SE501782C2 - Method and apparatus for selectively removing hydrogen sulfide from a gas - Google Patents

Abstract

A method and an apparatus for selectively removing, by liquid absorption, hydrogen sulphide from a gas containing both hydrogen sulphide and carbon dioxide, are disclosed. In a first stage of the method, the gas (3) is in a container (1) contacted, in at least one step, with a carbonate-containing alkaline solution whose pH is adjusted during the absorption by the addition of a hydroxide, such as sodium hydroxide. In a second stage of the method, the gas is combusted in a combustion device (32) for converting the hydrogen sulphide remaining in the gas to sulphur dioxide, which then is absorbed in an alkali-containing solution, such as a sodium hydroxide solution, in a wet cleaner (36). By combining the absorption of hydrogen sulphide in a carbonate-containing solution in a first stage, and the combustion of the remaining hydrogen sulphide to sulphur dioxide and the absorption of the formed sulphur dioxide in an alkaline solution in a second stage, one may attain a total hydrogen-sulphide separation of high selectivity, high degree of separation and good process economy.

Description

The sulfur hydrogen content is only 10%, ie 90% of the gas absorbed consists of carbon dioxide, which means that as much as 90% of the sodium hydroxide is needed for the absorption of carbon dioxide.

In order to counteract the above problems in the absorption of hydrogen sulphide from gases, which contain both hydrogen sulphide and carbon dioxide, attempts have been made to develop methods for the selective absorption of hydrogen sulphide. Attempts have been made, among other things, to selectively absorb hydrogen sulphide by absorption in solutions of strong oxidizing agents, such as potassium permanganate, sodium dichromate or ferric salts. Other selective methods utilize alkaline solutions, such as sodium or potassium carbonate solutions, while carefully adjusting the operating conditions upon absorption. For further details on this prior art, see an article by C. Oloman, FE Murray, and JB Risk, "The Selective Absorption of Hydrogen Sulphide From Stack Gas", Pulp and 1969, page 69ff., "Selective Absorption- Paper Magazine of Canada, December 5, and E. Bendall, RC Aiken and F. Mandas, tion of H25 from Larger Quantities of C02 by Absorption and Reaction in Fine Sprays ", AICHE Journal (Vol. 29, No. 1), January 1983, p. 66ff.

By using a carbonate solution, such as a sodium carbonate solution, instead of a hydroxide solution, such as a sodium hydroxide solution, the selectivity for the absorption of hydrogen sulphide can be increased to about 30-50%. The reactions of such absorption can generally be written as follows: (5) (5) H s + 0032 '' r-: š us '+ 11003' 2 co + co 2 '+ H 2 O -ve 211003' 2 3 With a carbonate solution as absorption solution, the absorption of hydrogen sulphide takes place almost instantaneously, while carbon dioxide reacts only slowly with carbonate ions to hydrogen carbonate ions. The formation of a high content of bicarbonate using a carbonate solution as absorbent also gives the advantage of a "back pressure" (equilibrium pressure) against absorption of carbon dioxide, as shown in 3 of the equilibrium formula (6). above.

A problem with the use of a carbonate solution as an absorbent is that only a relatively low hydrogen sulphide content can be achieved in the solution due to the fact that the absorption capacity decreases as a result of the formation of bicarbonate ions. Achieving hydrogen sulfide levels above about 10 g / liter is very difficult. As a result, previously known methods for the selective absorption of hydrogen sulfide with carbonate solution as the absorption medium have not gained any general recognition, despite the fact that there is a great need in various fields in which hydrogen sulfide and carbon dioxide-containing gases are formed. Examples of areas in which this need exists are petroleum refining, light gas production and in particular in the sulphate pulp industry in the combustion of so-called black liquor.

When recycling chemicals according to the traditional so-called Tomlinson process in the sulphate industry, the black liquor is burned in a so-called recovery boiler during the generation of steam and the formation of a melt of mainly sodium carbonate and sodium sulphide. This melt is then dissolved in water and causticized, whereby the sodium carbonate is converted to sodium hydroxide and so-called white liquor is obtained, which can be used again for digestion of wood. For several reasons, including the risk of explosion in the event of a tube rupture in the boiler, attempts have recently been made to develop new processes for black liquor combustion, in which no melt is formed. These processes can be summarized under the name "black-end gasification" and an example is the so-called SCA-Billeruds process (E. Hornstedt and J. Gomy, Paper Trade Journal 158 (1974): 16, pages 32-34). In this process, the black liquor is pyrolyzed in a reactor under such temperature conditions that a dust is formed, which essentially consists of sodium carbonate and carbon, as well as a combustible gas, which contains sulfur compounds, among other things. Swedish Patent Application No. 8605116-0, which describes a method of thermally decomposing black liquor during the supply of oxygen in an amount below the stoichiometrically required, at a pressure above 10 bar, and at such a temperature that any melt does not form.

During the gasification, a solid phase is formed, which mainly consists of sodium carbonate, and a gas phase, which mainly consists of hydrogen sulphide, carbon monoxide, carbon dioxide, hydrogen, water vapor, methane, etc.

In order to be able to utilize the chemicals in a black liquor gasification according to the above and to produce white liquor from them for re-use in the mass production process, it is required that the hydrogen sulphide can be recovered from the gas formed. Since the gas also contains carbon dioxide, it will compete with the hydrogen sulphide in a liquid absorption of hydrogen sulphide, and since the gas has a low content of hydrogen sulphide (approx. 0.5-2%) while the carbon dioxide content is up to 20 times higher (approx. 10 -20%), the recovery of hydrogen sulphide by conventional liquid absorption becomes unsatisfactory.

There is thus a need to be able to separate the hydrogen sulphide with a high degree of separation and with a high selectivity in the case of gases which contain both hydrogen sulphide and carbon dioxide.

According to the unpublished Swedish patent application No. 9300533-8, filed on February 18, 1993, it has been found that if a carbonate-containing alkaline solution is used as the liquid absorption medium and during the absorption of hydrogen sulphide the pH of the solution is adjusted by adding a hydroxide, ie. not by adding new carbonate, an absorption can be achieved with high separation of hydrogen sulfide and high selectivity for hydrogen sulfide. Thus it is possible to obtain a selectivity for absorption of hydrogen sulphide of 60-70%, a sulphide content in the absorption solution of about 30 g / liter, and a degree of separation of hydrogen sulphide of about 90-99%.

More specifically, this is achieved by a method of the kind initially indicated, in which the gas is contacted in at least one step with a carbonate-containing alkaline solution, the pH of which during absorption is adjusted by adding a hydroxide. The method can be carried out with a device which is characterized in that it consists of a container with a gas inlet and a gas outlet, that the container contains filling which is arranged in several successively arranged steps, that the device means for supplying a carbonate-containing solution to the last stage, seen in the feed direction of the gas, that each stage includes means for feeding the carbonate-containing solution through the step in countercurrent to the gas and for recirculating the solution over the stage, that the device has conduit means between the stages for feeding a partial stream of the solution from one stage to a previous stage, seen in the direction of gas supply, and that the device comprises means for supplying a hydroxide to the carbonate-containing solution in at least one of the stages, and an outlet line from the first step, seen in the feed direction of the gas, for discharging hydrogen sulfide-containing liquid.

By "carbonate-containing alkaline solution" is meant an aqueous solution containing carbonate ions (CO32 '). Preferably, this solution is an alkali metal carbonate solution, such as a solution of sodium, potassium or lithium carbonate. Sodium carbonate is particularly preferred for reasons of readiness and economic reasons. The carbonate concentration of the solution is not critical, but suitably it is about 0.1-3 M with respect to carbonate, preferably about 1-2.5 M, and most preferably about 1.7 M with respect to carbonate.

It is important that the carbonate-containing alkaline about 9. At pH values below about hydrogen sulfide unsatisfactory- that already absorbed sulfur- The pH of the solution should not the whole solution has a pH of at least 9 is namely the absorption of land, and instead hydrogen is released from solution. may be too high, as this disadvantages the absorption of hydrogen sulphide in relation to the absorption of carbon dioxide. Thus, it is preferred that the pH of the solution is not higher than about 12 so that the absorption of carbon dioxide does not become too great. Preferably the pH of the solution is in the range of about 10.0-11.0, most preferably in the range of about 10.2-10.8. If the pH of the solution is regulated in the latter, narrow range, an optimal separation of hydrogen sulphide is achieved.

As can be seen from the equilibrium reactions (5) and (6), bicarbonate ions (HCO3_) are formed during the absorption of hydrogen sulphide and carbon dioxide, respectively. This means that the pH of the absorption solution decreases as the absorption of hydrogen sulphide and carbon dioxide progresses. When the pH of the solution drops below about 9, as pointed out above, the absorption of hydrogen sulphide becomes unsatisfactory and instead there is a risk that already absorbed hydrogen sulphide is released from the solution.

To avoid this, the solution must be regenerated, ie its pH raised above the permissible lower limit for equilibrium between H28 in gaseous form and sulphide content in liquid at current temperature and pH. At the same time, however, the pH value should not be raised above about 12 to avoid the competing absorption of carbon dioxide taking over. By raising the pH of the solution by adding a hydroxide, for example an alkali metal hydroxide such as NaOH, it is achieved that the hydrogen carbonate ions formed are recycled to carbonate ions according to the equilibrium reaction 2- 3 on '+ Hco': ri co 3 + H 2 O (7). After regeneration or regeneration, the solution can absorb additional hydrogen sulfide according to the reaction (5) above. By adjusting the pH of the solution by adding a hydroxide and keeping it within the stated range of about 9-12, preferably about 10.0-11.0 and most preferably about 10.2-10.8, the sorption of carbon dioxide at a relatively low level.

As stated above, the carbonate-containing alkaline solution is regenerated by adding a hydroxide. This can in principle consist of any / hydroxide which does not adversely affect the absorption of hydrogen sulphide and which is capable of raising the pH of the solution from the stated lower limit of about 9 and to the desired value, such as 10 15 20 25 30 35 501 782 '7 at most about 12.0, preferably at most about 11.0, and most preferably at most about 10.8. It is preferred in the invention to use hydroxides of alkali metals or alkaline earth metals. Examples which may be mentioned are sodium hydroxide (NaOH), potassium hydroxide (KOH), lithium hydroxide (LiOH), calcium hydroxide (Ca (OH) 2) and magnesium hydroxide (Mg (OH) 2).

Sodium hydroxide is most preferred in terms of availability and price. The liquid temperature during absorption is not particularly critical and may be within a wide range, but should preferably be below about 80 ° C, as at a temperature of about 80 ° C or higher there is a risk that hydrogen sulphide absorption is reduced. It is preferred that the temperature be in the range from room temperature, i.e. about 20 ° C, and up to about 80 ° C, more preferably about 40-70 ° C, and most preferably about 60-70 ° C.

In order to achieve as high a selectivity for hydrogen sulphide in the liquid absorption of hydrogen sulphide, the absorption should take place in a countercurrent ratio between the gas flow and the flow of the absorbent liquid, and furthermore the gas flow should be turbulent, while the liquid flow should be laminar. The separation of hydrogen sulphide is further favored if the volume of absorbent liquid is large in relation to the volume of gas from which hydrogen sulphide is absorbed. Such a desired high liquid-gas ratio is achieved by recirculating the absorbent liquid which is brought into contact with the hydrogen sulfide-containing gas.

The contact between the sulfur-containing gas and the absorbent liquid (the carbonate-containing solution) can furthermore take place in one or more steps, and preferably takes place in two or three steps, most preferably in three steps.

By dividing the contact into several steps, the advantage is achieved that the length of each individual step becomes shorter, whereby the pH of the carbonate-containing solution does not have time to fall below about 9 in the individual step, and, at the same time, the sulphide content can be kept low in the top step . Preferably, each step has such a range or length that the pH of the solution at the end of the step has dropped to about 10.0-10.2, after which the liquid is withdrawn from the step and regenerated with a hydroxide and then recycled to the stage.

According to the present invention, it has been found that the removal of hydrogen sulfide in a process of the above kind can be made more efficient and the chemical consumption is drastically reduced if the process is divided into two stages: a first stage, in which the main part of the hydrogen sulphide is removed according to the above described process, and a second stage, in which residual hydrogen sulphide is substantially removed by combustion to sulfur dioxide and its absorption in an alkaline solution in a scrubber. The reason for this is as follows.

As hydrogen sulfide is absorbed in the various steps of the first stage, the hydrogen sulfide content of the gas decreases, causing the previously defined selectivity for hydrogen sulfide to decrease. As the selectivity decreases, the consumption of alkali increases dramatically in accordance with the reactions (5), it consumes 1 mole of OH- per mole of absorbed hydrogen sulphide and 2 moles of OH- per mole of absorbed carbon dioxide. As the selectivity of hydrogen sulfide decreases and the absorption of carbon dioxide increases, (6) and (7) above. As can be seen from these reactions, as is the case in the last absorption step or steps in the first stage, the consumption of alkali (OH ') consequently increases sharply. This means that relatively much more alkali is required for the absorption of the "last" hydrogen sulfide of the gas than for the absorption of the "first" hydrogen sulfide of the gas. By not driving the absorption of hydrogen sulphide to the end in the first stage, but intentionally leaving a certain proportion of hydrogen sulphide in the gas leaving the first hydrogen sulphide stage, it is avoided that the selectivity for drops dramatically and consequently that the consumption of Alkali increases dramatically in the first stage.

The hydrogen sulphide absorption in the first stage is driven so that the majority of the hydrogen sulphide, i.e. at least 50%, is removed in this stage, and suitably about 60-97% is removed, preferably about 80-95%, preferably about 80-95%, and most preferably about 90% of the hydrogen sulfide in the first stage. This results in a low consumption of alkali in the first stage, which provides good process economy.

However, before the gas from the first stage can be released into the atmosphere, residual hydrogen sulfide must also be removed. According to the present invention, this is done by burning the hydrogen sulfide-containing gas from the first stage so that the hydrogen sulfide is oxidized to sulfur dioxide, after which the sulfur dioxide formed is absorbed with an aqueous solution, the pH of which is adjusted by adding an alkaline solution or alkaline substance. wet training device (scrubber).

According to the present invention there is thus provided a method of selectively removing hydrogen sulphide from a gas containing both hydrogen sulphide by liquid absorption, in which manner the gas, in a first stage for removing the major part of the hydrogen sulphide content of the gas, is contacted in at least one step with a carbonate-containing alkaline solution, whose pH during absorption is adjusted by the addition of hydroxide, and that the gas is then burned in a second stage to convert the remaining hydrogen sulphide to sulfur dioxide, which is absorbed in an alkali-containing solution.

According to the invention there is also provided a device for selectively removing hydrogen sulphide from a gas containing both hydrogen sulphide and carbon dioxide by liquid absorption, and the device is characterized in that it consists of a) a container with a gas inlet and a gas outlet, the container contains filling which is arranged in several successive steps; means for supplying a carbonate-containing solution to the last stage, seen in the direction of supply of the gas, each stage comprising means for feeding the carbonate-containing solution through the step in countercurrent to the gas and for recirculating the solution over the stage; conduit means between the steps for feeding a partial stream of the solution from one step to a previous step, seen in the direction of supply of the gas; and means for supplying a hydroxide to the carbonate-containing solution in at least one of the steps; and an outlet line from the first stage, seen in the direction of supply of the gas, for discharging liquid sulphide-containing liquid, b) a combustion device with an inlet for hydrogen sulphide-containing gas from the gas outlet on the container; combustion means for the combustion of feed gas and the conversion of its hydrogen sulphide into sulfur dioxide; and an outlet for discharging sulfur dioxide-containing gas from the combustion device, and c) a wet cleaning device for absorbing the sulfur dioxide formed during the combustion, said wet cleaning device comprising a container with an inlet for sulfur dioxide-containing gas from the combustion device; a purified gas outlet; means for supplying, circulating and atomizing an aqueous solution, the pH of which is adjusted by adding an alkaline solution or alkaline subs- to contact with feed, gas; and means for discharging solution, which has absorbed tans, sulfur dioxide-containing sulfur dioxide.

Further features of the invention appear from the following description and from the appended claims.

As stated previously, the carbonate-containing alkaline solution used in the first stage of hydrogen sulfide absorption is an aqueous solution containing carbonate ions, such as, for example, an alkali metal carbonate solution, preferably a sodium carbonate solution. When the method is used for removing hydrogen sulphide in the sulphate pulp industry, such as in connection with the combustion of black liquor, it has been discovered in the present invention that so-called green liquor can be used as the carbonate-containing alkaline solution, and this constitutes a special aspect of the invention.

Green liquor consists of an aqueous solution, which usually contains about 1.3-1.4 moles of Na2CO3 per liter, about 0.4-0.6 moles of Na2S per liter and about 0.3 -0.5 mol NaOH per liter. Furthermore, it has been discovered in the present invention that in the first stage, as hydroxide for regenerating the carbonate-containing alkaline solution, so-called white liquor can be used in a corresponding manner when the invention is used in the sulphate pulp industry to remove hydrogen sulphide. White liquor is an aqueous solution containing NaOH, Na2S and Na2CO3, usually in proportions of about 2.3-2.6 moles of NaOH per liter, about 0.4-0.6 moles of Na2S per liter and about 0.25-0.4 mol Na2CO3 per liter. The use of white liquor as a hydroxide for regenerating the carbonate-containing alkaline solution also constitutes a special aspect of the present invention.

Instead of white liquor, weak liquor or oxidized white liquor can also be used for regeneration of the carbonate-containing alkaline solution. By weak liquor is meant an aqueous solution containing NaOH, Na2S and Na2CO3, usually in the proportions of about 0.3-0.4 moles of NaOH per liter, about 0.05-0.08 moles of Na2S per liter and about 0.05 0.07 mol Na2CO3 per liter.

As both green liquor, white liquor and weak liquor contain both carbonate and hydroxide, they can be used both individually and together both as a carbonate-containing solution or for regeneration of the carbonate-containing alkaline solution.

The combustion in the second stage of the hydrogen sulphide-containing gas takes place in a manner known per se in a combustion device of known type, such as a lime kiln or bark boiler when the invention is used in the sulphate industry. If the heat content of the hydrogen sulfide-containing gas is too low, an auxiliary fuel, such as oil or natural gas, can be used in the combustion to achieve a satisfactory oxidation of the hydrogen sulfide to sulfur dioxide. During the combustion of the hydrogen sulphide-containing gas, the hydrogen sulphide is completely oxidized to sulfur dioxide at normal combustion temperatures, which is why the gas after combustion contains negligible amounts of hydrogen sulphide. After the combustion in the second stage of the hydrogen sulphide-containing gas, the sulfur dioxide formed must be removed, which in the invention takes place by absorption in an alkali-containing solution in a wet cleaning device (scrubber). Selectively absorbing sulfur dioxide from a gas, which also contains carbon dioxide, by absorption in an alkali-containing solution in a scrubber is a well-known technique, which need no more detailed or detailed description here. A scrubber with a high degree of recirculation of the absorbent, alkali-containing solution is suitably used. The pH of the alkali-containing solution in the SO 2 scrubber is preferably in the range of about 6-8, most preferably about 7-7.5. At these relatively low pH values, the absorption of carbon dioxide is negligible and consequently the selectivity for the sulfur dioxide separation becomes 100%. Upon absorption, the sulfur dioxide undergoes one of the following reactions: soz + on - Hsos (s) so + so2 '+ Ho-2nso' (9) 2 3 2 3 nso '+ on'-no + so2 "(10) 3 2 3 As As shown in reaction formulas (8) - (10), 2 moles of OH 'per mole of absorbed SO2 are required. economy is thus considerably better than if all the sulfur is absorbed as hydrogen sulphide in the first stage with poor selectivity as a result.

As the alkali in the alkali-containing solution, a hydroxide or a carbonate of an alkali metal or alkaline earth metal, such as sodium hydroxide, potassium hydroxide, calcium carbonate, calcium hydroxide or magnesium hydroxide, is suitably used. Sodium hydroxide is preferred, especially if it is desired to recycle the sulfur directly in the process, for example when using the method in the sulphate industry. A sodium hydroxide-based spray scrubber is suitably used, for example of the type used by ABB Fläkt Industri AB under the name MoDo scrubber. The scrubber may in this case also include one or more heat recovery steps, in which cold water is sprayed against the hot gas, so that hot water with a temperature of about 45-65 ° C is obtained. After absorption of sulfur dioxide in the scrubber, the leaving solution contains sodium sulphite (Na2SO3), sodium hydrogen sulphite (NaHSO3) and sodium sulphate (Na2SO4). In the sulphate industry, this leaving solution can be mixed with other combustible process liquids, for example mixed into the black liquor in a pulp mill. After combustion, the sulfur is obtained in the form of hydrogen sulphide if the combustion takes place under reducing conditions or in the form of sulfur dioxide if the combustion takes place under oxidizing conditions.

In the pulp industry, the process according to the present invention is suitably designed so that the various stages of the process are integrated into the existing equipment. This means that the combustion of the hydrogen sulphide-containing gas is carried out in an existing bark boiler or lime kiln. In this way, the need for other fuels in these process steps is reduced. The sulfur dioxide-containing flue gases from the bark boiler and the lime kiln can then be led to an existing SO2 scrubber. In this case, the need for new equipment is limited to a scrubber for hydrogen sulphide separation.

On pulp mills, there may be separate SO2 scrubbers for flue gases from the bark boiler and the lime kiln and from the recovery boiler, respectively. In cases where the hydrogen sulphide-containing gas is combusted in another place, for example in a separate boiler, the sulfur dioxide-containing flue gases can be led to the existing SO2 scrubber for the flue gases of the recovery boiler.

From the above description it appears that the present invention enables good process economy, a closed chemical system with recycling of all sulfur, and very low emissions to the environment. 501 782 10 15 20 25 30 35 14 To facilitate the understanding of the invention, it will now be described with reference to the accompanying drawings. On these, Fig. 1 shows a preferred device according to the invention for carrying out the first stage of the method according to the invention. Fig. 2 schematically shows a flow chart of the method according to the invention.

The device in Fig. 1 comprises a tower or a container 1 with an inlet 2 for a gas 3, which contains both hydrogen sulphide and carbon dioxide. At the other end of the device there is an outlet 4 for gas 5 from which hydrogen sulphide has been removed by liquid absorption. The contact between the hydrogen sulfide-containing gas and the carbonate-containing solution takes place in three steps 6, 7 and 8. Each step contains filling 9, as partially indicated in step 6 of the drawing. In order to achieve the best selectivity for hydrogen sulfide absorption, the filling 9 has such a shape that a laminar liquid flow through steps 6, 7 and 8 is achieved. It has been found that fillings in the form of corrugated sheets are particularly suitable for this purpose. The filling can be made of, for example, plastic or metal.

The contact between the hydrogen sulfide-absorbing, carbonate-containing solution and the hydrogen sulfide-containing gas takes place in countercurrent, and for this purpose each step includes means for feeding the carbonate-containing solution through the step in countercurrent to the gas and also for recirculating the solution across the stage. As shown in Fig. 1, these means are constituted by pumps 10, 11 and 12, which via lines 13, 14 and 15 feed carbonate-containing, alkaline solution to the respective stages 6, 7 and 8, 17 and 18, which divert the solution from the 20 and 21. From the collecting vessels, the solution is recirculated over the stages via lines 22, 23 and 24, 11 and 12, respectively. Fresh carbonate solution, as well as lines 16 and the stages respectively to collecting vessels 19, which are connected to the pumps 10, which preferably consist of sodium carbonate solution, are fed to the last step 6, seen in the direction of supply of the gas, via a line 25 from a store (not shown) for sodium carbonate solution. 10 15 20 25 30 35 501 782 - 15 As an alternative to feeding fresh carbonate solution to the last step, it is possible to generate the carbonate solution in the last step by instead feeding sodium hydroxide solution thereto and allowing this hydroxide solution to absorb carbon dioxide from the gas. so that a carbonate-containing solution is formed according to reactions (3) - (4) above.

To adjust (raise) the pH of the absorption solution, a hydroxide, preferably sodium hydroxide solution, can be added to the collecting vessels 19, 20 and 21 via lines 26, 27 and 28, respectively, which supply alkali from an undisclosed, preferably common storage of alkali. The supply of sodium hydroxide solution for adjusting the pH of the absorption solution is regulated on the basis of measurement of the pH of the solutions in the collection vessels 19, 20, 21 (not shown).

As also shown in Fig. 1, the various steps are further connected by lines 29 and 30 for feeding a partial stream of the absorption solution from one step to a previous step, ie between steps 6 and 7 and between steps 7 and 8, respectively.

Finally, an outlet conduit 31 is provided from the collecting vessel 21 for discharging hydrogen sulfide-containing liquid from the collecting vessel 21 and the step 8.

Fig. 2 shows a schematic flow diagram of the method according to the invention. As indicated by the flow chart, a hydrogen sulphide and carbon dioxide-containing gas 3 is treated in a first stage in a device with a container 1 for absorbing hydrogen sulphide, as described above in connection with Fig. 1. In accordance with the present invention, the absorption of hydrogen sulphide in the first stage not to 100%, ie so far that all the hydrogen sulphide is absorbed in the first stage, but still so far that at least the main part, ie at least 50%, of the hydrogen sulphide is absorbed in the first stage. Preferably about 60-97%, more preferably about 80-95%, and most preferably about 90% of the hydrogen sulfide in the first stage. In total, at least about 98% of the hydrogen sulfide content of the gas is preferably absorbed through the first and second stages. 501 782 10 15 20 25 30 35 16 The purified, still hydrogen sulfide-containing and carbon dioxide-containing gas 5 is then fed from the first stage to the second stage, in which the gas is first combusted to convert hydrogen sulphide to sulfur dioxide and then wet-purified in a scrubber for absorption. of sulfur dioxide formed.

As indicated in Fig. 2, the combustion takes place in a combustion device 32, which has combustion means 33, for example a burner means, for combustion of the gas 5 fed in via an inlet 34 and oxidation of the hydrogen sulphide to sulfur dioxide. As mentioned earlier, the combustion device 32 may be an existing device, such as a bark boiler or lime kiln. The sulfur dioxide-containing flue gas is discharged through an outlet 35 and led to a wet cleaning device 36, which includes a container 37 with an inlet 38 for the sulfur dioxide-containing gas, an outlet 39 for purified gas 40 and means 41 for supplying an alkali-containing gas. solution. As mentioned earlier, the alkali-containing solution contains a hydroxide of an alkali metal or alkaline earth metal, hydroxide. The solution is circulated with a pump 42 to nozzles 43, where the solution is atomized and brought into countercurrent contact with the sulfur dioxide-containing gas, preferably sodium, so that the sulfur dioxide is absorbed by the solution. In order for the absorption of sulfur dioxide to be optimal, without competitive absorption of carbon dioxide, the pH of the solution is adjusted to about 6-8, preferably 7-7.5. Solution, which has absorbed sulfur dioxide, departs from the second stage at 44.

To further illustrate the invention, a non-limiting exemplary embodiment is given below.

Example Attempts were made to selectively remove hydrogen sulphide from a gas, using an apparatus, at black liquor gasification of the kind shown in the accompanying drawing figures 1 and 2 and described previously. The absorption of hydrogen sulphide in the first stage of the process took place at atmospheric pressure and the gas contained had a temperature of about 60 ° C and contained 1.13 mol% of hydrogen sulphide and 16.9 mol% of carbon dioxide. The gas was saturated with water vapor at the current temperature, which corresponded to about 18.7 mol% water. The incoming gas flow was 38280 Nm3 / h, which gave a gas velocity in the absorption tower of about 3.1 m / s. The absorption tower had a height of 6.25 m and the first two steps each had a height of 1.5 m, while the last step, seen in the gas supply direction, had a height of 1 m. Each step was provided with filling of the type Mellapack 500 from Sulzer. The diameter of the tower was 2.3 m.

Fresh absorption solution, consisting of 8.8 m3 / h 2 M sodium carbonate solution with a temperature of about 60 ° C, was fed to the last stage of the tower together with recycled absorption solution, so that the total flow of absorption solution to the last stage in the tower was about 50 m3 / h. The pH of the added absorbent solution was about 11.0 and this value dropped to about 10.2 as the solution passed through the step due to the absorption of hydrogen sulfide. After passing through the step, the solution was fed to a collection vessel with a volume of 1.5 m 3, in which the solution was regenerated by adding a 2.5 M sodium hydroxide solution with a temperature of about 60 ° C so that the pH of the solution was again raised to approx. 11.0. The regenerated solution was then recirculated using a pump to the final stage of the absorption tower for re-absorption of hydrogen sulfide.

From the collecting vessel about 11 m3 / h of the absorption solution was diverted to the collecting vessel for the intermediate step, and from this collecting vessel, in the same way as in the previous step, about 50 m3 of absorption solution per hour with a pH value of about 11.0 to the intermediate step, from which the absorbent solution was derived at a pH of about 10.2 and returned to the collection vessel. In the collection vessel, the solution was regenerated by adding 2.5 M sodium hydroxide solution at a temperature of about 60 ° C in a manner similar to the previous step.

From the collecting vessel of the intermediate stage, about 13.5 m3 / h of absorbent solution was diverted to the collecting vessel of the first (bottom) stage, from which 50 m3 / h of absorbent solution with a pH of about 11.0 was pumped to the first stage, seen in the gas supply direction. After passing the step and absorbing hydrogen sulfide, the solution was drained at a pH of about 10.2 and returned to the collection vessel. In the collection vessel, the solution was regenerated in the same manner as in the previous steps by adding a 2.5 M sodium hydroxide solution having a temperature of about 60 ° C so that the pH of the regenerated solution became about 11.0. In total, about 8.6 m 3 / h of 2.5 M sodium hydroxide solution was added to the collecting vessels of the three stages.

From the first (bottom) stage collection vessel, about 17.4 m3 / h solution was drained, which had a concentration of HS- of 1 mol / liter. The gas leaving the absorption tower contained 0.113 mol% hydrogen sulfide and 16.4 mol% carbon dioxide. In the first stage of the process, the salt for hydrogen sulfide was obtained at the separation of about 0.67%.

After the first stage, the second stage of the process took place, whereby the gas leaving the absorption tower in the first stage was burned in a furnace with the addition of 31700 Nm3 / n air, whereby esooo mms / n flue gas containing 650 ppm SO 2 (1930 mol / hrs). This flue gas was passed on to an SO2 scrubber of the conventional and previously described type. The scrubber consumed 1.54 m3 / h 2.5 M sodium hydroxide solution, and in addition the scrubber was supplied with 2.0 m3 / h fresh water to cool the incoming flue gas to saturation temperature. The outgoing gas from the scrubber had an SO 2 concentration of 12 ppm, which corresponds to 35 mol / h (which converted to sulfur becomes 1.12 kg / h). The constituent absorbent solution was maintained at a constant pH of 7.2. The absorption solution was recycled in the scrubber in a flow of 170 m3 / h in total. From the scrubber 1.8 m3 / h was extracted from the solution and its concentration of total sulfur (SO32_ + HSO3_ + SO42-) was 1.05 mol / liter. The total sulfur precipitation, ie the precipitation of hydrogen sulphide in the first stage plus the precipitation of sulfur dioxide in the second stage, was therefore 99.8%. The consumption of sodium hydroxide in the first stage was 860 kg / h and in the second stage 152 kg / h, ie a total of 1012 kg / h of NaOH was consumed to achieve a sulfur separation of 99.8%.

For comparison, the process was repeated, but only using the first stage, ie some combustion of hydrogen sulphide to sulfur dioxide and absorption of sulfur dioxide formed in a wet scrubber was not carried out. Instead, the hydrogen sulfide separation in the first stage was pushed so far that 98% of the hydrogen sulfide was separated. The selectivity for hydrogen sulfide in the hydrogen sulfide separation between 90 and 98% is 0.15. The total selectivity for the whole stage will then be about 0.55. This resulted in a NaOH consumption of 1751 kg / h, ie a NaOH consumption which was more than 70% higher than in the process according to the invention, despite the fact that the total degree of precipitation of sulfur was lower.

Claims (10)

501 782 10 15 20 25 30 35 20 PATENT REQUIREMENTS A method of selectively removing hydrogen sulphide from a gas (3) containing both hydrogen sulphide and carbon dioxide, (3), in a first stage by liquid absorption, in order to remove the main part of the gas characterized in that the hydrogen sulphide content of the gas is brought into contact in at least one step (6, 7, 8) with a carbonate-containing alkaline solution (25) whose pH during absorption is adjusted by adding a hydroxide (26, 27, 28), and that the gas then in a the second stage is burned to convert the remaining hydrogen sulphide to sulfur dioxide, which is absorbed in an aqueous solution, the pH of which is adjusted by adding an alkaline solution or alkaline substance. A method according to claim 1, wherein in the first stage the gas (3) is brought into contact with a sodium carbonate-containing solution (25). 3. A method according to claim 2, wherein the gas (3) is brought into contact with green liquor, white liquor known or weak liquor, especially green liquor. 4. A method according to claim 1, in that in the first stage the pH of the carbonate-containing, characterized by alkaline solution is adjusted with an alkali metal hydroxide-containing liquid (26, 27, 28). 5. A method according to claim 4, characterized in that the pH is adjusted with white liquor, green liquor or weak liquor, preferably white liquor or weak liquor, especially white liquor. 6. A method according to any one of the preceding claims, characterized in that in the first stage the gas is brought into contact with the carbonate-containing solution (25) at a pH of about 9-12, preferably about 10-11. 7. A method according to any one of the preceding claims, characterized in that the gas in the second stage after combustion is absorbed in the aqueous solution at a pH of about 6-8, preferably about 7.0-7.5. 10 15 20 25 30 35 501 782 21 8. A method according to any one of the preceding claims, characterized in that about 60-97%, preferably about 80-95%, of the hydrogen sulphide content of the gas is removed in at least the first stage. 9. A method according to any one of the preceding claims, which comprises removing a total of at least about 98% of the hydrogen sulfide content of the gas. Device for selectively removing hydrogen sulphide from a gas (3) containing both hydrogen sulphide and carbon dioxide by liquid absorption, characterized in that it comprises a) a container (1) with a gas inlet (2) and a container (1) contains filling several successively arranged steps for supplying a carbonate inlet outlet (4), wherein (9) which is arranged in (6, 7, 8); means (25) holding solution to the last step, direction of direction, each step comprising means for feeding the carbonate-containing solution through the step in countercurrent to the gas and for recirculating the solution over the step; conduit means (29, 30) between the steps for feeding a partial stream of the solution from one step to a previous step, seen in the direction of supply of the gas; and means (26, 27, 28) for supplying a hydroxide to the carbonate-containing solution in at least one of the steps (6, 7, 8); and an outlet line (31) from the first stage, seen in the direction of supply of the gas, for discharging hydrogen sulphide-containing liquid, b) a combustion device (32) with an inlet (34) for hydrogen sulfide-containing gas (5) from the gas outlet (4) on the container (1), combustion means (33) for combustion of feed gas and conversion of its hydrogen sulphide to sulfur dioxide, and an outlet (35) for discharging sulfur dioxide-containing gas from the combustion device (32), and c) a wet cleaning device (36) for absorbing the sulfur dioxide formed during combustion, which wet cleaning device (36) comprises a container (37) with inlet (38) for sulfur dioxide-containing gas from combusted in the gas feeding device 501 782 10 15 20 25 30 35 22 (32): an outlet (39) for purified gas (40): 42, 43) for supplying, dividing an aqueous solution, the pH of which is adjusted by adding an alkaline solution or alkaline substance, to contact with feed, sulfur dioxide-containing g as, means (41, circulation and fines and means (44) for discharging solution, which has absorbed sulfur dioxide.