The invention relates to a device for cleaning gases by a liquid, i.e. a scrubber, of the type described in the preamble of the appended claim 1.

Cleaning of gases is needed either for separation of dust from a gas flow or for separation of gaseous chemical components from the gas flow, or for both purposes simultaneously. In practice, the last-men¬ tioned is the case e.g. in the cleaning of flue gases, whereby acidifying gaseous components are separated from the gas and so called fly dust present in the flue gases is separated in the scrubber simultaneously.

A scrubber is a previously known device. One known scrubber type is the so-called cyclone-shaped scrubber, where the gas flow is brought into a circulating motion. Thus, by the cyclone principle, the dust in the gas is washed onto the walls of the cyclone and away with water. In the present devices, however, the properties of the gas flow, e.g. the possible circul¬ ating motion, are controlled by the rate and inlet of the gas flow entering the scrubber. Further, because of the placement of nozzles in known scrubbers, maintenance of nozzles one at a time is not possible during operation of the scrubber.

Further, a scrubber is known where gas can be brought into cyclonic motion with no need to make special arrangements for the gas inlet. In this scrubber type, the circulating motion of the gas flow is substantially generated by tangential nozzles on the perimeter of the scrubber, whereby the generation of the tangential motion is independent of the flow rate of the gas input in the scrubber, because the mass of the liquid input is high in comparison with the gas input. The nozzles are placed in two separate stages, subsequent

when seen in the direction of the main gas flow. Between the stages, there is a narrow throat in the main flow direction of the gas flow, which is provided by shaping the scrubber so that the compartments for the subsequent stages taper towards the throat, whereby the flow opening at this point is formed of the narrow ends of the two compartments opposite each other. A venturi scrubber of this kind is known e.g. from German Application Publication No. 2045021.

It is an aim of the invention to present a scrubber, in which a reduced cross-sectional area between the stages is achieved by a simple construction, allowing a better design of the compartments in a desired way. For attaining this purpose, the invention is primarily characterized in what will be presented in the charac¬ terizing part of the appended claim 1. The cross- sectional flow area between the compartments of the scrubber is reduced by a blocking wall placed transver- sely to the main flow direction in a flow opening between the compartments, partly blocking the cross- sectional flow area of the flow opening. The blocking wall is placed advantageously in the center of the flow opening, whereby a free cross-sectional flow area remains between edges of the blocking wall and the inner wall of the cleaning space. In this area, advantageously, guides are placed to direct the gas in the direction of the circumferential circulating motion of the gas.

In an advantageous embodiment, also the nozzles of the second group are directed tangentially. In an advantageous embodiment, the scrubber is disposed in a way that the lowermost wall parts in the compartments of each stage slope gently down from the flow opening between the compartments, whereby the washing liquid being separated from each stage also tends to collect at the lowermost point of the respective compartment,

where the discharge point of the washing liquid is located. Thus the scrubber can be placed in horizontal position, i.e. the main flow direction of the gas is horizontal.

In an advantageous embodiment, the nozzles of at least one group of nozzles are fixed individually onto the walls of the cleaning space, to be replaceable irrespective of each other. Thus the maintenance of the device is simple, because the nozzles are accessible from the outside of the scrubber, and for instance a group of nozzles located inside a scrubber do not need to be demounted as a whole group for maintenance. Further, the nozzles are advantageously provided with their respective closing valves, whereby they can be changed during the operation of the scrubber.

The invention will be described more closely in the following with reference to the appended drawings, wherein

Fig. 1 shows a scrubber of the invention in a cross-section taken along in the main direction of the gas flow, Fig. 2 illustrates the direction of the nozzles, and

Fig. 3 shows the structure of the flow opening between the compartments in the scrubber of Fig. 1.

The scrubber of Fig. 1 comprises a cleaning space 1, limited by walls, in which the gas and the washing liquid are brought into contact with each other. The shape of the scrubber is a body of revolution symmetri¬ cal in respect of a central axis parallel to the main direction W of the gas flowing through the space 1. The end perpendicular to the axis is provided with a tubular inlet 2 for gas coming from the process, e.g. from a combustion boiler, the opposite end having a

corresponding tubular outlet 3 for cleaned gas. In the main flow direction W of the gas, the cross-sectional flow area between the inlet and the outlet is first tapered and then broadened again towards the outlet. Thus, two compartments la and lb of the cleaning space are formed, separated from each other by the flow opening 4 at the narrowest point and forming subsequent stages for the gas flowing through the cleaning space. The scrubber has thus a general shape of two truncated cones with their narrow ends placed against each other.

The wall of the first compartment la is equipped with nozzles 5 along the perimeter of the cleaning space, forming the first group of nozzles. As shown in Fig. 2, the nozzles of this group are circumferential by orientated so that their liquid flow (shown in broken lines) generates in the gas entering the area of influence of the group of nozzles a component which is parallel to the perimeter of the cross-sectional area perpendicular to the main flow direction, i.e. the gas is, in the compartment la forming the first stage, brought into a circumferential cyclonic movement. The nozzles are simultaneously directed diagonally to the main flow direction of the gas, but they can also be directed perpendicular thereto.

In this context, the concept "circumferential" or "parallel to the perimeter" means deviating from the radial direction, whereby the deviation between the directions is advantageously more than 30°, most advantageously more than 45°.

From the flow opening 4, the gas enters the second stage formed by the compartment lb, which has a second group of nozzles formed by nozzles 6. In the same way as the nozzles of the first group, these nozzles are parallel to the perimeter of the cross-sectional

area perpendicular to the main flow direction, as shown in Fig. 2, and in the axial direction also diagonally to the main flow direction W of the gas. Thus a motion component is generated in the gas in the middle of the compartment lb, directed towards the closed end of the compartment, as well as a peripheral back flow to the area of influence of the nozzles 6, the gas flows having met the end wall of the compartment (these axial flows are shown by broken lines) . The nozzles 6 directed towards the end wall also facilitate raising the static pressure in the second compart¬ ment lb.

The flow opening 4 is equipped with a blocking wall 11 which is placed transversely to the main flow direction of the gas to reduce the cross-sectional flow area of the opening 4. Thus a desired throttle is provided at the opening 4 to prevent the scrubber from discharg¬ ing gas directly from the first to the second co part- ment. At the same time, the desired volume and shape of the compartments can be maintained, because the free cross-sectional flow area of the throttling point is not dependent on how steeply the compartments la and lb taper towards the opening 4. The blocking wall 11 is formed of a disc disposed in the middle of the opening 4, whereby the free, reduced cross-section¬ al flow area remains as an annular area between the edges of the blocking wall and the inner wall of the scrubber.

Fig. 3 shows the structure of the part comprising the blocking wall 11 in more detail. From the edges of the blocking wall, guides 9 formed of suitably shaped blades extend outwards to be situated in the reduced point of the cross-sectional flow area. These are used to secure that the circulating motion of the gas is continued also in the second compartment lb. The guides are thus placed at an angle to the axial

direction, e.g. at an angle of 45°. The blocking wall 11 and the guides 9 are placed inside a short cylindrical element 12, the outer ends of the guides 9 being fixed to the inner wall thereof. The element 12 can be fixed to the scrubber by a flange 12a extending outwards therefrom, which, during assembly of the scrubber, is installed between the open ends of the scrubber halves forming the compartments la and lb, as shown in Fig. 1.

Another alternative is to install the blocking wall by its edges to the inner wall of the cleaning space, whereby an opening in the center of the blocking wall forms the free cross-sectional flow area.

Fig. 1 also shows outlets for the washing liquid, placed at the lowermost points of the compartments of the cleaning space 1 placed in a horizontal axial position, the outlets 7 and 8 in the compartments la and lb, respectively. The washing liquid, collected centrifugally on the walls of the compartments, flows slowly by gravitation to the respective outlets.

If the scrubber had a vertical structure, a cyclonic- type scrubber could not be successfully used to separate the washing water flows so that the water from the first stage would not flow to the second stage by gravitation, or vice versa, depending on the gas inlet. Thus, the advantage of staging is lost in certain types of separation. For instance, the gas may contain two chemical components, the first of which excludes the dissolution of the second one in the water phase. If the excluding component can be separated at the first stage, there are no obstacles at the second stage for the separation of the remain¬ ing component, excluded at the first stage, provided that also the discharged liquids are separated. This is the case when gas flows containing e.g. hydro-

chloric acid and sulphur dioxide are washed by water. Hydrochloric acid is dissolved in water much better and prevents the partial pressure of sulphur dioxide from falling to a sufficiently low level. Thus at the second stage, after the liquid flows have been fully separated from each other and hydrochloric acid has been discharged, the remaining small amounts of sulphur dioxide can be separated which would otherwise not be separated. In the above-mentioned case, the nozzles of both stages can have a common liquid input, because the same washing liquid can be used. A separate liquid input for each stage is used when the different components of the gas require different liquids at subsequent stages.

As the scrubber can have a horizontal structure, the washing stages can thus be fully separated for the liquid flows. However, the other properties of the scrubber can be applied to constructions in other posi- tions as well. It should also be noted that the scrubbers of Fig. 1 can be placed one after another, whereby the number of washing stages can be increased. Further, it is possible to construct a scrubber with three or more subsequent compartments, separated by a reduced cross-sectional flow area. For instance in a horizontal scrubber type, the middle compartments can taper towards both ends, whereby the outlets are placed in the broadest part, at the lowermost point of the bottom. A blocking wall can be installed between any successive compartments of such a scrubber with multiple compartments.

The perimeter of the scrubber is provided with so many nozzles 5 and 6 that the washing operation is not substantially impaired by disconnection of any single nozzle. The housing of the nozzles is con¬ structed so that the inner part 5a, 6a (Fig. 1) , provided with the water nozzle, can be disconnected

from the outside of the scrubber, the respective water pipe being closed by a separate valve 10 (Fig. 2) , whereby the maintenance of the nozzles is possible without stoppage of the scrubber. In many applications, such as in power plants, the continuous operation of the scrubber is fully necessary for economical reasons.

The compartments la, lb forming the stages of the scrubber are thus separated from each other by an opening 4 with reduced cross-sectional area of the gas flow. This point facilitates the full separation of the stages from each other. It is thus possible to process the gas at two stages so that the gas is brought at the first stage of the scrubber (compartment la) to its dew point. The gas flow velocity is increased at the reduced cross-sectional flow area of the opening 4, whereby the static pressure is reduced. Because the gas contains a small quantity of liquid droplets, some more liquid is correspondingly vaporized in the gas when the pressure is lowered at constant vapour pressure of the liquid. At the second stage (com¬ partment lb) , the flow velocity being slowed down again, the static pressure is increased according to Bernoulli's law and because the nozzles 6 of the second stage are directed at an angle with the axial direction towards the end wall of the compartment lb. Liquid is thus condensed from the gas phase on all surfaces, including the surfaces of small dust par¬ ticles in the gas flow. The mass of the particles is increased, facilitating the separation at the rear part of the scrubber.

As the nozzles, some nozzle types known in the field can be used which produce a liquid spray of a desired shape, e.g. at an angle of 30 to 90°.