WO1994021365A2 - Mixing device for gases and liquids - Google Patents

Abstract

A mixing device for gases and liquids has a feed tube (3) open both above and below the liquid level. Below the feed tube is located a turbo-stirring member (8) driven by means of an axial shaft (4). The inner stirring blades of the stirring member (8) are surrounded by a ring (10) which turns together with them. On the outer side of the ring (10) are mounted further axial blades (9) which also convey the gases and liquids in the axial direction. In order to generate appropriately divided swirls while improving gas feed, the feed tube (3) may be provided on its outer side with flow-disturbing baffles (11) arranged in a star configuration. This mixing device, suitable for example in waste water treatment plants for aerating the water, as well as for high-pressure reactors, allows gas feed to be substantially increased with an optimum gas bubble distribution. Solids may also be fed therewith and are remarkably well suspended.

Description

 Device for mixing gases and liquids

description

The invention relates to a device for mixing gases and liquids; it relates in particular to the formation and recirculation of gas bubbles / liquid mixtures.

A number of possibilities and techniques are known in order to achieve the mixing of gases with liquids. Saving systems, for example, for introducing a gas under a stirring element, such as a Rushton turbine or an axial stirring element, are widely used. The gas must be added under pressure, for which either a compressor or a pressure storage tank is required. Saving systems are generally circular and have several openings for the gas outlet or they are simple tubes with an open end. The agitator should break the gas into as small and as many gas bubbles as possible and disperse it in the entire mixing vessel, for example a tank. In a known economy system, the advantage of an axial flow generated by a known stirring element is used in order to enable gas to be introduced through four simple pipes near the bottom of the container. An axial flow is desirable in order to achieve short mixing times, good solids suspension properties and a movement of the gas bubbles upwards from the container bottom. If a suitable axial flow is used, local backflow under the mixing blades and thus the formation of a gas cushion under the individual mixing blades is also prevented, which would result in the stirrer being flooded. An overflowing mixing blade is no longer able to pump the liquid so that there is an adequate flow in the mixing container. There are two degrees of freedom for a given system in a gas / liquid reactor, namely the speed of the agitator and the gas rate.

Another basic type of economy system has one or more self-priming stirring elements with a hollow shaft. The negative pressure behind the blades of the agitator creates a vacuum in the hollow shaft. The gas is drawn in through an opening above the liquid level and supplied to the liquid in the vicinity of the mixing blades or blades. The amount of gas that can be dispersed depends on the stirrer speed and the negative pressure generated by the blades, which in turn depends on the shape of the blades. There is only one degree of freedom for a given system, namely the stirrer speed.

A third known device for mixing gases and liquid is the so-called turbo aerator (cf. DE-A 24 15 813), which is shown in the schematic drawing in FIG. 11 in an embodiment with only one stirring element . In this embodiment, a motor 1 is flanged to a bearing housing 2. The bearing housing 2 is only required from certain capacities and nominal speeds and can also be used to accommodate additional ring seals. A guide tube 3 of the aeration

SPARE BLADE RULE 26 ters has a first opening above a liquid level indicated by an arrow mark. This is an essential difference to all other known guide tube systems. The guide tube 3 opens inside the liquid briefly above a stirring element 5 designed as an axial flow. The space between the guide tube 3 and the stirring element 5, together with the diameter ratio of the two, determines the amount of gas that can be drawn in. A drive shaft 4 for the agitator 5 runs centrally within the guide tube 3; it can additionally be supported by a bearing within the guide tube 3 in order to ensure greater stability in the case of longer shafts.

A typical agitator 5 for the turbo-aerator shown in FIG. 11 is shown in FIG. 13, namely a three- or multi-bladed inclined-blade stirrer with a rotating ring 10 comprising the agitator blades. Reference 6 denotes a flange plate for fastening the door - Bo aerators on a container, not shown.

The known turbo aerator according to FIG. 11 works as follows:

With increasing speed, the amount of liquid pumped between the guide tube 3 and the stirrer 5 increases. This leads to a decrease in the static pressure above the wings. From a certain speed, the pressure has dropped to the level of the static water pressure and the system begins to draw in gas from the surface through the guide tube 3. Since the gas / liquid mixture passes through the blades of the stirring element 5, the gas bubbles become extremely small due to the mechanical shear. Such small gas bubbles can never be reached with liquid shear alone. A uniform distribution of the gas bubbles prevails within the mixing container. If the only task is. This type of turbo-ventilators have proven their worth in introducing gas into the liquid. Difficulties arise, however, from the fact that the system tends to flood easily. In addition, since the gas bubbles can only be pumped down by a distance corresponding to approximately half the blade diameter, this type of aerator is not suitable for a simultaneous suspension of solids. In order to fulfill the task of a solid suspension in addition to the aeration, it is known, as shown in FIG. 12, to equip the turbo aerator according to FIG. 11 with a further axial stirring element 7. In principle, this additional stirring element 7 corresponds to the stirring element 5 except for a conical ring 12 instead of the cylindrical ring 10. The additional stirring element 7 has the task of sucking in the gas bubbles formed by the stirring element 5 and passing them through the mixing container to distribute. Like all known axial stirring elements, however, this embodiment variant of the known turbo aerator also has the tendency to flood if the amount of gas introduced by the stirring element 5 becomes too large. Another disadvantage is that. that the weight of the overall system becomes greater as a result of the additional axial stirring element 7 and the substantially longer drive shaft 4. The critical speed becomes lower and the system tends to vibrate, which in turn leads to other mechanical difficulties.

So there is a need for better gas / liquid mixing systems.

The object of the invention - in particular based on the known turbo aerator systems described above - is to provide a device for mixing gases and liquids which, with a comparatively smaller drive power and lower overall weight, provides very good mixing of the gas and liquid phases made possible in a predetermined mixing container.

Improvements are especially for the area of high-pressure reactors, where an economy system arranged below the liquid level is not desirable, but on the other hand the recirculation of the gas from the upper area of the tank is very important, in order to be able to suspend solids in particular in the entire tank , as they are sought with the invention, of particular importance. This also applies in particular to the area of catalytic reactions.

SPARE BLADE RULE 26) Another requirement, to name just one more example, is for the aeration of lakes or wastewater treatment plants. Because of the great demand, there are in particular requirements for inexpensive solutions which are nevertheless able to suspend solids, for example digested sludge. No mixing systems equipped with stirrers are currently known which can simultaneously meet all of the requirements mentioned.

The invention relates to a device for mixing gases and liquids with a guide tube immersed in the liquid and open below and above the liquid level as well as with a drive shaft axially penetrating the guide tube for an axial arranged at a fixed distance below the lower guide tube opening ¬ stream as a conveyor and agitator with a plurality of enclosed by a co-rotating ring, vertically from top to bottom conveying inner axial blades, according to the invention characterized by a plurality of attached to the outside of the ring with the same mutual angular distance, also vertically from top to bottom ¬ changing outer axial blades.

With the same or different number of inner axial leaves with respect to the drive shaft, the outer axial leaves can be mounted in the same angular positions as the inner axial leaves or also in angular positions between the inner axial leaves. In the latter case, it is expedient to make the ring comprising the inner axial leaves stronger. The number of inner or outer axial blades is preferably chosen between three to six. The ring is required to ensure the flow between the guide tube and the inner axial blades.

From DE-PS 369 362 a stirring element in the form of a tube section is already known, which is covered with threaded blades inside and outside over the entire axial length of the tube section. However, this known stirring element cannot be used for self-priming ventilation. det, because the threaded blades on the inside cause an upward flow and the oppositely acting threaded blades on the outside cause a downward flow. In contrast to this, according to the characterizing part of claim 1 in the subject matter of the invention, the outer axial leaves are designed and arranged in such a way that, like the inner leaves, they also convey vertically from top to bottom. As a result, the particular advantages explained in more detail below, namely, above all, a self-priming ventilation with maximum distribution depth of the gas bubbles over the entire mixing container are achieved with the invention.

The geometric design of the outer axial blades is not tied to a fixed wing shape. A substantial improvement in the mixing is achieved in almost all of the wing shapes which are expediently possible. However, it has been shown that the so-called tip chord angle or tip pitch angle of the wing blades should generally be smaller than that of the inner axial blades. Even with a tip chord angle of only 5 ° there was a very substantial improvement in the flow within the mixing container. Increasing the tip chord angle to 16 ° results in a higher power input at the same speed. Six outer axial blades also increase the power input compared to only three blades.

The diameter ratio of the entire vane arrangement to the ring diameter also has a certain influence on the gas input.

On the basis of the invention, the degrees of freedom of the structural design become greater by increasing the variable structural parameters, so that the ratio of liquid shear. mechanical shear and pumping capacity can be customized individually and much better for each process. Larger inner axial blades, for example, ensure a larger gas input, higher shear and a larger power input. Smaller outer axial blades increase the power requirement only slightly, but already improve the flow conditions considerably. On the other hand, with larger outer leaves

SPARE BLADE (RULE 26) the emphasis is on mixing and the solid suspension with reduced mixing times.

Larger tip chord angles increase fluid shear. The exit angle is influenced by the type of wing for the axial blades. Its shape can depend on the container geometry or on the liquid properties. If an increased amount of gas is desired, the covering of the agitator becomes more and more important, so that, for example, six inner and outer axial blades at an high gas rate can represent an optimum in terms of gas entry, mixing and solid suspension.

The invention and advantageous details are explained in more detail below with reference to the drawing in exemplary embodiments. Show it:

1 shows a first embodiment of a gas / liquid mixing device with features according to the invention;

FIGS. 2 and 3 show two different embodiments of a stirring element with the features characterizing the invention;

4 shows a laser scan diagram of flow velocity vectors within a mixing container (provided with a viewing window) to clarify the advantageous ones

Effect of the outer axial blades;

5 shows a similar laser scan diagram of the flow velocity vectors within the mixing container to illustrate the effect of the inner axial blades;

SPARE BLADE (RULE 26) 6 shows a modified embodiment of a mixing device according to the invention, in which, on the one hand, additional constructive measures are provided for the targeted promotion of a distributed formation of troughs and, on the other hand, the possibility of controlled gas introduction under pressure is provided by means of an additional device or as an alternative;

7 to 9 show different embodiments of vertical sheets attached to the outside of a guide tube of a mixing device according to the invention in a star shape as baffles for the production of selectively controlled troughs;

10 to 12 already known embodiments of

Turbo mixing systems and

13 shows a laser scan flow velocity profile for a conventional mixing device when using the essentially the same mixing container to illustrate the advantageous effect of the invention when compared with the laser scan diagrams of FIGS. 5 and 6.

A comparison of FIGS. 11 and 1 immediately shows that the motor 1, the bearing housing 2, the flange plate 6 and the guide tube 3 and the drive shaft 4 in the same or similar embodiment are also used in the first exemplary embodiment according to FIG. 1 according to the invention available.

As stated in the above definition of the invention, the underlying task was to bring gas bubbles to greater depths and to achieve a practically complete suspension of solid particles by simply adding a plurality of on the outside of the ring 10 (cf.

SPARE BLADE (RULE 26) 2 and 3) achieved with the same mutual angular spacing attached further mixing blades or axial blades on the shaft 4.

As can be seen from the laser scan of the flow velocity profile for a known downward-conveying axial flow, the velocity vectors above the downward-moving axial agitator are always smaller than those on the underside. From this, however, it can be concluded that the further stirring element 7 according to the prior art (cf. FIG. 12) can only bring about a slight improvement. The introduction of gas bubbles on the pressure side (underside) would be much more sensible, but is not possible because the stirring element 5 draws the gas through the guide tube 3.

The invention solves the problem in a very simple manner, namely by the attachment of axial blades 9 on the outside of the ring 10. This very simple solution means that the stirring elements 5 and 7 according to FIG. 12 are provided by the only stirring element 8 (FIG. 1) outer axial blades 9 replaced.

2 and 3 show two possible configurations. 2 illustrates the mechanically more stable design, in which the outer axial blades 9 are attached at the same angular positions with respect to the drive shaft 4 as the inner axial blades. In the embodiment according to FIG. 3, the outer axial blades are in the middle between the inner blades. This configuration requires a more stable ring 10 and results in a slightly higher weight of the entire wing arrangement. However, configurations with, for example, six outer axial leaves 9 and up to nine inner leaves are also possible. It was found experimentally that the suction effect increases with the number of leaves.

The inclination of the outer axial blades 9, that is to say with regard to the tip chord angle, can be freely adjusted in order to be able to set the power consumption required for the respective mixing process. If the optimum is determined for a certain application area, provision can be made to weld the axial blades 9 firmly.

The graphically reproduced laser scan diagram of FIG. 6 clearly shows that the outer blades or axial blades 9 extend beyond the diameter of the guide tube 3. As a result, they rotate more in liquid than in the gas phase. You have the tendency to pump the liquid without being flooded, even if the inner axial blades are flooded. As can be seen from the speed vector diagram of FIG. 5, the outer axial blades 9 produce a ring of high axial flow around the guide tube 3, which extends up to three wing diameters under the entire stirring system. This prevents any backflow directly under the wing, which would lead to a premature stall and then to flooding.

The laser scan diagram of the flow velocity vectors according to FIG. 6 shows an exemplary embodiment for the agitating element of the inner axial blades with blades loaded at the tip ends. However, the invention is not restricted to this design of the inner axial blades. In order to prevent radial flow at the interface of the gas-liquid phase, however, the ring 10 should be present in any case which also ensures separation of the gas dispersion zone and the zone of the axial flow in the outer region of the inner axial blades. The following comparison test was carried out:

The embodiment according to FIG. 1 was tested in comparison to a conventional turbo mixing system according to FIG. 11. A container with the dimensions 4 mx 4 mx 4 m was used as the test tank, ie the liquid volume was 64 m ³ ; the liquid was water and air was used as gas. In the conventional system according to FIG. 11, the lower stirring element 7 had a ground clearance of approximately 1 m. The distance between the two stirring elements 5 and 7 was about half of the stirring element diameter. The diameter of the guide tube 3 was 180 mm in both cases with an immersion depth of approx. 2.7 m. In the conventional device according to FIG. 11, the system at 800 rpm with an estimated power consumption of 3 kW was unable to bring the air bubbles to the bottom, since there was effectively no more flow caused by the stirrers. The rising air bubbles caused a slight movement in the container. Solids could find and deposit low-speed zones.

The mixing device according to the invention used for comparison is shown in FIG. 6. The guide tube 3 had a diameter of 115 mm. The agitator β had, similar to that shown in FIG. 3, three outer axial blades 9 with a tip-chord angle of 5 °. The total diameter of the stirring element 8 was 340 mm and was arranged approximately 0.5 m below the water surface.

At a speed of likewise approximately 800 rpm and a measured power requirement of only 1.5 kW, the stirring element according to the invention not only sucked in more air visually, but rather an axial flow resulted in the entire container; the surface was moving and the observation window on the container showed that the gas bubbles were literally shot against the bottom of the container. Solid particles could not settle anywhere on the container. The air bubbles were pressed down far over 3.5 m or more than ten stirrer diameter. The gas bubbles were first pushed downwards in a jet-like cloud, which corresponded approximately to the diameter of the guide tube, which then opened in a cone.

A major advantage of the device according to the invention is that the residence time of the gas bubbles in the mixing container is significantly increased, since the gas bubbles are pressed approximately from the surface to the bottom of the mixing container and then covered again all the way to the surface. After the mixing device was switched off, it took about 40 seconds for the last gas bubbles to escape from the mixing container. This is a lot longer than with a conventional mixing device. The reason is that

REPLACEMENT TT (REGE the gas bubbles in the conventional turbo stirrer only form very close to the bottom of the container and then only cover the upward path. The residence time is approximately doubled with the mixing device according to the invention.

It was also observed that the gas bubbles are largely evenly distributed throughout the mixing container. In other words, the effective distribution diameter is much larger than the diameter of the mixing container used for the tests. This effect was achieved with a power input of only 1.5 kW / 64 m ³ - a result for which at least twice the power input would be required with a comparable conventional mixing device.

A similarly favorable result, which is considerably more advantageous for certain applications mentioned briefly below, was also achieved if the gas to be mixed in was supplied under pressure. In this case too, the gas bubbles were still shot to the bottom of the mixing vessel and then distributed evenly throughout the container.

As can be seen from Fig. 6, the mixing device can preferably be provided above the liquid level with a connection 15 for feeding gas into the guide tube 3 under pressure. The main advantage of convection thereby forced is that the amount of gas or gas rate to be introduced or dispersed into the liquid is no longer dependent on the differential pressure developing within the guide tube immediately above the axial blades 8, the suction effect being a function of the agitator speed , the size of the stirrer, the immersion depth and the spacing or annular gap δ between the ring 10 and the guide tube 3. Without this functional dependency, the gas input quantity can be determined and, in principle, is no longer a variable. Under such an alternative operating condition when the gas is supplied under pressure, more gas can be injected into the liquid and the amount of gas dispersed can be better controlled. In experiments, for example, it was found.

SPARE BLADE RULE 26) that with the same drive power for the agitator a good 10-fold gas input (e.g. air) could be achieved at a gas pressure of approx. 10 bar.

A particular advantage of this gas supply under pressure arises, of course, when the invention is applied to pressure vessels if the gas supply, which is not limited to oxygen or hydrogen, has to take place under pressure. The gas supply can then be injected directly into the area of the desired chemical reaction, connection 15 being carried out to the desired reaction point. In this case, no sparge ring is required. With this modification of the invention, the reaction rate can be increased considerably, in particular in the case of rapidly occurring reactions.

Since the mixing device according to the invention brings the gas bubbles to the bottom of the container and develops sufficient flow, long guide pipe systems are no longer necessary. This has an extremely positive influence on the overall mechanical stability of the overall system. However, when the guide tube is shortened, a new problem arises, namely the uneven formation of troughs. An uncontrolled formation of droplets can lead to mechanical problems.

This problem of the uneven formation of troughs could, however, be eliminated with a further improvement of the mixing device according to the invention in that on the outside of the guide tube 3 a plurality of sheets 11 arranged in a star shape with the same mutual angular distance and extending in the axial direction of the guide tube were attached.

These plates 11 arranged in a star shape (cf. FIG. 6) extend approximately from the surface of the liquid to just above the ring 10. The width of these plates 11 was chosen such that the diameter of the entire guide tube including the plates 11 corresponds approximately to that of the axial flow device according to the invention, that is, including the outer axial blades 9. It has now been observed that even then, could be promoted or was promoted if the radial extension of the sheets 1 1 was chosen smaller or larger.

The effect of these baffles, i.e. Sheet 11 is as follows:

Instead of preventing an uncontrollable trough, a plurality of troughs corresponding to the number of spaces between the sheets 1 1 develops, for example six evenly distributed troughs, as shown in FIG. 7. The center of one of these trombones lies on the geometric center of the guide tube 3 and in the middle between two such baffles, that is, between two sheets 1 1 If the outer tips of the outer axial blades 9 are closed, the trumpets hit the center of these outer axial blades 9. This uniform distribution of the trombones makes the entire system considerably more stable.

On the other hand, the trumpets that are created in a controlled manner have a great advantage, namely that the gas input becomes significantly higher, i.e. up to a factor of 10 higher than the gas input through the guide tube 3. In one experiment it was possible, for example 200 1 / min. to press over the guide tube 3. However, the approximately 10-fold amount was introduced via the trombones without the system being flooded.

The mixing device according to the invention can also be used, in particular because of the comparatively low manufacturing price, for a large number of mixing processes. One category will be one in which the price plays a major role, so that compressors and saving systems which have to be installed below the liquid level are not used. For example, sewage systems fall into this category, as do systems for aerating lakes, for example for fish and crab breeding. The new mixing system can be used just as well where there is a dangerous situation due to an increase in pressure, for example when handling ozone. Another area of application is high pressure reactors and fermentation tanks due to the fact that the gas can be introduced over a height difference of 4 m and more even with a relatively small immersion depth.

The disadvantage of self-priming stirring systems that has hitherto existed for some applications, namely the comparatively very low gas rate, is overcome with the invention, since the system does not flood even when gas is introduced under pressure through a closed guide tube. In this case, there is a further advantage that no separate piping is required and, accordingly, blocked outlet holes cannot occur at all.

Claims

PATENT CLAIMS 1. Device for mixing gases and liquids with - a guide tube (3) and which is immersed in the liquid and open below and above the liquid level - A drive shaft (4) axially penetrating the guide tube (3) for an axial flow (5) arranged at a fixed distance below the lower guide tube opening as a conveying and stirring element with a plurality of vertically encircled by a rotating ring (10) inner axial blades conveying from top to bottom, characterized by - A plurality of outer axial blades (9) arranged on the outside of the ring (10) with the same mutual angular distance and also vertically conveying from top to bottom. 2. Device according to claim 1, characterized in that the outer axial blades (9) with respect to the drive shaft (4) are attached in approximately the same angular positions as the inner axial blades. 3. Apparatus according to claim 2, characterized in that the outer axial blades (9) with respect to the drive shaft (4) are mounted in angular positions between the inner axial blades. 4. Apparatus according to claim 2 or 3, characterized in that the number of inner and outer axial blades is chosen the same. 5. Apparatus according to claim 2 or 3, characterized in that the number of inner and outer axial leaves is selected differently. E 6. Apparatus according to claim 2 or 3, characterized in that the number of inner axial leaves is chosen to be larger, in particular in the range from 6 to 9 and the number of outer leaves is smaller, in particular in the range from 3 to 6. 7. Device according to one of the preceding claims, characterized in that the number of the respective axial blades is between 3 and 9. 8. Device according to one of the preceding claims, characterized ge indicates that the diameter of the ring (10) corresponds approximately to that of the guide tube (3). 9. Device according to one of the preceding claims, character- ized in that on the outside of the guide tube (3) a plurality of with the same mutual angular spacing arranged in a star arrangement, in the axial direction of the guide tube (3) extending sheets (11) for Current disturbance and controlled formation of trombones are appropriate. 10. The device according to claim 9, characterized in that the sheets (11) extend substantially over the entire axial length of the guide tube within the liquid. 1 1. Apparatus according to claim 9 or 10, characterized in that the diameter of the guide tube including the star-shaped baffle plates (1 1) corresponds approximately to the diameter of the agitator including the outer axial blades (9). 12. Device according to one of the preceding claims 9 to 1 1, characterized in that the sheets (1 1) are curved. 13. Device according to one of the preceding claims, marked by a device (15) through which gas can be introduced under pressure into the guide tube and / or directly into the liquid. 14. Device according to one of the preceding claims, characterized ge indicates that the lower end of the guide tube (3) and the ring (10) of the stirring element overlap axially to form an annular gap (δ), the annular gap width also determining the gas entry rate. SPARE BLADE RULE 26