JPH0258966B2 - - Google Patents

Description

[Detailed description of the invention]

FIELD OF THE INVENTION This invention relates to a method for selectively removing hydrogen sulfide from a gas stream containing carbon dioxide in addition to hydrogen sulfide. More specifically, the present invention selectively removes even small amounts of hydrogen sulfide from a gas stream containing hydrogen sulfide by oxidizing the hydrogen sulfide with a polyvalent metal chelate solution to convert it to elemental sulfur. Regarding how to remove. In addition, this invention relates to the use of a special type of aeration flotation device to recover the formed sulfur particles from the metal chelate solution and to regenerate (oxidize) the reduced metal chelate solution. BACKGROUND OF THE INVENTION AND PRIOR ART The removal of hydrogen sulfide from gas streams such as waste gases generated during various chemical and industrial processes, such as wood pulping, natural gas and crude oil production, and petroleum refining processes, is It is becoming increasingly important to eliminate air pollution. Not only do hydrogen sulfide-containing gases have an unpleasant odor, such gases harm plant growth, painted surfaces and wildlife, and pose a significant threat to human health. Government regulations are constantly imposing lower limits on the content of hydrogen sulfide that can be released into the atmosphere, and are now under penalty of a complete ban on continuing operations such as plants that produce hydrogen sulfide-containing gas streams. Removal of virtually all hydrogen sulfide is an absolute necessity in many regions. The amount of hydrogen sulfide in the process gas stream is not very high. U.S. Patent No. 3,071,433 states that the flue gas obtained by concentrating black liquor, which is the waste pulping liquid of the kraft pulping method, is 500 to 2000 ppm.
This indicates that it contains hydrogen sulfide. However, the smell of hydrogen sulfide can be detected by humans at concentrations of about 0.01 ppm. Therefore, highly efficient methods of hydrogen sulfide removal are needed to remove small amounts of harmful hydrogen sulfide from process gases. Carbon dioxide may also be present along with hydrogen sulfide as a contaminant in gases such as gas from oil well drilling, flue gas, geothermal steam or tank steam. Not only is it desirable to remove H ₂ S from such gases, but it is often desirable to selectively remove hydrogen sulfide but not carbon dioxide.
One well-known technique for removing hydrogen sulfide from a gas stream involves contacting the gas stream with caustic soda, which removes acid gases from the gas stream. U.S. Pat. No. 2,747,962 describes a method for selectively removing hydrogen sulfide from acid gases that also contain carbon dioxide by using an alkaline solution, such as caustic soda, to remove acid gases such as hydrogen sulfide. .
Since absorption of CO ₂ is much slower than absorption of H ₂ S,
CO ₂ absorption is prevented by keeping the contact time between the gas stream and the lye very short (0.01-0.02 seconds). However _{, the disadvantage of this method is that H 2 S is produced when the lye is regenerated by heating it to 132 °C (270 °C), so the problem of disposing of H 2 S} has not been solved and has simply been postponed. It is nothing more than a Hydrogen sulfide may also be removed in a redox system by contacting the hydrogen sulfide-containing gas with a solution of polyvalent cations (e.g. iron) complexed with a chelating agent (e.g. ethylenediaminetetraacetic acid or its sodium salt). Known. In this way, 3
The iron in the valent state oxidizes the hydrogen sulfide to liberate sulfur, the iron is reduced to the divalent state, the chelate solution is regenerated by aeration returning the iron to the trivalent state, and the sulfur is removed by foam flotation. recovered from solution by a method. For example, U.S. Pat. No. 4,036,942 describes reacting a fluid with oxygen in the presence of a metal amino acid chelate in an aqueous solution containing an amine to convert hydrogen sulfide to sulfur and alkyl mercaptans to dialkyl sulfides, and then converting these into an aqueous solution of the metal chelate. A method for removing hydrogen sulfide and alkyl mercaptans from a fluid by separating them from a fluid is disclosed. However, the presence of oxygen in the reaction components is disadvantageous since it converts sulfur to sulfate and thiosulfate. Furthermore, the reaction described above requires a relatively long contact time between the metal chelate solution and the hydrogen sulfide-containing gas stream, and if carbon dioxide is present in the gas stream, the contact time required for the above reaction will not produce much carbon dioxide. Carbon is absorbed into the reaction solution, lowering the PH of the solution,
Decrease reaction efficiency. U.S. Pat. No. 4,009,251 (Mieury) also describes soluble alkali metal, alkaline earth metal, or ammonium or amine salts of acids (with a pK of about 1.2).
to about 6) and a metal chelate catalyst solution, the method comprises oxidizing hydrogen sulfide to sulfur without substantially producing oxidized sulfur. ing. Alkyl mercaptans are oxidized to dialkyl disulfides under the same conditions. The invention of US Pat. No. 4,009,251 attempts to prevent hydrogen sulfide from being oxidized to sulfur oxides by adding the above-mentioned acid salts. The addition of an acid salt to the metal chelate catalyst solution is necessary because Mieury recognizes that upon reacting a hydrogen sulfide-containing gas stream with oxygen, sulfur oxides are produced by such reaction mixture. Moreover, the patented process described above requires a relatively long contact time for oxidation, and the presence of carbon dioxide in the hydrogen sulfide-containing gas stream is therefore detrimental to the relatively long contact time.
Absorption of _CO2 also occurs, resulting in a decrease in the PH of the solution,
System efficiency decreases. OBJECTS OF THE INVENTION It is an object of the invention to provide an improved method for selectively removing hydrogen sulfide from a gas stream that also contains carbon dioxide together with hydrogen sulfide. SUMMARY OF THE INVENTION A gas stream containing carbon dioxide in addition to hydrogen sulfide is combined with a polyvalent metal chelate solution, and the polyvalent metal chelate solution oxidizes the hydrogen sulfide to elemental sulfur without absorbing appreciable amounts of carbon dioxide. The present invention resides in a method for selectively removing hydrogen sulfide from a gas stream containing carbon dioxide in addition to hydrogen sulfide by contacting an oxygen-containing gas without addition for a period sufficient to achieve the desired effect. An industrial foam flotation process conventionally used to separate the reduced metal chelate solution containing exhaust gases and sulfur particles from the process of this invention to recover sulfur from the solution and simultaneously separate oil from water. The apparatus is used to regenerate (oxidize) the reduced metal chelate by bubbling oxygen or oxygen-containing gas into the solution. The solid sulfur obtained is of high purity and can be sold without further chemical treatment after separation from the solution. The polyvalent metal chelate solution used in the method of this invention has the following general formula: (A) _3-o -N-(X) _o () (where n is a number from 1 to 3, and X is an acetate group and a propionic acid group. an amino acid selected from the class consisting of acid groups, where A is 2-hydroxyethyl, 2-hydroxypropyl, or an alkyl group of 1 to about 4 carbon atoms; and of the general formula: (In the formula, 2 to 4 X groups are selected from the class consisting of acetic acid groups or propionic acid groups, and zero to 2 X groups are 2-hydroxyethyl groups, 2
-Hydroxypropyl groups and groups: and R is ethylene, propylene or isopropylene, or cyclohexane or benzene in which the two hydrogen atoms replaced by nitrogen are in the 1,2-position. It is preferably a coordination complex formed by forming a chelate between one of the amino acids and a polyvalent metal. As polyvalent metals it is possible to use polyvalent metals in more than one oxidation stage, but iron, copper and manganese are preferred, with iron being particularly preferred. Polyvalent metals can reduce hydrogen sulfide to sulfur in a typical redox reaction, and at the same time the metal itself is reduced from a high valence state to a low valence state, and then from a low valence state to a high valence state by oxygen. It must be something that can be oxidized. Other polyvalent metals that can be used include lead;
These include mercury, palladium, platinum, tungsten, nickel, chromium, cobalt, vanadium, titanium, tantalum, zirconium, molybdenum and tin. Polyvalent metal chelates can be easily prepared by reacting a suitable salt, oxide or hydroxide of a polyvalent metal with a chelating agent in the acid form or its alkali metal or ammonium salt form in an aqueous solution. Can be manufactured. Examples of chelating agents are ammonia or aminoacetic acids derived from 2-hydroxyalkylamines, such as glycine, diglycine (aminodiacetic acid), NTA (nitrilotriacetic acid), 2-hydroxyalkylglycine; dihydroxyalkylglycine and hydroxyethyl- or biloxypropyldiglycine; aminoacetic acids derived from ethylenediamine, diethyletriamine, 1,2-propylenediamine and 1,3-propylenediamine, such as EDTA (ethylenediaminetetraacetic acid);
HEDTA (2-hydroxyethylethylenediaminetriacetic acid), DETPA (diethylenetriaminepentaacetic acid); aminoacetic acid derivatives of cyclic 1,2-diamines such as 1,2-diaminocyclohexane-N,N
-tetraacetic acid; and amides of polyaminoacetic acids as disclosed in US Pat. No. 3,580,950. Since the reaction proceeds quantitatively at room temperature, it is not necessary to use a temperature higher than room temperature, but a temperature higher than room temperature may be used as desired. For example, if care is taken to eliminate water loss through evaporation, high-temperature gases can be used. The metal chelate solution should be at least approx.
It is stable up to 100°C and therefore the reaction can be carried out at elevated temperatures up to at least about 100°C. The PH of the system should be within the range of about 7 to about 11. The upper limit on PH exists solely because metal chelate solutions are generally not stable at PH greater than 11. However, the oxidation of H ₂ S to sulfur is more efficient at pHs above 11, so metal chelates can be used if they are stable at pHs above 11. The most efficient range for a given set of conditions is a PH of about 8 to about 10.5. If the chelate solution is acidic, adjust the pH of the chelate solution to within an appropriate range by adding an alkali metal hydroxide such as sodium hydroxide, an alkali metal, or ammonium carbonate or ammonium bicarbonate or ammonium hydroxide. It is necessary to adjust the A gas-liquid contact system can be used that ensures good contact between the hydrogen sulfide-containing gas phase and the metal chelate-containing liquid phase. In both continuous and intermittent flow systems,
Co-current, counter-current and cross-current flow can all be used.
A preferred gas-liquid contacting system is a static mixer with internally fixed baffles, requiring no external power other than that necessary to create flow, and having no moving members. A special baffle design allows mixing or dispersion of all flowing substances with a predetermined precision.
Many static mixers of this type are commercially available. The contact time between the gas and liquid phases of the static mixer can be adjusted by adjusting the flow rate and the length of the static mixer. The method of the invention is applicable to gas streams containing hydrogen sulfide at any concentration, and even at very low concentrations of hydrogen sulfide, on the order of a few ppm. The method of the invention is particularly suitable for high concentrations (75
% or more) of carbon dioxide is present. Examples of gas streams containing hydrogen sulfide and carbon dioxide are sour gas and waste gas from oil refining, sier oil and tar sands processing, coal liquefaction waste gas, gas recovered during crude oil and natural gas production,
flue gases from cellulose pulping processes, effluent gases from sewage treatment plants, flue gases from Claus process equipment and hydrogen sulfide flue gases from other chemical and industrial processes. The following equation describes the reaction when iron chelate is used as a catalyst to convert hydrogen sulfide to elemental sulfur: Absorption: H ₂ S + 2OH ^- →S ^-- + 2H ₂ O Oxidation: 2[Fe( X)] ⁺⁺⁺ +S ^-- →S゜+2 [Fe (X)] ⁺⁺ Regeneration: 2 [Fe (X)] ⁺⁺ +1/2O ₂ +2H ⁺ →2 [Fe (X)] ⁺⁺⁺ +H ₂ O Total net reaction: H ₂ S+1/2O ₂ →S゜+H ₂ O In the above formula, X is a chelating agent. The invention will be explained in more detail below with reference to the figures. Referring now to the figures, the invention is best understood in terms of a circulatory system for removing hydrogen sulfide from a gas stream as illustrated in FIG. The circulation system shown in FIG. 1 is particularly useful in selectively removing hydrogen sulfide from hydrogen sulfide-containing gas streams that also contain large proportions of carbon dioxide. The hydrogen sulfide containing gas stream enters the system as stream 12 and enters three static mixer separator series 14, 16 and 18. Each static mixer separator linkage comprises a static mixer 20 followed immediately by a gas-liquid separator 22. PH7 or higher 2nd
Iron chelate solution static mixer separator connection device 1
4, 16, and 18 as stream 24. The contact time between the gas phase and the liquid phase in each static mixer 20 is kept below 0.1 seconds, preferably between 10 and 80 milliseconds. This contact time is adjusted by adjusting the flow rates of gas and liquid streams into each individual static mixer 20 of fixed length and inner diameter.
In the contact zone of the static mixer 20, the hydrogen sulfide in the introduced gas is instantaneously oxidized to elemental sulfur by the iron chelate solution. Solid sulfur precipitates as a slurry in the processing solution. Exhaust gas from stream 26 is removed from the top of the separator 22 of each static mixer separator linkage and sent to a second static mixer separator linkage 28, 30 and 32, respectively. These static mixer separator connection devices 28, 30, 3
2 each includes a first static mixer separator connection device 1
4, 16, 18 static mixers 20 and separators 22
It has a static mixer and separator structure equivalent to . The gas from stream 26 is contacted with the metal chelate solution from stream 24 in each second static mixer separator connection for preferably about 10 to 80 milliseconds. The exhaust gas removed from the top of each separator of static mixer separator connections 28, 30 and 32 flows through each separator 34.
This becomes stream 36 and is sent to the incinerator. Stream 38 is a bypass flow of stream 34 which supplies further exhaust gas 34 to feed stream 12 as needed to provide the necessary residence time in the static mixer. Often, the feed gas stream 12 is not at the precise volumetric flow rate to provide the residence time in the static mixer desired for selectively removing _H2S . Static mixers 14, 16 and 18 are used in parallel to provide the required gas flow through each mixer by splitting the feed stream. Valves 13, 15, 17 provide flow regulation of the feed gas flow to the desired static mixer. Liquid and solid sulfur containing reduced metal ions from each separator 22 in the first and second static mixer separator connections are routed through conduit 40 to conduit 42.
The liquid and solid sulfur in conduit 42 is then supplied to a pair of tanks 44 and 46. These tanks 44 and 46 are commonly referred to as the regeneration stage of the process of this invention. In the regeneration stage, the iron reduced during the oxidation of hydrogen sulfide to sulfur is regenerated (oxidized) to a highly oxidized state, and the sulfur particles suspended in a slurry are separated by a foam flotation method. The chelated iron contained in the liquid entering tanks 44 and 46 from conduit 42 consists of approximately 60% by weight ferric chelate and 40% by weight ferrous chelate. Tanks 44 and 4
6 consists of a number of floating tanks, and
The "WEMCO" tank sold by Envirotech Corp. is suitable. In each tank, an oxygen-containing gas such as air is carried into the liquid by the reduced pressure created during the movement of a rotor within the liquid. The oxygen-containing gas is dispersed into small bubbles which carry the suspended sulfur to the top of each tank and which oxidize the reduced iron. The regenerated solution is conveyed from tanks 44 and 46 via conduits 48 and 50, respectively, to surge tank 54 by conduit 52. The regenerated solution is returned via conduit 24 to the static mixer separator connection. Sulfur and residual metal chelate solutions from tanks 44 and 46 are transported to sulfur melter 56 via conduits 58 and 60, respectively. The molten sulfur is removed as stream 62 and any remaining iron chelate solution is returned to surge tank 54 by conduit 52. The iron chelate solution replenishment solution and the PH adjustment solution are stored in tanks 64 and 66, respectively, and supplied to the system when needed. As previously mentioned, the preferred regeneration device for use in this invention is a commercially available flotation tank sold under the tradename WEMCO. Such devices have been used to separate oil from oil and water mixtures.
It has now been discovered that this WEMCO flotation vessel can work very efficiently as a reactor for gas-liquid phase chemical reactions and can also separate solids present before and after the reaction is complete. The preferred regenerator, shown as tanks 44 and 46 in FIG. 1, consists of four flotation vessels, one of which is shown as 70 in FIG. Also, US Patent No. 3491880 and No. 3647069 disclosing this floating method device.
Please refer to the issue. Each floating tank 70 has a tank 72
, which includes an inwardly sloping lower side wall portion 74 . Each tank 70 includes a rotor 76 and a distribution plate 78 surrounding at least an upper portion of the rotor 76 and spaced from the rotor periphery. The distribution plate 78 includes a number of fluid passage holes 80 evenly spaced along substantially the entire circumferential surface of the distribution plate 78 . The rotor 76 is fixed to the bottom of the shaft 82 and is supported at a considerable distance above the bottom wall of the tank 72 for rotation about a substantially vertical axis. The rotor 76 is connected to pulleys 85 and 86 by a motor 88 supported above the top of the tank 72.
and rotated by belt 84. Standpipe 90 surrounds shaft 82 and forms a conduit for air from above the liquid level of tank 72 into the interior of tank 72 near rotor 76 . Air intake hole 92 is connected to tank 72
A perforated dispersion plate hood 94 is formed on the standpipe 90 above the liquid level of the dispersion plate 78 and is attached to the upper end of the dispersion plate 78 so as to extend downward and outward. A ferrous chelate solution containing suspended sulfur particles is introduced into tank 72 of vessel 70 . As the rotor 76 rotates, the rotational motion creates a swirl, forcing water through the holes 80 in the distribution plate 78, thus creating a vacuum in the standpipe 90. This reduced pressure draws air through the air intake hole 92 and down the standpipe 90 to be dispersed in the iron chelate solution, so that the gas (air) is well mixed with the solution. As the gas-liquid mixture moves through the distribution plate 78 at high velocity, shear forces are created and small bubbles are formed from the gas. As small air bubbles float through the metal chelate solution, sulfur particles are carried with them to the surface of the bath 70. Bubble plate 96 removes sulfur particles concentrated on the surface of the metal chelate solution. Additionally, the iron chelate can be regenerated since air or other oxygen-containing gases oxidize ferrous ions to ferric ions, and this regenerated iron chelate solution is recycled to the static mixer for use in hydrogen sulfide oxidation. be done. The WEMCO flotation bath was found to be highly effective in oxidizing ferrous chelate ions to ferric chelate ions. Figure 3 shows three types of regenerators: bubble-bubbling tanks, packed towers, and
Describe the time required to regenerate (oxidize) substantially all of the ferrous ions for WEMCO. In the figure, the x marks are the values obtained from the bubble bubbling tank, the ○ marks are the values obtained from the packed tower (continuous blowing type), and the b marks are the values obtained from WEMCO. WEMCO operates at 1200 rpm and air is used as the oxygen-containing gas dispersed in the solution. As can be seen from Figure 3, the WEMCO flotation tank regenerates the ferrous chelate solution in 1 to 2 minutes, but the bubble bubbling tank can oxidize and regenerate 87% of the total iron ions to ferric ions. It takes 50 minutes. In common applications, solutions are regenerated from a 40% ferrous state. The packed tower regeneration test is conducted in a flow system (continuous system), and various regeneration times can be achieved by changing the amount of liquid retained in the tower. The ferrous ion concentration at the inlet of the packed column was maintained at 40%. With a residence time of 10 minutes, the packed column regenerated 70% of the iron ions to the ferric state. The WEMCO floating reactor operates with the highest regeneration efficiency and has clear advantages over other regeneration equipment.
Air blowers are not required when using WEMCO tanks. The reason for this is that air is dispersed into the solution by the low pressure swirl created in the standpipe by the rotation of the rotor. Furthermore, if you use the WEMCO flotation method, sulfur will be concentrated by bubbling action,
Reduces the amount of solution that must be passed through. Effect of gas flow sucked into the liquid and air passing through the dispersion plate
The shearing action of the liquid mixture results in highly efficient regeneration (oxidation) of the reduced metal chelate. Aerators that create an air flow through the liquid without the gas-liquid shear of MEMCO are not as efficient as WEMCO in regenerating the reduced metal chelate. An example of an aeration device for oxidizing hydrogen sulfide in such a liquid is given in U.S. Pat.
Described in No. 4309285. As mentioned in Figure 2, sulfur is stored in tank 4.
The solution is collected from the surface of each of the floating baths 70 constituting 4 and 46. The sulfur particles and residual metal chelate solution are then heat treated at about 132°C (270°C) to melt the sulfur. It is advantageous to use an excess of chelating agent when the metal is in particular iron chelated with HEDTA. Excess amounts of chelating agent of 6% or more have been found to keep the iron stable in solution during gas-liquid contact, regeneration reactions, and sulfur melting so that it does not precipitate as hydroxide. Loss of iron is thus prevented. When using HEDTA as a chelating agent, the pH of the regenerated chelate solution
is about 8.8, an excess of 6 mol % is sufficient.
A greater excess of chelate solution is desirable as the pH increases since increasing pH reduces the stability of this iron chelate solution. The amount of excess depends on the chelating agent used. Therefore, other iron chelates require some excess chelate to keep the iron stable under operating conditions. Molten sulfur is 1.808
The molten sulfur is collected at the bottom of the melting vessel since it has a density of g/cc, which is significantly greater than the density of the residual chelate solution. The recovered sulfur is of high purity and can be recovered from the container and sold directly. Other separation devices such as filters or centrifuges may also be used. The inclusion of dihydroxybenzenes (hydroquinones), anthraquinones, and naphthoquinones (which represent an intermediate step between hydroquinone and dihydroxydurene) to further speed up the regeneration of ferrous chelate solutions to ferric chelate solutions. Oxidizing coagents can be added to the metal chelate solution. Since anthraquinone is water-insoluble, it is used by partially sulfonating it to make it water-soluble. 2,7 to speed up the oxidation rate of the reduced metal chelate.
- Preference is given to using disodium anthraquinone disulfonate (ADA). Of course, various isomers of anthraquinone disulfone can be used. As shown in Table 1, the regeneration (oxidation) rate of ferrous ions is accelerated by the addition of ADA. The ADA is then oxidized by the oxygen-containing gas in the regeneration stage. ADA also reacts with hydrogen sulfide to form elemental sulfur. However, the static mixer's
It has been found that the presence of ADA in the liquid contact area produces undesirable sulfur compounds.

Table: It is important that no oxygen be present in the area of contact between the polyvalent metal chelate and the hydrogen sulfide-containing gas stream to prevent the formation of sulfates, thiosulfates and sulfur oxides. This can be advantageously done by keeping the regeneration (oxidation) of the reduced iron chelate to less than 100% so that there is no excess oxygen in the regeneration solution. The invention will be explained below with reference to examples. EXAMPLE The method of this invention was used to selectively remove hydrogen sulfide from waste gas produced in an oil field. The hydrogen sulfide-containing gas stream under test was approximately 1.2% hydrogen sulfide, 88% carbon dioxide, 5% nitrogen, and hydrocarbons, expressed as volume %.
It has a composition of 5.8%. The metal chelate solution tested was ferric chloride and EDTA.
(ethylenediaminetetraacetic acid). Table 2 shows the mixture composition of the iron chelate solution used in the experiment. The pH of the solution is sodium carbonate,
Changed using sodium bicarbonate and sodium hydroxide.

[Table] Oil field waste gas supplied at a pressure of 11Kg/cm ² (25 pounds) is passed through the first trap to collect condensate, and then passed through a pressure regulator to reduce the pressure to 3Kg/cm ² (6 pounds).
dropped to The condensate formed by the pressure drop in the pressure regulator was collected by passing through a second trap and then the dried saturated gas was fed to the static mixer, and the metal chelate solution was also fed to the static mixer. Gas-liquid contact time for absorption in static mixer from 0.006 seconds by varying gas flow rate
The optimal residence time required to selectively absorb hydrogen sulfide was determined by varying it to 0.08 seconds. The PH was varied from 7.75 to 10.2 to observe the effect of PH on hydrogen sulfide absorption. Two lengths (5 cm (2 inches) and 10 cm (4 inches)) of static mixers with 0.6 cm (1/4 inch) diameter were used in the experiment. Finally 2
Iron chelate solutions with seed concentrations (0.065N and 0.13N) were used. Table 3 shows the combinations of these variables that were tested.

[Table] The results of the above experiments are shown in graphs in FIGS. 4 to 8. As can be seen from Figure 4 (iron concentration 0.065N),
As the pH increases, the absorption of hydrogen sulfide decreases with the corresponding gas/
increased relative to liquid ratio. This indicates that the rate of hydrogen sulfide absorption is primarily limited by the hydroxyl ion concentration. At high pH and high gas/liquid ratios, all ferric ions are instantaneously converted to ferrous ions, explaining that the conversion of sulfur ions to sulfur is also instantaneous. From Figure 5 (PH=8.5), the iron chelate concentration is 2
Even if doubled, the amount of hydrogen sulfide removed from the gas stream is 2
It turns out that it doesn't double. Although the higher the concentration of the iron chelate solution, the more hydrogen sulfide is removed, the conversion (H ₂ S removal) efficiency of the high iron chelate concentration solution is lower and it is not as economical as the dilute solution.
To determine an economical iron ion chelate solution concentration, pumping costs and chemical costs must be balanced against hydrogen sulfide absorption effectiveness. The liquid-gas contact time was varied to determine the optimal residence time in the static mixer required to selectively absorb hydrogen sulfide with minimal carbon dioxide absorption, and the results are shown in Figure 6. . In the figure, △ mark is a solution with iron concentration of 0.065N and PH8.5, □ mark is iron concentration
0.13N, PH8.5 solution, 〇 mark is iron concentration 0.065N, PH
The data for the solution in 10.2 are shown. FIG. 6 illustrates the dependence of carbon dioxide on the residence time of the three different solutions mentioned above. At pH 10.2, the maximum residence time required for selective absorption of hydrogen sulfide without absorption of carbon dioxide is 10 to 20 milliseconds (marked with a circle), while at pH 8.5 at the same iron concentration. In the case of the solution (△ mark), it is 40 to 80 milliseconds. Both of these solutions are identical except that more sodium carbonate is added to the higher PH solution to obtain the higher PH. Figure 6 also shows the absorption of CO ₂ by another solution (marked □) with twice the iron chelate concentration at pH 8.5. The maximum residence time for this solution for selective absorption of hydrogen sulfide without appreciable absorption of carbon dioxide is 20-30 milliseconds. This latter solution contains different amounts of sodium carbonate. Since iron chelate concentration has little or no effect on carbon dioxide absorption, the dependence of carbon dioxide absorption on residence time is based on the carbonate concentration in the solution.
Using sodium hydroxide to adjust the pH reduces the stability of the solution. When sodium hydroxide is added to an iron chelate solution in any form, the pH locally increases and iron hydroxide precipitates. Iron hydroxide is difficult to dissolve in the iron chelate solution used and very slowly returns to the solution to form a chelate complex. Study of mixing characteristics to determine whether complete mixing was achieved in the shorter mixer by doubling the static mixer length (PH of the iron chelate solution =
8.75 and the concentration was 0.065N). Figure 7 shows that doubling the length of the mixer makes no difference in hydrogen sulfide absorption; therefore, a 5.0 cm (2 inch) mixer with 6 baffle elements provides a complete It explains that mixing has been achieved. Percent hydrogen sulfide removal is a function of the amount of liquid in contact with the gas, ie, the gas/liquid ratio. As can be seen in Figure 8, the percentage of hydrogen sulfide removed from the gas stream (% _H2S removed) is an exponential function when the pH of the solution is less than 10. Thus, at a pH of 10 or above, sufficient hydrogen sulfide absorption can be maintained at more economical gas/liquid ratios. Oil field gas containing hydrogen sulfide was tested using a static mixer 5.0 cm (2 inches) in diameter and 53 cm (21 inches) long. At a flow rate of 225 MCFD of oilfield gas, the residence time in the static mixer was 15 ms. The iron chelate solution tested was supplied by the Dow Chemical Company. This iron chelate solution removes iron.
It is chelated with HEDTA (n-hydroxyethylethylenediaminetriacetic acid). This chelate solution contained 0.25% iron by weight. 106 (28 gallons) per minute of iron chelate solution in a static mixer
transported at a speed of This flow rate combined with a gas feed rate of 225 MCFD gives a contact time of 15 milliseconds at a gas/liquid ratio of 50:1. Sufficient sodium carbonate was added to the iron chelate solution to bring the pH to 9.6. As shown in Table 4, the pH of the solution decreased to 8.5 after 21 hours of use. PH
Additional carbonate is added to bring the value back to 9.0, and 11
After time use, the solution stabilized at PH 8.8, and at the same time a 98% H ₂ S removal rate from the gas stream was achieved. The effect of PH on the hydrogen sulfide removal rate is summarized in a graph in Figure 9.

【table】

[Table] As can be seen from the foregoing, the present invention combines a gas stream containing even a large proportion of carbon dioxide together with hydrogen sulfide into a polyvalent metal chelate solution and hydrogen sulfide without appreciable absorption of CO ₂ . It is possible to selectively remove hydrogen sulfide from the gas stream by contacting it for a sufficient period of time to allow absorption. Absorption of CO ₂ is disadvantageous because it lowers the pH. As can be seen from FIG. 4, even if the pH of the polyvalent metal chelate solution is slightly lowered, the absorption of hydrogen sulfide is significantly reduced. The efficiency of the method of this invention decreases significantly with decreasing PH. Preferably, the contact time between the hydrogen sulfide-containing gas and the polyvalent metal chelate solution is limited to about 80 milliseconds (0.08 seconds) or less. Otherwise, absorption of carbon dioxide by the chelate solution will occur, lowering the pH of the solution.

[Brief explanation of drawings]

FIG. 1 is a schematic explanatory diagram of a process for removing hydrogen sulfide from a gas stream according to a method according to an embodiment of the present invention; FIG. 2 is a partial vertical sectional view of a regenerator used in the process shown in FIG. 1; Figure 3 is a graph showing the relationship between the regeneration rate (%) of trivalent iron and the regeneration time when the regeneration equipment shown in Figure 2 is compared with other types of regeneration equipment. Figure 5 is a graph illustrating the change in hydrogen sulfide removal rate (%) from the PH of the chelate solution and gas flow; Figure 5 is a graph illustrating the effect of iron concentration in the iron chelate solution on the hydrogen sulfide removal rate (%). Figure 6 is a graph explaining the influence of the residence time of the gas flow in the static mixer on the carbon dioxide absorption rate when an iron chelate solution is used, Figure 7 is the graph showing the effect of the length of the static mixer Figure 8 is a graph explaining the effect of gas-liquid ratio on hydrogen sulfide removal rate, Figure 9 is a graph explaining the effect of PH change on hydrogen sulfide removal rate. It is a figure which shows the graph explaining the effect on rate (%). In the figure: 12...Hydrogen sulfide-containing gas flow (supply gas flow), 14, 16, 18... (first) static mixer separator connection device, 20... static mixer, 22...
Gas-liquid separator, 24... (of ferric chelate solution)
Stream or conduit, 26... Stream [from (first) static mixer separator linkage] 28,3
0, 32...(second) static mixer separator connection device, 34...exhaust gas from the second static mixer separator connection device separator, 36...stream of the exhaust gas 34 sent to the incinerator, 38...exhaust gas 34 bypass flow, 40, 42... (reduced metal ion containing liquid and solid sulfur transport) conduit, 44, 46... tank (regeneration stage), 48, 50... (regeneration solution) conduit, 52... Conduit, 54... Surge tank, 56
... sulfur melter, 58, 60 ... (sulfur + residual chelate transport) conduit, 62 ... molten sulfur flow,
64... Tank for replenishing iron chelate solution, 66...
PH adjustment solution tank, 70...flotation tank (flotation tank or tank), 72...tank, 74...lower side wall section, 76...rotor, 78...dispersion plate, 80...
...hole, 82...shaft, 84...belt, 8
5, 86...Pulley, 88...Motor, 90...
Standpipe, 92... Air intake hole, 94... Hood,
96... Foam board.

Claims (1)

[Claims] 1. A gas stream containing carbon dioxide in addition to hydrogen sulfide is mixed with a polyvalent metal chelate solution, and the polyvalent metal chelate solution oxidizes the hydrogen sulfide without absorbing appreciable amounts of carbon dioxide. for a period sufficient to form elemental sulfur;
A method for selectively removing hydrogen sulfide from a gas stream containing carbon dioxide in addition to hydrogen sulfide by contacting without addition of an oxygen-containing gas. 2. The method according to claim 1, wherein the polyvalent metal is iron. 3. The method according to claim 1 or 2, wherein the chelate is ethylenediaminetetraacetic acid. 4. The method according to claim 1 or 2, wherein the chelate is N-hydroxyethyl-ethylenediaminetriacetic acid. 5 Contact time between gas flow and metal chelate is 0.006
5. A method according to any one of claims 1 to 4, wherein the time is between 0.08 seconds and 0.08 seconds. 6. The method according to any one of claims 1 to 5, wherein the polyvalent metal chelate solution has a pH of 7 to 11. 7. The method according to any one of claims 1 to 6, wherein the polyvalent metal chelate solution that is brought into contact with the hydrogen sulfide-containing gas contains a buffer that maintains the pH of the solution at 8.5 to 9.0. 8. The method of claim 2, wherein the chelate in solution is present in at least 6 mole percent excess. 9. A gas stream containing carbon dioxide in addition to hydrogen sulfide is treated with a polyvalent metal chelate solution, and the polyvalent metal chelate solution oxidizes the hydrogen sulfide to elemental sulfur without absorbing appreciable amounts of carbon dioxide. selectively removes hydrogen sulfide from a gas stream containing carbon dioxide in addition to hydrogen sulfide, after which the used polyvalent metal chelate solution is regenerated by passing an oxygen-containing gas through the solution. How to remove. 10. The method according to claim 9, wherein the polyvalent metal is iron. 11. The method according to claim 9 or 10, wherein the chelate is ethylenediaminetetraacetic acid. 12. The method according to claim 9 or 10, wherein the chelate is N-hydroxyethyl-ethylenediaminetriacetic acid. 13 The contact time between the gas flow and the metal chelate is
Claim 9 between 0.006 seconds and 0.08 seconds
The method according to any one of Items 1 to 12. 14. The method according to any one of claims 9 to 13, wherein the polyvalent metal chelate solution has a pH of 7 to 11. 15 Passing an oxygen-containing gas through the solution and dispersing the gas in the solution by forming a low pressure zone below the surface of the polyvalent metal chelate solution and directing the oxygen-containing gas into the low pressure zone. The method according to any one of claims 9 to 14, wherein the elemental sulfur particles are suspended and recovered. 16. The method according to any one of claims 9 to 15, wherein the solution is separated from the sulfur particles, the sulfur particles are melted, and the molten sulfur is separated from the residual solution. 17. The method of claim 7, wherein less than 100% of the polyvalent metal chelate in the polyvalent metal chelate solution used is regenerated so that excess free oxygen is not present in the regeneration solution. 18. The method according to any one of claims 9 to 17, wherein an oxidation aid is added to the solution in order to increase the regeneration efficiency of the polyvalent metal chelate. 19. The method of claim 18, wherein said additive comprises an oxidizing agent selected from the group consisting of hydroquinone, anthraquinone, naphthoquinone and mixtures thereof. 20. The method of claim 10, wherein the chelate in solution is present in at least 6 mole percent excess. 21 Claims 9 to 2, wherein the polyvalent metal chelate solution brought into contact with the hydrogen sulfide-containing gas contains a buffer to maintain the pH of the solution at 8.5 to 9.0.
The method according to any of item 0.