DE3004757A1 - Ammonia and hydrogen sulphide removal from gases - with two-stage hydrogen sulphide scrubbing and combined acid gas stripping - Google Patents

Abstract

Removal of NH3 and H2S from gases (esp. coke-oven gas) is effected by scrubbing with water to remove NH3 and 2-stage scrubbing with a NH4OH soln. and an alkali metal hydroxide soln. to remove H2S, with the improvement that the liquors from both H2S scrubbing stages are combined and freed of H2S and HCN in at least one stripper column. The gas is passed successively through 4 scrubbers. The 1st is operated with deacidified NH3-contg. recycle water and liquor from the 2nd scrubber to remove 30-60% of the H2S. The 2nd scrubber is operated with recycled NH3 gas and liquor from the 3rd scrubber to remove 30-80% of the H2S. The 3rd scrubber is operated with water to remove NH3. The 4th scrubber is operated with NaOH soln. to remove residual H2S. The liquors from the 1st and 4th scrubbers are combined and introduced into 2 parallel stripper columns for removal of H2S and HCN. The bottoms stream from one column is recycled to the 1st scrubber and the bottoms stream from the other column is passed to a NH3 stripper column, and the NH3 is recycled to the 2nd scrubber. The process uses no more than the optimum amt. of alkali metal hydroxide.

Description

Re: procedures for removing ammonia and sulfur

hydrogen from gases, especially carbon distillation gases The invention relates to a method for removing ammonia and hydrogen sulfide from gases, especially carbon distillation gas, by absorption of the ammonia using water and hydrogen sulfide in at least two desulfurization stages, whereby in one stage with ammonia solutions and in another stage with alkali hydroxide solution is desulfurized.

It is known to produce hydrogen sulfide by means of sodium hydroxide or potassium hydroxide solution To remove gases and the resulting waste to release bound To use ammonia.

This is done together with the drain water from the ammonia wash in so-called ammonia drivers. In the ammonia expeller, the water is removed from everyone freed of volatile constituents and then discharged into the receiving waters.

Such processes produce large amounts of sodium sulphite waste liquor at. This is particularly the case if, according to the German patent specification 250959 is the procedure described. After that, it is contained in the coke oven gas - Hydrogen sulphide washed out mainly by alkali carbonate solutions. In the In practice, however, this is uneconomical because caustic soda and potassium hydroxide are particularly expensive Resources are.

In addition, the cycle management chosen in this process requires of the detergent due to the long residence time and the secondary reactions caused by it an increased need for detergent. The invention is therefore based on the object the economy of the known processes for cas desulphurisation with ammonia and alkali to improve.

The invention is based on the idea, only as far as alkali to use for the cleaning of gases, as it is economically optimal. That is achieved in that the washing solutions from the dewaxing stage with ammonia as well as from the desulfurization stage with alkali hydroxide together in a deacidification column are freed from hydrogen sulfide and hydrogen cyanide. Furthermore, the desulfurization carried out with particular advantage in two # steps.

The first step is known per se and consists in the following regenerated ammonia water is only used to the extent that a certain pre-desulphurisation, preferably from 40 to 50> and thereafter free of hydrogen sulfate Ammonia gas is supplied and the gas thus treated is then subsequently e-iner ammonia washing stage is fed. In the second step, after the ammonia wash, there is an additional one Desulphurisation stage provided, which is treated with sodium or potassium hydroxide solution.

This process allows a selective desulphurization of the gas with #Alkaiilauge and short dwell times. As a result of the short residence times, harmful side reactions, such as the formation of sodium carbonate through the absorption of carbon dioxide v-avoided. Bound ammonia is only released to the extent necessary for extraction of additional ammonia for casesulfurization is economical and to the extent that it is necessary to achieve it given wastewater quality becomes necessary.

Not only a small proportion of bound is used for water protection Ammonia useful, but also a pH in the range between 7 and 8. By contrast pH values between 9.5 and 11.5 were often measured using conventional methods.

Compared to the disadvantages of such a pH value is that of the invention Process-related water pollution by ammonia salts is comparatively negligible small amount.

In addition, the inventive release of ammonia in the upstream deacidification column compared to the release rate of ammonia after the known processes in the ammonia stripper have three major advantages. It will the content of free ammonia in the washing solution increases significantly. Furthermore, the Hydrogen sulphide removal from the scrubbing solution through the action of carbon dioxide, this also applies to selective desulphurisation in a certain amount during the washing process is absorbed and released during desorption, is significantly improved. About that In addition, it becomes possible to concentrate the hydrogen swell from the deacidifier direct processing to sulfuric acid or sulfur, without going through the Add ammonium sulfate saturator and keep it away from the ammonia expeller. This is particularly important for the production of weight-free ammonia. In the drawing an embodiment of the invention is shown.

The gas coming from a gas suction device or a gas aftercooler (carbon distillation gas) becomes an absorber with, for example, 8 g H2S / Nm3 and 6 g NH3 / Nm3 via a line 4 1 fed to a first washing stage, which is used for pre-desulfurization. The gas will in this stage with deacidified NH3 circuit water and an NH3 washing solution washed an absorber 2 of a subsequent desulfurization stage and thereby pre-desulfurized to 30 to 50%.

The circuit water arrives via a line 5 (in the upper area) in the absorber 1. There the liquid composition corresponds to the enrichment of the circulating water. The NH3 washing solution of the absorber 2 flows via a line 18 to the absorber 1.

The enrichment of the scrubbing solution in the absorber 1 depends on the gas composition and the temperature. Increased during the residence time in the absorber 1 the H2S content in the washing solution increased from 2.3 to 7 g H2S / 1. Through BE surcharge with 0.6 l / Nm3 circulating water from line 5, 35% of that contained in the raw gas is Washed out hydrogen sulfide. Arrived from the H25 pre-washer or absorber 1 the gas via a line 15 into the absorber 2. In the absorber 2 is with the drainage water a third stage forming absorber 3 with the simultaneous addition of largely Washed ammonia free of hydrogen sulfide. The ammonia is dosed through pipes 6 blown into the lower and middle area of the absorber 2 at several points.

In the absorber 2, which is used for desulfurization, the gas can at sufficient addition of pure ammonia and use of relatively large amounts of water largely freed from hydrogen sulfide for backwashing the additional ammonia are For economic reasons and because of greater operational safety, is after the proposal of the invention, the gas desulfurization in the absorber 2 not so far operated as technically possible, but it is the fine desulphurization in one further stage 31 carried out. The drain water of the absorber 3 passes through a Line 16 into the absorber 2.

The gas enters the absorber 3 through a line 17.

There it is exposed to fresh water from a line 7. A Part of the fresh water can be caused by weakly enriched NH3 water, for example Coal water as gas condensate. These waters are through a pipe 8 is pumped into the absorber 3. In terms of loading with NH3 and H2S, the waters correspond roughly the concentration of the wash water.

All washing solutions given into the process are in countercurrent to the Gas led. They reach a sump 22 of the first sulfurization stage via a line 9 (Absorber 1), from which it is taken via a line 19. That in the cycle flowing NH3-LBasser fed to the absorber 1 via the line 5 is via the Line 19 and line 20 are fed to a column 10 and here in a known manner This way, about 60 to 70% of the acid is deacidified by thermal decomposition.

It then returns to the absorber 1 through the line 5.

The excess washing water, which - in the amount of about the fresh and Coal water, must be used for the production of hydrogen sulphide-free ammonia vapors be deacidified more than the circulating water. This is done in the known way according to the method disclosed in German patents 11 55 426 and 11 75 381- with the generation of pure ammonia vapors and the simultaneous generation of about 20 <B-igem Ammonia strong water.

The raw ammonia water withdrawn from the sump 22 via lines 19 and 21 is desulfurized in a column 11 and via a line 23 to a column 12 supplied, in which it is freed from all volatile constituents. Then it worked the-water through a pipe 14 in-s sewage.

In the column 12 there is direct cooling and subsequent deacidification of the rising vapors, so that pure ammonia gas is withdrawn from the top of the column and can be blown into the absorber 2 via the line 6. For ammonia production becomes ammonia starch water from the cooling circuit 13 or ammonia vapor from the head taken from column 12.

In the case of gas desulfurization of up to 80%, the deacidification column may be unnecessary 11. It is then sufficient that the total ammonia raw water in column 10 is reduced to about 70% to deacidify and a subset of it in the column 12 for the production of ammonia gas to process.

The gas treated in the absorber 3 with fresh water and coal water passes through a line 30 into a scrubber 31. In the scrubber 31, the gas treated with an alkali hydroxide solution. The alkali hydroxide solution is a 2 up to 10% solution.

with the alkali hydroxide solution the hydrogen sulfide is depending on Necessity washed out of the gas except for town gas purity. The resulting Clean gas is fed through a line 32 to any use. The last one Washer 31 accumulating washing solution is drawn off at the washer foot and through a line 33 fed to both the column 10 and the column 11.

The line 33 opens into branches both in the line 19 as also in the line 21, so that a mixing of the withdrawn from the scrubber 31 Washing solution enters with the washing solution drawn off from the sump 22. Consequently is the scrubbing solution from the desulfurization stage with ammonia together with the Wash solution from the desulfurization stage with alkali hydroxide in the deacidification column freed from hydrogen sulfide and possibly hydrogen cyanide.

In the lower third of the deacidification column, they are dissolved in the water Gases that are predominantly in the form of ammonium hydrosulfide and ammonium carbonate pass through Exposure to heat. The vapors rising in the column are in the upper Third by direct sprinkling with concentrated aqueous ammonia solution like this cooled so that a return occurs. The long residence times achieved in this way allow the CO to go into solution and thereby drive off the hydrogen sulfide and to promote the binding of ammonia.

Ammonium carbonate is initially formed from ammonia and carbon dioxide, that is converted to ammonium carbonate or bicarbonate in the rectification column.

This procedure makes it possible to remove the hydrogen sulfide selectively to remove from the washing solution and in concentrations greater than 80% to win the head of the column.

The NH leaving the bottom of the deacidification column contains water only a little H2S (approx. 0.5 - 3.0 g / l), but considerable C02 (approx. 4> g / l) and is therefore not particularly suitable as a scrubbing solution for gas desulfurization.

CO forms ammonium carbonate and reduces the content of free NH3 in the solution and thus the absorption capacity for H This disadvantage is due to Joint deacidification of the washing solutions from the desulfurization stages with ammonia solutions and the alkali hydroxide solution step balanced by cleavage of the fixed ammonium compounds, additional NH3 is released and thereby the ratio NH3: C02 in the washing solution is improved. An undesirably high enrichment the washing solution of alkali salts is prevented from continuously entering Part of the deacidified washing solution is discharged via the ammonia expeller.

In the ammonia expeller 12 are heated to 105-1080C aíls volatile components removed from the water.

The vapors rising in the column, mainly NH3, C02, water vapor and small amounts of H2S are cooled directly with a concentrated ammonia solution.

The cooling solution conducted in the circuit accumulates very strongly here with NH3, G02 and H25. This makes it possible to enter the water with the Ajfgabewasser Column brought acidic components in a relatively small amount of liquid to bind and to remove from the cooling circuit through continuous removal. A pure ammonia gas escapes from the top of the column, which is used as additional ammonia is very suitable for binding the H2S in gas cleaning.

The following is an operational case of the system according to the invention in detail described.

Example calculation for 1000 m3 of gas / h 3 1000 m of coke oven gas are produced in the coking of 2.86 t of coal.

Due to coal moisture of approx. 10% and formation water, approx. 4%, the result is a coal water that contains about 3 g of free and about 3 g of contains bound ammonia / l.

Free ammonia is predominantly in the form of ammonium carbonate and Ammonium sulfide and can be driven off by distillation without the addition of a base.

Fixed ammonia compounds can only be obtained through strong bases such as calcium hydroxide and alkaline solutions are digested.

Example of the composition of fixed NH3 compounds in coal water: Ammonium sulphate (NH4) 2S04 9.5% ammonium thiosulphate (NH4) 2S203 25.0% ammonium chloride (NH4) Cl 55.5% ammonium rhodanide (NH4) CNS 11.0% Because of the harmfulness of the ingredients the coal water cannot be discharged into public waters in the form it accumulates will. It must at least be freed from all volatile constituents and gradually the future minimum requirements for wastewater, in accordance with the Water Resources Act, to a large extent also be freed from the fixed ammonia compounds.

The necessary processing of the coal water is carried out with a special one Beneficial connected. First, the coal water is used as an absorbent in the Gas cleaning used and enriched with ammonia and hydrogen sulfide. Thereafter it can be used particularly economically with other absorption solutions from the ammonia and process H2S laundry.

In the example, the coal water is in an amount tn Lf 1000 m gas placed in the lower zone of the NH3 scrubber 3 and flows through the H2S absorber one after the other 2 and 1 and accumulates with, for example: 20 g NH / l, 7 g H2S / l, 7 g C02 / l and with small amounts of HCN The carbon water contains before the feed into the Absorber about 0.5 H2S / l and caused by the enrichment to 7 g H2S / l and through Addition of gaseous ammonia removes about 30% H2S from the gas.

The processing of the coal water is done together with the others Washing solutions in the deacidification column 10 or 11 and the ammonia stripper 12.

The difference between the deacidification columns 10 and 11 is in that in the deacidification column 10 only the ammonia water is deacidified, which is returned to the absorber 1 as a washing solution through the line 5. here with a lower heat requirement than in column 11, only about 60-70 goat achieved hydrogen sulfide removal from the ammonia water, but for a pre-dewulfurization of the gas to about 50% is sufficient.

In the deacidification column, the hydrogen sulfide can without ammonia loss can be largely removed from the solution and by separating off the carbon dioxide In the booster column of the ammonia stripper 12, it is possible, on the one hand, to produce pure To produce ammonia gas and on the other hand the ammonia production in the form of an ammonia kerf to win.

With a feed water with 20 g free NH3t 1, 7 g H2S / l and 7 g co2 / i, the water leaves the deacidification column 10 with 17 g NH3 / 1, 2.3 g H2S / l and 4 g of CO 2/1 and the deacidification column 11 with 20 g of NH 3 / l, 0.5 g of H 2 S / l and 6.5 g C02 / 1.

The lower NH 3 content in the column 10 is due to the ammonia losses attributed, which leave the head of the column with the acidic components. When the H2S binds to ammonium sulfide and the C02 to ammonium carbonate, they remain in the outlet of the deacidifier 10 only 11.6 g of ammonia not bound to acids, d. H. a maximum of 68% of the ammonia designated as free is sufficient for H2S absorption usable with the gas. 14.5 gil of free ammonia remain in the outlet of the column 11, accordingly 72.5%. The cyano compounds have been neglected here for the sake of simplicity.

According to the invention, the content of free ammonia in the discharge of the deacidification column is increased by the fact that the washing solution from the absorption stage 31, which predominantly consists of Na2S, which is added to the entire wash water before deacidification, and so that the fixed ammonia compounds are split at the same time as the deacidification process and the ammonia is released in the process.

For each molecule of NH3, one mole of NaOH is required for the release, which corresponds to a weight ratio of aOH to NH3 = 2.35.

If the caustic soda is previously used to remove H2S from the gas, stoichiometrically, 2 moles of NaOH are necessary for each mole of H2S when Na2S is formed. That corresponds to 2.35 kg tYesOH / kg H2S.

This means that the same amount of NaOH is required to bind 1 kg of H2S is required, which is also required for the release of 1 kg of fixed ammonia is. It is taken into account that not only Na2S, but also NaHS, NaCN and forms Na2CO3.

In the example, alkali solutions are used in the desulfurization stage 31 1.2 kg H2S / 1000 m3 gas corresponding to 15% of the hydrogen sulfide contained in the raw gas washed out. By processing this alkali washing solution in the NH3 mass deacidifiers 1.2 kg of fixed ammonia are released, with the stoichiometric additional 15% of the Hydrogen sulphides contained in the gas can be removed.

The proposed procedure allows a relative low use of alkali lye increase the desulphurisation effect of the gas, and The operating costs are reduced by splitting the fixed ammonia compound in the deacidification column thereby lowering the circuit wash solution for pre-desulfurization and generation of the ammonia required for binding H2S can be significantly reduced.

Claims (8)

Claims 1. A method for removing ammonia and hydrogen sulphide from gases, in particular carbon distillation gases by absorption of the ammonia by means of water and hydrogen sulfide in at least two desulfurization stages, in one stage with ammonia solutions and in another stage with an alkali hydroxide solution is desulfurized, characterized in that the washing solutions from the desulfurization stage with ammonia (3) as well as from the desulfurization stage with alkali hydroxide (31) together in at least one deacidification column (10, 11) of hydrogen sulfide and hydrogen cyanide to be freed. 2. The method according to claim 1, characterized in that the alkali hydroxide wash (31) is arranged behind the ammonia wash. 3. The method according to claim 1 or 2, gekennzeichnat by an known pre-desulphurization (1,2) with ammonia circulation. 4. Process according to claims 1 to 3, characterized by a) pre-desulphurisation to 30 to 60% b) desulphurisation in the intermediate stage with hydrogen sulfide-free ammonia to 30 to 80% c) residual desulfurization with alkali 5 The method according to one or more of the claims 1 to 4, characterized in that in the lower area of the deacidification column (11) the acidic components of the washing solution are released and in the upper part the column (11) by direct cooling of the rising vapors with one in the circuit led ammonium sulfide solution the carbon dioxide by rectification in the lower Half. is returned. 6. The method according to one or more of claims 1 to 5, characterized characterized in that the washing solution in the deacidifying column (10, 11) with ammonium hydroxide is enriched by releasing the bound ammonia. 7. The method according to claim 5 or 6, characterized in that the from the deacidifier 11) running washing solution in an ammonia column (12) in this way is further treated that by heating all volatile constituents from the water removed and largely H2S-free by direct cooling from the top of the column Ammonia vapors are removed. 8. The method according to claim 7, characterized in that the residual deacidification of the ammonia in the ammonia column by direct cooling of the vapors with a circulating formed Amconiumcarbonatiösung is effected, from the continuously or at intervals the excess that forms is withdrawn.