Method and composition for the removal of hydrogen sulfide and carbon dioxide from gaseous streams

Abstract

A method of removing hydrogen sulfide and carbon dioxide from a sour gaseous stream in a contact zone by contacting the sour gaseous stream with a lean CO.sub.2 -selective absorbent in an aqueous alkaline solution containing a polyvalent metal chelate at a pH of about 7 to about 10 wherein the polyvalent metal chelate is in one embodiment of the invention in the reduce or lower valence state in a contact zone and is oxidized to the oxidized or higher valence state in an oxidation zone so as to convert hydrosulfide and/or sulfide salts present in the contact zone to sulfur. In a second embodiment, the aqueous alkaline solution and CO.sub.2 -selective absorbent contact in the gaseous stream is a mixture containing a minor amount of higher valence polyvalent metal chelate together with a major amount of a lower valence polyvalent metal chelate.

Description

BACKGROUND OF THE INVENTION

(1) Field of the Invention

The present invention relates to the removal of carbon dioxide and hydrogen sulfide from gaseous streams utilizing a CO.sub.2 selective absorbent in admixture with an aqueous alkaline solution containing a chelated polyvalent metal.

(2) Description of the Prior Art

In U.S. Pat. No. 4,091,073, the use of an absorbent for carbon dioxide and preferably for hydrogen sulfide as well, is taught in a process for removal of hydrogen sulfide from gaseous streams by contact with a polyvalent metal chelate. The use of carbon dioxide absorbents in a process for the removal of hydrogen sulfide from a gaseous stream is also taught in U.S. Pat. No. 4,518,576 and U.S. Pat. No. 4,368,178.

The removal of hydrogen sulfide and carbon dioxide from sour gaseous streams is disclosed in U.S. Pat. No. 4,402,930 by scrubbing the gaseous streams with an aqueous solution containing a CO.sub.2 absorbent and a polyvalent metal chelate in a higher valence state and oxidizing said chelate in a regeneration zone. It is known in the prior art that iron in the ferric state acts as a catalyst for the oxidation of ethylenediamine tetraacetic acid in aqueous solutions from Motekaitis, et al. Canadian Journal Of Chemistry, volume 58, No. 19, Oct. 1, 1980.

One of the main disadvantages of the processes for removing hydrogen sulfide from gaseous streams utilizing polyvalent metal chelates is the instability of the chelating agent under the process conditions. In order to overcome the instability of the chelating agents, particularly those complexed with polyvalent metal ions such as iron, the prior art has taught the use of mixtures of certain chelating agents. In U.S. Pat. No. 4,421,733 and U.S. Pat. No. 4,455,287, methods are disclosed for reducing the instability of polyvalent metal chelating agents under the reaction conditions in which these agents are utilized to remove hydrogen sulfide from gaseous streams. In U.S. Pat. No. 4,421,733, the use of a stabilizing amount of bisulfite ion is suggested and in U.S. Pat. No. 4,455,287, the use of a biocide is suggested as means of stabilizing a polyvalent metal chelate for use in the removal of hydrogen sulfide gas from a fluid stream. In U.S. Pat. No. 3,068,065 there is disclosed a process for the removal of hydrogen sulfide from gaseous streams by washing the gas stream with a solution containing an iron chelate wherein the iron is present in the chelate in the ferric state.

British Pat. No. 999,800 issued July 28, 1965 to Humphreys and Glasgow Ltd. teaches the benefit of employing a high proportion of a polyvalent metal chelate in the reduced valence state in conjunction with a polyvalent metal chelate in the oxidized or higher valence state, to reduce degradation of the chelating agent in a process for the removal of hydrogen sulfide from a gas. The gaseous stream is contacted with an aqueous solution containing iron complexed with an amino polycarboxylic acid in which the iron is a mixture of the higher and lower valence state. The hydrogen sulfide gas is converted to sulfur by contact with the iron chelating agent in which the iron is present in the higher oxidation state. In turn, the iron is reduced to the lower oxidation state. Subsequently, the iron is converted from the lower oxidation state to the higher oxidation state in an oxidation zone and it is at this point, that, as the iron chelating agent is exposed to oxidation, there results a progressive loss of the chelating agent from the aqueous solution. Precipitation of insoluble iron compounds occurs as the result of the decomposition of the iron chelate. The British Patent teaches the controlled, oxidative regeneration of the iron chelate so as to prevent localized, intensive, oxidative decomposition of the chelating agent. Generally from 15 to 75% by weight of the total iron present in the iron chelate solution can be ferrous iron with the preferred proportion of ferrous iron chelate remaining in the solution after regeneration being between 20 and 50% by weight, based upon the total iron chelate present in said solution.

There is no suggestion in any of these prior art references for the use of a polyvalent metal chelate in a contact zone, particularly an iron chelate wherein all the iron is present in the chelate as the reduced state of the metal. In addition, there is no suggestion for the use of mixed higher and lower valence state polyvalent metal chelates wherein the amount in moles of said chelate present in the lower valence state is greater than about 5 times the amount present of the higher valence polyvalent metal chelate. After hydrogen sulfide is absorbed in the process of the invention in a contact zone by contacting a gaseous stream with an aqueous alkaline solution and converted to hydrosulfide and/or sulfide ions, some or all of these ions may be converted in the contact zone to elemental sulfur by reaction with any iron chelate which may be present in the higher valence state. The remainder of these ions are converted in an oxidation zone to elemental sulfur. The conversion is carried out in the oxidation zone by contact with an iron chelate present in a higher valence state only in at least an effective amount.

SUMMARY OF THE INVENTION

In accordance with the present invention, it has now been found that hydrogen sulfide and carbon dioxide can be removed from a sour gaseous stream by contacting said stream with a lean CO.sub.2 -selective absorbent in admixture with an aqueous alkaline solution containing at least one polyvalent metal chelate, especially a chelate wherein the polyvalent metal is iron, can be utilized in a continuous process for the liquid phase oxidation of hydrogen sulfide in a gas stream to elemental sulfur without substantial oxidative degradation of the chelating agent. In one embodiment of the process of the invention, a polyvalent metal chelate is present in all or substantially all in the lower valence state of the metal in a contacting zone together with an aqueous alkaline solution. The hydrogen sulfide in the gas stream is converted to the hydrosulfide and/or sulfide by the aqueous alkaline solution. In a second embodiment of the invention, a polyvalent metal chelate in the higher valence state, is present in the contact zone in an amount at least equal to about the stoichiometric amount in moles required to convert the hydrogen sulfide present to sulfur, provided the amount in moles of said lower valence polyvalent metal in said chelate is greater than about 5 times the amount of said higher valence polyvalent metal. In each of the embodiments of the invention, hydrosulfide and/or sulfide not converted to elemental sulfur in the contact zone are thereafter reacted in an oxidation zone wherein the lower valence polyvalent metal chelating agent from the contact zone is oxidized to the higher valence state in an effective amount in order to complete the oxidation of said hydrosulfide and/or sulfide to elemental sulfur. The carbon dioxide which is absorbed can be removed by several different methods, the choice of which method is mainly dependent upon the amount of carbon dioxide present and whether the CO.sub.2 is to be recovered. Preferably when the carbon dioxide is present in an amount less than 5%, it is stripped from the oxidation zone by the stripping action of the oxygen-containing gas utilized in the oxidation of the iron chelate. The carbon dioxide exits the process together with the vent gases from the oxidation zone. If it is desired to recover carbon dioxide from the process, this can be done in a carbon dioxide removal zone in which another vessel is placed to receive it between the contact zone and the oxidation zone where by stripping, reducing pressure, and heating, if necessary, the carbon dioxide is removed from the process. Removal of carbon dioxide at this point in the process is more efficient because of the lower pH of the aqueous scrubbing solution at this point in the process. The pH rises in the oxidation zone as hydroxyl ions are formed. Various other methods known to those skilled in the art can be used to remove carbon dioxide from the aqueous solution in the above described carbon dioxide removal zone.

Simultaneously with the removal of hydrogen sulfide in a contact zone from a gaseous stream, carbon dioxide is removed by the presence of a lean carbon dioxide absorbent in the aqueous scrubbing solution of the contact zone comprising an alkali and a polyvalent metal chelate. The carbon dioxide absorbent may also be a solvent for hydrogen sulfide as well as carbon dioxide, thus improving the efficiency of hydrogen sulfide removal. This is beneficial especially when an iron chelate is present in the scrubbing solution of the contact zone in the reduced or lower valence state.

BRIEF DESCRIPTION OF THE DRAWINGS

In FIGS. 1 and 2 there are shown two embodiments of the process of the invention in schematic form.

In FIG. 1, a separate contact zone 10 and oxidizing zone 18 is shown.

In FIG. 2, the zone 50 functions as both a contact zone and an oxidizing zone.

DETAILED DESCRIPTION OF THE INVENTION

A process is disclosed for the removal of hydrogen sulfide and carbon dioxide from a sour gaseous stream by contacting said stream with a lean CO.sub.2 selective absorbent in admixture with an aqueous alkaline solution containing at least one polyvalent metal chelate. When a portion of the polyvalent metal chelate is present in the higher valence state of the metal, some or all of the hydrogen sulfide is converted in the contact zone to elemental sulfur. Any hydrogen sulfide remaining is converted to hydrosulfide and/or sulfide by the aqueous alkaline scrubbing solution. In one embodiment of the invention, when the polyvalent metal chelate is present in all or substantially all in a lower valence state, the alkalinity of the scrubbing solution is used to absorb the hydrogen sulfide from the sour gaseous stream and convert it to hydrosulfide and/or sulfide. Certain carbon dioxide solvents which also absorb hydrogen sulfide also aid in the removal of hydrogen sulfide from the gaseous stream. The contact zone, in one embodiment of the invention, can contain an amount up to equal to or greater than a stoichiometric amount of the polyvalent metal chelate in the higher valence state of the metal which is required to convert the hydrogen sulfide present to sulfur. However, the lower valence polyvalent metal chelate present must be present in an amount in moles greater than about 5 times the amount of higher valence state polyvalent metal chelate present. The molar amount of polyvalent metal chelate present in the lower valence state is preferably greater than about 10, and most preferably greater than about 30 times the amount of polyvalent metal chelate present in the higher valence state.

The process of the invention is operated in one embodiment of the invention in a manner contrary to the teachings of prior art processes for hydrogen sulfide removal wherein the polyvalent metal of the polyvalent metal chelate is present in the contact zone in all or substantially all in an oxidized, or higher valence state. The polyvalent metal chelate, when present in the contact zone of the process of the invention in all or substantially all of the lower valence state polyvalent metal, is ineffective in converting hydrogen sulfide, hydrosulfide and/or sulfide to elemental sulfur in the contact zone but is believed to act as a scavenger for oxygen radicals which are considered to be responsible for the degradation of the chelating agent. Upon oxidation of the lower valence state polyvalent metal chelate to the higher valence state in an oxidation zone, the polyvalent metal chelate becomes effective to convert hydrosulfide and/or sulfide to sulfur. The hydrosulfide and/or sulfide formed in the contact zone by reaction of the hydrogen sulfide with the aqueous alkaline solution and the hydrogen sulfide absorbed, if any, by the carbon dioxide solvent is thus oxidized to sulfur in the oxidation zone. In this embodiment of the invention, at least an effective amount of polyvalent metal chelate in an oxidizing or higher valence state is present in the oxidation zone. Said effective amount is defined as at least about the stoichiometric amount required to convert the hydrogen sulfide present in the contact zone to sulfur and preferably up to about 5 to about 10 mole percent in excess thereof. When the higher valence state polyvalent metal chelate concentration in the contact zone of the process is zero, absorption of hydrogen sulfide is obtained by the formation of hydrosulfide and/or sulfide in the presence of the aqueous alkaline solution present in the contact zone of the process.

It is known in the prior art that polyvalent metal chelating agents, particularly those in the class of polyamino polycarboxylic acids, are subject to oxidative decomposition with precipitation of insoluble iron compounds as the chelating agent is decomposed. The decomposition of the polyamino carboxylic acid portion of the chelating agent is known to be accelerated in the presence of iron ions in the higher valence state. Such decomposition is discussed in British Pat. No. 999,800 and in the Canadian Journal Of Chemistry Vol. 58 No. 19 for Oct. 1, 1980 Motekaitis et al-The Iron (III)--Catalized Oxidation Of EDTA In Aqueous Solution. The available evidence indicates that chelate degradation occurs through several mechanisms, the most important likely involving oxygen radicals. Maximizing the proportion of ferrous iron (II) to ferric iron (III) in the process of the invention has been found to minimize chelate degradation.

In another embodiment of the invention, at least about a stoichiometric amount of a chelate in the higher valence state of the metal is present in the contact zone in order to convert some or all of the hydrogen sulfide to sulfur so as to achieve a greater H.sub.2 S absorption rate. The presence in the contact zone of higher valence state chelate is important, especially where the process is operated at the lower end of the pH range, in order to provide a more economical solution flow rate. Recirculation from the oxidation zone to the contact zone of the process of the invention of at least about said stoichiometric amount of higher valence state polyvalent metal chelate can be used.

For example, when a ferrous iron chelate is oxidized to the ferric iron chelate in the oxidation zone in an effective amount, which is sufficient to oxidize the remaining hydrosulfide and/or sulfide present in the aqueous alkaline solution fed to the oxidation zone from the contact zone, the oxidative degradation of the chelating agent in the contact zone is substantially avoided. This is accomplished by the control of oxidizer conditions so as to keep the presence therein of the iron chelate in the higher valence state to a minimum while maintaining a large excess of iron chelate in the lower valence state.

In one embodiment of the process of the invention, hydrogen sulfide is absorbed from the gaseous phase of a sour gaseous stream in a contact zone primarily by reaction of hydrogen sulfide with hydroxide ion present in an aqueous solution containing a base. The carbon dioxide which may be present together with the hydrogen sulfide is absorbed by a lean carbon dioxide selective absorbent. Some carbonate and bicarbonate may be formed by reaction of carbon dioxide with hydroxide ion. Hydrosulfide and sulfide are formed. The iron chelating agent present in the aqueous solution is present in this embodiment of the invention in all or substantially all in the reduced or lower valence state rather than as is conventional in the oxidized or higher valence state. Even in a second embodiment of the invention in which the higher valence state iron chelate is present in the contact zone in an amount at least equal to about the stoichiometric amount required to convert the hydrogen sulfide present to sulfur, the absorptivity of the aqueous alkaline solution is reduced over that of prior art aqueous absorption solutions containing a major amount of the polyvalent metal chelate in the higher valence state in comparison with the lower valence state polyvalent metal chelate present. Thus, some of the hydrogen sulfide is not oxidized to sulfur. To compensate for the reduced absorption capacity of the aqueous alkaline solution, the process of the invention provides for the use of a water soluble carbon dioxide selective absorbent which acts primarily to absorb carbon dioxide and secondarily to absorb hydrogen sulfide.

After absorption of carbon dioxide and hydrogen sulfide in the contact zone, the alkaline solution is fed to an oxidation zone wherein absorbed hydrosulfide and sulfide is exposed to a large volume of an oxygen containing gas such as air, with or without additional heating, so as to produce an effective amount of the polyvalent metal chelate in an oxidizing or higher valence state in order to produce by reaction with the hydrosulfide and sulfide a sulfur product and a polyvalent metal chelate in a reduced or lower valence state. In one embodiment of the process of the invention wherein the polyvalent metal chelate is present in all or substantially all in the lower valence state in the contact zone of the process, the formation of a sulfur product occurs primarily in the oxidation zone. In a second embodiment of the invention wherein the polyvalent metal chelate is present in the contact zone in an amount at least equal to about the stoichiometric amount required to convert to sulfur the hydrogen sulfide present in the contact zone, the oxidation zone of the process is used primarily to regenerate the polyvalent metal chelate so as to convert it from the reduced or lower valence state to the oxidized or higher valence state for recycling to the contact zone of the process as well as to strip carbon dioxide from the absorbent mixture and regenerate the CO.sub.2 selective absorbent for recycling to the contact zone of the process. Carbon dioxide may also be removed in a carbon dioxide removal zone located either before or after the oxidation zone in the process.

The lean carbon dioxide selective absorbent can be either a physical solvent or a regenerable chemical solvent but a physical solvent is preferred. The vapor pressure of the solvent should be low enough so that it is not stripped from the solution in substantial amounts during the contact stage of the process or during the oxidation stage of the process. The carbon dioxide solvent can be either an organic or an inorganic solvent or a solvent which, in combination with the aqueous alkaline solution, increases the solubility of the combined solution with respect to carbon dioxide. Non limiting examples of suitable carbon dioxide solvents are as follows: dimethylformamide, dimethylsulfoxide, diethylene glycol monoethyl ether, polyethers of ethylene and propylene glycols, tetraethyleneglycol dimethyl ether, sulfolane, N-methyl pyrrolidone, 2,4-pentanedione, 2,5-hexanedione, cyclohexanone, methylisopropyl ketone, propylene carbonate, and glycerine acetate. The particular carbon dioxide absorbent chosen is a matter of choice given the qualifications that the carbon dioxide solvent must not affect the activity of the polyvalent metal chelate and the carbon dioxide solvent must increase the absorption of carbon dioxide by the aqueous alkaline solution.

DETAILED DESCRIPTION OF THE DRAWINGS

Referring to one embodiment of the process of the invention illustrated in FIG. 1 of the drawings, a sour gas is introduced through line 32 into a venturi scrubber 34 so as to mix with a polyvalent metal chelate alkaline solution containing a carbon dioxide solvent which enters scrubber 34 through line 29 which is fed by line 26 from pump 24. The gas and liquid mixture passes into bubble tower contact zone 10 for further contact. A gas essentially free of hydrogen sulfide and carbon dioxide leaves bubble tower 10 through line 12 and polyvalent metal chelate solution in admixture with absorbed CO.sub.2, hydrogen sulfide, hydrosulfide and/or sulfide and sulfur passes through line 14 to oxidation zone 18. Air or other oxygen containing gas is fed to oxidation zone 18 through line 20 and is distributed within oxidation zone 18 by means of sparging apparatus 21. Spent air or other oxygen containing gas and CO.sub.2 are vented through line 16. The lower valence state metal in the polyvalent metal chelate solution present in oxidation zone 18 is oxidized to the higher valence state of the metal to provide at least an effective amount to convert the remaining sulfide and/or hydrosulfide present therein to sulfur. In the oxidation zone, the CO.sub.2 is stripped from the mixture and the amount of oxidation of said polyvalent metal chelate is controlled so as to preferably provide an excess of at least the stoichiometric amount of polyvalent metal chelate in the higher valence state needed to convert to sulfur the hydrosulfide and/or sulfide present in said contact zone 10. The polyvalent metal chelate solution comprising sulfur and all or substantially all of the polyvalent metal chelate in the reduced or lower valence state exits oxidation zone 18 through line 22 and is pumped by means of pump 24 through line 26 into line 29 and thence to the venturi scrubber 34. A bypass is shown through sulfur removal zone 28 by way of line 27 for removal of at least a portion of sulfur in a sulfur recovery zone. Sulfur is removed from the system through line 30. The polyvalent metal chelate solution exits sulfur removal zone 28 through line 29, is joined by line 26, and is recycled thereafter to venturi scrubber 34 and then to contact zone 10.

Referring to FIG. 2 which is particularly suited for the removal of hydrogen sulfide and carbon dioxide from waste gases, there is shown another embodiment of the invention in which a combined contact and oxidation zone 50 is fed through line 60 with an oxygen containing gas such as air which is distributed within said zone 50 by sparging apparatus 52. Sour gas is fed through line 58 into said zone and distributed therein through sparging apparatus 54. A mixture of gases containing CO.sub.2, spent air or other oxygen containing gas and essentially free of hydrogen sulfide is discharged through vent 72. A small amount of the polyvalent metal chelate solution is removed for sulfur recovery through line 56 by means of pump 62 and passes through line 64 to sulfur removal zone 68 from which sulfur is removed through line 66. The polyvalent metal chelate solution is returned to the contact/oxidation zone 50 through line 70.

In one embodiment of the process of the invention, hydrogen sulfide is absorbed from the gaseous phase in a contact zone by reaction with hydroxide ion present in an aqueous alkaline solution. Hydrosulfide and/or sulfide are formed. Carbon dioxide is absorbed by the lean carbon dioxide absorbent. Some carbonate and bicarbonate can also be formed by reaction with the hydroxide ion present in the aqueous alkaline solution. All or substantially all of the polyvalent metal chelate can be present in the contact zone in the reduced or lower valence state of the metal. Thus, when the oxidized polyvalent metal chelate is present in the contact zone in less than the stoichiometric amount needed to convert all the hydrogen sulfide present to sulfur, there is a reduction in absorptivity of the aqueous alkaline solution since oxidation of the hydrosulfide and/or sulfide does not take place so as to produce water insoluble sulfur.

The absorption capacity of the contact zone aqueous alkaline soultion need not be reduced in the process of the invention where the higher valence state polyvalent metal chelate is present in at least the stochiometric amount required to convert the hydrogen sulfide present to sulfur. Excessive chelate degradation is avoided provided the molar amount of polyvalent metal chelate present in the lower valence state of the metal is generally greater than about 5 times the molar amount of polyvalent metal chelate present in the higher valence state of the metal. To increase the absorption capacity of the aqueous alkaline solution, one embodiment of the process of the invention provides for the use of up to at least a stoichiometric amount, based upon the hydrogen sulfide present in the contact zone, of the polyvalent metal chelating agent in the higher valence state. In addition, the use of a carbon dioxide absorbent, which may absorb hydrogen sulfide, improves the absorption capacity of the aqueous alkaline solution. The aqueous alkaline solution is thereafter removed from the contact zone and sent to the oxidation zone wherein an effective amount of polyvalent metal chelate in an oxidizing or higher valence state is produced, said amount being at least a stoichiometric amount required to produce by reaction of said chelate in said higher valence state with said hydrogen sulfide, hydrosulfide and/or sulfide, a lean carbon dioxide absorbent, a sulfur product, and a polyvalent metal chelate product in a reduced or lower valence state.

In order to convert the polyvalent metal chelating agent from the lower valence state to the higher valence state, in which it is effective as a reactant for the oxidation of hydrosulfide and/or sulfide, the polvalent metal chelate can be exposed to an oxygen containing gas such as air so as to promote the oxidation process. Control of the amount of air introduced in the oxidation zone allows an effective amount of polyvalent metal chelate to be present in the higher valence state or oxidized state which is sufficient to function as a reactant in the oxidation of hydrogen sulfide, hydrosulfide, and/or sulfide to elemental sulfur. The polyvalent metal chelate is simultaneously reduced to the lower valence state. Carbon dioxide is preferably stripped from the aqueous alkaline solution by the air or other oxygen containing gas in the oxidation zone and removed from the process. If carbon dioxide recovery is desired, it is preferably recovered prior to the oxidation zone of the process. Thereafter the sulfur is separated in a sulfur recovery zone by conventional separation processes such as filtration, flotation, and the like and the residual aqueous alkaline solution, containing said carbon dioxide absorbent and all or substantially all of said polyvalent metal chelate in the reduced or lower oxidation state, is returned to the contact zone.

The particular type of gaseous stream treated is not critical, as will be evident to those skilled in the art. Streams particularly suited to removal of H.sub.2 S by the practice of the invention are naturally-occurring gases, synthesis gases, process gases, and fuel gases produced by gasification procedures, e.g., gases produced by the gasification of coal, petroleum, shale, tar sands, etc. Particularly preferred are natural gas streams, waste gases, refinery feedstocks composed of gaseous hydrocarbon streams, coal gasification streams, and other gaseous hydrocarbon streams. The term "hydrocarbon stream(s)", as employed herein, is intended to include streams containing significant quantities of hydrocarbon (both paraffinic and aromatic), it being recognized that such streams contain significant "impurities" not technically defined as a hydrocarbon. Streams containing principally a single hydrocarbon e.g., methane, are eminently suited to the practice of the invention. Streams derived from the gasification and/or partial oxidation of gaseous or liquid hydrocarbon may be treated by the invention. The H.sub.2 S and CO.sub.2 content of the type of streams contemplated will vary extensively, but, in general, will range from about 0.1 percent to about 10 percent by volume. The amount of H.sub.2 S and CO.sub.2 present in the gaseous stream is not generally a limiting factor in the practice of the invention.

Temperatures employed in the contact zone wherein hydrogen sulfide is absorbed utilizing an aqueous alkaline solution containing a carbon dioxide absorbent and a polyvalent metal chelate are not generally critical, except that the reaction is carried out at a temperature below the melting point of sulfur and the process temperature should be low enough to prevent loss of significant amounts of carbon dioxide absorbent from the system. Generally, the operating range temperature is from about 10.degree. centigrade to about 90.degree. centigrade. The preferred temperature range is from about 25.degree. to about 50.degree. centigrade and the most Preferred range is about 20.degree. to about 40.degree. centtigrade. Contact times in the contact zone can generally range from about 1 second to about 270 seconds or longer, preferably about 2 seconds to about 120 seconds, and most preferably about 2 seconds to about 60 seconds.

In the oxidation zone, temperatures are not generally critical and can vary widely. Preferably, the oxidation zone should be maintained at substantially the same temperature as the contact zone wherein hydrogen sulfide is absorbed by an aqueous alkaline solution. Where heat is utilized to assist the oxidation of the hydrosulfide, and/or sulfide to elemental sulfur, cooling of the aqueous alkaline solution is not required before return of said solution to the contact zone although it is preferred that the contact zone be cooler to increase the rate of hydrogen sulfide and carbon dioxide absorption. In general, the temperatures in the oxidation zone are similar to those utilized in the contact zone. The preferred and most preferred temperatures are also similar. Pressure conditions in the contact zone and the oxidation zone can vary widely. The range of operating pressure in these zones is generally about atmospheric pressure to about 100 atmospheres. The preferred pressure is about atmospheric pressure to about 20 atmospheres and the most preferred pressure is about atmospheric to about 3 atmospheres. At high pressures, the liquifaction or absorption of hydrocarbon components of the feed gas can take place. The pressure-temperature relationships involved are well understood by those skilled in the art and need not be detailed here.

The process operating range for pH is generally about 7 to about 10. The preferred range is about 7 to about 9 and the ost preferred range of pH is from about 8 to about 9. In general, operation at the highest portion of the range is preferred in order to operate at a high efficiency of hydrogen sulfide absorption. Since the hydrogen sulfide is an acid gas, there is a tendency for the hydrogen sulfide to lower the pH of the aqueous alkaline solution. The optimum pH also depends upon the stability of the particular polyvalent metal chelate chosen. The ability of the amino acid portion of the polyvalent metal chelate to protect the metal from precipitation as an insoluble sulfide or hydroxide at high pH values will determine how high in pH the aqueous alkaline solution can be used. At pH values below 6, the efficiency of hydrogen sulfide absorption is so low so as to be impractical. At pH values greater than 10, for instance, with iron as the polyvalent metal, the precipitation of insoluble iron hydroxide may occur resulting in decomposition of the iron chelate.

The minimum effective amount of polyvalent metal chelate in the higher valence state which is released in the oxidation zone in one embodiment of the invention is at least an amount sufficient to convert all of the hydrogen sulfide present in the gas stream fed to the contact zone in the process of the invention. The maximum effective amount is generally about 10 mole percent, preferably about 5 mole percent, and most preferably about 2 mole percent in excess of the required stoichiometric amount.

In an embodiment of the invention where greater than the required amount of polyvalent metal chelate in the higher valence state is released in the oxidation zone and recirculated to the contact zone, the lower valence polyvalent metal chelate is maintained at a concentration of greater than about 5 times the molar amount of said chelate present in the higher valence state. In this embodiment of the process of the invention, the amount of polyvalent metal chelate in the higher valence state which is present in the oxidation zone is controlled so as to form an amount of higher valence polyvalent metal chelate equal to or in excess of that required for conversion of the hydrogen sulfide, hydrosulfide, and/or sulfide to sulfur.

Any oxidizing polyvalent metal chelate can be used but those in which the polyvalent metal is iron, copper, and manganese are preferred, particularly iron. Other useful metals which can provide the polyvalent metal of the polyvalent metal chelate are generally those that are capable of undergoing a reduction/oxidation reaction, that is, those metals capable of being reduced to a lower valence state by reaction with hydrosulfide and/or sulfide ions and which can be regenerated by oxidation with an oxygen containing gas to a higher valence state. Specific examples of useful metals include, besides the preferred metals listed above, nickel, chromium, cobalt, tin, vanadium, platinum, palladium, and molybdenum. The metals which are normally supplied as the salt, oxide, hydroxide, etc. can be used alone or as mixtures.

The preferred polyvalent metal chelates are coordination complexes in which the polvalent metals form chelates by reaction with an amino carboxylic acid, an amino polycarboxylic acid, a polyamino carboxylic acid, or a polyamino polycarboxylic acid. Preferred coordination complexes are the amino polycarboxylic acids in which the polyvalent metal forms a chelate with an acid having in formula: ##STR1## where n is two or three; A is a lower alkyl or hydroxyalkyl group; and B is a lower alkyl carboxylic acid group.

A second class of preferred acids utilized in the formation of the polyvalent metal chelates utilized in the process of the invention is an amino polycarboxylic acid represented by the formula: ##STR2## wherein two to four of the X groups are lower alkyl carboxylic groups, zero to two of the X groups are selected from the group consisting of lower alkyl groups, hydroxyalkyl groups, and ##STR3## and wherein R is a divalent organic group. Representative divalent organic groups are ethylene, propylene, isopropylene or alternatively cyclohexane or benzene groups where the two hydrogen atoms replaced by nitrogen are in the one or two position, and mixtures thereof.

The polyvalent metal chelates useful in the process of the invention are readily formed in an aqueous solution by reaction of an appropriate salt, oxide, or hydroxide of the polyvalent metal and the amino carboxylic acid present in the acid form or as an alkali metal or ammonium salt thereof. Exemplary amino polycarboxylic acids include (1) amino acetic acids derived from ammonia or 2-hydroxy alkyl amines, such as glycine, diglycine (imino diacetic acid), NTA (nitrilo triacetic acid), 2-hydroxy alkyl glycine; di-hydroxyalkyl glycine, and hydroxyethyl or hydroxypropyl diglycine; (2) amino acetic acids derived from ethylene diamine, diethylene triamine, 1,2-propylene diamine, and 1,3-propylene diamine, such as EDTA (ethylene diamine tetraacetic acid), HEDTA (2-hydroxyethyl ethylenediamine tetraacetic acid), DETPA (diethylene triamine pentaacetic acid); and (3) amino acetic acid derivatives of cyclic 1,2-diamines, such as 1,2-diamino cyclohexane N,N-teraacetic acid, and 1,2-phenylenediamine-N,N-tetraacetic acid. The iron chelates of NTA, EDTA and HEDTA are preferred.

Buffering agents which are used alone or in mixtures are useful as components of the aqueous alkaline scrubbing solution of the invention. In general those are useful which are capable of maintaining the aqueous alkaline solution at a pH generally in the operating pH range of about 7 to about 10. The buffering agents should be water soluble at the concentrations in which they are effective. Examples of suitable buffering agents operable in the process of the invention are the ammonium or alkali metal salts of carbonates, bicarbonates, or borates. Examples of useful specific buffering agents within these classes of buffering agents are sodium carbonate or bicarbonate or sodium borate. Where the hydrogen sulfide containing feed gas also contains carbon dioxide at a volume percent of greater than about 5%, the carbonate or bicarbonate buffers are the preferred buffers for use in the process of the invention. These may be produced in situ by the use of an alkali in an amount suitable to provide a pH of about 7 to about 10, preferably an alkali metal hydroxide such as sodium hydroxide is used in the preparation of the aqueous alkaline scrubbing solution. Where the hydrogen sulfide containing feed gas contains carbon dioxide only in a minor amount, (less than about 5%) then the borate buffers, for example, borax or sodium borate (Na.sub.2 B.sub.4 O.sub.7) are useful.

In the oxidation zone of the process, the preferred oxygen containing gas utilized is air. In addition, any inert gas may be utilized in combination with pure oxygen as an oxidizing gas. The operating range of oxygen concentration in the oxidation zone is from about 1% to about 100% by volume. The preferred range of oxygen concentration is about 5% to about 25% by volume and the most preferred range is about 5% to about 10% by volume. In general, mild oxidizing conditions are preferred in the process of the invention. The oxygen containing gas should be introduced to the oxidation zone in such a manner so as to disperse it throughout the aqueous alkaline solution and to minimize intense, localized oxidation. The total amount of oxygen fed to the oxidation zone is dependent upon the amount of hydrosulfide and/or sulfide absorbed in the aqueous alkaline solution which is fed to the oxidation zone from the contact zone. The minimum amount that can be fed to the oxidation zone is one-half mole of oxygen per mole of sulfide or hydrosulfide in the aqueous alkaline solution feed liquid. The operating range of total oxygen fed to the oxidation zone is dependent upon the efficiency of oxygen mixing and absorption into the aqueous alkaline solution present in the oxidation zone. In the process of the invention, essentially all the dissolved sulfide and/or hydrosulfide present in the oxidation zone is converted to crystalline sulfur. Since mild conditions are preferred, the operating range of total oxygen fed can be broad while carefully controlling the heating and oxygen concentration conditions in the oxidation zone. Over oxidation can result in the formation of undesirable thiosulfate and sulfate salts. The operating range for oxygen present in the oxidation zone is generally about one-half mole of oxygen per mole of sulfide or hydrosulfide up to about five moles, preferably about 1 mole to 3 moles of oxygen per mole of sulfide or hydrosulfide present in the aqueous alkaline solution fed to the oxidation zone. A preferred amount of oxygen utilized is that amount which results in zero of the polyvalent metal chelate in the higher valence state leaving the oxidation zone.

Any of the conventional methods for recovery of elemental sulfur employed in processes similar to the process of the invention can be employed in the present process. For example, sulfur can be recovered by settling subsequent to flocculation, centrifugation, filtration, flotation, and the like. The method of sulfur recovery is not critical to the process of the invention. It is desirable to recover as much of the aqueous alkaline scrubbing solution as possible for recycle back to the contact zone of the process to minimize physical losses of the polyvalent metal chelating agent and the carbon dioxide absorbent.

While this invention has been described with reference to certain specific embodiments, it will be recognized by those skilled in the art that many variations are possible without departing from the scope and spirit of the invention, and it will be understood that it is intended to cover all changes and modifications of the invention disclosed herein for the purposes of illustration which do not constitute departures from the spirit and scope of the invention.

Claims

1. An oxidation stable aqueous alkaline scrubbing solution suitable for removing hydrogen sulfide and carbon dioxide from a sour gaseous stream in a contact zone without substantial oxidative degradation of a higher valence polyvalent metal chelate present in said scrubbing solution, said scrubbing solution consisting of:

an alkali, a lean carbon dioxide selective absorbent, a mixture of a lower valence polyvalent metal chelate and a higher valence polyvalent metal chelate, said lower valence polyvalent metal chelate present in said mixture in an amount which is greater than about five times the amount of said higher valence polyvalent metal chelate, and at least one buffering agent to maintain said aqueous alkaline solution within a pH range of about 7 to about 10.

2. The composition of claim 1 wherein said polyvalent metal chelate is a coordination compound of said polyvalent metal with an acid selected from the group consisting of amino carboxylic acid, amino polycarboxylic acids, polyamino carboxylic acids, and polyamino polycarboxylic acids.

3. The composition of claim 1 wherein said polyvalent metal is selected from the group consisting of iron, manganese, copper, nickel, chromium, cobalt, tin, vanadium, platinum, palladium, molybdenum, and mixtures thereof.

4. The composition of claim 3 wherein said polyvalent metal is iron, said buffering agent is selected from the group consisting of at least one of an ammonium or an alkali metal carbonate, bicarbonate, or borate and said alkali is selected from the group consisting of ammonium hydroxide and an alkali metal hydroxide.