FI84434B - Method and installation for recovery of sulfur and/or nitrogen oxides from gases containing them - Google Patents

Description

84434

Method and apparatus for recovering sulfur and / or nitrogen oxides from gases containing them

The invention relates to a process according to the preamble of claim 1 for recovering acid gases, in particular sulfur and nitrogen oxides, from gases.

The invention also relates to an apparatus according to the preamble of claim 13.

The present process is useful for recovering acidic gaseous compounds such as sulfur and nitrogen oxides from gas mixtures. The temperature of the gas mixture can be -50 ... + 120eC and it can be rich in carbon dioxide, nitrogen and oxygen. The gas mixture is most usually a mixture of carbon dioxide and sulfur and nitrogen oxides without air, which can come from a process in which the above-mentioned acidic compounds are formed as waste. as tail gas, such as in sulfuric acid factories or process parts using sulfuric acid. The gas can also come from fossil fuel power plants, such as coal, so-called thermal or electric power stations using burning rock or fuel oil.

Numerous methods have been developed for the removal and / or recovery of acid gases and nitrogen oxides of sulfur, some of which are given below: 1. Grate methods

Sulfur oxides can be bound already at the combustion stage to a basic compound, such as limestone, magnesite or a similar mineral, which reacts with sulfur dioxide to form a sulphate of the general formula MeSO 4 ·

The MeSC> 4 can then be removed as waste and the flue gas from the furnace is substantially free of sulfur dioxide.

Such a method is, for example, the "Pyroflow®" process developed by Ahlström Oy, in which limestone or compounds made from it, such as CaO, are used to bind sulfur oxides.

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The nitrogen oxides generated in the combustion process can be removed either by combustion techniques or by various chemical additions, such as by injecting NH3 gas, which reacts with the nitrogen oxides, into the flue gas after combustion.

2. Flue gas methods

The sulfur oxides in the gases can be further bound to the above-mentioned basic compound by treating the process gas with this compound. In this case, the compound is reacted with the gas phase either dry (e.g. CaO, MgO, ZnO), semi-wet (Ca (OH) 2) or in solution (e.g. MeOH,

Me 2 SO 3 and NH 3).

Numerous indirect lime methods are also known which use lime. Sulfur-reactive Me solutions were regenerated according to the following reactions: 2MeOH + SO2 ------> Me2SO3

Me 2 SO 3 + Ca (OH) 2 ------> CaSO 3 + 2MeOH

The first descaling method was developed as early as 1884 (Schroeder and Hanisch) and ICI installed the first Ca (OH) 2 slurry desulphurisation plant in Swansea in the 1930s.

The so-called semi-dry methods are known e.g. LIFAC process developed by Tampella Oy.

The most well-known wet processes are General Electric's NaOH process, the Wellman-Lord process, in which the adsorbent is sodium bisulphite and the product is sulfur dioxide or sulfuric acid, the NH3 process in UHDE, the product is (NH4) 2SO4 · Outokumpu Oy is also known: n Sulfred process in which the product is elemental sulfur and RR / CAT process (FI patent publication 73603, FI patent application 874363) in which the product is potassium sulphate.

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We will look in particular at the latter process. The process can be divided into separate entities: a) preparation of potassium hydroxyacetone sulfonate, b) decomposition of potassium bisulphite into potassium sulphite and sulfur dioxide, c) oxidation of potassium sulphite and d) scrubbing of SC> 2 and NOx from flue gases.

The potassium hydroxyacetone sulfonate from which the potassium sulfite used for flue gas scrubbing is prepared is made in this process by an ion exchange method known per se, using potassium chloride and alpha-hydroxyacetone sulfonic acid as starting material.

Potassium acetone sulfonate decomposes on heating to potassium bisulphite according to equation (I): (1) K (Ac) SO 2 OH ------------> khso3 + Ac KHSO3 can be decomposed, for example, by drying it to its anhydride: K2S2O5, which decomposes further on heating to K2SO3 and SC> 2.

In the process according to FI patent 73603, the K 2 S 2 O 5 crystals are separated in a scrubber and oxidized directly to I 2 SO 4 at 200 ° C. However, in the decomposition process by indirect heating of K2S20s, as a result of side reactions, e.g. potassium thio compounds and potassium sulfate. Thio compounds can accumulate in the process cycle and impair absorption yield. On the other hand, the K2SO4 formed can be removed from the solution as the most difficult to crystallize and thus obtain the finished end product of the process.

Furthermore, in the process according to the above-mentioned patent, saturated K2SC> 3 solution has been used for the purification of SC> 2 -containing gases, whereby crystals easily form in the process, which clog the process equipment.

In the process according to BE patent publication 706449 (Beckwell Process Co.), the K2S205-H2O crystal mass obtained from the washing of sulfur dioxide-containing gases is heated in an autoclave at a temperature of 110 to 191 ° C and an overpressure of 1 to 11 at. In this case, according to the factors, the K 2 S 2 O 5 crystals "dissolve in their water of crystallization" and the desired decomposition reaction to recycle SO 2 takes place. In the latter process, in addition to the inconveniences caused by the processing of the above-mentioned crystals, e.g. continuous operation of a large pressure vessel with indirect heating.

The decomposition process of K2s2 ° 5 in the above-mentioned manner can therefore only be run in batch mode, which is difficult as a technical solution. In our experiments we have been able to find that the desorption yield of this prior art process is low (only about 6 ...

13%, cf. Example 7), in which case the process is subjected to uneconomically high rotational loads.

The object of the present invention is to eliminate the drawbacks associated with the prior art and to provide a completely new solution for the recovery of acid gases, in particular oxides of sulfur and nitrogen.

The invention starts from the same starting point as in FI patent 73603, i.e. the gases to be purified are washed in the reaction state with an aqueous solution, the solute of which consists mainly of potassium sulphite. When potassium sulfite is reacted with oxides of sulfur and / or nitrogen, potassium bisulfite is formed. According to the method, at least a part of the potassium sulfite in the washing liquid is prepared from a potassium hydroxyacetone sulfonate solution by first removing acetone and decomposing the resulting potassium bisulfite solution into potassium sulfite.

The invention is based on the idea that sulfur dioxide containing 5,84434 of the potassium bisulfite-containing solution obtained from the above-mentioned gas treatment is desorbed by subjecting the solution to a steam heating treatment. In practice, the potassium bisulfite is decomposed by heating it directly at 101 to 800 ° C, preferably at 125 to 250 ° C with steam to release sulfur dioxide. Steam heating can be carried out, for example, in a countercurrent desorption device at an overpressure of 0.1 to 250 atoms, preferably 1 to 25 atoms and particularly preferably about 8 to 12 at. The reaction can also be carried out under normal pressure.

In the process of this application, the decomposition of KHSO 3 by pressurized steam takes place in the following reactions in the liquid phase: (2) 2KHS03 ------------> K2SO3 + SO2 + H2O The rise in temperature causes a strong increase in the H + ion concentration of the solution, which in turn drive the reactions (3) 2HSO3 + 2H + ----------> 2H2S03 (4) in the direction of H2S03 --------------> H20 + S02 of the arrow.

When scrubbing NOx-containing flue gas with K2SO3, 70-80% of NOx compounds can be removed from the flue gas. Elimination of NOx occurs mainly through the following reactions:

(9) 2NO + 2KHS03 ----------> 2KHS03 * NO

(10) 2KHSO3 * NO ----------> KHSO3 (N0) 2 + KHSO3 (11) KHSO3 (N0) 2> KHSO4 + N2O (12) 2K2SO3 + NO2 -------- -> 2K2SO4 + 0,5N2 (13) 2K2SO3 + NO2 + 02 ----> KNO3 + K2SO4 + K2SO3

Water vapor is usually used in the invention, but other suitable vapors may also be considered. It is important that enough thermal energy is quickly introduced into the solution phase. Thus, as can be seen from the equations above, the desorption reactions are based primarily on the effect of the amount of heat introduced by the steam, and not on the possible chemical reaction of the steam and bisulfite.

The potassium bisulphite in the solution decomposes at least partially into potassium sulphite through heat treatment. A portion of the resulting potassium sulfite / potassium bisulfite solution is separated and treated to prepare crystalline sulfur compounds and sulfur dioxide. The latter compounds are recovered. Preferably, at least a portion of the sulfur dioxide is used to prepare the potassium hydroxyacetone sulfonate solution.

An amount of fresh potassium sulfite from the potassium hydroxyacetone sulfonate process is introduced into the recycling of the potassium sulfite wash liquor, corresponding to the separable portion of potassium sulfite and potassium bisulfite.

More specifically, the method according to the invention is mainly characterized by what is set forth in the characterizing part of claim 1.

Based on the idea according to the invention, it is also possible to provide a completely new type of apparatus for recovering sulfur and / or nitric oxide. An essential part of the apparatus is then a steam desorption device with which the potassium bisulfite solution can be decomposed.

The apparatus according to the invention is characterized by what is set forth in the characterizing part of claim 13.

For the purposes of this application, "washing" means essentially the removal of sulfur and / or nitrogen oxides and similar impurities from gases by contacting them with solutions so that the impurities are absorbed into the liquid.

The product obtained from the washing tower is called an aqueous solution of "potassium bisulfite". However, it is clear that some potassium bisulfite anhydride, potassium pyrosulfite, i.e. potassium (meta) bisulfite, K2S2O5, may also be present.

The potassium sulphite-potassium bisulphite solution obtained from the steam desorption of potassium bisulphite may, despite the heat treatment carried out during the desorption stage, contain reasonable amounts of potassium bisulphite, which are thus returned to the washing tower through recycling. It should be noted that while a high concentration of potassium sulfite is an advantage for the absorption process in the scrubber, reasonable amounts of potassium bisulfite are not very detrimental to absorption. However, in the process according to the invention, the aim is always to keep the amount of potassium sulphite in the washing solution higher than the amount of potassium bisulphite. On the other hand, a high concentration of potassium bisulphite solution is advantageous for the release of sulfur dioxide in the desorption device. The higher the potassium bisulphite / potassium sulphite ratio, the more sulfur dioxide is released. The method according to the invention seeks to balance these factors acting in partially opposite directions and to optimize the compositions of the solutions fed to both the scrubber and the desorption device.

By "recycling" of a washing liquid is meant the cycle of the process in which the solution from the scrubber is regenerated by desorption, the desired products are separated, and the remainder of the regenerated solution is returned to the scrubber.

"Fresh feed" means a scrubbing liquid, i.e., an aqueous solution containing substantially potassium sulfite, that has not yet been used to treat gases in a scrubber.

The pressure of the pressurized solution obtained from the desorption process is allowed to dissipate either by bringing the solution to normal atmospheric pressure or by cooling the solution to a temperature of 0 to 115 ° C, preferably about 20 to 40 ° C.

The process according to the invention is an integrated whole, an essential part of which involves the preparation of a fresh feed e 84434 of potassium sulphite. This is described in more detail in the detailed explanation below. It should be noted in this connection, however, that the potassium bisulphite solution obtained when acetone is separated from the potassium hydroxyacetone sulphonate mixture (e.g. by distillation) is preferably decomposed by heating, preferably by direct steam heating, before being combined with the washing liquid recycle. The steam heating is carried out at 101 to 800 ° C, preferably 101 to 250 ° C, preferably in a counter-current desorption apparatus.

According to a preferred embodiment of the invention, the steam heating of said potassium bisulfite solution is carried out in the same desorption apparatus as the steam heating of the potassium bisulfite solution to be removed from the reaction space. Preferably, the fresh feed is then recycled by combining the potassium bisulfite solution from the potassium hydroxyacetone sulfonate process with the potassium bisulfite solution from the reaction space and decomposing the combined potassium bisulfite solution by direct steam heating. The amount of potassium equivalent in the fresh feed to be washed for recycling is at least approximately equal to the total amount of potassium equivalents of the potassium sulfite and potassium pyrosulfite compounds to be separated.

According to the invention, considerable advantages are achieved. Thus, most of the excess thermal energy used in the decomposition of K2S20s by direct steam heating can be recovered. SO2 is recovered from the desorption part of the process in high yield, thus avoiding high rotational loads. The formation of thio compounds is minimized. The process is easy to run continuously.

With regard to the apparatus, it can be stated that it is generally suitable for use in processes in which a solution containing potassium bisulphite is obtained from the washing of sulfur and / or nitrogen oxides. Thus, it can be used not only in the process described above but also in those processes in which potassium hydroxide and / or potassium carbonate are used as the washing solution.

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The invention will now be examined in more detail with the aid of the accompanying drawing, which shows a simplified flow diagram of the method according to the invention.

Sulfur or nitrous oxide-containing gas, i.e. the gas to be purified, which may be e.g. flue gas from the fossil fuel combustion process, is passed under pressure through a blower to a scrubber 1, i.e. a scrubber, where sulfur and nitrogen oxides are absorbed into an aqueous solution of potassium sulphite. The scrubber 1 used in the method can be of any conventional type, i.e. e.g. an empty tower with spray or spray nozzles at the top or filled with filling pieces. The scrubber 1 can also be of the type in which spacers are arranged in the tower, by means of which sudden changes in the direction of the gas flow, which enhance the transfer of matter, are effected.

The potassium sulphite solution fed to the upper end of the scrubber 1 is sprayed or sprayed in the scrubber into the gas flowing from the bottom upwards, whereby most of the sulfur or nitrogen oxides are absorbed into the liquid droplets. When using a downstream scrubber, such as a so-called Venturi scrubber, both the gas and the potassium sulfite solution flow in the same direction, i.e. from top to bottom. Upon reaction with the potassium sulfite-water system, sulfur dioxide forms potassium bisulfite. The nitric oxide reactions are described above in Equations 9-13. It should be noted, however, that potassium bisulfite is also formed in the reactions of nitrogen oxides. The gas exits the upper end of the scrubber tower 1 and can either be discharged directly into the open air or used as a process gas. The operating parameters of the scrubber, i.e. the pressure and temperature prevailing therein, as well as the solution and gas flow rates, are chosen so that the potassium bisulfite-containing solution to be removed from the lower end of the tower is enriched in bisulfite but has not yet reached saturation.

The potassium sulphite content of the solution used as absorbent is preferably optimally high. As mentioned above, it promotes a high concentration of sulfur and nitrogen oxides absorption of 84434 tlota. On the other hand, however, it is important to avoid crystallization of potassium sulfite in the scrubber. For this reason, the process according to the invention does not use a saturated solution or a solution which, under the conditions prevailing in the scrubber, would become saturated with respect to potassium sulphite. Thus, the potassium bisulphite concentration of the washing solution is preferably about 1 to 20% by weight.

Although the scrubber 1 is usually operated at normal atmospheric pressure, pressurized or vacuum solutions can also be considered. The temperature in the scrubber is maintained at about 20 to 150 ° C, preferably about 20 to 100 ° C. Low temperature promotes absorption. Preferably the temperature of the solution is about 20 ... 40eC. The flue gases are also cooled efficiently before they are fed to the scrubber.

The absorption is preferably continued by circulating the washing liquid in the scrubber 1 until no more sulfur and / or nitrogen compounds are absorbed in the potassium sulphite solution. At this stage, about 90% or more of the sulfur and / or nitrogen compounds originally contained in the gas are absorbed into the scrubbing liquid. As the sulfur and nitric oxide contents of the gas are quite low, as mentioned above, the washed gas can be removed from the scrubber and discharged to the outside air. The exhaust gas is then preferably heated above the dew point of the vapors therein to prevent condensation otherwise occurring in the chimney 2. Alternatively, the exhaust gases are used, for example, as process gases.

Flue gases are known to contain silicate-based solids such as fly ash. Many of these can be removed, for example, with electrostatic precipitators before the flue gases are fed into the scrubber. However, for the present process, it is not necessary to completely remove fly ash from the flue gases before washing. Thus, the solids in the gases form a slurry-like bottom fraction in the scrubber, which can be removed from the bottom of the scrubber 1. The bottom fraction is then passed to clarifier 3, where the aqueous phase is separated from it.

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A potassium bisulfite-containing liquid stream from scrubber 1 is passed through a pump to a heat exchanger 4, where it is heated to a higher temperature. As the heat exchanger 4, a counter-current device can be used, the warm side of which is connected to the exhaust stream of the next device, the steam stripper 5. The operation of the steam stripper is described in more detail below. A hot stream of potassium sulphite from the desorption of sulfur dioxide is used to heat the liquid stream from scrubber 1, which at the same time cools down. This solution is particularly advantageous because the latter potassium sulfite solution is recycled to the scrubber 1, where it is used as a washing liquid.

The potassium bisulphite flow to the heat exchanger 4 is usually about 50 * ... 80eC. In a heat exchanger, its temperature is raised at least approximately to the level required for steam stripping, i.e. usually to about 150 ... 200eC. In this case, the pressure of the potassium bisulphite stream must also be raised to a corresponding level in order to prevent the water in the liquid phase from evaporating. A pump fitted before the heat exchanger is used to increase the pressure. The pressure is increased to about 1 ... 25 bar, depending on the steam used in the stripper.

As mentioned in the preamble of the description, the potassium bisulfite solution is decomposed with steam. This is done in a sulfur dioxide stripper, i.e. a steam desorber 5. The desorption device used, which preferably operates on the countercurrent principle, can be of any conventional type, such as, for example, an empty tower with spray or spray nozzles at the top or a column filled with screen bottoms or fillers. In place of said screen bases, bell bases or similar spacers can be used, on which a thin layer of liquid is formed. The fillers may be of a type known per se; in one embodiment of the invention (Example 3) rings made of polytetrafluoroethylene material are used. It is important for the invention that good contact is made in the stripper between the liquid and the water vapor to be fed, without the flow resistance of either becoming too high.

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The preheated and pressurized potassium bisulfite solution is fed to the top of the stripper 5 and the stripping steam to be used to the bottom of the column, respectively. The potassium bisulfite solution is sprayed or sprayed in a scrubber onto bottom-up flowing water vapor. In the stripper 5, the bisulfite compounds decompose to form sulfur dioxide, which exits the stripper with water vapor through an outlet pipe fitted to the top of the column. The stripper preferably uses pressurized steam at a temperature of 101 to 800 ° C, preferably 125 to 250 ° C, and a pressure of 0.1 to 250 aty, preferably 1__ to 25 aty, respectively. Preferably, steam is introduced into the stripper at a temperature at least somewhat higher than the potassium bisulfite solution to be fed. An increase in the temperature of the solution then causes an increase in the concentration of hydrogen ions. Bisulfite ions are converted to the labile form of sulfuric acid, which in turn decomposes into water and sulfur dioxide (cf. Reactions 3 and 4). Saturated steam is usually used, in which case the conditions prevailing in the stripper are determined by the saturation temperature of the water vapor and the corresponding pressure. Typically, it operates under conditions where the temperature of the solution fed to the desorber is about 150 ° ... 200eC. The temperature of the strip vapor is then about 160 ... 210eC and its pressure is about 7 ... 21 bar.

If necessary, superheated steam can also be used for stripping.

As mentioned above, it is desired in the invention to achieve the best possible contact between the liquid and the water vapor during desorption. Thus, according to a preferred embodiment of the invention, the flow rate of the potassium bisulfite solution fed to the column 5 is kept relatively small relative to the volume of the column. Preferably, the bisulfite solution is applied to the column at a flow rate of 0.1 to 10 column volumes per hour. The steam can be introduced into the column 5 in an amount of about 0.01 to 10 times the amount of sulphite solution fed, the amount of steam is preferably about 5 to 100% of the amount of the solution and particularly preferably 5 to 20% thereof. According to a preferred embodiment of the invention, the amount of steam to be fed is about one tenth of the amount of solution. In the same embodiment, the operating parameters of the column are set so that about half of the steam is removed with the sulfur dioxide released. The exhaust gas then contains about half water vapor and half sulfur dioxide.

The desorption exhaust gases are passed to a condenser 6 to separate the water from the sulfur dioxide. The pressure of the gases is reduced to normal atmospheric pressure and the temperature is lowered to less than 100 ° C, preferably 20 to 50 ° C, in order to condense the water. Sulfur dioxide is passed to the fresh feed (eluent) used in scrubber 1 and the water is combined with the potassium sulfite-containing solution obtained from desorption.

The bottom of the desorption device 5, which essentially contains potassium sulfite, sulfur dioxide and water, is passed to a heat exchanger 4, where its heat content is used to heat the product stream fed to the desorption. Preferably, the substituent is still considered to be under pressure at this stage. After the heat exchanger, the potassium sulfite-water-sulfur dioxide mixture is depressurized by bringing it to normal atmospheric pressure and / or cooling the mixture. This releases sulfur dioxide from the liquid phase. A portion of the solution is then passed to a mixing tank 7, where some water, e.g. water from a condenser 6, is added if necessary. The product obtained from the mixing tank 7 is used in the scrubber 1 as a flue gas scrubbing liquid.

The other part of the potassium sulphite-water-sulfur dioxide mixture is in turn passed to the crystallizer 8, where water is evaporated from it so that the potassium sulphite content of the solution becomes about 30 to 70% by weight, preferably about 40 to 60% by weight. At the same time, the solution is cooled to a temperature of 0 to 50 ° C, preferably 20 to 40 ° C. Sulfur dioxide is removed from the crystallizer 8 along with the water vapor, and the gas mixture is passed to a condenser 9, where the sulfur dioxide and water are separated from the other 84434 as described above. Again, at least a portion of the sulfur dioxide and water are passed to produce acetone hydroxysulfonic acid. During evaporation, the solution containing potassium sulfite becomes saturated with potassium sulfite and differs from the crystals as evaporation continues. In addition to K2SO3 crystals, the crystal mass also contains some K2SO2 and K2SO4.

As the crystallizer 8, any crystallization equipment known per se can be used. According to a preferred embodiment of the invention, a vacuum crystallizer is used.

The crystals are separated from the bottom product of the crystallizer 8 and further fed to a drying-oxidation drum 14, where the crystalline mass is oxidized to produce potassium sulfate. The oxidation product may still contain some potassium pyrosulfate. The oxidation / drying is typically performed in a stream of air at a temperature of 100 to 800 ° C, preferably 400 to 650 ° C.

The sulfur dioxide or part thereof obtained from the drying-oxidation drum 14 is passed to the production of acetone hydroxysulfonic acid as described above.

The potassium sulfite used in the process is known to be prepared, for example, by introducing sulfur dioxide gas into a potassium hydroxide or potassium carbonate solution. However, according to the invention, the potassium sulfite is preferably prepared by the ion exchange method mentioned in the introduction to the description, in which potassium chloride and alpha-hydroxyacetone sulfonic acid are used as starting materials. In this connection, we refer to FI patent 73603.

Sulfur dioxide from the steam desorption, drying-oxidation drum and other SO 2 collection points, which may already already contain some water, is fed to a regenerator 10 where it is absorbed into the water-acetone mixture. The strong hydroxysulfonic acid solution thus obtained (denoted as PF acid in the figure) is pumped into an ion exchange resin 11 in K + form, which can be prepared, for example, by adsorbing potassium ions i5 84434 from an aqueous solution of potassium chloride 13 into a cation exchange resin. Elution of the potassium adsorbed on the ion exchange resin gives potassium hydroxyacetone sulfonate, which decomposes on heating according to reaction 1. The hydrochloric acid obtained from the ion exchanger can be neutralized, e.g. with a suitable carbonate compound. The hydroxyacetone sulfonate product is therefore heated until the acetone has distilled off. Some of the potassium bisulfite obtained further decomposes on heating to potassium sulfite and sulfur dioxide. The acetone and sulfur dioxide from the heating are returned to prepare the hydroxysulfonic acid solution. The removal of acetone and sulfur dioxide can be carried out in the same device, the so-called acetone stripper 12.

The potassium sulfite solution obtained as described above can be fed directly to the scrubber for use as a washing liquid.

However, virtually all of the potassium bisulfite cannot be decomposed in the acetone stripper 12, so it is preferable to combine the above solution with the potassium bisulfite solution from scrubber 1 and pass it through heat exchanger 4 to desorption device 5. Part of the potassium sulfite stream

Alternatively, the potassium bisulfite solution prepared by the ion exchange method is decomposed in a separate steam stripper. In this case, the finished potassium sulphite solution is introduced into the washing liquid only after the point at which part of the potassium sulphite and bisulphite is separated. Thus, the potassium sulfite solution can be fed e.g. to the mixing tank 7.

Among the conditions of the process for preparing the potassium sulfite solution, the following can be mentioned: The concentration of the potassium chloride solution used in the ion exchange is about 1 to 300 g / l, preferably about 200 to 300 g / l. The hydroxyacetone sulfonic acid used to elute the ion exchanger has an acetone content of 1 to 80% by weight, preferably about 10 to 30% by weight, and a sulfur dioxide content of 10 to 200 g / l, preferably about 50 to 30% by weight.

ie 84434 150 g / l. The acetone of the ion exchange eluate is distilled at a temperature of 58 to 120 ° C, preferably at a temperature of 90 to 110 ° C,

From the material balance of the process, it should be mentioned that the amount of potassium leaving the system through the crystallizer 8 and the drying and oxidation 9 is compensated by the amount of potassium introduced into the ion exchange. That is, an amount of potassium sulfite corresponding to the amount of potassium ion introduced for the production of hydroxyacetone sulfonate is passed from the heat stream obtained from the heat exchangers to crystallization to maintain the mass balance. The rest of the potassium sulphite is passed to the scrubber 1. In practice, the amount of potassium sulphite recycled to the scrubber 1 is 1 to 5 times, preferably 2 to 3 times, relative to the amount passed to the crystallizer 8.

The following examples further illustrate the process of this invention.

Example 1. Flue gas scrubbing

A column with a diameter of 22.5 cm and a total height of 3.8 m was fed with flue gas with the following composition: SO2 1163 ppm CO2 15.2% 02 4.3%

The gas temperature was 90 ° C and the flow rate was 36.5 m3 / h. Flue gas flowed from the bottom up in the column.

At the same time, a washing solution of potassium sulfite / potassium bisulfite with a total content of 20% by weight (w / w) and a ratio of K 2 SO 3 / K 2 S 2 O 5 = 3 was fed to the column from the top via spray nozzles. The flow of the washing solution was 4.1 l / h.

The SO2 was absorbed into the wash solution by 91% and the ratio between the absorbed sulfur dioxide and the potassium sulfite fed was 0.244 mol / mol.

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The concentration of K2SO3 ~ in the washing solution, more precisely the ratio K2SO3 / (K2SO3 + K2S2O5) (mol / mol), changed, being 0.82 in the feed and 0.56 in the effluent.

Example 2. Vapor desorption of a potassium bisulfite solution A K2s2 ° 5 / K2SO3_ solution 'having the following composition was fed to a steam desorption apparatus:

25% by weight K2S2O5 10% by weight K2SO3 65% by weight H2O

The solution was fed at a rate of 5 ml / min.

At the same time, steam with a temperature of 200 eC was fed to the device. The steam feed rate was 70 g / min.

The analysis of the subcondensation in which the desorption product obtained mainly accumulated was as follows: 3.2% by weight K2S2O5 20.1% by weight K2SO3

76.7% by weight H2O

The desorption rate was thus 84%.

The heat of the steam from the process was collected with a heat exchanger.

Example 3. Vapor desorption of potassium bisulfite solution

In the above-mentioned steam desorption apparatus, 15 cm long Teflon tube sections with a diameter and a length of 7 mm were placed as fillers. Steam was fed to the column at a rate of 70 g / min and, in the opposite direction to the inflow of steam, a solution of

25% by weight K2S2O5 10% by weight K2SO3 65% by weight H2O

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The solution was fed at a rate of 5 ml / min. The temperature in the test apparatus was 180 ° C.

The subcondensation analysis was as follows: 7.75 wt% K2S2O5 16.5 wt% K2SO3 remaining water.

The desorption rate of SO 2 was 69%.

Example 4. Preparation of potassium sulfite solution

A KCl solution with a concentration of 252 g KCl / 1 was introduced into an Amberlite 200 ion exchange mattress with a height of 74.5 cm and a volume of 387 ml at a flow rate of 4.8 BV / h (BV = bed volume). The potassium of the solution was adsorbed on the ion exchange resin in the H + form.

The HCl formed in the ion exchange was then washed off, after which the mass was eluted with an aqueous solution of 20% by weight of acetone and 115 g / l of sulfur dioxide.

A sample of 0.5 to 1.5 BV was collected from the eluate. The composition of the solution was as follows: K 52.6 g / l SO 2 93.0 g / l

Acetone 146 g / l H2O remaining

From a solution of 1 BV, i.e. 387 ml, acetone was now distilled off by heating it to boiling point and keeping it at 105-108 ° C until 145 ml of excess had accumulated. At that time, the acetone had been distilled in its entirety to the excess.

The alite was 240 ml. Alite was now fed at a rate of 5 ml / min to the steam desorber described in Example 2.

The steam condensation subcondensation was 300 ml and had the following composition: i9 84434 2.2% by weight K2S2O5 13.7% by weight K2SO3

84.1% by weight H2O

Thus, a potassium sulfite solution suitable for flue gas scrubbing had been prepared in the process.

Example 5. Comparative example

According to Finnish patent 73603, a synthetic mixture of K 2 S 2 O 5 and K 2 SO 3 with 80% K 2 S 2 O 5 and 20% K 2 SO 3 was prepared and heated at 220 ° C for 10 minutes.

The salt then reacts mainly according to equation (5) K2S205 --------> K2SO3 + SO2.

However, disproportionation of K2S205 also occurred according to the following equations: (6) 3 K2s205> 2 K2SO4 + K2S203 + 2 SO2 (7) 2 K2S205> K2S306 + K2SO4

(8) K2S306> K2SO4 + SO2 + S

so that about 75% reacts according to Equation (5), 15% according to Equation (6) and about 10% according to Equations (7) and (8).

In particular, the formation of S in the process is detrimental because, when it enters the fertilizer, it forms harmful compounds in the soil upon oxidation, which can be harmful to the crop.

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Example 6. Gas scrubbing, steam desorption and preparation of potassium sulfite

A K2SO3 solution was fed to a continuous scrubber with transverse plates which caused sudden changes in the direction of gas flow and thus improved the mixing of gas and liquid with each other at a feed rate of 30 l / min. At the same time, SO2-containing flue gas was fed to the scrubber at a feed rate of 233 m3 / h. The SC> 2 content of the gas was 1899 ppm.

The washing solution leaving the scrubber is passed through a heat exchanger to an SC> 2 stripper at a spray rate of 24 l / min. At the same time, steam with a temperature of 220 ° C and a pressure of 21 aty was blown into the reactor.

The KHSO3 of the solution then decomposed according to the equation 2KHSO3 ---------> K2SO3 + SO2.

At the same time, a KHSO 3 solution prepared from Amberlite 200 ion exchange resin (cf. Example 3) was fed to the stripper - by feeding to the ion exchanger 3.4. 4-molar (250 g / l) KCl solution at a rate of 4,5 to 5 BV / h, - washing the resin in K + form with water to remove HCl - eluting the K + resin with acetone-hydroxysulphonic acid - distilling off the potassium hydroxyacetone sulphonate thus obtained At a temperature of 105 ... 110β lämpöt: η.

The solution was moistened simultaneously with the solution from the scrubber at a rate of 12 l / h.

A portion of the solution obtained by steam desorption was passed to a crystallizer, from which K 2 SO 3 crystals, which still contained K 2 S 2 O 5 (12.4% of the total weight of the crystals) and K 2 SO 4 (2.5% of the total weight of the crystals), were separated and fed to a drying oxidizer. the product was oxidized so that the final product contained 97% K2SO4 and about 3% K2S2O7.

The H 2 O-SO 2 gas streams from the steam desorption, drying-oxidation drum and other SO 2 collection points were passed through condensers to produce acetone hydroxysulfonic acid, to which acetone from the distillation of acetone was also passed.

Example 7. Comparative example

According to the process presented by Beckwell (BE patent 706449), K2S2O5 - K2SO3 - K2SO4 - H2O - crystal mass having the following composition was taken: 45% by weight K2S2O5 2.5% by weight K2SO3 2.5% by weight K2SO4 was heated in an autoclave at 160 ° C and pressure 4.7 bar. 13% of K2s2 ° 5 Sea was desorbed.

Alternative solutions:

Within the framework of the invention, solutions that differ from the above can also be considered. Thus, K2S2O5 can be crystallized from the K2S2C> 5 / K2SO3 solution obtained after steam desorption and heat exchanger. These crystals are decomposed into SC 2 and K 2 SO 3 by heating them indirectly, e.g. in a microwave field.

Alternatively, a catalytic surface is used to decompose the K2S20s crystals, which accelerates the release of SC> 2 from K2S2O5, thus preventing the formation of harmful thio compounds in a slower reaction. Of course, plasma technology, for example, can also be used to decompose potassium bisulfite crystals.

Other crystalline sulfur compounds, such as potassium pyrosulfate, can also be produced by the process of the invention.

22 84434

Instead of a cation exchanger, the potassium hydroxyacetone sulfonate formed in the preparation of potassium sulfite can be obtained by binding hydroxyacetone sulfonic acid to an anion exchanger eluting with potassium chloride. Cl adsorbed on the anion exchanger is eluted with Ca (OH) 2.

The KHSO3-HUOS leaving the scrubber is neutralized with Ca (OH) 2 or Mg (OH) 2 and the resulting CaSO3 or MgSO3 is decomposed with ion-exchange HCl solution.

Claims (17)

23 84 434 A process for recovering sulfur and / or nitrogen oxides from gases containing them, which process comprises - washing the gases in a reaction space with an aqueous solution in which the solvent is predominantly potassium sulphite to produce potassium bisulphite. - potassium bisulphite-containing aqueous solution is removed from the reaction space and the potassium bisulphite is decomposed by heat treatment to potassium sulphite, - at least part of the potassium sulphite and potassium bisulphite is separated after possible further treatment, is recycled to the reaction space for use as a scrubbing liquid, tu - the potassium bisulphite-containing solution to be removed from the reaction space is decomposed by direct steam heating, - at least part of the sulfur dioxide obtained from the treatment of potassium sulphite and potassium bisulphite is used Process according to Claim 1, characterized in that the potassium bisulphite-containing solution is decomposed by heating it directly at 101 to 800 ° C, preferably at about 101 to 250 ° C, to release sulfur dioxide. 24 84 434 Process according to Claim 2, characterized in that the steam heating is carried out in a countercurrent desorption device. Method according to one of Claims 1 to 3, characterized in that the steam heating is carried out at an overpressure of 0.1 to 250 atoms, preferably 1 to 25 atoms and particularly preferably about 8 to 12 atoms. . Process according to Claim 4, characterized in that the pressure of the pressurized solution obtained from the steam heating is allowed to discharge by bringing it to normal atmospheric pressure. Method according to Claim 4, characterized in that the pressure of the pressurized layer obtained from the steam heating is allowed to discharge by cooling it to a temperature of 0 to 115 ° C, preferably about 20 to 40 ° C. Process according to Claim 1, characterized in that the potassium bisulphite solution obtained from the potassium hydroxyacetone sulphonate mixture after separation of the acetone is decomposed by heating, preferably by direct steam heating, before it is combined with the recirculation of the washing liquid. Process according to Claim 7, characterized in that the heating is carried out with steam at 101 to 800 ° C, preferably at about 101 to 250 ° C. Process according to Claim 7 or 8, characterized in that the steam heating is carried out in a countercurrent desorption device. Process according to Claim 9, characterized in that the steam heating is carried out in the same desorption apparatus as the steam heating of the potassium bisulphite solution to be removed from the reaction vessel. Process according to one of the preceding claims, characterized in that the fresh feed is recycled to the washing liquid by combining the potassium bisulphite solution obtained from the potassium hydroxyacetone sulphonate mixture with the potassium bisulphite solution obtained from the reaction space and decomposing the combined potassium bisulphite solution. Process according to Claim 1 or 11, characterized in that the amount of potassium equivalent of the fresh feed to be recycled to the washing liquid is at least approximately equal to the total amount of potassium equivalents of the potassium sulphite and potassium bisulphite compounds to be separated. 13. An apparatus for recovering sulfur and / or nitrogen oxides from gases containing them by absorbing said oxides in a scrubbing liquid to form potassium bisulfite, the apparatus comprising - a gas scrubber (1), - scrubber and scrubbing combined heating devices (4, 5) by which the potassium bisulphite contained in the solution can be at least partially decomposed into potassium sulphite, - a crystallizer (8) by which the part to be separated from the potassium sulphite / potassium bisulphite solution from the heating devices can be crystallized, - pipe fittings and a crystallizer (8), - recycling means for returning an integral part of the potassium sulphite / potassium bisulphite-containing solution to the washing unit (1) via the supply means, and - separable potassium sulphide from the apparatus (10, 11, 12, 13) for preparing a fresh feed to replace a t-t / potassium bisulphite-containing solution, characterized in that - the heating means (4, 5) comprise a steam desorption device (5) in which the potassium bisulphite-containing solution can be decomposed by direct steam heating. 26 84434 Apparatus according to Claim 13, characterized in that the steam desorption apparatus (5) is a counter-current steam stripper. Apparatus according to claim 13 or 14, characterized in that the heating elements (4, 5) further comprise a heat exchanger (4) arranged before the steam desorption device (5). Apparatus according to Claim 15, characterized in that the warm side of the heat exchanger is connected to the outlet side of the steam desorption device (5). Apparatus according to one of Claims 13 to 16, characterized in that the heating elements (4, 5) and the crystallizer (8) are connected to a fresh feed production plant (10, 11, 12, 13) for supplying sulfur dioxide for use in connection with the production of the fresh feed. 27 84434