Note: Descriptions are shown in the official language in which they were submitted.
i3i~
BRIEF DISCLOSURE '~
The metal oxides CeO2, either supported or unsupported,
used to remove sulfur oxides from waste gas effluent streams by
conversion into cerium sulfate and/or cerium oxysulfate, is re~
generated to the starting cerium oxide by means of a reducing
regenerating atmosphere consisting essentially of H2S as the re
ducing component at a concentration of from 0.5 to 100 vol. ~,
preferably 1-70 vol. ~, most preferably 5-70 vol. ~, the balance
comprising non-regenerating non-reactive and inert gases such as
helium, argon, CO2, nitrogen, water vapor, etc. at a temperature
of from 3~0-700 C, preferably 350-700C, most preferably 450-600 C
in the presence of sufficient oxygen to convert any SO2 in said
gas to SO3, (in the SO2 removal step) the reducing-regenerating
atmosphere passing at any convenient rate, such as from 50 to
50,000 V/V/Hr, preferably 100 to 50,000 V/V/Hr, most preferably
100-20,000 V/V/Hr.
Regeneration is typically conducted on the cerium oxide
sorbent which has been converted to cerium sulfate, and/or oxy-
sulfate to the extent of 10 to 100 mole ~, preferably 10-70 mole %.
Regenera~ion of the cerium-sulfur oxide compounds to the cerium
oxide is accompanied by the liberation of SO2 which is conveniently
used with additional H2S in a Claus plant for conversion to ele-
mental sulfur.
DISCLOSURE
Metal oxides selected from the group consisting of CeO~,
copper oxides, iron oxides, preferably CeO2, either supported or
unsupported, which have been used as a sorbent to scrub suIfur
oxides (i~e. SO2, SO3, etc.) from waste gas effluent streams and are
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thereby converted to the metal sulfate, and/or metal oxysulfate
~for use of CeO2 see U.S. Patent ~,001,375).
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are regenerated to the metal oxide by means of a reduc-
ing regenerating a~mosphere consisting essentially of H2S
as the reducing-regenerating component present at from 0.5
to 100 vol. %, most preferably 5 - 70 vol. %, the balance
co~prising nonregenerating inert gases such as helium, neon,
argon, C02, nitrogen, wa~er vapor, etc. a~d mixtures thereof,
at a temperature ~rom 300-700C, preferably 3S0-700C, most
preferably 450-600C, the reducing-regenerating atmosphere
~ stream passing through the sorbent at any convenient rate, such
as ~rom 50 to 50,000 V/V/Hr, preferably 100 to 50,000 V~V/Hr.,
most-preferably 100 to 20,000 Vlv!Hr~ This regeneration proce-
dure can be practiced on either a cyclic or continuous basis.
The regeneration procedure of the instant invention
is utilized in flue gas desulfurization processes in which the
flue gas is contacted with a sorbent comprising cerium oxide in
elther the ~3 or the ~4 oxidation state. Preferably,
the ceriumoxide is supported on an inert support. The support
LS preferably an inorganic refractory oxide, for example,
various aluminas, silica, etc. The support can be
of various shapes, such as pellets, extrudates, Raschig rings,
saddles or monoliths, e.g. honeycombs. The most preferred
support is ~ -alumina, especially in the shape of Raschig
rings:
The support will have a surface area of from lcm2/c
to 300 m2/~, oreferably from 100 m2!8 to 200 m2!g, The cerium
oxide is co~bined with the support at from 1 to 40 wt. ~/0 of
said support. Pree~rably, the sorbent will comprise .rom
2 to 20 wt. ~ cerium oxlde.
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In the ~ollowing discussion CeO2 will be used as the specific
example, it being understood that e~uivalent arguments
and descriptions are available and operable for other
metal oxide sor~ents unless specifically indicated other-
wise . T~e sup30rted cerium oxide sorbent may be prepared by
methods known Ln the art for preparing supported catalysts fo~
use in petroleum processes, e.g. re~ormingl hydsocrac~ing, etc.
For example, an aqueous s~lution of a cerium oxide precursor
may be impregnated onto an alumina support. The ~m~regnated
support may be subsequen~ly separated from excess solution,
dried ae a temperature o~ ~ro~ abou~ 20 to llO~C and calclned
at a te~perature of rom ab~ut 300C ~o 600C. ~uring the drying
and/or the calcin~ng step, the supported catalyst may be contacted
with air or 2 to convert the cerium oxide prec~rsor co~pound
on the support into the oxide.
An alter~ative approach to the preparation o~ a ceri~m
oxide impregnated support which olaces the CeO2 on the outer
surface of a porous support involves prefilling of the pores
wieh an inert liquid as described in U.S. Patent 2,746,936,
For convenie~ce, the catalyst i5 imp~egnated with an
aque~us solution of t~e ceriu~.o~ide precursor. However, or~anic
solvents may be utilized provided the cexi-nm oxide?recursor is
soluble eherein. Precursors of the preferred sor~er.t, ceriu~
o~ide, tdhich are soluble in aqueous solutions, include ce ic
am~onium ~itraee, cerous nitrate, basic ceric ~itrate, cerous
acetate, ecc. For other m~tal oxides, similar metal salts
may be utiliz~d.
I The waste gas effLuent stream scr~boed is typically a
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1 gas, preferably a flue gas, which comprises from 0 01 to 2.0%
2 by volume (100 ppm to 20,000 ppm) sulfur oxides. T'nis waste
3 gas stream is contacted with th~ sorbent described above
4 Addi~ionally, the waste gas stream contains 2 su~ficient to
convert all S02 to S03 and may also comprise ~2~ C02, C0, H~O~
6 N0X, etc. It should be noted that none o~ these additional
7 co~ponants w;~ll interfere with t:he scrubbing o~ tbe gas stream.
8 In practice, at least a stoichiometric quantity o~ oxygen in the
9 waste gas is needed to per~it the absorption o~ S02 on CeO2
to form the sulfate Or oxysulfate. During the initial contacting
11 step, the temperatu~e is maintained at from 300C to 700C,
12 most pre~erably from 450C to 600C. The pressure is not
13 critical. For convenience, whatever pressure is ob~ained at
14 the flow and temperatures utilized will be acceptable. The
~low of the flue gas through the initial contacting zone, i.e
16 the zo~e in which the sorbent is contained, ~ay vary from 50
17 to 50,000, preferably f~om 500 to 50,000, and most preferabl~
18 from 500-20,000 V/V/~r. In the initial contacting zone, the
lg catalyst may be present in the form of pellets, extrudates,
etc. After a certain time, depending on the above contacting
21 condi~ions, the cerium oxide will be conver~ed subs~antially
22 to cerium sulfate and/or cerium oxysulfate.
23 When the conversion of the cerlum
24 oxide to the corresponding sulfate or oxysulfate reaches ~ lQ-
100%, preferably 10~70r/~ of capacity, the sorbent is regenera~ed
26 by the prOC9Ss of the instant invention utilizing an H2S con-
77 taini~g gas wherein the reducing-regeneratlng agent consists
28 essentially of the H2S present at a concentration o_ f~o~ 0.;
29 to lC0 vol. %, preferably, 1-70 vol ~/" most preferably S~
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1 70 vol %, the balance of the stream being nonregenerative,
2 nonreactive inert gas such as nitrogen, helium~ argon, neon,
3 water vapor, C02, etc. The ceri~m sulfate, and/or oxysulfate
4 is converted substantially to cerium oxide while the sul~ur
is removed as sulfur dioxide from the sorbent.
6 When dealing with the cerium~system for example,
7 if reaction of the spent cerium sorbent with H2S goes too far
8 and begins to convert the regenerated cerium oxide to the
9 sulfide, a treatment with air, air/steam mixture, or steam
alone can be used to restore the sorbent to full capacity.
11 Preferably the atmosphere is steam. The temperature at which
12 this final s~ep is performed (if it is necessary) ranges from
13 300-700C, preferably, 400-700C, most preferably 450 to 600C.
14 This step converts any sulfide bac~ to the oxide with only
small amounts of sulfate formation. The reaction of cerium
16 sulfide with oxygen to ~ive almost quantitatively the oxide
17 is unique for cerium among the lanthanide sulfides which
18 generally burn to give oxysulfates.
19 Cerium oxysulfates have the general formula:
CeO2_y(SO4)y where 0 C y ' 2.
21 The overall general regeneration reaction is:
22 CeO2_y(S04)y ~ Y3 H2S -> CeO2 + 4Y S02 + Y H20
23 so tha~ the overall method is to sequester S02 from a flue
24 gas and recover it as concentrated S02 as follo~s;
S0~ + 2 2 + 3 H2S~ 4 S02 ~ 1 H20
26 Any S03 present in the flue gas will also be re~oved.
27 The S02 thus formed can be reacted with additional
28 H2S over the amount needed to regenerate the sorben~ to for~
29 elemental sulfur by the Claus reaction:
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l S2 + 2~T2S --~ 3S + 2H20
2 which can be made to procePd in the same or a separate
3 reactor from the CeO2 containing vessel. Note that a mole
4 of H2S is able to reduce three times as much sulfate as a
mole of hydrogen, a major advantage o~ this method of regen-
6 eration over the prior art.
7 Alternatively, the S2 thus formed ma~J then be fed
8 to a separate Claus plant for conversion to elemental sulfur
9 In the Claus plant H2S is mixed with the SO2 to bring the
H2S:SO2 mole ratio to 2:1 prior to the catalytic converter.
11 One advantageous method of regenerating the spent
12 metal oxide sorbent is to pass an excess of H2S-containing
13 gas over it, that is, in an amount in excess of the volume
14 of H2S needed to just regenerate the sorbent so that H2S/SO2
mixture is produced, which can be fed directly to the Claus
16 plant. Indeed, some of the 2H2S + SO2 Claus reaction takes
17 place over the metal oxide sorbent resulting in the production
18 of some elemental sulfur in the sorbent vessel.
19 As previously stated, this regeneration procedure
can be practiced in either a cyclic or continuous manner.
21 When operated in a cyclie manner the scrubber-regenerator
22 ccmprises multibed units, wherein a gas mixture containing
23 sulfur oxides is passed through one or more fixed beds of sup-
24 ported cerium oxide. ~hile these beds are scrubbing sulfur
oxides, the other beds of the unit are being ~egenerated with
26 an H2S containing gas as described. The roles o~ the scrubber
27 and regenerator are reversed when both have completed their
28 task. Purging with a gas stream such as steam, between these
29 two steps may be advantageous both to prevent explosive conditions
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as well as for converting any cerium sulfide which may have
~ormed in the regeneration step to cerium oxide.
In another embodiment of the instant invention, ~he
catalyst is continuously removed and regenerated. For example,
see the apparatus described in U.S. Patent No 3,989,798
As stated above, ~he cerium oxide is preferably sup-
ported on an inert support material to most economically use
the cerium o~ide. However, unsupported cerium o.~ide may be
used provided adequate surface areàs are obtained. Preferably
the unsupported cerium oxide should have a sur~ace area of at
least 10 m2/g, preferably ~rcm 20 m2/g to 50 m2/g. Such un-
supported cerium oxide is regenerated by the same H2S pro-
cedure as is supported cerium oxide.
EXAl'ilPIES
A 5.5 g (8.2 cc) sample of 20~/~ CeO2 supported on
extruded ~ -A1203 is held in place by quartz wool within a
vertical quartz tube (rVl" dia.). A gas blend containing
3700 ppm S02, 5% 2 and balance Ar is passed upward through
the heated sample at 4000-5000 V/V/Hr. during the SO2 scrubbing
mode. The S02 content of the exit gas is analyzed using an
electrochemical method containing a Faristor which is cali-
brated to read L00% at 5000 ppm S02. ~uring regeneration, a
1% H2S in ~e regeneration gas i~s run through the bed at r-1000
~/VIhr and the exit gas is bubbled through Pb(M03)2 solutions.
.~ white precipitate (PbS0~) indicates S0~ while a black pre-
cipitate ~PbS) indicates H2S in the gas stream and sio~nals
the end of a regeneration.
A number of S02 scrubbings, H2S regenerations,
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1 and optional burns w ere carr ied ou t un der
2 the condi~ions described above. Typically, duri~g the SO2
3 scrubbing, the SO2 content in the exit gas as a function of
4 time ~as as follows: 500!ppm/20 mi.n; 1000 ppm/40 min; 1500 ppm/
65 min; and 2000 ppm/95 min. The regeneration at 500-600C with
6 1% H2S~He too~ about 1 hr. '~efore breakthrough of H2S. The for-
7 mation of cerium oxysulfate, oxide, and sulfide ~ere all moni-
8 tored by removing a small sa~ple of extrudate at appropriate
9 times and examining the product by X-ray diffraction.
When a dry or we t H2S containing gas is used for the regen-
11 eration, prolonged treatment of the spent sorbent may first
12 convert it to the oxide, followed by further conversion to
13 the sulfide, CeS2. It was also shown tha~ the resultant sul-
14 fide can be converted back to the oxide by passing an oxygen
and/or steam containing gas over it a~ 300-700C.
16 The present inven~ion is particularly well suited
17 for the removal of SO2 from gases in an installation where
18 stoichiometrically adequate amounts of H2S are also available
19 from other operations. Typical examples æe as follows:
(a) Claus plant tail gas cleanup.
21 (b) So2!sO3 removal from refinery flue gases.
22 (c) So2!SO3 removal from flue gases in coal gasi-
23 fication or liquefaction plant, tar sand refineries, and the
24 like where H2S is available as a byproduct
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