AU687297B2

AU687297B2 - H2S absorption with subsequent oxalate removal

Info

Publication number: AU687297B2
Application number: AU45034/96A
Authority: AU
Inventors: M.C. Allen; S.A. Bedell
Original assignee: Dow Chemical Co
Current assignee: Dow Chemical Co
Priority date: 1994-12-21
Filing date: 1995-11-28
Publication date: 1998-02-19
Anticipated expiration: 2015-11-28
Also published as: BR9510304A; CA2207898A1; WO1996019280A1; NO972901D0; NO972901L; MX9704680A; EP0799085A1; AU4503496A; NZ300326A

Description

H2S ABSORPTION WITH SUBSEQUENT OXALATE REMOVAL.

The invention relates to the processing of gas streams containing H2S to remove the I^S, and particularly to the treatment of aqueous oxidizing solutions utilized in H2S removal. The invention further relates to a process for aqueous solution oxidation of H2S in such streams, the process being characterized by control of and efficient removal from the aqueous solution of oxalate ions generated as a degradation product in the process. Industrial and natural sources produce a wide variety of non-condensible gas streams containing significant amounts or concentrations of H2S. Although various procedures have been developed to remove and recover these contaminants, most such processes are deficient, for a variety of reasons. Where the gas streams contain lesser quantities of H2S, aqueous reactant systems which comprise regenerable reactants which react with the H2S (or hydrosulfide in solution) to produce solid free sulfur are preferred. Suitable reactants include polyvalent metal ions, such as iron, vanadium, copper, manganese, and nickel, and include polyvalent metal chelates. Preferred reactants are coordination complexes in which polyvalent metals form chelates with particular organic acids.

Because such processes generally represent significant costs to manufacturing operations, any improvements in such processes which increase their efficiency may have great importance. For example, where chelates of polyvalent metals are employed, ligand degradation or decomposition represents an important cost in the process, as well as requiring measures for decomposition product bleed, removal or treatment, and addition of fresh reactant solution. Even in the case of preferred chelates such as those of N- (2-hydroxy- ethyl) ethylenediamine triacetic acid and nitrilotriacetic acid, ligand decomposition, over a period of time, requires attention to prevent buildup of oxalate ion and loss of efficiency. As will be recognized, the bleed from such processes contains, along with the decomposition products, a considerable amount of the valuable metal chelate or chelates. U.S. patent 4,485,082 and U.S. patent 4,485,083 disclose processes for the removal of oxalate ion from solutions of polyvalent metal chelates of nitrilotriacetic acid. While the presence of limited quantities of oxalate ion appears beneficial (see, for example, U.S. patent 4,009,251), in practice, the concentration of oxalate ion is significant in determining the bleed rate of the solution, and it appears that excess concentrations of oxalate ion tend to inhibit efficient regeneration. According to the removal patents mentioned, removal of the oxalate ion is achieved by the addition to the oxidizing solutions of sufficient hydrogen ion to precipitate ferrous oxalate, but not in an amount which will remove the bulk of the polyvalent metal chelate or chelates of nitrilotriacetic acid. While this procedure results in oxalate removal, it requires significant lowering of the pH of the solutions. A procedure that would accomplish the removal of oxalate without lowering the pH might have significant advantage. The invention is such a procedure.

Accordingly, in one embodiment, the invention relates to a process in which an aqueous solution or admixture containing a composition selected from polyvalent metal chelates, and mixtures thereof, and containing decomposition product oxalate ions, is contacted or blended with an amount of alkali metal composition sufficient to and under conditions to precipitate alkali metal oxalate. Alkali metal oxalate is precipitated, and may then be separated from the solution or admixture. As used herein, the phrase "alkali metal" is understood to mean sodium, potassium, and mixtures thereof, and the phrase "alkali metal composition" is intended to include any compound or complex of these metals, or mixtures thereof, which provides their ions, or mixtures thereof, in the solutions treated. Again, the term "under conditions to precipitate", and variants thereof, merely implies solution conditions, such as temperature, pH, ionic strength, solution type, etc. at which the alkali metal oxalate exceeds its solubility in the solution or admixture treated. As will be understood by those skilled in the art, precipitation of the alkali metal oxalate will occur when the product of the square of the alkali metal concentration and the free oxalate concentration ([Na]^[free oxalate]) exceeds the K_Sp of the alkali metal oxalate under the solution conditions. Precipitating solution temperatures may exist initially, or may be accomplished simply by cooling. It is not necessary that all the oxalate be precipitated; some oxalate, as noted, is beneficial, and the invention is accomplished in this embodiment if oxalate ion is removed to the level desired. Solutions containing oxalate ion derived from the decomposition of the oxidizing polyvalent metal chelates of N-(2- hydroxyethyl) ethylenediamine triacetic acid and nitrilotriacetic acid, and mixtures thereof, are admirably suited to the invention.

As indicated, the stream or admixture treated is preferably a bleed stream from one of the aqueous reactant solution processes mentioned, supra. In particular, the invention is admirably suited to the removal of oxalate ions in a bleed stream from a cyclic process for H2S removal from a gas stream wherein the oxidizing reactant is a polyvalent metal chelate or chelates of an organic acid, especially preferred chelates being those of N- (2-hydroxyethyl) ethylenediamine triacetic acid and nitrilotriacetic acid. In such cyclic processes, the regenerated oxidizing reactant solution, containing an increased concentration of oxidizing polyvalent metal chelate, is returned to the contacting zone for use therein as aqueous reaction solution. The treated bleed stream solution or admixture having reduced oxalate ion content may simply be returned to a suitable point in the process. Because the alkali metal composition will not lower the pH significantly, and because such a bleed stream will be small in relation to the volume of solution in the system, minimal pH adjustment will be required, if any, and may be conducted as part of the overall pH adjustment of the system. The polyvalent metals of the chelates are preferably selected from iron, copper, vanadium, and manganese, particularly iron.

Accordingly, in this context, the invention comprises, in one embodiment, a process for the removal of H2S from a sour gaseous stream including:

(a) contacting the sour gaseous stream in a contacting zone with an aqueous reaction solution, the solution comprising an effective amount of an oxidizing reactant selected from the group consisting of oxidizing polyvalent metal chelates of organic acids, and mixtures thereof, to produce a sweet gas stream and an aqueous admixture containing sulfur and reduced reactant;

(b) removing the aqueous admixture from the contacting zone and regenerating the aqueous admixture in a regeneration zone, producing a regenerated oxidizing reactant solution, and returning regenerated oxidizing reactant solution to the contacting zone;

(c) removing a bleed stream containing a composition selected from polyvalent metal chelates of organic acids, and mixtures thereof, and containing oxalate ions, from one or more loci in steps (a) or (b) ;

(d) mixing with said bleed stream an amount of an alkali metal composition sufficient to and under conditions to precipitate alkali metal oxalate, and precipitating alkali metal oxalate and separating precipitated solid oxalate from the bleed stream.

As will be evident to those skilled in the art, the bleed stream may be removed continuously or intermittently, and the particular location or point of removal of the bleed in such a process is not critical, although removal of the bleed "subsequent" to the contact zone and "prior" to return of regenerated solution or the contact zone in the cyclic process is preferred. Again, the bleed may be removed from a portion of the process stream if the stream is divided for any purpose, for example, a portion for sulfur removal, and a portion sent directly for regeneration. The specifics of the sulfur removal procedure are not critical; for example, sulfur may be removed prior to or subsequent to regeneration. Moreover, the sulfur may first be concentrated in a portion of the liquid in circulation in the process, and this may be done prior to or subsequent to regeneration. If the sulfur-containing liquid is first concentrated into a slurry before final separation of the sulfur, the liquid, or a portion thereof, from the slurry may be utilized as a "bleed" stream. Statement herein that precipitated alkali metal oxalate is separated from the bleed stream does not imply that it must be separated alone; for example, the alkali metal oxalate may be quite suitably separated with the sulfur. The rate and volume of bleed will depend on a variety of factors, but, as indicated, the concentration of oxalate ion is the predominant consideration. Accordingly, a precise volume of bleed (and make-up) , although obviously a quite minor portion of the total volume of liquid in the system, cannot be given, but, in general, twenty percent to 0.5 percent, by volume, of the total liquid capa-city in the process will suffice. The bleed, upon removal of the oxalate, may be returned to any point in the system. Fresh make-up chelate or chelate-contaming solution may similarly be supplied continuously or intermittently.

In a much preferred embodiment, the level or concentration of oxalate ion m this type of process is regulated or controlled by controlled addition of alkali metal composition. More particularly, alkali metal composition may be added to the aqueous oxidizing polyvalent metal chelate solution at a suitable location in the process on a continuous or intermittent basis, preferably after or in response to a determination of the oxalate level or concentration in the polyvalent metal chelate solution or admixture. For example, the alkali metal composition may be supplied on a continuous or intermittent basis to the contacting zone after the degradation rate of the particular oxalate producing chelate is known, the amount of alkali metal ion being supplied being sufficient to precipitate sufficient oxalate and maintain the oxalate level at a desired or predetermined concentration, that is, below a level at which undesired effects occur, a level which is usually at or below the saturation level. Again, and preferably, the oxalate level or concentration of the process solution or admixture may be monitored on a continuous or intermittent basis, and the alkali metal composition may be added thereto in response to an indicated unacceptable or undesired predetermined concentration of the oxalate ion. In each case, the precipitated alkali metal oxalate, because the volumes precipitated are small, may be removed routinely in the process, such as in the sulfur removal procedure, for example, by filtration. Accordingly, here also, statement that precipitated alkali metal oxalate is separated from solution or admixture does not imply that it must be separated alone; for example, the alkali metal oxalate may be quite suitably separated with the sulfur. In general, the oxalate level or concentration should be maintained at a level below which significant undesired effects occur, as mentioned, and this level will generally depend on the particular solutions being used, etc. By way of example only, more than 8000 ppm to 12000 ppm of oxalate in some treating solutions may begin to produce the undesired effects mentioned. Accordingly, in this embodiment, the invention comprises a process for the removal of H2S from a sour gaseous stream including:

(a) contacting the sour gaseous stream in a contacting zone with an aqueous reaction solution, the mixture comprising an effective amount of an oxidizing reactant selected from the group consisting of oxidizing polyvalent metal chelates of organic acids, and mixtures thereof, to produce a sweet gas stream and an aqueous admixture containing sulfur and reduced reactant;

(b) removing aqueous admixture from the contacting zone and regenerating the aqueous admixture in a regeneration zone, to produce a regenerated oxidizing reactant solution, and returning regenerated oxidizing reactant solution to the contacting zone;

(c) regulating the concentration of oxalate ions in any polyvalent metal containing solution or admixture of steps a) and b) by blending an alkali metal composition with said solution and/or admixture in an amount sufficient and under conditions to precipitate alkali metal oxalate;

(d) and separating precipitated oxalate.

The blending of the alkali metal composition may be carried out, as indicated, continuously or intermittently during the operation of the H2S contacting procedure, as may the separation of the oxalate from the solution and/or admixture.

As indicated, in each of the embodiments herein, the solution or admixture containing the chelate or chelates is contacted, blended or mixed with a composition capable of providing alkali metal ions in solution. Any composition capable of providing sufficient alkali metal ions in the solution may be employed, and compositions commonly utilized to regulate pH in the system may be utilized. The composition may be supplied as a finely divided solid, but is preferably supplied as a slurry or aqueous solution. The reaction proceeds according to the general equation:

2Me + 2θ4= > Me2C2θ/}Ψ wherein Me is alkali metal. Compositions which may be used to supply the required alkali metal ions include, but are not limited to, NaOH, NaCl, KOH, Na2S2θ3, and NaHSθ3. A much preferred source of the alkali metal ions is so-called "free" ligand supplied to the treating solution, that is, the alkali metal salt of the organic acid which may be employed in maintaining chelate concentration in the contacting zone. The use of such "free" ligand in each of the embodiments mentioned may also provide additional oxalate removal; while not wishing to be bound by any theory of invention, it is believed that the ligand frees iron complexed oxalate, which will allow precipitation of additional alkali metal oxalate. Thus, even if, for example, alkali metal ions are supplied or present from some other source, and precipitate alkali metal oxalate, it may be possible, by the addition of some free ligand, for example, sodium nitrilotriacetate or sodium N- (2-hydroxyethyl) ethylene diamine triacetate, to precipitate additional alkali metal oxalate. In an additional aspect of the invention, the "free" ligand may also be supplied as some other soluble form, such as the ammonium salt of the organic acid, or the complexing acid itself if it is sufficiently soluble, provided there is sufficient alkali metal ion in the solution treated (or is supplied thereto) to precipitate the freed oxalate as the alkali metal oxalate.

In the embodiments of the invention where a separate stream is treated, the alkali metal composition is supplied in an amount sufficient to precipitate a significant or major proportion, say 50 percent by weight to 90 percent by weight of the oxalate in the stream on a continuous basis, while leaving sufficient oxalate in the stream to provide benefits attributed thereto in the contacting and regeneration zones. The precipitated alkali metal oxalate may be separated by any suitable method, such as by filtration, and the supernatant liquid is recovered and returned to the process. Suitable temperatures are those employed in the H₂S removal process, it being unnecessary to lower the temperature to any significant extent, although this may be done if desired. Since the aqueous reactant solutions employed for H2S removal are normally alkaline to slightly acid, it will normally be unnecessary, prior to return of the solution to the process, to adjust pH, the overall pH adjustment of the removal step being adequate to maintain proper pH. An additional advantage is that the polyvalent metal need not be replaced to any significant extent. In the case where the oxalate concen-tration is controlled directly in the contacting zone, regeneration zone, or circulating solution or admixture, the amount of alkali metal composition or soluble acid or ammonium form to be added may be determined routinely. For example, continuously or intermittently, the oxalate content of the solution in any zone or site may be monitored, and sufficient alkali metal composition may be added to the polyvalent metal containing solution or admixture in response to an undesired increase or concentration to maintain the concentration of oxalate at the desired level, preferably a concentration below the saturation level in the solution. The particular types of solutions or admixtures treated according to the invention do not appear critical. Virtually any solution containing the specified polyvalent metal chelate or chelates and decomposition product oxalate ion or ions and in which it is sought to regulate the oxalate concentration may be treated according to the invention. The polyvalent metal chelate or chelates may be present in more than one species, that is, the solution or admixture may and most likely will contain oxidized and reduced forms of such metals, such as Fe and Fe . As indicated, it is an advantage of the invention that the chelate or chelates do not precipitate, but remain in solution, thereby assuring an effective separation. The preferred aqueous chelate oxidation procedure is under-stood to be a cyclic procedure carried out in an aqueous liquid oxidation zone, such zone including provision for re-generation of the chelate, with appropriate removal of sulfur formed from a suitable locus in the aqueous liquid oxidation zone. For example, the procedures employed in U.S. patent 4,830,838 and U.S. patent 4,774,071 may be used. Single vessel H2S oxidation and regeneration procedures may also be em¬ ployed.

The reactions for conversion of the H2S by the polyvalent metal chelate and regeneration of the chelate may be summarized, as follows: 2Me^xchelate + H₂ S > 2Me^{x 1}chelate + S° + 2H⁺ water and x-1 2 Me chelate + 2H+ + _z > 2Me^xchelate + H₂0 wherein Me is a positive metal ion and x is a whole number. The preferred polyvalent metal chelates are coordination complexes in which the polyvalent metal forms chelates by reaction with an amino carboxylic acid, a amino polycarboxylic acid, a polyamino carboxylic acid, or a polyamino polycar-boxylic acid. One preferred class of coordination complexes is that in which the polyvalent metal forms a chelate with an acid having the formula:

(A)₃__n N-B_n wherein 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 forming the metal chelates employed are acids having the formula

X X

\ / -R-N

/ \

X X wherein from two to four of the X groups are selected from lower alkyl carboxylic acid groups, zero to two of the X groups are selected from the group consisting of lower alkyl groups, lower hydroxyalkyl groups, and

Y

/

-CH2CH2-N,

\

Y wherein Y is selected from lower alkyl carboxylic acid groups, and R is a divalent organic group containing 2 through 8 carbon atoms, preferably 2 through 6 carbon atoms. Representative divalent organic groups include ethylene, propylene or isopropylene, or, alternatively, cyclohexane or benzene where the two hydrogen atoms replaced by nitrogen are in the 1,2 position. The metal chelates are present in solution as solubilized species, for example, solubilized ammonium or alkali metal salts (or mixtures thereof) of the metal chelates, and references herein to "polyvalent metal chelate", "Fe⁺⁺⁺chelate", or to "iron chelate", etc., in solution indicate dissolved chelates. whether as a salt or salts of the aforementioned cation or cations, or in some other form, n which the metal chelate or chelates exist in solution.

The polyvalent metal chelates useful in the invention are readily formed in aqueous solution by reaction of an appropriate salt, oxide, or hydroxide of the particular polyvalent metal or metals and the amino carboxylic acid present in the acid form or as an alkali or ammonium salt thereof. As noted, the polyvalent metals of the chelates are preferably selected from iron, copper, vanadium, and manganese, particularly iron, and mixtures of the metals may be used. Exemplary aminocarboxylic acids include (1) aminoacetic acids derived from ammonia or 2-hydroxyalkyl amines, such as glycine, diglycine; 2- hydroxyalkyl glycine, dihydroxyalkyl glycine, and (2) nitri¬ lotriacetic acid; and (3) aminoacetic acids derived from ethylene diamine, diethylene triamine, 1,2-propylene diamine, and 1, 3-propylene diamine, such as ethylenediamine tetra-acetic acid, 2-hydroxyethyl ethylenediamine triacetic acid, and diethylenetriamine pentaacetic acid; and (4) aminoacetic acids derived from cyclic 1,2-dιamιnes, such as 1,2-dιamιno cyclohexane N,N-tetraacetιc acid, and 1,2- phenylenedιamιne-N,N-tetraacetιc acid. The iron chelates of 2- hydroxyethyl ethylenediamine triacetic acid and nitrilotriacetic acid are preferred.

The H2S, when contacted, is quickly converted by the polyvalent metal chelate, or chelates, to elemental sulfur. The amount of the polyvalent metal chelate, or mixtures thereof, supplied is an effective amount, that is, an amount sufficient to convert all or substantially all of the H2S in the gas stream, and will generally be on the order of at least one mole per mole of H2S. Ratios of from 1 or 2 moles to 15 moles of polyvalent metal chelate per mole of H2S may be used, with ratios of from 2 moles per mole to 5 moles of polyvalent metal chelate per mole of H2S being preferred. The manner of preparing the aqueous solution or admixture is a matter of choice. The polyvalent metal chelate solution will generally be supplied as an aqueous solution having a concentration of from 0.1 molar to 2.0 molar, and a concentration of 0.5 molar is preferred. As indicated, the invention is suitable for the processing of a variety of gas streams containing H2S; the particular type of stream treated is not critical. As will be evident to those skilled in the art, the ϊ^S-comprising gas treated preferably will be, other than the H2 , non-reactive or substantially non-reactive with and of limited solubility (that is, insoluble or substantially insoluble) in the aqueous polyvalent metal chelate solution, but these are not absolute requirements. For example, streams containing minor quantities of reactive components may be treated if appropriate provision is made, while soluble gases can be stripped. Suitable gas streams include naturally-occurring gases, fuel gases, vent gases, hydrocarbon gases, stack gases, and gases produced, for example, after the condensation of a desired component such as a hydrocarbon or steam. Other gases to which the invention may be applied are described more fully in U.S. patent 4,705,676, and, given the teachings herein, may readily be selected by those skilled in the art. The concentrations of H2S in the streams treated may vary from trace or minimal to heavy, but, not by way of limitation herein, commonly encountered streams range from 200 ppm by volume to 50 percent by volume, pre-ferably from 0.5 percent by volume to 10 percent by volume.

H2S-contaminated steam will contain minor concentrations or amounts of H2S and other non-condensible gases. The steam and non-condensible streams derived after condensation of the steam are particularly suited to the treatment of the invention. In general, steam processed according to the invention will contain H2S and other non-condensible fluids in quite minor amounts, that is, less than 15 percent by weight. Normally, the H2S will be present in an amount of less than 5 or 6 percent by weight, most commonly less than 3 percent by weight. Conditions of temperature and pressure and their relationship for condensing the steam are well understood by those skilled in the art, and need not be recited herein.

The temperatures employed in the contacting zone are not generally critical. Preferably, however, the reaction is carried out below the melting point of sulfur. In many applications, such as the removal of H₂S from natural gas to meet pipeline specifications, treatment at ambient temperatures is preferred. In general, o o temperatures of from 10 C to 80 C are suitable, and temperatures of o o from 20 C to 55 C are preferred. Contact times will range from 1 second to 120 seconds, with contact times of from 2 seconds to 60 seconds being preferred.

Similarly, in the regeneration or stripping zone or zones, temperatures may be varied widely. Preferably, the regeneration zone should be maintained at substantially the same temperature as the contacting zone. If heat is added to assist regeneration, cooling of the aqueous admixture may be required before return of the admixture to the contacting zone. In general, temperatures of from 10 C to 80 C, o o preferably from 20 C to 55 C, may be employed.

Pressure conditions in the contacting zone may vary widely, depending on the pressure of the gas to be treated. For example, pressures in the contact zone may vary from one atmosphere up to one hundred fifty or even two hundred atmos-pheres. In the regeneration zone or zones, pressures may also be varied considerably, and will preferably range from 0.5 atmosphere to three or four atmospheres. The pressure-temperature relationships involved are well under-stood by those skilled in the art and need not be detailed herein. In general, suitable conditions, proportions, and parameters for the polyvalent metal chelate removal of H2S are well described in the patent literature, especially U.S. patent 4,705,676. Other conditions of operation for this type of reaction process, for example, pH, etc., are further described in U.S. patent 3,068,065, to Hartley, et al, dated December 11, 1962, and U.S. patent 4,009,251, to Meuly, issued February 22, 1977, which disclosures are incorporated herein by reference. Preferably, if the iron chelate of nitrilotriacetic acid is used, pH in the contact zone will range from 6 to 8.5, and the molar ratio of the nitrilotriacetic acid to the iron, in the absence of other chelating materials, will pre-ferably be from 1.2 to 1.6. In the case of the iron chelate of N- (2-hydroxyethyl) ethylene diamine triacetic acid, pH in the contacting zone will preferably range from 6 to 8.5, and the molar ratio of the N-(2- hydroxyethyl) ethylenediamine triacetic acid to the iron will preferably range from 0.2 to 1.0. The contacting process is preferably conducted on a continuous basis.

As indicated, the invention provides, in the H2S removal embodiments, for the regeneration of the reactant. Preferably, solution or admixture containing reduced reactant is removed from the contacting zone and contacted in a regeneration zone with oxygen. As used herein, the term "oxygen" includes oxygen-containing gases such as air, or air enriched with oxygen. The oxygen will accomplish two functions, the oxidation of the reduced polyvalent metal ion to a higher valence state, and the stripping of any minor quantities of absorbed gas in the admixture. The oxygen (in whatever form supplied) is supplied in a stoichiometnc equivalent or excess with respect to the amount of reduced metal ion of the chelate or chelates present in the mixture. Preferably, the oxygen is supplied in an amount of from 1.2 to 3 times excess.

If a bleed stream is the solution to be treated, the "bleed" is preferably treated before sulfur removal and regeneration. The manner of sulfur recovery herein is a matter of choice. For example, the sulfur may be recovered by settling, filtration, or by suitable devices such as a hydroclone. Moreover, it s advantageous to concentrate the sulfur first in a portion of the mixture, either before or after regeneration. For example, the sulfur containing admixture from the contacting zone (or from the regeneration zone) may be separated into two portions, a portion or stream having reduced sulfur content, and a portion or stream containing increased sulfur content, preferably a slurry. The manner of separation is a matter of choice, and equipment such as a hydroclone or centrifugal separator may be employed. If a slurry is produced, the "slurry" or concentrated stream will comprise 2 percent to 30 percent, by volume, (on a continuous basis) of the total stream from the contact or regeneration zone. It is not necessary that absolutely all sulfur be removed on a continuous basis in the process; the process may suitably be operated with a very minor inventory or significantly reduced content of sulfur in the system. In the case where a slurry is produced, the slurry may be filtered or subjected to further treatment to remove the sulfur, and the recovered admixture may be used as all or portion of the bleed treated herein, or it may be returned to the process cycle, either before or after regeneration.

In order to demonstrate the effect of alkali metal composition addition to oxalate containing solution, the following experiments were conducted. Examples I

A sample (15.1045g) of solution from a cyclic process for the removal of H2S from a gas stream which had reached equilibrium composition through make-up and bleed was obtained after sulfur removal. The solution contained 0.95 percent total iron [of which 34.6 percent was chelated by N- (2-hydroxyethyl) ethylenediamine triacetic acid], 5.27 percent thiosulfate, 1.48 percent oxalate, 7.56 percent nitrate, and 6,11 percent sodium ion (all percentages by weight), and minor portions of other degradation products.

Sodium chloride (2.0093g) was added to the cyclic process solution. The resulting precipitate was determined to be sodium oxalate, and the oxalate concentration of the process solution was reduced to 0.61 percent by weight. II

The experiment was repeated utilizing a 15.0098g sample of process solution and 2.9930g of sodium chloride. The concentration of oxalate in solution was reduced to 0.29 percent by weight. ill

In a further experiment in which the potassium salt of N- (2-hydroxyethyl) ethylenediamine triacetic acid was added to a similar process solution, the concentration of the oxalate was reduced.

While the invention had been illustrated with particular apparatus, those skilled in the art will appreciate that, except where otherwise specified, other equivalent or analogous apparatus may be employed. The term "zones," as employed in the specification and claims, includes, where suitable, the use of segmented equipment operated in series, or the division of one unit into multiple units because of size constraints, etc. For example, a contacting unit or column might comprise two separate columns in which the solution from the lower portion of the first column would be introduced into the upper portion of the second column, the gaseous material from the upper portion of the first column being fed into the lower portion of the second column. Parallel operation of units, is, of course, well within the scope of the invention. As indicated, contacting and regeneration may be carried out in the same zone.

As will be understood by those skilled in the art, the solutions or mixtures employed may contain other materials or additives for given purposes. For example, U.S. patent 3,933,993 discloses the use of buffering agents, such as phosphate and carbonate buffers. Similarly, U.S. patent 4,009,251 describes various other additives, such as sodium formate, sodium thiosulfate, and sodium acetate, which were beneficial, and other additives, such as antifoaming and/or wetting agents, may be employed.

Claims

1. A process comprising contacting an aqueous solution or admixture containing a composition selected from polyvalent metal chelates of an organic acid, and mixtures thereof; and oxalate ions, with an amount of alkali metal composition sufficient to and under conditiors to precipitate alkali metal oxalate, and precipitating alkali metal oxalate.

2. A process comprising contacting an aqueous solution or admixture containing a composition selected from polyvalent metal chelates of an organic acid, and mixtures thereof; and oxalate ions, with a composition selected from the alkali metal salt of an organic acid, and mixtures thereof, in an amount sufficient to and under conditions to precipitate alkali metal oxalate, and precipitating alkali metal oxalate.

3. A process comprising contacting an aqueous solution or admixture containing a composition selected from polyvalent metal chelates of an organic acid, and mixtures thereof; alkali metal ions, and oxalate ions, with a composition selected from the organic acid, alkali and ammonium salts thereof, and mixtures thereof, in an amount sufficient to and under conditions to precipitate alkali metal oxalate, and precipitating alkali metal oxalate.

4. A process for the removal of H2S from a sour gaseous stream comprising

(a) contacting the sour gaseous stream in a contacting zone with an aqueous reaction solution comprising an effective amount of an oxidizing polyvalent metal chelate of an organic acid, and mixtures thereof, producing a sweet gas stream and an aqueous admixture containing sulfur and reduced reactant;

(b) removing aqueous admixture from the contacting zone and regenerating the aqueous admixture in a regeneration zone, producing a regenerated oxidizing reactant solution, and returning regenerated oxidizing reactant solution to the contacting zone;

(c) removing a bleed stream containing a composition selected from polyvalent metal chelate of said organic acid, and mixtures thereof, and containing oxalate ion, from one or more loci in steps (a) or (b) ; (d) mixing with said bleed stream an amount of an alkali metal composition sufficient to and under conditions to precipitate alkali metal oxalate, and precipitating alkali metal oxalate, and separating precipitated solid alkali metal oxalate from the bleed stream.

5. A process for the removal of H2S from a sour gaseous stream comprising

(c) removing a bleed stream containing a composition selected from polyvalent metal chelate of said organic acid, and mixtures thereof, and containing alkali metal ions and oxalate ions, from one or more loci in steps (a) or (b) ;

(d) mixing with said bleed stream an amount of an composition selected from the organic acid, alkali metal and ammonium salts thereof, and mixtures thereof, in an amount sufficient to and under conditions to precipitate alkali metal oxalate, and precipitating alkali metal oxalate, and separating precipitated alkali metal oxalate from the bleed stream.

6. In a process for the removal of H2S from a gas stream wherein the gas stream is contacted with an aqueous solution containing an oxidizing polyvalent metal chelate of an organic acid, wherein oxalate ions are formed as a degradation product of said organic acid chelate in process solution, the improvement comprising controlling oxalate concentration in said process solution by adding alkali metal composition to process solution in an amount sufficient and under conditions to precipitate alkali metal oxalate and maintain the oxalate concentration at a desired level.

7. A process for the removal of H2S from a sour gaseous stream comprising

(d) and separating precipitated oxalate.

8. A process for the removal of H2S from a sour gaseous stream comprising

(a) contacting the sour gaseous stream in a contacting zone with an aqueous reaction solution, the mixture comprising an effective amount of an oxidizing reactant selected from the group consisting of oxidizing polyvalent metal chelates of organic acids, and mixtures thereof; and alkali metal ions, to produce a sweet gas stream and an aqueous admixture containing sulfur and reduced reactant;

(c) regulating the concentration of oxalate ions in any polyvalent metal containing solution or admixture of steps a) and b) by blending therewith a composition selected from the organic acid, alkali metal and ammonium salts thereof, and mixtures thereof, in an amount sufficient to and under conditions to precipitate alkali metal oxalate;

(d) and separating precipitated oxalate.

9. A process as in any one of the preceding claims wherein the organic acid is an organic acid having the formula

(A)₃__n—N-B_n wherein n is two or three, A is a lower alkyl or hydroxyalkyl group; and B is a lower alkyl carboxylic acid group: or the formula

X X

\ /

N-R-N

/ \

Y

/

-CH₂CH2-N,

\

Y wherein Y is selected from lower alkyl carboxylic acid groups, and R is a divalent organic group containing 2 through 8 carbon atoms, preferably 2 through 6 carbon atoms.

10. The process of Claim 9 wherein the organic acid is 4- (2- hydroxyethyl) ethylenediamine triacetic acid or nitrilotriacetic acid.

11. The process of Claim 1 wherein precipitated alkali metal oxalate is separated from the aqueous solution or admixture.

12. The process of claim 7 or 8 wherein the oxalate ion concentration of the polyvalent metal containing solution or admixture is monitored, and the alkali metal composition is blended with the solution or admixture in response to an oxalate ion concentration at or above a predetermined concentration.

13. The process of claim 1 or 12 wherein the polyvalent metal is iron.

14. The process of any one of claims 1, 6 or 7 wherein the alkali composition is selected from sodium nitrilotriacetate or sodium N- (2-hydroxyethyl) ethylenediamine triacetate.