DE2240025A1 - METHOD OF WASHING SULFUR DIOXYDE FROM A GAS FLOW - Google Patents

Description

The invention relates to a continuous process for the selective removal of SO ₂ from a gas stream containing it, in particular an SO ₂ scrubbing stage with an aqueous alkali-containing absorbent, coupled with a peculiar regeneration process. This comprises: a pretreatment step in which the sulfite compound contained in the rich washing solution is converted into the corresponding thiosulfate compound by selective reaction with H ₂ S, a primary reduction step in which the obtained

Thiosulphate is catalytically converted into the corresponding sulphide compound by reaction with E, and a stripping stage in which the adsorbed solution is _{regenerated by reaction with CO 2} with the release of hydrogen sulphide. It is particularly au stress that di © has to do with a washing process sau _f the with

rear: Frankfurt / Main 6763 Esnhi Dresdner Esnfc AG, Wletbadea, iContc-Ke> 876 SOf

ORIQINALJNSPECTED

effective aqueous absorbent, containing ammonium bicarbonate and / or ammonium carbonate, has to do, in which the washing solution withdrawn from the washing stage contains airanonium sulfite and / or ammonium bisulfite, while the pre-treatment stage involves a selective reaction of the ammonium sulfite and / or ammonium bisulfite with an H ₂ S and CO ₂ -containing gas stream with the production of ammonium thiosulfate, wherein the primary reduction stage comprises a selective catalytic reduction of the resulting ammonium thiosulfate with hydrogen to ammonium sulfide and / or hydrosulfide and wherein H ₂ S is stripped from the resulting solution with CO ₂ , part of which originates from the primary treatment step to form the regenerated absorbent and at least a portion of the H _{2 S used in the thiosulfate formation reaction.}

In many industrial fields today there is a serious problem with the creation of exhaust gas flows that Contain sulfur dioxide. The problem essentially involves eliminating these off-gas streams without substantial Air pollution, however, is extremely complex because of it great diversity of industrial origins which emit gas streams containing sulfur dioxide. One of the more common sources is associated with the combustion of sulphurous fuels in boilers, internal combustion engines, heating systems, etc., resulting in flue gas streams which contain considerable amounts of sulfur dioxide. Similarly, exhaust gas flows of this type are common in others

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Industrial processes / e.g. in the melting of sulphurous Ores, in the refining of coke, production of sulfur in a CLaÜs process, in the production of Paper produced via a wood pulp process and similar industrial processes.

It is known that the indiscriminate discharge of these gas streams into the atmosphere leads to significant air pollution because the sulfur dioxide has extremely harmful effects on animal and plant life. ¹ In addition, the discharge of these gas streams into the atmosphere is a waste of valuable material because the sulfur they contain is an industrial product. Many methods have been proposed for removing sulfur dioxide from such gas streams. Much of these proposals involve treating the sulfur dioxide-containing gas stream with an aqueous absorbent stream which usually contains materials which chemically or physically react with the sulfur dioxide in order to absorb it in the liquid solution. A very well studied method involves the use of a solution of an alkaline reagent such as ammonium hydroxide or carbonate, an alkali or alkaline earth salt such as sodium hydroxide or carbonate, potassium hydroxide or carbonate, and the like, to produce a rich absorbent stream containing the appropriate sulfite - and / or contains bisulfite compound. For example, an aqueous absorbent containing sodium bicarbonate and carbonate is used to form a rich wash solution containing sodium sulfite and bisulfite.

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Although the simple idea of leaching SO ₂ on the gas stream in question with an aqueous absorbent containing an alkaline reagent has many related advantages, such as simplicity, high degree of effectiveness and versatility, the widespread adoption of this type of solution for the problem of SOZ contamination hampered by the lack of a regeneration process for the rich absorbent stream, because this should be regenerated by selective and economical conversion of the absorbed SO ₂ into a conveniently traded and tradable product with high selectivity, that is, it is necessary that the regeneration process in the operation of the washing system allowed a closed loop with respect to the absorbent. In particular, it is required that a useful regeneration process be able to not only continuously produce a regenerated absorbent stream, but also to drive back undesirable by-products, as well as prevent the build-up of undesirable, untreatable and difficult to remove constituents in the absorbent stream when the system operated once in a closed circuit. The by-product that is mainly troublesome in this regard is sulfate. For example, in a system using an aqueous solution of ammonium hydroxide or ammonium carbonate as the absorbent, ammonium sulfate and ammonium bisulfate, once formed in the system, can cause serious trouble unless special measures are taken in order to remove them or not to prevent them from occurring.

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In particular, these salts can build up until finely divided solids are formed which then leads to corrosion, erosion and pollution difficulties being able to lead.

A solution to the problem of regeneration of the rich absorbent streams of the type discussed above is to use the appropriate one Reducing agent that reacts with the sulfite compounds it contains to selectively generate elemental sulfur and / or to produce the corresponding sulfide compound. However, if you use common reducing agents, such as hydrogen, a suitable sulfide compound or carbon monoxide is used in an attempt to directly apply these sulfite compounds elemental sulfur or to reduce the corresponding sulfide compounds, so form despite a number of precautionary measures unwanted sulfate compounds in unacceptable amounts. These sulfates presumably arise from the sulfite compounds by auto-oxidation reduction, at the conditions required for direct reduction.

The invention has therefore set itself the task of creating an exhaust gas scrubbing system which is capable of continuous operation - with a closed absorbent circuit, which selectively produces an easily removable substance as the main product of the counter-regeneration section and which reduces the amount of waste in the regeneration absorbent as a by-product can reduce unwanted sulfate formed to a minimum . - 6 - ■

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The solution found was a continuous scrubbing of SO ₂ from a gas stream, in which a conventional aqueous absorbent stream is used in a closed circuit, which comprises a high-washing stage, coupled with a new regeneration process, which allows the recovery of hydrogen sulfide in high yield, undesired sulfate by-products is largely pushed back from the regeneration section and produces a regenerated absorbent stream which has a relatively low total sulfur content and consequently a high capacity for SO. removal. The basic idea of the invention is based on the discovery that the sulfite compound contained in the rich washing solution withdrawn from the hash stage is easily converted into the corresponding thiosulfate compound by a selective reaction with H ₂ S without the formation of substantial amounts of undesired, non-salable sulfate compounds at relatively low heat can. Associated with this finding are additional observations, namely that the thiosulfate compound can be catalytically reduced by hydrogen in a highly selective, economical and effective manner to form the corresponding sulfide compound, and that hydrogen sulfide can easily be stripped _{from the resulting solution with a stripping gas containing CO 2.} The key point of the present process thus comprises the use of a conventional washing process, coupled with a regeneration process, in which thiosulfate is used as an intermediate product in a multi-stage operation, which is aimed at

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current contained suIfit compound in hydrogen sulfide instead of trying to reduce the sulfite compound directly to sulfide in a one-step operation. This path from sulphite via thiosulphate to sulphide results in a regeneration process that allows careful control of sulfur product formation during the regeneration operation facilitated and allows the generation of a regenerated absorbent stream that goes directly to the washing stage can be recycled. As a result, the system can be described in a closed cycle of the absorbent flow.

It is therefore the object of the invention to provide a simple, effective, efficient and selective method for treating a _{gas stream containing SO 2} , which can selectively produce hydrogen sulfide and can be operated with a closed absorbent circuit between the scrubbing section and the regeneration section, the amount of undesired In the regeneration section formed non-tradable by-products is largely reduced. At the same time, however, there is a regeneration process for a SO, washing stage which keeps the sulfur difference to a maximum over the regeneration process, so that the capacity and efficiency of the regenerated absorbent is increased.

The invention consists in one embodiment of a method for treating an incoming SO ₃

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Gas stream for the purpose of continuous removal of a substantial portion of the SO ₂ * The first stage is a scrubbing stage in which the inlet gas is treated in a suitable liquid-gas contact zone with an aqueous absorbent stream which contains an alkaline reagent, so that a treated gas stream with a substantially reduced _{amount of S0 2} and an outflowing Hasserstrom with a water-soluble sulfite compound. The next stage is a pretreatment in which at least part of the effluent _{stream from the scrubbing stage is reacted with a gas stream containing HS and CO 2} under conditions of thiosulfate production, so that a liquid effluent with a thiosulfate compound and a gas stream containing _{CO 2 are formed.} The liquid effluent from the pretreatment stage is then catalytically reacted with hydrogen in the first reduction stage in such a way that a suIfid-containing liquid stream is generated. Then, CO _{2 is mixed with the CO 2} -containing gas stream formed in the pretreatment step to form a stripping gas stream. In the next stage, hydrogen sulfide is stripped from the liquid effluent from the first reduction stage by its countercurrent contact with the stripping gas to provide an _{overhead gas stream containing H 2} S and CO ₂ and a regenerated aqueous absorbent stream. In the last stage, at least a portion of _{the overhead gas stream is passed to the pretreatment stage in order to provide B 2} S for this, and at least a portion of the regenerated aqueous absorbent

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Stromes is passed to the washing stage, so that a closed Absorbentkreis results.

In another embodiment, the invention is described as Method as set out in the first embodiment, carried out, whereby the alkali used in the aqueous absorbent stream is among the carbonates and bicarbonates of ammonia, alkali and alkaline earth metal is selected, which in the water to form a basic solution, for example ammonium carbonate, Hydrolyze sodium carbonate and the like.

In a particular embodiment, the method according to the invention for treating a _{gas stream containing SO 2} for the purpose of removing a substantial portion of the SO ₂ consists in that in the first stage the inlet gas stream is washed with an aqueous absorbent containing ammonium bicarbonate or carbonate, so that the treated Gas stream contains a significantly reduced _{amount of S0 2} ~ and the outflowing water stream contains ammonium sulfite or bisulfite. At least part of the water runoff from the scrubbing stage is then reacted in a first treatment stage with a _{gas stream containing H 2} S and CO ₂ under conditions of thiosulphate formation, so that a liquid _{runoff containing ammonium thiosulphate and a CO 2} -rich gas stream arise. The first reduction stage then comprises a catalytic conversion of the liquid effluent stream from the pretreatment stage with aaa hydrogen sfcom under reducing conditions to generate efees liquid effluent.

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stream containing ammonium sulfide or hydrosulfide. The gas flow from the pretreatment stage is then combined with additional CO ₂ to form a stripping gas. The liquid effluent from this first reduction stage is then stripped in countercurrent treatment with the gas, so that a regenerated aqueous absorbent containing ammonium bicarbonate or carbonate and an overhead gas stream containing _{H 2} S and CO _{2 are generated.} The last stage then comprises the line of at least part of the overhead gas stream from the stripping stage to the pretreatment stage and the line of at least part of the regenerated absorbent stream produced to the scrubbing stage, so that a closed absorbent circuit results.

Yet another embodiment of the invention is a method as set out above for the first embodiment, wherein the first reduction stage in Is carried out in the presence of a catalyst which is a catalytically effective amount of a metal of the transition elements of groups VI and VIII of the periodic table or their compounds combined with a porous support.

Other objects and embodiments of the invention will become apparent from the detailed explanation of the detailed below Streams of preferred compounds of preferred reactants of outgoing streams and measures that with each of the essential and preferred stages of

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discovery are connected.

The starting point for the method of the invention is a washing step in which an incoming _{gas stream containing SO 2} ′ is contacted in a gas-liquid contact device with an aqueous absorbent stream containing an alkaline agent. As explained above, the gas stream entering this stage usually consists of exhaust gas or flue gas. For example, a typical furnace gas stream contains about 1 to 10% O ₂ , about 5 to 15% or more CO ₂ / about 3 to 10 % or more H ₂ O, about 0.01 to 1% or more SO ₂ . In many cases the inlet gas will also contain carbon monoxide, nitrogen oxide, entrained fly ash and other components known from flue gas flows. _{The SO 2} contained in the inlet gas stream can vary within a wide range, namely from about 0.01 to 1 mol% or more, with a particularly typical amount of about 0.05 to 0.5 mol%. In many cases, this inlet gas stream occurs at a relatively high temperature of about 90 to 260 ° C (about 200 to 500 ° F) or more, and since it is desirable that the temperature of the inlet gas be at a relatively low level, because of this If the performance of the absorbent solution is increased, it is often advantageous to cool the inlet gas in a suitable manner, for example by presaturation with water under adiabatic conditions.

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The aqueous absorbent used in the washing step is generally an aqueous solution of a suitable water-soluble alkaline reagent which reacts with water to form a basic solution (ie more basic than a solution of SO ₂ in water) such as ammonium hydroxide, ammonium carbonate and bicarbonate, alkali hydroxide , Alkali carbonates and water soluble alkaline earth hydroxide, carbonate and bicarbonate and similar alkaline reagents. Among the alkali compounds, sodium hydroxide, sodium carbonate, sodium bicarbonate, potassium hydroxide, potassium carbonate and potassium bicarbonate are particularly preferred. In most cases excellent results are obtained when the alkaline agent is ammonium hydroxide or ammonium carbonate or ammonium bicarbonate. It should be emphasized that it is within the scope of the invention to use a mixture of the above-mentioned alkaline agents. Since it is also provided that the washing stage is to be operated with the absorbent circulating continuously around the washing device, it is possible that the absorbent can accumulate considerable amounts of sulfite and bisulfite compounds. In the latter case, only a small proportion of the rich effluent flow from the washing stage would be sent to the regeneration section of the process, and the majority of the rich absorbent withdrawn from the wash step would be mixed with the regenerated absorbent stream and recycled to the wash step.

The amount of the alkaline agent contained in the washing solution depends on the particular requirements,

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connected to the respective gas flow to be treated Acceptable results are usually obtained when the alkaline agent is about 1 to 50% by weight of the absorbent solution, preferably about 1 to 15% by weight. Of course you can get absorbent solution which is a lot of alkaline Contains agent up to the solubility limit of the agent in question under the conditions maintained in the washing stage, use if desired. If the absorbent continuously circulated around the washing stage and only a Sfaleppstrom is branched off for regeneration, The total amount of alkaline agent contained in the solution (i.e., fresh and used) can be quite high Achieve values; for example, the total amount can easily represent 30 to 50% by weight of the absorbent solutions.

This washing stage can be carried out in a suitable washing zone Manner, e.g. with several stages. The washing solution can either go up or into the washing zone flowing downwards, and the inlet gas stream can simultaneously enter the scrubbing zone in countercurrent to the Wash solution are introduced. In a particularly preferred method, the wash solution flows downward in a counter-current to the gas flow to be treated. The scrubbing zone is preferably a conventional gas-liquid contact zone with suitable ones Facilities for intimate contact between one descending liquid flow and an ascending gas flow. Suitable contact devices include bell bottoms, baffles and various types of filling material,

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as they are known in the professional world. With this countercurrent become a treated gas stream from the upper area the wash zone and a rich absorbent solution is withdrawn from the lower area. For those under consideration here Coming class of alkaline agents, the rich absorbent solution contains substantial amounts of a water-soluble one Sulfite compound such as ammonium sulfite and / or bisulfite, sodium sulfite and / or bisulfite and the like. As already mentioned, According to one embodiment of the operation of the washing stage, only a drag flow from the rich absorbent withdrawn from the stage to the regeneration section of the process sent. The rest is recycled around the washing stage. The drag current is usually withdrawn at a rate which is at least sufficient to continuously remove the net sulfur taken up in the washing stage.

This washing step is generally carried out under conventional washing conditions which are selected on the basis of the properties of the alkaline agent used, the sulfur dioxide content of the inlet gas, the proportion of sulfur dioxide to be removed in the washing step and the physical properties of the washing zone. Usually, the washing stage is preferably operated at a relatively low temperature of about 10 to 10O ⁰ C, under a relatively low pressure which appropriately approaches atmospheric pressure (although better results are obtained at higher pressures), with a volume ratio of inlet gas to washing solution of about 100 : 1 to about 10,000: 1 and

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a pH of about 4 to 7 or more. When the inlet gas flow an exhaust gas or furnace gas, usual measures must be taken to reduce the inlet gas to a to cool relatively low temperature before it is introduced into the washing stage. When the typical operation of the washing stage involves the treatment of large amounts of gas which contain only relatively small amounts of sulfur dioxide, it is also advisable to that the pressure drop through the washing zone is kept to a minimum, thus eliminating the need for compression to avoid large gas volumes to overcome the pressure gradient within the scrubbing zone.

Subsequent to the washing stage, the next step is the pretreatment stage, and this comprises the conversion of the sulphite or bisulphite compound in a highly selective manner, which is contained in at least part of the effluent stream withdrawn from the washing stage, into the corresponding thiosulphate compound. Usually the inlet stream for this stage contains sulphite or bisulphite compounds in an amount of about 0.01% by weight, calculated as sulfur equivalent, up to the solubility limit of the sulphite compound in question in water under the conditions used in the washing stage; for example, the feed stream to this stage can contain about 1 to 20% by weight or more sulfur than ammonium sulfite and / or ammonium bisulfite. According to the invention, a reaction takes place in this stage between the sulfite compound contained in this rich absorbent stream and a gas stream containing _{H 3} S and CO _2. It is also a feature of the

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present process that after touching the process of this H ₂ S and CO ₂ -containing gas stream is obtained from a stripping stage described below. The active ingredient in this gas stream is H ₂ S and it acts as a reducing agent to the sulfite compound contained in the absorbent stream. The amount of this sulfide reactant used in this step is at least sufficient to provide 0.5 moles of sulfide compound per mole of sulfite compound in the sequential absorbent stream, with best results being obtained at a mole ratio corresponding to about 0.6: 1 to 1 , 5: 1 or more. Likewise, good results are usually obtained when the pH of the drainage water is in the range of 4 to about 7 or more.

The conditions used in this first stage of treatment can be referred to as thiosulfater generation conditions denote and include: a temperature of about 20 to 150 C, sufficient pressure to keep the drainage water in to keep the liquid phase, and a contact time corresponding to about 0.05 to 1 or more hours. For example are excellent results at a temperature of 65 C, a pressure of 3.5 kg / cm (50 psig), a contact time of 5 minutes, a molar ratio of sulfide to Sulphite of 1.1: 1 and pH 9 has been obtained.

Subsequent to this pre-treatment stage is a relatively large one. Amounts of a thiosulfate compound-containing liquid | effluent stream withdrawn and to the first

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Passed reduction stage of the present process, wherein the stream is catalytically treated with hydrogen under reducing conditions to produce the corresponding sulfide compound. A gas stream containing _{CO 2} was also withdrawn from this pretreatment stage. This gas stream is relatively rich in CO, because practically all of the H ₂ S originally contained in it is fully reacted in this pretreatment stage; for example, a designated inlet gas stream for this stage contains 92% CO ₂ and 8% H ₂ S, while the outlet gas contains more than 99% CO ₂ .

The liquid effluent stream is brought into the first reduction stage and a hydrogen stream with a suitable one Reduction catalyst under reducing conditions in contact, so that the thiosulfate compound contained in this process is converted into the corresponding sulfide and water. This stage can be appropriately following the teachings of the Technique for the contact of a liquid stream and a gas stream with a solid catalyst.

A particularly preferred method consists in a catalyst system with a fixed layer. One in which the catalyst is in the reaction zone is particularly useful is arranged and the thiosulfate-containing liquid either upwardly Sjradial or downward flowing with a stream of hydrogen is passed, which simultaneously either is introduced in countercurrent or cocurrent to the aqueous stream. A particularly preferred embodiment is downward flow and countercurrent flow of the aqueous

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Stream and the hydrogen stream through the reduction zone.

This Reduktlonsstufe can also with a suitable Reduction catalyst are carried out, its ability to reduce a thiosulfate in the corresponding Sulfide is known. On the basis of the available investigations it has been found that particularly good results with achieved verden a catalyst which has a catalytically effective amount of a metal component from the group the transition metals of groups VI and VIII, such as chromium, molybdenum, tungsten, iron, cobalt, nickel, platinum, palladium, Iridium, etc., combined with a porous support material. Are very particularly preferred for the desired Reduction catalysts which are a combination of a catalytically effective amount of a Group VI transition metal or VIII with a suitable porous support such as alumina or activated carbon. Among the transition metals are for the present process best those of the iron group of group VIII, especially iron, cobalt, nickel and their compounds, are suitable. In particularly preferred embodiments of the invention, catalysts are used in which the metal component is in the form of a metal sulfide, such as cobalt sulfide, iron sulfide, carbon sulfide, molybdenum sulfide or tungsten sulfide, combined with a carrier, is present. The preferred carrier materials are activated carbons or charcoals, such as those sold under the name "Norite", "Nuchar", "Darco" are available, or other similar pro-

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dukes In addition, normal or artificial highly porous, inorganic, refractory oxide carriers for the metal component, such as alumina, silicic acid-zirconium oxide, silicic acid-alumina, Bauxite tori, etc., can be used. The best results is usually obtained with a catalyst which combines an iron group metal with relatively small particles a suitable solid support, i.e. excellent good results are obtained with a catalyst containing iron, cobalt or nickel, and particularly expediently in the form of their oxides and / or sulfides. Excellent Results were obtained when the reducing catalyst contained a catalytically effective amount of cobalt sulfide, combined with a suitable, refractory inorganic oxide carrier such as alumina or cobalt sulfide combined with Contains activated carbon.

A useful method of making this reduction catalyst is to treat the carrier with a aqueous solution of a soluble salt of the metal compound such as acetate, chloride, nitrate, etc., soak. The metal component of the resulting mass can then be treated with hydrogen sulfide, preferably at room temperature, converted into the sulfide. The resulting sulphided mass is then treated with aqueous and / or ammoniacal Solution washed and dried. If the carrier material consists of a refractory inorganic oxide, can it is advantageously calcined or oxidized after impregnation at a relatively high temperature in order to

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To achieve division of the metal component on the support, which can then be sulfided with a suitable sulfur compound in order to obtain the desired catalyst.

In general, the metal component is preferably combined with the carrier in an amount sufficient to form one Reduction catalyst with about 0.1 to 25 wt .-% of the metal component, calculated as elemental metal, to lead. For the preferred cobalt sulfide catalyst, the amount of cobalt introduced is preferably sufficient to result in a To lead reduction catalyst with about 1 to 10 wt .-% cobalt.

An essential reaction partner for the primary reduction stage is hate substance. The hydrogen stream introduced into this stage can be essentially pure hydrogen or a mixture of hydrogen with other relatively inert gases, such as a mixture of hydrogen with C ^ - bis (^ -Hydrocarbons, a mixture of hydrogen and nitrogen, a mixture of hydrogen and carbon dioxide, a mixture of hydrogen and hydrogen sulfide, etc., be. The cycle gas obtained from various petroleum processes, in which a net hydrogen replenishment takes place, such as in a reforming process, a dehydrogenation or dehydrocyclization process, etc., can also be used if desired. Preferably the hydrogen is in an equivalent or greater amount than that used for the reduction of

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Thiosulfate to sulfide required stoichiometric amount. The latter is 4 moles of hydrogen per mole of thiosulfate. In general, it is preferred to work with a substantially larger molar ratio of hydrogen to thiosulfate than this stoichiometric amount. It is therefore preferred to use about 4 to 10Q moles of hydrogen per mole of thiosulfate compound in the aqueous stream introduced to the reduction stage; the best results are obtained with a molar ratio of about 20: 1 to 50: 1. It goes without saying that any unconverted hydrogen can be recovered from the effluent from this reduction stage and then, if necessary, recycled through suitable evaporation equipment in order to supply at least part of the hydrogen "required" for the reduction stage, ^as shown in the flow diagram.

The conditions used in this first reduction stage are commonly referred to as the reduction conditions for conversion called from thiosulfate to sulfide. The temperature used is preferably selected in the range from about 125 to 350 ° C. The pressure applied is sufficient to make the aqueous stream containing the thiosulfate compound liquid to keep. In general, it is preferable to work at over-

Press 2, preferably at about 7.0 to 210 kg / cm (about 100 to 3000 psig). It is also useful to use a liquid hourly space velocity (defined on The reason for the volumetric feed rate of the aqueous stream divided by the total volume of the reduction

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catalyst within the reduction zone) in the range of use about 0.5 to 10 hours with best results being obtained in about 1 to 3 hours.

In a preferred embodiment for the first reduction stage, in which the aqueous stream containing the thiosulfate compound and the hydrogen stream countercurrently are passed on contact with the reduction catalyst, the effluent stream from the reduction zone contains the Sulphide product of the reduction, a very small amount of unreacted thiosulphate, unreacted hydrogen, water and the alkaline agent. For example, if the aqueous stream contains ammonium thiosulfate, this is typically the case Sulphide product of the reduction stage as ammonium hydrosulphide or as hydrogen sulphide, or a mixture thereof, the amount of ammonium hydrosulfide present therein mainly depends on the amount of ammonia present in the inlet to the reduction stage. Unreacted hydrogen will expediently separated from the effluent stream from the reduction stage in a separation zone and recycled to the reduction zone. The liquid portion of the effluent stream from the first reduction stage is also recovered from the separation zone. It contains the sifid product of the reduction reaction.

In the next stage according to the invention, the liquid effluent stream from the first reduction stage is obtained is subjected to a stripping step designed to release hydrogen sulfide therein. Although ir-

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Since a suitable stripping gas such as steam, nitrogen, air and the like can be used, carbon dioxide is particularly preferred because it serves to lower the pH of the solution and form the corresponding carbonate. For example, if the effluent stream from the first reduction stage contains ammonium hydrosulfide, the stripping with carbon dioxide releases hydrogen sulfide and creates a regenerated absorbent stream containing ammonium bicarbonate. According to the invention, at least part of the CO ₂ required for the stripping is diverted from the effluent gas stream obtained from the first treatment stage. It has been observed that this gas stream provides a superior component of the gas used in the stripping stage because the reaction in the pretreatment stage between sulfite and sulfide _{acts to enrich the CO 2} content of the gas stream exiting from this treatment stage. As a result, a feature of the invention is that the required stripping _{gas is prepared by mixing carbon dioxide with the CO 2} -rich gas stream which is generated in the first treatment stage. The amount of other CO _2, which is added to this mixture, which is obtained from the first Behändlungsstufe depends on the C0 ₂ amount from the ₂ ~ rich gas stream is contained in the CO, and contained by the liquid flow from the primary reduction step Amount of sulfide. In general, the amount of CO ₂ added should be sufficient to produce a molar ratio of CO ₂ to sulfide introduced into the stripping stage essentially above 0.5: 1, preferably in the range

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from 1: 1 to 10: 1 or more, certainly maintained.

This stripping stage is carried out by countercurrent contact of the liquid drain from the first reduction stage with the stripping gas stream under suitable stripping conditions for HS. In general, superior results are obtained when this step is carried out at a relatively low temperature and under a relatively high pressure. For example, excellent results are obtained at this stage at a temperature of 40 ° C, a pressure of 14 kg / cm (200 psig) and a molar ratio of CO ₂ to sulfide of 5: 1. This stage is operated in such a way that an overhead gas stream with H ₃ S and CO ₃ - in many cases also H ₂ O and a bottom stream containing the regenerated aqueous absorbent are obtained.

This regenerated aqueous absorbent stream is in total * " sulfur content is substantially reduced in relation to the entering rich absorbent stream and usually contains less than 10% of the incoming absorbent stream contained amount of sulfur. Because carbon dioxide in the stripping stage is used as a stripping agent, this regenerated absorbent stream can contain substantial amounts of alkali carbonate or -bicarbonate from the absorbent stream originally in the incoming stream contained alkali - for example, if the alkali agent is ammonia, the regenerated absorbent stream becomes / urarionium carbonate and / oflei -bicarbonaf. contain

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th, and in the case where the alkaline agent consists of sodium hydroxide or carbonate, the treated water stream is Contain sodium carbonate and / or bicarbonate.

In accordance with the present process, at least a portion of the regenerated absorbent stream is passed to the SC ^ scrubbing stage or recycled. Similarly, a portion of the overhead gas stream generated in the stripping stage which contains I ^ S and CO- is passed to the pretreatment stage to provide at least a portion of the HjS reagent used therein. The remaining part of this hydrogen sulfide-containing stream is then obtained as one of the product streams from the present process. The hydrogen sulfide contained in this product stream can be converted into elemental sulfur or sulfuric acid by a suitable oxidation method, such as a customary Claus process, or it can be further used per se.

After basically the essential stages of the presentjiden Method according to the invention have been set forth, is used to explain an embodiment in more detail Reference is made to the drawing with a preferred flow sheet of the invention. This drawing is only for general representation of the flow of proceedings without it When it comes to details of heaters, pumps, valves and similar equipment, it is thin that a knowledge this device is essential for understanding the invention txlur not for the person skilled in the art

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would be taken for granted.

According to the drawing, a flue gas flow enters the system through line 1 and goes into the lower area of a conventional liquid gas scrubber 2. In this zone it is guided in countercurrent to a descending stream of absorbent solution, which enters the upper area of zone 2 through line 4 entry. The inlet gas stream contains approximately 5% O ₂ , 12% CO ₂ , 6% H ₂ O, 76.8% N ₂ and 0.2% SO ₃ . Zone 2 is a conventional gas-liquid contact zone, which is equipped with conventional devices such as baffles, bubble caps, filling material and the like. is equipped to bring about intimate contact between an ascending stream of gas and a descending stream of liquid. The treated gas stream obtained is withdrawn from zone 2 through line ^ .

A liquid stream containing the aqueous absorbent solution is also introduced into zone 2 through line 4. He wLrd supplemented by two separate Htröme, one of which ULN regenerated Absorben & Etrom is composed of the RegenerLeruhschnLtt the stems before lying system while the zweLte t; Ln HauptfcoLL the ruLchen Absorbensstromoij, the ana the lower IiutoLcli dm Zon '2 via LeLtung 1 deducted wLrd. The alkaline medium used in the absorption medium is mainly composed of <ui :; Ammonium carbonate with oLner (jo amount of AmmonLuinbLcutbondt.

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BATH ORIGINAL

Uach the embodiment illustrated in 4-drawing operation of the zone 2, the rich absorbent stream ^ om the bottom of the zone 2 through 4 Iteitimg abgezoge ©, xmä a Hampfcteil <uies.es stream is continuously passed around zone 2 in a circle "mn a construction of the concentration of the Sulphite salts to allow the absorbent flow to be relatively high. This measure increases the capacity of the absorbent and minimizes the amount of absorbent flow that must be passed through the regeneration section of the system in the KiELse consequently contain a substantial amount of sulphite salts together with the alkaline agent- Usually zone 2 is controlled by monitoring the pH of the absorbent stream at the inlet to zone 2 and controlling the amount of rich absorbent stream going to the regeneration section of the system at the junction between the Lines 4 and 5 are branched off according to an increase in pH. The preferred pH range is about 4 to 7 or more, with best results usually being obtained in the range of about 5 to 7. If the system is operated so that the absorbent stream introduced into zone 2 through line 4 is maintained at a pH value within this range, the led cha 7ibsorbenti3t withdrawn continuously from zone 2 through line 4 in rom about 1 to 35 or more wt .-? Schwel el. Mainly a3a mixture of ammonium sulphite and ammonium thisuli it ent hall on. Juiikirdein are minor in Zone 2; Amounts of Ammoniintii-υΐί nt and -tliiosuliai formed. In. the special duich the Ecdrlim-mg erliiuloi ten rail e «t-

The rich absorbent preferably holds about 8% by weight sulfur as ammonium sulfite and bisulfite.

Zone 2 is operated at a temperature of about 50 C under a pressure of about air pressure and a volume ratio of gas to absorbent of about 500: 1. Under these conditions it is found that the treated gas stream withdrawn from the upper region of zone 2 through line 3 contains less than 5% of the SO ₂ originally contained in the incoming gas stream.

The rich absorbent stream withdrawn from the lower region of zone 2 through line 4 is discharged at the junction of Line 5 and line 4 split. The main part continues through line 4 and is filled with regenerated absorbent at the junction of line 18 with line mixed. The resulting mixture of recycled and regenerated absorbent is then returned to through line 4 Zone 2 introduced. The small portion of the rich absorbent is passed through line 5 to the first reaction zone 6. The days that enter there are usually enough for Absorbas reJdit at least to avoid the net sulfur inlet to zone 2 to be removed from the inlet gas stream in order to keep the sulfur concentration in the absorbent stream balanced. By doing the case considered here, the amount of absorbent emitted through line 5 for regeneration is about 0.1 to 10% of the rich stream of absorbent withdrawn from the bottom of zone 2. It should be emphasized that the

309809/1048

Wash zone 2 the stock of wash solution required for single-line operation through lines 20, 18 and 4 is introduced into the system. It can also be seen that a net water replenishment in the regeneration section of the System as a result of the implementation of Ammoniuntfeiosulfat with Hydrogen takes place. At least a portion of the net water product of the present process can be withdrawn from the system be removed in the treated gas stream, which is by conduit 3 is withdrawn if the original gas stream entering the process through line 1 is not already saturated with water is. When the amount of water withdrawn from the system through line 3 is insufficient to make up the net water supplement to remove, a portion of the regenerated absorbent can be removed from the system through line 19 and placed in the usual way for the purpose of separating the water therefrom, if desired with recycling of recovered alkaline agent.

The rich absorbent stream introduced into zone 6 through line 5 contains in the particular case discussed here about 8 wt .-% sulfur as a mixture of ammonium sulfite and ammonium bisulf it. He is to a temperature of about 65 ° C by not shown hot equipment in front of his Heated entry to zone 6. Zone 6 is designed in this way usual liquid-gas contact ^ one ^ that one in it intimate contact between an ascending gas stream and a liquid flow takes place, which enters the upper region of zone 6 and to an outlet opening in the bottom @ a d @ r

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Zone flows downwards. A gas stream containing _{a mixture of H 2} S and CO _{2 is} introduced into the lower region of zone 6. This gas flow flows upwards through the descending liquid flow to an outlet opening in the upper region of the zone, where it is drawn off through line 21. During the start-up of this zone, sufficient H ₂ S is introduced through lines 17 and 15 to initiate the desired reaction. Part of an _{overhead gas stream containing H 2} S and CO ₂ , which is generated in a stripping stage described below, is then fed from line 14 through line 15. Regardless of the origin of the reducing agent added to zone 6, it occurs in at least a sufficient amount to convert about 0.5 mol of H ₂ S per mole of sulfite added to zone 6. It should be emphasized that excess amounts of H ₂ S above this minimum amount can be used in a favorable manner, because excess H ₂ S is at least partially absorbed in the liquid stream being treated and this H ₂ S absorbed in zone 8 described below ongoing reaction is significantly supported.

Zone 6 is kept on thiosulphate production conditions, which in this particular EaIl a temperature of about 65 C,

a pressure of about 14 kg / cm (200 psig) and a residence time of the liquid flow in zone 6 means of about 0.1 hour.

309809/1041

An overhead gas stream containing relatively large amounts of CO ₂ and very small amounts of H ₂ S is then withdrawn from zone 6 through line 21 and passed to the junction of line 21 with line 22 where it is mixed with additional CO, which is fed into the system from line 22 enters. The resulting mixture then goes through line 21 to zone 14. A liquid effluent stream is withdrawn from the bottom of zone 6 through line 7, ^{heated to approximately 200 °} C. by heating devices (not shown) and introduced into second reaction zone 8. This aqueous effluent stream contains ammonium thiosulfate in a teenge, corresponding to a conversion of more than 95% of the incoming ammonium sulfite and bisulfite to ammonium thiosulfate in zone 6. Furthermore, the amount of undesired ammonium sulfate formed by selective conversion within zone 6 is less than 3% of the input sulfite. The aqueous effluent stream from zone 6 thus mainly contains ammonium thiosulphate with small amounts of unconverted ammonium sulphite and bisulphite and traces of ammonium sulphate. Because the gas introduced into zone 6 contains carbon dioxide, this liquid effluent stream also contains minor amounts of ammonium carbonate and bicarbonate.

Another liquid-gas reaction follows in the second reaction zone 8 from, and it is designed so that an intimate contact between the countercurrent flowing hydrogen-containing Gas flow and liquid flow is achieved on a solid layer of a reduction catalyst.

309809/1048 " ³² "

The liquid effluent stream from zone 6 is introduced into the top of zone 8 through line 7. After the process has been initiated, a substantial proportion of the necessary hydrogen consists of circulating hydrogen, which is obtained from the outlet stream withdrawn from zone 8. During the cranking of the process, sufficient hydrogen is introduced into zone 8 through lines 12, 11 and 7 to provide a circulating inventory of hydrogen which circulates through lines 9, 11 and 7. After this inventory is established, line 12 is used to introduce make-up hydrogen into the system in order to maintain the desired molar ratio of H ₂ to thiosulfate within zone 8. The latter contains a catalyst made of Darco activated carbon of about 1.7 to 0.85 mm in size (12 to 20 mesh) with combined cobalt sulfide component in a sufficient amount for a catalyst content of 2.3% by weight of cobalt. The catalyst is prepared by a conventional impregnation method with a water-soluble cobalt compound followed by sulfidation at a relatively low temperature. The total amount of hydrogen introduced into zone 8 corresponds to a molar ratio of hydrogen to ammonium thiosulfate of 40: 1. The reduction conditions β maintained in zone 8 ind in this particular case a temperature of 200 C,

2
a pressure of 20 kg / cm (300 psig) and a liquid hourly space velocity of 2 h /.

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An effluent stream consisting of a mixture of liquid and gas is exited from zone 8 through line 9 withdrawn / cooled by cooling devices not shown to a temperature of about 100 C and into the separation zone 10 initiated. A hydrogen gas phase separates here from an aqueous liquid phase. The gas phase contains unreacted hydrogen and hydrogen sulfide and water vapor; it is withdrawn from zone 10 dunh line 11 and sent back to zone 8 by a compressor not shown. Zone 10 is under pressure which is roughly the same as used in Zone 8 will.

The -FXUssigk@itgpha.se formed in zone 10 is then exhausted through line 13. " Cooled by cooling devices not shown to siae leaiperstösr ψοά © about 40 ° C and passed to the upper area of the Äbstrsifsoas 14" This liquid flow is the aqueous portion of the discharge flow from zone 8 and contains ammonium hydrosulfide, traces of unconverted ammonium thiosulfate, ammonium hydroxide hydroxide _B siumbium Analysis of this aqueous stream shows that 99 % of the ammonium silosulfate fed to zone 8 has been converted therein, with 89% of the thiosulfate converted to ammonium hydrosulfide. In addition, analysis shows that less than 5% of the -Ammoniuinthiosulfäts in zone 8 was converted into the undesired refractory ammonium sulfate-

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In the stripping zone 14 comes the introduced therein aqueous Discharge flow in countercurrent contact with an ascending stripping gas that enters the lower region of zone 14 by means of Line 21 is introduced. The stripping gas is made by mixing by CO. with the overhead gas stream from zone 6 in sufficient Amount prepared in order to get a pick ratio of

lead to 5: 1 sulphide introduced into zone 14.

The zone 14 is in a conventional manner at a relatively low temperature of about 40 C under a pressure of about

14 kg / cm (200 psig) operated. It also contains useful suitable device sur achieve a ·· intimate contact between the descending liquid stream and an ascending gas stream. Carbon dioxide is particularly preferred for use as the stripping gas because it acts to lower the pH of the liquid stream to the point where H ₂ S is released from solution and ammonium carbonate or bicarbonate is formed; it occurs to. Elimination of difficulties with the NH ₃ transfer in the stripping gas stream. In some cases, a portion of the bottom stream withdrawn therefrom through line 18 is preferably introduced at the top of zone 14 in order to reduce the transfer of gas in the top gas stream as far as possible.

An overhead gas stream is then withdrawn from zone 14 through line 15 and to the junction of line 16 with Line 15 out. The majority of this overhead stream is then removed from the system through line 16. This Ab

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gas stream 16 contains the net sulfide product of the present Method and can be any suitable process for Sulfur extraction or production of sulfuric acid are spent; for example, this stream can be a usual Claus plant for the purpose of sulfur recovery by an oxidation process. This head gas flow contains a relatively large amount of hydrogen sulfide, Carbon dioxide and small amounts of ammonia and water. Another portion of this overhead stream is through Line 15 passed to zone 6 in order to convert the hydrogen sulfide required for the conversion of sulfite into thiosulfate in zone 6 to deliver, as has already been stated.

A current is regenerated from the lower area of zone 14 Absorbent withdrawn through line 18 and returned to zone 2 through lines 18 and 4. It contains mainly a mixture of ammonium carbonate and bicarbonate with small amounts of unreacted ammonium thiosulphate, unreacted ammonium sulfite, ammonium hydrosulfide and ammonium sulfate. The said sulfur content this regenerated Absorbent flow is less than 10% of the total sulfur content from the scrubbing zone of the system through line 5 withdrawn rich absorbent stream. Also, the amount of unwanted ammonium sulfate that is found in the regeneration section of the system, i.e. in the section with the zones 6, 18 and 14, less than 3% of the sulphite added in the regeneration section through line 5.

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The washing process according to the invention thus allows the continuous leaching of SO2 from the into the system
gas stream entering through line 1 with continuous regeneration and recycling of absorbent in a closed loop. In addition, the amount of undesirable ammonium sulfate formed in the regeneration section of the system is kept to an extremely low level.

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Claims (12)

Claims 1.) Process for the treatment of an SO ₂ -containing inlet gas stream for the continuous SO ^ removal, characterized by the following stages: a) the inlet gas stream is brought with an aqueous absorbent stream containing an alkaline agent under scrubbing conditions and draws a treated gas stream containing a reduced amount of SC ^ contains, and a draining water stream with a water-soluble SuTfit compound, b) at least part of the effluent stream from stage a) is reacted with a _{gas stream containing H 2} S and CO _{2 to} form thiosulfate in the effluent liquid stream and a CO ₂ -containing gas stream, c) the liquid effluent stream from stage b) is catalytically treated with hydrogen and practically produced thiosulfate-free liquid effluent stream with a sulfide compound, d) the CO ^ -rich gas stream from stage b) is added to form a stripping gas CO ₂ ; - 38 - 3 0 9 8 0 9/1048 224D025 e) hydrogen sulfide is stripped from the liquid effluent stream generated in stage c) by continuous contact treatment with the stripping gas from stage b) and an _{overhead gas stream containing H 3} S and CO ₂ and a regenerated aqueous absorbent stream are generated f) at least part of the regenerated aqueous absorbent stream obtained is then passed back to stage a) and at least part of the overhead gas stream from stage / S) to stage b). 2.) Method according to claim 1, characterized in that the alkaline agent consists of ammonium carbonate or ammonium bicarbonate consists. 3.) Method close to claim 1, characterized «indicates that the alkaline agents made of alkali carbonate or alkali bicarbonates, preferably made of sodium or potassium carbonate or bicarbonate. 4.) The method according to claim 1, characterized in that the alkaline agent consists of a water-soluble alkaline earth carbonate or bicarbonate. 5.) The method according to claim 1, characterized in that in step b) a temperature of about 20 to 150 C and a sufficient pressure must be maintained to keep the stream draining from station a) liquid. - 39 - 309809/1048 6.) The method according to claim 1, characterized in that in stage c) a temperature of about 125 to 350 C and Sufficient pressure can be maintained to keep the from stage b) to keep running stream liquid. 7.) The method according to claim 1, dadur.ch characterized in that the amount of hydrogen introduced in stage c) has a molar ratio of hydrogen to thiosulfate introduced of delivers at least 4: 1. 8.) The method according to claim 1, characterized in that the amount of H-S introduced in step b) is at least sufficient, a molar ratio of sulfide to sulfite of 1: 2 to deliver at this stage. 9.) The method according to claim 1, characterized _g that in step c) the current flowing off in step b) and a hydrogen stream are treated with a reduction catalyst on contact, which in combination is a catalytically effective amount of a metal compound from the transition metals and compounds of the Contains groups VI and VIII of the periodic table and a porous support and the reduction is carried out to form a virtually thiosulfate-free aqueous effluent stream with a sulfite compound. 10.) The method according to claim 9, characterized in that the - 4o - 309809/1048 - 4ο. Reduction catalyst contains a metal of the iron group, preferably cobalt or a cobalt compound. 11.) The method according to claim 10, characterized in that the reduction catalyst contains a catalytically effective amount of cobalt sulfide on activated carbon or alumina as a carrier. 12.) The method according to claim 1 and 2, characterized in that in step c) hydrogen in an amount corresponding to a molar ratio of hydrogen to ammonium thiosulfate introduced therein of about 4: 1 to 100: 1. 309809 / KK8