Activated carbon process for removing hydrogen sulfide from gases
FIELD OF THE INVENTION 
     Our present invention relates to a process for desulfurizing a gas which contains H 2  S and water vapor by means of activated carbon, wherein elemental sulfur is formed with which the activated carbon is laden and which is removed from the activated carbon as the latter is regenerated. 
     BACKGROUND OF THE INVENTION 
     A process in this activated carbon is used to remove H 2  S from gases is known and has been described, e.g., in Published German Application 15 67 774. 
     OBJECT OF THE INVENTION 
     It is an object of the invention to provide an improved process of the above-described type which can be used to treat various gases which contain H 2  S and which permits a loading of the activated carbon not only with elemental sulfur but also with sulfuric acid. 
     SUMMARY OF THE INVENTION 
     This object is accomplished in accordance with the invention in that oxygen is supplied to the gas that is to be desulfurized so that the gas contains 1 to 20 moles of O 2  per mole of H 2  S and the gas at temperatures in the range from 50° to 180° C. is contacted with the activated carbon to provide the activated carbon with a loading which contains elemental sulfur and at least 3% by weight of the activated carbon of sulfuric acid. 
     By contrast, in known processes care is taken to suppress a simultaneous formation of sulfuric acid in addition to elementary sulfur on the activated carbon and special measures are required for that purpose. Such measures can be avoided in the process in accordance with the invention and the share of the sulfuric acid in the entire loading on the activated carbon may preferably amount to 5 to about 50% by weight. Surprisingly, this has been found not to give rise to difficulties in the regeneration of the activated carbon. In most cases the gas supplied to the activated carbon contains 500 to 5000 ppm H 2  S. 
     According to a further feature of the invention the gas which is to be desulfurized and contains at least 1000 ppm H 2  S+SO 2  is initially passed at a temperature from 100° to 180° C. through a catalytic prepurification, in which elemental sulfur is adsorbed on a high-metal-oxide catalyst (i.e. a catalyst having a high content of metal oxides and, therefore, a catalyst high in the metal oxide component), oxygen is admixed to the H 2  S-containing gas coming from the prepurification and the gas at a temperature of 50 to 180° C. is contacted with the activated carbon for a final desulfurization. 
     Suitable high-metal oxide catalysts for use in the prepurification may mainly consist of substances consisting mainly of Al 2  O 3  or TiO 2 . Such catalysts are preferably impregnated in addition with about 0.5 to 5% by weight iron, cobalt or nickel. Such catalysts are known per se and have been described, e.g., in European Patent 0 215 317. 
     The activated carbon which is laden with elemental sulfur and sulfuric acid may be regenerated in various ways. In one method the activated carbon at a temperature of 100 to 400° C. is treated with an oxygen-free reducing gas. That reducing gas may consist, e.g., of hydrogen, methane or H 2  S. 
     In a preferred processing of the adsorbent, the activated carbon laden with elementary sulfur and H 2  SO 4  is re-generated by treating it with a partial stream or the entire stream of the H 2  S-containing gas which is to be desulfurized and which is at a temperature from 100° to 400° C. In that case the sulfuric acid is initially reduced to elemental sulfur or in part also to sulfur dioxide 
     
         (H.sub.2 SO.sub.4 +3H.sub.2 S=4S+4H.sub.2) or 
    
     
         3H.sub.2 SO.sub.4 +H.sub.2 S=4SO.sub.2 +4H.sub.2 O) 
    
     and the elemental sulfur is subsequently removed as vapor by the same gas. 
     In the first regenerating step, in which the sulfuric acid is reduced, the H 2  S-containing gas in contact with the laden activated carbon is preferably at a temperature from 100° to 180° C., at which the gas will also be desulfurized 
     The second regenerating step, in which the same gas is used to remove the elemental sulfur still present on the activated carbon, is preferably carried out at a temperature in the range of 200° to 400° C. 
     If the process includes a prepurification, the high-metal-oxide catalyst will have to be regenerated too. In this case the reducing gas used for the regeneration is suitably contacted with the laden activated carbon and with the high-metal-oxide catalyst and at least part of the gas is suitably circulated. The reducing gas may first be contacted with the activated carbon or with the high-metal-oxide catalyst. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The above and other objects, features and advantages of my invention will become more readily apparent from the following description, reference being made to the accompanying highly diagrammatic drawing in which: 
     FIG. 1 is a flow diagram of a process according to the invention comprising a gas purification and regeneration and 
     FIG. 2 illustrates a modified final purification. 
    
    
     SPECIFIC DESCRIPTION 
     The H 2  S-containing gas which is to be regenerated is supplied in line 1 as shown in FIG. 1. That gas contains SO 2  and also water vapor and in addition may contain other sulfur compounds, such as COS and CS 2 . The gas can come from a Claus process plant and in most cases is at a temperature of 120° to 180° C. 
     At such elevated temperatures all or part of the gas is initially fed in line 2 to a prepurification stage 3, which contains a high-metal-oxide catalyst in a fixed bed. The main component of the catalyst used in the prepurification stage 3 consists of Al 2  O 3  or TiO 2 . Elemental sulfur will be deposited on that catalyst 
     
         (2H.sub.2 S+SO.sub.2 =3S+2H.sub.2 O). 
    
     Partly purified gas leaves the prepurifier 3 via line 5 and in many cases will have a residual H 2  S content of only 500 to 3000 ppm. Air in a controlled proportion is supplied to that gas through line 6. The final purification stage contains a bed of granular activated carbon having a particle size in the range from 1 to 6 mm and a BET surface area in the range from 500 to 1500 m 2  /g. 
     Care is taken to form in the final purification stage at a temperature of 100° to 180° C. preferably 130° to 160° C., a loading of elemental sulfur and at least 3% by weight sulfuric acid on that activated carbon. 
     In the loading on the activated carbon the weight ratio of elemental sulfur to sulfuric acid lies in the range from 20:1 to 1:2 in most cases. 
     Purified gas having a H 2  S content usually not in excess of 20 ppm, preferably not in excess of 8 ppm, is discharged via line 10. The SO 2  content of the purified gas is not in excess of 600 ppm, preferably not in excess of 200 ppm. 
     The regeneration of the laden high-metal-oxide catalyst in stage 3a from a prior prepurification and the regeneration of a laden activated carbon in stage 8a from a prior final purification stage 8 are illustrated on the right in FIG. 1. In the present case the regeneration is effected by means of a partial stream of the H 2  S-containing gas, which has been branched in line 11 from the H 2  S-containing gas supplied in line 1. 
     The gas used for the regeneration enters the line 13 and is fed by a fan 14 through line 15 to a by-pass line 16, which contains an open valve 17. The gas then flows in lines 18 and 18a into the bed of the laden activated carbon catalyst in the stage 8a. 
     If required, a reducing gas from an external source, such as a high-H 2  S gas, is admixed from line 19. The temperature in line 18a is in the range of 120° to 180° C. and preferably in the range from 130° to 160° C. The temperature is controlled by a heater 20, through which a partial stream of the gas may be passed when the valve 21 is opened to a larger or smaller extent and the valve 17 has properly been adjusted That partial stream will then be heated and subsequently admixed with the gas in line 16. 
     In the first regenerating step the activated carbon in stage bed 8a is at a temperature from 120° to 180° C. and care is taken that the sulfuric acid which constitutes a part of the loading is entirely or substantially reduced to elemental sulfur. Effluent gas is supplied in line 23 to the catalyst bed of stage 3a and further in line 24 through the cooler 27 and then back to line 13. A partial stream of the gas can be removed in line 26 from the cycle and can be admixed with the gas which flows in line 2 and is to be treated. 
     To remove the elemental sulfur loading from the catalysts in stages 3a and 8a, the supply of gas in line 19 is interrupted and the temperature of the gas in lines 18 and 18a is increased to 200° to 400° and preferably to at least 300° C. For that purpose the gas in line 15 is passed through the heater 20 and the by-pass line 16 is throttled. 
     The gas which flows in line 23 and contains elemental sulfur is contacted with the high-metal oxide catalyst in stage 3a to remove the loading of elemental sulfur from this catalyst. The elemental sulfur is removed from the gas in the cooler 27, in which the elemental sulfur is condensed and from which it is drained through line 28. 
     For the sake of clearness the arrangement of lines permitting a switchover of the adsorbers between the loading and regenerating modes is not shown as such arrangements are well known to those skilled in the art. 
     FIG. 2 shows the use of two beds 81 and 82, which contain activated carbon for the final purification. Before the gas is supplied through line 5a to the first bed, air is supplied to line 6 to the first bed 81 and in line 6a to the region 9 between the beds 81 and 82. The H 2  S:O 2  molar ratio in line 5a is about 1:1 to 1:3. A H 2  S:O 2  molar ratio of about 1:3 to 1:19 with respect to the H 2  S content in line 5 is maintained in the region 9. The H 2  S content of the gas has substantially been removed from the gas before it enters the intermediate region 9. 
     EXAMPLE 
     A processing system as shown in FIG. 1 is used to treat exhaust gas from a Claus process plant. That gas flows at a rate of 7545 sm 3  /h (sm 3  =standard cubic meter or m 3  STP) in line 1. The prepurifier 3 contains an Al 2  O 3  catalyst which is impregnated with 1% by weight nickel. The residence time of the gas in the prepurifier 3 to 6 seconds and the residence time of the gas in contact with the activated carbon of the final purifier 8 is 3 seconds. Air at 135° C. is supplied in line 6 at a rate of 120 sm 3  /h. 
     The following Table contains data for the gas mixtures in the various lines. The beds or stages 8a and 3a are assumed to be subjected to the first regenerating step for reducing the sulfuric acid on the activated carbon or stage 8a. The activated carbon is laden with 5% by weight H 2  SO 4  at the beginning of the regeneration. 
     
                       TABLE
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LINE        1       5      11    18A  19    26
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Rate (sm.sup.3 /h)
            7545    7564   200   2310 10    243
Pressure (bars)
            1.18    1.14   1.18  1.30 1.5   1.20
Temperature (°C.)
             135     148   135    145 49    135
COMPOSITION
H.sub.2 S (vol. %)
            0.85    0.17   0.85  0.48 95.4  0.07
SO.sub.2 (vol. %)
            0.15    0.02   0.15  4.95 --    6.25
H.sub.2 O (vol. %)
            34.35   35.08  34.35 32.81
                                      3.0   32.52
N.sub.2 (vol. %)
            58.75   58.82  58.75 56.08
                                      --    55.58
CO.sub.2 (vol. %)
            5.90    5.91   5.90  5.63 1.6   5.58
S (kg/h)    16.40   1.08   0.43  4.62 --    0.53
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     The pure gas in line 10 still contains 0.02% by volume SO 2  and 8 ppm H 2  S. To remove the elemental sulfur from the beds of stages 3a and 8a, the gas in line 18 is heated to 320° C. and the gas stream in line 19 is shut off. In the cooler 27, a total amount of 3200 kg sulfur is separated at 135° C. during a regenerating phase.