METHOD AND APPARATUS FOR FORMING SMALL BUBBLES IN LIQUID

Abstract: The invention relates to a method and apparatus for forming small gas bubbles in liquid, and for creating effective material transfer between gas and liquid in large tanks or basins. Part of the liquid is made to flow through a stationary aerator unit (1) where the pumping member (2) sets to motion the liquid flowing in the flow channel, and simultaneously gas is fed into the liquid through small apertures (5), with a diameter of less than 2 mm, or through a porous surface, provided at the bottom (4) of the flow channel subsequently after the pumping member.

Description: The present invention relates to a method and apparatus for forming small gas bubbles in liquid, and in the creation of efficient material transfer between gas and liquid in large tanks or basins. Part of the liquid is made to flow through a stationary aerator unit, where a pumping member gives kinetic energy to the liquid flowing in the flow channel, and simultaneously gas is fed in the liquid through small apertures (less than 2 mm) or through a porous surface provided at the bottom of the flow channel subsequently after the pumping member. It is known in the prior art that the efficiency of the oxygen transfer of any gas, such as air, to be fed in a liquid, for instance wastewater or concentration sludge, is remarkably increased when the bubble size is reduced. For example, a decrease of the bubble diameter from 4 mm to 2 mm roughly doubles the amount of oxygen dissolved from the same amount of air. This means that the capacity of the gas supply equipment is doubled, and the energy efficiency is increased by 30-50%. In the prior art there is known the US patent publication 4,066,722 where the gas injected in liquid is attempted to break into small bubbles by means of a rotatable bell. In order to feed and mix the gas into liquid, in the bottom part of the aerator tank there is installed a hollow bell rotating around its axis, and coaxially above the bell there is provided a mixer. The rotating bell and the mixer make the liquid flow along the outside surface of the bell, and gas is fed in the liquid from inside the bell through the apertures provided on the bell surface. The gas supplied from inside the bell is sparged in the liquid in relatively small bubbles. In order to give the liquid sufficient kinetic energy, so that the oxidized liquid should flow further away from the bell and new, unoxidized liquid could be made to flow along the bell surface, comparatively high power is required in the rotation of the bell and the mixer. Thus the rotating bell is not an economical arrangement. On the other hand, the same patent publication also mentions an experiment where the same bell was used as a stationary installation, in which case the oxygen transfer readings collapsed. The experiments carried out during our examinations confirm the fact. The present invention relates to a method for conducting the liquid under treatment and contained for instance in a tank into a stationary flow channel comprising a pumping member. The gas is fed into the liquid flowing in the flow channel through small apertures - less than 2 mm - provided in the bottom part of the flow channel, or through a porous surface, so that the flowing liquid cuts the gas injected through the apertures into small bubbles. The flow channel is provided with a cover, which extends over the point of gas supply. The invention also relates to an apparatus for realizing the method. The essential novel features are apparent from the appended claims. When aiming at an efficient material transfer, the small bubble size alone is often not enough, but - particularly in large liquid tanks - the delay time of the bubbles in the liquid is another very important factor. In large tanks, there easily arises a so-called air-lift phenomenon, which means that the gas bubbles rise rapidly up to the surface in the vicinity of the bubble generator. Consequently the delay time of the gas bubbles in the liquid is shortened, and the material transfer is drastically declined. The purpose of the method and apparatus of the present invention is to create as small bubbles as possible with as low energy consumption as possible, and to move the resulting gas-liquid mixture sufficiently far and distribute it in a sufficiently large area in order to attenuate the air-lift phenomenon. Accordingly, it is essential for the cover part of the flow channel of the aerator of the present invention, that - opposite to the apparatus of the US patent 4,066,722, for instance - the flow channel always comprises a stationary top part, which is extended over the point of gas supply. Thus it can always be secured that the gas bubbles do not rise directly up through the liquid, but flow sideways along with the liquid and are simultaneously mixed therein. The cover also enables the creation of essentially smaller bubbles (0.5 - 3 mm) than could be possible without the cover. In the currently used wastewater aerating units, the oxidizing efficiency E in standard conditions into pure water varies within the range of 2.0 - 2.5 kg O₂/kWh. By employing so-called fine-bubble aerators, even remarkably higher readings are achieved in suitable test conditions, but on one hand blinding and slime formation in the porous plates or the like, and on the other hand the special requirements of the process in question, for instance special demands for mixing, result in a performance corresponding to the rate of 2.0 kg O₂/kWh in standard conditions. When applying the method of the present invention to forming gas bubbles with an average size of 3 mm, the E rate rises up to the range of 2.4 - 2.8 kg O₂/kWh, and with an average bubble size of 2 mm, the obtained E rate is over 3.0 kg O₂/kWh, which means remarkable savings in energy, when the suitability of the method and the apparatus in industrial-scale conditions is also taken into account. The invention is further described with reference to the appended drawings, where figure 1 is an illustration in principle of the realization of the method of the invention, figure 2 is an illustration in principle of the realization of another method of the invention, figure 3 is a top-view illustration of the embodiments of figures 1 and 2, when the aerator is completely of the same shape as the revolving body, figures 4 and 5 are top-view illustrations of the embodiments of figures 1 and 2, when the aerator unit is partly formed of several pipes, figure 6 is an illustration of an aerator unit where the flow channel is turned less than 90°, figure 7 illustrates an aerator unit where the liquid enters the aerator unit from below, and figure 8 illustrates an aerator unit where the flow channel, in the area of the pumping member, is annular in cross-section. As is seen from the schematical illustration of figure 1, the aerator unit 1 is open at the top so that the surrounding tank liquid can flow from around the axis of the pumping member 2, fitted inside the aerator unit and pressing the flow downwards, into the aerator unit. It is naturally clear that an actuator unit of this pumping member can be installed above the aerator unit, although the drawing does not include a detailed description thereof. The diameter of the propeller of the pumping member 2 is indicated by the letter D. From the gas supply unit 3 from below, gas is conducted into the aerator unit through small apertures arranged in the bottom part 4 of the aerator unit and having a diameter of less than 2 mm, or through a porous surface 5. The fact is that when arranged in a suitable fashion, small apertures form a porous surface. The apparatus includes a cover 6, its diameter being at least 2D, and gas inlet openings in an area which is located below the cover so that the cover part is extended after the gas inlet openings for a given length which is at least half of the diameter of the pumping member (i.e. ½D), which means that at this point the flow channel is closed towards the top. The cover and bottom parts of the aerator unit are so designed that the flow channel is first vertical in direction but curves flexibly to the side at least 30°, often about 90°, so that the aerated liquid is discharged from the aerator until in a more or less horizontal direction. In the cases of figures 1 and 2, the cover 6 or the aerator unit is uniform throughout its casing and equal in shape with the revolving body. In the aerator unit of figure 2, the structures have been made lower by making use of the partly sideways directing pumping member. Figure 3 illustrates the embodiments of figures 1 and 2 when seen from the top. In the cases of figure 4 and 5, both the bottom and the cover part of the aerator unit are first uniform from the pumping member axis onwards, but are further divided into several separate parts, and together these cover and bottom part form separate pipes 7. Thus the liquid flow is divided into several separate partial flows, which flow radially into different directions. Gas can be fed in either onto the uniform part of the aerator unit, before dividing the liquid flow into partial flows, as is illustrated in figures 1-4, or gas can be fed to the starting point of the separate pipes 7, i.e. separately to each partial flow, as is seen in figure 5. Figure 6 introduces an aerator unit where the flow channel is turned at least 30°, but clearly less than 90°. In the case of figure 7, the aerator unit is stationary at the top, and the liquid is conducted into the pumping unit from below the aerator unit. Figure 8 illustrates an embodiment which is one of the most advantageous as for the technical point of view. The flow channel 8 is annular in cross-section, and the pumping member 9 is correspondingly designed so as to match this arrangement. The gas supply into the liquid is still carried out in the bottom part of the apparatus. As is apparent from the above drawings and specification, it is an essential feature of the embodiments of the aerator unit according to the present invention that by means of them, a large liquid volume can be made to proceed with a small energy, because the created back pressure is low. In the majority of the cases, the most advantageous device for setting liquid into motion is a propeller pump. By thus setting the liquid into motion, and by simultaneously shifting the flow direction by means of reshaping the aerator unit, so that the peak of the liquid velocity profile is achieved near the flow channel wall, the gas supply into the liquid can be made as efficient as possible by adjusting the point of supply to fall within the said peak of the liquid velocity profile. Generally the flow is turned in the flow channel about 90°, but always at least 30°. Moreover, it is essential of the apparatus of the present invention that the apparatus itself is stationary, that it comprises a pumping member which gives motional energy to the liquid, and that it is provided with a cover part which extends over the point of gas supply. The most advantageous pump and flow channel combination is formed of a pump duct which is annular in cross-section, illustrated in figure 8. The shape of the flow channel at the point of gas supply may vary, and it may also be other than round or elliptical in cross-section, for instance a polygon.

Claims: 1. Method for forming small bubbles in liquid, wherein liquid contained in a tank is conducted into a stationary flow channel provided with a pumping member, which flow channel is at least partly formed of a revolving body, and where the liquid flow is flexibly turned at least 30° sideways; gas is fed into the liquid through small apertures with a diamter of less than 2 mm which are formed at the bottom of the flow channel subsequently after the pumping member; the flow channel extends in radial direction over the point of the gas supply for a length of at least half of the diameter of the pumping member; and the flowing liquid cuts the gas flow emerging through these apertures into small bubbles, generally of a size whithin the range of 0,5 - 3,0 mm, into a flow channel which is closed at the top. 2. The method of claim 1, characterized in that the flow channel is curved with respect to the vertical axis. 3. The method of claim 1, characterized in that the flow channel is annular in shape. 4. The method of claim 1, characterized in that the initially uniform liquid flow is divided into several separate partial flows. 5. The method of claims 1 and 2, characterized in that the liquid flow is divided into several separate partial flows after the point of gas supply. 6. The method of claims 1 and 4, characterized in that the liquid flow is divided into several separate partial flows prior to the point of gas supply, so that the gas is fed separately into each partial flow. 7. The method of claim 1, characterized in that the liquid flow direction in the flow channel is turned about 90°. 8. The apparatus for carrying out the method of claim 1, characterized in that the flow channel of the aerator unit (1) is formed of a stationary cover part (6) which is at least partially of the same shape as the revolving body, as well as of a bottom part (4), and that inside the aerator unit there is fitted a pumping member (2, 9); that immediately outside the bottom part there is arranged a gas supply unit (3), wherefrom gas is made to flow through small apertures (5), with a diameter of 0.5 - 2 mm, provided on the bottom part surface, into the liquid flowing in the flow channel, and that the cover part (6) and the bottom part (4) extend, in the breadthwise direction, over the gas supply unit for a length which is at least half of the diameter of the pumping member (2, 9). 9. The apparatus of claim 8, characterized in that the cover part (6) is uniform throughout the casing of the aerator unit. 10. The apparatus of claim 9, characterized in that the initially uniform cover part (6) and the bottom part (4) are divided into several separate parts forming separate pipes (7). 11. The apparatus of claims 8 and 10, characterized in that the apertures (5) wherethrough gas is fed into the aerator unit are located within the area of the uniform bottom part (4). 12. The apparatus of claims 8 and 10, characterized in that the apertures (5) wherethrough gas is fed into the aerator unit are located at the beginning of the separate pipes (7). 13. The apparatus of claim 8, characterized in that the apertures (5) wherethrough gas is fed into the aerator unit form a porous surface. 14. The apparatus of claim 8, characterized in that the cover part (6) extends outwards from the pumping member axis over an area with a diameter at least twice the diameter of the propeller of the pumping member (2). 15. The apparatus of claim 8, characterized in that the pumping member (2) presses the flow downwards. 16. The apparatus of claim 8, characterized in that the pumping member (2) presses the flow obliquely sideways. 17. The apparatus of claim 8, characterized in that within the area of the pumping member (2), the flow channel (8) of the aerator unit is annular in shape.

Inventors: ANDERSSON, ARVO EINARI, KIVISTOE, TUOMO VEIKKO JUHANI, LEIPONEN, MATTI OLAVI, Kivistö, Tuomo Veikko Juhani
Applicants: OUTOKUMPU OY
Application year: 1989
Main classification: C02F3
IPC codes: C02F3, B01F5