WO1996016721A1

WO1996016721A1 - Method and apparatus for liquid regulation in wet cleaning of a gas

Info

Publication number: WO1996016721A1
Application number: PCT/SE1995/001404
Authority: WO
Inventors: Sven Ragnarsson; Anders Johansson
Original assignee: ABB Fläkt AB
Priority date: 1994-11-28
Filing date: 1995-11-24
Publication date: 1996-06-06
Also published as: SE9404101L; JP3009474B2; SE504408C2; AU4126596A; EP0794825A1; SE9404101D0; JPH10500065A

Abstract

A method and an apparatus for regulating the amount of liquid in the wet cleaning of a gas in at least two stages are disclosed. In the method, liquid from at least one of the stages after the first stage is brought into contact with air, such that the liquid is partly evaporated and cooled, the evaporation being carried out in such a manner that the amount of liquid in the stage remains the same, whereupon the remaining liquid is heated before being recycled to the wet cleaning. The apparatus comprises a scrubber (6) having at least two stages and a liquid-circulation circuit (53) in at least one of the stages after the first stage, the liquid-circulation circuit (53) including an evaporator (20) adapted to evaporate liquid to such an extent that the amount of liquid in the stage remains the same and to cool liquid from the circuit by contacting it with air, as well as a heater (44) adapted to heat the cooled liquid in the circuit.

Description

METHOD AND APPARATUS FOR LIQUID REGULATION IN WET CLEANING OF A GAS

The present invention relates to a method and an apparatus for liquid regulation in the wet cleaning of a gas, and more specifically concerns a method and an apparatus for regulating the amount of liquid in the wet cleaning of a flue gas containing hydrogen chloride and sulphur dioxide and being first wet-cleaned in a scrubber with a view to removing hydrogen chloride and then wet- cleaned with a view to removing sulphur dioxide.

When wet-cleaned, the gas is brought into contact with a liquid for removing impurities in the gas. The wet cleaning takes place in a wet-cleaning apparatus, also referred to as a scrubber or a washer, which operates in one or more stages with liquid as separating medium. In order to achieve a high degree of efficiency in the wet cleaning, the liquid should preferably be finely divided and brought into contact with the gas countercurrently. Furthermore, the liquid is often water-based and may con¬ tain an agent intended to improve the absorption of impu¬ rities in the gas. Wet cleaning is especially used for cleaning flue gases, such as flue gases generated upon refuse incinera¬ tion. Such flue gases contain, inter alia, hydrogen chlo¬ ride and sulphur dioxide, which have to be removed before the flue gases may be discharged into the surrounding atmosphere.

In actual practice, such flue gases are wet-cleaned in large scrubber towers having a diameter of approxi¬ mately 5-6 m and a height of approximately 20 m. The scrubber is arranged in a cold place, i.e. either out- doors or indoors in a cold place, which normally means that the temperature is approximately 10°C or below, the incoming flue gas often having a temperature of approxi¬ mately 150-250°C, especially 200°C. In the first stage, the flue gas is cooled by the washing liquid absorbing hydrogen chloride in the gas. Simultaneously, there is an evaporation of washing liquid, such that the flue gas is saturated. In the second stage (and subsequent stages, if any), the flue gas from the first stage is wet-cleaned with a view to removing sulphur dioxide.

SE 9300169-1 (Publ. No. 470,565) teaches an instance of the above-mentioned technique for the wet cleaning of flue gases. According to the SE publication, the flue gases are, in a first stage, washed with an aqueous acid washing solution for the absorption of hydrogen chloride and, in a second stage, washed with an aqueous alkaline washing solution for the absorption of sulphur dioxide. A partial flow of the acid washing solution is drawn off in the first stage and is regenerated in a secondary cir¬ cuit by neutralisation, separation of heavy metals and evaporation with a view to separating sodium chloride. Furthermore, a partial flow of the alkaline washing solu¬ tion is drawn off in the second stage and is regenerated in a secondary circuit by the addition of lime for the precipitation of gypsum and by the addition of soda for the precipitation of excess calcium. SE 9300169-1 is dis¬ tinguished by the fact that the washing liquid in the first stage is kept essentially separate from the washing liquid in the second stage, the condensate from the eva¬ poration in the secondary circuit of the first stage being recycled to the primary circuit of the first stage, and the fact that use is made of a so-called indirect lime process for regenerating the washing liquid in the secondary circuit of the second stage.

As is explained in some detail in SE 9300169-1, one strives, in the wet cleaning of gases, to keep the liquid employed in a system that is as closed as possible, i.e. one strives not to discharge any polluted liquid to the surroundings. To this end, the liquid in each stage is circulated in a circuit. However, one problem is that, while liquid is evaporated in the first stage and satu- rates the gas, liquid is instead condensated from the saturated gas in the subsequent stages as a result of cooling in the scrubber. This condensation results in an increase in the amount of liquid in the stages at issue, and liquid thus has to be removed if the amount of liquid in the liquid-circulation circuit is to be maintained the same. If liquid is to be removed by being discharged into the surroundings, it first has to be cleaned, for instance by evaporation, which is a costly procedure requiring special equipment. It is also con¬ ceivable to eliminate excess liquid in the stage at issue by transferring it to the first stage, which normally has a shortage of liquid owing to the evaporation. For seve¬ ral reasons, however, such a transfer of liquid is not without its problems. First, a transfer of liquid from the stage at issue would mean that chemicals dissolved in the liquid were drained from this stage. These chemicals naturally have to be replaced, which would render the cleaning process more expensive. In addition, the chemi- cals may cause problems in the first stage. Thus, the transfer of, say, dissolved sulphate to the first stage may give rise to scaling problems in the evaporation or distillation of liquid in the first stage. Moreover, there may not be a balance between the first stage and the subsequent stage, i.e. it is not absolutely certain that all the excess liquid in the subsequent stage can be transferred to the first stage.

In two- or multi-stage wet cleaning of gases, espe¬ cially flue gases, one thus needs to be able to regulate the increase in the amount of liquid that is due to con¬ densation in the stage or stages coming after the first wet-cleaning stage, without encountering the above prob¬ lems associated with the prior art, such as loss of che¬ micals, risk of scaling and high installation costs. According to the present invention, this need is met by evaporating, in stages with an increased amount of liquid due to condensation, liquid by contacting it with air and heating the remaining liquid before this is recycled to be used once again in the wet cleaning of gas.

To be more specific, the invention provides a method for regulating the amount of liquid in the wet cleaning of a gas in at least two stages, said method being char¬ acterised in that liquid from at least one of the stages after the first stage is brought into contact with air, such that the liquid is partly evaporated and cooled, the evaporation being carried out to such an extent that the amount of liquid in the stage remains the same, whereupon the remaining liquid is heated before being recycled to said stage.

The invention further provides an apparatus for regulating the amount of liquid in the wet cleaning of a gas, said apparatus comprising a scrubber having at least two stages and a liquid-circulation circuit in at least one of the stages after the first stage, said apparatus being characterised in that the liquid-circulation cir- cuit includes an evaporator adapted to evaporate liquid to such an extent that the amount of liquid in said stage remains the same and to cool liquid from said circuit by contacting it with air, as well as a heater adapted to heat the cooled liquid in the circuit. Preferably, the gas being wet-cleaned is a flue gas containing hydrogen chloride and sulphur dioxide and being first wet-cleaned with a view to removing hydrogen chloride and then wet-cleaned with a view to removing sulphur dioxide, liquid from the sulphur-dioxide wet cleaning being partly evaporated and cooled, whereupon the remaining liquid is heated before being recycled for renewed sulphur-dioxide wet cleaning.

It is furthermore preferred that the sulphur-dioxide wet cleaning takes place in a primary circuit and a secondary circuit in accordance with the indirect lime process as described in e.g. SE 9300169-1 mentioned above, and that the evaporation and the cooling of liquid take place in the secondary circuit before the liquid is treated with a view to separating absorbed sulphur dioxide, preferably by precipitation. Even though heat can be supplied in optional manner to the liquid after the evaporation, it is especially preferred, in accordance with a special aspect of the invention, that heat be supplied by means of heat ex¬ change with the liquid in the first stage, i.e. the heat is indirectly taken from the flue gas itself.

According to the invention, the evaporation can be so controlled that an optional amount of liquid is eva¬ porated. If, say, it is desirable that a certain amount of liquid be transferred to the first stage and a certain amount of liquid is in addition drawn off along with the precipitates formed, the evaporated amount of liquid need not correspond to the condensated amount of liquid, but should instead correspond to the condensated amount of liquid minus the transferred and the drawn-off amount of liquid. Regardless of whether liquid is transferred or drawn off, the liquid is, in accordance with the inven¬ tion, evaporated to such an extent that the amount of liquid in the stage remains the same.

Preferably, the apparatus according to the invention is a scrubber comprising a cleaning stage for hydrogen chloride and at least one cleaning stage for sulphur dioxide, the evaporator being connected to the cleaning stage for sulphur dioxide.

In agreement with what has been stated above, it is preferred that the sulphur-dioxide cleaning is designed according to the indirect lime process, the evaporator being connected to the secondary circuit before the sepa¬ ration of sulphur dioxide, which preferably takes place in form of gypsum precipitation. According to the invention, it is furthermore, espe¬ cially preferred that the heater adapted to heat the cooled liquid is a heat exchanger for heat exchange with liquid from the first stage, and that this heat exchanger is disposed after the separation of sulphur dioxide in the secondary circuit. The evaporator according to the invention preferably comprises a tower for countercurrent contact between air and finely-divided liquid, the tower having a liquid inlet, a liquid outlet, an air inlet, an air outlet, as well as a fan for feeding the air. In a special aspect of the invention, the evapora¬ tor, which preferably is a cooling tower, serves not only to evaporate liquid, but also to oxidise the sulphite found in the liquid. In order that the evaporator should be able to perform this double function, the liquid flow, the air flow and the temperature have to be controlled, necessitating the use of a complicated control algorithm with ensuing problems. If, however, part of the air flow¬ ing through the evaporator is recirculated and control¬ led, the required conditions can be met with the aid of simpler control algorithms. At the same time, the air flowing through the evaporator is sufficient to enable the above-mentioned sulphite oxidation to be performed. There is no need of any recirculation of liquid in the evaporator. The moisture content in the evaporator may be sufficiently high to enable the avoidance of crystalli¬ sation in the liquid, and the system can be so dimension¬ ed that all temperature and moisture conditions in the supplied air can be controlled.

Further distinctive features and advantages of the invention will appear from the following description, in which reference is made to the accompanying drawings. For clarity purposes, the following description focuses on a method and an apparatus for two-stage wet cleaning of a flue gas containing hydrogen chloride and sulphur dioxide. The invention is not, however, restrict¬ ed to this particular embodiment. In the accompanying drawings,

Fig. 1 schematically illustrates the method and the apparatus according to the invention when used for wet- cleaning a flue gas in two stages, and Fig. 2 schematically illustrates a preferred embodi¬ ment of the invention.

Thus, Fig. 1 shows an apparatus for two-stage wet cleaning of a flue gas which contains, inter alia, impu¬ rities in the form of hydrogen chloride and sulphur dioxide. In the first stage, the flue gas 1 is conduct¬ ed into a quencher 2, where it is brought into contact with an acidic, aqueous washing liquid 3 which is fine¬ ly divided by means of spray nozzles 4 as well as spray nozzles 5 in the scrubber 6 itself. The washing liquid is circulated with the aid of a pump 7 to the spray nozzles 4 and with the aid of a pump 8 to the spray nozzles 5 via conduits 9 and 10, respectively. Fresh washing liquid can be supplied at 11. When entering the quencher 2, the flue gas 1 is cooled from approximately 150-200°C to approxi- mately 55-60°C by the cooler washing liquid sprayed through the nozzles 4. When the gas is thus cooled, part of the liquid evaporates, and the flue gas is saturated with water vapour. As the flue gas passes through the first stage and is contacted with the finely-divided washing liquid 3, the latter absorbs hydrogen chloride in the gas. The washing liquid containing hydrogen chloride is drawn off at 12 to be further treated and cleaned.

The flue gas thus rid of hydrogen chloride is then conducted from the first stage to the second stage in the scrubber 6, where it is brought into contact with an alkaline, aqueous washing liquid 13, which is finely divided by means of spray nozzles 14. The washing liquid absorbs sulphur dioxide from the flue gas and is then collected and drawn off via a conduit 15 to a storage vessel 16, whence it can be recirculated to the spray nozzles 14 via a conduit 18 with the aid of a pump 17. As mentioned in the foregoing, the flue gas conduct¬ ed to the second stage is saturated with water vapour. When the flue gas is brought into contact with the cooler washing liquid, moisture in the flue gas is condensed. This condensed moisture is, along with the washing liquid, conducted to the storage vessel 16 and thus increases the volume of liquid in the second stage. A partial flow is drawn off from the conduit 18 via a con¬ duit 19 and is, in accordance with the invention, sup- plied to an evaporator 20. Excess water supplied to the washing liquid by condensation from the flue gas in the second scrubber stage is removed in this evaporator 20. Thus, the water is removed by bringing the washing liquid in the evaporator 20 into intimate contact with air. In order to achieve this intimate contact between the wash¬ ing liquid and the air, the washing liquid is introduced at the upper part of the evaporator 20 and is distributed over a considerable surface e.g. of packing or various sorts of bottoms or by being finely divided by means of nozzles. In Fig. 1, this is indicated by reference nume¬ ral 21. Air is supplied at the lower part of the evapora¬ tor, as indicated by the arrow 22, and is, with the aid of a fan 23, drawn countercurrently to the liquid flow towards the upper part of the evaporator where the air is discharged. When the air comes into contact with the liquid in the evaporator, liquid evaporates and leaves the evaporator along with the air. As a result, the volume of liquid is reduced, i.e. excess liquid from the condensation in the second scrubber stage is eliminated. The elimination of liquid in the evaporator 20 can be regulated by varying the relationship between the air flow and the liquid flow, the contact surface and the time of contact between the liquid and the air.

Preferably, the evaporator is designed in the manner described above, such that part of the air flow through the evaporator is recirculated. In Fig. 1, this is indi¬ cated by the recirculation conduit 72. The size of the recirculated air flow is regulated with the aid of regu¬ lating valves, as indicated by the valves 73 and 74 in Fig. 1.

During the evaporation in the evaporator 20, the liquid is cooled, so that the liquid drawn off from the evaporator via the conduit 24 is cooler than the liquid supplied to the evaporator via the conduit 19 (approxi¬ mately 40"C and approximately 55°C, respectively).

After the evaporator 20, the washing liquid is rege- nerated by the removal of absorbed sulphur dioxide, which is carried out by precipitating the sulphur dioxide in the form of gypsum (CaS04•2H2O) . In Fig. 1, this regene¬ ration treatment is schematically indicated by the box 25. Some liquid may be drawn off from the regeneration treatment 25, for instance together with the separated gypsum precipitate, as indicated by the arrow 26. After the regeneration treatment 25, the washing liquid is recycled via a conduit 27 in order to be used once again in the wet cleaning of the second scrubber stage. As men- tioned above, the washing liquid has, however, been cool¬ ed in the evaporation in the evaporator 20. Unless this cooling of the liquid is compensated for, it will result in increased condensation in the second stage of the scrubber 6. To this end, heat is supplied to the cooled washing liquid which thus is reheated by heat exchange in a heat exchanger 28. Basically, the heat in the heat exchange can be supplied from any suitable source. As illustrated in Fig. 1, it is, however, especially prefer¬ red in the invention that the heat is supplied from the washing liquid in the first stage, thus indirectly coming from the flue gas itself. This is carried out by drawing off a partial flow from the conduit 10 and conducting it via conduits 29, 30 through the heat exchanger 28. The heat exchange with washing liquid from the first wet- cleaning stage is advantageous in that it not only serves to reheat the washing liquid in the second stage but also to cool the washing liquid in the first stage, which reduces the need of fresh water in the first stage as well as improves the absorption of hydrogen chloride and other acid components in the first stage. It should fur¬ thermore be added that the evaporation of washing liquid due to the intimate contact with oxygen in the evaporator 20 also results in excellent oxidation of oxidisable com¬ ponents, such as sulphur dioxide. Furthermore, the cool¬ ing of the remaining washing liquid improves the quality of the gypsum subsequently precipitated in the regenera- tion treatment 25, gypsum being stable at temperatures below approximately 40°C.

The invention having now been described in general terms with reference to the embodiment shown in Fig. 1, the especially preferred embodiment schematically illu- strated in Fig. 2 will now be described in some detail. As in Fig. 1, a gas 1 containing hydrogen chloride and sulphur dioxide enters, in Fig. 2, a scrubber 6, where it is cleaned of hydrogen chloride in a first stage. In this first cleaning stage, the gas is brought into contact with an aqueous, acid washing liquid 3 sprayed through spray nozzles 5. With the aid of a pump 8, the washing liquid 3 is circulated in a primary cir¬ cuit 31 from the scrubber 6 to the spray nozzles 5 via conduits 32 and 33. A partial flow of washing liquid is drawn off from the primary circuit 31 and conducted, via a conduit 34, to a secondary circuit 35 comprising a con¬ tainer 36 for neutralisation with the aid of soda ( a2Cθ3), which is added to the liquid, as indicated by the arrow 37. After neutralisation, the partial flow of washing liquid is conducted to a container 38, where sodium sulphide is added, as indicated by the arrow 39, with a view to precipitating heavy-metal impurities in the form of sulphides. The thus-precipitated heavy-metal sulphides are separated in a container 40 in the form of a sludge that is removed, as indicated by the arrow 41. Then, the partial flow of washing liquid is conducted to an evaporation plant 42. The condensate obtained upon the evaporation is, via a conduit 43 and a heat exchanger 44, conducted to the primary circuit 31 of the first stage. In the concentrated liquid obtained upon the evaporation, sodium chloride crystallises and is separated at 45, for example by centrifugation or filtration. The thus-sepa¬ rated sodium chloride is recovered in the form of a pro¬ duct, as indicated by the arrow 46. The mother liquor obtained upon the separation of sodium chloride can be conducted directly back to the vessel 36 via a conduit 47. Preferably, however, the mother liquor is first treated by being, via a conduit 49 and in a container 48, joined with a partial flow from the secondary circuit of the sulphur-dioxide cleaning. This partial flow contains sulphate and chlorides. By cooling the mixture in the container 48, sodium sulphate is precipitated in the form of Glauber salt, which is separated and, via the conduit 50, is recycled to the secondary circuit of the sulphur- dioxide cleaning. The purpose of thus feeding a partial flow from the secondary circuit of the sulphur-dioxide cleaning to the secondary circuit of the hydrogen-chlo¬ ride cleaning is to avoid an accumulation of chlorides in the washing liquid in the second stage. Even if the main part of the hydrogen chloride in the gas 1 is absorbed in the first stage in the scrubber 6, a residual amount of hydrogen chloride normally remains and is absorbed in the second stage of the scrubber (the sulphur-dioxide clean¬ ing stage). The above-mentioned preferred drawing-off of the partial flow from the secondary circuit of the sul¬ phur-dioxide cleaning is insignificant as to volume and constitutes but a per cent or so of the flow in the secondary circuit.

As indicated in the foregoing, the sulphur-dioxide cleaning, which now will be described, consists of a so- called indirect lime process which, for example, may be designed as described in SE 9300169-1.

A container 13 holds an aqueous, alkaline washing solution which to begin with, i.e. before the absorption of sulphur dioxide, consists of an alkaline aqueous solu¬ tion of an easily-soluble alkali, such as an easily- soluble alkali metal compound; an easily-soluble alkaline earth metal compound, such as a magnesium compound; ammo- nia; or an ammonium compound. As easily-soluble alkali, use is preferably made of an easily-soluble alkali metal compound, such as an easily-soluble sodium or potassium compound. Using a sodium compound, such as sodium hydroxide (NaOH), is most preferred. With the aid of a pump 17, the alkaline washing liquid is circulated in a primary circuit 51 through a conduit 18 to spray nozzles 14, through which the alkaline washing liquid is finely divided and brought into contact with the gas containing sulphur dioxide. When contacted with the gas, the washing liquid absorbs sulphur dioxide and is then collected and recycled to the container 13 via a conduit 15. The gas thus rid of sulphur dioxide leaves at 52.

In order to regenerate the alkaline washing liquid, a partial flow thereof is drawn off from the primary circuit 51 to a secondary circuit 53 via a conduit 19. Before being subjected to the regeneration treatment in the secondary circuit 53, the washing liquid is, in accordance with the invention, treated with a view to removing excess water having condensed in the second stage, i.e. the sulphur-dioxide cleaning stage, in the scrubber 6. In this treatment, the partial flow of wash¬ ing liquid in the conduit 19 is conducted to an evapora¬ tor 20 which, at the bottom, has a liquid supply 54, whence liquid can be supplied with the aid of a pump 55 to spray nozzles 56 and in a tower 57 be distributed over packing 58, thereby to achieve a considerable contact surface for the liquid. Moreover, the evaporator 20 includes a suction fan 23 which sucks in air through louvres 59 in the lower part of the evaporator, whereupon the air is drawn upwards through the tower 57 and brought into intimate contact with the liquid flowing downwards. In the contact between the liquid and the air, part of the liquid is evaporated and leaves with the air at 60, resulting in a reduction in the amount of liquid in the secondary circuit 53. In the evaporation of liquid in the evaporator 20, the remaining liquid is cooled (from approximately 55°C to approximately 40°C), and then leaves via a conduit 61 for the regeneration treatment.

It will be appreciated that the evaporator 20 shown in Fig. 2 may be designed to recirculate part of the air flow, as has been described in the foregoing as well as illustrated in connection with Fig. 1.

The regeneration treatment chiefly amounts to the separation of absorbed sulphur dioxide in the form of gypsum (CaS04 • 2H2O) and the restoration of the alkali content of the washing liquid. On account of this, the absorbed sulphur dioxide should be oxidised and be pre¬ sent in the form of sulphate ions. To ensure the oxida¬ tion of the absorbed sulphur dioxide, it is known to treat the washing liquid with air before it is regene¬ rated in the secondary circuit. However, this prior-art air treatment does not correspond to the inventive treat¬ ment in the evaporator 20. Whereas the prior-art air treatment has only resulted in the oxidation of absorbed sulphur dioxide, which preferably has been carried out by introducing air in the form of bubbles into the liquid via nozzles, there being no evaporation of the liquid, the inventive treatment involves an evaporation of liquid in the evaporator 20, causing liquid to be eliminated and the remaining liquid to be cooled. At the same time, oxi- disable components of the liquid are oxidised. The evapo- ration according to the invention requires a considerable contact surface and a considerable contact time, as well as a high flow ratio of air to liquid. All in all, it should be emphasised that the evaporation treatment according to the invention bears no comparison with the oxidation treatment of the prior art.

From the evaporator 20, the remaining washing liquid is conducted to a container 62 via the conduit 61. In this container, calcium ions are added, as indicated by the arrow 63, preferably in the form of lime, for in¬ stance burnt lime (CaO) or, which is most preferred, slaked lime (Ca(0H)2). As a result of the addition of calcium ions, part of the sulphate content of the partial flow is precipitated as gypsum, which is left to sediment in a container 64 and then is separated, as indicated by the arrow 65. The thus-separated gypsum is dewatered and may, for instance, be used for producing gypsum board. Thereafter, the partial flow of washing liquid is con¬ ducted further in the secondary circuit to a container 66, where a minor amount of carbonate, preferably soda (Na2Cθ3), is added, as indicated by the arrow 67. This results in the precipitation of excess calcium ions as sparingly-soluble carbonate, which then is left to sedi¬ ment in the container 68. The thus-sedi ented precipitate is removed from the container 68 and recycled to the con¬ tainer 62 via a conduit 69. The partial flow of alkaline washing liquid thus rid of absorbed sulphur dioxide and regenerated with respect to its NaOH content, is then recycled via a conduit 70 to the container 13 for washing liquid in the primary circuit 51.

Before reaching the container 13, the regenerated washing liquid is, in accordance with the invention, heated in the heat exchanger 44 through heat exchange with hot condensate from the secondary circuit 35 of the first stage, i.e. heat from the liquid in the first stage is used for heating the cooled washing liquid in the second stage. As explained in the foregoing, such heating of the washing liquid in the second stage is needed in order to avoid the increased condensation in the second stage of the scrubber 6 that would otherwise ensue. Accordingly, the heating of the cooled washing liquid in the second stage constitutes an important aspect of the invention, and even if the heat required for this purpose need not necessarily be taken from the washing liquid in the first stage, it is nevertheless especially preferred to do so, since it reduces the need of fresh water in the first stage and improves the absorption of hydrogen chlo¬ ride and other acid substances in the first stage.

In order to avoid any accumulation of chloride in the washing liquid of the second stage, it is, as indi¬ cated in the foregoing, preferred in the invention to draw off a minor amount of the regenerated washing liquid of the secondary circuit from the conduit 70 and, via a conduit 71, conduct it to the secondary circuit of the first stage, where it is mixed with the mother liquor after the separation of sodium chloride crystals in the container 48. The partial flow thus admixed via the con¬ duit 71 contains not only chloride but also sulphate. By cooling the mixture in the container 48, sodium sulphate may be precipitated in the form of Glauber salt, which is separated and, via the conduit 50, is recycled to the container 68 in the secondary circuit of the second stage.

It should be emphasised that, whereas the drawing- off of liquid from the second stage, such as the drawing- off of liquid from the secondary circuit of the second stage via the conduit 71, was in the prior art condition¬ ed by the increase in the amount of liquid due to the condensation in the second scrubber stage, this is not so in the present invention, since the increase in the amount of liquid due to condensation in the second scrub¬ ber stage is here easily adjusted and compensated for by the evaporation in the evaporator 20. In accordance with the invention, the drawing-off of liquid from the second stage may therefore be optional and instead be condition¬ ed by the salt concentration in the second stage, i.e. the need of removing salts, such as chlorides, from the second stage. It is further possible to adjust the eva¬ poration in the evaporator 20 in such a manner that the drawing-off of liquid taking place in the separation of gypsum sludge at 65 is sufficient. Since the amount of liquid drawn off in accordance with the invention is con- ditioned by the salt concentration instead of the amount of water condensed, the present invention results in a considerable reduction in the amount of liquid drawn off. This reduction may amount to as much as 75%, or even exceed this figure. When the liquid drawn off is trans¬ ferred from the secondary circuit 53 via the conduit 71 to the secondary circuit 35 of the first stage, this reduction according to the invention involves a highly significant sulphate reduction, which is of great impor- tance in plants having no separation of Glauber salt of the type shown in Fig. 2, since this reduces the risk of scaling. Actually, the amount of liquid drawn off, and hence the sulphate concentration, may in accordance with the invention be reduced to such a level that the sul- phate concentration becomes subcritical, i.e. is well below the limit of approximately 1,000 mg/1 required for the precipitation of gypsum. It thus becomes possible to completely eliminate the difficult scaling problems pre¬ viously associated with the transfer of drawn-off liquid from the second stage to the first stage.

To further illustrate the invention, it may be mentioned that a plant, which generates approximately 100,000 Nm.3 of flue gas/h having a sulphur-dioxide con¬ centration of approximately 500 mg/h and a hydrogen-chlo- ride concentration of approximately 1,000 mg/h, yields approximately 700 1 of condensate/h in the second scrub¬ ber stage (the cleaning stage for sulphur dioxide) . With¬ out the use of the present invention, a corresponding amount of liquid would have to be drawn off from the second stage which, when the indirect lime process is used in the second stage, would involve a chemical loss (of sodium sulphate) of approximately 40 kg/h. The cost for replacing this chemical loss is considerable, as is the cost for treating the liquid drawn off, usually in order to be cleaned. When use is made of the present invention and the liquid flow is approximately 30 ^/ in the secondary circuit of the second stage and through the evaporator and the air flow is approximately 10,000 Nm³/h through the evaporator, the amount of liquid drawn off is reduced from approximately 700 kg/h to approximately 70 kg/h, i.e. to 1/10 of the former. This amount of liquid roughly corresponds to the water leaving with the precipitated gypsum (approximately 175 kg/h). In order to reheat the cooled washing liquid in the secondary circuit of the second stage, two heat exchangers of totally 400 kW are required. It is evident from the foregoing that the present invention enables considerable savings and that the invention can be implemented in existing as well as new plants at a fairly small cost.

Claims

1. A method for regulating the amount of liquid in the wet cleaning of a gas in at least two stages, c h a r a c t e r i s e d in that liquid from at least one of the stages after the first stage is brought into contact with air, such that the liquid is partly evapo¬ rated and cooled, the evaporation being carried out to such an extent that the amount of liquid in the stage remains the same, whereupon the remaining liquid is heat¬ ed before being recycled to said stage.

2. A method as set forth in claim 1, c h a r a c ¬ t e r i s e d in that the gas is a flue gas which con- tains hydrogen chloride and sulphur dioxide and which is first wet-cleaned with a view to removing hydrogen chlo¬ ride and then wet-cleaned with a view to removing sulphur dioxide, liquid from the sulphur-dioxide wet cleaning being partly evaporated and cooled, whereupon the remain- ing liquid is heated before being reintroduced for renew¬ ed sulphur-dioxide wet cleaning.

3. A method as set forth in claim 2, c h a r a c ¬ t e r i s e d in that the sulphur-dioxide wet cleaning is performed in a primary circuit and a secondary circuit in accordance with the indirect lime process, and that the evaporation and the cooling of the liquid are performed in the secondary circuit before the liquid is treated with a view to separating absorbed sulphur dioxide.

4. A method as set forth in claim 3, c h a r a c - t e r i s e d in that the separation of absorbed sulphur dioxide is carried out in the form of gypsum precipita¬ tion.

5. A method as set forth in claim 3 or 4, c h a r ¬ a c t e r i s e d in that the heating of the remaining liquid is carried out after the separation of absorbed sulphur dioxide.

6. A method as set forth in any one of claims 1-5, c h a r a c t e r i s e d in that the heating of the cool¬ ed liquid is brought about by heat exchange with liquid from the first stage.

7. An apparatus for regulating the amount of liquid in the wet cleaning of a gas, said apparatus comprising a scrubber (6) having at least two stages and a liquid-cir¬ culation circuit (53) in at least one of the stages after the first stage, c h a r a c t e r i s e d in that the liquid-circulation circuit (53) includes an evaporator (20) adapted to evaporate liquid to such an extent that the amount of liquid in said stage remains the same and to cool liquid from said circuit by contacting it with air, as well as a heater (28, 44) adapted to heat the cooled liquid in the circuit.

8. An apparatus as set forth in claim 7, c h a r ¬ a c t e r i s e d in that it includes a scrubber (6) hav¬ ing a hydrogen-chloride-cleaning stage and at least one sulphur-dioxide-cleaning stage, and that^" the evaporator (20) is connected to the sulphur-dioxide-cleaning stage.

9. An apparatus as set forth in claim 8, c h a r ¬ a c t e r i s e d in that the sulphur-dioxide-cleaning stage is designed in accordance with the indirect lime process and thus includes a primary circuit (51) and a secondary circuit (53), and that the evaporator (20) is connected to the secondary circuit ( 53) before the sepa¬ ration of sulphur dioxide.

10. An apparatus as set forth in claim 9, c h a r ¬ a c t e r i s e d in that the sulphur-dioxide separation in the secondary circuit is carried out in the form of gypsum precipitation (65).

11. An apparatus as set forth in any one of claims 7-10, c h a r a c t e r i s e d in that the heater (28, 44) is a heat exchanger for heat exchange with liquid from the first stage, and that said heat exchanger is disposed after the sulphur-dioxide separation in the secondary circuit (53) .

12. An apparatus as set forth in any one of claims 7-11, c h a r a c t e r i s e d in that the evaporator (20) comprises a tower (57) for countercurrent contact between air and finely-divided liquid, said tower having a liquid inlet (19), a liquid outlet (24, 61), an air inlet (22, 59), an air outlet (60) as well as a fan (23) for feeding the air.

13. An apparatus as claimed in claim 12, c h a r ¬ a c t e r i s e d in that the evaporator (20) comprises a recirculation conduit (72) arranged between the air out¬ let (60) and the air inlet (22, 59) to recirculate some of the air.