CA1219128A - Gas composition modification - Google Patents

Abstract

A B S T R A C T

GAS COMPOSITION MODIFICATION

An integrated process for modifying the H2/CO ratio in specified gaseous streams is disclosed, the process being charac-terized by bulk removal of H2S, adjustment of H2/CO ratio by water gas (or carbon monoxide) shift, and removal of remaining H2S. CO2 may be recovered, and the process may be operated to produce a gas comprising principally hydrogen.

Description

~Z~9~28 GAS COMPOSITION MODIFICATION

A number of gasification processes in existence or being developed, e.g., gasification of coke, residues, coal, etc., produce synthesis gases having various quantities of H2, CO, C02 and H2S, 2S well as minor "impurity" components of NH3 and HCN.
In the case of synth~sis gases derived from the gasification of coal, for example, the ratio of CO to H2 may range from 0.9 to 12:1, and the gas may contain from 0.05 per cent to 10 per cent by volume of H2S. If the "syn-gas" is to be used for fuel purposes, the ratios mentioned are generally satisfactory, and little need be done except elimination of contaminants such as H2S and HCN.
On the other hand, if other uses for the synthesis gas are contemplated, such as hydrogen production or use as a feedstock for synthesis operationsJ the ratio of H2 to CO may become critical, and ad~ustment of the H2/CO ratio to the right range may require great expense. Accordingly, a process that provided a ready method of adjustment of the H2/CO ratio from such gaæes, even to the production of hydrogen alone, could have great economic importance. The inven~ion relates to such a processO
Accordingly, the lnvention relates to a process comprising a) contacting a gaseous stream containing H2, CO, and H2S with an H2S--~elective absorbent in an absorption zone and absorbing the bulk of the H2S in said stream, producing a partially purified gas stream containing a minor portion of H2S;
b) contacting at least a portion of the partially purified gas stream with a water shift catalyst under condltions to react CO and water in a conversion zone, and converting CO and water to H2 and C02, and producing a modified gas stream having an increased ratio oE H2 to CO and containing a minor portion of ~2S;

~r lZ19~;28 c~ passing the modified gas stream to a contacting zone and contacting the modified gas stream with an aqueous reactant solution, t-ne solution containing an effective amount of an oxidizing reactant comprislng oxidizing polyvalent metal S ions or a polyvalent metal chelate of nitrilotriacetic acid or of an acid having the formula Y Y

N-R-N

Y Y, wherein - from two to four of the groups Y are selected from acetic and propionic acid groups;
- from zero to two of the groups Y are selected from 2-hydroxy ethyl, 2-hydroxy propyl, and X

wherein X is selected from acetic acid and propionic acid groups; and - R is ethylene, propylene or isopropylene or alternatively cyclohexane or benzene where the two hydrogen atoms replaced by nitrogen atoms are in the 1,2 position; and mixtures thereof, and converting H2~ in the modified gas stream in the contacting zone to sulphur, and recovering a substantially sulphur-free gas stream having an increased ratio of H2 to C0. In an additional embodiment, the substantially sulphur-free gas stream having an increased ratio of H2 to C0 is passed to an absorption zone which contains an absorbent selective for C02. Carbon dioxide is absorbed, and a gas stream having a high H2/C0 ratio or comprising H2 having a substantially reduced C02 content i8 ~Z~91Z8 produced. The invention thus provides an efficient method of producing a product stream containing a wide range of H2/CO
compositions, ranging to the point of virtually pure hydrogen. Additionally, an optional embodiment provides for removal of minor quantities of COS, if present, in the streams.
The source of the gaseous stream (containing H2, CO, and H2S) is not critical. Thus, the streams mentioned, i.e., streams derived from the gasification of coke, residues, coal, etc., are eminently suited to the invention. Other streams containing the components mentioned, and in which it is desired to adjust the ratio of H2 to CO and remove H2S, may also be treated according to the invention, so long as other components therein do not sub-stantially adversely affect the absorbents, catalysts, etc. em-ployed herein. In this regard, if the absorbents chosen are sen-sitive to HCN, removal of this contaminane before the stream is treated according to the invention is preferred. For example, the stream may be treated as described in U.S. Patent Serial ~o.
4~497,784 entitled ~emoYa1 of HCN from Gaseous Streams, by 20 Diaz, filed November 29, 1983. Streams derived from the gasification and/or partial oxidation of gaseous or liquid hydrocarbon may be treated by the invention. The H2S content of the type of streams contemplated will vary e~tensively3 but, in general, will range from about 0.05 per cent to about 10 per cent 25 by volume. CO content may vary considerably, and may range from about 30 per cent to over 80 per cent by volume. H2 content may also vary, but normally will range from about lO per cent to about 50 per cent by volume. C02, of course, may be present.
Obviously, the concentrations of H2S, CO and H2 present are not 30 generally a limiting factor in the practice of the invention. In some of the most economically attractive gasification processes, the CO to H2 volume ratios may be quite high, as mentioned previously.

,~r~, ', ~Z191~8 In the first step of the process, the gas stream selected is contacted or mixed with an absorbent selective for H2S in a manner or under conditions that will absorb the bulk of the H2S, preferably at least 80 per cent by volume. Any of the known H2S-selective absorbents conventionally used (or mi~tures thereof) which do not react substantially with the other components of the gas stream, may be employed. Those skilled in the art will recognize that most H2S-selective absorbents tend to absorb C02, and if any of this gas is present, it will also be absorbed. Given these qualifications, the particular absorbent chosen is a matter of cholce. Aqueous alkali metal carbonate and phosphate solutions, e.g., aqueous potassium and sodium carbonate and phosphate, carbitol (diethylene glycol monoethyl ether), and certain aqueous alkanolamines, such as alkyl diethanolamines, may be used. Suitable alkanolamines include methyldiethanolamine, triethanolamine, or one or more dipropanolamines, such as di-n-propanolamine or diisopropanolamine. Aqueous methyldiethanol-amine, triethanolamine and dipropanolamine solutions are preferred absorbents, particularly methyldiethanolamine and diisopropanolamine solutions. The solutions may ontain very minor amounts of physical solvents, such as substituted or unsubstituted tetra-methylene sulphones.
If diisopropanolamine is used, either high purity diiso-propanolamine may be used, or technical mixtures of dipropanol-amine such as are obtained as the by-product of diethanolamine production may be employed. Such technical mixtures normally consist of more than 90% by weight of diisopropanolamine and 10%
by weight or tess of mono- and tri-propanolamines and possibly trace amounts of diethanolamine. Concentrations of aqueous alkanolamine solutions may vary widely, and those skilled in the art can adjus~ solution concentrations to achieve suitable absorption levels. In general, the concentration of alkanolamine in aqueous solutions will be from 5 to 60% by weight, and preferably between 25 to 50% by weight. If COS is present in the gas, it may be removed in the absorbent, or may be hydrolyzed, as described herein.
Suitable temperature and pressu}e relationships for different hydrogen sulphide-selective absorbents are known, or can be calculated by those skilled in the art. In general, the temperatures employed in the absorption zone are not critical, and a relatively wide range of temperatures, e.g., from 0 to 100 C may be utilized. A range of from about 0 to 85 C is preferred.
Similarly, 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 one atmosphere up to 150 or even 200 atmospheres. Pressures of from 1 atmosphere to about 100 atmospheres are preferred. As indicated, what is required in the absorption zone is that the bulk of the 15 H2S, preferably at least 80 or 90 per cent by volume, be absorbed. Given the solvents and parameters mentioned, those skilled in the art may adjust the conditions of operation to achieve this result. It is thus an advantage of the invention that 811 of the H2S need not be removed at this point.
The absorption step thus produces a "purified" gas stream which has most of the H2S removed, leaving a minor portion of H2S, e.g., less than about 10 per cent to 20 per cent by volume N2S in the stream. The absorption liquid or solvent, being "loaded" or "semi-loaded", is preferably "regenerated" in suitable cyclic techniques, producing a stream rich in H2S and a "lean" absorbent which can be recycled for use in the absorption steps. Suitable techniques for these procedures are well known, and form no part of the present invention. See, for example, Canada patent 729,090, U.S. patent 3,989,811 and U.S. patent 4,085,192. Thus, in the regeneration or stripping zone, temperatures may be varied widely, the only requirement being that the temperatures be sufficient to reduce the H2S content in the absorbent to a level sufficient so that, when returned to the absorption zone, the absorbent will effectively absorb H2S from the gas to be treated. Preferably, the temperature should be ~21912~

sufficient to reduce the H2S content in the load absorbent to a level which ~orresponds to an equilibrium loading for an H2S
content having less than 50 per cent (preferably 10 per cent) of the H2S content of the treated gas. Equilibrium loading conditions for H2S and C02 at varying concentrations, temperatures and pressures for different hydrogen sulphide-selective absorbents are known or can be calculated by known methods and hence need not be detailed herein. In general, temperatures of from about 90 C to 1~0 C, preferably from 100 C to 170 C, may be employed.
Similarly, in the regeneration or desorption zone, pressures will range from about 1 atmosphere to about 3 atmospheres. As noted J the pressure-temperature relationships involved are well understood by those skilled in the art, and need not be detailed herein. Contact times in the absorption zone, insofar as meaningful, will depend, inter alia, on the velocity of the gas stream treated, the absorbent employed, and the type of contactor employed. In a tray column, for examples contact time might usefully be described as the total time a given volume of gas is present in the given absorber, recognizing the gas liquid contact may not occur coneinuously in such a unit. Given these qualifi-cations, "contact" times will normally range from 1 second to 30 seconds, preferably from 1 second to 20 seconds.
In sum, the conditions for the absorption and regeneration should be so specified that the bulk of the ~2S, preferably 80 to 90 per cent and most preferably at least 95 per cent~ by volume, of the H2S in the gas is absorbed. Such conditions, including choice of solvents and, e.g., number of trays, if a tray contactor is used, will provide that very little C02 is absorbed.
Any C02 or other gases ab40rbed will be released on regeneration, and are treated wieh the ~2S, e.g., in a Claus unit.
The partially "purified" gas i4 now passed to a conversion zone wherein it contacts water, preferably as vapour, in the presence of a catalyst for the reaction of water and C0, and under conditions suitable for the conversion. Since one mole of water reacts with one mole of C0 to produce the hydrogen and C0 1219~

and since equilibrium is not easily reached, the volume of H2 produced varies directly with the water and CO supplied. Suitable conditions, i.e., temperatures, pressures, contact times, catalysts, etc., are known to those skilled in the art. For example, Kirk-Othmer, Encyclopedia of Chemical Technology (~nd Edition), Volume 4, pages 431 and 432 (1967), the Catalyst Handbook, Chapter 6 (1970), and Catal. Rev. - Sci. Eng., Volume 21(2) pages 275-318 (19803 describe suitable conditions and catalysts for treating the purified stream. Appropriate ~atalysts include Fe/Cr for high temperature shift, and Cu/Zn for low temperature shift. The high temperature Fe-based supported catalystY have a higher sulphur tolerance than the Cu/Zn catalyst. However, the latter system, since it operates at low temperature~, can convert a higher proportion of CO and thus achieve a pronounced modification of the CO/H2 ratio. This is possible because the equillbrium of the water-shift reaction C~+H O ~ CO +H
lies to the right at lower temperatures. As indicated, the ratio of H~/CO is ad~usted to the extent desired by controlling the volume of water supplied to the conversion zone. Depending on the conditions applied and the volume of H2S remaining in the stream, at least some COS, if present, may be converted. Optionally, a COS conversion zone may be employed after the shift zone to remove any COS present in the stream. The hydrolysis of COS is shown by the following formula:
COS+H20 ~ H2S+C02 Water is added, in the COS conversion zone, in the required amount. Any catalyst demonstrating activi~y for this reaction may be employed. Preferred catalysts are Ni, Pd, Pt, Co, Rh or In. In general, most of these materials will be provided as solids deposited on a suitable support material, preferred amorphous support materials being the aluminas, silica aluminas, and silica. Crystalline support materials such as the alumino-silicates, known as molecular sieves (zeolites), synthetic or natural, may also be used. The selection of the particular 12~L9~28 catalyst (and support, if employed) are within the skill of those working in the field. Platinum on alumina is preferred.
The temperatures employed in the optional hydrolysis zone are not critical, except in the sense that the temperatures employed will allow substantially complete conversion of the COS.
Temperatures will range from about 50 ~C to 150 C or even 200 C, although a range of from about 50 C to about 150 ~C is preferred. Those skilled in the art may adjust the temperatures, as needed, to provide efficient reaction temperatures. Contact times will range from about 0.5 second to about 10 seconds, with contact times of 1 second to 3 seconds being preferred. Pressures employed ln the hydrolysis zone may be atmospheric, below atmospheric, or greater than atmospheric. If higher temperatures and a high temperature catalyst are employed in the shift zone, the gas stream exiting the shift reactor or the optional COS
hydrolysis zone should be pa~sed through a heat exchange zone, the heat from the gas preferably being utilized to heat the gas stream entering the shift zone.
In accordance with the invention, the remainder of the H2S
in the gas stream (and any ~2S produced by hydrolysis) is removed by contacting the stream with an aqueous reactant solution> the solution containing an effective amount of a reactant comprising oxidizing polyvalent metal ions, such as iron, vanadium, copper, manganese, and nickel, or a specified polyvalent metal chelate, and mixtures thereof. As used herein, the term "mixtures thereof"
includes mixtures of the polyvalent metal ions, mixtures of the polyvalent metal chelates, and mixtures of polyvalent metal ions and polyvalent metal chelates. The specified chelates are chelates of a polyvalent metal and ~itrllotriacetic acid or an acid having the formula:
Y

~-R-N

Y Y wherein :12~ 28 - from two to four of the groups Y are selected from ace~ic and propionic acid groupsj - from zero to two of the groups Y are selected from 2-hydroxy ethyl, 2-hydroxy propyl, and -CH2CH2N~
X, ~herein X is selected from acetic acid and propionic acid groups; and - R is ethylene, propylene or isopropylene or alternativelycyclohexane or benzene where the two hydrogen atoms replaced by nitrogen atoms are in the 1,2 position; and mixtures thereof. The oxidizing polyvalent metal should be capable of oxidizing hydrogen sulphide, while being reduced itself from a higher to a lower valence state, and should then be oxidizable by oxygen from the lower valence state to the higher valence state in a typical redox reaction. Iron, copper and manganese and particularly iron are preferred as polyvalent metals for the polyvalent metal chelate of nitrilotriacetic acid. Most preference is given to the Fé(lII) chelate of nitrilotriacetic acid. Iron(III) is preferred as polyvalent metal for the polyvalent metal chelate of an acid having the aforementioned formula. Most preference is given to the Fe(III) chelate of N-(2-hydroxy-ethyl)ethylenediamine triacetic acid. O$her polyvalent metals (or chelates thereof) which can be used include lead, mercury, palladium, platinum, tungsten, nickel, chromium, cobalt, vanadium, titanium, tantalum, zirconium, molybdenum, and tin.
In accordance with the invention, a substantially sulphur-free gas stream having an increased H2/~0 ratio is recovered. The conditions of operation of the oxida~ive removal of the remainder of the H2S from the gas stream, sulphur recovery, and regeneration of the oxidizing reactant solution are adequately ~219~2~3 described in U.S. patent 4,409,199 (Blytas), issued October 11, 1983, and U.S. patent 4,356,155 (Blytas and Diaz), issued October 26, 1982.
The product produced, from this stage, will depend on the degree of conversion in the previous shift step. The gaseous stream is treated under appropriate conditions with an absorbent selective for CO2 in the presence of H2 or H2 and CO. If the shift reaction has been utilized to adjus~ the H2/CO ratio to a given point, the product will be H2 and CO, in the given ratio.
On the other hand, if the CO is reacted to extinc~ion, the gas stream product will be comprised predominantly of hydrogen. Those skilled in che art may select appropriate CO2-selective absorbents, pressures, temperatures, etc., to separate the hydrogenJCO2 or hydrogen/CO2/CO mi~tures. Suitable absorbents include aqueous alkanolamines, sodium or potassium carbonate solutions, tri-potassium phosphate, or solutions of sterically-hindered amlnes in aqueous or organic solvents, or in combinations of,amines and potassium carbonate. Conditions for designing absorption and regeneration may be selected on the 20 basis of the specific case considered. Characteristics of the aqueous alkanolamines, alkali metal carbonates, and potassium metaphosphate are wel] known, as described in Gas Purification by A.L. Kohl and F.C. Riesenfeld (1960)o Use of sterically-hindered amines for CO2 absorption is described in U.S. patent 4 ~ 112 ~ 050 (1978)~ U~S~ patent 4~112~051 (1978)~ and U.S. patent 4~100~257 (1978)~ Preferably, temperatures will range from 10 C to 80 C, and pressures will preferably range from 1 atmosphere to 100 atmospheres. The CO2 absorption is preferably conducted as a cyclic process in which the CO2-"loaded" absorbent is regenerated or stripped, the "lean" absorbent being returned for use, and the C2 being recovered or vented.
Off-gases from the bulk H2S absorption-regeneration procedure are preferably oxidized to produce sulphur. The liberated H2S is preferably treated by that process known as the "Claus" process. In the "Claus" process, elemental sulphur is ~2~9~2~3 prepared by partial oxidation of the H2S to sulphur dioxide, using an oxygen-containing gas (including pure oxygen), followed by the reaction of the sulphur dioxide with the remaining part of the hydrogen sulphide, in the presence of a catalyst. This process, which ls used frequently at refineries, and also for the workup of hydrogen sulphide recovered from natural gas, is carried out in a plant which typically comprises a combustion chamber followed by one or more catalyst beds between which are arranged one or more condensers in which the reaction products are cooled and the separated liquid elemental sulphur is recovered. To some extent, the amount of elemental sulphur recovered depends on the number of catalyst beds employed in the Claus process. In principle, 93% of the total sulphur available can be recovered when three beds are used.
Since the yield of recovered elemental sulphur, relative to the hydrogen sulphide introduced, is not quantitative, a certain amount of unreacted hydrogen sulphide and sulphur dioxide remains in the Claus off-gases. These gases may be inclnerated in a furnace or treated in other ways known to those skilled ~n the art.
In order to describe the invention with greater particulari-ty, reference is made to the accompanying schematic draw~ng. All values are merely exemplary or calculated, and should not be taken as delimiting the invention.
As shown, a gas stream containing 2 per cent ~12S, 5 per cent C02, 48 per cent C0 and 35 per cent H2 (all by volume), enters absorber or contactor (1) via line (2). Absorber (1) is a tray contactor, although any suitable contacting device (such as a venturi) may be employed. An absorbent mixture, e.g., a mixture comprising 45 per cent by volume of water and 55 per cent by volume of sulfolane, enters contactor (1) via line (3). For illustrative purposes, it will be assumed that the gaseous stream enters at 200 mscf per hour, while the absorbent mixture enters at 20 M gallons per hour. Pressure of the gas in line (2) ls 100 psig, and the temperature is 45 C. The countercurrent flow of ~LZ~91;~8 liquid and gas, 2S illustrated, provides for good contact and absorption of the H2S in the stream. Approximately 97 per cent by weight of the H2S in the stream is absorbed, and the partially purified gas ls removed overhead via line (4).
The H2S-containing ("loaded") absorbent exits absorber (1) via line (5), and passes to stripping or regeneration column (6) wherein the H2S is stripped from the absorbent, preferably by heat supplied as steam. "Lean" absorbent is returned via line (3) for re-u~ilization in absorber (1) while H2S is removed via line ~7). The H2S in line (7) may be treated in any suitable fashion, but is preferably sent to a Claus unit. If C02 has been absorbed to any extent, provision may also be made for its removal or recovery.
Upon exit from rontactor (1), the gas stream, which has a substantially reduced H2S content, passes via line (4) to reactor or contact zone (8) wherein it is contacted with water supplied via line (9) and with a catalyst containing Fe/Cr on activated alumina. The gas in line (4) is preferably heat exchanged with the exit gas in line (10) before entry into reactor (8). The temperature of the exit of reactor (8) is about 300 C, pressure about 1000 psig, and total contact time in zone (8) is 2 seconds.
In this illustration, sufficient water~ as vapour, is supplied in a ratio of 0.3 mols per mol of C0 in the gas stream. More or less water may be supplied, the determining factor being the de8ree of conversion desired. If the COS in the original stream has not been absorbed by the absorbent in contactor (1), or if some remains in the gas stream in line (4), it may be hydrol~zed also in zone (8) to some de8ree. An optional COS hydrolysis zone (11) i8 shown (dotted lines) in line (10), the outlet line from zone 3G (8). Suitable catalysts and conditions for such removal are as described, supra; see the aforementioned U.S. patent 4,409,199.
In accordance with the invention, the gas stream, containing the modified gas stream, and posæible COS hydrolysis products, passes via line (10) to contactor (12) where it is contacted with an aqueous reactant solution to produce sulphur. Contactor (12) lZ8 is a tray contactor, although any suitable contacting device (such as a venturi) may be employed. An aqueous oxidizing reactant solution, e.g., a solution containing 0.4 molar of the Fe(III) chelate of N-(2-hydroxyethyl)ethylenPdiamine triacetic acid or of nitrilo-triacetic acid, enters contactor (12) via line (13). The gaseousstream enters at 225 mscf per hour, while the reactant solution enters at 400 gallons per hour. Pressure of the gas in line (10) is 800 psig, and the temperature of the gas, having exchanged heat with line (4), is 50 C. Reactant solution is supplied at a temperature of 40 DC. The countercurrent flow of liquid and gas, as illus~rated, provides for good contact and reaction of the ~2S
in the stream to sulphur. As will be understood by those skilled in the art, water and the Fe(II) complex or chelate of N-(2-hydroxyethyl~ethylenediamine triacetic acid or of nitrilo-triacetic acid are also produced by the reaction.
Upon exit from con~actor (12), the modified gas stream,which is now substantially free of H2S, passes through line (14) to absorption zone (15), as more fully described hereinafter.
Concomitantly, reactant mixture, containing some Fe(II) chelate of N-(2-hydroxyethyl)ethylenediamine triacetic acid or of nitrilo-triacetic acid and sulphur, is forwarded via line (16) to re-generation zone (17). As shown in dotted line boxes, the sulphur may be removed prior to regeneration or after regeneration.
Preferably, sulphur is removed before regeneration.
In regenerator (17), oxygen is supplied, via line (18), in molar excess. Preferably, the oxygen is supplied as air, in a ratio of about 2.0 or greater per mole of Fe(II) chelate in the mixture. Temperature of the mixture is preferably around 40 C, and pressure is suitably 20 to 30 psig. Regeneration in this manner has the added advantage of removing some water vapour, thus aiding in prevention of water build-up in the system and reducing bleed and make-up problems. It is not necessary that all of the Fe(II) chelate be converted.
Regenerated absorbent mixture, i.e. 9 an absorbent mixture in which at least the bulk of the Fe(II) chelate has been converted 9~'8 to the Fe(III) chelate, is removed via line (13) and returned to contactor (12).
Any suitable absorbent for removing C02 from the H2/CO
mixture in the stream may be employed in absorber (15). For example, aqueous diisopropanolamine/sulfolane mixtures may be employed. Suitable C02 absorption removal procedures and conditions are known to those skilled in the art, and form no part of the present invention. Suitably, the C02 removal procedure ls conducted with a regenerable absorbent, the desired modified stream being removed via line (18), and the loaded absorbent being removed for regeneration via line (19).
While the invention has been illustrated with particular apparatus, those s~illed in the art will appreciate that, except where specified, other equivalent or analogous units may be employed. The term "zone", 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. ~or example, an absorption colu~n 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, e.g., the oxidizing reactant solutions, may contain other materials or additives for given purposes. For example, U.S. Pat. No. 3,933,993 discloses the use f buffering agents, such as phosphate and carbonate buffers.
Similarly, U.S. Pat. No. 4,009,251 describes various additives, such as sodium oxalate, sodium formate, sodium thiosulphate, and sodium acetate, which may be beneficial.

Claims (5)

C L A I M S

1. A process comprising a) contacting a gaseous stream containing H2, CO, and H2S with an H2S-selective absorbent in an absorption zone and absorbing the bulk of the H2S in said stream, producing a partially purified gas stream containing a minor portion of H2S;
b) contacting at least a portion of the partially purified gas stream with a water shift catalyst under conditions to react CO and water in a conversion zone and converting CO
and water to H2 and CO2, and producing a modified gas stream having an increased ratio of H2 to CO and containing a minor portion of H2S;
c) passing the modified gas stream to a contacting zone and contacting the modified gas stream with an aqueous reactant solution, the solution containing an effective amount of a reactant comprising oxidizing polyvalent metal ions or a polyvalent metal chelate of nitrilotriacetic acid or of an acid having the formula - from two to four of the groups Y are selected from acetic and propionic acid groups;
- from zero to two of the groups Y are selected from 2-hydroxy ethyl, 2-hydroxy propyl, and wherein X is selected from acetic acid and propionic acid groups; and - R is ethylene, propylene or isopropylene or alternatively cyclohexane or benzene where the two hydrogen atoms replaced by nitrogen atoms are in the 1,2 position; and mixtures thereof, and converting H2S in said modified gas stream to sulphur, and recovering a substantially sulphur-free modified gas stream having an increased ratio of H2 to CO.

2. The process of claim 1 wherein the substantially sulphur-free modified gas stream is passed to an absorption zone containing an absorbent selective for CO2, CO2 is absorbed, and a gas stream comprising H2 having substantially reduced CO2 content is produced.

3. The process of claim 2 wherein CO2 is recovered.

4. The process of claim 2 or 3 wherein the oxidizing reactant is the iron(III) chelate of N-(2-hydroxyethyl)ethylenediamine triacetic acid or of nitrilotriacetic acid.

5. The process of claim 1 wherein the modified gas stream produced in step (b), prior to passing to step (c), is contacted with a COS hydrolysis catalyst under conditions to hydrolyze COS.