Process for removing ammonia and hydrogen sulfide from gases, in particular coal distillation gases.

0001The invention relates to a process for removing ammonia and hydrogen sulfide from gases, in particular carbon distillation gases, by absorption of the ammonia by means of water and the hydrogen sulfide in at least two desulfurization stages, with desulfurization in one stage using ammonia solutions and in another stage using alkali metal hydroxide solution.

0002It is known to remove hydrogen sulfide from gases by means of sodium or potassium hydroxide solution and to use the waste liquor formed in this way to release bound ammonia.

0003This takes place together with the waste water from the ammonia wash in what are known as ammonia cleaners. In the ammonia strippers, the water is freed of all volatile constituents and then released into the receiving water.

0004Large amounts of sodium sulfite waste liquor are obtained in such processes. This is particularly the case if the process described in German patent specification 2505959 is used. The hydrogen sulfide contained in the coke oven gas is then washed out predominantly by alkali carbonate solutions. In practice, however, this is uneconomical because caustic soda and potassium hydroxide are particularly expensive input materials. In addition, the cycle of the detergent selected in this process requires an increased detergent requirement due to the long residence time and the resulting side reactions. The invention is therefore based on the object of improving the economy of the known processes for gas desulfurization with ammonia and alkali metal hydroxide solution.

0005The invention is based on the idea of ​​using alkali metal lye for cleaning gases only to the extent that it is economically optimal. This is achieved by washing the washing solutions from the desulfurization stage with ammonia and from the desulfurization stage with alkali hydroxide together in a deacidification column of hydrogen sulfide and hydrogen cyanide. Desulfurization is also particularly advantageously carried out in two steps.

0006The first step is known per se and is that first. only as far as regenerated ammonia water is used that a certain pre-desulphurization, preferably of 40 to 50%, is effected and then ammonia gas free of hydrogen sulfide is added and the gas treated in this way is then passed to an ammonia washing stage. In the second step, an additional desulfurization stage is provided after the ammonia wash, which is treated with sodium or potassium hydroxide solution.

0007This process allows selective desulfurization of the gas with alkali lye and short residence times. As a result of the short residence times, harmful side reactions, such as, for example, sodium carbonate formation by absorption of carbon dioxide, are avoided. Bound ammonia is only released to the extent that it is economical for the production of additional ammonia for gas desulfurization and to the extent that it is necessary to achieve specified wastewater qualities.

0008Not only a small proportion of bound ammonia is useful for water protection, but also a pH value in the range between 7 and 8. In contrast, with conventional methods, pH values ​​between 9.5 and 11.5 are often measured. Compared to the disadvantages of such a pH value, the water pollution from ammonia salts associated with the method according to the invention is comparatively negligible.

0009Moreover, the release of ammonia according to the invention in the upstream deacidification column has three major advantages over the release of ammonia by the known processes in the ammonia stripper. The free ammonia content in the washing solution is increased considerably. Furthermore, the removal of hydrogen sulfide from the washing solution is considerably improved by the action of the carbon dioxide, which is absorbed in a certain amount during the washing process even with selective desulfurization and is released during desorption. In addition, it becomes possible to concentrate the hydrogen sulfide directly from the deacidifier for further processing into sulfuric acid or sulfur, without going through the ammonium sulfate saturator and to keep it away from the ammonia stripper. This is particularly important for the extraction of sulfur-free ammonia. In the drawing, an embodiment of the invention is shown.

0010The gas coming from a gas suction device or a gas aftercooler (carbon distillation gas) is, for example, 8 g H₂S / Nm³ and 6 g NH₃/ Nm³ Via a line 4 to an absorber 1 of a first washing stage, which is used for pre-desulfurization. The gas is deacidified with NH in this stage₃-Circulation water and an NH₃-Washing solution washed from an absorber 2 of a subsequent desulfurization stage and thereby desulfurized to 30 to 50%.

0011The circulating water reaches the absorber 1 via a line 5 (in the upper area), where the liquid composition corresponds to the accumulation of the circulating water. The NH₃Washing solution of the absorber 2 flows to the absorber 1 via a line 18.

0012The enrichment of the washing solution in the absorber 1 is influenced by the gas composition and the temperature. During the residence time in the absorber 1, the H increases₂S content in the wash solution from 2.3 to 7 g H₂S / 1. By applying 0.6 1 / Nm³ Circulation water from line 5 is washed out 35% of the hydrogen sulfide contained in the raw gas. From the H₂S pre-scrubber or absorber 1, the gas reaches the absorber 2 via a line 15. In the absorber 2, the waste water from an absorber 3 forming a third stage is washed with the simultaneous addition of largely hydrogen sulfide-free ammonia. The ammonia is metered in through lines 6 at several points into the lower and middle area of ​​the absorber 2.

0013In the absorber 2 which serves for the desulfurization, the gas can be largely freed of hydrogen sulfide with sufficient addition of pure ammonia and the use of relatively large amounts of water for the backwashing of the additional ammonia. For economic reasons and because of greater operational safety, according to the proposal of the invention, the sulfurization in the absorber 2 is not operated as far as technically possible, but the fine sulfurization is carried out in a further stage 31. The drain water of the absorber 3 reaches the absorber 2 via a line 16 ′.

0014The gas enters the absorber 3 through a line 17. There it is supplied with fresh water from a line 7. Part of the fresh water can be caused by weakly enriched NH₃-Water, for example coal water as gas condensate, to be replaced. This water is pumped through a line 8 into the absorber 3. In the loading with NH₃ and H₂S the water corresponds approximately to the concentration of the wash water.

0015All washing solutions given in the process are led in countercurrent to the gas. They go via a line 9 into a sump 22 of the first desulfurization stage (absorber 1), from which it is removed via a line 19. The NH flowing in the circuit and fed to the absorber 1 via the line 5₃Water is fed via line 19 and line 20 to a column 10 and is deacidified in a known manner by about 60 to 70% by thermal decomposition. It then passes back through line 5 into absorber 1.

0016The excess washing water, which corresponds approximately to the amount of fresh and coal water, must be deacidified more than the circulating water for the production of ammonia vapors free of hydrogen sulfide. This is done in a known manner according to the processes disclosed in German patents 11 55 426 and 11 75 381, producing pure ammonia vapors and simultaneously producing about 20% strength ammonia water.

0017The ammonia raw water drawn off from the sump 22 via lines 19 and 21 is desulfurized in a column 11 and fed via line 23 to a column 12 in which it is freed of all volatile constituents. Thereafter, the water passes through a line 14 into the waste water.

0018In the column 12 there is direct cooling and post-deacidification of the rising vapors, so that pure ammonia gas can be removed from the top of the column and blown into the absorber 2 via the line 6. For the ammonia production, ammonia heavy water is taken from the cooling circuit 13 or ammonia steam from the top of the column 12. In the case of gas desulfurization up to 80%, the deacidification column 11 may not be necessary. It is then sufficient to deacidify the entire raw ammonia water in the column 10 to about 70% and to process a part of it in the column 12 to obtain ammonia gas.

0019The gas treated with fresh water and coal water in the absorber 3 passes through a line 30 into a scrubber 31. In the scrubber 31, the gas is treated with an alkali hydroxide solution. The alkali hydroxide solution is a 2 to 10% solution.

0020With the alkali hydroxide solution, the hydrogen sulfide is washed out of the gas as necessary, except for city gas purity. The resulting clean gas is fed through a line 32 for any use. The washing solution obtained in the last scrubber 31 is drawn off at the scrubber foot and fed to both the column 10 and the column 11 through a line 33.

0021The line 33 opens into branches in both the line 19 and the line 21, so that mixing of the washing solution drawn off from the scrubber 31 with the washing solution drawn off from the sump 22 occurs. As a result, the washing solution from the desulfurization step with ammonia is freed of hydrogen sulfide and, if appropriate, hydrogen cyanide together with the washing solution from the desulfurization step with alkali hydroxide in the deacidification column.

0022In the lower third of the deacidification column, the gases dissolved in the water, which are mainly present as ammonium hydrosulfide and ammonium carbonate, are released by the action of heat. The vapors rising in the column are cooled in the upper third by direct sprinkling with concentrated aqueous ammonia solution so that a reflux occurs. The long residence times achieved in this way enable the CO₂ to go into solution and thereby promote the stripping of the hydrogen sulfide and the binding of the ammonia.

0023Ammonium carbonate is initially formed from ammonia and carbon dioxide and is converted to ammonium carbonate or bicarbonate in the rectification column. This procedure makes it possible to selectively remove the hydrogen sulfide from the wash solution and to obtain it in concentrations of over 80% from the top of the column.

0024The NH water leaving the bottom of the deacidification column contains only a little H₂5 (approx. 0.5 - 3.0 g / l), but considerably C0₂ (approx. 4% g / l) and is therefore not particularly suitable as a washing solution for gas desulfurization. C0₂forms ammonium carbonate and reduces the free NH content₃ in the solution and thus the absorption capacity for H₂S.

0025This disadvantage is compensated for by jointly deacidifying the washing solutions from the desulfurization stages with ammonia solutions and the stage with alkali hydroxide solution in that additional NH is eliminated by cleaving the fixed ammonium compounds₃ becomes free and thereby the ratio NH₃ : C0₂ is improved in the washing solution. An undesirably high concentration of alkali salts in the washing solution is prevented by continuously removing part of the deacidified washing solution via the ammonia stripper.

0026In the ammonia stripper 12, all volatile constituents are removed from the water by heating to 105-108 ° C. The vapors rising in the column, mainly NH₃, C0₂, Water vapor and small amounts of H₂S are cooled directly with a concentrated ammonia solution.

0027Here, the cooling solution in circulation is enriched very strongly with NH₃, CO₂ and H₂S on. This makes it possible to bind the acidic components brought into the column with the feed water in a relatively small amount of liquid and to remove them from the cooling circuit by continuous removal. A pure ammonia gas escapes from the top of the column and is used as an additional ammonia to bind the H_Z.S is very suitable for gas cleaning.

0028An operation of the system according to the invention is described in detail below.

0029example

0030Bill for 1000 m³ Gas / h 1000 m³ Coke oven gas is produced when coking 2.86 t of coal.

0031Coal moisture of approx. 10% and formation water, approx. 4%, result in a coal water which, in addition to other substances, contains approx. 3 g free and approx. 3 g bound ammonia / 1.

0032Free ammonia is predominantly in the form of ammonium carbonate and ammonium sulfide and can be removed by distillation without adding base.

0033Fixed ammonia compounds can only be broken down by strong bases such as calcium hydroxide and alkali lye.

0034Example of the composition of fixed NH₃-Connections in coal water:<tables id="tabl0001" num="0001"><img file="EP0062684A1_D0001.tif" /></tables>

0035Due to the harmfulness of the ingredients, the coal water cannot be discharged into public waters. It must at least be freed from all volatile constituents and, according to the future minimum requirements for waste water, in accordance with the Water Resources Act, to a large extent also from the fixed ammonia compounds.

0036The necessary treatment of coal water is associated with a special benefit. First, the coal water is used as an absorbent in gas cleaning and enriched with ammonia and hydrogen sulfide. Then it can be particularly economical with other absorption solutions from the ammonia and H₂Refurbish suburban laundry.

0037In the example, the coal water is used in an amount of 0.4 m³/ ₁₀₀0 m³ Gas into the lower zone of the NH₃-Washer 3 and flows through the H one after the other₂S-Absorbsr 2 and 1 and enriches itself with, for example: 20 g NH₃/ 1.7 g H₂S / 1.7 g CO₂/ 1 and with small amounts of HCN

0038The coal water contains about 0.5 H before being placed in the absorber₂S / 1 and caused by enrichment to 7 g H₂S / 1 and by adding gaseous ammonia approximately 30% H2S removal from the gas.

0039The coal water is worked up together with the other washing solutions in the deacidification column 10 or 11 and the ammonia stripper 12.

0040The difference between the deacidification columns 10 and 11 is that in the deacidification column 10 only the ammonia water is deacidified, which is returned as a washing solution through line 5 into the absorber 1. Here, with a lower heat requirement than in column 11, only about 60-70% hydrogen sulfide removal from the ammonia water is achieved, which is sufficient for a pre-desulfurization of the gas to about 50%.

0041In the deacidification column 11, the hydrogen sulfide can be largely removed from the solution without ammonia losses, and by removing the carbon dioxide in the booster column of the ammonia stripper 12, it is possible, on the one hand, to produce pure ammonia gas and, on the other hand, to obtain ammonia production in the form of a high ammonia water.

0042With a feed water with 20 g free NH₃₁ 1.7 g H₂S / 1 and 7 g CO₂/ 1 the water leaves the deacidification column 10 with 17 g of NH₃/ l, 2.3 g H₂S / 1 and 4 g CO₂/ 1 and the deacidification column 11 with 20 g of NH₃/ 1, 0.5 g H₂5/1 and 6.5 g C0₂/ l.

0043The lower NH₃Content in column 10 is due to the ammonia losses which leave the top of the column with the acidic constituents. When the H₂S to ammonium sulfide and the C0₂ to ammonium carbonate only 11.6 g of ammonia not bound to acids remain in the drainage of the deacidifier 10, ie a maximum of 68% of the ammonia designated as free is for one H₂S absorption from the gas can be used. 14.5 g / l of free ammonia, corresponding to 72.5%, remain in the outlet of column 11. The cyan compounds have been neglected here for the sake of simplicity.

0044According to the invention, the free ammonia content in the discharge of the deacidification column is increased in that the washing solution from the absorption stage 31, which consists predominantly of Na₂ exists, the entire wash water is added before the deacidification, and thus the fixed ammonia compounds split simultaneously with the deacidification process and the ammonia is released.

0045The molecule NH₃ a mole of NaOH is required for the release, which corresponds to a weight ratio of NaOH to NH₃ = 2,35.

0046If the caustic soda is used for the H₂S removal from the gas is used, so are stoichiometric per mole of H₂S 2 moles of NaOH with formation of _Na2S necessary. This corresponds to 2.35 kg NaOH / kg H₂S. This means that for the binding of 1 kg H₂S the same amount of NaOH is required that is also required for the release of 1 kg of fixed ammonia. It is taken into account that not only_Na2S, but also NaHS, NaCN and Na₂CO₃ forms.

0047In the example, alkali lye in the desulfurization stage 31 1.2 kg H₂S / 1000 m³ Gas washed out corresponding to 15% of the hydrogen sulfide contained in the raw gas. By working up this alkali wash solution in the NH₃-Water deacidifiers release 1.2 kg of fixed ammonia, with which another 15 of the hydrogen sulfide contained in the gas can be removed stoichiometrically. The proposed procedure can be used to increase the desulfurization effect of the gas with a relatively low use of Akalilauge, and to reduce the operating costs by splitting the fixed ammonia compound in the deacidification column in that the circuit washing solution for the pre-desulfurization and the generation of the H₂S-Sensation necessary ammonia can be significantly reduced.