The spacing of the mouth 28 from the centre axis 30 is designated bye, and, if the mouth 28 extends obliquelyto the longitudinal axis 32 of the tubular lance 26, as illustrated in Fig. 3, then e means the spacing of the centre of the mouth 28 from the centre axis 30 of the agitator 22.
The ratio e/d2 is preferably in the range from 0.080.18, particularly0.13.
The air blown into the suspension 36 through the tubular lance 26 is dispersed by the propeller 24 as a result of the shearing forces emerging in the propeller jet and is carried away by the agitator 22, 24, forming air bubbles 38.
The system is non-coalescent, that is to say the air bubbles formed remain small and stable; large bubbles are not formed and flooding of the agitator is thus ruled out.
Therefore, not only is the air blown in through the tubular lances 26 dissipated into small bubbles in the region of the shearing zones 34 of each individual agitator by the shearing forces arising there, but also the suspension 36 is thoroughly agitated and the particles of gypsum formed are kept in suspension by the agitators 22.
As already mentioned, at least one tubular lance 26 is associated with each agitator 22,24.
The known form of construction as shown in Fig. 1 and Fig. 2 will now be compared below with the form of embodiment according to the invention shown in Fig. 3, with reference to an example.
The basis for the comparison was the same material transition factor k,a of 70h-' in both experiments. ("Material transition factor" is equivalent to the term "oxygen transition factor" which is a measure of transfer of oxygen from gaseous to dissolved form.)The plants used had the following dimensions:Diameter of the absorber tank D=13.6 mFilling height H= 6.5 mPropeller diameter d2= 1 mThree agitators were usedIn the case of the experiment with the known gastreatment cross shown in Figs. 1 and 2, the specificagitator output is lower because the agitators serveonly for the suspension. Their power consumptionwas 3x8.5 kW=25.5 kW. The air throughput of the gas treatment cross was Q=8200 m3/h. The blower power was P=193 kW.
Thus the total power of agitators and blowers in the experiment with the known gas treatment cross shown in Figs. 1 and 2 is 218.5 kW.
If, on the other hand, according to the invention, tubular lances are used instead of the known gas treatment cross shown in Figs. 1 and 2, as illustrated in Fig. 3, then the specific agitator power rises to 3x 18.7 = 56.1 kW, because the agitators are used both for the suspension and for the treatment with gas, that is to say, in addition to the suspension, energy must be provided by the agitators in order to disperse the air blown in through the tubular lances 26.
With the method according to the invention, however, only an air throughput of 3100 m3/h is necessary so that the corresponding blower power can be reducedto 74 kW. Thus the total power necessary when using the invention is 130.1 kW in comparison with a total power of 218.5 kW when using the conventional installation.
This means that distinct savings result not only on the investment side but also in the operating costs.
The mouth 28 may possibly be provided with a perforated cover in order to improve the bubble formation.
The above description is couched in terms of a liquid being treated with a gas (air) but it will be appreciated that similarly a gas may be treated with a liquid and such usage falls within the present invention.