CA1299842C - Reducing regeneration effluent rate by supplemental cooling and oxidantintroduction during regeneration of metal oxide absorbent - Google Patents

Abstract

ABSTRACT
Sulfur species are removed from a Claus plant tailgas stream by contacting with ZnO producing ZnS. ZnS
is regenerated to ZnO by contacting with dilute O2 under conditions for reducing the rate of regeneration recycle to the Claus plant.

GMB:tb/mh

Description

1~9~842 Lee "REDUCING RE~ENERATION EFFLUENT RATE BY
SUPPLEMENTAL COOLING AND OXIDANT INTRODUCTION
DURING REGENERATION OF METAL OXIDE ABSORBENT"
FIELD OF THE INVENTION
The invention relates to the removal of sulfur 15 and sulfur compounds from gaseous streams containing such compounds. In one aspect, the invention relates to the removal of sulfur compounds including ~2S (hydrogen sul-fide) and SO2 (sulfur dioxide) from Claus plant tail gas.
In another aspect, the invention relates to the use of 20 solid high surface area contact materials (absorbents), for example, ZnO-based (zinc oxide-based) absorbents, for absorbing sulfur compounds such as SO2 and H2S. In a fur-ther aspect, the invention relates to regenerating sul-fided ZnO-based absorbents (zinc sulfide, ZnS) under con-25 ditions for reducing the time required for regeneration,and/or for reducing the rate of regeneration effluent returned to the Claus plant, while increasing the concen-tration of sulfur and sulfur compounds in such regenera-tion effluent.
SETTING OF THE INVENTION
A developing area of sulfur recovery technology is that of tail gas cleanup, that is, of removing trace quantities of sulfur compounds from gaseous effluent streams (tail gas) of Claus process sulfur recovery 35 plants. Tail gas may contain substantial amounts of sulfur compounds. Tail gas from Claus or extended Claus plants (having at least one Claus catalytic reaction zone operated under conditions, including temperature, effec-lX998A2 tive for depositing a preponderance of formed sulfur oncatalyst therein) typically can contain about 0.5-10% of the sulfur present in feed to the plant as elemental sulfur, H2S, SO2, COS (carbonyl sulfide), CS2 (carbon 5 disulfide), and the like. Tail gas cleanup processes remove at least part of such residual sulfur compounds from Claus tail gas.
In U. S. Patent 4,533,529, Claus tail gas is contacted with ZnO (zinc oxide) in an absorber reducing 10 average overall emission levels from the absorber to less than 250 ppm sulfur species. When ZnS is being regener-ated to its active ZnO form, producing SO2, the resulting regeneration effluent stream comprising SO2 can be returned to the Claus plant for removal of SO2 by conver-15 sion to elemental sulfur.
Reducing the rate of return of such a regenera-tion effluent stream to the Claus plant can result in highly significant cost savings in plant construction and retrofits as well as in operation and maintenance. This 20 is because providing the regeneration effluent stream to the Claus Plant in addition to the Claus Plant acid gas feed increases the total feed rate of gaseous streams to the Claus Plant. The result is an increase in size of the Claus Plant downstream of the site at which the regenera-25 tion effluent stream is returned. Hence, reducing therate of return of such a regeneration effluent stream to the Claus plant can reduce the size of Claus plant and downstream processing equipment required.
There is provided a process which can reduce the 30 rate of regeneration effluent, and increase the concentra-tion of sulfur compounds therein, returned to a Claus plant and can achieve the significant economic and tech-nical benefits resulting therefrom.
In according with further aspects of the inven-35 tion, there is provided a process in which the advantagesof reducing the rate of regeneration effluent return to the Claus plant and of reducing the time required for regeneration can both be achieved, thus providing a 129984~

greater flexibility in design and operation of such plants.
SUMMARY OF THE INVENTION
The present invention comprises an improved pro-5 cess and apparatus for the recovery of sulfur and sulfurcompounds from a gaseous stream. The sulfur compounds are removed from the gaseous stream in the presence of metal oxide absorbent (ZnO based absorbent) in an absorber to produce sulfided absorbent (ZnS) and a gaseous stream 10 reduced in sulfur compound content. The laden absorbent ZnS is regenerated in a regeneration zone by passing a dilute oxygen containing stream in contact with a first portion of sulfided absorbent, removing heat evolved during the conversion of ZnS to ZnO from the resulting 15 stream, and enriching the resulting stream with oxygen producins a second dilute oxygen-containing stream which is passed in contact with. Then, the second dilute oxygen-containing stream is passed in contact with a second portion of sulfided absorbent in the regeneration 20 zone, producing regenerated absorbent (ZnO) in each of the first and second portions of absorbent, and regeneration effluent. In accordance with further aspects of the invention, the steps of removing from the gas-in-process heat evolved during conversion of ZnS to ZnO in a portion 25 of sulfided absorbent and of using the gas-in-process to dilute a further portion of oxygen producing a further dilute oxygen-containing stream for regeneration of a fur-ther portion of absorbent can be repeated two or more times. Regeneration effluent is returned to the Claus 30 plant where the sulfur compounds therein are converted to elemental sulfur and removed.
By introducing supplemental oxygen to successive portions of absorbent being regenerated and removing heat evolved by the regeneration reaction in each portion of 35 absorbent, the volume of regeneration effluent produced during regeneration can be dec.eased without exceeding a predetermined temperature during regeneration. This is due to the fact that the volume of diluent required during regeneration is reduced by, in effect, using the same diluent for each supplemental successive introduction of oxygen into successive portions of sulfided absorbent. By reducing the volume of regeneration effluent produced 5 during the regeneration period, but keeping the regenera-tion period about the same or even less, the rate of regeneration effluent to be returned to the Claus plant can be reduced in comparison with the case where supple-mental oxygen is not introduced with cooling for removal 10 of heat evolved during regeneration in each successive portion of absorbent being regenerated. As a result, the Claus plant as well as downstream equipment such as the ZnO absorbers themselves can be sized to process a smaller volume of gas, resulting in significant capital and oper-15 ating savings.
In one aspect, the invented process comprisesremoving sulfur compounds from the Claus plant tail gas comprising H2S, SO2, organic sulfides such as carbonyl sulfide (COS), carbon disulfide (CS2) and the like and 20 elemental sulfur. The tailgas is then provided to an absorber containing a zinc oxide absorbent (ZnO-based absorbent) effective for removal of at least H2S from the stream. H2S in the stream is absorbed by the zinc oxide absorbent (ZnO) producing sulfided absorbent (ZnS) and 25 absorber effluent. The sulfided absorbent is then regen-erated in a regenerator by passing the dilute oxygen-containing gas in contact with a first portion of the sul-fided absorbent, regenerating ZnS to ZnO in the first portion, removing from the gas-in-process heat evolved 30 during conversion of ZnS to ZnO, and using the gas-in-process as diluent for producing a second dilute oxygen-containing stream which is then passed in contact with a second portion of ZnS, producing a regenerated absorbent ZnO and regeneration effluent. Further steps of intro-35 ducing oxygen and cooling can also be used. Thus, thegas-in-process is successively enriched in oxygen during passage through successive portions of absorbent under-going regeneration. The final regeneration effluent is 12998~2 provided back to the Claus plant where sulfur compounds therein are converted to elemental sulfur and removed.
In a further aspect, the process comprises removing sulfur compounds from a Claus plant tail gas 5 stream having at least one Claus low temperature catalytic absorption zone operated under conditions, including tem-perature, such that a preponderance of elemental sulfur formed is deposited on the Claus conversion catalyst.
Reducing the rate at which recycle is provided 10 to the Claus plant results in a reduction in the total gaseous feed rate to the Claus plant. Hence, the rate at which gaseous effluent is produced from the Claus plant is reduced, and therefore the rate at which such effluent is processed in the absorber is likewise reduced. The end 15 result is a significant reduction in the sizing of equip-ment required for a Claus plant with metal oxide (ZnO) absorbers for tail gas cleanup with a corresponding reduc-tion in capital and operating costs.
In further aspects, the benefits of supplemental 20 introduction of oxygen with cooling prior to reintroduc-tion into a successive portion of absorbent being regener-ated can be utilized in achieving a reduction in the time required for regeneration. In most cases a combination of reducing the time required for regeneration and reducing 25 the rate of regeneration effluent recycled to the Claus plant will be desirable. Further, other combinations of these advantages will be apparent to those skilled in the art from the present disclosure.
The invention accordingly comprises the pro-30 cesses and systems, together with their steps, parts, and interrelationships which are exemplified in the present disclosure, and the scope of which will be indicated in the appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGURE 1 shows schematically a process employing a first embodiment of the invention.
FIGURE 2 shows schematically a second embodiment of the invention.
DETAILED DESCRIPTION OF THE INVENTION

~29~

The invention comprises introducing a sulfur andsulfur compound containing gaseous stream into an absorber zone containing ZnO effective for removing at least H2S
from the gaseous stream under conditions, including tem-5 perature and composition, for such removal, producing sul-fided absorbent (ZnS) and absorber effluent reduced in sulfur compound content. ZnS is regenerated in a regener-ation zone by introducing a dilute oxygen-containing stream into contact with successive portions of the ZnS
10 absorbent, with heat removal to remove heat evolved during regeneration of ZnS to ZnO in such portions, and use of the gas-in-process to produce a cooled dilute oxygen-containing stream for use in regeneration of a further portion of absorbent. The final regeneration zone effl-15 uent is recycled back to the Claus plant for furthersulfur recovery.
Any suitable apparatus can be used in the regen-eration zone for practicing the method of the invention.
Thus, multiple beds in a single reactor (regenerator) can 20 be used with partitions in the reactor permitting with-drawal of the gas-in-process from one bed, cooling, enrichment with oxygen, and return to the reactor upstream of another bed. Alternatively, two or more reactors (regenerators) can be employed with coolers between the 25 reactors. Alternatively, heat removal means such as coils can be embedded in successive layers of the absorbent itself, and oxygen can be incrementally introduced at suc-cessive intervals into the absorbent bed. Alternatively, fluidized bed regeneration means can be employed with a 30 portion of sulfided absorbent being passed in contact with a dilute oxygen-containing stream, followed by cooling of the gas-in-process to remove heat evolved during regenera-tion, and enrichment with oxygen, followed by introduction of an additional portion of sulfided absorbent in contact 35 with the resulting cooled dilute oxygen-containing stream produced using the resulting stream for diluent.
In plants of the type described herein, an important consideration is operating temperature during 1~99842 regeneration. Operation at high temperatures can necessitate increased costs due to materials of construc-tion, reduction of absorbent life, and the like. Accord-ingly, it is desirable to operate at or below a predeter-5 mined temperature level which can vary from plant toplant.
The cause of temperature rise during regenera-tion is the heat evolved during regeneration by reaction of oxygen with zinc sulfide. The temperature rise is pro-10 portional to the concentration of oxygen in the regenera-tion stream. Consequently, excessive temperature rise can be avoided by limiting the concentration of oxygen in the regeneration stream. Thus, as discussed below in further detail, oxygen concentration can be limited to less than 15 about 3.5 mol%, depending on the inlet temperature of the regeneration stream, when it is desired to limit the tem-perature in the regenerator to less than about 1500F.
Limiting temperature rise by limiting oxygen concentration, however, results in a larger ratio of dil-20 uent to oxygen in the regeneration stream. Since theamount of oxygen required for regeneration of absorbent is stoichiometrically determined, the result of limiting oxygen concentration is an increase in the volume of regeneration effluent produced during regeneration due to 25 an increased volume of diluent. Even if the regeneration period is the same as the absorptiGn period to minimize the rate of regeneration effluent returned to the Claus plant, the result of controlling temperature rise by lim-iting oxygen is an increase in the regeneration effluent 30 rate.
Since the regeneration rate is concentration related, it is further not feasible to introduce, for example, 10-15 mol% oxygen at a lower rate since the regeneration front moving through the bed will exceed the 35 desired temperature of operation. In accordance with the invention, however, these disadvantages are avoided by introducing oxygen at a concentration effective to maîn-tain the temperature in each portion of sulfided absorbent being regenerated below a desired temperature. The effect is to cause 2, 3, or more regeneration fronts to be moving concurrently through successive portions of the sulfided absorbent, avoiding excessive temperature rise in each 5 portion, and using the same gas-in-process as diluent.
The result is that regeneration can be accomplished using a smaller total volume of gas without exceeding desired temperatures of operation. By distributing this volume of regeneration gas across the regeneration period, the rate 10 of return of regeneration effluent to the Claus plant can be very significantly decreased. Thus, by causing two regeneration fronts with intercooling to be moved through the sulfided absorbent, the rate of return of regeneration effluent to the Claus plant while maintaining a predeter-15 mined regeneration temperature can be roughly one-half that of the rate where only one regeneration front is employed. Similarly, use of three regeneration fronts with intercooling between the fronts can reduce the rate of return of regeneration effluent to the Claus plant to 20 roughly one-third of that otherwise occurring.
In a Claus plant, sulfur is recovered from an H2S-containing stream by introducing the stream into a thermal reaction zone (Claus furnace) and at least one Claus catalytic reaction zone. The Claus thermal reaction 25 zone can be, for example, a Claus muffle furnace, a fire tube (tunnel) furnace, or the like. Generally, the Claus thermal reaction zone functions for converting a portion of H2S, preferably about 1/3, to SO2 for thermal or cata-lytic Claus reaction with H2S to form elemental sulfur.
In the Claus furnace, the H2S-containing gas and oxidant can be reacted at a temperature generally in the range of about 1800-2600F. The effluent from the Claus thermal reaction zone can be cooled, for examplel in a waste heat boiler, and optionally passed through a sulfur 35 condenser to condense and remove liquid sulfur.
The gaseous effluent can then be fed into a Claus catalytic reaction zone operated above the sulfur dewpoint having an inlet temperature in the range, for ~9~2 example, of about 350-650F. In the high temperature Claus catalytic reactor, sulfur is formed by the Claus reaction (shown below) in the presence of an effective Claus reaction-promoting catalyst such as alumina or 5 bauxite:

2 H2S + SO2 ~ 3/2 S + 2 H2O

10 Elemental sulfur vapor can be continuously removed from the reactor and provided to a sulfur condenser where it is condensed and removed as a liquid. Gaseous effluent from the sulfur condenser can be reheated, if desired, and passed to further high temperature Claus reactors and 15 associated sulfur condensers as is known in the art. The effluent gas from the final sulfur condenser is then the Claus plant tail gas.
Preferably, the Claus plant tail gas is from a Claus plant which includes at least one Claus catalytic 20 reactor operated under conditions, including temperature, effective for depositing a preponderance of the formed sulfur on Claus catalyst therein. Such a Claus low tem-perature adsorption zone can be broadly operated in the range of from about 160 to about 330F, preferably in the 25 range of from about 260-320F.
The operation of such Claus plants having Claus high temperature reactors and Claus low temperature adsorption reactors is well known in the art and need not be further described here.
The tail gas from such Claus plants comprises H2S, SO2, organic sulfides, and reducing species such as H2 and CO. Tail gas from plants having only Claus high temperature reactors can contain H2S in the range of about 0.4 to about 4 mol%, SO2 in the range of about 0.2 to 35 about 2 mol%, water in the range of about 20 to about 50 mol~ (typically 30-40 mol%), as well as organic sul-fides such as COS and CS2, and elemental sulfur. Where the tail gas is from a plant having one or more Claus low 1299~2 temperature adsorption reactors, the tail gas may have equivalent of about 0.4 mol%, preferably about 0.2 mol%, or less single sulfur species.
Use of at least one Claus low-temperature 5 adsorption reactor is preferable in part because such reactors remove significant amounts of organic sulfides, such as COS, CS2, and the like from the gas in process.
These organic sulfides are not removed by sulfur recovery processes such as the IFP process described in DeZael, et 10 al., U. S. Patent 4,044,114 (1977) which forms elemental sulfur in the presence of polyethylene glycol and sodium benzoate.
For the same reason, it is also preferred to operate at least one Claus high temperature reactor so 15 that effluent has a temperature in the range from about 550 to 700 F, preferably from about 600 to 650F to diminish the amount of organic sulfides in the effluent.
Both H2S and SO2, as well as organic sulfides, can be concurrently removed in the absorber in the pres-20 ence of reducing species for reducing the SO2 and othersulfur species to H2S. Alternatively, sulfur species other than H2S can be converted to H2S in a hydrogenation zone prior to introduction into the absorber. In either case, it is preferred to operate the Claus plant so that 25 about a 2:1 ratio of H2S:SO2 is maintained in the Claus plant tail gas to maximize sulfur recovery in the Claus plant and to minimize the amount of sulfur remaining in the Claus plant tail gas to be removed by the ZnO absor-bers. By reducing the organic sulfide and other sulfur 30 content in the feed to the absorbers, the volume of regen-eration effluent returned to the Claus plant can be dimin-ished. An effect of operating at about a 2:1 ratio is that significant quantities of both H2S and SO2 are present in the Claus plant tail gas.
The reducing species, for example, H2, and/or CO
required for conversion of sulfur compounds in the tail gas to H2S can be obtained from any convenient source including that present in the tail gas as H2, or available ~Z998~2 from a donor such as CO, which can react with water to yield H2. H2 is preferred, whether contained in the tail gas or internally generated or provided from an outside source.
The Claus plant tail gas can contain sufficient reducing species where the Claus plant is appropriately operated. For most Claus plants, by operating the Claus furnace so that slightly less air is utilized than that required for reaction (1) ~2S + 2 ~ H2O + 1/2 S + 1/2 SO2 (1) and by insuring that the tail gas leaving the final sulfur condenser of the Claus plant has a low level of residual 15 elemental sulfur, the Claus plant tail gas will contain sufficient reducing species. By further reducing the amount of oxidant introduced into the Claus furnace or by other methods which will be apparent to persons skilled in the art, the amount of reducing species can be further 20 increased if desired.
The Claus plant tail gas having sufficient reducing species to reduce all sulfur compounds therein to H2S can be heated, for example, directly by means of direct fired heaters, or indirectly by heat exchange, for 25 example, with other process streams such as absorber effl-uent, to produce a heated Claus plant tail gas effluent stream having a temperature effective for removal of both H2S and SO2 in the presence of a solid particulate prefer-ably high surface area (for example, pellets, extrudates, 30 and the like) ZnO absorbent effective for such removal.
This simultaneous removal of both H2S and SO2 is consid-ered to proceed by hydrogenation of sulfur compounds present in the tail gas to H2S in the presence of ZnO, ZnO
in this respect acting as a catalyst, followed by absorp-35 tion of the thus-formed H2S by the ZnO by sulfiding the ZnO to ZnS, the ZnO acting as an absorbent. Preferably, the Claus plant tail gas can be heated to above about 1000F. By operating at these absorber temperatures, a 12~98~:

hydrogenation reactor is not required before removal of H2S and other sulfur compounds in the absorber. Con-versely, temperatures below about 1000F can be used during absorption with the addition of a separate and dis-5 tinct hydrogenation reactor or zone prior to the absor-bers. When operating at temperatures above about 1000F, H2S emissions can set a practical upper limit on the absorption temperature which will be used. Currently for these reasons it may be appropriate that the upper limit 10 during absorption be about 1200F. Higher temperatures can also be used. Absorber operation above about 1000F
is preferred because such higher temperatures favor higher absorption capacity and the hydrogenation reactor can be eliminated. Also, since absorption and regeneration will 15 then be conducted at approximately the same inlet tempera-ture (1000-1200F), temperature stress on equipment can be reduced. As a result, there will be no significant heating and cooling period. Hence, the time available for regeneration and absorption will be increased and the rate 20 of regeneration effluent returned to the Claus plant can be decreased.
Where the Claus plant tail gas is introduced into a hydrogenation zone prior to the ZnO absorbers, the principal reaction will be the conversion of SO2 to H2S as 25 shown by Reaction (6) below; other sulfur compounds including elemental sulfur, COS, CS2, and the like will also be reduced to H2S. Hydrogenation can be carried out at a temperature of from about 450 to about 1200F or even higher, preferably from about 580F to about 800F, 30 depending on the conditions and the source of H2 chosen.
Hydrogenation by contacting with a bed, either supported or fluidized, of effective hydrogenation catalyst is pre-ferred. Useful catalysts are those containing metals of Groups VB, VIB, VIII and the Rare Earth series of the 35 "Periodic Table of the Elements" in Perry and Chilton, Chemical Engineers Handbook, 5th Ed. The hydrogenation catalyst may be supported or unsupported. Catalysts sup-ported on a refractory inorganic oxide, such as on a lZ9984Z

silica, alumina or silica-alumina base are preferred. The preferred catalysts are those containing one or more of the metals, cobalt, molybdenum, iron, chromium, vanadium, thorium, nickel, tungsten (W) and uranium ~U) added as an 5 oxide or sulfide of the metal, although the sulfide form appears to be the active form. Particularly preferred are cobalt-molybdenum hydrogenation catalysts such as are com-mercially available for use in the refining industry for desulfurization processes in the refining of oil.
After hydrogenation, the resulting stream now containing substantially all sulfur compounds in the form of H2S can then be contacted in an absorber zone with a suitable ZnO absorbent (either fixed or fluidized bed) to absorb H2S and to produce a laden (sulfided) absorbent at 15 temperatures in the range of about 600F to about 1000F.
Alternatively, where absorption is conducted at a tempera-ture above about 1000F, for example, in the range of about 1000F to about 1200F, the absorption of H2S can be accomplished in an absorber simultaneously with removal of 20 the other sulfur compounds without prior hydrogenation.
In either event, while a first absorption zone is func-tioning as an absorber, a second absorption zone can be functioning as a regenerator.
As used herein, and in the claims, the terms 25 "metal oxide absorbent", "ZnO", "ZnO absorbent", and the like shall mean an absorbent effective for removal of both H2S and SO2 in the presence of reducing species. A major portion of the active absorbent, for example, fifty per-cent or more, is in the form of ZnO which is the active 30 form. The absorbent can also contain binders, strength-eners, and support materials, for example, alumina (A12O3), calcium oxide (CaO) and the like. Zinc sulfide and zinc sulfate can be used as starting materials and treated with heat and/or oxygen to produce an active ZnO
35 absorbent. Other suitable starting materials can also be used. The ZnO absorbent is effective for absorbing H2S by undergoing sulfidization to produce a laden (sulfided) absorbent; simultaneously, if desired, hydrogenation of i2~BA~

other sulfur compounds to H S followed by such absorption can occur. Preferably, the 2nO absorbent is capable of a high level of removal of sulfur compounds and is rela-tively insensitive to water.
Particularly preferred are ZnO absorbents which are thermally stable, regenerable, and capable of absorbing substantial amounts of sulfur compounds. An acceptable absorbent is United Catalysts, Inc., G72D*
Sulfur Removal Catalyst, available from United Catalysts, 10 Inc., Louisville, KY, having the following chemical compo~
sition and physical properties:
CHEMICAL COMPOSITION

wt% Trace Metal Impurities wt~
15 ZnO........... 90.0 +5% Pb..................... <0.15 Carbon........ <0.20 Sn..................... <0.005 Sulfur........ <0.15 As..................... <0.005 Chlorides..... <0.02 Hg..................... <0.005 Al O ......... 3-7 Fe..................... ~0.1 20 CaO........... 0.5-3.0 Cd..................... <0.005 PHYSICAL PROPERTIES
Form Pellets Size 3/16 in.
25 Bulk Density 65 +5 lbs/ft3 Surface Area 35 m2/g minimum Pore Volume 0.25-0.35 cc/g Crush Strength 15 lbs minimum average Representative chemical reactions considered to occur during absorption, regeneration and purging are shown below:

35 During Absorption:

H S + ZnO ~ ZnS + H O (3) COS + ZnO ~ ZnS + CO (4) .~ ~
' *G72D is a trademark.

1299~3~2 CS2 + 2ZnO ~ 2ZnS + CO2 (5) S2 + 3H2 ~ H2S + 2H2O (6) H2S + Sulfated Absorbent ~ SO2 + ZnO Absorbent (7) 10 During absorption, H2S, COS and CS2 in the stream can react with ZnO to form ZnS as shown in Eqs. (3) to (5).
S2 can react directly with H2 to form H2S as shown by Eq. (6), and the resulting H2S can then react with ZnO.
COS and CS2 may also be hydrogenated to H2S before absorp-15 tion by ZnO. When elements in the absorbent such as zinc,calcium, aluminum, or other elements become sulfated during regeneration, SO2 may be produced during absorption as indicated by Eq. (7) due to the presence of effective reducing species in the absorber feed. Sulfation is 20 reversed by purging the regenerated absorbent with effec-tive reducing species before returning regenerated ZnO to absorption and returning the produced SO2 to the Claus plant for sulfur formation and removal.

25 During Regeneration:

ZnS + 3/2 2 ~ ZnO + SO2 (8) Absorbent + SO2 + 2 ~ Sulfated Absorbent ~9) Regeneration of sulfided absorbent is effected by oxi-dizing ZnS to ZnO as shown by Eq. (8). Absorbent sulfa-tion can also occur, as shown by Eq. (9) during regenera-tion in the presence of 2 and SO2. Temperature rise 35 during regeneration can suffice if unchecked to destroy both the physical integrity and the chemical activity of the absorbent as well as to exceed metallurgical limits of preferred materials of construction. Consequently, tem-lZ99842 perature rise during regeneration is preferably controlled to less than about 1500F.

During Purging:

Sulfated Absorbent + H2 ~ Absorbent + SO2 + H2O (10) Sulfated Absorbent + CO ~ Absorbent + SO2 + CO2 (ll) Sulfated Absorbent + H2S ~ Absorbent + SO2 + H2O (12) Reduction of the sulfated absorbent will occur at a temperatures above about 1000F in the presence of H2, CO or other reducing species such as H2S. Reduction 15 of the sulfated absorbent does not occur at lower tempera-tures such as 900F or lower or in the absence of such effective reducing species.
Methane, although a reducing gas, may not be effective in reasonable periods of time under process con-20 ditions for purging. Further, purging with an inert gaswill not prevent the SO2 emissions increase upon returning to absorption. Rather, upon switching to absorption, the sulfated ZnO absorbent will be contacted with a stream containing the effective reducing species (H2, CO, and 25 H2S) and SO2 emissions will occur. Accordingly, for the purging, it is essential that effective reducing species be present and that the temperature be greater than about 1000F.
The absorber zone containing ZnO can comprise at 30 least a first absorption zone (functioning as an absorber) and a second absorption zone (functioning as regenerator) and the process can comprise contacting H2S with absorbent in the absorber to remove it and other sulfur species pro-ducing a laden absorbent and absorber effluent lean in 35 sulfur species. Absorption can be continued for a period of time (absorption), preferably less than that required for H2S breakthrough from the absorber. For practical purposes, H2S breakthrough can be defined as occurring when the H2S concentration in the absorber effluent stream reaches a preset low value, such as for example, 50 ppm H2S. Concurrently with absorption in the absorber, laden absorbent in the regenerator can be regenerated by intro-5 ducing a regeneration stream comprising dilute 2 ther-einto at a temperature effective for converting laden sul-fided absorbent to active absorbent. Regeneration effluent comprising SO2 is returned from the regenerator to the Claus plant, for example, to the thermal reaction 10 zone or to a downstream Claus catalytic reaction zone.
Thereafter, the absorber and the regenerator can be inter-changed, with the second absorption zone now functioning as absorber and the first absorption zone now functioning as a regenerator, and the process can be repeated and con-15 tinued. Prior to interchanging the absorber and theregenerator, freshly regenerated absorbent in the regener-ator is purged with an effective reducing gas. For example, purging can be conducted by discontinuing the introduction of 2 into the portion of absorber effluent 20 or other diluent used during regeneration, and continuing flowing of the absorber effluent or other reducing gas for a period of time effective for reducing to a desired level an otherwise observed temporary increase in SO2 occurring where a freshly regenerated ZnO-based absorbent is 25 returned to absorption without such purge.
During regeneration of ZnS to ZnO, a temperature rise of about 145F occurs for each mol percent of oxygen consumed in converting ZnS back to ZnO. To avoid exceeding about about 1500F and to maintain absorbent 30 physical and chemical integrity during regeneration, a maximum of about 3.5 mol% oxygen can be used during regen-eration of each portion of sulfided absorbent when the regeneration stream is introduced at about 1000F, and a maximum of about 2.75 mol% 2 when the regeneration stream 35 is introduced at about 1100F. Thus, preferably oxygen is introduced during regeneration at a concentration of about 0.4 or less to about 3.5 mol%, more preferably at about 1 to about 2.75 mol%. Higher temperatures of regeneration 12998~2 up to about 2100F can also be used and the amount of 2 introduced increased accordingly.
In accordance with the invention, oxygen is therefore introduced into each portion of absorbent in the 5 regenerator at a concentration effective for maintaining temperature rise in that reactor below a predetermined temperature. While the predetermined temperature as indi-cated can be any temperature up to about 2100F, it is currently preferred to maintain temperatures below about 10 1500F. Consequently, in accordance with the invention, it is preferred to introduce oxygen into each portion of sulfided absorbent in the regenerator in an amount of about about 2.75 mol% to about 3.5 mol% depending on the temperature of the stream being introduced. For each por-15 tion of sulfided absorbent being regenerated, heat is thenremoved from the gas-in-process concurrently with or sub-sequent to regeneration in that portion of absorbent, for example, to a resulting temperature of about 1000-1100F.
In accordance with an aspect of the invention, the heat 20 removal apparatus can be controlled responsive to the desired temperature of the dilute oxygen-containing stream, for example, after introduction of supplemental oxygen and cooling prior to introduction into a further portion of sulfided absorbent for regeneration in accor-25 dance with the invention.
The flow rate during regeneration is preferablya rate sufficient to complete regeneration and purging as described herein of a ZnO absorber while absorption is conducted in another ZnO absorber. In this way, only two 30 absorption zones will be required. Some time can also be allowed for the contingency of process upsets (slack time). Preferably, the flow rate during regeneration is such that the period during which regeneration is occur-ring is substantially equal to the period during which 35 absorption is occurring less the period required for purging as herein set forth and such slack time.
As indicated, regeneration effluent comprising S2 is returned from the regenerator to the Claus plant 12~842 for conversion of the SO2 to elemental sulfur which is removed from the gas in process. Dilution of the 2 during regeneration by introducing the 2 supplementally with heat removal reduces the rate of regeneration effl-S uent being returned to the Claus plant and increases theconcentration of sulfur and sulfur compounds returned to the Claus plant. This has the desired result of decreasing the size of the Claus plant and equipment down-stream of the locus where the regeneration effluent is 10 reintroduced resulting in significant cost decreases as compared with the use of additional diluent for each addi-tion of oxygen or in comparison with the amount of diluent required to dilute the oxygen if all of the oxygen is introduced at a single point, while permitting operation 15 in a desired temperature range during regeneration.
Using the method of regeneration in accordance with the invention, the rate of regeneration effluent returned to the Claus plant can be still further reduced where the Claus plant comprises at least one Claus low-20 temperature absorption reactor.
By use of such a low-temperature Claus adsorp-tion reactor, the absorption rate for a ZnO absorber is decreased in comparison with the rate where such a Claus low-temperature adsorption zone is not used, allowing the 25 regeneration stream to be introduced to portions of sul-fided absorbent in the regenerator at a still lower rate, further reducing the rate at which regeneration effluent is returned to the Claus plant.
Regeneration can be continued until substan-30 tially all of the sulfided absorbent is regenerated, forexample, until ZnS is substantially reconverted to ZnO.
Completion of regeneration can be conveniently determined by monitoring 2 or SO2 content or temperature of the regenerator effluent stream. Preferably, an 2 analyzer 35 is employed downstream of the regenerator to determine the presence of 2 in the regenerator effluent, which is an indication of completion of regeneration.

lX99842 As will be appreciated by those skilled in the art from the foregoing discussion, materials of construc-tion for the valves, vessels, and piping for the process according to the invention can require special attention.
5 The material preferably has the capability of withstanding high temperatures, for example, in the range of about 800F to about 1500F or higher while being repeatedly exposed to reducing and oxidizing atmospheres in the pres-ence of sulfur compounds.
Following regeneration, prior to returning the regenerated absorbent for use during the absorption cycle, the regenerated absorbent is treated (purged) by passing a reducing stream in contact with the regenerated, albeit sulfated absorbent, for a period of time effective for 15 reducing a temporary increase in SO2 emissions otherwise occurring when freshly regenerated ZnO absorbent is returned to absorption without such purging with a reducing gas. Preferably, the purge time used should be effective for reducing SO2 emissions to below about 20 250 ppm at all times. Most preferably, the time is effec-tive for substantially eliminating the increase in SO2 emissions, that is, for reducing the increase in SO2 emis-sions by 90~ or more from the level occurring where such a reducing gas purge is not used prior to returning to 25 absorption.
The effective purge time can be readily deter-mined by one skilled in the art by monitoring SO2 emis-sions from an absorber following returning a freshly regenerated reactor to absorption function and increasing 30 the purge time prior to returning to absorption until the S2 emissions are reduced to a desired level upon returning to absorption.
The purge stream can comprise a portion of absorber effluent or any other suitable reducing gas 35 stream. In accordance with the invention, the purge is preferably effected by using the same portion of absorber effluent used for diluent during regeneration, by discon-tinuing the flow of Q2 to the regenerator during the purge period.

lZ99842 The invention will be further described and further advantages and applications and equivalents will be apparent to those skilled in the art from the descrip-tion of FIGURES 1 and 2.
Referring now to the drawings and specifically to FIGURE 1, FIGURE 1 represents an embodiment of the invented process in which absorption of H2S by the metal oxide absorbent can be carried out at a temperature above about 1000F, preferably in the range of about 1000F to 10 about 1200F.
An acid gas stream 110 containing H2S is intro-duced into a Claus plant furnace 112 and combusted, in the presence of oxygen-containing gas, for example, atmos-pheric air (source not shown), and/or SO2 (provided, for 15 example, via line 111), to produce elemental sulfur, SO2, and water. The elemental sulfur is recovered and uncon-verted H2S and SO2 are processed by Claus catalytic sulfur recovery 114, including at least one Claus catalytic reac-tion zone operated above the sulfur dewpoint and at least 20 one low-temperature Claus adsorption reaction zone. Ele-mental sulfur is thus produced and removed, for example, by sulfur condensers (shown schematically by the arrow S).
A Claus plant effluent stream is removed by line 116 con-taining sufficient reducing equivalents for reduction of 25 sulfur containing compounds remaining therein to H2S in a hydrogenation zone (not shown) or in the absorber zone as illustrated.
The Claus plant effluent stream in line 116 can then be heated to an effective temperature as described 30 herein. Preferably at least a portion of the heating requirements can be met by passing the Claus plant effl-uent stream 116 in direct heat exchange with the absorber effluent stream in line 156, for example, in recuper-ator 158, as indicated schematically by the line marked A.
35 Following heating in recuperator 158, the heated Claus plant effluent stream can be provided by the lines marked to heater 117 for further heating to above 1000F, prefer-ably in the range of about 1000-1200F. Alternatively, ~299842 of course, the Claus plant effluent stream 116 can be provided directly (as indicated by the dashed line) and can be heated in heater 117 to a temperature in the range of about 1000F to about 1200F and introduced by 5 lines 125, 126, valve 126V, and line 130 into first absorber 134. That other provision can be made for heating the Claus plant effluent stream in accordance with the invention will be clear to those skilled in this art.
First absorber 134 contains a ZnO absorbent 10 effective to absorb H2S present in the inlet stream to produce a sulfided absorbent and to produce an absorber effluent stream 138 containing, for example, less than about 50 ppm H2S. Simultaneously with absorption in first absorber 134, after heating to a temperature in the range 15 of 1000 F to 1200F, SO2 present in Claus effluent stream 116 can be hydrogenated to H2S utilizing reducing equivalents present in Claus effluent stream 116 and the resulting H2S can also be absorbed by the absorbent.
As illustrated, first absorber 134 contains two 20 beds of ZnO absorbent 202a and 202b, and the beds of absorbent are separated in the absorber by a partition, 203. During absorption, effluent from first bed 202a can be removed by line 204 and reintroduced into the reactor by lines 209 and 208 into contact with a second bed of 25 absorbent in bed 202b. During absorption, flow through cooler 206 can be prevented by a closed valve in the inlet line to the cooler as illustrated. Also, during absorp-tion, oxygen is not introduced via line 202, and the valve is accordingly also illustrated closed.
The absorber effluent stream 138 can be con-ducted by line 142, valve 142V, lines 152 and 156, heat recuperator 158, and line 160 for discharge, for example, to the atmosphere. The heat recuperator 158 provides at least a portion of the heat required for heating the Claus 35 plant effluent stream as described above, or for producing high pressure steam. A portion of the absorber effluent stream can be withdrawn from line 152, by way of, for example, line 154 and valve 154V, for dilution of atmos-~299842 pheric air 172, via compressor 170, line 168, and valve 168V, to produce a dilute air regeneration stream 166. During regeneration, valves 154V and 168V
control the recycle rate to the Claus plant. In this way, 5 in accordance with the invention, recycle of re~eneration effluent from the regenerator to the Claus plant can be controlled and reduced to a fraction of what otherwise is returned.
The regeneration stream 166 can be heated in 10 heater 174 to regeneration temperatures and can be con-ducted by lines 176, 178, 180, valve 180V, and line 132 to second absorber 136 shown on regeneration. The heated regeneration stream 176 is thus passed in contact with sulfided absorbent in second absorber 136 to produce a 15 regeneration effluent stream 146 having a reduced 2 con-tent and an increased SO2 and/or sulfur content.
During regeneration, oxygen introduced into the first bed 210a of regenerator 136 reacts with sulfided absorbent producing ZnO and heat which is removed in the 20 effluent stream by line 214 upstream of partition 213. A
supplemental portion of oxygen, for example, to a final concentration in the range of about 2.75-3.5 mol~ can be introduced by controlling the valve in line 212. The resulting stream can then be cooled in cooler 216 to a 25 temperature suitable for introduction into second sulfided absorbent bed 210b, for example, to a temperature of 1000-1100F in line 218. Valve 219 is closed during regeneration or can be used for temperature control. In the second bed 210b, sulfided absorbent is again converted 30 to ZnO with a concomitant temperature rise and the effl-uent from the regenerator is removed by stream 146. Other alternatives can also be employed in accordance with the invention. For example, the oxygen 5air) can be intro-duced after cooling and the like.
Stream 146 is conducted by line 144, valve 144V, heat recuperator 190, compressor 192, and line 111 to the Claus plant furnace 112. Alternatively, the regeneration effluent stream can be introduced into a catalytic zone in 129984;~

the Claus plant 114 as indicated by dotted line 111';
however, operation should insure that no free or molecular oxygen is introduced thereby into the catalytic zone.
Absorption is continued in first absorber 134 5 and regeneration is continued in second absorber 136 until prior to H2S breakthrough occurs in effluent stream 138 from first absorber 134. Preferably, the oxygen content and regeneration stream flow rate is established so that the regeneration time (plus purge and slack time) is equal 10 to absorption time prior to H2S breakthrough. H2S break-through can be determined by monitoring the H2S content of first absorber effluent stream 138 until H2S content can exceed a predetermined limit which can be, for example, that suitable to meet emission requirements for discharge 15 of stream 160. Prior to breakthrough, first absorber 134 can be placed on regeneration and second absorber 136 can be placed on absorption by closing valves 126V, 142V, 180V, and 144V in their respective lines 126, 142, 180, and 144; and by opening valves 128V, 182V, 140V, and 148V
20 in the respective lines 128, 182, 140, and 148. The valves in line 202, 204 and 219 are also opened, and the valves in lines 212, 214 and 209 are closed.
Prior to interchanging the first absorber and the second absorber, purge of the second absorber zone can 25 be effected by discontinuing 2 flow to the second absorber, for example, by closing valve 168V, and the valve in line 212, by continuing flow of absorber effluent by line 154 and valve 154V to the second absorption zone 136 for a period effective to reduce SO2 emissions, 30 upon interchanging the absorbers, to a desired level.
Referring now to FIGURE 2, FIGURE 2 represents a second embodiment of the invention in which regeneration can preferably be conducted, for example, using two stages of intercooling with two supplemental introductions of 35 oxygen (three regeneration fronts). The reference numerals for FIGURE 2 are the same as for FIGURE 1 except as may be indicated below. Thus, a regeneration stream is introduced into the reactor 136 by line 132. In reactor lZg9842 136, the sulfided absorbent is disposed in three beds, 210A, 210B, and 210C. The beds are not in flow communica-tion with one another except via intercoolers 216' and 226. Accordingly, when the dilute oxygen-containing 5 stream in line 132 passes through bed 210A and regenera-tion of ZnS to ZnO occurs, the resulting heated stream is removed by line 214', a further portion of oxygen is introduced, for example, by a line 212' and then the resulting blended stream is cooled in intercooler 216' to 10 a suitable temperature, for example, 1100F. The valves in line 212' and 214', as also in lines 224 and 226 (not shown), are accordingly open. The temperature of the stream in line 218' which is provided to the second bed of absorbent can be suitably controlled by a temperature-15 controlled valve 220 controlling the coolant to inter-cooler 216' as illustrated. Regeneration of ZnS to ZnO in bed 210b can similarly be followed by removal of the resulting heated stream in line 224, introduction of an additional portion of oxygen by line 222, cooling in 20 intercooler 226, and reintroduction of the cooled oxygen enriched stream into contact with a third portion of absorbent in bed 210c by line 228. Temperature control of cooler 226 can be provided by temperature controlled valve 230 as illustrated. The resulting stream in line 146 can 25 then be returned to the Claus plant as hereinabove described.
While the invention has been described herein in terms of specific and preferred embodiments as required, the invention is not limited to the specifics herein 30 described but in accordance with the claims appended hereto.

Claims (5)

1. Process for the recovery of sulfur and sulfur compounds from a gaseous stream comprising:
removing sulfur compounds from the gaseous stream in the presence of a ZnO absorbent in an absorber and pro-ducing sulfided absorbent comprising ZnS and absorber effluent reduced in sulfur compound content;
regenerating the sulfided absorbent ZnS in a regeneration zone by passing a dilute oxygen- containing stream in contact therewith producing regeneration effluent;
and returning the thus produced regeneration effluent to a Claus plant in which sulfur compounds therein are con-verted to elemental sulfur and removed;
wherein the regenerating step comprises:
regenerating ZnS to ZnO by introducing first and second regeneration streams respectively into first and second portions of the ZnS, each regeneration stream being at a temperature above about 1000°F and comprising O2 in the range of 2.75 to 3.5 mol%;
wherein the second regeneration stream is produced using effluent of the first regeneration stream removed from the first portion of the ZnS as diluent and adding supplemental O2 thereto.

2. The Method of Claim 1 wherein the temperature during regeneration is less than about 2100°F.

3. The Method of Claim 1 wherein:
the temperature during regeneration does not exceed about 1500°F, the temperature of introduction of a regenerating stream into a portion of absorbent being regenerated being in the range of 1900°F - 1100°F.

4. The Method of Claim 1 further comprising:
controlling the removal of heat from streams resulting from each of the first and second portions of absorbent being regenerated responsive to the temper-ature of the gas-in-process following heat removal.

5. The Method of Claim 1 wherein the rate of returning the regeneration effluent to the Claus plant during regeneration is reduced to about one-half of that occurring for a given temperature of regeneration in the absence of using first regeneration stream effluent as diluent for supplemental O2.