CN1118146A - Hydrogen sulfide absorption method and equipment - Google Patents

Abstract

A process and an apparatus in black-liquor evaporation for the selective removal, by liquid absorption, of hydrogen sulphide from the generated gas (3) containing hydrogen sulphide as well as carbon dioxide, are disclosed. In the process, the gas is countercurrently brought into multistage contact, with circulating carbonate-containing alkaline solutions, preferably sodium carbonate solutions (25), the pH of which is adjusted during the absorption to about 9-12 by the addition of a hydroxide. It is preferred that the gas flow is turbulent and the liquid flow is laminar when contacted with one another. The apparatus comprises a container (1) and a packing (9) arranged in several successive stages (6, 7, 8). The apparatus has means (25) for supplying a carbonate-containing solution to the last stage and through this stage countercurrently to the gas, and for recycling the solution across this stage; conduits (29, 30) arranged between the stages for supplying a partial flow of the solution from one stage to a preceding stage; means (26, 27, 28) for supplying a hydroxide to the carbonate-containing solution in at least one of the stages ; and an outlet conduit (31) from the first stage, as seen in the feed direction of the gas, for discharging a sulphide-containing liquid.

Description

Hydrogen sulfide absorption method and equipment

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

Hydrogen sulfide is absorbed mainly in the form of hydrogen sulfide ion (HS ^- ), but also in the form of sulfide ion (S ^2- ). The set of total contents of these ions is referred to as "total sulfide content" or simply "sulfide content" below.

The removal of hydrogen sulfide from hydrogen sulfide-containing gases is well known using aqueous alkaline solutions such as aqueous sodium hydroxide or absorption with ethanolamines such as monoethanolamine or diethanolamine. This absorption method can be used, for example, to produce pure hydrogen sulphide, which can also be further processed to sulfur in the Claus process. In addition to hydrogen sulfide, carbon dioxide is also absorbed by the alkaline solution if the gas contains carbon dioxide. Carbon dioxide has about the same solubility in water as hydrogen sulfide, so absorption of carbon dioxide in solution will compete with hydrogen sulfide. The absorption of hydrogen sulfide and carbon dioxide in alkaline aqueous solution (such as sodium hydroxide) is carried out in the following way:

(1)

(2)

(3)

(4) The selectivity of hydrogen sulfide, that is, the ratio of the absorbed hydrogen sulfide to the absorbed (hydrogen sulfide + carbon dioxide) moles is directly proportional to the hydrogen sulfide and carbon dioxide content of the gas. The competitiveness of carbon dioxide is therefore particularly pronounced when the gas contains more carbon dioxide than hydrogen sulphide, as is the case in most cases in practice. If the gas contains 1% hydrogen sulfide and 10% carbon dioxide (both by volume), and try to absorb hydrogen sulfide in sodium hydroxide solution, the selectivity of hydrogen sulfide is only 10%, that is, 90% of the absorbed gas is composed of carbon dioxide composition, indicating that as much as 90% of the sodium hydroxide is consumed to absorb carbon dioxide.

In an effort to remedy the aforementioned disadvantages of absorbing hydrogen sulfide from gases containing hydrogen sulfide and carbon dioxide, methods for selective absorption of hydrogen sulfide have been developed. For example, efforts have been made to selectively absorb hydrogen sulfide in solutions of strong oxidizing agents such as potassium permanganate, sodium dichromate or iron salts. In other methods of selective absorption is the use of alkaline solutions such as sodium or potassium carbonate, with careful adjustment of operating conditions during absorption. For more detailed information of this prior art, please see C.Oloman.F.E.Murray and J.B.Risk article "The Selective Absorption of Hydrogen Sulphite from. Stack Gas", reported in "Pulp and Paper Magazine of Canada" (1969.12.5.) P .69 ff. and E.Bendall, R.C.Aiken and F.Mandas article "Selective Absorp-tion of H2S from Larger Qantities of CO2 by Absorption and Reaction in Fine Sprays", reported in AICHE Journal (Vol.29, No.1), 1983.1, p.66 ff.

An example of the prior art is described in US 3,554,859 which relates to the recovery of sulfur from furnace gases such as those produced by the combustion of black liquor. The combustion gas containing hydrogen sulfide and carbon dioxide is contacted with a gas containing molecular oxygen and with an aqueous alkaline solution containing sodium ions, for example in the form of sodium hydroxide and sodium carbonate or sodium carbonate and sodium bicarbonate. Thus, hydrogen sulfide is absorbed from the gas and oxidized to thiosulfate. With the addition of iron oxide (Fe ₂ O ₃ ), the absorption is even more effective and the concentration of sulfide ions in the solution is thus kept at a very low level, ie the sulfide ion content should be as low as possible.

The hydrogen sulfide absorption selectivity can be increased to 30-50% by using a carbonate solution such as sodium carbonate solution instead of a hydroxide solution such as sodium hydroxide solution. The reaction that occurs for this absorption is generally as follows: (5) (6)

If the absorbing solution is a carbonate solution, the absorption of hydrogen sulfide is almost instantaneous, while carbon dioxide reacts only slowly with carbonate ions to form bicarbonate ions. When using a carbonate solution as the absorption medium, its "back pressure" (equilibrium pressure) for the absorption of carbon dioxide is another advantage due to the high content of bicarbonate ions produced, as seen in the equilibrium equation (6) above .

A problem with the use of carbonate solutions as absorption medium is that only relatively low sulfide ion contents can be achieved in the solution, since the formation of bicarbonate ions causes a reduction in the absorption capacity. Therefore, it is extremely difficult to obtain a sulfide ion content in excess of about 0.3 mol/liter. As a result, prior art methods of carbonates for the selective absorption of hydrogen sulphide in the absorption medium have not been very successful, although there is a strong demand for such methods in many areas where gases containing hydrogen sulphide and carbon dioxide are produced. Examples of these areas of application are petroleum refining, gas production and black liquor combustion, especially in the kraft pulp industry.

When recovering chemicals in the sulfate industry according to the conventional Tomlinson process, the black liquor is burned in a soda ash recovery unit, resulting in the generation of steam and the formation of molten metal mainly composed of sodium carbonate and sodium sulfide body. The melt is then dissolved in water and causticized so that the sodium carbonate is converted to sodium hydroxide and a white liquor is obtained which can be reused to digest the woody raw material. For many reasons, including the risk of sudden explosion of the pipes in the soda ash recovery unit, a new method of black liquor combustion has been studied in recent years. The black liquor is pyrolyzed under reducing conditions without forming a melt. This set of methods is called "black liquor evaporation", one of which is the so-called SCA-Billerud method (E. Horntvedt 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 soda ash mainly composed of sodium carbonate and carbon and combustible gases, especially sulfur-containing compounds. Another example of black liquor evaporation is found in US4, 872,950, which relates to the method of thermally decomposing and evaporating black liquor under a pressure greater than 10 bar and at a temperature that does not form a melt with an amount of oxygen lower than the stoichiometric requirement. It consists of a solid phase and a gaseous phase mainly composed of hydrogen sulfide, carbon monoxide, carbon dioxide, hydrogen, water vapor and methane.

EP 459,962 relates to the purification of process gases in the evaporation of black liquor in which the gas is contacted with an alkaline solution containing hydrogen sulfide and hydroxide ions to remove sulfur and sodium compounds therefrom. The purification method consists of two stages. In the first stage, the gas is passed through a Venturi nozzle together with the alkaline solution, so that the black liquor melt aerosol droplets in the gas are absorbed by the solution, and then the gas is mixed with hydroxide ions and hydrogen sulfide ions ( molar ratio greater than 4:1) solution contact. The high molar ratio of hydroxide ions to hydrogen sulfide ions can result in an absorption solution with a low sulfide concentration. The alkaline solution used for absorption, such as white liquor or wash liquor, has a high pH value of about 13-14, resulting in poor selectivity of hydrogen sulfide absorption. Furthermore, the absorption solution used has a low content of carbonate ions, and it is specifically stated that green liquor with a high content of carbonate ions cannot be used as a washing solution.

In order to recover the chemicals used for the evaporation of black liquor, ie the evaporation of black liquor burned in a reducing atmosphere, and to produce from these chemicals white liquor for making pulp, hydrogen sulfide must be removed from the gas produced. Since the gas also contains carbon dioxide, in liquid absorption, carbon dioxide will compete with hydrogen sulfide, and because the hydrogen sulfide content in the gas is low (about 0.5-2%), while the carbon dioxide content is about 20 times higher (about 10-20%), Hydrogen sulfide recovery is not satisfactory by conventional liquid absorption methods.

Therefore, there is a need for a method of separating hydrogen sulfide with high resolution and selectivity from a gas containing hydrogen sulfide and carbon dioxide produced from the evaporation of black liquor and absorbing the hydrogen sulfide in a liquid having a high sulfide ion content therein.

In accordance with the present invention, it has been found that countercurrent multistage absorption using a carbonate-containing alkaline solution as the absorption medium and adjusting the pH of the solution to about 9-12 by addition of hydroxide rather than fresh carbonate has a high sulfidation Hydrogen separation and high hydrogen sulfide selectivity. Therefore, the present invention makes the absorption selectivity of hydrogen sulfide reach 60-70%, and the separation degree of hydrogen sulfide is about 90-99%. According to the present invention, the total sulfide ion content in the effluent absorption liquid is high, i.e. greater than 0.30 mol/liter, preferably greater than about 0.47 mol/liter, usually in the range of about 0.30-1.30 mol/liter, preferably about 0.47 - 1.1 mol/liter, especially about 0.65-1.0 mol/liter. This solution can be used to produce white liquor in kraft pulp making.

More specifically, the invention provides a process of the type described in the introduction, which involves the multistage contacting of gases with a circulating alkaline solution containing carbonates, with the addition of hydroxides during the absorption process. The pH of the solution is adjusted to about 9-12 such that the total sulfide ion content of hydrogen sulfide absorbed exceeds about 0.30 moles per liter in the effluent solution in contact with the feed gas.

The invention further provides a device of the type mentioned in the introduction, characterized in that the device comprises a container with a gas inlet and an outlet, in which there are packings arranged in a plurality of successive stages, and the device has the function of supplying the last stage with a carbonate-containing solution The device (see gas feed description), each level has a device that provides a carbonate-containing solution through the level countercurrent to the gas and circulates the solution through the level, and the equipment has between the layers from one level to the previous level Pipelines providing solution splitting (see gas feed description), equipment having means to supply hydroxide to the carbonate-containing solution on at least one stage to adjust the pH of the solution to 9-12, and an outlet from the first stage Pipelines (see Gas Feed Instructions) to discharge liquids with a total sulfide ion content exceeding 0.30 mol/l.

Further distinctive features of the invention are described below and in the claims.

As used herein, "carbonate-containing alkaline solution" means an aqueous solution containing carbonate ions (CO ₃ ²⁻ ). This solution is preferably an alkali metal carbonate solution, such as sodium carbonate, potassium carbonate or lithium carbonate solution. Sodium carbonate is particularly preferred because it is readily available and very inexpensive. The concentration of carbonate in the solution is not critical, suitably about 0.1-3M carbonate, preferably about 1-2.5M, especially about 2M.

According to the present invention, it is important that the carbonate-containing alkaline solution has a pH of at least about 9. At pH values below about 9, hydrogen sulfide absorption is unsatisfactory and there is a risk of liberating absorbed hydrogen sulfide from solution. However, the pH of the solution should not be too high, since the absorption of hydrogen sulphide will have a negative effect compared to the absorption of carbon dioxide. Therefore, the pH of the solution should not exceed about 12, so that the absorption of carbon dioxide is not too significant, the pH of the solution is preferably in the range of about 10.0-11.5, especially about 10.0-11.0, most preferably about 10.2-10.8. If the pH of the solution is adjusted within the last narrow range, the best separation of hydrogen sulfide can be obtained.

It can be seen from the equilibrium reaction equations (5) and (6) that bicarbonate ions (HCO ₃ ^- ) are formed in the absorption of hydrogen sulfide and carbon dioxide. This means that the absorption of hydrogen sulfide and carbon dioxide proceeds by lowering the pH of the absorption solution. When the pH of the solution drops below about 9, the uptake of hydrogen sulphide becomes unsatisfactory and, as already indicated, there is a risk that the absorbed hydrogen sulphide will be released from the solution. If this is to be avoided, the solution must be regenerated, that is, the pH must be increased above the lower allowable limit to achieve a state of equilibrium between gaseous _H2S and the sulfide ion content of the solution at the temperature and pH in question, but the pH cannot be high At 12, carbon dioxide absorption in this case predominates. As a result of the addition of a hydroxide such as an alkali metal hydroxide (e.g. NaOH) according to the invention to achieve an increase in pH, the bicarbonate ions formed are in turn converted to carbonate ions according to the following equilibrium reaction:

(7)

The carbonate solution regenerated in this way can absorb more hydrogen sulfide according to the above reaction formula (5). Because the pH of the solution is adjusted and maintained in a given range of about 9-12 with hydroxide, preferably about 10.0-11.5, especially about 10.0-11.0, most preferably about 10.2-10.8, the absorption of carbon dioxide is maintained at A negligibly low level.

As mentioned above, the alkaline solution containing carbonate can be regenerated by adding hydroxide. Essentially any hydroxide that 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 not exceeding about 12.0, preferably not exceeding About 11.5, especially not more than 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 hydrogen Magnesium oxide (Mg(OH) ₂ ). Sodium hydroxide is most preferred due to its availability and low cost.

The temperature of the liquid used in the absorption of the present invention is not a critical issue, it can vary within a wide range, but preferably should be below about 80°C, because at a temperature of about 80°C or above, there is a decrease Hazard of hydrogen sulfide absorption. The best temperature range is from room temperature (ie 20°C) to about 80°C, preferably about 40-70°C, especially about 50-70°C, most preferably about 60-70°C.

The inventors have discovered that hydrogen sulfide selectivity in liquid absorption can be optimized by contacting the gas stream and the absorbing liquid stream in countercurrent, with turbulent gas flow and laminar liquid flow. Furthermore, the separation of hydrogen sulfide can be facilitated if the volume of absorption liquid is larger compared to the volume of gas from which hydrogen sulfide is absorbed. This high liquid/gas ratio is obtained by recirculating the absorption liquid in contact with the hydrogen sulfide-containing gas.

Furthermore, the contact between the hydrogen sulphide-containing gas and the absorption liquid (carbonate-containing solution) involves several stages, preferably two or three stages, but most preferably three stages. The advantage of this multistage contact is that the length of the individual stages is shortened so that the pH of the carbonate-containing solution does not have time to drop below about 9 in a single stage, while the sulfide ion content of the uppermost stage can be kept low . The extent or length of each stage is preferably such that the pH of the solution drops to about 10.0-10.2 at the end of that stage and is then recirculated to that stage after the liquid is drawn off and regenerated with hydroxide.

For the sake of clarity, the invention will be described with reference to the accompanying drawing, which shows a preferred embodiment of the apparatus of the invention.

The apparatus of the invention comprises a column or vessel 1 having an inlet 2 for gas 3 which is produced by the evaporation of black liquor and contains hydrogen sulphide and carbon dioxide. The opposite end of the apparatus has an outlet 4 for gas 5 which has been absorbed by the liquid to remove hydrogen sulphide. The contact between the hydrogen sulfide-containing gas and the carbonate-containing solution involves three stages 6 , 7 and 8 . Each level contains filler 9 (shown as level 6 in the figure). To optimize hydrogen sulphide selectivity in absorption, the shape of the packing 9 is such that the fluid flows through the stages 6, 7 and 8 in a laminar manner. The inventors have found that corrugated sheet-like packings are particularly suitable for this purpose. Fillers can be made, for example, of plastic and metal.

The contact between the carbonate-containing solution absorbing hydrogen sulphide and the hydrogen sulphide-containing gas is carried out in countercurrent. To this end, each stage has means for passing the carbonate-containing solution through the stage countercurrent to the gas, and means for circulating the solution through the stage. As shown, these devices consist of pumps 10, 11, 12, which feed carbonate-containing alkaline solution to the stages 6, 7, 8 via lines 13, 14, 15 and fed by lines 16, 17, 18 Lead solutions from the various stages into collection containers 19 , 20 , 21 . The solution is recycled to each level from the collection container through the pipelines 22, 23, 24 connected to the pumps 10, 11, 12 respectively. Fresh carbonate solution, preferably sodium carbonate solution, is obtained from the sodium carbonate solution reservoir (not shown). shown) is fed to the final stage 6 via conduit 25.

In addition to providing fresh carbonate solution to the last stage, sodium hydroxide solution can be provided to this stage to generate carbonate solution and let the hydroxide solution absorb carbon dioxide from the gas, so according to the reaction formula (3)—(4) A carbonate-containing solution is obtained.

In order to adjust (increase) the pH value of the absorption solution, a hydroxide solution, preferably a sodium hydroxide solution, can be supplied to the collection vessels 19, 20, 21 via the conduits 26, 27, 28, respectively, and the alkali is transferred from the reservoir (not shown) import. The reservoir is preferably common to each pipeline. The supply of sodium hydroxide solution to adjust the pH of the absorption solution is based on measuring the pH of the solution in the collection containers 19, 20, 21 (not shown).

As shown, the different stages can further be connected by pipes 20 and 30 to feed a split flow of absorption liquid from one stage to the preceding stage, ie from stage 6 to stage 7 and from stage 7 to stage 8.

Finally, an outlet pipe 31 is provided to discharge sulphide-containing liquid from vessel 21 and stage 8 .

The invention will now be further elucidated by way of non-limiting examples.

Example 1

In this example, the test of selective removal of hydrogen sulfide from the gas generated by the evaporation of black liquor was carried out. The equipment used was of the type described above and is shown in the accompanying drawings.

The absorption is carried out at atmospheric pressure, the inlet temperature is about 60°C, and it contains 1.13 (mol)% hydrogen sulfide and 16.9 (mol)% carbon dioxide. The gas is saturated with water vapor at said temperature, corresponding to about 18.7 mole percent water. The inlet gas flow is 38,280Nm ³ /h, so that the gas flow velocity in the absorption tower is about 3.1m/s. The absorption tower is 6.25m high, the first two stages are 1.5m high each, and the last stage is 1m high, as shown in the gas feed description. Each level has Mellapack 500 type packing supplied by Sulzer. The diameter of the absorption tower is 2.3m.

Fresh absorption solution consisting of 8.8 m ³ /h of 2M sodium carbonate at a temperature of about 60° C. is fed to the last stage of the absorption column together with circulating absorption liquid, so that the total absorption solution in the last stage of the column is about 50 m ³ /h. The pH of the absorbing solution is about 11.0, and the pH of the absorbing solution drops to about 10.2 as the solution passes through this stage due to the absorption of hydrogen sulfide. After the solution passes through this stage, it enters a 1.5m ³ collection vessel, where a 2.5M sodium hydroxide solution at a temperature of about 60°C is added for regeneration, after which the pH of the solution rises to 11.0. The regeneration solution is then pumped back to the final stage of the absorber to reabsorb the hydrogen sulfide.

About 11 m ³ /h of absorption solution flows from the last stage collection vessel into the middle stage collection vessel, whereby about 50 m ³ /h of absorption solution with a pH of about 11.0 is pumped (as in the previous stage) into the middle stage, whereupon regeneration is required The absorption solution with a pH of about 10.2 flows into the collection vessel. In the collection vessel, the solution is regenerated as in the previous stage by adding a 2.5 M sodium hydroxide solution at a temperature of about 60°C.

From the collection vessel of the middle stage, about 13.5 m ³ /h of absorption solution flows into the first (lowest) stage, whereby 50 m ³ /h of absorption solution with a pH of about 11.0 is pumped into the first stage (as stated in the description of the gas feed Show). After passing through this stage, hydrogen sulfide is absorbed and the solution (pH at this point is about 10.2) flows into the collection container. The solution is regenerated by adding 2.5M sodium hydroxide solution at a temperature of about 60°C in the collection container as in the previous stage, so that the pH of the regenerated solution is about 11.0. A total of approximately 8.6 m ³ /h of 2.5M sodium hydroxide solution is supplied to the three levels of collection vessels.

About 17.4 m ³ /h of a solution with a sulfide ion concentration of 1 mol/l is discharged from the collection vessel of the first (lowest) stage. The gas leaving the absorption tower contains 0.113 (mol)% hydrogen sulfide and 16.4 (mol)% carbon dioxide. The separation degree of hydrogen sulfide in this test is about 90%, and the selectivity of hydrogen sulfide in the separation is about 67%.

Solutions having a high sulfur content together with a high content of alkali metal carbonates, preferably sodium carbonate, which have been absorbed according to the process of the invention are particularly suitable for use in the production of white liquor in the manufacture of kraft pulp. According to the present invention, the solution leaving the absorber after absorbing hydrogen sulfide, as previously indicated, has a sulfide ion content in excess of about 0.30 moles/liter, preferably in excess of 0.47 moles/liter. The usual sulfide ion content is in the range of about 0.30-1.30 mol/liter, preferably in the range of about 0.47-1.1 mol/liter, most preferably in the range of about 0.65-1.0 mol/liter. As noted above, a suitable carbonate concentration is about 0.1-3M, preferably about 1-2.5M, most preferably about 2M.

Claims (10)

1. A method for selectively removing hydrogen sulfide from the hydrogen sulfide and carbon dioxide-containing gas (3) in black liquor evaporation by liquid absorption, characterized in that the gas (3) is mixed with the recycled carbonate-containing alkaline The solution (13, 14, 15) is subjected to multi-level (6, 7, 8) countercurrent contact, and the pH of the solution is adjusted to about 9-12 by adding hydroxide (26, 27, 28) during absorption, so that hydrogen sulfide is absorbed , the total sulfide ion content in the effluent solution in contact with the feed gas exceeds about 0.30 moles per liter. 2. The method according to claim 1, characterized in that an alkali metal hydroxide, preferably sodium hydroxide, is added. 3. The method according to claim 1 or 2, characterized in that the pH of the solution is adjusted to 10-11.5. 4. A method according to any one of the preceding claims, characterized in that hydrogen sulphide is absorbed to a sulphide ion content in the effluent solution of about 0.47 - 1.1 mol/l. 5. A method according to any one of the preceding claims, characterized in that the carbonate-containing alkaline solution has a carbonate content of about 0.1-3M. 6. A method according to any one of the preceding claims, characterized in that the gas is brought into contact with a sodium carbonate solution. 7. The method according to any one of the preceding claims, characterized in that the gas is contacted with the solution in three stages (6, 7, 8). 8. A method according to any one of the preceding claims, characterized in that the contacting of the gas with the solution is carried out under conditions of laminar liquid flow and turbulent gas flow. 9. A device for selectively removing hydrogen sulfide from the generated gas (3) containing hydrogen sulfide and carbon dioxide in black liquor evaporation by liquid absorption, characterized in that the device includes a gas inlet (2) and a gas outlet (4) container (1), containing packing (9) arranged in a plurality of successive levels (6, 7, 8), the device having means for supplying a carbonate-containing solution to the last level (see gas feed instructions), each stage has means to provide carbonate-containing solution and gas countercurrently through the stage and to circulate the solution through the stage, the device is arranged to provide solution flow from one stage to the previous stage Pipelines (29, 30) between the stages (see gas feed description), the plant has means (26, 26, 26, 27, 28) to adjust the pH of the solution to about 9-12, and discharge the liquid with a total sulfide ion content exceeding about 0.30 moles/liter from the outlet pipe (31) of the first stage (see gas feed description). 10. The device as claimed in claim 9, characterized in that it comprises three levels in which the packing (9) is in the shape of corrugated boards.

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