SE463699B - PROCEDURE AND DEVICE FOR GAS CLEANING - Google Patents

Abstract

A method and an apparatus are used for removing solid, liquid and/or gaseous pollutants from a gas or gas mixture. By means of a gas distributing system (14), the gas is passed below the surface (34) of a bath (32) of washing or cleaning liquid in such a manner that it is caused to rise through the bath (32) in several partial flows, each partial flow having a volume flux independent of the size of the total gas flow. The polluted gas is caused to bubble through the liquid bath at a velocity which is higher than the velocity at which a free bubble rises through the bath. Each partial flow of the polluted gas is completely or almost completely screened off by partitions (48) from adjacent parts of the bath (32). This screening is preferably performed during a substantial part of the passage of the partial flows through the bath (32).

Description

463 699 2 physical phenomena, sedimentation and diffusion, whereby particles in the size range 0.5-10 μm are only slightly affected. Such a filter will therefore be relatively large and entails a considerable cost. Despite this, a relatively poor separation of the finest particles is obtained, especially particles below 0.5 μm. Furthermore, such a rock bed filter has poor efficiency in the separation of gases, which here takes place by adsorption on the rock material.

According to the present invention, it is proposed to use wet cleaning by means of a liquid bath, wherein the contaminated gas is introduced below the surface of the liquid. The separator will thus function as a water trap, which when there is no overpressure in the space connected to the gas purifier keeps it separate from the surrounding atmosphere.

In order to obtain a good cleaning efficiency in a wet cleaner, an intimate mixture between liquid and gas in the cleaner is required. The purification liquid must therefore be distributed as fine droplets in the gas or the gas must be caused to pass through the liquid in the form of small bubbles. For wet cleaners where the gas is introduced under a liquid surface, it also applies that the cleaning efficiency is very flow-dependent. Good purification is obtained only within a relatively narrow flow interval. This offers special problems with an emergency system, as these systems must be able to provide satisfactory purification regardless of the flow without any active control or regulation.

A device where the polluted gas is lowered below the surface of a liquid bath and purified when passing up through the liquid bath is, however, for other reasons very suitable as an emergency system. Such a device can be made to function completely passively and can be constantly ready for operation, as well as automatically come into operation in the event of an accident or malfunction.

Gas purification devices have previously been proposed in which a contaminated gas is introduced below the surface of a liquid bath and bubbles out through a number of openings or nozzles. However, these previously proposed devices have a poor cleaning effect because usually relatively large low-velocity air bubbles pass through the liquid bath and there is therefore poor contact between liquid and gas.

In such a known purification device according to DE-PS 228 733, incoming 463 699 3 gas flow is divided into a number of partial flows and is caused to strike impingers or abutment surfaces like fine jets. However, this device has the above-mentioned limitation that it has low efficiency at low gas flows, since the gas will then bubble up through the liquid relatively unaffected in the form of large bubbles. For such a system to provide high purification, gas velocities of 50-100 m / S are required.

Wet cleaning devices have further been proposed according to USA 3,216,181 (see Fig. 4), USA 3,520,113 (see Fig. 7) and USA 4,182,617 (see Fig. 2), where the gas is led under the surface of the liquid through a number of openings. at gradually increasing depths. With increasing inlet pressure, the incoming gas presses down the liquid surface in the inlet line until the first outlet opening is exposed and the gas is caused to flow up through the liquid. With increasing flow, the liquid surface is pressed down further and more openings are exposed, so that the gas can flow out through these as partial flows. Thus, to a certain extent, the flow variations can be prevented from affecting the purification efficiency.

These prior art devices have the disadvantage that when the liquid surface in the inlet line is pressed down under an outlet opening, the gas will flow out into the purification bath at low speed. The gas will therefore flow up through the liquid as relatively large, slowly rising bubbles.

There is therefore a poor separation of fine particles and gaseous pollutants. The object of the present invention is to provide an improved gas purification process of the above kind. A method that provides intimate contact between gas and purification liquid, as well as high purification efficiency both at partial load and full load, and which has a high capacity within a small device volume. A further object is to provide a device for carrying out the method.

In order to achieve this object, the method according to the invention is characterized above all by the features stated in the characterizing part of claim 1. A device for carrying out the method according to the invention is characterized above all by the features stated in the characterizing part of the first device requirement.

In the process and the device according to the invention, a high purification efficiency is always obtained regardless of the size of the total gas flow, by causing the polluted gas to pass through a number of inlet openings adapted to the size of the gas flow at a gradually increasing depth below the liquid surface and thereafter. through higher located outlet openings and then in the form of bubbles are caused to flow up through the liquid.

Usually this means that the speed of the bubbles is quite low and that the separating capacity especially for particulate pollutants is very low. According to the present invention, however, the bubbles are caused to rise upwards at a speed which is substantially higher than that at which an undisturbed bubble rises upwards.

The proposed method is based on the surprising discovery that when a stream of bubbles passes through a liquid at a speed which at the surface passage is higher than 0.1 m / s, by surface velocity is meant the volume flow of the gas divided by active liquid surface, such a violent interaction occurs between the bubbles, merging and decomposition, that a very good particle separation is obtained in the liquid bath.

However, this velocity is not achieved in a freely rising flow of bubbles.

These rise in water at a speed not exceeding 0.3-0.5 m / s, and in view of the fact that the bubbles occupy only a small part of the volume in the liquid, the surface velocity becomes significantly lower.

According to the proposed procedure, however, the rising speed of the bubbles can be significantly increased. This is achieved by introducing lateral constraints, so that a partial flow of bubbles from an outlet opening cannot spread over more than a predetermined area. The area limitation increases the possibility that the bubbles reach a high velocity, but this does not prove to be sufficient for a satisfactory degree of separation to be achieved.

Therefore, in order to further increase the rising speed of the bubbles, it is proposed according to the present invention that the space for rising bubbles is limited, so that a substantial part of the surface of the bath is not passed by rising bubbles even at maximum load. By this method, which is illustrated in the example below, a mammoth pump effect can be obtained.

The bath is therefore divided into a number of cells, hereinafter referred to as riser cells, in which bubbles rise upwards, and between these arranged spaces, hereinafter referred to as backflow channels, into which no bubbles are introduced. The riser cells are separated from each other and from backflow channels and surrounding passive fluid with shields. These shields consist of walls which completely or almost completely enclose a partial flow of gas bubbles during the passage from the outlet opening to the liquid surface. The walls preferably end slightly below the level of the surface of the liquid bath as this is not loaded.

When a certain riser cell is loaded, the flow of gas bubbles causes the liquid level in the cell to rise because the static pressure at the lower edge of the shields must be maintained at the value determined by the surrounding liquid.

This value is determined on both sides by the shielding of the weight of the liquid column extending from this level up to the surface. Since the presence of gas bubbles reduces the bulk density in the riser cell, this must be compensated by a higher level of liquid surface in the cell. If this means that equilibrium is reached at a level that is higher than the upper edge of the shields, liquid will overflow into adjacent spaces, backflow channels.

There will also be no equilibrium at the lower edge of the screens.

The pressure in the riser cell will be lower there than in the surroundings, which is why new liquid flows in.

In this way a liquid flow upwards through the riser cell is obtained. Due to this flow, the bubbles will rise at a higher speed, as the speed given by the lifting force of the displacement is added to the speed of the upwardly flowing liquid.

Repeated experiments have shown this to give an unexpectedly intense interaction between the bubbles and a surprisingly large improvement in the degree of separation, especially for small particulate pollutants.

It has also proved to be of little importance how the nozzles through which the gas is released into the liquid are formed. After a short distance, for example 0.5 m, the flow pattern is essentially the same for highly varying outlet nozzles. A significant advantage is thus offered by the proposed procedure in that cheap distribution devices can be used.

In order to obtain good separation at varying flow of polluted gas, it is proposed that the polluted gas be distributed to riser cells via a distribution device with inlet openings arranged at successively increasing depths, for example as shown in the following description of an embodiment of the invention.

A consequence of such an arrangement is that the flow through an individual riser cell increases slightly as the total gas flow increases, since this means that the pressure drop across the outlet devices increases due to the liquid level inside the distributor being lowered. However, this does not impair the separation, rather the opposite, since the rising speed of the bubbles then also increases. However, the variation in efficiency that this leads to can be kept small if the vertical distance between the inlet opening and the outlet opening is of the same order of magnitude as the level difference between the highest and lowest located inlet openings, or at least not significantly smaller.

The invention will now be described in more detail with reference to the accompanying drawings.

Figure 1 shows a vertical section of a first embodiment of a gas purification device according to the present invention.

Figure 2 shows a cross section in section along the line II-II in figure 1.

Figure 3 shows a detail view of a manifold with nozzle included in the gas purification device according to figure 1.

Figure 4 shows a vertical section of a second embodiment of a gas purification device according to the invention.

Figures 1-3 schematically show a gas cleaning device with a pressure vessel 1, which consists of a cylindrical outer wall 10, a slightly curved bottom 11 and a curved lid 13. The gas cleaning device has an inlet 12 for polluted gas, connected to a distributor 14, an outlet 16 for purified gas and a means 84 for supplying and removing liquid, to and from the pressure vessel 1, respectively.

The manifold 14 consists of a number, in example six, of radial, obliquely downwardly directed manifolds 18. The manifolds have an outer, vertical, downwardly directed pipe section 20, with a downwardly directed free opening 22. On their oblique parts 465 699 to transverse distribution pipes 26, connecting risers 24, which v * gas inlets or inlet openings 64 are connected to the manifolds 18.

The distribution pipes 26 are, as best seen in Figure 3, provided with nozzles 28 with outlet openings or orifices 31.

The distributor 14 is immersed in a liquid bath at a certain depth, depending on the desired degree of purification, below its surface 34 and has an inner liquid surface 46, the position of which depends on the pressure of the incoming gas. When the purification device comes into operation, the gas will flow out into the liquid with a pressure drop corresponding to the height difference h between the respective outlet opening 31 and the inner liquid surface 46.

A number of partitions 48 are so arranged in the liquid bath 32 that a riser cell 50 is formed around each nozzle 28 and that reflux channels 52 are formed radially between the riser cells 50. The partitions 48 have the task of enclosing the upwardly flowing gas for a large part of the passage to the surface, but does not necessarily have to be quite dense. It may be advantageous for liquid from the backflow channels 52 to be sucked into the rising stream of bubbles.

As shown in Figure 2, the backflow channels 52 together occupy a smaller area than the riser cells 50. The ratio between the total area of the backflow channels and the total area of the riser cells is between 0.05 and 0.5, preferably between 0.05 and 0.2. An individual riser cell has an area which may be between 0.01 and 0.10 square meters, preferably between 0.01 and 0.04 square meters.

Figure 2 also shows that the partitions 48 are supported by radial support walls 54 arranged between the manifolds 18.

In the embodiment shown in Figure 1, the outlet openings 31 of all nozzles 28 are at the same level in the pressure vessel 1. The distance between the liquid surface 34 and the outlet openings 31 is between 1 and 10 m, preferably between 2 and 5 m. It should be noted, however, that The decisive factor for gas purification is the height of the partitions and not the depth of the bath. The upper edge of the partitions 48 is slightly below the level of the surface 34 of the bath in the unloaded condition. The position of the upper edge should be adapted to the operating conditions so that variations in the amount of liquid due to condensation of, for example, water vapor in the polluted gas or evaporation of washing liquid do not affect the function in a disturbing manner. Therefore, if the contaminated gas is hot, the partitions 48 should close some distance below the liquid surface 34, preferably 0.1 to 1.0 m.

The lower edge of the partitions 48 lies at a level below the outlet openings 31 of the nozzles 28, this means that the gas flowing out through a certain nozzle 28 can only rise through the liquid in the riser cell 50 corresponding to this nozzle 28.

In the embodiment shown in Figure 4, the outlet openings 31 of the nozzles 28 are not at the same level. There, instead, the distance between the outlet openings 31 and the inlet openings 64 is the same for all nozzles 28. This thus means that with increasing load, the outlet openings of the nozzles taken into operation are at a gradually increasing depth. The lower edges of the partitions 48 are correspondingly adapted to this increasing depth so that the partitions 48 also in this case extend to a level below the outlet opening 31 of each nozzle 28. However, the upper edges of the partitions 48 are in this case all at the same level.

The function of the gas purification device is as follows. In the unloaded state, washing liquid penetrates through the openings 31 of the outlet nozzles 28 and through the lower openings 22, on the vertical parts 20 of the distributor tubes 18, so that the inner liquid surface 46 in the distributor 14 is at the same level as the outer liquid surface 34. water trap function. In the event of a gas discharge malfunction, the pressure in the inlet 12 increases and the inner liquid surface 46 is pressed down until it falls below a sill edge 65 at the highest inlet port (s) 64 to the nozzles 28. The gas then exits through the outlet ports 31 of these outlet nozzles 28. and rises in the form of bubbles through the washing liquid in the riser cells 50 surrounding the active nozzles 28. The washing liquid in the riser cells 50 in question is caused to flow upwards by the mammoth pump effect (see description introduction) obtained by cooperating with the liquid in the backflow channels 52 adjacent to the riser cells.

Due to this liquid flow, the gas bubbles rise through the actual riser cells at such a high speed that their surface velocity, i.e. the volume flow of the gas over the surface divided by the active liquid surface, becomes greater than 0.1 m / s, preferably 0.2 to 0.5 m / S. This means that a violent interaction, ie aggregation and decomposition, occurs between the gas bubbles, whereby a very good separation, even for the usually very hard-separated particulate pollutants with a size less than 0.5 micrometers, is obtained. with the gas bubbles rising through the actual riser cells 50 until they pass the upper edges of the partitions 48 after which the washing liquid flows back down through the backflow channels 52. The bubbles continue to rise through the bath 32 and pass through the surface 34 to the space 35 under the lid 1 of the vessel 13 .

From the space 35, the gas is led via the outlet 16, possibly through an additional, not shown, purification device, to a chimney or other emission means.

As the pressure and / or flow of gas entering the gas purifier increases, the inner liquid surface 46 will be further depressed in the manifold 14, whereby a larger number of inlet openings 64 will flow through gas in an increasing number of substreams. This means that the flow through a single riser cell 50 is relatively independent of the total gas flow through the gas purification device.

The space 35 is dimensioned to be able to absorb the volume increase the bath receives due to the rising gas bubbles and, among other things, the condensation that occurs if the polluted gas is moist.

If the contaminated gas is hot, liquid can instead be evaporated, thereby lowering the liquid surface 34. For this reason, as previously mentioned, the upper edges of the partitions should be arranged at a suitable distance below the level of the surface 34 of the bath in an unloaded condition, so that the washing liquid will safely pass over these edges during the time the cleaning device it must be able to operate without monitoring or supply of liquid via means 84.

Separation of gaseous components can be facilitated by mixing suitable substances into the washing liquid, which substances react with the contaminants. 463,699 m For example, for the absorption of gaseous iodine, sodium thiosulphate can be mixed into the washing liquid.

A gas purification device according to the invention can be made arbitrarily large. It may comprise several distributors 14 separated from each other and independent of each other. Furthermore, a gas purification plant may consist of several separate gas purification devices. The latter facilitates maintenance work and enables the shutdown of individual units while maintaining preparedness.

The invention is of course not limited to the embodiments described above, but can be modified in many ways within the scope of the appended claims.

The distributor and associated construction details can, for example, be designed in accordance with the embodiments described in patent application PCT / SE87 / 00420 and in patent application PCT / SE87 / 00421.

The partitions 48 between the riser cells 50 and the backflow channels 52 can further be provided with gaps which allow some limited interaction or with so-called gills, through which liquid can flow into the riser cells 50, but which are so designed that gas cannot bubble out. (ä

Claims (10)

10 15 20 25 30 35 463 699 ll P A T E N T K R A V A method of purifying a gas or gas mixture from solid, liquid and / or gaseous pollutants, the gas being passed below the surface (34) of a bath (32) with washing or purifying liquid by means of a gas distribution system (14) having on gradually increasing depth below the liquid surface (34) arranged inlet openings (64) for it to be brought up through the bath in several sub-streams, each sub-stream having a volume flow independent of the size of the total gas flow, characterized in that the contaminated gas is brought bubbling up through the liquid bath at a rate exceeding that at which a free bubble rises through the bath, by causing each partial stream of the polluted gas during its passage through the liquid bath to flow through a mass of liquid which, completely or almost completely, is shielded from, in the horizontal plane, adjacent parts of the bath, during a substantial part of its passage through the bath (32). Method according to claim 1, characterized in that the partial current is caused to bubble up through the liquid bath at a speed which just above the surface of the bath gives the gas a speed exceeding 0.1 m / s, preferably 0.2 to 0.5 m / s. s. 3. A method according to claim 2, characterized in that all substreams are shielded and caused to pass through the liquid individually, within predetermined areas. A method according to claim 3, characterized in that the liquid carried upwards by the gas substreams, according to the principle of a mammoth pump, is caused to flow over screens (48) back into the liquid bath by each area (50) intended for a partial stream of polluted gas adjacent to an area (52) intended for recirculation. Device for use in purifying gas or gas mixture, in a method according to any one of the preceding claims, comprising a partially liquid-filled vessel (1) with an inlet (12) for polluted gas and an outlet (16) for purified gas and a gas distribution system (14) with inlet openings (64) and outlet openings (31) arranged at successively increasing depths below the liquid surface (34), distributed over the main part of the cross-section of the bath, cooperating with said inlet, characterized in that the outlet 465 10 15 20 The openings (31) are arranged at a substantial depth below the surface (34) of the liquid bath (32) and that substantially vertical shields (48), forming cells (50) for rising bubbles, are provided in the bath, which cells (50) ), completely or almost completely encloses a partial stream of the polluted gas for a substantial part of the distance between the respective outlet opening (31) and the surface (34) of the liquid bath (32). Device according to claim 5, characterized in that spaces (52) in the bath (32) are arranged between and / or next to cells (50) for rising bubbles, under which there is no outlet opening (31). Device according to claim 6, characterized in that the upper edges of the screens (48) are located slightly below the level of the surface of the bath (34) in the unloaded condition. Device according to claim 6 or 7, characterized in that the shields (48) form cells (50) with completely dense walls, which lower openings of cells (50) are located at substantially the same depth below the surface of the bath (32) (34). ) as corresponding outlet openings (31), preferably 2-5 m. Device according to claim 6, 7 or 8, characterized in that all outlet openings (31) are arranged at the same depth below the surface of the bath (34) or that the outlet openings (31) are arranged at successively increasing depths below the surface of the bath (34), so that with gradually increasing flow of polluted gas, the gas distribution system (14) is arranged to distribute gas to successively located outlet openings (31). Device according to claim 6, 7 or 8, characterized in that the shields (48) are arranged with their upper edges at a gradually increasing level, so that at higher loads in operation cells put into operation are surrounded by higher shields (48). L)