DE2524124A1 - Recovery of sulphur from gases contg. sulphur cpds. - by treating gas with polybasic organic acid - Google Patents

Abstract

Hydrogen sulphide is removed from a gas by treating it with an aq. soln. of a polybasic organic acid having a dissociation constant of 10-2 - 10-5, the soln. being buffered with a base. The molar ratio between H2S and SO2 in the gas is >=2:1 and it is injected into the soln. which is at pH 2-6 (3.3-3.9), at 10-100 degrees C forming solid sulphur which is removed by a current of the soln. from the reaction chamber. Gas contg. SO2 and having the soln. is heated to 700-1000 degrees C to transform S-contg. cpds. into SO2, and the gas is recycled. The rest of gas leaving the soln. is treated with a second absorption step with the soln. having the reaction chamber to remove the last traces of S. The process enables gas mixtures richer in H2S than cleavable by the Claus process to be purged of sulphurous gases.

Description

Process for cleaning waste gases containing hydrogen sulphide The present invention relates to a process for cleaning gases containing hydrogen sulphide, which may also contain sulfur dioxide and / or organic compounds. The hydrogen sulfide is in a buffered, polybasic organic acid containing aqueous solution in the presence of SO2 according to the gross reaction: converted to solid sulfur, while the organic compounds are thermally decomposed to carbon dioxide water and possibly SO2 with such a proportion of hydrogen sulfide as is necessary according to the aforementioned gross equation to convert the hydrogen sulfide with sulfur dioxide to sulfur depending on the origin of the gases The organic compounds can optionally also contain sulfur themselves, which then results in a corresponding sulfur dioxide content corresponding to the sulfur content of these compounds in the thermally decomposed gas.

For the treatment of gases containing hydrogen sulfide, various Proposed method such. B. the absorption in alkaline detergents below Conversion of the hydrogen sulfide to sulfides or hydrogen sulfides, which may be oxidizing adsorption on activated carbon, the thermal oxidation of exhaust gases with a subsequent removal of the Light Dioxide, the Absorption in organic solvents, the so-called Claus process and absorption in weakly alkaline aqueous solution and oxidation of the hydrogen sulfide with Atmospheric oxygen in the presence of catalysts such as anthraquinone disulfonic acid and sodium vanadate in the Stretford process.

Since more and more exhaust air flows with sometimes considerable sulfur contents must be desulphurized, it is becoming increasingly difficult to make such a process economical to be used where sulfur chemicals such as sodium hydrogen sulfide, sodium sulfite, Sodium sulfate, etc. arise. The obsession with these sulfur chemicals leaves at least not in the long term to infiltrate the economic process again. on the other hand is the problem-free disposal of such substances without any further chemical Realizations impossible. There are therefore such processes advantageous in which the Elemental sulfur is obtained because sulfur is used technically in large quantities is and on the other hand as an inert material until its use without too great Effort can be stored. For gases with a relatively high hydrogen sulfide content the Claus process has therefore acquired considerable practical importance.

In the Claus process, a partial flow of the hydrogen sulfide-containing Gas thermally - for the production of SO, treated with this so obtained sulfur dioxide catalytically convert the remaining proportion of NS in the exhaust gas to sulfur.

As for the implementation of 2 moles of hydrogen sulfide 1 mole of sulfur dioxide are necessary, are the high demands on the control of the Claus process Quantities disadvantageous. If the amount and concentration of hydrogen sulfide in the exhaust gas, Oxygen and optionally organic Pollutants fluctuates greatly, a complex gas buffering is necessary. In general, it is also used for security reasons post-treatment of the end gases may be necessary.

The Stretford process mentioned at the beginning, which also uses sulfur arises, is easier to handle in this regard, is disadvantageous here however, the poor water solubility of the absorption chemicals and the relative slow response speed. So only a maximum of about 0.5 g of hydrogen sulfide absorb per liter of washing solution and circulation. Therefore are for higher hydrogen sulfide concentrate ions extremely large and technically hardly controllable specific amounts of detergent required, for concentrated gases up to 760 1 solution per 2 exhaust gas. With intermittent Accumulation of exhaust gas or fluctuating exhaust gas concentrations must be due to the reaction time the maximum amount of solution required for at least 20 minutes be pumped around.

US Pat. No. 2,563,437 describes a method in which first SO produced from H28, up to a concentration of about 5, preferably 1 to 2 percent by weight in an aqueous solution is absorbed, the small amounts of aluminum sulfate and sulfuric acid. This solution is then used subsequently reacted in a second stage with H2S, from which the sulfur is converted into filterable Form should arise.

A further development of this process, in which an aqueous organic acid buffered to pH 4 to 6, such as citric acid, lactic acid, maleic acid, tartaric acid, oxalic acid, glutaric acid and other aqueous solutions, is used as the absorption solution, describes the process of US Pat. No. 2,729,543 In this known process, thiosulfate and alkali are also added to the absorption solution in order to accelerate the conversion rate between H2S and SO2 for the second stage. Furthermore, for the same reason, the oxidation of the hydrogen sulfide is carried out at temperatures from 400 to the boiling point of the reaction solution. According to this US patent specification, at pH values of 4 to 6, the sulfur dioxide initially reacts very quickly with a deficit of hydrogen sulfide according to the equation to thiosulfuric acid. The reaction leading to sulfur formation according to the gross equation H2S203 + 2H2S = 4S + 3H20 is much slower and the rate of this reaction can be significantly influenced by the concentration of thiosulfuric acid and the reaction temperature according to the explanations of US Pat. No. 2,729,543. An improvement in the process known per se for reacting H2S with SO in buffered organic solutions containing polybasic acids is therefore achieved by setting a thiosulfate concentration of up to about 3 to 3.5 mol / l in the absorption solution, while in the second stage - the regeneration of the SO2-containing absorption solution - the reaction with H2S takes place at elevated temperatures, the hydrogen sulfide being added up to a maximum of the stoichiometrically necessary amount - based on SO2. According to this reference, an excess of H2S must be carefully avoided in order to avoid the formation of sulfur in the solution returned to the absorption step after the sulfur has been separated off.

This known method was essentially used for the purification of SO, -containing Exhaust gases developed. So can during the SO2 absorption with volume ratios from gas to liquid from about 100: 1 to 5000: 1 worked will. A 0.5 molar aqueous solution of sodium sulfosuccinate with a pH of about 4.5 for example with a gas: liquid volume ratio of 1700 : 1 the SO2 content of the exhaust gas from 0.25 percent by volume to 0.05 percent by volume. Conversely, this process can of course also be used to remove hydrogen sulfide can be used from exhaust gases. It should be noted here, however, that the so-called Regeneration of the solution containing 802 relatively long contact times with the hydrogen sulfide (up to 15 minutes) are necessary.

According to US Pat. No. 3,757,488, it should be possible to convert from exhaust gases, which contain both hydrogen sulfide and sulfur dioxide, the sulfur dioxide to absorb selectively if one uses an aqueous solution to treat the exhaust gases used, which was adjusted to a pH of 4 to 6 and the rest of the polyvalent organic acids such as citric acid, maleic acid, lactic acid, etc. in one Contains concentration from about 5 up to 95%.

For example, at a pH of 5.7, there was complete absorption of S02, however, no absorption of H2S was observed while approaching the neutral point the H2S absorption increased.

In Hydrocarbon Processing 53 (1974) No. 4, pages 75 to 77, is a method for the post-purification of exhaust gases from Claus plants described in which the exhaust gases are first thermally decomposed to remove all sulfur compounds contained in the exhaust gas to be transferred to S02. This gas is then mixed with an aqueous sodium citrate-citric acid solution washed, the washing solution then with hydrogen sulfide regenerated, whereby elemental sulfur precipitates, that from the circulated Solution is filtered off.

The procedure of the German Offenlegungsschrift 2 432 749 concerns essentially a process for the regeneration of absorption solutions containing sulfur dioxide with hydrogen sulfide, the preferred absorbent being aqueous solutions of sodium phosphate, Sodium citrate or mixtures thereof with pH values between about 3 and 4 are used will. The main aim of this known process is to remove the aqueous, sulfur dioxide-containing Rapidly reacting absorbent with gaseous hydrogen sulfide and to minimize the deposition of sulfur in the reaction zone itself. A regeneration of the Absorption liquid is achieved with a contact time of 4 seconds, however, in order to compensate for concentration fluctuations at k out, dwell times in the regeneration zone from 10 to 300 seconds can be suggested. For complete regeneration of the Absorption liquid are, based on sulfur dioxide, stoichiometric excesses of hydrogen sulfide - in particular 5 to 50% - introduced into the regenerator.

It has now been found that the methods described above also work with very high demands on the purity of the residual gas with good controllability of the process, even in the case of intermittent attacks the exhaust gases with different hydrogen sulfide concentrations. In the inventive Process are aqueous, with inorganic bases, such as. B. caustic soda or Potash lye to pH values between 2 and 6 buffered solutions of organic polyvalent Acids such as oxalic acid, malonic acid, glutaric acid, adipic acid, maleic acid, fumaric acid, Citraconic acid, phthalic acid or mixtures thereof, hydrogen sulfide and sulfur dioxide, at the same time supplied in finely divided form, with elemental sulfur precipitating out. An embodiment in which the sulfur dioxide is particularly advantageous by burning some of the hydrogen sulphide is produced and this Sulfur dioxide is introduced into a first absorber together with the hydrogen sulfide will. The conversion of sulfur dioxide takes place in this first absorption zone and hydrogen sulfide, preferably under approximately stoichiometric conditions, however, on average over time with a minimum excess of NS in the escaping gas from 0.05 to 2 percent by volume, preferably 0.5%. That emerges from the following Combustion of the hydrogen sulphide fraction with mixed air resulting in flue gas with an excess of sulfur dioxide, the combustion zone becomes one directly second absorber fed to the washing solution of the first absorber after separation of the precipitated sulfur flows in. This washing solution is then used in the circuit first absorber returned. The one emerging from the first absorber, even more than 0.05 volume percent hydrogen sulfide-containing gas is expedient together with the partial flow branched off for the direct generation of sulfur dioxide in given a combustion chamber so that all the exhaust gas at least once the combustion chamber passes through and thus all organic pollutants are decomposed while the end gas after passing through the second absorber only carbon dioxide, water and traces Contains sulfur dioxide.

The present invention is therefore a method for Removal of hydrogen sulfide, if necessary Sulfur dioxide and / or organic compounds from gases, in which the sulfur compounds in solid elemental sulfur can be converted by treating the gases with a aqueous solution, buffered by the addition of bases, the organic polybasic acids a dissociation constant between 10 2 and 10 5, which is characterized is that a) the gas having a hydrogen sulfide to sulfur dioxide molar ratio of approximately equal to or greater than 2: 1 in a first absorption and reaction zone Temperatures 0 from 10 to 100 C in finely divided form with an addition of base to a pH between 2 and 6 buffered aqueous solution of one or more of organic polyvalent acids brought into contact; b) from the solution sulfur accumulating in solid form is removed from at least a partial stream of the solution; c) the gas leaving stage a), optionally mixed with previously untreated Gases for converting the sulfur compounds contained in the gases to sulfur dioxide as well as for the oxidation of any organic compounds that may be present to form carbon dioxide and heated water to temperatures of about 7000C to 10000C; d) the S02-containing Flue gas from stage c) optionally in such a partial flow amount as it is used for Adjustment of the hydrogen sulfide / sulfur dioxide molar ratio desired in stage a) is required, fed to stage a); e) the remaining flue gas partial flow of c) for the most quantitative possible SO2 absorption in a second absorption stage e) treated with the solution leaving step b); and f) the solution containing SO2 of stage e) is returned to stage a).

As organic polyvalent acids, oxalic, malonic, Glutaric, adipic, maleic, fumaric, citraconic and / or phthalic acids are used. not one is preferred only for economic, but also for technical reasons Mixture of organic polybasic acids used, which contains about 40 to 60% glutaric acid, Contains 15 to 30% adipic acid and about 20 to 40 ß succinic acid. One such Mixture is also known technically as undistilled glutaric acid. It was found that when using such an acid mixture even with several days of operation no lye had to be added to a test plant to stabilize the pH value and therefore no by-products occur. It didn’t have to use sodium sulphate sodium thiosulphate can still be discharged.

Except for the low consumption of chemicals and the technical use The process according to the invention still has easily accessible organic acids a number of other advantages.

The molar ratio of hydrogen sulfide to sulfur dioxide does not have to be are constantly regulated as precisely as possible to a value of 2: 1, but only in Time average, since the washing solution has a considerable buffer capacity. the Capacity of both the first and the second stage is relatively large and it can also large quantities of gas with relatively small contents of hydrogen sulfide and SO2 be treated. E.g.

an exhaust gas with a hydrogen sulfide content of 80 percent by volume and an SO2 content of 3 percent by volume, which is achieved by adding to SO2-containing Exhaust gas to an H2S: S02 molar ratio of about 2: 1 and thus to an H2S content of 15.85 and an SO2 content of 7.73 volume percent was set, with a specific gas load in the absorber a) of max.

150 m3 gas / m2 cross-section and hour and a volume ratio of Introduced gas: supplied (i.e. pumped over) liquid of 40 and at one Contact time of -v5 seconds converted almost quantitatively to sulfur be, with the sulfur floating under these flow conditions and in the upper Part of the absorption and reaction zone in the form of an easily filterable suspension accrues. In general, the pH of the reaction solution becomes more usual with the addition Alkalis such as caustic soda, potassium hydroxide, soda or potash to a pH value of 2 to 6, preferably from 3.3 to 3.9.

The temperature of the reaction solution in the first stage becomes 0 at about 30 ° C to the boiling point of the solution, preferably kept at 4000 to 6000. Depending on the hydrogen sulfide content of the exhaust gas, the volume ratio can be gas : Absorbent liquid fluctuate in a relatively wide range. As opposed to to the known processes the absorption of H2S and SO? as well as their reaction and Conversion to sulfur in column a) takes place simultaneously, depends on the required Volume ratio of introduced gas to circulated liquid only of the following Conditions from: 1) the achievable sulfur concentration in the flotation suspension produced at the top of column a) and 2) from that in column e) absorbing residual S02.

For the requirement 1) are about 15 1 / ion? (H, S + S02) accordingly 100 100g 8/1 solution required. Requirement 2) then results in larger ones Volume ratios, i.e. larger specific circulating quantities if more than 5% of the all of the S02 in the absorber e) must be absorbed by the solution. In general is the volume ratio of the amount of gas fed in to the amount of liquid pumped around in the absorber a) between 8: 1 and 60: 1, preferably between 30: 1 and 40: 1, the contact times of the gas generally being between 3 and 30 seconds, preferably between 4 and 6 seconds.

Through the thermal treatment of the one leaving the first absorption stage Exhaust gases are any organic compounds that may be present in the gas to be scrubbed decomposed. The thermal decomposition takes place in a combustion chamber, in which if necessary by adding liquid or gaseous fuels to a temperature of above 7000C up to 10000C is maintained.

By combining two absorption stages with one combustion chamber is the system against fluctuations in the amount and composition of the items to be cleaned Exhaust gases are very insensitive, as there is a momentary excess of S02 in the second Absorption zone removed and further the first absorption zone leaving excess H2S is converted to S02 in the combustion chamber, which is either the first or but is fed to the second absorption zone, a tail gas is produced in each case high purity, which is less than 1 ppm hydrogen sulfide and a maximum of 0.1 percent by volume Contains sulfur dioxide.

The method is simple to carry out in terms of apparatus and essentially consists (see Figures 1 to 3) from two absorbers (1) and (2), a combustion chamber (3) and one or two filters (4) for sulfur separation. There can be several such units can be connected in parallel or in series. In these figures the numbers given have the following meaning: 1 = first absorber 2 = second absorber 3 = combustion chamber with heat exchanger if necessary 4 = sulfur filter, -decanter 5 = cooler (condenser) 6 = exhaust gas to be treated 7th = Air supply 8 = cleaned exhaust gas 9 = sulfur discharge 10 = alkali supply 11 = Washing water 12 = supply of fresh absorption solution 13 = cooled flue gas - partial flow to the first absorber 14 = cooled flue gas - partial flow to the second absorber 15 = Partial flow of the exhaust gas to be treated 16 = exhaust gas from the first absorber 17 = preheated Exhaust gas from first absorber 18 = flue gas 19 = air 20 = condensate In some special Embodiments, the method is explained in more detail with reference to FIGS. FIG. 1 describes a procedure for treating a containing only H2S and N2 Exhaust gas. The incoming exhaust gas (6) with an average of 80% hydrogen sulfide, remainder N2, is divided in the ratio of 27.1: 72.9, the larger part of the flow goes into the absorber (1), which is designed as a gas bubble scrubber with a gas dispersion stirrer is. The washing liquid consists of a solution of 10 g of undistilled glutaric acid (~ 50% glutaric acid, 20% adipic acid and 30% succinic acid) 1 water each, adjusted with sodium hydroxide solution to pH 3.5. Your temperature is kept at 580c. That the absorber (1) Leaving above, still containing 0.576 percent by volume of hydrogen sulfide Gas (16) is with the smaller partial flow of the raw exhaust gas (15) together with a, based on the amount of H2S at (6), 2.62 times the amount of air (7) in the Brerin chamber with preheating to 4500C (3) passed (17) and there at 7800C to a 2.42% sulfur dioxide, Flue gas containing 15.55% water vapor, 1.76% oxygen, the remainder nitrogen. Then the flue gas (18) cooled to 480c in the cooler (5), where 0.565 kg water / Nin? condense the supplied H28 (20). The flue gas will divided roughly in the ratio 20.7: 79.3. The 79.3% are on a rule cap and a fan returned (13) to the absorber (1). The remaining 20.7 ffi of the flue gas (14) are passed into the smaller absorber (2) and there accordingly Purified the pH of the solution at 480c down to 0.04 volume percent sulfur dioxide (8th). The specific gas loading in both absorbers is 150 m3 / m2 cross-section x hour and the liquid circulation 25 l / 2 H2S.

The one in the absorber (1) after the gross reaction Elemental sulfur formed is floated under these process conditions in the absorber (1) and can be removed with part of the liquid as a suspension that can be easily filtered at the head of the absorber (1). The absorption solution filtered in the rotary filter (4) is fed to the absorber (2) for the residual absorption of the sulfur dioxide. The solution then enriched with sulfur dioxide flows back into the absorber (1). The sulfur is washed (and removed at (9).

By choosing the ratio of the partial flows (13): (14) and their Temperature behind the cooler (5), the temperatures in the absorbers (1) and (2) and thus also control the water balance in such a way that the entire reaction occurs resulting water is discharged as condensate (20) and as steam with the flue gas at (8) will. Corresponding to the formation of small amounts of sodium thiosulphate or sodium sulphate Sodium hydroxide solution (lo) is added to the absorber (1) in a pH-controlled manner. In the present However, this was not the case even after operation for more than 100 hours necessary. Loss of organic acids, e.g. with the filtered sulfur (9) compensated by the regular addition of fresh absorption solution (12).

In the example of Figure 2, the inventive method for Treatment of an exhaust gas which, in addition to hydrogen sulfide, also contains a considerable amount of sulfur dioxide contains, and exhaust air containing traces of hydrogen sulfide, which is used as combustion air serves, explained. The exhaust gas (6) with an average of 59.0 percent by volume #S, 29.3 percent by volume SO2 and 11.7 percent by volume N2 are fed into the main absorption tower (1) below and there with a dispersing stirrer finely divided with a gas load of about 50 m3 / m2 cross-section x hour and an average residence time of the gas bubbles in the Absorbent solution of about 5 to 10 seconds. The composition of the absorption solution is the same as in Example 1, its temperature is kept at 600c. That the The exhaust gas (t6) leaving the absorber (1) at the top contains only traces of 0.5% H25 of 502. It is used without preheating together with the pollutant-containing one preheated to 4500C Combustion air (7), which contains 0.267 volume percent H2, and natural gas as fuel passed into the combustion chamber (3). Volume ratio exhaust gas (6): exhaust air (7): natural gas = 1: 0.87: 0.0355.

The flue gas (18) emerging from the combustion chamber at 7850C has the Composition 3.08 percent by volume C02 0.294 percent by volume S02, 9.1 percent by volume H20, 10.5 percent by volume 02, remainder N2. It is related to the exhaust gas (6) 2.04 times the amount of air (19) mixed and at a temperature of 2400C into the column (2) headed. There, at 600c, the S02 is reduced to 0.04 percent by volume in the flue gas outlet (8) washed out.

Liquid circulation, sulfur filtration (4), washing (11) and removal (9) and the addition of chemicals (10 and 12) correspond to Example 1.

The combustion heat supplied to the gas in the combustion chamber is used to decompose the hydrogen sulfide as well as any other organic ones that may be present Traces of pollutants in the gas streams (7) and (16) and for discharging the water of reaction from 0.524 kg / m) of the exhaust gas stream (6) by evaporation from the column (2) into the flue gas stream (8th).

In the example of Figure 3, the inventive method for Treatment of an exhaust gas stream (6) which, in addition to hydrogen sulfide, is still on average 6.23 volume percent carbon oxide sulfide and 6.23 volume percent ethyl mercaptan as Example of sulfur-containing pollutant admixtures explained in more detail. The combustion air (7) contains 0.175 percent by volume of carbon oxide sulfide and 0.175 percent by volume of ethyl mercaptan.

The entire exhaust gas (6) is in the equipped with a gas dispersing stirrer Absorber (1) passed. The washing solution is the same as in Example 1. The gas outlet (16) on the absorber (1) has the following composition at 530C: 1.13 percent by volume H2S, 0.338 volume percent CoS; 0.338 volume percent C2H5SH; 3.18 percent by volume C02; 1.92 percent by volume 02; 14.12 percent by volume H2O, traces SO2, remainder N2. That Because of the risk of explosion, gas (16) is fed into the combustion chamber (3) without preheating. routed together with the, based on the exhaust gas volume (6), 5.93 times, to 4500c, preheated exhaust air volume (7). The flue gas leaving the combustion chamber (3) at 7800C (18) has the following composition: 1.405 volume percent SO2; 12.58 percent by volume u 0; 3.25 volume percent CO 2; 1.96 percent by volume 02, remainder N2. It gets in the cooler (5) cooled to 480C, with 0.365 kg water / m3 of the exhaust gas fed in at (6) condense out (20). The flue gas is divided in the ratio 75.6: 24.4. the 75.6% is returned to the absorber (1) via a control flap and a fan (13). The remaining 24.4% of the flue gas (14) is in the smaller absorber (2) and purified there at 480C down to 0.04 volume percent sulfur dioxide. The specific gas loading in both absorbers is 150 in? / M2 cross-section. h and the liquid circulation 75 l / 2 of the hydrogen sulfide supplied in (6).

The separation of the sulfur (4), the washing (11), the discharge (9) and the chemical additions (10) and (12) as well as the discharge of the Reaction water, partly as a condenser (20) and partly as vapors with the Exhaust gas (8), the example of Figure 1.

Claims (7)

Patent claims: 1) Procedure for removing hydrogen sulfide, if necessary Sulfur dioxide and / or organic compounds from gases, in which the sulfur compounds can be converted into solid elemental sulfur by treating the gases with a aqueous solution, buffered by the addition of bases, the organic polybasic acids contains a dissociation constant between l0 # 2 and lO 5, characterized in that that a) the gas with a hydrogen sulfide to sulfur dioxide molar ratio of about equal to or greater than 2: 1 in a first absorption and reaction zone at temperatures from 10 to 10000 in finely divided form with an addition of base to a pH value between 2 and 6 buffered aqueous absorption solution one or more of the organic polyvalent acids brought into contact; b) from the solution in Sulfur accumulating in solid form is removed from at least a partial stream of the solution; c) the gas leaving stage a), optionally mixed with previously untreated Gases for converting the sulfur compounds contained in the gases to sulfur dioxide as well as for the oxidation of any organic compounds that may be present to form carbon dioxide and heating water to temperatures from about 70,000 to 100,000; d) the S02-containing Flue gas from stage c) optionally in such a partial flow amount as it is used for Adjustment of the hydrogen sulfide / sulfur dioxide molar ratio desired in stage a) is required, fed to stage a); e) the remaining flue gas partial flow of c) for the most quantitative possible SO2 absorption in a second absorption stage e) treated with the solution leaving step b); and f) the solution containing SO2 the step eß is returned to step a). 2) Method according to claim 1, characterized in that as organic polyvalent acids oxalic, malonic, glutaric> adipic, maleic> fumaric, citraconic and / or phthalic acid can be used. 3) Method according to claim 1 or 2, characterized in that as organic polybasic acids a mixture of 40 to 60 ffi glutaric acid, 15 to 30% adipic acid and 20 to 40% succinic acid can be used. 4) Method according to one of claims 1 to 3, characterized in that that the absorption solution is adjusted to a pH of 3.3 to 3.9. 5) Method according to one of claims 1 to 4, characterized in that that the absorption solution is kept at a temperature of 40 to 600c. 6) Method according to one of claims 1 to 5, characterized in that that the volume ratio of the amount of gas fed in to the amount of liquid pumped around in the absorber is between 8: 1 and 60: 1, preferably between 30: 1 and 40: 1. 7) Method according to one of claims 1 to 6, characterized in that that the residence time of the gases in the absorber is between 3 and 30 seconds, preferably between 4 and 6 seconds.