Gas treating process and composition
This is a continuation of application Ser. No. 937,446 filed Dec. 5, 1986, now abandoned, which is a continuation of application Ser. No. 790,763 filed Oct. 23, 1985, now abandoned, which is in turn a continuation of Ser. No. 614,363, filed May 29, 1984, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     The presence of significant quantities of H 2  S and CO 2  in various &#34;sour&#34; industrial gaseous streams poses a persistant problem. Although various procedures have been developed to remove and recover these contaminants, most such processes are deficient, for a variety of reasons. 
     In one cyclic method currently attracting attention, the sour gas is contacted, preferably with a solvent-reactant system which comprises a regenerable reactant, to produce solid free sulfur which is recovered either prior or subsequent to regeneration. Suitable reactant materials include polyvalent metallic ions, such as iron, vanadium, copper, manganese, and nickel, and include polyvalent metal chelates. Preferred reactants are coordination complexes in which the polyvalent metals form chelates with specified organic acids. 
     In yet another type of process, aqueous alkaline solutions are employed to absorb the H 2  S from the gas stream and to convert the absorbed H 2  S to elemental sulfur. In general, this type of process employs an aqueous alkaline solution containing vanadium ions and an anthraquinone disulfonate. This type of process usually requires dilute solutions and low H 2  S loadings, resulting in high circulation rates, large capital investments, and high costs. 
     One of the reasons for dilute solution use in the latter type of process is the real problem of maintaining sufficient levels of vanadium in solution. Another problem associated with this process is that the solid sulfur produced, if special measures are not taken, may be of poor quality, i.e., it may be finely divided and difficult to separate from the aqueous reactant solution. Finally, the breakdown of the anthraquinone disulfonate represents extra expense, as well as itself representing a disposal problem. The invention seeks to overcome these problems, and provides an efficient and economic process for H 2  S removal. 
     SUMMARY OF THE INVENTION 
     Accordingly, the invention, in one embodiment, relates to a process for the removal of H 2  S from a sour gaseous stream comprising, contacting the sour gaseous stream in a contacting zone with an aqueous reaction solution, the reaction solution containing an effective amount of vanadium V-containing ions. A sweet gas stream is produced, and an aqueous admixture containing sulfur and vanadium IV-containing ions is removed from the contact zone. The solution further comprises an amount of phosphate ion or ions which is sufficient to enhance the solubility of the vanadium IV-containing ions produced in the solution, at the same time, if solid sulfur is produced, improving sulfur quality, the molar ratio of the phosphate ions to vanadium IV-containing ions produced being at least 0.1. At least a portion of the sulfur may be removed before regenerating the reactant, during regeneration, or at least a portion of the sulfur may be removed after regeneration. Solid sulfur obtained directly, i.e., if the contacting zone is operated below the melting point of sulfur, due to the presence of the phosphate ions, is of improved quality, i.e., the particles or crystals have greater purity and improved filterability. The reduced reactant, i.e., the vanadium IV-containing ions, is regenerated, preferably by contacting the mixture in a regeneration zone or zones with oxygen. The term &#34;oxygen&#34;, as used herein, includes oxygen-containing gases, such as air or air-enriched with oxygen. The oxygen oxidizes the reduced vanadium-containing ion to the higher valence state, and the regenerated mixture is returned to the contact zone for use as the aqueous reaction solution. Electrochemical regeneration may also be employed. 
     A key feature of the invention is the use of reaction solutions of the type described which are free or substantially free of anthraquinone disulfonate (ADA), thus avoiding or substantially avoiding the problems attendant thereto. As used herein, the term anthraquinone disulfonate refers to anthraquinone disulfonate acid or its water soluble salts, such as the disodium salt, while the term &#34;substantially-free&#34; indicates hereinafter that the solutions contain less than 0.2 grams per liter of this material. It has been discovered that, in addition to avoiding the problems related to the anthraquinone disulfonate, the elimination of this material, coupled with the use of phosphate ions, as specified, provides real advantages, as set out more fully hereinafter. Very small amounts of anthraquinone disulfonic acid, e.g. 0.1 g/liter, may, however, be &#34;catalytic&#34; in this system. 
     In another embodiment of the invention, a sour gaseous system containing H 2  S and CO 2  is contacted with a selective absorbent-aqueous reactant mixture, the reactant mixture and procedure being similar to that described, supra. Broadly, this is accomplished by the use of an absorbent mixture containing a selective absorbent for CO 2  (and preferably for H 2  S, as well), an effective amount of vanadium V-containing ions, and an amount, as mentioned, supra, of phosphate ion or ions, and in the absence or substantial absence of ADA. A purified or &#34;sweet&#34; gaseous stream is produced which meets general industrial and commercial H 2  S and CO 2  specifications. The CO 2  is absorbed and the H 2  S is immediately converted to sulfur by the vanadium V-containing ions. In the process, the vanadium V-containing ions are reduced, producing vanadium IV-containing ions, and the sulfur may be treated, as described, supra. As in the previous embodiment, the sulfur may be removed prior to, during, or subsequent to regeneration of the admixture, and, if the reaction is carried out below the melting point of sulfur, the particles or crystals produced will be of increased size. Preferably, if the volume of CO 2  absorbed is large, the reactant-containing solution is treated, such as by heating or pressure reduction, to remove the bulk of the CO 2  before regeneration of the reactant (either prior or subsequent to sulfur removal). Alternately, or if small quantities of CO 2  are absorbed, the CO 2  may simply be stripped in the regeneration zone. 
     As indicated, supra, the invention also provides in this embodiment for the regeneration of the reactant and the absorbent. Preferably, the loaded absorbent mixture and the reduced polyvalent metal chelate are regenerated by contacting the mixture in a regeneration zone or zones with an oxygen-containing gas. The oxygen-containing gas may be air, oxygen, or air-enriched with oxygen. The oxygen-containing gas accomplishes two functions, the stripping of the CO 2  from the loaded absorbent mixture, and the oxidation of the reduced reactant to a higher oxidation state. The oxygen (in whatever form supplied) is supplied in a stoichiometric equivalent or excess with respect to the amount of reactant present in the mixture. Preferably, the oxygen-containing gas is supplied in an amount of from about 1.2 to 3 times excess. The regenerated absorbent mixture is returned to the contact zone for use as the aqueous absorbent mixture. 
     It has also been discovered that a limited or minor amount of thiosulfate ion may provide improved results on start-up in low H 2  S concentration treatment in the embodiments described. Accordingly, the invention also comprises a composition for use in aqueous or aqueous alkaline solutions for removing H 2  S from sour gaseous streams, the composition comprising or consisting essentially of 
     (a) an alkali metal, an alkaline earth metal, alkali metal ammonium, or ammonium vanadate; 
     (b) an alkali metal, alkaline earth, alkali metal ammonium, or ammonium phosphate, polyphosphate, -hydrogen or -dihydrogen phosphate; and 
     (c) an alkali metal, alkaline earth, alkali metal ammonium or ammonium thiosulfate, bisulfite, dithionite or tetrathionite, the ratio of (a) to (b) to (c) being from about 1:0.3 to 6:0.01-1. 
     Other aspects of the invention will become apparent to those skilled in the art. 
     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 2  S and CO 2  by the practice of the ivention are, as indicated, naturally occurring gases, synthesis gases, process gases, waste or stack 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 coal gasification streams, natural gas streams and refinery feedstocks composed of gaseous hydrocarbon streams, especially those streams of this type having a low ratio of H 2  S and CO 2 , and other gaseous hydrocarbon streams. The term &#34;hydrocarbon streams&#34;, 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 &#34;impurities&#34; not technically defined as a hydrocarbon. Again, streams containing principally a single hydrocarbon, e.g., ethane, 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 2  S content of the type of streams contemplated will vary extensively, but, in general, will range from about 0.01 percent up to approaching 100 percent by volume. CO 2  content may also vary, and may range from about 0.5 percent to over 99 percent by volume provided appropriate pH, as noted herein, is maintained. Obviously, the amounts of H 2  S and CO 2  present are not generally a limiting factor in the practice of the invention. 
     The temperatures employed in the contacting or absorption-contact zone are not generally critical. For example, temperatures may range above the melting point of sulfur, say from about 113° C. to 160° C., in which case provision will be made for removal of molten sulfur. Preferably, however, the reaction is carried out at a temperature below the melting point of sulfur, and, if a CO 2  -selective absorbent is used, temperatures employed must permit acceptable absorption of CO 2 . In many commercial applications, such as the removal of H 2  S and CO 2  from natural gas to meet pipeline specifications, contacting at ambient temperatures is desired, since the cost of refrigeration would exceed the benefits obtained due to increased absorption at the lower temperature. In general, temperatures of from about 0° C. to about 160° C. are suitable, and temperatures from about 10° C. to about 80° C. are preferred. Contact times may range from about 0.01 second to about 270 seconds or longer, with contact times of about 0.1 second to 120 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 absorption zone. If heat is added to assist regeneration, cooling of the absorbent mixture may be required before return of the absorbent mixture to the absorption zone. In general, temperatures of from about 0° C. to about 160° C., preferably about 10° C. to about 80° C. may be employed. 
     Pressure conditions in the absorption zone may vary widely, depending on the pressure of the gas to be treated. For example, pressures in the absorption zone may vary from about 0.1 atmosphere (absolute) up to one hundred fifty or even two hundred atmospheres (absolute). Pressures of from 0.7 atmosphere (absolute) to about one hundred atmospheres (absolute) are preferred. In the regeneration or desorption zone or zones, pressures may be the same as in the absorption zone, or may be varied considerably. Preferably, they will range from about 0.1 atmosphere (absolute) to about four or five atmospheres (absolute), most preferably from about 0.5 atmosphere to about three or four atmospheres (absolute). The pressure-temperature relationships, pH, etc. involved may be managed by those skilled in the art, and need not be detailed herein. Preferably, pH in the process of the invention will be at least 7, and will range from about 7 to about 14, with a pH range of from about 8 to about 11 being preferred. The procedure is preferably conducted continuously. 
     As indicated, the H 2  S, when contacted, is quickly converted by an oxidizing reactant comprising vanadium V-containing ions to elemental sulfur. The amount of vanadium V-containing ions supplied is an effective amount, i.e., an amount sufficient to convert all or substantially all of the H 2  S in the gas stream, and will generally be on the order of at least about two moles per mole of H 2  S. Ratios of from or about 2 moles to about 15 moles of vanadium-containing ions per mole of H 2  S may be used, with ratios of from about 2 moles per mole to about 5 moles of vanadium V-containing ions per mole of H 2  S being preferred. However, it is an advantage of the invention that, at least at low vanadium concentrations, the solutions employed are stable to over reduction, i.e., oxidative capacity is not degraded by absorption of amounts of H 2  S in excess of the theoretical stoichiometric amount. 
     Insofar as the process of the invention is concerned, any suitable manner of providing the vanadium as a vanadium V-containing ions in solution may be employed. Thus, for example, a vanadium V-containing compound may be dissolved in water, preferably to which phosphorus containing compounds of the invention have already been added. If employed, the thiosulfate species of the invention may then be added. Alternatively, a composition comprising a vanadium IV-containing material may be utilized, it being understood that such a composition can simply be &#34;regenerated&#34; by oxidizing the solution in the regeneration zone. 
     Compositions suitable for providing the vanadium V-containing ions include vanadium pentoxide, the common alkali metal and ammonium orthovanadates, pyrovanadates, and metavanadates, and vanadium sulfates, halides, phosphates, hydroxides, and oxy-halides, and mixtures thereof. If a vanadium IV-containing compound is utilized, similar types of compounds may be used. Preferably, the compositions of the invention employ sodium, lithium, potassium, or ammonium vanadates. 
     Similarly, any suitable sources of phosphate and the optional thiosulfate ions may be employed. Such materials include, but are not limited to, ammonium, sodium, potassium, and lithium phosphate, polyphosphate, -hydrogen or -dihydrogen phosphate, and mixtures thereof, and ammonium, sodium, potassium, or lithium thiosulfate, bisulfite, sulfite, dithionite, or tetrathionite, and mixtures thereof. Precursors of any of these materials may be employed. The term &#34;polyphosphate&#34;, as used herein, is understood to include ortho- and meta-phosphates, and the various phosphoric acids may be employed. The ratios of the various components are, as noted, supra. 
     When making the compositions employed in the invention, it is advantageous to dissolve the phsophate species first, adjust pH if necessary to near the desired final pH, dissolve the finely divided vanadate species, and then dissolve the thiosulfate species, if present. Heating and stirring the solution facilitates the dissolution of the added species. Alternatively, the phsophate species can be added to an existing solution comprised of the vanadate species and the thiosulfate species to make the claimed composition. If so, it is advantageous for the vanadate species to be in the oxidized form [vanadium (V)]. If the claimed composition is dissolved in water, the water can be evaporated to dryness by heating and/or reduced pressure, leaving the claimed composition as a dry solid. This dry solid redissolves faster than the original constituents and possesses the same properties as the original claimed composition. 
     An important feature of the invention is the discovery that phosphate ions increase the utilization of the oxidative capacity for a given concentration of vanadium V-containing solution, at the same time enhancing the solubility of the vanadium IV-containing ions produced in solution by the reaction of the H 2  S and the vanadium V-containing ions. If the reaction is carried out below the melting point of sulfur, such an amount also tends to effect an improvement in sulfur quality. According to the invention, sufficient phosphate ion is employed to provide a molar ratio of phosphate ion to vanadium IV-containing ion of at least 0.1. Preferably, the ratio will range from 0.3 or 0.5 to 6:1, most preferably from 1.5 to 3:1. The effects of the use of phosphate ions in the prescribed molar ratio are thus the inhibition of precipitation of the vanadium IV-containing ion, thereby allowing higher loadings of vanadium containing compounds, and the increase in the oxidative capacity of solutions employed at a given vanadium concentration by increasing utilization of the vanadium V-containing ions to approach the theoretical 2 to 1 vanadium V/H 2  S stoichiometry. Other benefits, such as increased oxidation rate, decreased salt production, and increased solution stability will accrue at various phosphate concentrations and ratios to Vanadium IV above the minimum given here. Thiosulfate ion, if used, will be present in a molar ratio of 0.1 to 1:1 with respect to the vanadium V-containing ions. 
     The manner of preparing an admixture containing an absorbent is a matter of choice. For example, the vanadium compound may be added to the absorbent, and, if necessary, then water added. The amount of water added will normally be just that amount necessary to achieve solution of the compound, and can be determined by routine experimentation. Since the vanadium compound may have a significant solubility in the solvent, and since water is produced by the reaction of the H 2  S and the vanadium V-containing ion, precise amounts of water to be added cannot be given. Preferably, however, the vanadium compound is added as an aqueous solution to the liquid absorbent. Where the vanadium V-containing ions are supplied as an aqueous solution, the amount of solution supplied may be about 20 percent to about 80 percent by volume of the total absorbent admixture supplied to the absorption zone. The vanadium containing solution will generally be supplied as an aqueous solution having a concentration of from about 0.05 molar to about 2 molar, and a concentration of about 0.5 to 1.0 molar is preferred. 
     The absorbents employed in this invention are those absorbents which have a high degree of selectivity in absorbing CO 2  (and preferably H 2  S as well) from the gaseous streams. Any of the known absorbents conventionally used which do not affect the activity of the agents employed and which exhibit sufficient solubility for the reactant or reactants may be employed. As indicated, the absorbent preferably has good absorbency for H 2  S as well, in order to assist in the removal of any H 2  S present in the gaseous streams. The particular absorbent chosen is a matter of choice, given these qualifications, and selection can be made by routine experimentation. For example, diethylene glycol, tetraethylene glycol, diethylene glycol monoethyl ether, propylene carbonate, tetraethylene glycol-dimethyl ether, N-methyl pyrrolidone, sulfolane, methyl isobutyl ketone, 2,4-pentanedione, 2,5-hexanedione, diacetone alcohol, hexyl acetate, cyclohexanone, mesityl oxide, and 4-methyl-4-methoxy-pentone-2 may be used. Suitable temperature and pressure relationships for different CO 2  -selective absorbents are known, or can be calculated by those skilled in the art. The manner of recovering the sulfur is a matter of choice. If molten sulfur is produced, the liquid sulfur may be separated by decanting. If the sulfur is solid, the sulfur may be recovered by settling, filtration, liquid flotation, melting, or by suitable devices such as a hydroclone, etc. 
    
    
     In order to describe the invention in greater detail, reference is made to the accompanying schematic drawing. The values given herein relating to temperatures, pressures, compositions, etc., are calculated or merely exemplary and should not be taken as delimiting the invention. 
     FIG. 1 illustrates the first embodiment of the invention, wherein solid sulfur is removed prior to regeneration. 
     FIG. 2 illustrates removal of the sulfur after regeneration. 
     Removal of sulfur &#34;during&#34; regeneration, i.e., from the regeneration zone, although not shown, is within the scope of the invention. Techniques such as those described in U.S. Pat. No. 4,243,648, Fenton, issued Jan. 6, 1981, may be employed. 
    
    
     In FIG. 1, sour gas, e.g., natural gas containing about 0.5 percent by volume H 2  S, in line (1) enters contactor or column (2) (tray type) into which also enters, from line (12), an aqueous admixture comprising an aqueous solution containing 25 grams per liter of vanadium V (added as NaVO 3 ), phosphate ion, added as 261 grams per liter of dipotassium hydrogen phosphate, and thiosulfate ion, added as 25 grams per liter of sodium thiosulfate. The pressure of the feed gas is about 1200 p.s.i.g., and the temperature of the aqueous admixture is about 55° C. A contact time of about 10 seconds is employed in order to react all the H 2  S. Purified or &#34;sweet&#34; gas leaves column (2) through line (3). The &#34;sweet&#34; gas is of a purity sufficient to meet standard requirements. In the admixture, the H 2  S is converted to elemental sulfur by the vanadium V-containing ions, vanadium V-containing ions in the process being converted to vanadium IV-containing ions. The aqueous admixture containing elemental sulfur is removed continuously and sent through line (4) to a depressurization and degassing unit (5), and then through line (6) to sulfur recovery unit (7). If CO 2  is present in the solution, degassing will have the effect of raising solution pH. This effect is beneficial on regeneration. Degassing may be accomplished prior to, during, or after sulfur removal, if sulfur removal is performed before regeneration. 
     Sulfur recovery unit (7) may be of any suitable type. Preferably, unit (7) comprises a filtration unit. It is not necessary that all the sulfur be removed in the sulfur recovery step, and some sulfur retention may be beneficial. The type of unit chosen is thus flexible. Preferably, the amount of sulfur removed in the separation step is simply balanced with the rate of sulfur intake in reactor (2), which is, of course, dependent on the amount of H 2  S in the gas stream (1). Those skilled in the art may adjust the appropriate rates of withdrawal of the streams. From unit (7) the sulfur-free or substantially sulfur free solution is sent via line 8 to regeneration zone (9). In regeneration zone or column (9) the admixture is contacted with excess air from line (10) to convert the vanadium IV-containing ions in the admixture to vanadium V-containing ions. The temperature of the regeneration column is about 45° C., and pressure in the column is maintained at about 2 atmospheres. Spent air is removed from column (9) through line (11), while regenerated aqueous admixture is returned via line (12) to contactor (2). Compressed air or O 2  under increased pressure may be used in the regeneration zone if it is not desired to depressurize the system, and this has the benefit of reducing the regeneration time. 
     As indicated, FIG. 1 illustrates the aspect of the invention wherein the sulfur recovery is carried out prior to regeneration. Removal of the sulfur after regeneration may be preferred in some instances, and may be accomplished by positioning of the sulfur recovery unit &#34;after&#34; the regeneration zone. Thus, in a separate embodiment, regenerated liquid, still containing sulfur, may be passed to units analogous or equivalent to unit (7), sulfur recovered, and regenerated sulfur-free solution returned to contactor (2). 
     Accordingly, in FIG. 2, numbers 1 through 5 refer to equipment or units identical to or substantially equivalent to that described in relation to FIG. 1. The sulfur-containing liquid is passed, after degassing in (5), via line (13) to regenerator (14) where it is regenerated, as previously described. The regenerated sulfur-containing admixture is removed via line (15), and passed to sulfur recovery unit (16). Spent air is removed via line (17). Regenerated reactant solution is returned via line (18) to contactor (2). 
     While the invention has been illustrated with particular apparatus, those skilled in the art will appreciate that, except where specified, other equivalent or analogous units may be employed. For example, although one type of contactor has been illustrated, other types of contactors, such as packed beds, venturis, etc., may be employed, and, in some instances, may actually be preferred. The term &#34;zones&#34;, 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 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. 
     Again, as will be understood by those skilled in the art, the solutions or mixtures employed may contain other materials or additives for given purposes. Make-up and bleed of solution may be carried by those skilled in the art. Pumps, valves, surge vessels, knock-out pots, etc., have not been illustrated in the drawing.