CA1300345C - Sulfur recovery process using metal oxide absorbent with improved purge - Google Patents

Abstract

ABSTRACT
Sulfur species are removed from a Claus plant tail gas stream by contacting with ZnO producing ZnS
which is regenerated to ZnO by dilute O2. Following regeneration, freshly regenerated ZnO in one ZnO
absorber is purged with an effective reducing gas stream producing SO2 emissions while another ZnO
absorber is downstream operating under effective absorption conditions for removing thus produced SO2 from the first ZnO absorber effluent.

GMB:sdg/tb/jdb

Description

~3003~i PendergraEt "SULFUR RECOVERY PROCESS USING METAL OXIDE
ABSORBENT_WITH_IMPROVED PURGE"
FIELD OF THE INVENTION
The invention relates to the removal of sulfur and sulfur compounds from gaseous streams containing such 15 compounds. In one aspect, the invention relates to the removal of sulfur compounds including H2S (hydrogen sul-fide) and SO2 (sulfur dioxide) from Claus plant tail gas.
In another aspect, the invention relates to the use of solid high surface area contact materials (absorbents), 20 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 preventing increased S2 emissions from appearing in plant emissions when treating freshly regenerated ZnO absorbent with a reducing 25 gas stream under conditions for reducing the time required.
SETTING OF THE INVENTION
A developing area of sulfur recovery technology is that of tail gas cleanup, that is, of removing trace 30 quantities of sulfur compounds from gaseous effluent streams (tail gas) of Claus process sulfur recovery plants. Tailgas may contain substantial amounts of sulfur compounds. Tailgas from Claus or extended Claus plants (having at least one Claus low temperature adsorption 35 reactor) 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 disulfide), and the like. Tailgas cleanup processes remove at least part of such residual sulfur compounds from Claus tail gas.

~300:~A5 In prior U. S. Patent ~,533,529, Claus tail ga~
is contacted with ZnO (zinc o.Yide) in an absorber reducing average overall emission levels from the absorber to less than 250 ppm sulfur species. It is desirable, however, 5 and necessitated by certain environmental requirements, that not only average but instantaneous emissions be con-tinuously maintained at a very low level.
It has been discovered, after ZnS (zinc sulfide) is regenerated to ZnO, that an increase in SO2 emissions 10 occurs from the absorber upon returning regenerated ZnO to absorption where it is contacted with reducing gases.
These SO2 emissions interfere with continuously main-taining instantaneous emissions at a very low level.
Accordingly, there is provided a process capable 15 of diminishing such an increase in SO2 emissions and main-taining effluent from the absorber at a continuous low level of emissions.
SU~IP RY OF THE. NV~N r ION
The invention comprises a process for continu-20 ously removing sulfur compounds, for example, H2S and SO2,from a Claus plant gaseous effluent stream to an extremely low level. In this process, the sulfur compounds are removed in the presence of an absorbent based on ZnO as active absorbent (herein referred to as ZnO or ZnO-based 25 absorbent) to produce a laden, sulfided absorbent (ZnS) and a purified gaseous stream (absorber effluent) continu-ously having on the order of 250 ppm or less total resi-dual H2S and SO2.
; There is provided a new and advantageous combi-30 nation of steps for preventing higher emissions of sulfur dioxide from freshly regenerated ZnO absorbent, the ZnO
absorbent having been regenerated in the presence of mole-cular oxygen and sulfur dioxide, from appearing in effl-uent from the plant. The invention comprises a new and 35 advantageous combination of-steps for use in a method for absorbing at least H2S from a stream by sulfidization of ZnO producing ZnS, the ZnS then being regenerated to ZnO, producing sulfur dioxide (SO2) in the presence of 2 and , ~

.,: .

1300:~AS

retu!ned to absorbincJ ~2S from the stream. The new and advanta~eo~s combination of steps comprises (1) following regeiner.ltion of ZnS to ZnO, passing the strearn containing at least H2S in contact with thus regerlerated ZnO, pro-S ducing effluent comprising SO2 and (2) pa,~ing the e~fl-u~nt comprising SO2 in contact with ZnO It~ r conditions, including temperature and compositior~, for rernovirlg SO2 in the presence of the ZnO.
In accordance with another aspect: o~ th~ inv~n-10 tion, the invention comprises a new alld advantayeous com-bination of steps for use in a method for removing at least sulfur dioxide SO2 from a stream ~omprising SO2 and reducing species effective for converting SO2 to H2S in the presence of an effective ZnO absorbent producing ZnS.
15 The ZnS is then regenerated to ZnO, producing SO2, in the presence of 2~ and then returned to absorbing at least S2 from the stream. The new and advantageous combination of steps comprises: (l) following regeneration of ZnS to ZnO, passing a stream comprising reducing species in con-20 tact with thus regenerated ZnO, producing effluent com-prising SO2 therefrom, and (2) passing the effluent com-prising SO2 in contact with ZnO under conditions including temperature and gas composition for removing SO2 there-from.
In accordance with further aspects of the inven-tion, steps (1) and (2) are continued Eor a period of time for reducing SO2 levels in the effluent from the plant to less than about 250 ppm, and more preferably, to less than about 50 ppm.
In a further aspect, a process for the recovery of sulfur from a H2S containing gaseous stream comprises converting H2S to elemental sulfur by the Claus reaction in a Claus plant comprising a Claus thermal reaction zone (furnace) and at least one Claus catalytic reaction zone 35 and producing a tail gas comprising significant amounts of both H2S and SO2. As used herein, significant amounts of H2S and SO2 means that each is present in excess of 250 ppm. The tail gas can then be treated to remove each ,~ ' ' ' . ' 1300:~S

of H2S and SO2 by ren:'ion with ZnO in a ~irst absorptio zone containing ZnO (functioning as an absorber) in the presence of reducing species for converting substantially ail sulfur species in the ta;l gas to H2S, ?ro~ucing ZnS
and absorber effluent. The res~lting ZnS can be regerler--ated, that is, returned to the active ZnO form of the absorbent, by introducing 2 (molecular ox~gen) into a second absorQtion zone (~unctioning as a reg~nerat:or) and reger.erating the zinc sulfide to ZnO, producing regenera--10 tion effluent. Regeneration effluent comprising SO2 isreturned to the Claus plant. Followin~ regeneration of absorbent (converting ZnS to ZnO) in the second absorption zone, the tail gas is introduced into the second absorp-tion zone, producing ZnS and absorber effluent which, 15 during an initial emissions period, contains an elevated level of SO2. Effluent from the second absorption zone is introduced into the first absorption zone operating under absorption conditions during the emissions period and the increased level of SO2 is removed by the ZnO in ~he first 20 absorption zone in the presence of reducing species.
Thus, purging in accordance with the invention is accom-plished by beginning absorption in a freshly regenerated absorber zone and passing effluent from the freshly regen-erated absorber zone to another absorption zone where the 25 elevated levels of SO2 are removed to an acceptable level.
After the emissions period, the first absorption zone can be removed from absorption function and regenerated by introducing 2 thereinto and regenerating the ZnS to ZnO, producing regeneration effluent. The process is addition-30 ally characterized by the fact that the purge periodduring which effluent from a freshly regenerated absorber is provided to a downstream absorber on absorption, lasts for a period of time effective for removing at least 10~
of an increase in SO2 emissions from the freshly regener-35 ated absorber during an emissions period at the start ofabsorption following interchanging of the absorber and the regenerator.

~3~0345 The invention accordin~ly co~prises the pro-cesses and systems, together with their steps, parts, and interrela~ionships which are exemplified in the present disclosure, and tne scope of which will be indicated in 5 the appended cla;ms.
BRIEF D~SC~IPTION OF _HE DRAWINGS
FIGURE 1 shows schematically apparatus for prac-ticing the invented process.
FIGURG` 2 shows graphically an increase ir- SO~
10 emissions occurring where ~reshly regenerated absorbent is not purged before absorption with an effective reducing gas.
FIGURE 3 shows graphically that such an increase in SO2 emissions as shown in FIGURE 2 can be eliminated by 15 purging before absorption with an effective reducing gas.
DETAILED DESCRIPTION OF THE INVENTION
In accordance with the invention, SO2 emissions are prevented from appearing in effluent from a sulfur recovery plant by steps following regeneration of ZnS to 20 ZnO. The steps comprise operating an absorber containing freshly regenerated ZnO under conditions, including tem-perature and gas composition, for causing SO2 emissions to occur from the freshly regenerated absorbent. The effl-uent from such zone containing the SO2 is then provided to 25 a second ZnO absorber zone operating under conditions effective for removing such SO2 in the presence of ZnO and effective reducing species. Thus, the SO2 emissions are prevented from appearing in emissions from the plant by providing them during an emissions period to another ZnO
30 absorber downstream thereof. Following the emissions period, the ZnS in the downstream absorber zone can be regenerated by introducing dilute molecular oxygen ther-einto, producing effluent comprising SO2, which is returned to a Claus plant from which the tail gas is der-35 ived and in which the SO2 is converted to sulfur and - removed from the gas-in-process.
Sulfur is recovered from an H2S-containing stream by introducing the stream into a Claus plant com-:
.~

: ::

~30034!;

prising a thermal r~ tion zone (Claus fur,l.~ce~ and atleast one Claus ca~alytlc reaction zone. The C~aus thermal reaction zone can be, for example, a Claus muF~le furnace, a fire tube (tunnel) furnace, or the like. Gen-5 erally, the Claus thermal reaction zone functions for con--verting a portion of H2S, preferably about 1/3, to SO~ for thermal or catalytic Claus reaction with H2S to form ele-mental sulfur.
In the Claus furnace, the H2S-containing gas and 10 oxidant can be reacted at a temperature generally in the range of about 1800-2600F. The effluent from the Claus thermal reaction zcne can be cooled, for example, in a waste heat boiler, and optionally passed through a sulfur 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 example, o~ about 350-650F. In the Claus high tempera-ture catalytic reactor, sulfur is formed by the Claus 20 reaction (shown below) in the presence of an effective Claus reaction-promoting catalyst such as alumina or bauxite:

2 H2S + SO2 -~ 3/2 S + 2 H2O
t 25 Gas containing elemental sulfur vapor can be continuously removed from the reactor and provided to a sulfur con-denser where sulfur is condensed and removed as a liquid.
Gaseous effluent from the sulfur condenser can be 30 reheated, if desired, and passed to further high tempera-ture Claus reactors and associated sulfur condensers as is known in the art. The effluent gas from the final sulfur condenser is then the Claus plant tail gas. Where a Claus low-temperature adsorption zone is used, it may or may not 35 be followed by a sulfur condenser. Thus, adsorber effl-uent may be the Claus plant tail gas.
Preferably, the Claus plant tail gas is from a Claus plant which includes at least one Claus catalytic 1300:~45 reactor operated untlf~lr collditions, includ;ng temperature, effective for depositing a pceponderance Ol- t~e formed sulfur on Claus catalyst therein. Such a Claus low tem-perature adsorption zone can be broadly operated in the 5 range of from about 160 to about 330~1, preferably in the range of from about 260-300 F.
The operation of such Claus plar,Ls having Claus furnaces, Claus high temperature reactors, and Claus low temperature adsorption reactors is well kno~n in th~ art 10 and need not be further described here. See, for example, U.S. Patents 4,044,114; 4,426,369; 4,430,317; 4,473,541;
4,482,532; 4,483,844; 4,507,275; 4,508,69~, and numerous others.
The tail gas from such Claus plants comprises 15 H2S, SO2, organic sulfides, and reducing species such as H2 and CO. Tailgas 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 about 2 mol%, water in the range of about 20 to about 20 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 temperature adsorption reactors, the tail gas may have equivalent of about 0.4 mol%, preferably about 0.2 mol%, 25 or less single sul~ur species.
Use of at least one Claus low-temperature 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.
30 These organic sulfides are not removed by sulfur recovery processes such as the IFP process described in DeZael, et al., U. S. Patent 4,044,114 (1977) which forms elemental sulfur in the presence of polyethylene glycol and sodium benzoate. See, e.g., Kohl and Riesenfeld, Gas Purifica-35 tion, pages 491-493 (3d Ed. 1979).
For the same reason, it is also preferred to operate at least one Claus high temperature reactor so that the effluent has a temperature in the range from ' ~`
, : :

~30034~;

about 550 to 700F, preferably fr(~)m anout 600 to 650E to diminish the amount o~ organic sulfides in the effl~lent.
See, e.g., Kunkel, et al., U.S. Patent 4,035,474 (1977).
Both H2S and SO2, as well as organic sulfides, 5 can be concurrently removed in the absorber containing ZnO
in the presence of reducing species for reducing the SO2 and other sulfur species to H2S. It is preferre~ to operate the Claus plant so that about a 2:1 ratio of H2S:SO2 is maintained in the Claus plant tail gas to max-10 imize sulfur recovery in the Claus plant and to millimizethe amount of sulfur remaining in the Claus plant tail gas to be remove~ by the ZnO absorbers. Such ratio can be maintained by control systems well known in the art and need not be further described here. By reducing the 15 organic sulfide and other sulfur content in the feed to the ZnO absorbers, the volume of regeneration effluent returned to the Claus plant can be reduced or diminished.
An effect of operating at about a 2:1 ratio, however, is that quantities of both H2S and SO2 are present in the 20 Claus plant tail gas, i.e., more than about 250 ppm of each of H2S and SO2.
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 25 including that present in the tail gas as H2, or available 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 producing Claus plant tail gas having a 2:1 ; 35 H2S:SO2 ratio and by insuring that the tail gas leaving the final sulfur condenser of the Claus plant has a low level of residual elemental sulfur, the Claus plant tail gas will contain sufficient reducing species. By further ~';
., ' `

` ' ~3~034S
_9_ reducirlg the amour-t of o~idant introd~lced into the Claus t furnace or by other methods which will be apparent to per-sons skilled i!l the art, the amount of reducing species can be further increased if desired.
S The Claus plant tail gas having su~ficient reducing species to reduce all sulfur compollnds therein to H2S can be heated, for example, directly by mf~ans o~
direct fired heaters, or indirectly by heat exchange, for example, with other process streams such as absorber effl-10 uent, to produce a heated Claus plant tail gas efluent stream having a temperature effective for removal of each of H2S and SO2 in the presence of a solid part;culate preferably high surface area (for example, pellets, extru-dates, and the like) ZnO absorbent effective for such lS removal. This removal of each of 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-tion of the thus-formed H2S by the ZnO by sulfiding the 20 ZnO to ZnS, the ZnO acting as an absorbent. Preferably, the Claus plant tail gas is heated to above about 1000QF.
As illustrated in EXAMP~E I below, by operating at these absorber temperatures, a hydrogenation reactor is not required before removal of sulfur compounds other than H2S
25 in the absorber. When operating at temperatures above about 1000F, H2S emissions and the reduction of ZnO to Zn vapor under a reducing environment can set a practical upper limit on the absorption temperature which will be used. Currently for these reasons it may be appropriate 30 that the upper limit during absorption be about 1200F.
Higher temperatures can also be used. Absorber operation above about 1000F is preferred because such higher tem-peratures favor higher absorption capacity of the absorber and the hydrogenation reactor can be eliminated. Also, 35 since absorption and regeneration will then be conducted at approximately the same inlet temperature (1000-1200~F), temperature stress on equipment can be reduced. As a result, there will be no significant heating anù cooling : .
' ' .

periods. Hence, the time available for regeneration will be increased and the rate of regeneration effluent returned to the Claus plant can be decreased.
While a first absorption zone is functioning as 5 an absorber, a second absorption zone can be functioning as a regenerator.
As u~ed herein, and in the claims, the terms "absorbent", "ZnO", "ZnO absorbent", and the like ~hall mean an absorbent effective for removal of both H S and 10 SO in the presence of reducing species. A major portion of the active absorbent, for example, fifty percent or more, is in the form of ZnO which is the active form. The absorbent can also contain binders, strengtheners, and support materials, for example, alumina (Al o ), calcium 15 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 sorbent. Other ~uitable starting material~ can al~o be used. The ZnO
absorbent i~ effective for absorbing H2S by underqoing 20 ~ulfidization to produce a laden (sulfided) absorbent;
~imultaneously, if desired, hydrogenation of other sulfur compounds to H2S followed by such absorption can occur.
Preferably, the ZnO absorbent is capable of a high level o removal of sulfur compounds and is relatively insensi-25 tive to water vapor.
Particularly preferred are ZnO absorbents whichare thermally stable, regenerable, and capable of absorbing substantial amounts of sulfur compounds. An acceptable absorbent is United Catalysts, Inc., G72D*
30 Sulfur Removal Catalyst, available from United Catalysts, Inc., Louisville, KY, having the following chemical compo-sition and physical properties:
CHEMICAL COMPOSITION

wt~ Trace Metal Impurities wt~
ZnO........... 90.0 +5% Pb..................... <0.15 Carbon........ ~O. 20 Sn..................... ~0. 005 ~ Sulfur........ <0.15 As..................... ~0.005 `~ ~ Chlorides..... <0.02 Hg..................... ~0.005 * G72D is a trademark.

~:~00345 A12O3......... 3-7 Fe.............. ~...... <0.1 CaO........... 0.5--3.0 Cd...................... <0.005 PH~S r CAL PRO~'.E~'LI~S
Form Pellets Size 3/16 in.
Bulk Density 65 +5 lbs~ft3 Surface Area 35 m2/g mini~T~lu Pore Voiume 0.25-0.35 cc,/g 10 Crush Strength 15 lbs minimum avera~e Representative chemical reactions considered to occur during absorption, regeneration and purging are shown below:
During Absorption:

H2S + ZnO ~ ZnS + H2O (l) 20 COS + ZnO -~ ZnS + CO2 (2) CS2 + 2ZnO -~ 2ZnS + CO2 (3) S2 + 3H2 ~~ H2S + 2H2O (4) H2S + Sulfated Absorbent -~ SO2 + ZnO Absorbent (5) 30 During absorption, H2S, COS and CS2 in the stream can react with ZnO to form ZnS as shown in Eqs. (l) to (3).
S2 can react directly with H2 to form H2S as shown by Eq. (4), and the resulting H2S can then react with ZnO.
COS and CS2 may also be hydrogenated and/or hydrolyzed to 35 H2S before absorption by ZnO. When elements in the absor-bent such as zinc, calcium, aluminum, or other elements become sulfated during regeneration, SO2 may be produced during absorption as indicated by Eq. (5) due to the pres-ence of effective reducing species in the absorber feed.
Sulfation is reversed by purging the regenerated absorbent with effective reducing species before returning regener-ated ZnO to absorption and providing the produced SO2 to a 5 downstream ZnO absorption zone.

DU ring Regeneration:

ZnS + 3/2 2 ~ ZnO + SO2 (6) Absorbent + SO2 + 2 ' Sulfated Absorbent (7) Regeneration of sulfided absorbent is effected by oxi-dizing ZnS to ZnO as shown by Eq. (6). Absorbent sulfa-15 tion can also occur, as shown by Eq. (7) during regenera-tion in the presence of 2 and SO2. Temperature rise 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 20 preferred materials of construction. Consequently, tem-perature rise during regeneration is preferably controlled to less than about 1500F.

During Purging:
Sulfated Absorbent + H2 ~ Absorbent + SO2 + H20 (8) Sulfated Absorbent + CO ~ Absorbent + SO2 + CO2 (9) Sulfated Absorbent + H2S ~ Absorbent + SO2 + H2O (10) Purging in accordance with the invention is accomplished by introducing Claus plant tail gas feed to an absorption zone into the freshly regenerated reactor 35 while another ZnO absorber is downstream operating under absorption conditions for removing SO2 produced. Such Claus plant tail gas contains H2, CO, and H2S and there-- fore contains the effective reducing species for causing S2 emissions.

1~0034S

Reduction of the sulfated absGrbent will occur at temperatures above about 1000F in the pr-sence of H2, CO or other reducing species such as H2S. Re~uction of the sulfated absorbent is not effected at lower tempera- -5 tures such as 900P or lower or in the absence Oe such effective reducing species.
Methane is not effective in reasonable periods of time under process conditions for causing the p~oduc-tion of SO2 ~rom freshly regenerated ZnO. Further, an 10 inert gas will not cause such SO2 emissions to occur.
Upon switching to absorption, however, the freshly regen-erated, sulfated ZnO absorbent will be contacted with a stream containing the effective reducing species (~2' CO, and H2S) and SO2 emissions will occur. Accordingly, for 15 causing SO2 emissions, during the purging, it is essential that effective reducing species be present and that the temperature be greater than about 1000F, but preferably not much greater than about 1200F since significant losses of zinc can occur above that temperature in the 20 presence of reducing species. Nevertheless, higher tem-peratures can be used.
The absorber zone containing ZnO can comprise at least a first absorption zone (functioning as an absorber) and a second absorption zone (functioning as regenerator) 25 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 sulfur species. Absorption can be continued for a period of time (absorption period), preferably less than that 30 required for H2S breakthrough from the absorber. For practical purposes, H2S breakthrough can be defined as occurring when the H2S concentration in the absorber effl-uent stream reaches a preset low value, such as for example, 50 ppm H2S. As shown in EXAMPLE I and II below, ; 35 breakthrough time and absorption capacity for an absorber increase with increasing absorber temperature. Concur-rently with absorption in the absorber, laden absorbent in the regenerator can be regenerated by introducing a regen-' : `~
:
~:
~: :
,~

eration stream ce~rising dilute 2 thereinto at a temperature effective for converting laden sulfided absor-bent to active absorbent. Regeneration effluent corn-prising SO2 is returned from the regenerator to the Claus 5 plant, for example, to the thermal reaction zone or to a downstream Claus catalytic reaction zone. Thereafter, the tail gas can be introduced into the second absorption zone functioning as absorber and the first absorption zone can for a period, receive effluent from the second absorption 10 zone. During this purge period which is characterized by increased emissions of SO2 from the freshly regenerated ahsorbent, contacting of freshly regenerated ZnO with the tail gas which comprises H2S, H2, and Co, causes the pro-duction of SO2 which can be removed by the downstream 15 first absorption zone operating as an absorber. There~
after, 2 can be introduced into the first absorption zone for regeneration, and the process can be continued and repeated for the regenerated ZnO in the first absorption zone.
During regeneration, a temperature rise of about 145F occurs for each mol percent of oxygen consumed in converting ZnS back to ZnO. To avoid exceeding metallur-gical limits of nonrefractory lined vessels and to main-tain absorbent physical and chemical integrity during 25 regeneration, a maximum of about 3.5 mol% oxygen can be used during regeneration when the regeneration stream is introduced at about 1000F, and a maximum of about 2.75 mol% 2 when the regeneration stream is introduced at about 1100F. Thus, preferably oxygen is introduced 30 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%. Due to the exothermic nature of the regeneration reaction, suitable methods for diluting the oxygen can be used. Where metallurgical limits are not 35 controlling, the maximum temperature during regeneration can be as high as about 2100F.
Suitable methods for diluting the oxygen during regeneration include the following: (1) a portion of the .
: ' ' ' ~ :`
~ ~' ' ' .

regenerator effluent can be recycled back to the regenerator to dilute 2 in the regeneration stream; (2) a portion of absorber ef~luent can be used to dilute 2 in the regeneration stream.
When method (1) is employed, the SO2 level during regeneration in the regenerator is higher than when method (2) is used since SO2 produced during regeneration is recycled to the regenerator. Reference to EXAMPLE VI
indicates that higher SO2 levels during regeneration 10 favors sulfation of the absorbent. It has also ~een found when method (1) is used that SO2 emissions are larger upon returning to absorption, and/or that a lo~ger purge time can be required to eliminate an increase in SO2 e~lssions following return to absorption. Accordingly, method (2) 15 is preferred for diluting the 2 to a suitable concentra-tion for regeneration. However, in accordance with the invention, absorber feed is used for producing SO2 from freshly regenerated ZnO absorbent in one absorber while another ZnO absorber is downstream operating under absorp-20 tion conditions for removing the produced SO2. Hencemethod (1) can also be used since the higher level of reducing compounds present in the absorber feed will shorten the purge period in accordance with the invention.
Further, by using absorber feed for purging, purging and 25 the start of absorption for an absorber occur concur-rently.
The flow rate during regeneration is preferably a rate sufficient to complete regeneration and purging as described herein of a ZnO absorber during effective ~- 30 absorption in another ZnO absorber. In this way, only two 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-35 ring is equal to the period during which absorption is occurring less the period required for purging as herein set forth and such slack time.
~' ., ~, .
.'`.~`.~ ' .
.,, ~ ~ .

1300~45 As indicated, regeneration effluent comprising S2 is returned from the regenerator to the Claus plant for cc,nversion oi the SO2 to elemental sulfur which is removed fro~! the gas in process. Dilution of the 2 using 5 absorber effluent minimiæes purge time and/or magnitude of the SO2 emissions at the start of absorption but can result in a large volu~e of regeneration ef~luent being returned to the Claus plant. This has the undesired result of increasing the size of the Claus plant and 10 equipment downstream of the locus wher~ the regeneration effluent is reintroduced resulting in signiEicant cost increases.
The rate of regenerator effluent returned to the Claus plant can, as indicated, be reduced by recycling 15 regenerator effluent as diluent back to the regenerator.
Absorber effluent can be used for diluent and the rate of regeneration effluent returned to the Claus plant can be minimized (1) where the Claus plant comprises at least one Claus low-temperature adsorption reactor, 20 (2) where 2 introduced during the regeneration period when ZnS is being converted to ZnO is in an amount about equal to the stoichiometric amount for such conversion, that is, about 3/2 moles 2 for each mole of ZnS to be regenerated, and (3) where the rate of absorber effluent 25 diluent introduced during the regeneration is such that the rate of regeneration effluent during the regeneration period is less than the rate of regeneration effluent returned in the absence of treating in a Claus low-temperature adsorption reactor prior to treatment in a 30 ZnO-containing absorber. As absorber effluent typically comprises residual H2 and CO, the amount of 2 introduced can further include about the stoichiometric amount required for combusting H2 and CO to water and CO2. When using such regeneration, further benefit can be realized 35 by purging in accordance with the invention since using full stream purging can shorten purge time requirements, permitting a longer regeneration period and further reducing the rate of regeneration effluent recycle to the Claus plant.

,~ .

:~ :

By us~ of a l~w-t~mp~rat~re Clalls acl~;orption reactor, the absorption rate for a ZnO absorb~r i~
decreased since the sul~ur content of the fee~ to the absorber is reduced, allowing 2 to be introduced to the 5 regenerator at a lower rate. By introducing 2 during the regeneration period in a total amount effective for oxi-dizing Zn~ to ZnO and, as appropriate, also ~or combusting any residual H2 and CO to H2O and CO2, the total volume oE
2 is minimized. This permits the rate of absorber effl-10 uent introduced as diluent into the regenerator duringregeneration to be such that the volume of regeneration effluent returned to the Claus plant during regeneration can oe reduced in comparison with the volume where a Claus low-temperature adsorption reactor is not used. Further, 15 as indicated, purging in accordance with the invention can further reduce the rate of return of regeneration effluent to the Claus plant.
Regeneration can be preferably continued until substantially all of the sulfided absorbent is regener-20 ated, for example, until ZnS is substantially reconvertedto 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 is employed downstream of the regenerator to det-25 ermine the presence of 2 in the regenerator effluent,which is an indication of completion of regeneration.
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 30 according to the invention can require special attention.
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-35 ence of sulfur compounds.
Following regeneration, prior to returning theregenerated 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 (see Examples VI-VII), while another ZnO absorber is downstream receiving the produced SO2 under conditions for removing SO2 from the gaseous stream.
S The purging can be conducted for a period of time effec-tive for reducing by at least 10% a temporary increase in S2 emissions otherwise occurring from the plant when freshly regenerated ZnO absorbent is returned to absorp-tion without such purging with a reducing gas while 10 another ZnO absorber is downstream operating under effe~-tive absorption conditions. Preferably, the time is effective for reducing SO2 emissions to below about 250 ppm at all times. Most preferably, the time is effec-tive for substantially eliminating the increase in SO2 15 emissions, that is, for reducing the increase in SO2 emis-sions above the usual level during absorption by 90% or more from the level occurring where such a reducing gas purge is not used prior to returning to absorption.
The effective purge time can be readily deter-20 mined by one skilled in the art by monitoring SO2 emis-sions from a freshly regenerated absorber during at least the increased SO2 emissions period at the start of absorp-tion function, while another absorber is downstream oper-; ating under absorption conditions for removing SO2 there-25 from, until the SO2 emissions from the freshly regenerated absorber are reduced to a desired level, for example, to below about 250 ppm SO2, preferably to below about 50 ppm SO2. Thereafter the downstream absorber can be removed from absorption function and can be regenerated by intro-30 ducing a dilute oxygen gas thereinto and the effluent fromthe now purged, freshly regenerated absorber can be disc-harged to the atmosphere or, preferably, a portion of that stream can be used as diluent for oxygen during regenera-tion. Generally it is expected that the purge period in 35 accordance with the invention will be in the range of about 0.3 to about 3 hours, preferably in the range of ;~ about 0.5 to about 1 hour.
, '~'~, ~, ,;~; ~ .

` :~

~300345 Preferably, the purge stream can comprise at least a portion of absorber feed. Most preferably, the purge can be effected by using the entire absorber feed, that is, the entire tail gas stream.
Thus, it will be appreciated that purging in accordance with the invention uses a portion o~ th~
absorber feed and preferably the entire absorber fee~.
During the purging period, the ZnO absorbers would be placed in series on absorption, with the freshly regener-10 ated ZnO being in the absorber in the first position.
Purging and the beginning of absorption occur concur-rently. The effluent from the first absorber is fed to the other absorber to absorb the increase in SO2 emis-sions. The effluent from the second absorber can then be 15 vented to the atmosphere, thereby eliminating the recycle stream during operation in the purge mode or, alterna-tively, a slip stream of the effluent from the downs~ream absorption zone can be recycled to the Claus plant. The purge time in accordance with the invention can be greatly 20 reduced where the entire absorber feed is used because of the larger flow rate of reducing compounds flowing through the freshly regenerated ZnO absorber. Reducing the purge time can allow an increase in the regeneration time and thereby allow a decrease in the recycle rate of e~fluent 25 during regeneration to the Claus plant, reducing the size of the Claus and the ZnO absorber portions of the plant.
The invention will be further understood by the EXAMPLES which are set forth below.
EXAMPLE I - ABSORPTION: EFFECT OF TEMPERATURE
The effect of temperature on H2S breakthrough is studied using a laboratory catalyst holder/reactor made from type 304 stainless steel tubing 2" (inch) diameter (O.D.) x 0.068" thick wall, 27" long overall. Calculated catalyst volume for 18" depth is 805 ml (milliliters), and 35 the catalyst is supported by a 20 mesh stainless steel screen. Catalyst used is G72D*Sulfur Removal Catalyst described above. The reactor is wrapped by six heaters (22 gauge nichrome wire) for preventing radial heat loss, *G72D is a trademark.
, .
~, 1300:~45 and is insulated with fi'oerglass. The total flow rate for absorption is 10 l./min (liters/min) and for regeneration 5 l./min. The reactor is placed in a large Blue M~ oven, available f~o~l Blue M Electric Company, Blue Isiand, IL.
5 All gas flow through the catalyst bed is downflow. Provi-sions for side draw of gas samples are available near the reactor a~is each 1.5" of catalyst depth.
The effect o~ reaction temperatures on H2S
breakthrough time during absorption is illustrated by 10 introducing a feed gas having the followiny composition into the reactor inlet:

H2S 0.8 mol%
S2 0 4 mol%
CO 1.0 mol~
H2O 30.0 mol~
N2 45.8 mol~
H2 2.0 mol%
C2 20.0 mol%
The feed gas is introduced at 850F, at 1000F, and at 1150F. Breakthrough, defined for purposes of these runs as 50 ppm H2S in the absorber effluent, and H2S concentra-tion in the effluent gas at equilibrium, are determined.
25 Results are set forth in the following Table IA.
TABLE IA
Combined SO2 and H2S Concentration Absorption Capacity Time (Hrs) for (Dry Basis) mols absorbed/
Run Temp. Breakthrough at Equilibrium wt~ mols sorbent 1 850F (Immed. SO2 -- --- --Breakthrough) ~ 2 1000F 25.5 hrs <10 ppm 33~ 0.84 ; 3 1150F 27.5 hrs <20 ppm 36% 0.92 The results indicate that higher temperatures favor increased absorption capacity as indicated by 13003~5 increased breakthrough times and that lower temperatures favor lower equilibriurn concentrations of H2S in the absorber effluellt streams. It is also noted t'nat at 1000F and at 1150F, SO2 present in the inlet stream is 5 substantially completely absorbed or converted to H2S and absorbed while at 850F, SO2 appears immediately in the absorber effluent stream. Thus at temperatures at least abcut 1000F and higher hydrogenation of SO2 to H2S is not required prior to absorption.
EXAMPLE II - ABSORPTION: EFFECT OF TEMPERATURE

_ _ . . _ ________ __~___ ___ _ _ _ The effect of temperature on H2S breakthrough is further in~estigated by the following runs using the apparatus described in EXAMPLE I and using an inlet stream having the following composition:
H2S1.2 mol%
H2O29.5 mol%
H 1.06 mol%
CO1.01 mol%
C220.39 mol%
N246.83 mol%

This inlet stream can be used to simulate the condition where SO2 present in a Claus plant eefluent stream is 25 hydrogenated to H2S prior to absorption. Breakthrough time for various temperatures below 850F are determined and are shown in Table IIA below:

TABLE IIA
Time (Hrs) for Absorption Capacity Run Temp. Breakthrough w mols/mol sorbent 4 625F 3 4% 0.10 700F 11 14~ 0.36 6 775F 17 22% 0.46 These results further confirm the dependence of absorption capacity and breakthrough on absorption temper-ature.
' ~`

, ~

~, :

13~3~S

E~XA~?riF Ill - ABSORE'TION: ~FE'ECT OF WATER
The efEect of the presence of water vapor on sulfur corlpG~ d breakthrough is illustrated in part by EXA~'PrE I above in which a feed gas stream containiny 5 30.0% water is contacted with a ZnO absorbent and, at 1000E' to 1150F, the sulfur compounds in the effluellt stream are reduced to 20 ppm or lower.
To further investigate the effect of water on sulfur compound breakthrough using a metal oxide absor-10 bent, the apparatus of EXAMPLE I can be used with a zincferrite absorbent containing about 45% iron oxide and about 55% amorphous silica. About 15% of the 45% iron oxide is in the form of zinc ferrite. A feed gas having the following composition is introduced into the reactor 15 inlet at 1000F:

H2S 1.2%
CO 1%
H2 2%
CO21 20% (42%) H2O1 22% (o~) N2 53.8%

lCO2 content of inlet stream is increased from 20% to 42% when 22% H2O is eliminated from the feedstream.
After about 5-1/2 hrs, water is eliminated from the feedstream. The results are shown in Table IIIA
below.

; 35 ~A4rF IIIA
Time~l2S Concentration (rlr~s)ln Reactor Ef~luellt 2.3 733 3.4 ~18 4.1 9~4 5.51 1682 5.7 9 7.1 9 8.6 9 Water eliminated from feeds~ream.
The results indicate that the iron oxide (zinc lS ferrite) absorbent is sensitive to the presence o water in the feedstream as compared with the ZnO oE EXA~PLE I.
After water is removed from the feedstream, H2S in the effluent stream is reduced to 9 ppm. These results indi-cate that ZnO is less sensitive to water than is iron 20 oxide (zinc ferrite).
EXAMPLE IV - REGENERATION
Regeneration is investigated using the apparatus described in EXAMPLE I by passing a dilute air stream in contact with the sul~ided absorbent. The effect of tem-25 perature on regeneration is investigated. For a dilute air regeneration stream containing about 5 molO oxygen having an inlet temperature of about 1000F, the sulfur recovered as SO2 in the regeneration effluent stream is only 0.75 mol%. However, when the inlet temperature is raised to 1150F after 5-1/2 hrs, about 3 mol-O o sulfur as SO2 appears in the regeneration effluent stream. This higher regeneration temperature is considered preferred to overcome the high activation energy required or Reac-tion (8) above. During regeneration, the concentration of S2 in the regeneration effluent stream remains above about 3.5 mol% and the concentration of 2 in the regener-ation effluent stream remains about 0 mol~, indicating substantially complete consumption of 2~ for about ;

22 hrs. After about 22 hrs, when regeneration is about complete, 2 starts to breakthrough and SO2 content begins to decline in the regeneration effluent stream.
EXAMP E V - EFFECT O_ PURGE
Effluent tail gas from a Claus sulfur recovery plant having two catalytic reactors operated abo~e the sulfur dewpoint and one Claus low temperature adsorption reactor on-stream at all times is provided to an absorber containing ZnO. A portion of absorber effluent is used as 10 a diluent for 2 to a regenerator containing ZnS~ In a first run, upon completion of reg~neration, the regener-ator and absorber are interchanged in function. Upon interchanging the absorbers, an emissions level from the freshly regenerated catalyst, now functioning as an 15 absorber, of about 350 ppm SO2 is observed. SO2 emissions decline to less than about 50 ppm in about two (2) hours.
See FIGURE 2. In a second run, upon completion of regen-eration and prior to interchanging the absorber and the regenerator, 2 flow into the regenerator is discontinued 20 and the flow of absorber effluent is continued for a period of about two (2) hours. Upon interchanging the absorber and regenerator, SO2 emissions from the absorber are initially less than about 50 ppm and continue at that low level. See FIGURE 3. This example indicates that 25 discontinuing 2 flow and continuing absorber effluent, or other reducing gas flow, prior to interchanging an absorber and a regenerator eliminates a temporary increase in SO2 emissions above a baseline level otherwise observed from the absorber after interchanging the two reactors.
EXAMPLE VI - EFFECT OF REGENERATION GAS COMPOSITION ON

PURGE
The effect of SO2 levels during regeneration upon purge time requirements at the end of regeneration is ; investigated by regenerating sulfided absorbent using 35 regeneration feedstreams having various SO2 levels fol-lowed by purging with a reducing gas stream having 1.1% H2 and 0.5~ CO at a space velocity of about 1. The results are set forth in the following table:

~, .. . .
:, Run SO2 in ~c3erler~ition Feed Purge Tim~ (Hours) _ . . . .. ....... .. . ... . . . .. . . . .

1 0 % 2.0 2 2.9% ~.5 5 3 13.2% >12 The results indicate that the SOz level in the re~en~ratioll feed greatly affects the purge time and that 10 increased levels of SO2 during regeneration increase the purge time requirements. The results indicate that the use of absorber effluent or other reducing gas having little or no SO2 present at the inlet is advantageous in reducing purge time.
_AMPLE VII -_EFFECT_OF_REGE ERATION TEM?ERATURE ON
PURGINGjS~BSEQ~ENT A~SORPTION
_ _ _ _ _ _ _ .
Purging runs are made after regeneration at 900E and 1150F using absorber effluent as the purge gas. The test results show that by purging at 900F, the 20 increase in SO2 emissions is not removed, whereas by purging at 1150 F, increased SO2 emissions were not observed upon returning to absorption. Based upon these results, it is considered that purging should occur at temperatures from about 1000 F to about 1200E consistent 25 with the temperatures required for hydrogenation of other species in the presence of ZnO absorbent as set forth in Example I above.
EXAMPLE VIII - EFFECT OF Hq ON SOq EMISSIONS
To investigate the effect of H2 on SO2 emis-30 sions, laden ZnO (ZnS) is regenerated at 1150F with a regeneration stream having the following inlet composi-tion:
TABLE VIIIA

2 5 mol%
NH3 720 ppm C2 85 mol%
H2O 10 mol%

1300;~4S

After SO2 emissions decrease~ to about 50 ppm, 1 mol% H2 was added. SO2 emissions immediatel~ increased to about 450 pprn and then decreased with time. (Note:
the NH3 was present to simulate refinery gas in this run;
5 howeverr the presence of NH3 is not considered to affect the results from the addition of H2 reported he~ein.) These results indicate that reduciny eq.livalents such as H2 result in SO2 emissions from a freshly regener-ated absorbent. Thus, these results indicate tha~ the 10 effect of reducing gases during the purge period is to cause the production of and allow the removal 3f SO2 from regenerated sulfated absorbent in the purge effluent stream prior to return to absorption. SO2 removed during purge in regeneration effluent is sent to the Claus plant 15 where sulfur is formed and removed from the process. In this way, SO2 emissions from regenerated absorbent will not appear as emissions from the plant.
EXAMPLE IX - EFFECT OF HYDROGEN SULFIDE ON REDUCING SO
EMISSIONS
The effect of H2S on reducing SO2 emissions is investigated by contacting freshly regenerated absorbent with a stream containing H2S but no SO2. An SO2 emissions peak of about 100 ppm is observed initially, diminishing to about 20 ppm after six (6) hours. These results indi-25 cate that H2S will be effective as a purge gas. It is noted that H2S will also result in absorbent loading. See Eq. (3)-EXAMPLE X - EFFECT OF METHANE ON REDUCING SO EMISSIONS
The effect of methane on reducing SO2 emissions 30 is investigated by contacting absorbent, freshly regener-ated with a stream comprising about 13% SO2, with methane for six (6) hours. At the end of the six (6) hours, SO2 emissions are about 2000 ppm. Upon switching to absorp-tion, with a stream comprising 0.39 mol% H2S, 0.16 mol%
35 SO2, 1.69 mol% H2, and 0.26 mol% CO, SO2 emissions of about 8000 ppm are observed which decrease to about 1000 ppm in about 7 hours. Mass spectrographic analysis of the effluent stream during purge with methane indicates .

, ~
.,~ .

, ~300345 that methane is not cracked to H2 and CO at regeneration temperatures of about 1100F. These results indicate that methane alone is relatively ineffective for purginy to reduce SO2 e.~issions under process conditions.
S E:.XA~PLE' XI - ANALYSIS OF SUL~IDED_ABSORB~NT
Samples of fresh absorbent and regenerated absorbent, regeneration having been conducted at ]150F in the presellce of oxygen and 13% SO2 are analyzed by X-ray diffraction. The fresh absorberlt is largely crystailine 10 ZnO (zincite). The regenerated absorbent contains ZnO as the major co~ponent, with minGr concentrations of zinc oxide sulfate Zn3O (SO4)2, anhydrite CaSO4, and gahnite, ZnA12O4. These results indicate that sulfated compounds may be the cause of SO2 emissions when reduced by con-15 tacting with a reducing gas stream.

The invention will be further described and fur-ther advantages and applications and equivalents wlll be apparent to those skilled in the art from the description 20 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 25 about 1000F, preferably in the range of about 1000F to about 1200F.
An acid ~as 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-30 pheric air (source not shown), and/or SO2 (provided, forexample, 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-35 tion zone operated above the sulfur dewpoint and prefer-ably at least one low-temperature Claus adsorption reac-tion zone. Elemental 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 containing sufficient redllcing equivalents for reduction o~ sul~ur containing compounds remaining therein to H2S in the hydrogenation zone or in the absorber zone.
The Claus plant effluent stream in line 116 can then be heated to an effective temperature as described herein. Pre~erably 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 10 effluent stream in line 156, for example, in recuper-ator 158, as indicated schematically by the line marked A.
Following heating in recuperator 158, the heated Claus plant effluent stream can be provided by the lines marked B to heater 117 for further heating to above 1000F, pref-15 erably in the range of about 1000-1200F. Alternatively, of course, the Claus plant effluent stream 116 can be pro-vided 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 lines 125, 20 126, 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 25 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 30 of 1000F to 1200 F, 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.
The absorber effluent stream 138 can be con-35 ducted by lines 142, valve 142V, lines 152, 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 ,.
~' .

.
~ '' .~.
: .

130034~

plant e~]uent stream as described above, or for producing high press~lre steam. A portion of the absorber e~fluent stream can be withdrawn from line 152, by way of, for exarlple, line 154, having valve 154V, for dilution of 5 atmospheric air 172, via compressor 170 and line 168, having valve 168V, to produce a dilute air reyeneration strearn 166. During regeneration, valves 154V and 168V
control the recycle rate to the Claus plan~ and the rate of regeneration.
The regeneration stream 166 can be heated in 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 15 sulfided absorbent in second absorber 136 to produce a regeneration effluent stream 146 having a reduced 2 con-tent and an increased SO2 and/or sulfur content.
Stream 146 is conducted by line 144, valve 144V, heat recuperator 190, compressor 192, and line 111 to the Claus 20 plant furnace 112. Alternatively, the regeneration effl-uent stream can be introduced into a catalytic zone in 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 until prior to or just before 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 30 slack time) is equal to absorption time prior to H2Sbreakthrough. H2S breakthrough can be determined by moni-toring 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 35 emission requirements for discharge of stream 160.
Prior to placing the first absorber on regenera-tion, purge of the second absorber zone can be effected by discontinuing 2 flow to the second absorber, for example, ~' .

. ~

~3003~5 by closing valve 168V, and by stoppiny ~low of absorber effluent for diluent by closing valve 154V. Then, by opening valve 128V, the tail gas can be provided to the second absorption zone. During this period, valves 194V
5 and 182V should be open, and valves 196V and 126V should be closed. (I~ only partial flow is used ~or pur~e, valve 126V can be partially closed to control the rate.) Effluent from the second absorption zone cGntaining SO2 produced during the emissions period can then be provided 10 by line 144 having valve 144V, line 194 having valve 194V, and line 182 having valve 182V to the first absorption zone 134 in which the SO2 is removed in the presence of the ZnO and effective reducing species. Effluent from the first absorption zone 134 can then by line 138, 142, 15 having valve 142V be exhausted. A portion of the efluent from the first absorption zone can be provided to the Claus furnace by line 198, valve 198V, and line 111 to maintain a steady rate of gas to the Claus plant. Alter-natively, valve 196V in line 196 could be partially opened 20 to provide a constant rate of recycle to the Claus plant from the effluent of absorber 136. This method, however, has the disadvantage of providing a varying SO2 content to the Claus plant.
Therea~ter, first absorber 134 can be placed on 25 regeneration and second absorber 136 can be placed on absorption by closing valves 126V (if not already closed), 142V, 180V, 144V, 194V, 196V, and 198V in their respective lines 126, 142, 180, 144, 194, 196, and 198, and by opening valves 128V, 182V, 140V, and 148V in the respec-30 tive lines 128, 182, 140, and 148. Valve 194V in line 194(which can be closed during normal operation) can be uti-lized to minimize pressure shock during valve switching.
It will be appreciated by those skilled in the sulfur recovery art that a Claus plant tail gas cleanup 35 process is provided which is not sensitive to water con-tent in the effluent stream and which is capable of con-tinuously maintaining low levels of emission while -~ reducing costs. Other embodiments and applications in the 1300;~45 spirit of the invention and within the scope of the appended claims will be apparent to those skillecl in the art from the description herein.

Claims (20)

1. In a method for absorbing at least H2S from a Claus plant effluent stream by (1) sulfidization of ZnO, thereby producing ZnS, (2) the ZnS then being returned to ZnO producing SO2 in the presence of molecular oxygen O2 and (3) the ZnO regenerated to absorbing at least H2S
from the Claus plant effluent stream, said method additionally comprising the steps of:
(a) following the steps of regeneration of ZnS to ZnO in a first reactor, passing at least a portion of the total Claus plant effluent stream in contact with regenerated ZnO producing a purge effluent comprising increased amounts of sulfur dioxide SO2 relative to SO2 concentrations in the Claus plant effluent; and (b) passing the purge effluent in contact with the ZnO in a second reactor under conditions including temperature and composition for removing SO2 therefrom.

2. The method of Claim 1 further comprising:
continuing (a) and (b) for a period of time for reducing SO2 levels in the purge effluent from freshly regenerated ZnO absorbent to less than about 250 ppm and then regenerating ZnS produced in step (b) in the presence of O2 and concurrently absorbing at least H2S
from the stream by sulfidization of ZnO producing ZnS and repeating the steps (a) and (b).

3. The method of Claim 2 wherein the period of time is effective for reducing SO2 levels in the purge effluent to about 50 ppm.

4. The method of Claim 1 further comprising:
regenerating ZnS to ZnO by diluting O2 with effluent produced during regeneration of ZnS to ZnO.

5. The method of Claim 1 further comprising:
regenerating ZnS to ZnO by diluting O2 with effluent produced from absorbing at least H2S in the presence of ZnO producing ZnS.

6. The method of Claim 1 wherein:
substantially all of the effluent produced during step (a) is used in step (b).

7. The method of Claim 4 further comprising:
returning effluent produced during regener-ation to a Claus plant from which said stream compris-ing at least H2S is derived.

8. The method of Claim 5 further comprising:
returning effluent produced during regener-ation to a Claus plant from which said stream compris-ing at least H2S is derived.

9. The method of Claim 7 further comprising:
during the period of steps (a) and (b), returning effluent from step (b) to the Claus plant at about the same rate at which effluent is returned to the Claus plant during regeneration of ZnS to Zno.

10. The method of Claim 8 further comprising:
during a period of steps (a) and (b), return-ing effluent from step (b) to the Claus plant at about the same rate at which effluent is returned to the Claus plant during regeneration of ZnS to ZnO.

11. In a method for removing at least sulfur dioxide SO2 from a Claus plant effluent stream comprising SO2 and reducing species effective for converting SO2 and reducing species effective for converting SO2 to H2S in the presence of an effective ZnO absorbent, said method compris-ing (1) sulfidization of ZnO, thereby producing ZnS, (2) the ZnS then being regenerated to ZnO producing SO2 in the pres-ence of molecular oxygen O2, and (3) the ZnO returned to absorbing at least SO2 from said Claus plant effluent stream, said method additionally comprising the steps of:
(a) following the step of regeneration of ZnS to ZnO in a first reactor, passing at least a portion of the Claus plant effluent stream in contact with regenerated ZnO producing a purge effluent comprising increased amounts of SO2 relative to SO2 concentrations in the Claus plant effluent; and (b) passing the purge effluent in contact with ZnO
in a second reactor under conditions including temperature and gas composition for removing SO2 therefrom.

12. The method of Claim 11 further comprising:
continuing (a) and (b) for a period of time for reducing SO2 levels in the purge effluent from freshly regenerated ZnO absorbent to less than about 250 ppm and then regenerating ZnS produced in step (b) in the presence of O2 and concurrently absorbing at least SO2 from the stream by sulfidization of ZnO treated in step (a) producing ZnS and repeating steps (a) and (b).

13. The method of Claim 12 wherein the period of time is effective for reducing SO2 levels in the purge effluent to about 50 ppm.

14. The method of Claim 11 further comprising:
regenerating ZnS to ZnO by diluting O2 with effluent produced during regeneration of ZnS to ZnO.

15. The method of Claim 11 further comprising:
regenerating ZnS to ZnO by diluting O2 with effluent produced from absorbing at least H2S in the presence of ZnO producing ZnS.

16. The method of Claim 11 wherein:
substantially all of the effluent produced during step (a) is used in step (b).

17. The method of Claim 14 further comprising:
returning effluent produced during regener-ation to a Claus plant from which said stream compris-ing SO2 is derived.

18. The method of Claim 15 further comprising:
returning effluent produced during regener-ation to a Claus plant from which said stream compris-ing SO2 is derived.

19. The method of Claim 17 further comprising:
during the period of steps (a) and (b), returning effluent from step (b) to the Claus plant at about the same rate at which effluent is returned to the Claus plant during regeneration of ZnS to Zno.

20. The method of Claim 18 further comprising:
during the period of steps (a) and (b), returning effluent from step (b) to the Claus plant at about the same rate at which effluent is returned to the Claus plant during regeneration of ZnS to ZnO.

LSC