CA2078966C - Process for elimination of low concentrations of hydrogen sulfide in gas mixtures by catalytic oxidation - Google Patents

Abstract

A process is described for the elimination of hydrogen sulfide from gas mixtures by catalytic oxidation over activated carbon catalyst which converts the hydrogen sulfide to elemental sulfur and water, the former being sorbed by the activated carbon while the latter is transported with the gas mixture and may be removed by known dehydration processes. The above oxidation process is conducted at elevated temperatures and pressures and with sufficient residence time to assure virtually complete conversion of the hydrogen sulfide with minimal production of by-product sulfur dioxide. Traces of heavy hydrocarbons in the feed gas mixture which may reduce the life of the catalyst and the quality of the sulfur product may be removed by cryogenic means or by sorption on an activated carbon guard bed. Both the activated carbon catalyst used to oxidize the hydrogen sulfide and the activated carbon used to remove heavy hydrocarbons from the feed gas may be regenerated by passing inert gas or product gas through the beds of these materials at elevated temperatures for sufficient time to remove the sorbed sulfur or the heavy hydrocarbons, respectively for use as by-products of the process.

Description

This invention relates to a cyclic process for direct conversion of low concentrations of hydrogen sulfide (H~~') in gas mixtures to elemental sulfur by catalytic oxidation over an activated carbon catalyst, sorption of the sulfur product by the said carbon and subsequent clesorption and recovery of the sulfur during regeneration of the catalyst.
Current practice in the sour natural gas processing industry is to remove the acid components (HAS and C0~) from the natural gas by a sweetening process. The removed i0 hydrogen sulfide, if present in small quantities, is incinerated to sulfur dioxide (S02) and vented to the atmosphere provided that the amount of released S02 is acceptable according to .regulations with respect to the environment. If the quantity of H~,~S removed by a sweetening process is sufficiently large, it is generally fed to a Claus plant and recovered as elemental sulfur.
Recent more stringent regulations in certain jurisdictions concerning the release of sulfur as 50~, to the atmosphei°e have made the sour gas processing industries aware that they will be recluired to reduce sulfur-containing emissions to the environment substantially. With the increasing demand for elemental sulfur and the need to meet the existing environmental regulations, considerable attention has been given to the development o.f inexpensive and effective methods for the recovery of elemental sulfur -z-from natural gases containing HAS.
It has been known for some time that hydrogen sulfide in natural gas or other gases can be oxidized in the presence of various catalysts to sulfur dioxide or sulfur.
Examples of some of these processes may be found in the patent literature.
In Canadian Patent 1,172,428 by R.F. Jagodzinski and R.K. Kerr issued on 84-08-14, a process is disclosed whereby hydrogen sulfide in sour gas is reacted with oxygen at pressures greater than 5 atmospheres over an activated alumina or a vanadium pentoxide catalyst. The catalyst is c:omtinuously soaked and submerged in liquid sulfur in a reactor at temperatures between 25(x° and 550° C. Elemental sulfur is produced along with a substantial fraction of SO2.
The unreacted H2S from this first reactor is then reacted with the produced S02 to produce elemental sulfur and water in a second reactor.
Tn Canadian Patent 1,063,321 by 4J.H. Prowlesland and J.~~d. smith issued on 79-10-02, a process is disclosed where E13S from a "fouled gas" as removed by passing the gas through hydrated hematite (Fez03) pellets in a chamber, thereby producing water and forming elemental sulfur which coats 'the pellets. Pellets are continuously withdrawn from the bottom of the chamber to a tumbler where continuous tumbling of the pellets abrades the elemental sulfur from their surface. The sulfur is recovered and the abraded pellets are then continuously .returned to the top of the chamber. The process is complex and the composition of the product stream is not given. It is statecl that during low temperature regeneration of the product Fe2S3 by oxidation, the possibility of S0~ production is high because of the high temperature rise in the following reaction (1) which in turn can initiate reaction (2) Fe?S3 + .Z . 50 + Fey 0~ + 3S + 144 kcal . ( 1 ) Fez S3 + 4 . 5 02 + Fey 03 + 3 50~. + 34 7 kca 1 . ( 2 ) It is advised in the patent that reaction (2) be avoided if possible because of SD2 production.
In Canadian Patent 722,113 issued on 65-11-23, E.E.
Baker and W.A. Duncan describe a process in which hydrogen sulfide in natural gas is oxidizecl in a bed of molecular sieve (crystalline zeolite) pellets having an apparent pore size of at least 4.6 Angstrom unites (AU) at temperatures below 150 ~' and at a "first higher pressure" (100 - 1000 prig) thereby absorbing the hydrogen sulfide. A hydrogen sulfide-depleted natural gas stream is discharged :from the first bed. The pressure in the first bed is then reduced to a "second lower pressure" (50 psig) at which hydrogen sulfide and other gases are desorbed. The .released gases are then absorbed in a second molecular sieve bed of crystalline zeolite again having an apparent pore size of at least 4.6 A.U. at temperatw.ros in the range of 350 °-750 ° F.
T~'le sOrptlOn is COndLlCteCI in. the presence of free oxygen so as to produce and recover elernental sulfur. This is a two-step process. There is no mention of S02 production.
In Canadian Patent 1,117,276 by K.D. Henning et al.
issued on 82-02-02, a process is disclosed for elimination of sulfur aompaunds, in particular hydrogen sulfide, From gases containing the same, by reaction with oxygen and/or SO~ in the presence of activated carbon at elevated temperatures l0to produce elemental sulfur. The process is performed at temperatures between 120° and 240 ° C and at pressures ranging from 1 to 50 bars and with OZIH~S molar ratios from 1.53 to 2.2 (i.e. 3.06 to 4.4 times the stoiChiometric ratio). A two-step process is necessary if the FI~S content in the feed gas exceeds 1318 ppm (i,e, 2 g H28 per m3 of feed gas). The regeneration of the carbon in the first adsorber is less frequent than that in the seoond adsorber because of its autoregeneration. When regeration is required, it is carried out with a hot inert gas. The preferred particle size of the activated aa.rbon is 3 to 6 mm. In the .first adsorber the activated carbon has a medium pore radius between 7 - 12 A.U. while in the second adsorber it is 5 - 8 A.U. There is no mention of the effect of pressure an HAS conversion and S0~ production.
The disadvantages of the above-mentioned pror.esses are that they are complicated and that some of them produce substantial amounts of 502. None, except the last one, uses activated carbon as a catalyst during the catalytic oxidation of H.~S. Although Patent 1,117,276 describes a process which is similar to the process being disclosed herein, it fails to recognize the positive effect of elevated pressure operation to achieve (a) high hydrogen sulfide conversions to elemental sulfur and (b) reduced S02 production. In addition, it has been found that by use of lower OZIH~S ratios than the above patent describes, lower S0~ production can be achieved at high H2S conversion levels.
The above mentioned patents tail to recognize the deleterious effects of having traces of heavy hydrocarbons in the feed gas. Unless these components are removed by means of cryogenic equipment or a guard bed, the overall lif a of the catalyst will be reduced, the time between regenerations of the catalyst will be shortened and the quality of the product sulfur will deteriorate.
Canadian Patent 1,117,276 fails to recognize the value of operation at pressures beyond the .range specified (1 to 50 bars), in terms of being able to use lower 02/H2S molar ratios which favour lower S02 production, in terms of more effective utilization of the activated carbon catalyst between regenerations and in terms of reduced energy _ 6 _ requirements in processing the gas ready for delivery to a pipeline for sale at pressures of, for example, 65 to 70 bars. Furthermore, the use of higher ratios than are required leaves more unconverted oxygen and the associated nitrogen in the gas causing dilution and a lower calorific value of the product.
The objectives of the present invention are to provide a simple and efficient process to oxidize H2S in gas mixtures catalytically in the presence of air and an activated carbon catalyst so as to produce substantially pure elemental sulfur and, simultaneously, to reduce the production of S02 to acceptable levels so that the product gas can be fed directly into pipelines ready for use by consumers in the case of natural gas or it can be burned, flared or otherwise vented to the atmosphere.
In accordance with an aspect of the invention, a process for the oxidation and elimination of hydrogen sulfide from gas mixtures comprises adding oxygen to the said mixture to obtain an oxygen/hydrogen sulfide molar ratio in the mixture between 0.5:1.0 and 1.5:1.0 (1.0 to 3.0 times the stoichiometric ratio), passing the gas mixture into at least one activated carbon bed contained in a reaction vessel and subjecting the said hydrogen sulfide to the catalytic action of the activated carbon under reaction conditions which produce elemental sulfur so that the elemental sulfur provided by the reaction is sorbed by the catalyst while the purified gas is recovered as product, said reaction conditions being selected from a gas pressure range of from about 100 kPa to 7000 kPa and a temperature in the range of about 130° to 220° C, the activated carbon being subjected to periodic regenerations so that the sorbed sulfur is removed therefrom as another product.
It has been discovered that the higher the operating pressure in the reactor, the lower is the S02 production. This is unexpected since Reaction 4 should be favoured by higher pressure.

I I i I

The invention will now be described by way of example only with reference to the drawings in which:
Figure 1 shows the arrangement of the apparatus according to the invention, as used to conduct operations at pressures up to 640 kPa.
Figure 2 shows the effect of pressure on conversion of H2 S and S02 production using the apparatus of Figure 1.
Figure 3 shows a schematic diagram of the apparatus as used at pressures of 5600 kPa.
Figure 4 shows the effect of pressure on conversion of HZ S and S02 production, using the apparatus of Figure 3.
Figure 5 shows in graphical form the influence of capillarity in sulfur vapour pressure over a range of sulfur loading.

Figure 6 shows in graphical form the influence of 02 supply on FI~S conversion and S0~ production at steady state at different temperatures; and Figure 7 shows a flow diagram of a purification plaint for natural gas containing H2S.
With reference to the drawings, the invention is explained as follows: , Figure 1 shows the arrangement of the apparatus used to conduct the operations at pressures up to 640 kPa. Prior to entering the reactor, air and hydrogen sulfide are mixed so that the desired 02/FIZS ratio is achieved. Initially, most of the produced sulfur i.s deposited in the micropores of the activated carbon catalyst. Gradually the catalyst loses its activity due to increased sulfur loading of the micropores.
Figure 2 provides results of experiments conducted at various pressures up to 640 kPa. It shows that with increased aperating pressures in the reactor, virtually complete. conversion of the hydrogen sulficle can be maintained for a longer period than is possible at lower pressures and with a substantial reduction in the SQL
production.
In view of the advantages observed from operation at higher pressures, further experiments were conducted at 5600 kPa using mass flowmeters to monitor the gas flows and a back pressure regulator to control the system pressures as _ c~ _ can be seen in Figure 3. The results of this e~perimemt are given in Figure 4. The results further confirm the advantages of elevated pressure operation in terms of high F12S conversion for longer periods accompanied by reduced S0~
production.
Figure 5 shows the reduction in the vapour pressure of deposited sulfur due to the capillarity effect in the micropores of the catalyst. We have discovered that when these pores are filled, the vapour pressure of sul:Eur increases so that there is a greater tendency for Reaction 4 to proceed producing SOz. It is recognized that by increasing the reaction operating pressure the partial pressure of sulfur vapour in the reactor is reduced and that this decreases the rate of S02 production. The Catalyst may be loaded with sulfur up to 80-150% of its mass before its activity decreases significantly so that it requires regeneration. Operata.ng conditions in the reactor are maintained so that virtually complete conversion of the HAS
in the sour gas stream is achieved. The allawable sulfur laading of the catalyst will depend on the catalyst itself and the operating parameters employed such as space velocity, residence time, temperature and pressure in the reactor.
Figure 6 shows the effects of varying the ~2/H3S ratio in the feed gas at reaction temperatures of 160°and 1.75°C on conversion of hydrogen sulfide to sulfur and to sulfur dioxide. These data published in the Proceedings of -the 9-th International Congress on Catalysis, June 28, 1988 were obtained under so called °°steacly state°' conditions in which the rate of p-roduct sulfur deposition on the catalyst surface was equal to the rate of desorption of sulfur :From the catalyst. The conditions chosen were such that the effects of varying the O~IHzS ratio and the reaction temperature on the conversion of the HsS in the feed gas could be observed as shown in Figure 6. It is seen that higher temperature and higher 02/F12S .ratios increase H2S
but at the expense of increases S02 production. These findings indicate that it would be necessary to adjust the temperature and 02/FI~S stoichiometric ratio within the limits mentioned in clause 1 depending on the concentrations of hydrogen sulfide and the operating pressure in the reactor to achieve virtually complete conversion of the FIsS
to sulfur and water and to minimize S0~ production. It was observed, under certain conditions, that the amount of 2p sulfur depositing on the catalyst from Reaction 3 can be made equal to that vaporizing from the catalyst surface, a condition referred to as the "steady state". From Figure 2 it is seen that at the "steady state" condition it is possible to oxidize the HaS to sulfur and water with a comparatively lower level of H2S conversion on a continuous basis without catalyst regeneration. Under these r_.irCUmstances the product sulfur may drip out of the catalyst an a continuous basis. L7tilizing this steady state conversiorx of HAS in 'the first reactor and using one or more reactors in series after the first one, it is possible to achieve virtually cornplete conversion of the HzS for long periods of time.
Uecause of the exothermic nature of Reactions 3 and 4 producing sulfur and sulfur dioxide, temperature control is of prime importance in maintaining low levels of S02 production. Calculations have shown that with an initial reaction temperature of 160°C the reactor can be operated adiabatically with up to approximately 1.0% .HAS in the feed, the final reaction temperature reached being less than J_5 200° C. Under such conditions there is low production of 502. With greater concentrations of H2S in the feed gas, it is necessary to provide heat transfer to control the bed temperature below about 200 C or to do the reaction in stages with heat exchange between stages to cool the reactants and to prevent excessive rise of temperatures.
Where feasible the operating pressure in the reactor should be approximately the same as the pressure at which the gas mixture is to be delivered to a processing plant or to a pipeline at pressures up to approximately 6300 kPa. In this way energy losses due i~o compression or expansion of the gas axe minimized or eliminated and the advantages of operating at higher pressures are gained.
The regeneration of the sulfur-loaded catalyst in the reactor should be conducted at 25(x' - 450 C and at slightly above ambient pressure by passing an inert gas or some of the product gas taken from the system through the catalyst bed. The amount of sulfur allowed to remain in the regenerated catalyst under the conditions chosen may range from 25-500 of the original mass of the catalyst. This sulfur loading of the regenerated catalyst is suggested because of the difficulty encountered in removing the residual sulfur from the microspores of the oatalyst due to the "capillary effect".
Experiments show (Figure 2) that the oxidative activity of the catalyst remains sufficiently high to achieve essentially complete conversion at an intermediate operating pressure such as 640 kPa and even at 1300 sulfur loading on the catalyst. Shortly after this point, conversion of the H2S begins to decrease so it is necessary to regenerate the catalyst or to add a second reactor in series to maintain conversion levels.
When the calorific value required for sale of the gas is not attained due to the amount of nitrogen added with the air required for oxidation of higher concentrations of H2S, oxygen or enriched air may be used to perform the oxidation.

2~~~~~~~
In some cases the feed gas mi;~tLa.res may contain heavy hydrocarbons which on passage into the reactor may partially reduce the catalyst activity due to fouling. These hydrocarbons may be removed by well-known aryocJenic methods or by using in guard bed containing an activated carbon to absorb the heavy hydrocarbons and other sulfur compounds prior to mixing of the sour gas mixture with air/oxygen before the mixture enters the reactor. In the case o:E
natural gas containing heavy hydrocarbons it has been found that the guard bed also absorbs other readily oxidizable components in the condensate fraction which may darken the product sulfur on decomposition. It should be recognized that in purifying gas mixtures which do not contain an appreciable quantity of heavy hydrocarbons, it would not be necessary to include cryogenic equipment or a guard bed.
In practice two guard beds could be used; while one is in operation, the other would be in the regeneration cycle.
The proper time for regeneration of the guarcl bed is indicated when analysis of the gas stream prior to the reactor indicates the presence of traces of heavy hydrocarbons containing six or more carbon atoms.
The regeneration of the activated carbon in the guard beds may be conducted by utilizing a small continuous stream of purified gas, depressurizing it through a pressure-passing it through the bed to desorb the heavy hydrocarbons 1 and other sorbed components from the activated carbon. The 2 desorbed heavy hydrocarbons along with the regeneration gas may

3 be sent to a high pressure separator via a compressor and a cooler.

4 The desorbed heavy hydrocarbons separated from the regeneration gas may be sent to a liquefied petroleum gas (LPG) recovery unit 6 while the regeneration gas may be sent back to the pipeline or 7 recycled.
8 ft has been found that the activated carbon used in the guard 9 bed (CALGON SGL TM Trademark of Calgon Corporation or similar material) can absorb heavy hydrocarbons equivalent to 20°l0 of its 11 mass before requiring regeneration. The size of the guard bed and 12 the amount of activated carbon required depends on the amount of 13 heavy hydrocarbons present in a particular sour gas.
14 The activated carbon used for the guard bed is capable of absorbing traces of mercaptans present in the feed gas. It has been 16 discovered, however, that if the guard beds are not used, these 17 men:aptans are oxidized in the reactor to less odoriferous 18 compounds.
19 The invention disclosed herein may also be utilized to oxidize the H2S removed in the acid gases during a conventional 21 sweetening process in the natural gas industry. Such gases are 22 currently incinerated or burned in a flare thereby converting the 23 hydrogen sulfide to sulfur dioxide which is discharged to the 24 atmosphere thereby causing 1 additional atmospheric pollution. Additionally, the process may also 2 be employed as a tail gas clean-up unit after a Claus sulfur plant to 3 oxidize the residual H2S to elemental sulfur and thereby to increase 4 the overall sulfur recovery efficiency.
Referring to Figure 7 describing the process of the invention 6 as applied to sour natural gas, the gas containing hydrogen sulfide is 7 introduced to conduit 1 at pressures available from the well head or 8 at a reduced pressure such that the purified gas can be fed directly 9 to a pipeline without a booster. The gas is directed either through branch 2 or 3 to one of the two guard beds 4 and 5. The heavy 11 hydrocarbons are absorbed by the activated carbon (CALGON SGL
12 T"" or the equivalent) in one of the guard beds at ambient 13 temperature and at the supply pressure. The resulting lean gas is 14 then passed through conduit 6 where the total flow is recorded and controlled by a flow controller FRC-1. The concentration of HZS in 16 the stream is analyzed and recorded by analyzer recorder AR-1. The 17 required amount of 02 or air is supplied by air compressor 7 at the 18 desired pressure to conduit 8. The amount of air fed is controlled by 19 the 021H2S ratio analyzer AR-2 and flow recorder controller FRC-2.
The mixed stream is then passed through heater 9 to raise the 21 temperature to between 140-170° C depending on the other 22 parameters employed. The heater gas mixture .is introduced into reactor R1 via conduits 10 and 11. The E32S is reacted with oxygen in reactor R1 in the presence of the activated carbon catalyst in catalyst bed 12. The sulfur produced during the catalytic oxidation is deposited on the catalyst surface.
The purified gas is sent to the pipeline via conduit 13 and cooler 14 at the desired temperature.
When the catalyst has lost some of its activity as indicated by the presence of H2S in the purified gas stream beyond the allowable limit, the HAS analyzer alarm ~R~1 located at the top of the reactor provides a signal and the flow of the mixture of the gases is then switched to reactor R2 via conduit 10 and 15. Simultaneously, valves 16 and 17 are closed and the pressure in reactor R1 is reduced to about 20 kPa above ambient pressure through a relief mechanism (not shown). The valves 18 and 19 are opened to initiate the catalyst regeneration cycle in .reactor R1. The gas remaining in the reactor R:L is then cirr_ulated through blower 20 and heater 21. The gas is heated to about 250 -450° C and then continuously circulated from the top o.E
reactor R1 downward. Sulfur is desorbed from the catalyst surface due to reduction of pressure in the reactor as well as due to the increased temperature of the catalyst bed. The sulfur is collected in sulfur collector 22 via valve 18 and cooler 23. The sulfur is kept in the sulfur collector 22 in ~p~~~~
molten condition and sent to a sulfur pit via a level control valve 2~.. Trace amounts of sulfur in mist form escaping the sulfur collector along with gas are trapped in the cyclone separator 25. The sul:~ur-free gas is then sent to reactor R1 via blower 20 and in heater 21 to continue the catalyst regeneration cycle. As soon as catalyst regeneration is complete, reactor R1 is allowed to cool. Similarly when the catalyst in reactor R2 is loaded with sulfur, reactor R3 is put on stream to ensure high levels of conversion of the l0 hydrogen sulfide to sulfur.
A small stream of purified gas is taken through conduit 26 from conduit 13, and pressure .reducing valve 27 to reduce the pressure to about 20 kPa above ambient pressure. The a gas is then heated in heater 28 to about '350 C and passed through one of the guard beds requiring regeneration. The desorbed heavy hydrocarbons along with 'the regenerating gas are passed through the conduit 29, compressor 30, cooler 31 and the high pressure (HP) separator 32. The gas separated from the HP separator 32 is either fed to the purified gas stream via conduit 33 prior to cooler 14 or is used again for guard bed regeneration. The demisters 34, 35, 36 are located above the catalyst bed in each reactor to trap sulfur mist as well as catalyst dust. Any sulfur and water separated during Gaoling in cooler 14 are collected in the sulfur and water collector 37 for disposal. The condensate - 1$ -1 recovered in HP separator 32 is sent to the LPG recovery unit.
2 To demonstrate the invention the following experiments were 3 conducted.

Examate 1A: Oaeration of the~rocess atnressures ua to 640 kPa.
6 A natural gas containing 1.114% hydrogen sulfide was 7 passed through a guard bed containing CALGONT'" activated carbon 8 at different pressures up to 640 kPa. The stream was mixed with the 9 required amount of air prior to entering the reactor. The flows of both streams were controlled by means of rotameters as shown in the 11 Figure 1.
12 The reactor was constructed from a stainless steel tube, 12.5 13 mm in diameter, 250 mm long with a screen fitted 2 cm from the 14 bottom end of the tube to support the catalyst. The reactor was snugly fitted into a hole drilled through an aluminum block 55 mm in 16 diameter and having the same length as the reactor. The aluminum 17 block was heated externally by an electrical, beaded, resistance 18 wire, coiled in grooves provided in the outside surface of the block.
19 The aluminum block provided a more uniform temperature throughout the reactor. The temperature was measured by means of 21 a thermocouple, placed in a thermowell immersed in the catalyst bed 22 from the top, The lines to transport the gases consisted of 0.25-inch 23 O.D. stainless steel or high 1 pressure nylon tubing. The sulfur formed was condensed and 2 collected in the sulfur condenser.
3 The catalyst used in the reactor was 5.00 g dry granular 4 HYDRODARCOTM (lignite based activated carbon, a trademark of NORIT Americas Inc.) manufactured by ICI America Inc. The 6 temperature in the reactor and the 021H2S ratio were 175 °C and 7 1.2 times stoichiometric, respectively.
8 Referring to Figure 2, it may be seen that when the operation 9 was conducted at 98 kPa, hydrogen suede conversion was approximately 90% during the initial 300 minutes of operatbn and 11 then gradually dropped to approximately 50% at steady state.
12 Conversion to sulfur dioxide also increased rapidly from zero 13 initially and reached 5.5% after 500 minutes. During operation at 14 502 kPa virtually 100% conversion of the H2S was achieved for about 800 minutes of operation with less than 0.3% of H2S feed 16 being converted to S02 for the initial 300 minutes and then rapidly 17 increased to 4.5%. When the catalytic oxidation was conducted at 18 640 kPa, virtually 100% conversion of the H2S was achieved for 19 1000 minutes of operation with less than 0.3% of H2S feed being converted to S02 throughout the initial 700 minutes. Prior to the 21 increase in S02 production, the sulfur loading on the catalyst 22 was approximately 100% of the mass of the catalyst. Although 23 there was high conversion of the H2S for another 300 minutes 24 beyond the initial 700 minutes in the case of a full scale plant as described in Figure 7, the operation would have to be stopped for. cata..lyst regeneratp.on at the point where SUB production starts to increase.
The concentration of S02 in product stream was calculated as follows:
Total gas stream = 500 mL/min, with 1.1140 HAS
Total H2S = 500 mL x 1.114 = 5.57 mL/min.

Air added with 02/F12S ratio o:~ 1.2 x stoichiometria

- 5.57 x 1.2 = 15.92 mL/min.
2 x 0.21 S02 formed - 0.30 of H2S feed = 5.57 x 0.3 = 0.0167 mL/min.

Approx. concentration of .502 in product stream -0.0167 x 10~ - 32.4 ppm This concentration is acceptable in the purified natural gas.
Example 1B: Advantanes of operating the process at pressures at 5600 kPa.
Based on the findings from experiments made at pressures up to 640 kPa, additional runs were condctcted at 5600 kPa with the same reactor to further confirm the advantages of elevated pressure operation in terms of higher H2S conversion and reduced S02 production. The additional items of equipment used are shown in Figure 3 and the results of the operation are shown in Figure 4. Further improvements were observed in terms of high H2S conversion and reduced S02 production compared with results from runs done at pressures up to 640 ~cha.
Example 2. Influence of 0~/FI~S Ratio on FI~S Conversion and SO-~ Production.
Figure 6 shawl EI~S conversion and SOv production profiles with increasing O.~lFl2S ratio under "steady state"
conditions of operation. The F32S conversion increases rapidly as the ratio is increased up to 2.0 times the stoichiametriC value. The rate of increase 'then gradually declines as the ratio increases from 2.0 to 3.0 and levels off above 3Ø On the other hand, S0~ production rate increases slowly with 02/FI~S ratios below 2.0 times stoichiometric but increases more rapidly above this ratio.
It is concluded therefore, that to limit S02 production, it is desirable to keep the 02/FIaS ratio within the range 1.0 to 2.0 times the stoichiometriC ratio, but preferably between 1.1 and 1.5 times the ratio.
Example 3.Tnfluence of the Capillary Effect on V'apau.r Pressure of Sulfur.
Figure 5 shows the influence o:f 'the capillary effect in reducing the sulfur vapour pressure in a nitrogen atmosphere with decreasing sulfur loading. The experiment was conducted with a catalyst which was loaded with sulfur equal to 85.8% of its mass and placed in the reactor. The removal of sulfur from the catalyst with a low flow of nitrogen at various temperatures was measured from time to time to calculat a the percentage saturation of sulfur vapour in the nitrogen. It was found that as the sulfur loading in the catalyst is reduced the vapour pressure of sulfur in the gas flowing over 'the catalyst is reduced appreciably due to the micropores. This phenomenon explains the low .~02 production when the sulfur loading on the catalyst is low. As shown in the figure the percentage saturation of sul:Eur in the nitrogen carrier gas at a given sulfur loading is higher at higher temperatures but the difference tends to diminish with lower sulfur loading. This plot indicates the difficulty which is encountered in regeneration of the catalyst if attempts are made to remove all of the sulfur from the catalyst. For practical reasons leaving 20 to 30%
of the sulfur on the catalyst after regeneration circumvents this problem.

Claims (15)

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

1. A process for the oxidation and elimination of hydrogen sulfide from gas mixtures comprising:

adding oxygen to the said mixture to obtain an oxygenlhydrogen sulfide molar ratio in the mixture of between 0.5:1.0 and 1.5:1.0;
passing said gas mixture into at least one activated carbon bed contained in a reaction vessel; and subjecting said hydrogen sulfide to the catalytic action of the activated carbon under reaction conditions which produce elemental sulfur with minimal production of SO2 so that the elemental sulfur produced by the reaction is sorbed by the catalyst while the purified gas is recovered as product, the activated carbon having a mean pore radius of about 2.9 nm, and a pore volume of about 1.0 cm3/g, the activated carbon being subjected to periodic regenerations so that the sorbed sulfur is removed therefrom as another product.

2. A process according to claim 1, said reaction conditions being selected from a temperature range of between about 130°C to about 220°C
and a gas pressure range of between about 100 kPa to 7000 kPa.

3. A process according to claim 1 wherein the oxygen/hydrogen sulfide molar ratio in the mixture is between 1.1 and 1.5 times the stoichiometric ratio.

4. A process according to claim 1 wherein the gas mixture is natural gas.

5. A process according to claim 1 wherein a concentration of hydrogen sulfide is above 1% further comprising providing heat exchange to control the temperature of the reaction.

6. A process according to claim 1 wherein a concentration of hydrogen sulfide is above 1 % further comprising providing two or more reactors in series for complete conversion of the hydrogen sulfide with heat exchange between said reactors to control the reaction temperature.

7. A process according to claim 1 wherein the activated carbon catalyst is loaded with produced sulfur from H2S oxidation up to 80 to 150% of the mass of the catalyst before regeneration.

8. A process according to claim 1 wherein the activated carbon catalyst in the reactor becomes loaded with produced sulfur from hydrogen sulfide oxidation to about 150% of the mass of the catalyst under which conditions sulfur will flow from said reactor when operated at elevated pressure of between about 100 KPa to 7000 kPa and the product gas stream being passed into a second reactor in series to provide virtually complete conversion of the hydrogen sulfide.

9. A process according to claim 8 in which the catalyst is regenerated with gas selected from the group consisting of inert gas and the purified gas that remains in the system after heating to 250 to 450°C
and at pressures above atmospheric pressure, the regeneration time being sufficient so that sulfur remaining absorbed on the catalyst is no more than 50% of the mass of the catalyst.

10. A process according to claim 1 which utilizes three reactors and in which at any time one reactor is in the operation mode oxidizing H2S in the feed gas, while a second is in the regeneration mode and the third is in the cooling.

11. A process according to claim 1 which employs two alternatively-used guard beds filled with an activated carbon to absorb heavy hydrocarbons of carbon number equal to or greater than six and to protect the catalyst in the reactors from being fouled so that the product sulfur is not darkened.

12. A process according to claim 11 in which activated carbon in the two guard beds is regenerated using a continuous stream of purified gas which was depressurized through a pressure reducing valve, said gas being heated to between 300 and 400°C and then passed through the bed to desorb the heavy hydrocarbons from the activated carbon, the desorbed heavy hydrocarbons being separated from purred gas in a pressure separator following recompression of said purified stream containing the desorbed heavy hydrocarbons.

13. A process according to claim 11 in which the guard bed carbon is capable of absorbing mercaptans present in the feed gas.

14. A process according to claim 1 utilized to oxidize hydrogen sulfide removed from acid gases after a conventional sweetening process.

15. A process according to claim 1 utilized as a tail gas clean-up unit for removal of residual hydrogen sulfide.