CA1102094A - Process for removal of hydrogen sulfide from gas streams - Google Patents

Abstract

"ABSTRACT OF THE DISCLOSURE"

A process for the removal of H2S from a feed gas, and the production of sulfur therefrom, is effected by oxidation with oxygen at temperatures between 250° and 450°F. The oxidation is conducted in the presence of an extremely stable oxidation catalyst comprising an oxide and/or sulfide of vanadium supported on a non-alkaline porous refractory oxide. Sulfur deposition and consequent catalyst deactivation are prevented by maintaining the partial pressure of free sulfur in the oxi-dation reactor below that necessary for condensation. H2, CO, and light hydrocarbons present in the feed gas are not oxidized.
Typical uses of the process include the removal of H2S and the production of sulfur from sour natural gases or gases obtained from the gasification of coal.
Feed gases which contain SO2 and H2S in mole ratios greater than 0.5, or which contain other gaseous sulfur com-pounds such as COS, CS2, SO3 and mercaptans, are desulfurized by catalytically converting all such sulfur components to H2S
and subsequently removing the H2S from the product gas by catalytic oxidation to elemental sulfur. This two-step catalytic process is especially contemplated for the desul-iurization of Claus tail gases.

Description

a The present invention relates to the removal of H2S
and recovery of free sulfur from feed gases containing H2S.
More specifically, it is concerned with a process for the selective oxidation of H2S to elemen-tal sulfur in gas streams which may also contain H2, CO or light hydrocarbons, said selective oxidation being conducted in the thermodynamically favorable tempera-ture range of 250-450F. The process is especially useful when Claus tail gas streams must be desulfurized.
The removal of H2S and recovery of sulfur from H2S-containing gases has been of major impor*ance to industry.
Petroleum refiners and natural gas suppliers in particular are concerned with such processes because H2S is present in many refinery gas streams and natural gases. Its presence in such gases is undesirable because of its noxious odor, toxicity, corrosive properties and, recently, because of its contribution to atmospheric pollution. As a result, numerous processes have been advanced to obviate the difficulties associated with the use or disposal of gases laden with H2S by removing it and effecting a conversion to marketable free sulfur.
The most successful method employed on a commercial basis which recovers sulfur from H2S-contaminated feed gases (especially sour natural gases and the like) is a process in which the H2S is first absorbed from the feed gases in solvents such as alkanolamines. These solvents are then stripped to recover a gas comprising about 85% H2S and 15% CO2 which is then processed for sulfur recovery in a Claus plant. Quite typically, this Claus process involves the combustion of a portion of the recovered gas to obtain sufficient SO2 to provide a 1:2 mole-ratio with H2S when the incinerated gases are recombined withthe remaining recovered gas. This mixture is processed through a series of two or three reactors containing a bauxite catalyst -1- ~

which effects the oxidation of H2S according to the well-known Claus reaction:
(I) 2H2S + S02~ 3S + 2H20 The sulfur produced in each reactor is condensed in sulfur con-densers situated af-ter each reactor, thus desulfurizing the recovered gas in successive stages. Although this process is used in many industries, the economical removal of H2S from the original feed gas stream to the Claus plant is limited to about 94 to 97% due to equilibrium limitations imposed by Reaction (I).
Because of the costly multi-step absorption-oxidation operations inherent in the Claus type of purification process, and the increasingly stringent environmental control standards, it has become a matter of great concern to develop a more economical process for the direct, and more complete, conversion of H2S present in feed gas streams to elemental sulfur. Idealiy, such a process would utilize only air or oxygen as an oxidant (without the necessity for separate facilities to produce SO2), would be entirely catalytic and be performed essentially in the gaseous phase. Also, the process should treat the feed gas directly, thus eliminating the costly absorption step. Hereto-fore, no process of this nature has been a practical possibillty.
Attempts directly to oxidize H2S with air according to:
(II) 2H2S + 2 ~2S (vapor) + 2H2O
necessarily also result in formation of some SO2 according to:
(III) 2H2S + 32 ~2SO2 + 2H2O
or (IV) S(vapor) + 2 ~SO2 The SO2 produced by Reaction (III) or (IV) then reacts with H2S
as in Reaction (I), and the final conversion of H2S is thus sti~
dependent to some extent upon the equilibrium limitations of Reaction (I). Also, light hydrocarbons, CO or H2, if present in 1 1~2~'~4 the feed gas, are usually oxidized to CO2, COS and water vapor, the formation of the latter further reducing conversion as defined by the thermodynamics of Reaction (I). The end result is not only a loss of sulfur recovery but also a possible loss of fuel gases and the production of an incompletely purified product gas.
Temperatures are of extreme importance in these oxi-dation processes because, as shown in FIGURE 1, the thermo-dynamics of Reac-tion (I) permits the highest conversion of H2S
to sulfur at relatively low temperatures of 250 to 450F. At these temperatures, however, the oxidation reaction kinetics are poor and no prior art catalysts are known which can effec-tively operate at these low temperatures. Additionally, at these low temperatures the condensation of sulfur on the catalyst may cause reactor plugging and/or catalyst deactivation. Efforts to deal with this condensation problem, such as by the use of swing reactors so as to permit frequent catalyst regeneration, increase the costs of operation. Hence, while low temperature operation is desirable, it is not without difficulties.
The several attempts to produce a competitive altern-ative to the Claus process and to effect direct catalytic oxidation of H2S in a feed gas with air or oxygen at low tem-peratures resulted in, at best, only marginal results. This was due in large measure to the difficulties just mentioned. The earliest methods, as disclosed in U. S. Patents 1,922,872 and

2,298,641, sought to employ bauxite catalysts of the kind utilized in the Claus process, or variations of those catalysts, to catalyze the desired oxidation. Such catalysts were found to be useful at temperatures above 450F if the feed gas con-tained relatively high concentrations ( ~10%) of H2S, but at low temperatures or with low feed concentrations of H2S the reac-tion tended to be sluggish and resulted in poor conversio~.
Generally, at such low tempera-tures the typical bauxite catalyst was found to be ineffective and required a follow-up operation, such as the absorption technique disclosed in U.S.
Patent 2,355,147, completely to remove the H2S. On a commercial basis, bauxite type catalysts alone have been used only at high temperatures between 800 and 1100F, at which temperatures H2 and light hydrocarbon gases are readily oxidized. Thus, although the conversion of H2S was more rapid due to the kinetics of Reaction (I) at such temperatures, the bauxite catalyst lost its selectivity for oxidizing H2S and conversions were necessarily poorer due to the thermodynamics of Reaction (I), as shown in FIGURE 1.
The use of other catalysts at the desirable low temperatures of 250 to 450F has only been of limited success.
For instance, sodium aluminosilicate zeolite catalysts, as described in U. S. Patent 2,971,824, have been reported to lose their effectiveness rapidly at below 450F. Other catalysts of an alkaline nature, such as the alkali metal sulfides disclosed in U. S. Patent 2,559,325, and the combination of alkali metal and alkaline earth metal oxides disclosed in U. S. Patent 2,760,848, have been reported to produce no SO2 in the product gas even in the presence of excess oxygen. Since the formation f S2 is inevitable even under conditions wherein insufficient oxygen (for Reaction (II)) is available, it is surmised that the SO2-reacted with the alkali present in the catalyst by an acid-base reaction. This would account for the lack of SO2 in the effluent gases and would result not only in a loss of alkalinity in the catalyst but also in rapid loss of catalytic activity and conversion efficiency.
In view of the foregoing, it can be seen that, although it is well-known that the highest theoretical conversion of H2S with air or oxygen to sulfur in the gas pnase occurs at temperatures in the range of 250-450F, i-t is also well-known that no catalyst has been totally successful in operating in this kinetically unfavorable temperature range.
Catalysts employed under the foregoing conditions either have little or no initial activity for high conversions of H2S to sulfur (assuming a commercially acceptable space velocity), or they lose their initial high activity within a relatively short period of time. As a result, no successful catalytic process is available in the prior art for obtaining high con-versions of H2S to sulfur in the gas phase at the relatively low temperatures of 250-450F.
Accordingly, it is the primary object of the invention to provide a gas phase, catalytic process for react-ing H2S with oxygen to produce elemental sulfur at temperatures in the 250 to 450F range.
It is another object of the invention to provide a catalyst for such a process that is highly active for the H2S to sulfur conversion in the gas phase at temperatures in the 250 to 450F range.
It is yet another o~j,ect of the invention to pro-vide a catalyst that will maintain its activity for extended time periods when H2S is being converted in the gas phase to sulfur at temperatures in the 250-450F range.
It is yet another object to provide an essentially all gas phase process for desulfurizing a feed gas containing assorted sulfur compounds, which process basically involves firstly converting the assorted sulfur compounds to H2S by contact with a catalyst containing Group VIB and iron group metal sulfides and subsequently contacting the produced H2S-containing product gas with a catalyst comprising a vanadium oxide or sulfide under conditions sufficient to convert the l~U~g~

H2S to sulfur.
It is yet another object to utilize a vanadium oxide or vanadium sulfide-containing catalyst in the final catalytic reactor of a Claus unit so as to improve the efficiency of the Claus unit.
Other objects and advantages of -the invention will be apparent from the following description and appended claims.
Briefly, the invention in its broadest embodiment is a process for the conversion of H2S to elemental sulfur, which process comprises contacting at a temperature between about 250 and 450F a feed gas stream containing H2S and a gaseous oxidant comprising elemental oxygen with a catalyst comprising one or more components selected from the group con-sisting of vanadium oxides and sulfides and further comprising a non-alkaline porous refractory oxide support, said contacting being conducted for at least 30 days under conditions such that at least some of said H2S is converted to elemental sulfur vapor, with said catalyst maintaining substantially undiminished activity for oxidizing H2S to sulfur under essentially the same conditions for at least 30 days of contacting.
In an alternative embodiment of the invention, useful for treating gas streams containing assorted sulfur compounds, the invention comprises a process for the desulfurization of a feed gas containing at least one sulfur component selected from the group consisting of SO2, COS, CS2, CH3SH, SO3, and sulfur vapor, which process, being carried out for at least 30 days, comprises the steps of: (1) converting at least some of said sulfur components to H2S by contacting said feed gas at a temperature between about 300 and 800F with a catalyst comprising a Group VIB metal sulfide and an iron group metal sulfide in the presence of gaseous reactants selected from the group consisting of H2, CO, and water vapor, said reactants 0~4 being present in a total proportion of H2 plus CO plus water vapor sufficient to convert a substantial proportion of said sulfur components -to H2S; (2) dehydrating the resulting product gas to a water vapor content of less than abou-t 15 volume percent; (3) contacting, at a -temperature between about 250 and 450F, a mixture of the dehydrated product gas produced in step (2) and a gaseous oxidant comprising sufficient elemental oxygen to oxidi~e at least 80% of said H2S to sulfur with a catalyst comprising one or more components selected from the group consisting of vanadium oxides and sulfides and further comprising a non-alkaline porous refractory oxide support, said contacting being carried out under conditions such that at least 80% of the H2S contained in said dehydrated product gas is converted to elemental sulfur vapor, with said catalyst maintaining substantially undiminished activity for oxidizing H2S to sulfur under essentially the same conditions for at least 30 days of contacting; and (4) separating free sulfur from the resulting gaseous effluent, and recovering a purified product gas.
In yet another embodiment more fully described herein~
after, the invention provides a novel control process for incr-easing the efficiency of a Claus unit. This embodiment may briefly be described as follows: In a Claus process wherein a mixture of gases comprising H2S and SO2 is passed, for a time period of at least 30 days, through a series of reactors, each of which contains a bed of catalyst, at a temperature and space velocity sufficient to produce elemental sulfur vapor in each of said reactors, and wherein the gases entering the final reactor in said series contain all three of the components H2S, SO2, and oxygen, the improvement comprising: (a) controlling said gases 09~

entering said final reactor such that the mole ratio of (SO2 +
O2)/H2S is about 0.5; (b) using as a catalys-t in said final reactor a composition comprising a vanadium oxide and/or sulfide supported on a non-alkaline porous refractory oxide, said cata-lyst being active for, and maintaining substantially undimin-ished activity for, 30 days for reacting ~12S with oxygen to produce sulfur; and (c) controlling the temperature in said final reactor at between about 250 and 450F and controlling the space velocity so that at least some of said oxygen reacts with said H2S.
FIGURE 1 is a graph showing the theoretical thermo-dynamic conversions of H2S to sulfur by air oxidation under anhydrous feed inlet conditions, said air oxidation occurring between 260 and 2420F.
FIGUR~ 2 and 2A are a flow diagram depicting the manner in which desulfurization of a gas similar to a ClauS
tail gas or a stack gas is accomplished in accordance with the invention. (As used herein, the term desulfurization is defined as the removal from a feed gas of the gaseous sulfur components contained therein, said gaseous sulfur components being selected from the group consisting of H2S,SO2, COS, CS2, SO3, sulfur vapor, and mercaptans).
FIGURE 3 is a graph depicting the percentage of the theoretical thermodynamic conversion at 400F one can expect to obtain if the oxidation of Reaction (II) is conducted in the presence of a gas stream having a water vapor dew point between 50 and 120F.
The present invention provides a novel process for oxidizing H2S to elemental sulfur with air or oxygen completely in the gaseous state in the temperature range of 250 to 450F.
H2S conversions over 99% are achievable. H2, CO, or light hydrocarbons, if present in the feed gas, remain substantially '9~

completely unoxidized and form none of the deleterious deriva-tive compounds, C02, COS, or water. No sulfur deposition occurs on the catalyst because the sulfur partial pressure is maintained below -that necessary for sulfur condensation.
Furthermore, the catalyst employed maintains its high activity for at least 30 days, normally for at least 90 days.
In general, the invention is most advantageously applied to those gas streams containing at least about 50 ppmv, usually at least about 100 ppmv, but less than about 10 mole percent of H2S, especially when at least about 500 ppmv of at least one normally oxidizable component selected from the group consisting of H2, C0, and light hydrocarbons is also present.
The process is thus most ideal for the treatment of sour natural gases, sour refinery gases, and gases obtained from the gasification of coal.
In addition to removing H2S from a feed gas stream, the oxidation process of the invention can also be used alone or in combination with the Beavon process described in Canadian patent 918,314 to desulfurize feed gases which con-tain other gaseous sulfur compounds. The oxidation processmay be used alone to treat those feed gases containing as essentially the only gaseous sulfur compounds therein, H2S
or both SO2 and H2S in a mole-ratio of SO2/H2S no greater than 0.5. Feed gases containing other gaseous sulfur compounds (such as COS, CS2, and mercaptans), or SO2 and H2S in a mole-ratio greater than 0.5, are most effectively desulfurized by hydrogenating and/or hydrolyzing all gaseous sulfur compounds in said feed gases to H2S by means of the Beavon process, and subsequently oxidizing the H2S in the hydro-genated feed gases to sulfur.

The feed gas streams to be treated herein, whetherby catalytic oxidation alone or by pretreatment by the Beavon l~U~14 process, may con-tain, in addition to one or more compounds of sulfur, one or more of the normally oxidizable components selected from the group consisting of H2, CO, and light hydro-carbons. (As used herein, the term light hydrocarbons refers to saturated hydrocarbons containing no more than six carbon atoms.) The sulfur components which can be removed by the process of the inven-tion include H2S, SO2, COS, CS2, SO3, sulfur vapor, and mercaptans. (As used herein, the term mercaptans refers to those saturated mercaptans containing no more than six carbon atoms.) Other components found in these and other feed gases usually include one or more of: CO2, N2, H2O, 2~ Ar, He, and NH3. As those skilled in the art will understand, these and many other components may be present in the feed gas to be desulfurized, but they should be present in chemically inert proportions under the desulfurization conditions to be utilized, and should not adversely affect or poison the catalyst or catalysts with which they come in contact.
The invention will now be described in relation to treating a feed gas, such as a Claus tail gas, requiring pretreatment by the Beavon process. Referring to the accom-panying flow diagram, FIGURE 2, a Claus tail gas is brought in via line 1 and combined at substantially atmospheric pres-sure, or at any other convenient pressure between about 5 and 500 psia, with water vapor and reducing gases from line 2, which reducing gases comprise, in the preferred method of operation, H2 and/or CO generated by partial combustion of a fuel gas.
In a preheater 3 the mixture of gases is heated to between 300 and 800F, preferably to between 500 and 800F, and more pre-ferably still, to between about 600 and 700F. The quantity of water vapor and reducing gas is controlled to provide at least a sufficient amount of each component stoichiometrically llUZ~

to convert all the sulfur-containing gases in the Claus tail gas to H2S by hydrogena-tion or hydrolysis. Preferably,about 1 to 2-1/2 times the stoichiometric amount of reducing gas, and 1 to 1-1/2 times -the stoichiometric amount of water vapor are provided. The mixture of heated gases is directed through line 4 to the hydrogenation reactor 5 wherein said sulfur-containing gases are converted to H2S according to the following reactions:

(V) S2 + 3H2 ~ H2S + 2H20 (VI) S H2 ~ H2S
(VII) CH3SH + H2- CH4 H2S
(VIII) CS2 + H2O ~COS + H2S
(IX) COS + H2O ~ CO2 + H2S
(X) SO3 + 4H2 ~ H2S + 3H2O
If CO is utilized as a component of the reducing gas, it is hydrolyzed in the hydrogenation reactor 5 to form H2 and C2 according to:
(XI) CO + H2O ~ C2 + H2' said H2 then being active for the conversion of the sulfur-containing compounds to H2S as previously described. In thiscase, of course, sufficient water must be available not only to convert the COS and CS2 to H2S, but also for the hydrolysis of CO to H2.
The hydrogenation reactor 5 preferably contains a pre-sulfided, sulfactive cobalt molybdate hydrofining catalyst, but any prereduced sulfactive hydrofining catalyst comprising one or more Group VIB metal sulfides and one or more iron group (i.e., iron, nickel, and cobalt) metal sulfides is suitable.
The preferred catalyst, however, is one composed of about 3-8%
cobalt oxide and about 8-20% molybdenum oxide on alumina in presulfided, reduced form. These catalysts in the preferred temperature range of 600 to 800F are extremely effective in 9g producing H2S according -to Reactions (V) through (X) from the sulfur con,?ounds present in Claus tail gases; equilibrium con-versions in excess of 80%, sometimes up to 100%, complete can be obtained. Lower temperatures in the range of 300-600F are also effective if the feed gas contains no COS, CS2 or mer-captans. Gaseous space velocities between 700 and 4000 v/v/hr can be utilized although preferred space velocities range between abou-t 1500 and 2500 v/v/hr. Pressures of about 0-200 psig are preferred.
After leaving hydrogenation reactor 5 through line 6 the hydrogenated Claus tail gases, now containing H2S as sub-stantially the only sulfur component therein, are cooled to re- -move water via line 8 by means of a condenser 7. Water could, of course, be eliminated from the gases in a number of ways, including by absorption in desiccants, but it is important at this point to remove as much water as is economically possible.
As shown in FIGURE 3 the conversion of H2S to sulfur at 400F
by the oxidation process to be described hereinafter is thermo-dynamically dependent upon the amount of water vapor present in the hydrogenated Claus tail gases, and conversions from as low as 80% to as high as 95% can be achieved. Thus, unless lower conversions are sufficient, or the gases leaving hydrogenation reactor 5 are inherently low in water vapor concentration, it is preferred that the hydrogenated gases be cooled to about 50-130F to condense sufficient water to yield a gas containing less than about 15%, preferably less than about 10% by volume of water vapor, so that at least an 80 to 90% conversion of H2S to sulfur can be accomplished. Better conversions, however, can be achieved if still lower condensation and/or reaction temperatures are utilized.

The hydrogenated Claus tail gases, dehydrated for example to a water vapor dew point in the range of 50-60F, 1 1~2~J~

are passed from line 9 to line 11 and are therein mixed with air or free oxygen introduced from line 10. It is highly desirable in carrying out the subsequent conversion in oxidation reactor 14 that the air or oxygen be supplied via line 10 such that the resultant mixture of gases in line 11 contains only the stoichiometric amount of oxygen necessary according to Reaction (II) to effect the conversion to sulfur of all the H2S in the mixture. It has been found that the use of air in amounts sub-stantially beIow or above that necessary for Reaction (II), al-though contemplated herein, generally results in poor conversions of H2S to sulfur. Additionally, as will be explained herein-after, the use of excess oxidant may deactivate some forms of the oxidation catalyst hereinafter to be described by sulfation.
Thus, although the use of oxygen in amounts other than the stoichiometric amount required for Reaction (II) may be found feasible or utilitarian in some instances, it is recommended that oxygen, preferably in the form of air, should be supplied in substantially the exact stoichiometric amount required by Reaction (II).
Other oxidants capable of being fed via line 10 are those comprising free oxygen and/or S02, the latter producing sulfur via Reaction (I). For the same reasons noted herein-before with respect to oxygen and air, these oxidants should be utilized only in the stoichiometric amounts necessary to pro-duce sulfur. Thus, gases comprising oxygen and/or SO2 should be added such that the hydrogenated Claus tail gas-oxidant mixture in line 11 will contain 2 and/or SO2 in a molar ratio with H2S substantially equal to 0.5, i.e., (2 + SO2)/H2S = 0.5.
The mixture of hydrogenated Claus tail gases and pre-ferably stoichiometric air (for Reaction (II)) is preheated in preheater 12 to a temperature of at least about 250F but no more than 450F, and fed via line 13 to the oxidation reactor 14 at a space velocity above about 100 v/v/hr, usually between about 250 and 2000 v/v/hr, but preferably between about 800 and 1000 v/v/hr. These gases contact a catalyst in the oxidation reactor 14 at temperatures ranging between about 250 and 450F, preferably about 300-400F. The catalyst comprises a vanadium oxide and/or sulfide supported on a non-alkaline, porous refrac-tory oxide. This catalys-t, described in fuller detail herein-after, is highly active for the conversion of H2S to elemental sulfur in the thermodynamically favorable, but kinetically unfavorable, temperature range of 250 to 450F. When operating within this temperature range, 90 to 95% conversions of H2S
to sulfur are easily achieved, even at high space velocities.
Furthermore, because the catalyst is selective for the oxidation of H2S, such highly oxidizable components as H2, CO, and light hydrocarbons, which all might be present, remain essentially completely unoxidized.
When H2S is oxidized in reactor 14 with air or oxygen oxidant, Reaction (II) defines the overall reaction by which the H2S is converted to sulfur. However, as with all gas phase oxi-dations of H2S to sulfur with air or oxygen oxidants, it isbelieved that only some of the H2S is oxidized to sulfur via Reaction (II) while the remainder is converted via Reaction (III) followed by (I). Thus, although the conversion of sulfur by oxygen via overall Reaction (II) or by SO2 via Reaction (I) are both dependent upon the equilibrium of Reaction (I), an advantage is gained when 2 rather than SO2 is fed as oxidant via line 10. For the same amount of H2S converted to sulfur, 50% more sulfur is formed by Reaction (I) with SO2 than by overall Reaction (II) with 2 The formation of 50% more sulfur by Reaction (I) necessitates higher operating tempera-tures for Reaction (I) than for Reaction (II) if the sulfur dew point is not to be exceeded. But by operating at higher ~i~Z~194 temperatures, as seen in Figure 1, the conversion of H2~ to sulfur decreases. Thus, because H2S can be converted to sulfur by overall Reaction (II) at lower temperatures than Reaction (I) without exceeding the dew point, more H2S can be removed in reactor 14 when 2 is utilized as oxidant via Reaction (II) than when S02 is utilized via Reaction (I).
The sulfur vapor con-tained in the gases leaving the oxidation reactor 14 through line 15 is first condensed, pre-ferably at about 250-270F, in a sulfur condenser 16 wherefrom -14a-11(~26~94 molten sulfur is discharged via line 17. Optionally and pre-ferably, the off-gases in line 18 are further purified of water, sulfur, and water-soluble sulfur compounds (notably SO2) in another condenser 19 operating below about 70F, more preferably between about 40-50F. Sour water containing some S2 is thus removed via line 20 for reprocessing to preheater

3. In addition, this second condensation in condenser 19 removes tin the case of Claus tail gases) an additional 50-150 ppm of sulfur vapor from the gases, leaving a gas purified of at least 80% but usually at least 90% of the original sul-fur components contained in the Claus tail gas feed. Also, by reprocessing the sour water, sulfur components are recovered for further treatment within the process, thus eliminating costly extraneous desulfurization procedures and improving overall desulfurization efficiency.
The off-gases from the sulfur condenser 16 can, because of the complete consumption of oxygen (i.e., when preferred operating conditions are utilized) in the oxidation reactor 14, be partly recycled for conversion of residual SO2 to H2S via lines 22, 24, 26, 28 and blower 27 to preheater 3.
The absence of oxygen in these recycle gases permits recycle without deactivation of the cobalt-molybdate hydrogenation catalyst in reactor 5, and by this recycle procedure a portion of the unconsumed reducing gas supplied via line 2 is utilized for conversion of the residual SO2. Other uses for the recycle gas, to be explained in more detail hereinafter, include direct temperature control of the exothermic oxidation reactions in reactor 14 by quenching via lines 18, 22 and 23, the in-direct temperature control by dilution of the hydrogenated feed gas-oxidant mixture entering the oxidation reactor 14 via lines 18, 22, 24 and 25.

ll~Z~94 The off-gases from condenser 19 can, as shown in FIGURE 2A, ei-ther be discharged to the atmosphere via lines 21, 30 and 31 or subjected to one of a number of post treatments.
One such post treatment consists in removing most of the remaining H2S, SO2 and sulfur vapor contained in these gases by means of a second stage of partial oxidation. This is accomplished by combining the gases transported via lines 21 and 32 with sufficient oxidant, preferably air or oxygen, from line 33 to provide in the resultant mixture a mole-ratio of (2 + SO2)/H2S of preferably about 0.5, thus meeting the stoichiometric requirements for Reactio~ (I) and (II). This mixture is then passed via line 34 to preheater 35 wherein it is heated to between about 250 and 450F and then fed via line 36 at a space velocity between about 250 and 2030 v/v/hr, but preferably between about 800 and 1000 v/v/hr, to the second oxidation reactor 37.
This second oxidation reactor 37 may contain any of the catalysts hereinafter described in fuller detail for use in the first oxidation reactor 14. Also, it can be operated in the same broad and preferred temperature ranges of 250-450F and 300-400F, respectively, as recommended for the first reactor. However, because the concentration of reactant sulfur compounds (i.e., H2S, SO2 and sulfur vapor) in the gases entering reactor 37 is much lower than that entering the first (thus producing lower sulfur vapor dew points upon being converted to sulfur), the lower thermo-dynamically favorable temperatures of about 250-350F can usually be safely maintained therein. When a second stage oxidation reactor is thus utilized, conversions therein of H2S to elemental sulfur between about 50% and 90% complete can be achieved. Usually however, conversions in excess of 80%

complete can be obtained. Overall desulfurization through the 2~9~

use of the hydYogena-tion reactor followed by tw~ oxidation reactors as described can result in the removal of greater than about 95%, even greater than 99%, of the total sulfur compounds in the original Claus tail gas feed. Following the oxidation in reactor 37, the gases are sent via line 38 to sulfur condenser 39 operating between 260 and 280F to pro-duce sulfur via line 40. The remaining purified gases are ; then sent ei-ther to atmosphere through lines 41 and 31 or to an incinerator 55 as described hereinafter.
The following two examples of a desulfurization process essen-tially as described above are provided to illus-trate the effectiveness of the invention. It will be shown how Claus tail gases can be desulfurized to meet present atmospheric pollution regulations (in the Los Angeles basin area) requiring the discharge of less than 500 ppm total sulfur compounds and no more than 10 ppm H2S. In all examples herein, the results are reported on a dry mole-percent basis, and although the purified gas streams shown therein contain more than 10 ppm of H2S, this level can easily be reached by conventional incineration of the H2S to SO2.

EXAMPLE I
After being blended with products from a reducing gas generator, a Claus tail gas had a composition, reported on a dry basis (30 mole % water vapor), as shown in column 1, Table I. This gas was mixed with an equal volume of recycle gases at atmospheric pressure and the resultant mixture passed at 400 scc/min (2000 GHSV) into a hydrogenation reactor con-taining 12 cc of catalyst comprising 12% molybdenum oxide (MoO3) and 6% cobalt oxide (CoO) on alumina. The temperature of the reactor was maintained at 720F. A hydrogenated gas having the dry composition shown in column 2, Table I was recovered. This hydrogenated gas was cooled to 55F to remove l:~V2~ 34 sour condensate water, then blended with 4.22 scc/min of air and finally fed at a space velocity of 928 v/v/hr into an oxidation reactor maintained at 400F by suitable external means and containing 21.43 cc of a catalyst comprising 10% V2O5, 70%
hydrogen Y-zeolite and 20% A12O3 (prepared as described in Example IV). Sulfur was recovered from the off-gas in a sulfur condenser maintained at 260F; sour water was then condensed at 55F and reprocessed back to the hydrogenation reactor. After seven days operation in such manner a gas having the dry compo-sition shown in column 3, Table I was obtained.

ll~Z~94 Table I

Mixture of Claus Tail Hydrogenated Gas, and Reducing Gases Mole %, (including Final Component Mole % Recycle Gas) Product H2 5.2333 4.8606 5.1045 CO 2.3524 0.9933 0.8781 CH4 0.0022 0.0195 0.0212 N2 85.9382 86.8064 87.2815 2 0.0245 0.0075 0.0036 H2S 0.6256 0.5332 0.0261 Ar 0.0085 0.0336 0.0385 C2 5.3700 6.7438 6.5888 CH3SH 0.0004 0.0004 0-0005 COS 0.0664 0.0014 0.0022 S2 0.3175 0.0003 0.0103 CS2 0.0610 0.0000 0.0007 Total sulfur compounds* 1.1319 0.5353 0.0405 % Overall Removal of Sulfur Compounds from Hydrogenated Gas(l) 92.43 % Overall Removal of H2S from Hydrogenated Gas(l) 95.10 % Overall Conversion of H2S in Hydrogenated Gas to Sulfur(l) 93.17 *Expressed as moles of monatomic sulfur compounds.

(1) Note: % overall removal and conversion results reported in all examples herein will be slightly higher than actual because the sulfur components concentrations in product compositions have been diluted with air oxidant; the effects of such dilution have been ignored.

This Example demonstrates that the theoretically determined conversion and removal of 90 to 95% of the H2S
present in the hydrogenated Claus tail gas is obtainable by the process of the invention and that H2, CO and light hydrocarbo~
remain substantially unreacted. These results were obtained despite the fact that only a relatively moderately active catalyst was utilized.
EXAMPLE II
To illustrate the effectiveness of the invention in the situation wherein a hydrogenated Claus tail gas is pro-cessed through two stages of oxidation, the 90 to 95% desul-furized gas obtained in Example I was fed to a second oxidation reactor containing the same type and amount of catalyst. This reactor, however, was maintained at 325F and no recycle gases from the second reactor were diverted back to the hydrogenation unit. The gas fed to the second oxidation reactor was blended with nitrogen-diluted air (0.89 mole % 2) entering at the rate of 2.0 scc/min, and flow rate through the reactor was at 560 v/v/hr space velocity. After condensing out sulfur at 260F
and water at 55F from the effluent of the second reactor, and operating in the manner described for seven days, a gas having the following molar-percent composition (reported on a dry gasis was obtained:
Table II
Component Mole % Component Mole %
H2 5.0564 C2 6.6861 CO 0.9652 CH8SH 0.0005 CH4 0.0205 COS 0.0005 N2 87.2224 S2 0.0052 H2S 0.0023 CS2 0.0007 Ar 0.0402 Total Sulfur Cmpds* 0.0099 % Overall Removal of Sulfur Compounds from Hydrogenated Gas of Table I............... 98.15 % Overall Removal of H2S from Hydrogenated Gas of Table I............................ 99.57 % Overall Conversion of H S in Hydrogenated Gas of Table I to Sul~ur.................. 98.60 'Expressed on a monatomic sulfur compound basis.

This Example demonstrates that by use of two stages in series, over 98% complete removal of sulfur compounds in hydrogenated Claus tail gases can be effected. Also, as in the case of Example I, no significant oxidation of H2, CO or light hydrocarbons occurs in the second oxida-tion stage.
As those skilled in the art will realize, it is of prime importance in this process to control the temperature rise in the oxidation reactor 14. Although the exothermic oxidatio~
of H2 and C0 are not significant factors, there still exists the difficulty of dissipating or controlling (in the case of Claus tail gases) the 130F temperature rise per mole of H2S
oxidized to elemental sulfur per 100 moles of reactor feed.
Exterior cooling apparatus could of course be utilized for this purpose, but this involves unnecessary expense. Prefer-ably, therefore, an inert diluent gas (i.e., inert under the oxidizing conditions in oxidation reactor 14) is supplied to oxidation reactor 14 from an external source to quench the temperature rise. Alternatively, recycle gases can be added directly to the oxidation reactor 14 via line 23 for tempera-ture control or indirectly via line 25 to preheater 12 tocontrol oxidation temperatures by reducing the H2S concentra-tion entering reactor 14. By any of these or equivalent means gas temperatures can be maintained below the thermodynamically favorable equilibrium temperature of 450F shown in FIGURE 1.
The concentration of H2S which must be maintained by recycle gas dilution via line 25 to permit complete indirect temperature control (when using recycle gases derived from a Claus tail gas feed as herein described) depends upon the exit gas temperature from preheater 12, and is determined by divid-ing the difference between the desired maximum reactor outlettemperature and the said preheat exit gas temperature by 130F/mole-percent of H2S. For example, if a preferred preheat ~2~jg4 temperature of 320F is utilized, and a maximum peak reactor temperature of 450F is desired, a concentration of H2S of 1.0 mole percent or less in the gases entering the oxidation reactor will be required to maintain the reactor temperature below the peak 450F temperature, and in such a case there will be no requirement for external cooling or quenching.
In the preferred mode of operation, recycle gases are utilized ra-ther than externally supplied inert gas to control oxidation temperatures. The advantages in using the recycle gases for this purpose may not at first be apparent. In addi-tion to allowing some SO2 and H2S present in these gases to be converted to sulfur (via Reaction (I)) and thus improving the overall desulfurization efficiency, the normally exothermically oxidizable components of H2, CO and light hydrocarbons present in the recycle gases remain unreacted with oxygen. As a result, those components do not contribute to the temperature rise in the oxidation reactor, and in fact serve as a heat sink. Since no water is formed by the oxidation of H2 or light hydrocarbons, the equilibrium of Reaction (I) is not un-favorably affected. Likewise, no reaction via:
(XII) CO + S ~ COShas been observed below about 450F in the oxidation reactor, so that there is substantially no COS, an unrecoverable sulfur component, in the purified gaseous effluent.
Dilution of the gases entering preheater 12 or in oxidation reactor 14 with the recycle gases from line 23 serves a purpose additional to temperature control. To prevent catalyst deactivation and possible pluggage in the oxidation reactor, it is necessary that condensation of sulfur vapor in the reactor be prevented. This can be accomplished by depressing the sulfur vapor dew point tem-perature by diluting the gases entering, or already in, ~l~Z~94 oxidation reactor 14 with recycle gases so that the partiai pr ssure of the sulfur vapor formed therein can never exceed that necessary for condensation. Generally, however, unless the hydrogenated Claus tail gases contain more than about 10 mole percent H2S, the use of recycle gases to prevent sulfur deposition is unnecessary, inasmuch as operating conditions can be selected from among those hereinbefore shown to accom-plish the same result.
The most critical aspect of the invention resides in the nature of the catalyst utilized in the oxidation reactor.
In general, catalysts comprising a vanadium oxide and/or sul-fide supported on a non-alkaline porous refractory oxide are operative. Suitable non-alkaline supports, as defined herein, include such refractory oxides as silica, alumina, silica-alumina, silica-magnesia, zirconia, silica-zirconia, titania, silica-titania, silica-zirconia-titania, or combinations of the aforementioned materials. Acidic metal phosphates and arsenates such as aluminum phosphate, boron phosphate, chromium phosphate, rare earth phosphates, aluminum arsenate, etc., may also be used, as also may certain amorphous and crystalline alumino-silicate zeolites, including such naturally occurring zeolites as mordenite, erionite, stilbite, faujasite and the like (in their "non-alkaline" forms - as hereinafter defined).
Synthetic forms of these natural zeolites can also be used with success. Synthetic hydrogen "Y" zeolites prepared by ion exchange with an ammonium salt followed by heating to decompose the zeolitic ammonium ion to leave hydrogen ions are partic-ularly contemplated as suitable supports, especially when composited with alumina to produce a support containing about 20-25 weight-percent alumina. These hydrogen "Y" zeolites are further characterized by a SiO2/A1203 mole-ratio preferably in the range of 4:1 to 5:1, but those in the range of 4:1 to 6:1 or even 3.5:1 to 6:1 are also contemplated. Preferred crystal-line aluminosilicate zeolites, whether natural or synthetic, consist of silica and alumina in a ratio between about 4:1 and 100:1. Especially contemplatecl are those natural and synthetic crystalline aluminosilicate zeolites having a silica-to-alumina ratio between about 6:1 and 100:1, mordenite and erionite, particularly in the hydrogen or decationized forms, being found to be most suitable. In general, zeolitic-supported catalysts are most active when feed gas-oxidant mix-tures are contacted therewith at temperatures in excess of350F, at space velocities below 500 v/v/hr, and at a pressure above about 50 psig.
The "non-alkaline supports" employed herein may be characterized as materials which contain no more than about 4 weight-percent, preferably less than about 2 weight-percent of alkali metal or alkaline earth metal compounds, calculated as oxides, which compounds are sufficiently basic to form salts with anionic oxides of the active metal component, e.g., vanadates. Such salt formation is believed to be at least one alkali-induced transformation leading to rapid deactivation of the catalyst. Sodium zeolites are exemplary of such undesirable basic compounds.
Alumina is a preferred support in the oxidation process of the invention, primarily because of its stability in the presence of water vapor. Furthermore, because of the relatively low temperatures and limited quantities of oxygen encountered in this process, one tends to avoid (as will be shown in Example IV) the sulfation problems which sometimes arise when alumina-based catalysts are used in the presence f S2 plus 2 Thus, unless it is necessary to treat a feed gas-oxidant mixture containing more than about 2 mole % H2S, alumina and other sulfatable supports such as silica-magnesia, zirconia, silica-zirconia, titania, silica-titania, sllica-zirconia-titania, etc., may be used with success.
The remaining catalyst supports hereinbefore mention~d also have been found to be very stable in the presence of SO2 plus 2' and even S03, and their use in treating feed gas-oxidant mixtures containing more than about 2 mole percent H2S
is normally feasible, even for time periods in excess of one year, depending upon other process factors. Silica, for example, does not sulfate but because of its well-known sus-ceptibility to decomposition and volatilization in the presence of water vapor, it should not be used in environments wherein the water dew point can exceed about 120F. Also, silica supported catalysts have been found to be of insufficient activity unless the operating pressure is above about 50 psig.
Silica-alumina supports containing at least lO
weight-percent alumina, preferably between about 20 and 30 weight-percent alumina as in high alumina, commercial cracking catalysts, are most preferred. Such catalysts not only are resistant to sulfation and water vapor attack, but they are also active when utilized under any operating pressure in the range of 0-200 psig.
The foregoing supports are compounded, as by impreg-nation, with from 0.2 to 30 weight-percent, preferably 2.0 to 20 weight-percent, of a vanadium promoter. Specifically, any oxide and/or sulfide of vanadium will perform satisfac-torily.
The preferred active metal promoter, however, is vanadium pentoxide (V205) when present in the catalyst between about 1 and 30 percent by weight. Especially preferred, however, is a catalyst comprising between 2 and 20 weight-percent V2O5, more preferably between about 5 and 15 weight-percent V2O5.
After being pelleted or extruded, the catalyst is subsequently dried, calcined for several hours, and then 11~2~94 preferably reduced. This reduction can be accomplished by preIeduction with hydrogen, or by utilizing hydrogen or carbon monoxide present in the feed gas. A typical prereducing procedure, and the one employed in all examples herein, comprises passing a mixture of gases consisting of 10 mole~
percent H2S and 90 mole-percent H2 at a temperature of about 400F and at a space velocity between about 400 and 600 v/v/hr over the catalyst for about two hours.
EXAMPLE III
This Example is cited to illustrate the superior performance of a catalyst of this invention versus the 13X
molecular sieve catalyst of U. S. Patent 2,971,824, promoted or unpromoted with vanadium. These alkaline "X" zeolite catalysts are also deemed to be comparable in most respects to the alkaline catalysts of U. S. Patents 2,559,325 and 2,760,848. ' The catalysts tested were as follows:
(1) 13X Molecular Sieve Catalysts:
This catalyst was a commercial 1/16" extrudate of Linde 13X molecular sieve.
(2) 66.6% Na "X" Zeolite, 20% A12O3 and 13.4% NaVO3 A solution of NaVO3 was prepared by dissolving 5.0 grams of V2O5 in a solution of 2.5 grams of NaOH in 50 ml water. 44.5 grams of Na "X"
zeolite (33.3 grams anhydrous) was soaked in the NaVO3 solution and evaporated to dryness.
Half the dried product was mixed with sufficient alumina hydrate to produce 20% A12O3 (anhydrous) in the finished product; the mixture was pelleted with 1% hydrogenated corn oil and calcined at 932F for 3 hours and then reduced.

(3) 5% V2Os on A123 A vanadium solution~as prepared by dissolving 13.5 grams of NH4VO3 in 500 ml of hot (75C) water. This hot solution was then poured onto 270 grams of spray-dried alumina hydrate (25.8~6 volatiles of H2O) to form a uniform paste. This was then dried at 110C. The dried product was mulled with enough water to form an extrudable paste, which was then extruded through a 1/8"
die. The extrudate was dried at 110C, calcined at 932F (500C) for 3 hours and then reduced.
Each of the above catalysts was tested in two oxida-tion reactors operating in series to desulfurize a partially dried (water removed at 550Fj hydrogenated Claus tail gas similar in dry composition to that shown in Table I. No recycle gases were utilized for any purpose and air was used as the oxidant. Sulfur and sour water were condensed at 260F and 55F, respectively, from the gases leaving each oxidation reactor. Other operating conditions and the results obtained 20 are tabulated in Table III.

Table III
Catalyst No.: (1) (2) (3) 66% Na- "X"
13X Molecular Zeolite . 5% V2O5 Sieves(l) 13.4% NaVO3(1 on A12O3(4) Days of Operation Cl(3) ~1(3) 1 6 1 6 Total sulfur compounds entering 1st oxidatio reactor, mol %(2) 1.6084 1.4806 1.3126 1.3170 .3521 1.3012 10 Temperature of 1st oxidation reactor,F 400 500 400 400 400 400 Space velocity of 1st oxidation reactor, v/v/hr. 2000 2000 1752 1752 1752 1752 Total sulfur compound leaving 1st ox~dation reactor, mol %-2) .3243 0.4546 0.0815 0.3341 0.0676 0.1749 1st oxidation, % re-moval of sulfur 20 compounds 79.84 69.30 93.79 74.63 5.00 86.56 Temperature of 2nd oxidation reactor,F X 325 325 325 325 Space velocity of 2nd oxidation reactor, v/v/hr. X 1752 1752 752 1752 Effluent of 2nd reactor, total sulfur compounds,mol %(2) X 0.0183 0.1011 .0131 0.0175 H2S, mol % X X 0.0127 0.0887 .0032 0.0024 CH3SH, mol % ~ X 0.0004 0.0003 .0003 0.0004 SO2, mol % X X 0.0033 0.0094 .0088 0.0129 COS, mol % X X 0.0019 0.0025 .0008 0.0018 CS2, mol % X X 0.0000 0.0001 0.0000 0.0000 2, mol % X X 0.0656 0.3224 .0000 0.0000 H2, mol % X X 5.67 5.52 5.75 5.41 Overall Sulfur Compds.
Removal, % X X 98.60 92.32 9.03 98.65 (1) Excess air required because when only stoichiometric air was used results were not as favorable.
0 (2) Expressed as moles of SO2 or as moles of monatomic sulfur compounds.
(3) Numerous attempts to utilize this catalyst ended in failure; no long term data available.

(4) Stoichiometric air employed.

11~2~9~

The data in this Example demonstrate the superiority of the catalysts used in the invention in several ways. For instance, in comparing the 66.6% Na-"X" zeolite, 13X molecular sieve and 5% V2O5 on A1203, it is found that only the latter gives optimum results when stoichiometric oxygen is used. This is considered essential because when excess air or oxygen is nec-essary, as is required with catalysts (1) and (2), control problems due to accumulation of SO2 develop in the system. When the conversions are compared, only the 66.6% Na-"X" zeolite initially removes H2S as effectively as the 5% V2O5, but the 66.6% Na-"X" zeolite shows much more rapid deactivation with time. The 13X molecular sieve is found to be incapable, under the operating conditions specified, of removing even 80% of the H2S in the feed gas.
EXAMPLE IV
Because the 5% V2O5 catalyst in Example III showed some deterioration with time, two 10% V2Os catalysts were pre-pared as follows:
(1) 10% V2O5, 70% hydrogen Y-zeolite, 20% A12_3:

A dry mixture of 70 grams (anhydrous) steam stabilized hydrogen "Y"-zeolite, containing substantially no metal cations, plus 13 grams of NH4V03 was prepared. After moistening with water to form a thin paste, the mixture was dried at 100C, mixed with sufficient alumina hydrate to form 20% A1203 on an anhydrous basis, pelleted, calcined at 932F for 3 hours and then reduced.

116~Z,~9~

(2) 10% V2_s on alumina:
200 grams of anhydrous A12O3 (as hydrated spray-dried alumina) was soaked in a hot solution of 28.5 grams of NH4VO3 in 500 ml water. The paste formed was dried at 90-100C, remoistened and extruded through a l/8-inch die, dried at 110C, calcined at 932F for 3 hours and then reduced.
Each of the above catalysts was tested in two oxidation reactors operating in series to desulfurize a partially dried (water removed at 55F) hydrogenated Claus tail gas similar in dry composition to that shown in Table I. No .-recycle gases were utilized, and stoichiometric air was used as the oxidant. Sulfur and sour water were condensed at 260F
and 55F, respectively, from the gases leaving each oxidation reactor. Other operating conditions and the results obtained are tabulated in Table IV.
The results shown in Table IV demonstrate an important feature of the invention. After 30 days operation, the activity of both catalysts was still remarkably high. As shown in the case of the 10% U2Os on A12O3 catalyst, absolutely no catalyst deactivation was detectable. Such results are typi-cal of those obtainable with any of the catalysts described hereinbefore as being suitable for use in the process of the invention. Hence, the catalysts of the invention are not only highly active, but maintain their activity over extended periods of time.

i ~2~

TABLE IV
10% V2Os on 70%
Hydrogen "Y" Zeolite 10% V2O5 and 20% A123 on A123 Days of Operation 30 1 30 Total sulfur compounds entering 1st oxidation reactor mol % 1.4459 1.3029 1.4042 Temperature of 1st oxidation reactor, F 400 400 400 Space velocity thru 1st oxidation reactor, v/v/hr. 876 876 876 Total sulfur compounds leaving 1st oxidation reactor, mol % -- 0.0623 0.0550 1st oxidation, % removal of sulfur compounds -- 95.22 96.08 Temperature of 2nd oxidation reactor, F 400 325 325 Space velocity thru 2nd oxidation reactor, v/v/hr 876 876 876 Effluent of second reactor, Total sulfur compounds, mol % 0.1617 0.0191 0.0127 H2S, mol % 0.1351 0.0018 0.0063 CH3SH, mol % 0.0004 0.0000 0.0005 SO2, mol % 0.0239 0.0167 0.0049 COS, mol % 0.0021 0.0002 0.0010 CS2, mol % 0.0001 0.0002 0.0000 2~ mol % 0.0000 0.0078 0.0000 H2, mol % 5.80 5.63 6.58 Overall Sulfur Compounds Removal, % 88.82 98.53 99.10 l~Z~94 Each of the following three Examples describes a method for producing a catalyst useful in the oxidation of H2S -to elemental sulfur as described hereinbefore.
EXAMPLE V
600 gm Zeolon, a commercial synthetic sodium mordenite manufactured by the Norton Company, was slurried in 5000 ml of 1.0 N HCl at room temperature for 60 minutes. It was then filtered and the treatment was repeated on the filter cake.
The filter cake from the second treatment was slurried in hot 1.0 N HCl (73C) for one hour, then filtered, and finally washed on the filter with four 1000 ml washes of hot water.
After the filter cake was dried, the Na2O content was 0.57% by weight (about 93% exchanged to the hydrogen form). The hot treatment was repeat ed twice more for 45 minutes each, after which time the Na2O level was 0.21% by weight (97.5% exchanged).
The amount of aluminum extracted was relatively small so the product had a SiO2/Al2O3 ratio of 11.5 compared to the original ratio of lO.
An amount of the dried hydrogen mordenite corres-20 ponding to 225 gm of anhydrous powder was mulled together with424 gm of a silica hydrogel (containing about 6% SiO2 or 25 gm of anhydrous silica) and 36.1 gm of NH4VO3 (or 27.8 gm of V2O5). The mixture was dried during mulling with a flow of hot air until it was of extrudable consistency. It was then extruded through a l/8-inch die, dried, and calcined at 932F
for 3 hours. The product containing 10% by weight of V2O5 had excellent physical properties and had a deep golden color.
It was then reduced.

l~Z~g~

EXAMPLE VI
A silica hydrogel was prepared in a manner similar to that used for the preparation of -the hydrogel binder of Example V. Two solutions (A) and (B) were prepared as follows:
(A) 70 ml of concentrated (96%) H2SO4 was diluted to 2500 ml with deionized water and cooled to 10C;
(B) 655 ml of 41 Be~ commercial sodium silicate (sp. gr. 1.394, 28.65 wt.% SiO2 and 8.90 wt.%
Na2O) was diluted to 2500 ml with deionized water and cooled to 10C. When equal volumes of (A) and (B) were mixed, the pH was too low for rapid gelation, so 3.0 gm NaOH was dissolved in solution (B).
Solution (B) was poured rapidly into Solution (A);
with stirring and after 4-1/2 minutes the mixture set to a vibrant hydrogel. After syneresis overnight, the hydrogel was cut into 1/2- to l-inch pieces and placed on a large Buchner funnel. It was washed free of sodium by soaking in 0.3 N HNO3 for half an hour, followed by draining and repeating of this sequence four times. The product so formed was then washed with water in the same way for a total of five times.
The hydrogel was partially dried and then mulled with enough NH4VO3 to give 10% by weight of V2O5 and 90% by weight of SiO2 in the final calcined product. The moisture content of the mulled mixture was adjusted until an extrudable product was formed. It was then extruded, dried, calcined and reduced as in Example V.

11~2~394 EXAMPLE VII
An aluminum phosphate hydrogel was prepared substan-tially as described in Example IV of U.S. Patent 3,147,227. A
slight excess of A12O3 (5-10%) remained in the preparation in order to preserve a high surface area. This hydrogel was combined with 10% V2O5 as in Example VI and finished in the same way.
The effectiveness of the process for desulfurizing or removing H2S from gases containing light hydrocarbons is aptly demonstrated in the treatment of sour natural gases. Since these gases generally comprise H2S, or SO2 and H2S in a SO2/H2S mole-ratio of 0.5 or less, as the only gaseous sulfur compounds contained therein, their treatment is much simpler than that required for Claus tail gases. No pre-hydrogenation is necessary. Also, dehydration is usually not necessary, inasmuch as sour natural gases generally contain only traces of water vapor. The gases need only be blended with sufficient air, oxygen and/or SO2, to provide an overall mole-ratio of (S2 + O2)/H2S therein of about 0.5, thus satisfying the stoichiometry of Reactions (I) and (II). The mixture of sour natural gas and, preferably stoichiometric air or oxygen, is processed through the oxidation reactor 14 at between about 250 and 450F, but preferably between about 300 and 400F, and at a space velocity between about 250 and 2000 v/v/hr, preferably between about 800 and 1000 v/v/hr. The gases leaving the oxidation reactor are then cooled, firstly to about 260F to remove molten sulfur, and then again, optionally, to below about 70F, preferably about 55F, to condense sour water.
Conversions of H2S to sulfur in reactor 14 in excess of 80%, and usually in excess of 90%, can be achieved.

l~Z~

EXAMPLE VIII
A sour natural gas having the composition shown in column 1, Table V was blended with sufficient air to provide a stoichiometric amount of oxygen (for Reac-tion (II)) and the resultant mixture was passed a-t atmospheric pressure and at a space velocity of 500 v/v/hr into an oxidation reactor maintained at 400F by external means. The catalyst in the reactor com-prised 10% V205, 70% hydrogen "Y" zeolite and 20% A1203, and was prepared as described in Example IV. No recycle gases were utilized. After condensing sulfur at 260F and water at 55F
from the gases leaving the oxidation reactor, a purified gaseous effluent having the composition shown in column 2, Table V was obtained.
In this Example a 93.25% removal of H2S and an 89.78%
removal of sulfur compounds were effected despite a relatively low concentration of H2S in the feed. Utilization of recycle gases would further improve the removal of sulfur gases by lowering the sulfur vapor dew point temperature and permitting safe operation at lower reactor temperatures. Of prime importance, however, is the complete lack of oxidation of the desired fuel gases.

OS~fl Table V
(1) (2) Gaseous Component Mole % of Feed Gas Mole % of Product Gas .
H2S 0.3805 0.0257 CH3SH 0.0008 0.0003 COS 0.0012 0.0012 S2 0 . 0000 0 . 0111 CS2 0.0000 0.0004 Total sulfur compounds* 0.3825 0.0391 CH4 88.135 87.6118 Ethane 4.70 4.60 Propane 0.92 0.83 N-butane 0.11 0.17 I-butane 0.08 0.00 N-pentane 0.00 0.26 I-pentane 0.06 0.00 Hexane 0.00 0.36 2 0.02 0.02 Water 0.02 0.03 H2 0.32 0.28 N2 4.59 5.46 C2 0.28 0.30 ;
*Expressed as moles of monatomic sulfur compounds.

11~2C~94 EXAMPLE IX
A natural gas from Casmalia, California having the dry composition shown in column 1, Table VI was blended at 150 scc/min with air at 3.0 scc/min. The resultant mixture containing slightly more than stoichiometric air for the conversion of H2S to sulfur, was fed at 100 psig and 450F
and 500 v/v/hr into an oxidation reactor containing the 10%
V2O5, 80% hydrogen mordenite, 10% silica catalyst prepared as described in Example V. The temperature in the reactor was maintained isothermally at 450F by external means. No recycle gases were used. After condensing sulfur at 260F, the product gas had the composition shown in column 2, Table VI.
As shown, an 84.04% removal of H2S and a 75.04% removal of sulfur compounds were effected.

~l~Z~9~

TABLE VI
(1) (2) Gaseous Component Mole % of Feed Gas Mole % of Product Gas H2S 0.4634 0.0740 CH3SH 0.0038 0.0014 COS 0.0005 0.0019 S2 0.0028 0.0336 CS2 0 0009 0.0035 Total Sulfur Compounds'; 0.4723 0.1179 CH4 63.42 62.82 Ethane 1.26 1.27 Propane 0.28 0.27 N-butane 0.16 0.09 I-butane 0.20 0.21 N-pentane 0.08 0.10 I-pentane 0.07 0.13 C6 Naphthenes 0.09 0.07 Hexanes 0.25 0.20 Air 0.02 0.05 H2 0.07 0.04 N2 6.30 7.37 C2 27.34 27.30 '~Expressed as moles of monatomic sulfur compounds.

ll~Z~94 EXAMPLE X
A natural gas from Casmalia, California having the dry composition shown in column l, Table VII was blended at 150 scc/min with air at 3.0 scc/min. The resultant mixture con-taining slightly more than stoichiometric air for the conversion of H2S to sulfur, was fed at lO0 psig and 450F and 500 v/v/hr into an oxidation reactor containing the 10% V205 on silica catalyst prepared as described in Example VI. The temperature in the reactor was maintained isothermally at 450F by external means. No recycle gases were used. After condensing sulfur at 260F, the product gas had the composition shown in column 2, Table VII. As shown, a 93.28% removal of H2S and an 88.35%
removal of sulfur compounds were effected.

2~)94 TABLE VII
(1) (2) Gaseous Component Mole % of Feed Gas Mole % of Product Gas H2S 0.5804 0.0390 CH3SH 0.0038 0.0012 COS 0.0009 0.0042 S2 0.0046 0.0183 CS2 0.0009 0.0031 Total Sulfur Compounds~:: 0.5913 0.0689 CH4 61.96 61.77 Ethane 1.40 1.36 Propane 0.20 0.21 N-butane 0.08 0.12 I-butane 0.12 0.14 N-pentane 0.01 0.00 I-pentane 0.25 0.28 Air 0.04 0.02 H2 0.02 0.02 2 6.06 7.41 C2 29.23 28.56 Benzene 0.0048 0.0053 C6 Naphthenes 0.07 0.07 *Expressed as moles of monatomic sulfur compounds.

11~2~94 Having shown the two general me-thods whereby feed gases such as Claus tail gases and sour natural gases can be most effec-tively desulfurized, it must be pointed out that the oxidation process for treating sour natural gases is not intended to be limited to those feed gases containing H2S, or H2S and SO2 as the only gaseous sulfur components therein.
It is possible that a feed gas could contain such components as COS, CS2, etc., and that the removal of these components may be unnecessary. For example, a waste gas might contain 5 mole %
H2S and less than 50 ppm of components such as COS, CS2, etc.
Thus, although the use of the hydrogenation-oxidation process shown previously for treating Claus tail gases would give most complete desulfurization, the use of hydrogenation reactor to convert only 50 ppm of the gases contained in the feed would, in all probability, be uneconomical. Hence, in this, and many other situations wherein the primary requirement is to remove H2S, or H2S and SO2 in an S02/H2S mole-ratio of 0.5 or less, the simple oxidation process described for sour natural gases and exemplified in Examples VIII, IX and X may be of greatest utility.
The Claus tail gas or sour natural gas (both as examples of two types of gases which can be desulfurized by processes hereinbefore described), after being desulfurized according to one of the embodiments hereinbefore shown, can be further treated by any of a number of post-desulfurization treatments. One such post-desulfurization treatment herein- .
before described consists of improving the efficiency of desulfurization by passing the gases through a second oxidation reactor 37 operating substantially in the manner described for the first, and then condensing sulfur from the product gas.
Referring once again to FIGURES 2 and 2A, some alternative post-treatment are as follows:

iZC~194 (1) Post-Hydrogenation Treatment. I-t is anticipated -tha-t if the gases leaving the oxidation reactor 14 are to travel through extensive piping systems, it may be desirable to avoid sulfur deposition therein by converting the remaining sulfur vapor leaving condenser 19 (or bypassed around the same via line 57) to H2S. One method of accomplishing this (as well as further reducing SO2 concentration, thus rendering the gas less corrosive) is to combine the gases -transported in lines 21 and 42 with a reducing gas from line 43 in preheater 44, and then to pass the resultant mixture preheated to between about 300 and 800F, preferably 300 and 600F, through line 45 to a hydrogenation reactor 46 which converts most of the remaining sulfur vapor and SO2 to H2S. The operating conditions and catalysts used in hydrogenation reactor 46 are essentially the same as those previously described for hydrogenation reactor

5. It will be understood however that if an excess of reducing gas was provided to prehe~ter 3, then further addition thereof via line 43 may be unnecessary because any excess of the reducing gases, H2 and CO, remains unoxidized in oxidation reactor 14 and thus passes as a reactant to hydrogenation reactor 46. Also, inasmuch as little or no COS, CS2 or mer-captans should be present in the gases in line 21, two results occur: (1) lower operating temperatures (i.e., between 300 and 600F) can be used in hydrogenation reactor 46, and (2) no water vapor need be added with the reducing gas via line 43 unless said reducing gas is CO and insufficient water vapor is present for Reaction XI.
(2) Post-Hydrogenation-Oxidation Treatment. If the original feed gas to be desulfurized contains more than about 10 mole % sulfur components, the concentration of the gaseous sulfur compounds (usually H2S, S, SO2) in the gases in line 21 may still be higher than is desired. One method for further ~l~ZIV94 desulfurizing these gases is to hydrogenate the sulfur compo-nents contained therein according to the method of post-treat-ment (1) above, but instead of diverting the H2S-containing gases in line 47 to a piping system 48, they may be passed, either directly via lines 49 and 50, or after lowering the water vapor conten-t to less than about lO volume percent in condenser 51, to line 52 from which they can be subsequently treated by oxidation in reactor 37 in the manner described hereinbefore.
(3) Post-Incineration Treatment. If the gases carried in lines 21 and 30, or recovered from sulfur condenser 39 via line 41, contain H2S in concentrations higher than allow-able for the particular locality for atmospheric discharge, it may be desirable to oxidize said H2S to S02, the latter usually having a much higher discharge concentration limit. Therefore, these gases can be combined in line 53 with excess air or oxygen (for Reaction (III)) from line 54 and then passed to a catalytic or thermal incinerator 55, wherein H2S is oxidized to S2 via Reaction (III) prior to atmospheric discharge via line 56.
To demonstrate the effectiveness of post-treatment (1), the following example is provided:
EXAMPLE XI
A natural gas from Casmalia, California having the dry composition shown in column 1, Table VIII was blended with sufficient air to provide a stoichiometric amount of oxygen (for Reaction (II)) and the resultant mixture was passed at 100 psig and at a space velocity of 500 v/v/hr into an oxi-dation reactor maintained at 401F by external means. The catalyst in the reactor comprised 10% V205 on alumina and was prepared as described in Example IV. No recycle gases were utilized. After condensing sulfur at 260F, the off-gases ~l~Z'~4 obtained (column 2, Table VIII) were combined with H2 such that ~he H2 concentra-tion in the off-gases was 600 ppmv. The H2-containing off-gases were preheated to 380F and the resultant mixture was fed to a hydrogenation reactor at 750 v/v/hr.
The hydrogenation reactor contained the 12% MoO3-6% CoO on alumina catalyst as described in Example I. The product gas from this reactor had the dry composition shown in column 3 of Table VIII.
Table VIII

Casmalia Oxidation Hydrogenation Natural Gas Reaetor Produet Reaetor Pr~uet Component (Mole %) (Mole %) (Mole %) H2 0.00 0.00 0.01 CH4 69.32 69.87 66.28 N2 1.52 2.25 5.99 Propane 0.20 0.20 0.24 Ethane 0.88 0.81 0.85 Air 0.17 0.05 0.04 H2S 0.2900 0.0057 0.0238 Isobutane 0.16 0.16 0.17 C2 26.54 25.67 25.38 CH3SH 0.0031 0.0044 0.0014 N-butane 0.02 O.00 0.02 COS 0.0008 0.0009 0.0009 S2 0.0080 0.0132 0.0023 N-pentane 0.28 0.26 0.26 CS2 0-0005 0.0000 C6 Naphthenes 0.15 0.14 0.17 Hexanes 0.50 0.50 0.50 Total Sulfur Compounds-:; 0.3014 0.0252 0.0284 % Sulfur Compounds Removal Overall 90.6 :Expressed as moles of monatomie sulfur eompounds.

Z,~94 As shown ~y the data in Table VIII, an 82.58% removal of S2 was effected. Unfortunately, the mass spectrometer does not record the proportion of sul:Fur vapor in any of the gaseous compositions of Table VIII. However, it is noted that the increase in sulfur compounds during hydrogenation was 32 ppm and it is assumed that this is due to the conversion of sulfur vapor to H2S. Since, by calculation, the sulfur vapor in the oxidation reactor product should be between 30 and 65 ppm, it is seen that an effective removal of sulfur vapor has b~en accomplished.
In a specific embodiment of the invention, the one or two stage oxidation-sulfur recovery process of the invention is used to desulfurize the gaseous effluent from a one or two stage Claus plant. In essence, this merely replaces one or two of the Claus plant reactors containing a bauxite catalyst with an equal number of oxidation reactors containing any of the vanadium oxide and/or sulfide catalysts hereinbefore described;
but the replacement of Claus plant reactors in this manner pro-duces a result that is not at first apparent. In the Claus .
process as described hereinbefore, it is necessary to incinerateby combustion one-third of the available H2S to SO2 (i.e., because the catalyst therein is essentially inactive for con-verting H2S to sulfur via Reaction (II)) and subsequently to recombine this portion with the other two-thirds. Theoretically, this should provide the exact stoichiometric amount of SO2 oxidant required to convert the H2S in the remaining two-thirds to elemental sulfur. In practice, however, the gases entering the first, and especially the second and third, Claus reactors seldom have the requisite ratio of H2S/SO2 of 2. As a result, inefficiencies in desulfurization occur, due either to the production of excessive SO2 during incineration or incomplete conversion of H2S in the catalytic reactors. The present ll~Z1~4 embodimen-t of the invention largely avoids this problem because oxidant is fed individually to each oxidation reactor used in series, thus providing better means for controlling the oxidant concentration throughout the entire process. As an example, if a combination of a Claus plant-oxida-tion process employing in series one Claus reactor and two oxidation reactors (with sulfur condensers situated after each reactor) were to be used, one would insure that slightly less than the 2:1 ratio of H2S:
S2 is provided to the Claus reactor, that slightly less than the amount of oxygen necessary for the (S02 + 02)/H2S = 0.5 ratio is provided to the first oxidation reactor and that the exact stoichiometric amount of oxygen for the (S02 + 02)/H2S =
0.5 ratio is provided to the second oxidation reactor. In so doing, the over-production of S02 (above the 0.5 ratio) is avoided, the only sacrifice being a small concomitant loss in efficiency of H2S removal in the first two stages. The overall efficiency, however, of the Claus plant-oxidation process herein described will be greater than that for a three-stage Claus plant treating the same feed.
It will be apparent to those skilled in the art from the foregoing that numerous modifications of the invention are contemplated. Accordingly, any such embodiments are to be construed as coming within the scope of the invention as defined in the appended claims or substantial equivalents thereto.

Claims (24)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A process for the conversion of H2S to elemental sulfur, which process comprises contacting at a temperature between about 250° and 450°F a feed gas stream containing H2S
and a gaseous oxidant comprising elemental oxygen with a catalyst comprising one or more components selected from the group consisting of vanadium oxides and sulfides and further comprising a non-alkaline porous refractory oxide support, said contacting being conducted for at least 30 days under conditions such that at least some of said H2S is converted to elemental sulfur vapor, with said catalyst maintaining sub-stantially undiminished activity for oxidizing H2S to sulfur under essentially the same conditions for at least 30 days of contacting.

2. A process as defined in claim 1 wherein said feed gas stream also contains one or more components selected from the group consisting of hydrogen, carbon monoxide, and light hydrocarbons, said one or more components remaining substan-tially unoxidized during said contacting.

3. A process as defined in claim 1 wherein said catalyst comprises between about 2 and 20 weight percent of V2O5.

4. A process as defined in claim 3 wherein said non-alkaline refractory oxide is selected from the group consisting of alumina, silica-alumina, silica, acidic metal phosphates, hydrogen "Y" zeolite, hydrogen mordenite, hydrogen erionite, or combinations thereof.

5. A process as defined in claim 1 wherein said feed gas stream contains substantially the stoichiometric amount of said gaseous oxidant required to oxidize all of said H2S to elemental sulfur.

6. A process as defined in claim 5 wherein said gaseous oxidant is air.

7. A process as defined in claim 1 wherein said catalyst comprises between about 2 and 20 weight percent of V2O5 on a silica-alumina support comprising between 20 and 30 weight percent alumina.

8. A process for the removal of H2S and the recovery of elemental sulfur from a feed gas mixture containing a minor proportion of H2S and a minor proportion of a gaseous oxidant comprising elemental oxygen, said oxygen being present in said mixture in essentially the stoichiometric proportion necessary to oxidize said H2S to sulfur, which process, being carried out for at least 90 days, comprises:
(1) contacting said feed gas mixture at a tem-perature between about 250° and 450°F and at a space velocity between about 250 and 2000 v/v/hr with a catalyst consisting essentially of one or more vanadium oxides and/or sulfides supported on a non-alkaline porous refractory oxide, said contacting being such that at least 80% of said H2S is converted to elemental sulfur vapor, with said catalyst main-taining substantially undiminished activity for oxidizing H2S to sulfur under essentially the same conditions for at least the 90 days of contacting; and (2) separating free sulfur from the resulting gas-eous effluent and recovering a purified product gas.

9. A process as defined in claim 8 wherein said contacting in step (1) is carried out at a space velocity correlated with temperature so as to oxidize at least about 90% of said H2S to elemental sulfur for at least 30 days.

10. A process as defined in claim 9 wherein said gaseous oxidant is air.

11. A process as defined in claim 8 wherein said non-alkaline porous refractory oxide is selected from the group consisting of alumina, silica, silica-alumina, silica-magnesia, zirconia, silica-zirconia, titania, silica-zirconia-titania, crystalline or amorphous aluminosilicate zeolites, acidic metal phosphates, and combinations thereof.

12. A process as defined in claim 8 wherein at least a portion of said puriifed product gas is subjected to a second oxidation substantially as described in step (1) to effect a further conversion of H2S to elemental sulfur.

13. A process as defined in claim 8 wherein remaining sulfur and/or SO2 in said purified product gas is at least partially converted to H2S by hydrogenation in contact with a hydrofining catalyst comprising a combination of a Group VIB
metal sulfide with an iron group metal sulfide.

14. A process as defined in claim 8 wherein said catalyst comprises between about 1 and 30 weight percent V2O5 on a silica-alumina support comprising between 20 and 30 weight percent alumina.

15. A process for the desulfurization of a feed gas containing at least one sulfur component selected from the group consisting of SO2, COS, CS2, CH3SH, SO3, and sulfur vapor, which process, being carried out for at least 30 days, comprises the steps of:
(1) converting at least some of said sulfur com-ponents to H2S by contacting said feed gas at a tem-perature between about 300° and 800°F with a catalyst comprising a Group VIB metal sulfide and an iron group metal sulfide in the presence of gaseous reactants selected from the group consisting of H2, CO, and water vapor, said reactants being present in a total propor-tion of H2 plus CO plus water vapor sufficient to con-vert a substantial proportion of said sulfur components to H2S;
(2) dehydrating the resulting product gas to a water vapor content of less than about 15 volume percent;
(3) contacting, at a temperature between about 250°
and 450°F, a mixture of the dehydrated product gas pro-duced in step (2) and a gaseous oxidant comprising sufficient elemental oxygen to oxidize at least 80% of said H2S to sulfur with a catalyst comprising one or more components selected from the group consisting of vanadium oxides and sulfides and further comprising a non-alkaline porous, refractory oxide support, said contacting being carried out under conditions such that at least 80% of the H2S contained in said dehy-drated product gas is converted to elemental sulfur vapor, with said catalyst maintaining substantially undiminished activity for oxidizing H2S to sulfur under essentially the same conditions for at least 30 days of contacting; and (4) separating free sulfur from the resulting gaseous effluent, and recovering a purified product gas.

16. A process as defined in claim 15 wherein said contacting in step (1) is controlled to convert essentially all of said sulfur components to H2S.

17. A process as defined in claim 16 wherein step (3) is carried out using substantially the stoichiometric amount of said gaseous oxidant required to oxidize all of the H2S contained in said dehydrated hydrofined gas to elemental sulfur.

18. A process as defined in claim 16 wherein said dehydrated product gas from step (2) contains at least one normally oxidizable component selected from the group consisting of hydrogen, carbon monoxide, and light hydrocarbons, said normally oxidizable component remaining substantially unoxidized during said contacting in step (3), and being recovered as one or more components of said purified product gas.

19. A process as defined in claim 16 wherein said catalyst of step (3) comprises between about 1 and 30 weight percent of V2O5.

20. A process as defined in claim 19 wherein said non-alkaline support is selected from the group consisting of alumina, silica, silica-alumina, silica-magnesia, zirconia, silica-zirconia, titania, silica-zirconia-titania, crystalline or amorphous aluminosilicate zeolites, acidic metal phosphates, acidic metal arsenates, and combinations thereof.

21. A process as defined in claim 19 wherein said catalyst comprises between about 2 and 20 weight percent of V2O5 on a silica-alumina support comprising between 20 and 30 weight percent alumina.

22. In a Claus process wherein a mixture of gases comprising H2S and SO2 is passed, for a time period of at least 30 days, through a series of reactors, each of which contains a bed of catalyst, at a temperature and space velocity suffi-cient to produce elemental sulfur vapor in each of said reactors, and wherein the gases entering the final reactor in said series contain all three of the components H2S, SO2, and oxygen, the improvement comprising:
(a) controlling said gases entering said final reactor such that the mole ratio of (SO2 + O2)/H2S
is about 0.5;
(b) using as a catalyst in said final reactor a composition comprising a vanadium oxide and/or sulfide supported on a non-alkaline porous refractory oxide, said catalyst being active for, and maintain-ing substantially undiminished activity for, 30 days for reacting H2S with oxygen to produce sulfur; and (c) controlling the temperature in said final reactor at between about 250° and 450°F and controlling the space velocity so that at least some of said oxygen reacts with said H2S.

23. A process as defined in claim 22 wherein sub-stantially all of said oxygen is reacted with H2S.

24. A process as defined in claim 23 wherein said mixture of gases is passed through said series of reactors for a time period of at least 30 days, with said catalyst maintain-ing substantially undiminished activity for 90 days for reacting H2S with oxygen to produce sulfur.