Patent Document

FIELD OF THE INVENTION 
     The present invention relates to a process for extracting sulfur from a gas stream containing hydrogen sulfide. More particularly, the present invention relates to a process for desulfurizing exhaust gas from a Claus unit using a solid adsorbent. 
     BACKGROUND OF THE INVENTION 
     Refinery and natural gas streams and are typically desulfurized by the Claus process wherein elemental sulfur is produced by reacting hydrogen sulfide and sulfur dioxide in the presence of a catalyst. 
     The Claus process was discovered over 115 years ago and has been employed by the natural gas and refinery industries to recover elemental sulfur from hydrogen sulfide-containing gas streams for the past 50 years. Briefly, the Claus process for producing elemental sulfur comprises two major sections. The first section is a thermal section where H 2 S is converted to elemental sulfur at approximately 1,800-2,200° F. No catalyst is present in the thermal section. The second section is a catalytic section where elemental sulfur is produced at temperatures between 400-650° F. over an alumina catalyst. The reaction to produce elemental sulfur is an equilibrium reaction; hence, there are several stages in the Claus process where separations are made in an effort to enhance the overall conversion of H 2 S to elemental sulfur. Each stage involves heating, reacting, cooling and separation. 
     In the thermal section of the conventional Claus plant, a stoichiometric amount of air is added to the furnace to oxidize approximately one-third of the H 2 S to SO 2  and also burn all the hydrocarbons and any ammonia (NH 3 ) present in the feed stream. The primary oxidation reaction is shown as follows:
 
2H 2 S+3O 2 →2SO 2 +2H 2 O  (1)
 
This reaction is highly exothermic and not limited by equilibrium. In the reaction furnace, the unconverted H 2 S reacts with the SO 2  to form elemental sulfur. This reaction is shown as follows:
 
2H 2 S+SO 2 ⇄3S 0 +2H 2 O  (2)
 
Reaction (2) is endothermic and is limited by equilibrium.
 
     In the catalytic section of the Claus process, the unconverted hydrogen sulfide and sulfur dioxide from the thermal stage are converted to sulfur by the Claus reaction (2) over an alumina catalyst. Typically, there are three stages of catalytic conversions. Important features of the Claus reaction in the catalytic stage are that the reaction is equilibrium limited and that the equilibrium to elemental sulfur is favored at lower temperatures. 
     The Claus process was modified in 1938 by I. G. Fabenindustrie and various schemes of the modified process are utilized today. For feed gas streams containing approximately 40% H 2 S, the balance carbon dioxide (CO 2 ) and water (H 2 O), the once through Claus process is generally employed in which all of the acid gas is fed directly to the burner. Three catalytic stages are typically utilized after the initial thermal stage. This scheme will generally produce an overall recovery of 95-97% sulfur. If this recovery efficiency is acceptable, no further processing is required. However, if the recovery efficiency is not high enough (for a variety of reasons and, in particular, environmental constraints) an advanced Claus process such as Comprimo&#39;s Super Claus process which has a sulfur efficiency of 99.0% can be utilized. This process consists of the replacement of the final Claus reaction stage by, or the addition of, a reaction stage featuring a proprietary catalyst to promote the direct oxidation of hydrogen sulfide to sulfur selectively in the Claus tail-gas. Air is injected upstream of the reactor. The hydrogen sulfide and oxygen react over the catalyst via the following reaction:
 
2H 2 S+O 2 →2S 0 +2H 2 O  (3)
 
If a sulfur recovery efficiency of greater than 99% is required, a tail-gas cleanup unit (TGCU) needs to be employed. This type of unit allows for an overall sulfur recovery efficiency of 99.8%. In the United States, a sulfur recovery efficiency of 99.8+% is required for Claus production units generating greater than or equal to 50 STSD of elemental sulfur, hence, a TGCU such as the Shell Scot process is often required. Such processes coupled with a sulfur recovery unit (SRU) can meet and exceed a sulfur recovery efficiency of 99.8+%.
 
     The Shell Claus Off-gas Treating (SCOT) process for removing sulfur components from Claus plant tail gas was first brought on stream in 1973. Since then, the process has been widely used in the oil refining and natural gas industries, with more than 150 units constructed all over the world. In the standard SCOT process, sulfur compounds in Claus plant tail gas are catalytically converted into hydrogen sulfide. After cooling, the hydrogen sulfide is removed by solvent extraction. The SCOT off-gas (the gas not absorbed in the absorber) is incinerated. 
     The standard SCOT process is able to recover 99.9% of total sulfur, resulting in a 250 ppmv sulfur concentration in the SCOT off-gas. In recent years, the demand for higher sulfur recovery efficiencies has resulted in the development of two improved versions to the SCOT process. These are the Low-sulfur SCOT and the Super-Scot processes. The new processes lower the total sulfur content in the SCOT off-gas to less than 50 ppmv. 
     An after treatment process which oxidizes all sulfur compounds into SO 2  is disclosed in U.S. Pat. No. 3,764,665. This patent disclosed a process for removing sulfur oxides from gas mixtures with a solid acceptor for sulfur oxides wherein the solid acceptor is regenerated with a steam-diluted reducing gas and the regeneration off-gas is fed to a Claus sulfur recovery process. The patent provides for cooling the regeneration off-gas to condense the water vapor contained therein, contacting the cooled off-gas with a sulfur dioxide-selective liquid absorbent, passing the fat liquid absorbent to a buffer zone and then to a stripping zone wherein the absorbed SO 2  is recovered from the liquid absorbent and is supplied to the sulfur recovery process. By operating in this manner, fluctuations in the sulfur dioxide concentration of the regeneration off-gas were leveled-out and a relatively concentrated sulfur dioxide stream was supplied to the sulfur recovery process at a substantially constant rate. Although this process supplies relatively concentrated sulfur dioxide to the sulfur recovery process at a substantially constant rate, the off-gas must be cooled and the fat liquid absorbent must be transferred to a buffer zone before the absorbed SO 2  can be stripped. Therefore, what is needed is a simpler process whereby these steps are eliminated and energy costs reduced. 
     In the acceptance apparatus as described in U.S. Pat. No. 3,764,665, solid acceptors are used which are able to accept sulfur oxides and release them again in the form of sulfur dioxide on being regenerated. To this end, carbon-containing adsorbents are disclosed as useful. In this case the sulfur oxides are retained as sulfuric acid in the pores of the carbon adsorbent. After saturation of the adsorbent with sulfuric acid, the carbon-containing adsorbent can be thermally regenerated at 400° C. with the exclusion of oxygen. This yields a sulfur dioxide rich regeneration of off-gas which also contains carbon dioxide, nitrogen, and water vapor. The removal of sulfur compounds in the form of sulfur oxides, under oxidative conditions, i.e., in the presence of oxygen, is preferably affected at temperatures from 325° C. to 475° C. Regeneration under reductive conditions takes place in the same temperature range. Preferably, acceptance and regeneration are affected within this range at the same or virtually the same temperature. At the temperature of adsorption as disclosed in this patent, it is likely the carbon adsorbent is acting as a reducing agent and being consumed as CO 2 , which is formed during regeneration of the adsorbent. Accordingly, continual replacement of the carbon adsorbent will be necessary. 
     U.S. Pat. No. 5,514,351 discloses a process of recovering sulfur from a Claus tail gas by forming a sulfur oxide enriched gas stream and contacting the sulfur oxide enriched gas stream with a solid adsorbent bed to extract the sulfur oxides and retain them as sulfur compounds, thus forming a sulfur oxide depleted stream. The sulfur compounds are retained in the bed in the form of inorganic sulfates, sulfur oxides or combinations thereof. The adsorbent bed is then contacted with a reducing gas stream to reduce the retained sulfur compounds to hydrogen sulfide and/or sulfur dioxide and thereby form a hydrogen sulfide and/or sulfur dioxide bearing stream. Sulfur is recovered from the hydrogen sulfide and/or sulfur dioxide bearing stream, and the sulfur oxide depleted stream may be sent to an incinerator or vented through a stack. While in the adsorbent mode, the adsorbent bed is at an elevated temperature of from about 900° F.-1,400° F. Similar to the previous patented process described immediately above, high temperature adsorption causes useful adsorbents such as carbon to react with and be consumed by the sulfur oxides, requiring significant and frequent replacement of the adsorbent. 
     The objective of this invention is to provide a lower cost solution to the recovery of sulfur from a Claus unit tail gas stream than possible using existing technology and the processes described in the above prior art patents. The current market leading solution for the recovery of sulfur from Claus tail gas streams is still the Shell SCOT process. Unfortunately, the Shell SCOT process costs approximately ½ to ⅓ the cost of the Claus plant itself. Accordingly, lower cost alternatives to the Shell SCOT unit to recover the last 5% of the sulfur leaving the Claus plant in the exhaust gas stream would be welcomed. 
     SUMMARY OF THE INVENTION 
     This invention is directed to a process for removing low concentrations of sulfur from a gas stream. In accordance with a broad aspect of the present invention, there is provided a method of recovering sulfur from a hydrogen sulfide containing gas stream, e.g., from an elemental sulfur recovery unit, comprising the steps of oxidizing the gas stream to convert the hydrogen sulfide therein to sulfur oxide, and thus form a sulfur oxide enriched gas stream. The sulfur oxide enriched gas stream is contacted at relatively low temperatures of about 90-250° C. with a solid, sulfation resistant adsorbent to extract the sulfur oxides and retain them as sulfur compounds, thus forming a sulfur oxide depleted stream. The sulfur compounds are believed retained in the bed in the form of sulfur oxides, sulfuric acid, combinations or complexes thereof. The adsorbent is then contacted with an inert or reducing gas stream to convert the retained sulfur compounds to sulfur and/or sulfur dioxide and thereby forms a sulfur dioxide bearing stream. The elemental sulfur is recovered and/or the sulfur dioxide bearing stream may be recycled to the Claus unit for further conversion. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic of the Claus process which shows both straight-through and split flow processing. 
         FIG. 2  is a schematic of the process of this invention used to treat the tail gas obtained from the Claus process; 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Claus feed gas typically has high concentrations of hydrogen sulfide, for example hydrogen sulfide concentrations of between 40% and 85% depending on plant and pretreatment processes. The pretreatment process may be an amine treater which provides a concentrated hydrogen sulfide output stream (acid gas). 
     A schematic of a typical three-stage Claus plant is shown in  FIG. 1 . The first step of the Claus process involves a controlled combustion of a feed gas which contains hydrogen sulfide and the noncatalytic reaction of unburned hydrogen sulfide with sulfur dioxide as depicted in reactions (1) and (2) above. In the straight through process, a feed gas containing hydrogen sulfide is directed via line  10  to reaction furnace  12  which contains a burner  14  where the feed gas is combusted. Oxygen is supplied to burner  14  by an air stream via line  16 . From the reaction furnace  12 , the products are cooled in a waste heat boiler  18  and the products condensed and separated in condenser  20  into a liquid sulfur stream  22  and gaseous product stream. Gaseous products are reheated via line  24  in reheater  26  and passed through a series of catalytic reactors  28 ,  30 , and  32  wherein the unreacted hydrogen sulfide and sulfur dioxide react over a catalyst, typically alumina, to produce sulfur and water as depicted in reaction (2). Subsequent to each reaction, the reaction products are condensed in respective condensers  29 ,  31  and  33  wherein liquid sulfur is separated and removed via respective lines  23 ,  25  and  27  and joined with liquid sulfur from line  22  to form a final sulfur stream  35 . Precedent to the respective catalytic reactions in reactors  30  and  32 , the product gas directed from the preceding condensers  29  and  31  is reheated in respective reheaters  34  and  36  which receive the cooled gas stream via lines  37  and  39 , respectively. Tail gas leaving condenser  33  via line  40  can be treated in accordance with this invention and as described below. 
     An alternative to the straight-through process is the split-flow process. In this process, 40-60% of the Claus feed bypasses the burner and is fed directly to the first catalytic stage. This process is shown in  FIG. 1  wherein line  42  directs a portion of the H 2 S-containing feed from line  10  into line  24  containing product gas from condenser  20 . The mixed stream is heated in reheater  26  and passed to first stage catalytic reactor  28 . 
     As shown in  FIG. 2 , the hydrogen sulfide-containing tail gas stream  40  from the elemental sulfur recovery unit or Claus process shown in  FIG. 1  is processed in accordance with this invention to recover sulfur values which remain in the tail gas. While tail gas stream  40  can come directly from the Claus process, it is contemplated that the tail gas stream  40  can be generated from a tail gas cleanup unit (TGCU) to increase overall sulfur recovery. Tail gas stream  40  is fed to oxidation reactor  41  to completely convert hydrogen sulfide and other sulfur-containing compounds to sulfur oxides, e.g., SO 2 . A temperature range of about 300 to 500° C. is used for the oxidation in reactor  41 . A sulfur oxide enriched gas stream  44  from oxidation reactor  41  is cooled in heat exchanger  46  to within a range of from about 90° C. to about 250° C. and is fed via line  48  to a fixed-bed reactor  50  containing a solid adsorbent bed (not shown). 
     The solid adsorbent bed in reactor  50  adsorbs substantially all of the sulfur oxide from the sulfur oxide enriched gas stream  44 , and provides a sulfur oxide depleted gas stream  52 . The sulfur oxide depleted stream  52  can be fed to an incinerator or to a stack (not shown). Alternatively, a portion of gas stream  52  can be treated to remove oxygen and CO 2  and used to regenerate the adsorbent as described below. 
     While in an adsorbent mode, the reactor  50  is operated at a temperature of from about 90° C. to about 250° C. A temperature of from about 90° C. to about 150° C. is preferred, and from 90° C. to 125° C. more preferred. These relatively low temperatures are effective for adsorption of the sulfur oxides and, importantly, are not so high as to cause appreciable reaction between the sulfur oxides and some useful adsorbents such as carbon and result in the eventual consumption of the adsorbent. Further, it is believed that by adsorbing the SO 2  in the presence of water and oxygen a higher level of sulfur oxide can be adsorbed in the solid adsorbent bed. It is postulated that the SO 2  is adsorbed as H 2 SO 4  most likely via reaction (4):
 
SO 2 +½O 2 +H 2 O→H 2 SO4  (4)
 
The tail gas from line  40  and oxidation tail gas from line  48  will often contain sufficient water for reaction (4) without the need for water addition. Oxygen may, however, have to be added to stream  48  entering reactor  50 . The oxygen content of the stream  48  entering the adsorbent bed  50  should be in an amount ranging from about 0.9 to 10 times the stoichiometric molar amount required in equation (4). Preferably, the oxygen content will range from about 1 to about 5 times the stoichiometric molar requirement. The amount of air or O 2  needed to meet the general requirements expressed above can be determined by measuring the sulfur content of the Claus tail gas stream  40 . Any analytical instrument known for measuring gas phase components can be used. For example, a Model 880-NSL tail gas analyzer from Ametek Western Research, Paoli, Pa., is one such instrument. Air supplied by line  54  may be the source of the oxygen. Typically, a water content of 10-50 vol. %, more typically, 20-30 vol. % is found in the tail gas stream from the third stage of a Claus reactor. Water vapor can be supplied, for example, from an external source of steam if needed. Pressure within the reactor  50  should be maintained at approximately atmospheric pressure, up to 100 psia.
 
     The adsorbent is most usefully present as a fixed bed in reactor  50  and can be in the form of balls, pebbles, spheres, extrudates, channeled monoliths, microspheres or pellets. A fluidized bed system is also possible with this invention wherein temperature and pressure conditions would remain similar to the fixed bed system. It is particularly important that the low temperatures of the fixed bed be used to avoid consumption of the adsorbent. The adsorbent provides absorbers or acceptors which absorb, and collect or otherwise remove sulfur oxides from the influent gaseous stream. 
     During regeneration of the adsorbent bed in reactor  50 , the temperature is maintained at least about the adsorption temperature or higher, preferably between 150° C. to about 550° C. To protect reactor metallurgy, temperatures of from 150° C. to 260° C. are preferred. The pressure in the reactor  50  is maintained at about atmospheric pressure. On regeneration of the adsorbent bed, it is important that SO 3 /H 2 SO 4  not be formed or released as these components can be deleterious to reactor metallurgy. Accordingly, the regeneration gas stream  56  passed through the adsorbent bed should not contain O 2 . An inert gas or reducing gas stream is therefore used to regenerate the bed. Preferably, a reducing gas is used, most preferably H 2 S since it is readily available. 
     As further shown in  FIG. 2 , the regenerating gas stream  56  is directed into the reactor  50  to liberate the adsorbed SO 2 . A regeneration gas stream flow provided at a volume of gas sufficient to heat the adsorbent bed is used and whereby the exit of the bed in reactor  50  is within 50° C. of the inlet. Preferred gases for regeneration include nitrogen, hydrogen, C 3 + hydrocarbons, and hydrogen sulfide. The off-gas stream  52  stripped of any O 2  and containing N 2  and CO can also be used for regeneration. Combinations of inert gas and reducing gas can be used. Regeneration with a portion of the Claus plant feed  10  is also acceptable. Regeneration with H 2 S or a reducing gas stream containing H 2 S is preferred. When regenerating with H 2 S, it has been found that only minimal, if any, amounts of SO 3 /H 2 SO 4  are released. Formation of elemental sulfur is observed, most likely occurring by reaction (5):
 
3H 2 S+H 2 SO 4 →4S+4H 2 O  (5)
 
     If carbon is used as the adsorbent and CO 2  is present at the exit of the adsorber during adsorption or regeneration, this indicates that the carbon was acting as a reductant and, therefore, it is postulated that the carbon is being consumed most likely via reaction (6):
 
C+2H 2 SO 4 →2SO 2 +CO 2 +2H 2 O  (6)
 
     The lower temperatures used during adsorption greatly minimize the formation of CO 2  and distinguish the process of this invention over the processes of U.S. Pat. Nos. 3,764,665 and 5,514,351 described above. 
     The invention contemplates that the regenerating gas  56  be back-flowed through the adsorbent bed in reactor  50  in a direction opposite the flow direction of the sulfur oxide enriched stream  48  through the bed. This would ensure that the last part of the bed that the sulfur oxide enriched stream sees is very active. 
     Regeneration of the adsorbent in reactor  50  provides sulfur and/or sulfur dioxide bearing stream through the outlet line  58 . The sulfur dioxide-containing stream  58  can be recycled to the Claus plant and line  10  for further recovery of sulfur. The hydrogen sulfide and/or sulfur dioxide bearing stream may also contain water and unconverted reducing gas. 
     The adsorbents useful in this invention can be characterized as being sulfation resistant. In other words, the adsorbents will not react with the SO 2  to form sulfates on the adsorbent surface. Therefore, alumina and alumina-containing adsorbents such as alumina-containing clays, spinels, and silica-alumina products are not useful in this invention. 
     Non-limiting examples of suitable sulfation resistant solid adsorbents for use in the present invention include the porous solids, silica, natural and synthetic zeolites, activated carbon, titania, zirconia, titania-silica, and zirconia-silica. 
     The adsorbents can be impregnated or otherwise coated with at least one oxidizing catalyst or promoter that promotes the removal of nitrogen oxides, the oxidation of SO 2  to SO 3  in the presence of oxygen, and the regeneration of the sorbent. It is believed that SO 3  is more readily adsorbed than SO 2 . One useful catalyst is ceria (cerium oxide). Another useful catalyst is platinum. Other catalytic metals, both free and in combined form, preferably as an oxide form, can be used, either alone or in combination with each other or in combination with ceria, such as rare earth metals, metals from Group 8 of the Periodic Table, chromium, vanadium, rhenium, tungsten, silver and combinations thereof. An even distribution of the promoter is preferred for best results and to minimize adsorbent erosion. 
     The specific amounts of the promoters included in the solid sorbent, if present at all, may vary widely. Preferably, the first promoter is present in an amount between about 0.001% to about 20% by weight, calculated as elemental metal, of the solid sorbent, and the second promoter is present in an amount between about 0.001% to about 10% by weight, calculated as elemental metal, of the solid sorbent. Preferably, the solid sorbent includes about 0.1% to about 20%, more preferably about 0.2% to about 20%, and still more preferably about 0.5% to about 15%, by weight of rare earth metal, calculated as elemental metal. Of course, if a platinum group metal is employed in the solid sorbent, very much reduced concentrations (e.g., in the parts per thousand to parts per million (ppm) range) are employed. If vanadium is included as the second promoter, it is preferably present in an amount of about 0.01% to about 7%, more preferably about 0.1% to about 5%, and still more preferably about 0.5% to about 2% by weight of vanadium, calculated as elemental metal. 
     The promoters may be associated with the solid sorbent using any suitable technique or combination of techniques; for example, impregnation, coprecipitation, ion-exchange and the like, well known in the art. Also, the promoters may be added during synthesis of the sorbent. Thus, the promoters may be an integral part of the solid sorbent or may be in a phase separate from the solid sorbent (e.g., deposited on the solid sorbent) or both. These metal components may be associated with the solid sorbent together or in any sequence or by the same or different association techniques. Cost considerations favor the preferred procedure in which the metal components are associated together with the sorbent. Impregnation may be carried out by contacting the sorbent with a solution, preferably an aqueous solution, of the metal salts. 
     It may not be necessary to wash the sorbent after certain soluble metal salts (such as nitrate, sulfate or acetate) are added. After impregnation with the metal salts, the sorbent can be dried and calcined to decompose the salts, forming an oxide in the case of a nitrate, sulfate or acetate. 
     The following examples are illustrative of adsorbents and process conditions useful to practice this invention. The scope of the invention, however, is to be determined from the appended claims. 
     EXAMPLE 1 
     The proposed mechanism for the adsorption of SO 2  on activated carbon in the presence of O 2  and H 2 O is the formation of an adsorbed sulfuric acid species, which is then thermally regenerated/reduced back to SO 2 . To test this theory, two adsorbent samples were impregnated with sulfuric acid: (1) an activated carbon with 35% H 2 SO 4  and (2) 1.9% Pt/ZSM-5 having a SiO 2 /Al 2 O 3  ratio of 270 with 20% H 2 SO 4 . Each acid loaded sample was placed in a column and then regenerated at 260° C. with wet N 2 . The SO 2 /SO 3  content of the off-gas was determined by wet analysis. 
     The loading for the activated carbon was 7.76 g (0.079 mol) of H 2 SO 4  on 13.7 g of carbon. The SO 2 /SO 3  split upon regeneration was determined to be 4.91 g SO 2  (0.077 mol) and 0.21 g of SO 3  (0.002 mol). Remarkably, 100% recovery of SO 2 /SO 3  (0.079 mol) was achieved with the formation of only 4% of undesirable SO 3 /H 2 SO 4 , a very favorable situation. 
     The loading for Pt/ZSM-5 was 6.76 g (0.069 mol) of H 2 SO 4  on 25.6 g of adsorbent. The SO 2 /SO 3  split upon regeneration couldn&#39;t be determined since the vent lines plugged up with a green solid. This negative result indicates that a significant amount of free sulfuric acid was liberated during regeneration and subsequently reacted with the metal lines. Unlike with the carbon adsorbent, this formation of undesirable H 2 SO 4 /SO 3  seen is a very unfavorable situation. Apparently, the structure/composition of activated carbon is more favorable for the reversible reactive adsorption of SO 2 . It is also likely, that the carbon was sacrificed before the reactor metallurgy. 
     EXAMPLE 2 
     This example compares the impact of the feed components during adsorption. SO 2  adsorption was compared with and without O 2  or H 2 O present in the fuel. Breakthrough times (detection of SO 2  in exit gas) were normalized to 20.0 g: 
     Sample: 15.6 g (dry basis) of Norit®RO activated carbon (0.8 mm extrudates) 
     Adsorption Temp: 90° C. 
     Adsorption Pressure: 20 psia 
     Feed Flow: 73 sccm 
     Duplicate SO 2  breakthrough tests on Norit®RO activated carbon using a feed stream containing 3,100 ppm SO 2 , ˜22% CO 2 , ˜22% H 2 O, balance N 2  resulted in an average breakthrough time of 219 minutes. Results were significantly better with O 2  present as shown next. Breakthrough tests were repeated using a feed stream containing 3,100 ppm SO 2 , 22% CO 2 , 9,000 ppm O 2 , ˜22% H 2 O, balance N 2 . In this case no breakthrough of SO 2  was noted even after 2,880 minutes, the point at which the run was stopped. In the presence of O 2 , loading of SO 2  was &gt;11.9 wt % SO 2  (g/g ads.) as compared to 0.9% wt % SO 2  (g/g ads) without O 2  present. The sample was regenerated at 260° C. overnight with dry N 2  between each breakthrough test. 
     In order to determine the effect of water on the SO 2  capacity of the activated carbon, a dry SO 2  breakthrough test was then run on Norit®RO activated carbon using a feed stream containing 3,100 ppm SO 2 , 22% CO 2 , 9,000 ppm O 2 , balance N 2 . A significantly reduced SO 2  breakthrough time of 589 minutes resulted. Thus, in the presence of O 2  but no H 2 O, SO 2  loading was to 2.4% wt % SO 2  (g/g ads.) 
     To more easily quantify the amount of SO 2  adsorbed on the Norit®RO activated carbon, a feed gas containing 5% SO 2 , 5% O 2 , ˜22% H 2 O, and balance N 2  was used. Even with this 16-fold increase in SO 2  concentration, the breakthrough time for SO 2  was still 1,042 min. This represents a ˜50% wt. loading of SO 2 . An analysis of the off-gas during subsequent regeneration indicated a reversible loss of SO 2  only. A survey of the literature confirms this result, i.e., activated carbons can pick up this amount of SO 2  when H 2 O and O 2  are present. The mechanism is reported to involve the reversible oxidation of SO 2  to SO 3  forming an “H 2 SO 4 ” like complex with the H 2 O that releases only SO 2  upon regeneration. It is important in the process of this invention that little or no free acid be released during regeneration. 
     EXAMPLE 3 
     In this example, the impact of inert gas regeneration of the adsorbent was studied. 
     Regeneration with N 2  (9 cycle life test): 
     Sample: 14.6 g (dry basis) of Norit®RO activated carbon (0.8 mm extrudates) 
     Adsorption Temp: 90° C. 
     Adsorption Pressure: 20 psia 
     SO 2  adsorption steps were run with a feed containing 5% SO 2 , 5% O 2 , 24% H 2 O, balance N 2  at 90° C. The feed flow was adjusted to 73 sccm so as to achieve a less than four hour breakthrough time. Regeneration steps were carried out at 260° C. with wet helium at 73 cc/min of He with 1 ml/min H 2 O for three hours. The final hour of the regeneration cycle was used for cooling the bed. Significant CO 2  was detected by the GC during regeneration. A GC scan of the regeneration off-gas from the 8 th  cycle showed that the production of CO 2  was directly associated with the release of SO 2 . Integration of the peaks indicated a ˜2.6/1 SO 2 /CO 2  molar ratio. This ratio is consistent with carbon oxidation by the adsorbed sulfuric acid, i.e., 2H 2 SO 4 +C→CO 2 +2SO 2 +2H 2 O, during thermal regeneration. It was also determined from peak integration that ˜0.30 wt % of the carbon was lost per the eight hour adsorption/regeneration cycle. This would add up to an intolerable 30 wt % loss of carbon adsorbent per month. 
     EXAMPLE 4 
     The benefit of H 2 S regeneration is shown in this example. Regeneration with_H 2 S was provided in a 17 cycle life test. 
     Sample: 14.7 g (dry basis) Norit®RO activated carbon (0.8 mm extrudates) 
     Adsorption Temp: 90° C. 
     Adsorption Pressure: 20 psia 
     Feed Flow: 73 sccm 
     SO 2  adsorption steps were run with 5% SO 2 , 5% O2, 24% H 2 O, balance N 2  at 90° C. and 50 cc/min. Regeneration steps were carried out at 400° C. with wet H 2 S at 50 cc/min of H 2 S with 1 ml/min H 2 O. No CO 2  or SO 2  was detected by the GC during regeneration. However the formation of sulfur was noted. Based on the GC detection limit, no more than a 12% annual loss of carbon would be expected. This result is consistent with the reaction of H 2 S with the adsorbed sulfuric acid, i.e., 3H 2 S+H 2 SO 4 →4S+4H 2 O, during thermal reaction. In addition, no loss in SO 2  capacity was noted after the 17 cycles. 
     EXAMPLE 5 
     In this example, the impact of adsorption temperature was measured using a 3 cycle test. 
     Sample: Darco® activated carbon (4/12 mesh granules) 
     Sample Wt: 13.8 g at 90° C./14.3 g at 150° C./15.8 g at 200° C. (dry basis) 
     Adsorption Pressure: 20 psia 
     Feed Flow: 73 sccm 
     SO 2  adsorption steps were run with 5% SO 2 , 5% O 2 , 24% H 2 O, balance N 2 , at the temperatures noted above and a gas flow of 73 cc/min. Regeneration steps were carried out at 260° C. with wet He at 73 cc/min of He with 1 ml/min H 2 O. A significant and undesirable reduction in performance was noted when the adsorption temperature was raised from 90° C. to 200° C. (˜85% loss after three cycles) and even to 150° C. (˜50% loss after 3 cycles). The loss in performance is undoubtedly correlated with the undesirable combustion of the activated carbon at the elevated adsorption temperatures of 150° C. and 200° C., as evidenced by CO 2  detection using GC analytical methods.

Technology Category: 8