FI118899B - A method for washing gas formed by gasification of black liquor - Google Patents

Description

! 118899

Method for scrubbing a gas formed by the gasification of black liquor - Förfarande för tvättning av gas alstrad genom förgasning av svartlut

The present invention relates to a process for recovering chemicals and energy from black liquor obtained in the manufacture of paper pulp by chemically boiling the fibrous raw material.

When the pulp is manufactured by the sulphate process, a waste liquor, commonly referred to as black liquor, containing organic matter and residual cabbage obtained by boiling the fibrous raw material is obtained. Usually, this black liquor is evaporated and passed to a separate process where the energy content of the organic material is recovered and the cooking chemicals are recovered as so-called soda liquor. The so-called Tomlinson process has long been a commercially predominant method of recovering energy and chemicals. However, one of the disadvantages of this already very old process is the fact that it requires very large combustion furnaces, which are complicated both technically and in terms of use.

The Swedish patent SE 448,173 describes a newer process which offers significantly improved energy and chemical recovery and, in addition, much simpler process equipment required. This process is based on a pyrolysis reaction in which black liquor is gasified in the reactor. Results •; Y; It produces a high-energy gas consisting predominantly of carbon monoxide, carbon dioxide, methane, hydrogen and hydrogen sulfide, and inorganic chemicals in the form of tiny droplets consisting mainly of sodium carbonate, sodium hydroxide, and sodium sulfide. The resulting mixture of gas and melt droplets is rapidly cooled in the first step by contacting it directly with the coolant formed from the water and soda liquor formed when the melt chemicals and hydrogen sulphide dissolve in the coolant. The gas is then washed in a second step in a scrubber-type scrubber. The gas is then used to fuel ···: * ·, 30 na steam and / or electric power. The physical calorific value of the gas can also be utilized by cooling the gas from the gasification temperature to the saturation temperature of the water vapor at the selected pressure. For example, at a saturation temperature of 200 ° C corresponding to 40 bar, steam can be made from 3 to 8 bar when cooling the soda liquor, and # · · ** when the gas is cooled and its water content is condensed after the scrubber.

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Although this process is much simpler and less problematic than the Tomlinson process, there is still room for improvement. One disadvantage, for example, is that undesirable hydrogen carbonate is formed in the soda liquor when the carbon dioxide of the pyrolysis gas comes into contact with the soda liquor when the gas and melt droplets are cooled in the first step. Further, the gas leaving the scrubber leaves extremely small, virtually hydrophobic particles present in the gas which cannot be effectively separated by the scrubber of SE 448,173. Another disadvantage is that the recovery of energy from the physical calorific value of the gas cannot be done in a fully optimal manner to produce high-quality process steam, but only relatively medium-pressure steam can be produced in the process.

The present invention further develops the idea of SE patent 448,173 and can effectively overcome the disadvantages of this prior art.

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The development of the process is based on the idea of making soda liquor without the formation of unwanted hydrogen carbonate in this liquor and optimally utilizing the thermal energy content and heat of vapor formation in the gas, while effectively separating the small particles contained in the gas, the so-called smoke particles.

A: The principle is that the mixture of gas and melt leaving the reactor is cooled in a first step by contacting it directly with the coolant, which mainly consists of water in the form of condensate. All intense contact between the gas and: 25 soda liquor formed by the droplets of melt and hydrogen sulphide dissolved in the coolant is avoided as much as possible. The carbon dioxide in the gas is thus prevented from reacting with the sodium carbonate of the soda liquor to form sodium hydrogencarbonate and the carbon dioxide is prevented from reacting with the sodium hydroxide and forming the sodium carbonate. In addition: I *. 30 carbon dioxide is prevented from reacting with sodium hydrogen sulfide to form sodium ·· * *. ···. umohydrogen carbonate and also to avoid desorption of hydrogen sulphide from the reaction between sodium hydrogen carbonate and sodium hydrogen sulfide. It is good if the sodium hydroxide formed * * * \ \ does not convert to sodium carbonate because sodium hydroxide is the desired end product after causticization of the salt. Sodium carbonate is converted into sodium hydroxide during causticization by reacting it with slaked lime.

Instead, the gas scrubbing in the second stage is configured such that the contact between the gas and the scrubbing liquid, which consists essentially of water in the form of condensate, is as thorough as possible. In a preferred embodiment, this object is achieved by quenching in two steps. In the first extinguishing step, gas is not allowed to bubble through the liquid bed which has accumulated at the bottom of the vessel. In the second quenching step, the gas is washed by passing its 10 bubbles through a second layer of liquid, mainly consisting of water in the form of condensate. In this way, hot gas can be effectively purified and saturated with moisture. The energy in the form of condensing heat and thermal energy is then preferably recovered from the hot, moisture-saturated gas using an indirect countercurrent condenser. For example, with a countercurrent film condenser 15, it is possible to efficiently generate high quality steam, preheat the feed water and produce hot water in a single unit. The small particles, or microparticles, which remain in the gas, mainly consisting of sodium carbonate, act as condensation nuclei in the condenser and are therefore effectively separated from the gas, after which they are recovered and dissolved in the condensate.

20: A: It is true that cooling and separating the reaction products from the gasification reaction in two successive quenching steps is already known per se in U.S. Patent No. 4,328,008 (Texaco). However, this patent differs from the present invention in that it relates to another field of application. Incineration of solid fuel · 1 · ·: 25, without the possibility of producing soda liquor, which is the main object of the present invention, and on the other hand in that it does not include the well-known idea of recovering energy from hot, moisture-saturated gas way using a countercurrent condenser. Also, the two extinguishing steps of U.S. Patent No. 4,328,008 are not assembled such that the first:! ·. 30 steps to achieve maximum contact between the gas and the liquid phase product ·· 1 ·. ··, and in the second step, contrary to what is described in the present invention.

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The invention will be described in more detail below, based on a preferred embodiment and with reference to the accompanying drawings.

In the drawings: 5

Figure 1 shows a preferred embodiment of the concept of the invention.

Figure 2 shows a preferred embodiment for recovering energy from combustion gas 10 by indirect back-flow cooling.

Figure 3 is an embodiment of the invention in which the same vessel has two extinguishing steps.

In Figure 1, reference numeral 1 denotes a pressure vessel containing a ceramic-lined gasification reactor 2. The reactor is provided with a black liquor inlet 3 and an oxygen or oxygen-containing gas inlet 4 and a burner (not shown). The opening at the bottom of the reaction chamber is in the form of a drainage salt 5 which opens directly above the surface of the liquid contained in the soda liquor chamber 6 below. A plurality of nozzles for cooling the liquid opens into the lowering salt 7. The formed soda liquor is transported from space 6 through pipe 8 through pump 9 and heat exchanger 10 to the subsequent process stages where the white liquor is formed or to other process stages. where soda liquor is used. The combustion gas from the first vessel is conducted through a conduit 11 to a second pressure vessel 12 for gas treatment and energy recovery:: · 25. This tube 11 opens below the surface of the liquid contained in the washer fluid · ···: * ♦ * in the pressure vessel 12. Above the space 13 there is a counter-flow film condenser-type indirect condenser 16. The cooled combustion gas outlet 17 is located at the top of the second pressure vessel 12.

The liquid in the second container washer fluid can be passed through the pipe 14 via the pump 15 to the first container, where it acts as a dilution liquid or coolant supplied through the spray nozzles 7. Feed water for boiling steam is supplied to condenser 16 through conduit 18 and vapor generated is discharged through conduit 19 (energy recovery is shown in more detail in Figure 2) Cold 118899 5 water is supplied to the upper portion of condenser 20 and the warm water formed is discharged through conduit 21. Water added to maintain fluid balance is introduced into the system via conduit 22.

5 Step 1 - Gasification and Cooling

As the gasification process itself is clearly disclosed in SE patent 44,8,173, it is not described in detail herein. However, the product obtained from flash pyrolysis in the presence of less than a stoichiometric oxygen content consists essentially of gaseous hydrogen, carbon monoxide, carbon dioxide, water vapor and hydrogen sulfide, and also sodium carbonate, sodium hydroxide and ca. temperature and 26 bar absolute pressure. The gas velocity is high, which facilitates the transport of melt droplets, some of which form a film on the reactor walls, to a soda liquor space 6 located below the gasification reaction 2. The outlet from the reactor consists of a drainage salt 5 into which the coolant is injected through the nozzles 7 in order to achieve the best possible contact with the melt and gas mixture. The coolant mainly consists of water. Some of this water evaporates when it comes into contact with the hot gas and the reactor temperature melt. The melt droplets and the melt film on the walls of the reactor dissolve into the remainder of the coolant, thus forming a soda liquor that falls into the liquid space 6. The melt droplets drop directly into the liquid space 6 and then dissolve in the soda liquor already present. . The evaporation of water in the soda liquor layer cools it • ··! * · Keen melting drops.

··· ·. 25 • · · ···; * j *; The trough 5 opens directly above the surface of the liquid in the liquid space 6. This * is very important in order to avoid strong contact between the gas and the · * · .. soda liquor formed. If the drainage salt opened below the liquid surface, the gas would be forced to **: pass through bubbles in the soda liquor. This would result in the formation of hydrogen carbonate ***. 30 through the reaction of carbon dioxide in the gas with sodium hydroxide in sodium hydroxide and sodium carbonate. The temperature after cooling is adjusted by an operating pressure, • · T, which depends on the temperature of the saturated steam at this pressure. Thus, at a working pressure of 26 J · 'bar, the equilibrium temperature of the soda liquor and gas can be assumed to be * · * J at 200 ° C for the partial vapor pressure of 60%.

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The soda liquor exits the first pressure vessel 1 through a conduit 8 and is pumped by means of a pump 9 through a heat exchanger 10 where heat energy is recovered from the soda liquor by cooling. A small part of the green liquor is used for bill passage 5 for humidifying the inside of returning it to the chute and allowing it to form a 5-en increase Sh film decrease passage on the inner surface.

Step 2 - Gas Purge The cooled gas, partially saturated with moisture, exits the first as-10 vessel 1 through a conduit 11 that opens to a second vessel 12. The conduit 11 opens as a liquid barrier consisting of a lowering salt and a rising tube, 13 below the liquid level. Due to the opening of the tube in this way below the surface of the liquid, the gas is forced to bubble through the liquid barrier, mainly through the liquid formed by the water in order to rise upwards. As a result, the gas is completely saturated with moisture while washing the chemicals that remain therein. Due to the effective contact made between the gas and the washing liquid, a large part of the small particles, so-called microparticles or smoke particles, which are contained in the gas and which are very difficult to separate, are washed away. The temperature of the washer fluid layer 13 and the gas exiting the layer 20 is largely the same as the temperature of the first vessel: v. 1 in the soda liquor chamber 6. The liquid of the liquor chamber 13 is returned to the first vessel 1 through pipe 14 to act as a . as coolant, which is sprayed into the lowering salt 5 through the nozzles 7. After that, ··· j 2 *; when the energy is recovered in the condenser 16, the gas is removed from the system by a stream of 2 * 25 · 17. Any remaining sulfur compounds are then washed off

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2 * 2 * 2 using alkaline washing fluid, for example sodium carbonate.

* · * · „Step 3 - Energy Recovery ························································ Chapter 2 30 Fig. 2 shows an energy recovery method according to the invention, wherein the system pressure is 26 bar. It is based on the use of a counter-conduction membrane condenser 16 mounted in a second pressure vessel above the solar fluid layer 13. Moisture-saturated gas at 200 ° C which exits

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* · *! washing fluid layer 13, is condensed by introducing indirect heat into the preheated feed water 23 at a temperature of about 180 ° C. This causes the feed water to evaporate and the generated steam, which is at about 10 bar overpressure and at about 184 ° C, can leave the system via steam hood 24 as a flow 25 for use elsewhere in the factory. The condensate leaving the gas drops into the washer fluid compartment 13 and thus forms a portion of the incoming gas 11 washer fluid. This gas will be washed in this way with its "own" hot condensate. Since the liquid in the washing fluid compartment 13 is predominantly composed of water, this liquid naturally also contains the washed chemicals that were present in the gas phase and were dissolved in the condensate.

The temperature of the gas after partial condensation is about 190 ° C and can now be used to preheat said feed water. This feed water, at a temperature of about 70 ° C, enters the condenser via conduit 18. After the heat has been indirectly transferred from the gas, the temperature of the feed water rises to about 180 ° C and, after passing through steam hood 24, can be used to generate steam as described above in Figure 15.

After preheating the feed water, the gas temperature is about 80 ° C, and the rest of the calorific value up to about 30 ° C can be utilized in the same condenser unit for making hot water. Cold water then enters the condenser through conduit 20 at a temperature of about 20 ° C to 15 ° C and exits condenser at about 70 ° C through conduit 21. Liquid loss from the system, e.g. removed as soda liquor 8, is replaced by the addition of water • · about 2.1 m3 per tonne mass to the upper part of the condenser at a flow of 22. This ♦ ·. · * ·. the water is heated to 2 ° C by the countercurrent method before it reaches the condensate. in washing liquid mode 13.

··· ·. 25 • · · ···. • s *. The gas exits the condenser at a flow of about 30 ° C as a flow 17. Its chemical energy is utilized by burning together with the steam and / or electricity produced, pre-j *. as disclosed in SE 448,173.

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. Because of the countercurrent process of energy recovery, the evaporative heat from the cooling of the first vessel is fully utilized as condensation heat in the second vessel. The countercurrent condenser provides a higher condenser heat ·. · Space, higher flow rate and thus higher steam output than the • · 1 * condenser. The high heat level, which enables the production of high quality steam, is based on the principle that the gas is, in the second stage, washed with its "own" hot condensate. Returning this condensate to the cooling stage also allows 90 to 100% of the cooling demand of the melt / gas from the reactor to be generated by the evaporation heat.

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Another advantage of returning the condensate from the second fluid layer 13 via the conduit 14 to the first vessel to act as a diluent or coolant in the spray nozzle 7 is that the extremely high temperature in the downer 5 where the spray nozzles are located causes gas to condense 10 cracking of tar-like compounds such as terpenes. Other compounds such as sulfur compounds can also be degraded in this way. This positive effect can, of course, be utilized to cook unwanted compounds in other condensates, filtrates, or effluents if they are sprayed into a lowering salt 5.

15

Moisture-saturated combustion gas contains about four times more water vapor than gas when based on specific volume. This means, as a result of the countercurrent procedure, that even if the velocity of the mixture of steam and gas is about 10 m / s, for example when entering the condenser, the velocity of the gas decreases when the moisture condenses, leaving its velocity about 4-5 m / s. Gas with .V. the droplets carried by them may then be more easily distinguished.

* • · • · »• · · • ·. · * ·. Small particles, so-called microparticles or smoke particles, ranging in size from 0.01 to 1.0 µm · ···: Cartridges that are still contained within the gas and which are otherwise extremely difficult to distinguish ··· ·. 25 are utilized as condensing cores, which allows the separation of these particles. The low gas velocity required by the counter-current principle gives these virtually hydrophobic particles a longer residence time on the wetting than is usually the case with cooling downstream.

30 • · · • · ·

A further advantage of the countercurrent method is that no layers are formed on the sensitive lower cooling surfaces of the condenser, which are easily soiled, since: the entire condensate stream rinses them at high temperature.

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The compact, multifunctional gas treatment tower, in which condensate is collected in a single location, represents a significant simplification from the conventional concept in terms of device design and process control and adjustment.

The formation of hydrogen carbonate in the soda liquor is avoided by separately quenching the melt in the first stage and the gas in the second stage.

Experience has shown that the liquid in the first-stage extinguishing step tends to foam when the pressure in the system is temporarily dropped. The present process makes it possible for the washing liquid in the second quenching step to absorb this foam from the first step.

The above-described embodiment is a preferred embodiment. However, the invention is not limited to this description but can be modified within the scope of claims 15. Of course, the number of different stages in the countercurrent condenser when the condensing heat and heat energy of the hot, moisture-saturated gas is recovered may be lower or higher, and, for example, the given temperatures of the feed water and gas may vary in value.

For the separation of microparticles, it may be advantageous to install structural layers between the various stages of the countercurrent condenser. This also has a positive effect in that it prevents channeling.

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:. ·. Although the gasification temperature in the reactor may be in the range of 500-1600 ° C, preferably 700-1300 ° C. . ·. 25 ° C, and more preferably 800-1000 ° C, and the system pressure may be up to 150 bar absolute to bone pressure, preferably 21-50 bar, although atmospheric pressure is possible, although it is not possible to produce high quality steam. Although the temperature of condensate and soda liquor should be as high as possible, each specific .2 ·. the saturation temperature of the pressure system limits this temperature. The temperature can be pre-

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, 30 for about 170-260 ° C if the pressure of the ice system is 21-50 bar and the partial vapor pressure iii * ". 1 for the cooling phase of the first vessel is about 35-90%. For example, the partial pressure of steam *» * "is 83% and absolute pressure 26 bar, temperature 216 ° C.

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In addition to being fed to the upper part of the condenser, the water added to maintain the liquid balance can also be fed directly into the condenser, for example, in conduit 14.

Further, it is possible to contemplate the use of a condenser 16 in combination with other solutions of the system having similar requirements.

The condenser 16 need not be in the same container as the washing liquid layer 13, although this is preferred. Instead, the cooling and condensation of the moisture-saturated gas may be carried out in a separate condenser, whereupon the condensate from this condenser is returned to the vessel containing the washing fluid layer 13. This separate condenser may optionally consist of a conventional condenser which is not a countercurrent film condenser. Of course, the benefits of using a countercurrent film condenser are naturally not achieved.

15

The structure of the two extinguishing layers / rapid cooling layers, using a salt and a liquid layer, can be designed differently with regard to the construction of the apparatus. For example, in the first vessel 1, separation of the dry material may be accomplished by allowing the droplets of melt to drop into the cooling layer 6 without cooling the fluid sprayed by the melt droplet 20. Thereafter, the gas is led away from the melt droplet stream and instead, the coolant is sprayed directly into this gas stream, whereby any entrained droplets of melt droplets dissolve and fall into the cooling bath. In the second vessel 12, the extinguishing can be designed to provide •; . ·. unlimited mammoth pump effect, i. so that the gas can raise «« i »M ·,, ·, 25 liquids and thus achieve good recycling and washing. Alternatively, one or both of the · · · «·· shut-off stages may be equipped with a so-called vent shut-off, possibly using a guide jacket.

·· • «• ··. ···. In addition, an additional gas treatment step may be installed in the gas pipe 11. This ···. 1. 30 additional steps could consist, for example, of a venturi to which the washing liquid is • 1 1 * ". · Fed from a second fluid bed 13. The use of such a venturi improves the separation of microparticles or smoke particles of the gas stream.

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The two liquid layers 6 and 13 may also be in one and the same container so that they are separated, for example, by a partition. One possibility is to use both pressure layers 6 and 13 with a horizontal pressure vessel provided with a partition. The one end of the vertical pressure vessel 1 containing the reactor 2 would then be connected to the Horison-5 talc vessel and the other end to the vertical pressure vessel 12 containing the condenser 16.

Another possibility to hold both liquid layers 6 and 13 in one and the same vessel is shown in Figure 3. The soda liquor layer 6 is located around the vertical axis of the vertical vessel containing the reactor 1. The gas is forced to pass through the washing liquid layer 13 10 fluid surrounding the periphery of the vessel. The dividing wall separates the two fluid layers completely and a plurality of additional, concentric walls 26 extending down to the washer fluid layer 13, together with the control shroud 27, act as a multistage mammoth pump to force the gas through the fluid.

15

Although it is desirable according to the invention that the wash liquid layer consists primarily of water in the form of condensate, it is contemplated that the liquid layer 13 could consist of a different type of soda juice than the soda juice of the first liquid layer 6. Since this different type of soda liquor is in effective contact with the gas 20, it could contain higher amounts of sodium hydrogen carbonate in this case; bonate and sodium hydrogen sulfide, which means that it could be used, for example, to · · remove hydrogen sulphide and carbon dioxide.

Of course, the concept of the invention can also be applied to the application of chemicals in processes where the waste liquors and the recovered chemicals are of a completely different type, such as bleaching plant waste liquors, semi-chemical pulp, for example, CTMP waste liquors or waste liquors from a pulp process based on the use of potassium as a base in place of sodium.

· 1 · * 1 ···: X 30 The concept of the invention can also be applied when a higher partial pressure of hydrogen sulfide in the reactor is utilized, resulting in equilibrium displacement, (1). SE 468,600) and formation of Na 2 S.

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Claims (14)

A process for the recovery of chemicals and energy from black liquor obtained in the production of paper pulp by chemical splitting of fiber raw material which gasifies black liquor, mainly forming CO, CO 2, CH 4 / H2 and H2 S in gaseous form and Na 2 CO 3 / NaOH and Na 2 S in the form of melt droplets; the resulting mixture of gas and molten is cooled in a first step by contacting directly with a coolant essentially water resistant, after which a portion of the coolant is displaced and the melt droplets are separated and released in the remaining portion of the coolant, 10 green liquor baths (6) are formed; in a second step, this gas is washed and moisture saturated by being brought into direct contact with a wash liquid bath (13) essentially consisting of water; after the gas has been washed in the second stage, energy is recovered in the form of thermal energy as well as condensate heat from the hot moisture saturated gas in an indirect cooler (16); The method is characterized in that the contact between the gas and the liquid bath (6), consisting of green liquor in the first stage, is substantially less than the contact between the gas and the washing bath (13), mainly the water-holding in the second stage. Process according to claim 1, characterized in that the energy content of the hot moisture saturated gas is recovered by means of an indirect cooler (16), the resulting hot condensate from the cooler (16) being utilized as part, preferably the main part, of the wash liquid bath (13) in the second step. • I • · * a a a • · aaa Method according to claim 1 or 2, characterized in that the indirect cooling •; One is achieved by a countercurrent process; the condensation heat in the gas as well! J *: the most high-quality part of the thermal energy of the gas is thereby utilized for the generation of high-quality gas; residual thermal energy is used to preheat feedwater for meadow generation and production of hot water. aa • a • «a a 30 Method according to claim 2, characterized in that the fluid balance is maintained in aaa; the liquid bath (6,13) by liquid addition (22) preferably consisting of water in the a / a (1) of the cooler (16) Upper part a a aa a aa a aaa a a a a a aa a is 1 1 8899 Process according to claim 2, characterized in that hot liquid is returned from the washing liquid bath (13) in a flow (14) to the first stage, such that coolant and solution liquid for this green liquor bath are obtained. 5 Process according to claim 1, characterized in that the gas in the first stage is not allowed to bubble through the liquid bath (6) consisting of green liquor. Process according to claim 1, characterized in that the gas in the second stage is forced to bubble through the washing liquid bath (13). 10 Method according to claim 2, characterized in that the indirect cooler (16) is a countercurrent case film condenser. Method according to claim 1, characterized in that the pressure in the system is up to 150 bar, preferably 21 - 50 bar. Method according to claim 1, characterized in that the temperature of the washing liquid bath (13) is substantially the same as the temperature of the gas entering the washing liquid bath and the temperature of the green liquor bath (6). 20 11. A method according to claim 1, characterized in that between the first and second stages a further gas treatment consisting of a gas scrubber is installed, the washing and cooling liquid being measured from the washing liquid bath (13). * ·· • i • · * · · ·. · · 25 Method according to Claim 1, characterized in that the liquid baths (6) and 1, (1) are located in one and the same vessel, where they are separated e.g. by means of an intermediate: T: wall. Method according to claim 7 or 12, characterized in that the gas in the second stage is forced to alternately bubble down and up, at least one thread in both directions, through the washing liquid bath (13), which bubble steps have been added. closed by means of a distribution jacket (27) and concentrically placed cylindrical walls (26), which reach into the wash liquid bath (13). ·· · • 1 · • · • · ♦ • »· ♦ ·· • i