CN1066634C - Method and apparatus for absorbing hydrogen sulphide - 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

Method and apparatus for absorbing hydrogen sulfide

The present invention relates to a method and apparatus for absorbing hydrogen sulfide, more particularly, to a method and apparatus for selectively removing hydrogen sulfide from gases containing hydrogen sulfide and carbon dioxide by liquid absorption.

It is well known that hydrogen sulfide can be removed from gases containing hydrogen sulfide by absorption in an alkaline aqueous solution (ie, an aqueous solution with a pH > 7), such as sodium hydroxide, or by the use of ethanolamines, such as monoethanolamine and diethanolamine. For example, this absorption can be used to produce hydrogen sulfide in pure form, which optionally can be further processed into sulfur using the Clans process. If the gas contains carbon dioxide in addition to hydrogen sulfide, the carbon dioxide is also absorbed in the alkaline solution. Carbon dioxide has about the same solubility in water as hydrogen sulfide, so carbon dioxide will compete with hydrogen sulfide for absorption in solution. Hydrogen sulfide and carbon dioxide are absorbed in an alkaline aqueous solution, such as sodium hydroxide solution, according to the following formula:

(1)

(2)

(3)

(4) The selectivity to hydrogen sulfide, that is, the ratio of the moles of hydrogen sulfide absorbed to the moles of absorbed (hydrogen sulfide + carbon dioxide), is proportional to the content of hydrogen sulfide and carbon dioxide in the gas. Thus, the competition for the CO2 portion occurs mainly when the gas contains more CO2 than H2S, which is the case in most cases. If a gas contains, say, 1% by volume of hydrogen sulfide and 10% by volume of carbon dioxide, and the test absorbs hydrogen sulfide in sodium hydroxide solution, the selectivity to hydrogen sulfide is only 10%, i.e. 90% of the absorbed gas is made up of Composition of carbon dioxide, which means that about 90% of the sodium hydroxide is used to absorb carbon dioxide.

In an effort to remedy the above-mentioned inconvenience of absorbing hydrogen sulfide from gases containing hydrogen sulfide and carbon dioxide, methods for selectively absorbing hydrogen sulfide have been developed. For example, efforts have been made to selectively absorb hydrogen sulfide in strong oxidizing agents, such as potassium permanganate, sodium dichromate, or iron salt solutions. In other selective absorption methods, overbasic solutions such as sodium or potassium carbonate solutions are used, and the operating conditions are carefully adjusted during the absorption process. For a more detailed introduction to this prior art see C. Oloman, FEMurray and JB Risk, entitled "The Selective Absorption of Hydrogen Sulphide from stack Gas", Pulp and Paper Magazine of Canada, P.69ff, December 5, 1969 and E.Bendall, RC Aiken and K.Mandas entitled "Selective Absorption of H ₂ S from Larger Quantities of CO ₂ by Absorption and Reaction in Fine Sprays” article.

By using a carbonate solution, such as sodium carbonate solution, instead of a hydroxide solution, such as sodium hydroxide solution, the selectivity for absorbing hydrogen sulfide can be increased to about 30-50%. The reactions that occur during such absorption are usually expressed as follows:

When the absorbing solution is a carbonate solution, hydrogen sulfide is absorbed almost instantaneously, however, carbon dioxide reacts only slowly with carbonate ions to form bicarbonate ions. When using a carbonate solution as the absorption medium, due to the high content of bicarbonate produced, the absorption of carbon dioxide has an additional benefit due to the existence of "counterpressure" (equilibrium pressure), just as the above equilibrium formula ( b) What you see.

A problem that arises when using carbonate solutions as absorption medium is that only relatively low hydrogen sulphide contents can achieve absorption in such solutions due to the reduction of the absorption capacity caused by the formation of bicarbonate ions. Therefore, it is extremely difficult to achieve a hydrogen sulfide content of more than about 10 g/l. As a result, existing technical methods for the selective absorption of hydrogen sulfide by absorption media in the form of carbonate solutions have not met with great success, although such a method has been highly regarded in various fields of production of hydrogen sulfide-containing and carbon dioxide-containing gases There is a great need. Examples of such areas of application are petroleum refining, gas production and, in particular, the combustion of black liquor in the kraft pulp industry.

When recovering chemicals in the kraft pulp industry according to the usual Tomlinson process, black liquor is burned in an alkali recovery unit, resulting in the generation of steam and the formation of a melt consisting mainly of sodium carbonate and sodium sulphide. This melt is then dissolved in water and causticized so that the sodium carbonate is converted to sodium hydroxide, resulting in a white liquid which can then be used again to dissolve wood. For a number of reasons, including the risk of explosion when pipes burst in caustic recovery plants, efforts have been made in recent years to develop new methods of black liquor combustion in which no melt is formed. These methods can collectively be referred to as "black liquor evaporation", an example of which is the so-called SCA-Billerud method (E. Hornstedt and J. Gomy, Paper Trade Journal 158 (1974): 16, pp32-34). In this method, black liquor is pyrolyzed in a reactor under temperature conditions that form dust mainly composed of sodium carbonate and carbon and a combustible gas containing especially sulfur compounds. Another example of black liquor evaporation is found in Swedish patent application 8605116-0, which covers a method of thermal decomposition of black liquor while supplying less than stoichiometrically required oxygen, at pressures in excess of 10 Pa and at temperatures where no melt is formed . Evaporation results in a solid phase consisting mainly of sodium carbonate and a gas phase consisting mainly of hydrogen sulfide, carbon monoxide, carbon dioxide, hydrogen water vapor and methane.

In order to be able to recover the chemicals used in the above-mentioned black liquor evaporation and to produce white liquor from these chemicals for pulp production, hydrogen sulfide must be removed from the gas produced. Since the gas also contains carbon dioxide, which competes with hydrogen sulfide for liquid absorption, and since the gas has a low content of hydrogen sulfide (approx. The liquid absorption method will lead to unsatisfactory recovery of hydrogen sulfide.

There is therefore a need for a highly separable and highly selective separation of hydrogen sulfide from gases containing hydrogen sulfide and carbon dioxide by liquid absorption.

According to the unpublished Swedish patent application 9300533-8 filed on February 18, 1993, it was found that a high degree of separation of hydrogen sulfide and a highly selective absorption of hydrogen sulfide can be obtained by using an alkaline solution containing carbonate as The liquid absorbs the medium, and the pH of the solution is adjusted during hydrogen sulfide absorption by the addition of a hydroxide, ie not freshly prepared carbonate. In this way, it is possible to achieve a selectivity of 60-70% for the absorption of hydrogen sulfide, a sulfide content of about 30 g/l in the absorption solution and a degree of separation of hydrogen sulfide of about 90-99%.

More precisely, this is achieved by a method of the type described in the introductory form, in which the gas is contacted in at least one stage with an alkaline solution containing carbonates, the pH of which is determined by the A hydroxide was added to adjust.

The method of the present invention can be carried out using a device characterized in that it comprises: a container with a gas inlet and an outlet containing packing arranged in successive stages; supplying a carbonate-containing solution to the last stage According to the feeding direction of the gas, each stage has a device for supplying the carbonate-containing solution to the gas in reverse through this stage and recirculating the solution through this stage; it can be seen according to the feeding direction of the gas, in Conduits arranged between stages for providing a partial flow of solution from one stage to a preceding stage, means for supplying a hydroxide to carbonate-containing solution in at least one stage; and by gas The feeding direction can be seen, and a first-stage outlet conduit is used to discharge the liquid containing hydrogen sulfide.

"Alkaline solution containing carbonate" refers to an aqueous solution containing carbonate ions (CO ₃ ²⁻ ). Most preferably, the solution is an alkali metal carbonate solution, such as sodium carbonate, potassium carbonate or lithium carbonate. Sodium carbonate is especially good, easy to use and fairly cheap. The carbonate concentration of the solution is not critical, but is conveniently about 0.1-3M, preferably about 1-2.5M, most preferably about 1.7M for carbonate. It is essential that the carbonate-containing alkaline solution has a pH of at least about 9. A pH value below about 9 leads to unsatisfactory absorption of hydrogen sulphide, and there is even a risk that already absorbed hydrogen sulphide will be released from solution. However, the pH of the solution should not be too high, as this would have a side effect on the uptake of hydrogen sulfide, compared to that of carbon dioxide. Thus, the pH of the solution should preferably not exceed about 12 so that the uptake of carbon dioxide is not 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. Optimal separation of hydrogen sulfide is achieved if the pH of the solution is adjusted within this last narrow range.

From the equilibrium reactions (5) and (6), it can be seen that bicarbonate ions (HCO ₃ ^- ) are formed in the absorption of hydrogen sulfide and carbon dioxide. This means that the pH of the absorption solution drops as the absorption of hydrogen sulfide and carbon dioxide continues. When the pH of the solution drops to about 9, the absorption of hydrogen sulphide becomes unsatisfactory and, as indicated above, there is a danger that absorbed hydrogen sulphide will be released from the solution. If this is to be avoided, the solution has to be regenerated, i.e. the pH of the solution is increased above the lower limit allowed by the equilibrium state between gaseous _H2S and liquid sulfide content at this temperature and pH. However, the pH must not be increased above about 12, since in this case the uptake of carbon dioxide will become dominant. As a result of the pH increase of the solution caused by the addition of a hydroxide, such as an alkali metal hydroxide, for example NaOH, the bicarbonate ions formed are converted back into carbonate ions, according to the following equilibrium reaction:

(7) After such conversion, the carbonate solution can absorb more hydrogen sulfide according to the above reaction (5). Since the pH of the solution is adjusted by the addition of a hydroxide and remains within a given range of about 9-12, preferably about 10.0-11.0, and most preferably about 10.2-10.8, the absorption of carbon dioxide will be possible kept at a relatively low level.

As previously mentioned, the carbonate-containing alkaline solution is regenerated by the addition of a hydroxide. Basically, any hydroxide can be used as long as it does not adversely affect the absorption of hydrogen sulfide and is capable of increasing the pH of the solution from a given lower limit of about 9 to the desired value, such as A value not exceeding about 12.0, preferably not exceeding about 11.0, most preferably not exceeding about 10.8. According to the present invention, it is preferable to use alkali metal or alkaline earth metal hydroxides, such as sodium hydroxide (NaOH), potassium hydroxide (KOH), lithium hydroxide (LiOH), calcium hydroxide (Ca(OH) ₂ ), and Magnesium Hydroxide (Mg(OH) ₂ ). For reasons of availability and cost, sodium hydroxide is the best.

The temperature of the liquid used for absorption is not particularly critical and can vary widely, but is preferably below about 80°C, since there is a risk that absorption of hydrogen sulfide will diminish at about 80°C and above. The temperature is satisfactorily in the range of room temperature, ie about 20°C, to about 80°C, more preferably about 40-70°C, most preferably about 60-70°C.

In order to optimize the selectivity to hydrogen sulfide in liquid absorption, the absorption should be performed in such a way that the flow of the gas and the flow of the absorbing liquid are countercurrent and the flow of the gas should be turbulent and the flow of the liquid laminar. Furthermore, the separation of hydrogen sulfide is facilitated if the volume of absorbing liquid is large compared to the volume of gas absorbing hydrogen sulfide. Such high liquid to gas ratios are achieved by recirculation of the absorption liquid in contact with the hydrogen sulphide containing gas.

Furthermore, the contact between the sulfur-containing gas and the absorbing liquid (carbonate-containing solution) may include one or more stages, preferably two or three stages (or stages), most preferably three stages. This multistage contacting has the advantage of shortening the length of each single stage so that the pH of the carbonate-containing solution does not have time to drop below about 9 in a single stage, while at the same time the sulfide content can be maintained at on a low level. Preferably, each stage has a range or length at which the pH of the solution has dropped to about 10.0-10.2 at the end of the stage and the liquid is then withdrawn to be regenerated with a hydroxide before being recycled to the stage.

According to the present invention, it has been found that the removal of hydrogen sulphide in a process of the type described above can be achieved more efficiently and drastically reduces the chemical consumption if the process is divided into two stages. i.e. a first stage in which the main part of the hydrogen sulphide is removed according to the method described above, and a second stage in which the remaining hydrogen sulphide is removed mainly by combustion to sulfur dioxide in a wet scrubber (scrubbing gas device), is absorbed in an alkaline solution. The reason for this is as follows.

As hydrogen sulphide is absorbed in the different stages of the first stage, the hydrogen sulphide content of the gas decreases, which causes the previously defined decrease in the selectivity to hydrogen sulphide. As selectivity decreases, base consumption increases dramatically according to reactions (5), (6) and (7) above. From these reactions it can be seen that 1 mole of OH ^- is consumed per mole of absorbed hydrogen sulfide and 2 moles of OH ^- are consumed per mole of absorbed carbon dioxide. As the selectivity for hydrogen sulphide decreases, the absorption of carbon dioxide increases, as is the case in the last absorption stage or stages of the first stage, and the alkali (OH ⁻ ) consumption increases substantially. This means that a considerable amount of base is required to absorb more hydrogen sulfide in the "last" gas than in the "first" gas. By not allowing complete absorption of hydrogen sulphide in the first stage, but deliberately leaving a certain amount of hydrogen sulphide in the gas discharged from the first stage, a sharp decrease in the selectivity to hydrogen sulphide, and thus avoiding the loss of alkali Consumption increases dramatically in the first phase. In the first stage hydrogen sulphide absorption takes place such that the majority of the hydrogen sulphide, ie at least 50%, is removed. Conveniently, about 60-97%, preferably about 80-95%, most preferably about 90% of the hydrogen sulfide is removed in the first stage. This results in a low consumption of base in the first stage, also including good process economy.

However residual hydrogen sulfide must be removed before the gas from the first stage is vented into the surrounding atmosphere. According to the invention, this is accomplished by burning the gas containing hydrogen sulphide from the first stage, so that the hydrogen sulphide is oxidized to sulfur dioxide, and the sulfur dioxide formed is subsequently absorbed in an aqueous solution in a wet scrubber (scrubber) , the pH of the solution is adjusted by adding an alkaline solution or an alkaline substance.

Thus, the present invention provides a method for the selective removal of hydrogen sulphide from a gas containing hydrogen sulphide and carbon dioxide by liquid absorption, characterized in that the gas is in a first stage for removal of the main part of the hydrogen sulphide content in the gas, Contact with an alkaline solution containing carbonates in at least one stage, the pH of which is adjusted during the absorption process by addition of hydroxides, the gas is then combusted in a second stage to convert residual hydrogen sulfide into sulfur dioxide, which is then absorbed in an alkaline solution.

The present invention further provides a device for selectively removing hydrogen sulfide from gas containing hydrogen sulfide and carbon dioxide by liquid absorption, characterized in that it comprises:

a) A vessel with a gas inlet and a gas outlet containing a packing arranged in a number of successive steps; means for supplying a carbonate-containing solution to the last stage, visible in the direction of the gas feed flow, supplied at each stage A device for passing a carbonate-containing solution countercurrently into the gas through this stage and through this stage for recirculating the solution; as seen in the flow direction of the gas feed, when part of the flow for supplying the solution passes from one stage (section) to a preceding one The conduits arranged between the various stages (sections) of the stages (sections); the means for supplying the hydroxide to the carbonate-containing solution of at least one of the stages; and a first The primary outlet conduit is used to discharge liquids containing hydrogen sulfide.

b) a combustion chamber having an inlet for gas containing hydrogen sulphide from the gas outlet of the vessel; combustion means for combusting the supplied gas and converting the hydrogen sulphide therein to sulfur dioxide; a combustion chamber for discharging gas containing sulfur dioxide from the combustion chamber; gas outlets;

c) a wet scrubber for absorbing sulfur dioxide formed by combustion, comprising a vessel having an inlet for sulfur dioxide-containing gas from the combustion chamber; an outlet for scrubbed gas; supply, circulation and Apparatus for subdividing an aqueous solution, the pH of which is adjusted by the addition of an alkaline solution or an alkaline substance, thereby bringing it into contact with a supply of gas containing sulfur dioxide; and apparatus for discharging the solution which has absorbed sulfur dioxide.

Further distinguishing features of the present invention appear from the following description and are set forth in the appended claims.

As mentioned above, the carbonate-containing alkaline solution used in the first stage of hydrogen sulfide absorption is an aqueous solution containing carbonate ions such as an alkali metal carbonate, preferably sodium carbonate. When the method of the invention is used for the removal of hydrogen sulphide in the kraft pulp industry, for example when burning black liquor, it is found that the green liquor is used by the invention as a carbonate-containing alkali solution, which constitutes a special aspect of the invention . Green liquor is _an aqueous solution that typically 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 moles of NaOH per liter. According to the present invention, it has also been found that when the present invention is used in the kraft pulp industry for the removal of hydrogen sulphide, the white liquor can also be used as hydroxide in the first stage to regenerate the carbonate-containing alkaline solution. White liquor is an aqueous solution containing NaOH, Na ₂ S and Na ₂ CO ₃ , usually in proportions of about 2.3-2.6 moles per liter NaOH, about 0.4-0.6 moles per liter Na ₂ S, about 0.25-0.4 moles per liter Na ₂ CO ₃ . The use of white liquor also as hydroxide for regenerating carbonate-containing alkaline solutions forms a particular aspect of the invention.

Instead of white liquor, weak liquor or oxidized white liquor for regenerating carbonate-containing alkaline solutions can be used. Dilute solution refers to an aqueous solution containing NaOH, Na ₂ S and Na ₂ CO ₃ , usually in proportions of about 0.3-0.4 moles per liter of NaOH, about 0.05-0.08 moles of Na ₂ S per liter, and about 0.05-0.07 moles per liter Na ₂ CO ₃ .

Since green liquor, white liquor, and weak liquor contain carbonate and hydroxide, they can be used separately or together, all as carbonate-containing solutions, to regenerate carbonate-containing alkaline solutions.

In the second stage, when the invention is used in the kraft pulp industry, the combustion of the gas containing hydrogen sulphide takes place in a known manner in a known type of combustion plant, such as a lime sludge reburner or a tree skin fired boiler. If the heat content of the hydrogen sulfide-containing gas is too low, a secondary combustion agent, such as oil or natural gas, can be used in the combustion, resulting in satisfactory oxidation of the hydrogen sulfide to sulfur dioxide. When the gas containing hydrogen sulfide is burned, hydrogen sulfide is completely oxidized to sulfur dioxide at normal combustion temperature, so that the gas contains only a small amount of hydrogen sulfide after combustion.

In the second stage, after the combustion of the hydrogen sulfide-containing gas, the sulfur dioxide formed should be removed, which according to the invention takes place by absorption in an alkaline solution in a wet scrubber (gas scrubber). The selective removal of sulfur dioxide from an otherwise carbon dioxide-containing gas by absorption in an alkali-containing solution in a scrubber is a well-known technique and need not be described in more detail here. Conveniently, a scrubber is used in which the absorbed alkali-containing solution is recirculated to a high degree. The pH of the alkali-containing solution in the SO2 scrubber is preferably in the range of about 6-8, most preferably in the range of about 7-7.5. At these rather low pH values, carbon dioxide uptake is negligible and the selectivity to sulfur dioxide separation is thus 100%. During absorption, sulfur dioxide undergoes one of the following reactions:

(8)

(9)

(10)

It is shown from reaction formulas (8)-(10) that 2 moles of OH ⁻ are consumed per mole of absorbed SO ₂ . Since no carbon dioxide is absorbed, sulfur dioxide absorption can be performed to almost 100% of the available sulfur dioxide without any noticeable increase in alkali consumption. This considerably improves the economics of the process compared to the situation when all the sulfur is absorbed as hydrogen sulphide in the first stage, resulting in poor selectivity.

The base in the base-containing solution is conveniently a hydroxide or a carbonate of an alkali or alkaline earth metal, such as sodium hydroxide, potassium hydroxide, calcium carbonate, calcium hydroxide or magnesium hydroxide. Sodium hydroxide is best especially if the sulfur is recycled directly in the process, for example when this method is used in the kraft pulp industry. If so, it is convenient to use a sodium hydroxide based liquid spray scrubber, such as that used by ABB Flakt Industri AB and designated as the MoDo scrubber type. The scrubber may then comprise one or more heat recovery stages (sections) in which cold water is sprayed onto the hot gas in order to obtain hot water at a temperature of approximately 45-65°C. After sulfur dioxide absorption, the solution leaving the scrubber contains sodium pentasulfate (Na ₂ SO ₃ ), sodium bisulfite (NaHSO ₃ ) and sodium sulfate (Na ₂ SO ₄ ). In the kraft pulp industry, this solution can be mixed with other flammable process fluids, such as black liquor in a pulp mill. Combustion under reducing conditions results in sulfur in the form of hydrogen sulphide and combustion in oxidizing conditions results in sulfur in the form of sulfur dioxide.

In the pulp industry, the method according to the invention is so conveniently designed that its different stages are combined in existing plants. This means that the combustion of the gas containing hydrogen sulphide takes place in an existing bark fired boiler or lime slag reburner. This reduces the need for other fuels in these process steps. The SO2-containing combustion gas from the bark firing boiler or lime slag reburner can then be directed into an existing SO ₂ scrubber. In this case, the only new equipment required is a scrubber to separate the hydrogen sulphide.

Pulp mills may be equipped with scrubbers for separating SO ₂ for fuel gas from bark burning boilers and lime residue reburning furnaces and from alkali recovery furnaces respectively. When the hydrogen sulphide containing gas is combusted elsewhere, for example in a split boiler, the sulfur dioxide containing fuel gas can be directed to the existing _SO2 scrubber for the fuel gas from the recovery furnace.

From the above, it is evident that the present invention achieves good process economics, a closed chemical system in which all sulfur is recycled, and little discharge to the ambient atmosphere.

For purposes of clarity, the invention will now be described in more detail with reference to the accompanying drawings, in which:

Figure 1 illustrates a preferred apparatus according to the invention for carrying out the first stage of the method according to the invention.

Figure 2 is a schematic flow diagram illustrating the method according to the invention.

The apparatus shown in Figure 1 comprises a column or vessel 1 having an inlet 2 for a gas 3 comprising hydrogen sulphide and carbon dioxide. At the other end of the device, there is provided an outlet 4 for gas 5, whereby gaseous hydrogen sulphide has been removed by liquid absorption. The contact between the gas containing hydrogen sulfide and the solution containing carbonate involves three stages (stages) 6, 7 and 8 . Each stage contains filler 9, as implied by stage 6 in the figure. To optimize the selectivity to hydrogen sulphide in the absorption, the packing 9 has such a shape as to produce a laminar flow of liquid through stages 6, 7 and 8. Packing in the form of corrugated plates has been found to be particularly suitable for this purpose. For example the packing can be made of plastic or metal.

The absorption of hydrogen sulphide, the contact between the carbonate-containing solution and the hydrogen sulphide-containing gas takes place in countercurrent. To this end, each stage has means to supply the carbonate-containing solution through this stage countercurrently to the gas and to recirculate the solution through this stage. As shown in Figure 1, these devices are made up of pumps 10, 11, 12, through conduits 13, 14 and 15, the alkaline solution containing carbonate is fed to 6, 7 and 8 each stage (segment), and Conduits 16, 17 and 18 lead solutions from the various stages to 19, 20 and 21 collection vessels. From these collection vessels, the solution is recirculated to the stages through conduits 22, 23 and 24, which are connected to pumps 10, 11 and 12, respectively. Fresh carbonate solution, preferably sodium carbonate solution, is fed to the last stage 6, which is fed via conduit 25 from a supply of sodium carbonate solution (not shown), in the gas feed direction.

Instead of supplying fresh phosphate solution to the last stage (stage), the carbonate solution can be regenerated in the last stage by supplying sodium hydroxide solution to this stage, allowing the hydroxide to absorb carbon dioxide in the gas, so that according to the above Reaction (3)-(4), a solution containing carbonate is obtained.

In order to adjust (increase) the pH of the absorption solution, a hydroxide, preferably sodium hydroxide solution, can be supplied to the collection vessels 19, 20 and 21 respectively through the pipelines 26, 27 and 28, the pipelines being at the supply (without Shown) leads to the base, and the supply is preferably shared by all pipelines. The supply of sodium hydroxide solution to adjust the pH of the absorption solution is adjusted based on the measured pH values of the solutions in the collection containers 19, 20 and 21 (not shown).

As can be seen from Figure 1, the different stages can be further interconnected by conduits 29 and 30, feeding part of the absorption solution fluid from one stage to subsequent stages, i.e. from stage 6 to stage 7 and from stage 7 to stage 8.

Finally, an outlet conduit 31 is arranged to discharge the liquid containing hydrogen sulphide from the collection vessel 21 and stage 8 .

Figure 2 is a schematic flow diagram illustrating the method of the present invention. As shown in the flow diagram, in a first stage, a gas 3 containing hydrogen sulphide and containing carbon dioxide is treated in a device comprising a vessel 1 for absorbing hydrogen sulphide, as previously described with reference to FIG. 1 . According to the invention, the absorption of hydrogen sulphide is not carried out to 100% in the first stage, i.e. hydrogen sulphide is not completely absorbed in the first stage, but is absorbed so much that at least the main part of hydrogen sulphide, i.e. at least 50%, is preferably Preferably about 60-97%, better about 80-95%, most preferably about 90% of the hydrogen sulphide is absorbed in the first stage. Preferably, at least about 98% of the total amount of hydrogen sulfide in the gas is absorbed through the first and second stages.

The gas 5 thus cleaned, still containing hydrogen sulphide and carbon dioxide, is then fed from the first stage to the second stage, in which the gas is first combusted in order to convert the hydrogen sulphide into sulfur dioxide, given the absorption of the formed sulfur dioxide, and then the gas Wet scrubbed in a scrubber.

As shown in Figure 2, combustion takes place in a combustion chamber 32 having combustion means 33, such as a furnace, to burn gas 5 supplied through an inlet 34, oxidizing hydrogen sulphide to sulfur dioxide. As mentioned previously, the combustion plant 32 may consist of existing equipment, such as a bark burner or a lime slag reburner. Fuel gas containing sulfur dioxide is discharged through an outlet 35 into a wet scrubber 36 comprising an outlet 39 having an inlet 38 for sulfur dioxide containing gas, an outlet 39 for scrubbed gas 40, and means 41 for supplying an alkaline solution. As mentioned above, the alkaline solution contains an alkali metal or alkaline earth metal hydroxide, preferably sodium hydroxide. The solution is circulated by a pump 42 to an injector 43 where it is subdivided and inversely brought into contact with a gas containing sulfur dioxide so that the sulfur dioxide is absorbed by the solution. To optimize the absorption of sulfur dioxide without any competing absorption of carbon dioxide, the pH of the solution is adjusted to about 6-8, preferably 7-7.5. The solution that has absorbed sulfur dioxide is withdrawn at 44 from the second stage.

The invention will now be further elucidated with the help of a non-limiting example.

example

A test was carried out for the selective removal of hydrogen sulphide from gases produced in black liquor evaporation. An apparatus of the type described above shown in Figures 1 and 2 was used.

In the first stage of the process, hydrogen sulfide absorption occurs at atmospheric pressure with a feed gas temperature of about 60°C containing 1.13 mole percent hydrogen sulfide and 16.9 mole percent carbon dioxide. The gas is saturated with water vapor at this temperature, corresponding to about 18.7 mole percent water. The feed gas flow rate is 38280 Nm ³ /h, so that the gas flow rate in the absorption tower is about 3.1 m/s. The absorber height is 6.25m, the two first stages (stages) are each 1.5m high, however the last stage is 1m high as indicated by the gas feed direction. Each section was supplied with a packing of model Mellapack 500 from Sulzer. The diameter of the tower is 2.3m.

Fresh absorption solution consisting of 8.8 m ² /h of 2M sodium carbonate solution at a temperature of approximately 60 °C is supplied in the column to the last stage together with recycled absorption solution for a total absorption of approximately 50 m ³ /h The solution is fed into the last stage in the column. The pH of the supplied absorption solution is about 11.0, and during the flow of the solution through this stage, the pH of the solution drops to about 10.2 due to the absorption of hydrogen sulfide. After passing through this stage, the solution is supplied to a 1.5 ^cm2 collection vessel where the solution is regenerated by the addition of 2.5M sodium hydroxide solution at a temperature of about 60°C so that the pH of the solution increases again to about 11.0. The regenerated solution is then recycled by a pump to the last stage (section) in the absorber for the restarted absorption of hydrogen sulphide.

About 11_m ³ /h of absorption solution is pumped from the collection vessel to the collection vessel of the intermediate stage (stage), whereby approximately 50_m ³ /h of absorption solution with a pH of approximately 11.0 is pumped into the middle as in the previous step section, whereby the absorption solution is withdrawn at a pH of approximately 10.2 and recycled to the collection vessel. In the collection vessel, the solution was regenerated by adding a 2.5M sodium hydroxide solution at a temperature of about 60°C, as in the previous step.

From the collection vessel of the middle section, about 13.5_m ³ /h of absorption solution is pumped into the first (lowest) stage of collection vessel, whereby 50 m ³ /h of absorption solution with a pH of about 11.0 is pumped to The initial stage, as indicated by the feed flow of gas. After passing through this stage, and after absorbing hydrogen sulphide, the solution, now at a pH of about 10.2, is drawn off and recycled again to the collection vessel. In the collection vessel, as in the previous stage, the solution is regenerated by adding a 2.5 M sodium hydroxide solution at a temperature of about 60° C., so that the pH of the regenerated solution is about 11.0. In total, about 8.6 m ³ /h of 2.5M sodium hydroxide solution were added to the collection vessels of the three stages.

From the first (lowest) primary collection vessel, about 17.4 m ³ /h of solution with an ^HS concentration of 1 mol/l are drawn off. The gas leaving the absorber contained 0.113 mole percent hydrogen sulfide and 16.4 mole percent carbon dioxide. In the first stage of the process, the degree of separation of hydrogen sulphide is about 90%, and the selectivity to hydrogen sulphide in the separation is about 0.67%.

Then comes the second stage of the method. Thus, the gas leaving the absorber in the first stage is combusted in a furnace fed with 31700 Nm ³ /h of air resulting in 66000 Nm ³ /h of fuel gas containing 650 ppm SO ² (1930 mole/h). This fuel gas is directed into a conventional type _SO2 scrubber as described previously. In this scrubber, 1.54m ₃ /h of 2.5M sodium hydroxide solution is consumed, in addition, 2.0m ³ /h of fresh water is supplied into the scrubber to cool the incoming fuel gas to saturation temperature. The gaseous _SO2 concentration leaving the scrubber was 12ppm, corresponding to 35mole/h (1.12Kg/h of sulfur). The supplied absorption solution was maintained at a constant pH of 7.2. The absorption solution is recirculated in the scrubber with a total flow rate of 170 m ³ /h. A solution with a total sulfur concentration (SO ₃ ²⁻ +HSO ₃ +SO ₄ ²⁻ ) of 1.05 mol/liter was withdrawn from the scrubber. The total sulfur separation, ie the separation of hydrogen sulfide in the first stage plus the separation of sulfur dioxide in the second stage, thus amounts to 99.8%. In the first stage, the sodium hydroxide of 860kg/h is consumed, and in the second stage, the sodium hydroxide of 152kg/h is consumed, that is, the total amount consumed is 1012kg/h for obtaining the separation degree of 99.8% sulfur h NaOH.

As a comparison, the process is repeated, but this time only for the first stage, ie without the combustion of hydrogen sulphide to sulfur dioxide and the absorption of the formed sulfur dioxide in a wet scrubber. The separation of hydrogen sulphide in the first stage instead proceeds to such an extent that 98% of the hydrogen sulphide is separated. The selectivity to hydrogen sulfide was 0.15 in the separation of 90-98% hydrogen sulfide. The overall selectivity of the whole process is then about 0.55. Despite the fact that the overall sulfur separation is lower, this results in a NaOH consumption of 1751 kg/h, ie 70% more NaOH consumption than in the process according to the invention.

Claims (10)

1. A process for the selective removal of hydrogen sulphide from a gas (3) containing hydrogen sulphide and carbon dioxide by liquid absorption, characterized in that it comprises the following first and second stages: In a first stage for removing a major part of the hydrogen sulphide content of the gas, the gas (3) is mixed in at least one of the stages (6, 7, 8) with an alkaline solution containing carbonate ( 25) contacting, the pH of the alkaline solution being adjusted during absorption by adding a hydroxide (26, 27, 28), and Then, in a second stage, the gas is combusted in order to convert the remaining hydrogen sulfide to sulfur dioxide, which is absorbed in an aqueous solution whose pH is adjusted by adding an alkaline solution or substance . 2. The method according to claim 1, characterized in that the gas (3) is brought into contact with a sodium carbonate-containing solution (25) in a first stage. 3. The method according to claim 2, the gas (3) is brought into contact with green liquor, white liquor or weak liquor in a first stage. 4. The method according to claim 1, characterized in that, in the first stage, the pH of the carbonate-containing solution is adjusted by means of a liquid (26, 27, 28) containing an alkali metal hydroxide . 5. The method according to claim 4, characterized in that the pH is adjusted by white liquor, green liquor or weak liquor. 6. A method according to any one of claims 1-5, characterized in that, in a first stage, the gas is brought into contact with a carbonate-containing solution (25) having a pH of 9-12. 7. The method according to any one of claims 1-5, characterized in that, in the second stage, the gas is absorbed in an aqueous solution of pH 6-8 after combustion. 8. Process according to any one of claims 1-5, characterized in that 60-97% of the hydrogen sulphide content of the gas is removed in the first stage. 9. A method according to any one of claims 1-5, characterized in that at least 98% of the hydrogen sulphide content of the gas is completely removed. 10. A plant for the selective removal of hydrogen sulphide from a gas (3) containing hydrogen sulphide and carbon dioxide by liquid absorption, characterized in that it comprises: a) Container (1) It has a gas inlet (2) and a gas outlet (4) and contains packing (9) arranged in successive stages (6, 7, 8); Device(25) For supplying the carbonate-containing solution to the last stage in the feed flow direction of the gas, each stage is used for supplying the carbonate-containing solution, through this stage, to the gas in the counter-current direction , and means for recirculating said solution through the stage; Catheter(29,30) It is arranged between the stages, and is used to supply a part of the liquid flow of the solution from one stage to the previous stage according to the gas feed flow direction, Devices (26, 27, 28) for supplying hydroxide to the carbonate-containing solution in at least one of said stages (6, 7, 8); Outlet Conduit(31) It is used to discharge the liquid containing hydrogen sulfide from the first stage according to the feeding direction of the gas, b) a combustion chamber (32) having One gas inlet (34) For introducing hydrogen sulfide-containing gas (5) from the gas outlet (4) of the container (1), Combustion Devices(33) for the combustion of feed gas and the conversion of hydrogen sulfide to sulfur dioxide, and one exit(35) for venting sulfur dioxide-containing gases from the combustion plant (32), and c) a wet scrubber (36) Used to absorb sulfur dioxide formed by combustion, it includes: A container (37) having an inlet (38) through which the sulfur dioxide-containing gas from the combustion container (32) is introduced; an outlet (39), from which the cleaned gas (40) is derived, Apparatus (41, 42, 43) for feeding, circulating and subdividing an aqueous solution, the pH of which is determined by adding an alkaline solution or alkaline regulated by the substance, and Device(44) Used to discharge the solution that has absorbed sulfur dioxide.

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