Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
`` lOS793~
IMPROVED PROCESS FOR THE PRODUCTION OF SULFUR
FROM SULFUR DIOXIDE EXTRACTED FROM GAS STREAMS
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Back~round of the Invention
The present invention i8 related to processes which
utilize heat regenerable liquids for the removal of sulfur
dioxide from gas streams and to the production of elemental
sulfur from sulfur dioxide extracted from gas streams.
Many processes employ aqueous solutions of ammonia,
alkaline salts of organic and inorganic acids, and aromatic
amines as absorbents for the extraction of sulfur dioxide from
gas streams. These solutions are readily stripped of sulfur
dioxide ~pon the application of heat and therefore regenerated
for reuse. The ~tripped sulfur dioxlde is proc-ssed for the
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¦ production of liquid sulfur dioxide, sulfuric acid or elemental
I sulfur.
¦ To produce elemental sulfur from the recovered sulfur
¦ dioxide, it is expedient to react the sulfur dioxide with hydrogen
5 ¦ sulfide by the Claus reaction. This requires two moles of
¦ hydrogen sulfide for each mole of sulfur dioxide.
¦ In a situation where a source of hydrogen sulfide is not
¦ available as, for example, in a power plant where sulfur
¦ dioxide must be removed from stack exhaust gases, hydrogen
10 ¦ sulfide has to be generated if sulfur is to be produced.
¦ This may be accomplished by converting two thirds of the
¦ stripped sulfur dioxide to hydrogen sulfide and reacting the
¦ balance with the formed hydrogen sulfide over an alumina
¦ catalyst in the catalytic zone of a typical Claus plant.
15 ¦ Another possibility, which has been proposed for the
¦ regeneration step of the Citrate process, is to produce hydrogen
¦ sulfide by reduction of elemental sulfur generated in the
¦ process for reaction with sulfur dioxide dissolved in the
I citrate solution.
201 Known methods of producing hydrogen sulfide from elemental
sulfur have been described, for instance, in "Canadian Mining
l and Metallurgical Bulletin", October 1957, p. 614 and follow-
¦ ing, and '~ining Engineering", January 1970, p. 75 and follow-
I ing.
25¦ The former process involves non-catalytic direct reaction
of hydrogen with sulfur to form hydrogen sulfide at temperatures
from 820 to 1000F. An admitted deficiency in the process is
¦ that the reaction products, i.e., a mixture of unreacted
I sulfur and hydrogen sulfide, are highly corrosive. Type 316
301 stainless steel, for instance, suffers severe corrosion. In
iO579 34
1 addition, hydrogcn of hi~h purity is required for the process
and this is expensive.
The process described in the latter publication involves
two conversion stages. In the first, sulfur is reacted with a
S hydrocarbon, such as methane, at a temperature from 600 to 700C
over a catalyst to form a mixture of hydrogen sulfide and
carbon disulfide. The gas stream is passed to a second stage
where, at a temperature of 200 to 300C, the carbon disulfide
reacts with water to form hydrogen suifide and carbon dioxide.
This process also sufers from severe corrosion problems and
the net gas stream can contain considerable quantities of
carbon-sulfur compounds such as carbonyl sulfide and carbon
disulfide.
As it pertains to the operation of the Citrate process,
it would be necessary to convert two thirds of the formed
sulfur to hydrogen sulfide for reaction with sulfur dioxide
in the liquid phase at ambient temperature. This is an
expensive operation from an energy conservation standpoint
since two-thirds of the product must always be recycled back
in the form of hydrogen sulfide. Further, the proposed means
to generate hydrogen sulfide from the formed sulfur leaves
much to be desired due to the corrosion and pOllution problems
attendant to the generation of hydrogen sulfide from the
elemental sulfur.
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l Summary of the Invention
In accordance with the present invention there is
provided a process for the production of sulfur from sulfur
dioxide contained in gas streams which comprises: -
(a~ passing a sulfur dioxide containing gas
stream through a sulfur dioxide absorption zone containing
an aqueous absorbent for sulfur dioxide to form a sulfur
dioxide rich absorbent and a sulfur dioxide lean gaseous
stream;
(b) separating the sulfur dioxide rich absorbent
from the absorption zone;
(c) passing about one-third of the sulfur dioxide
rich absorbent to a sulfur production zone;
(d) passing the balance of the sulfur dioxide rich
absorbent to a sulfur dioxide stripping zone to separate
sulfur dioxide from the absorbent to form a sulfur dioxide
lean absorbent for recycle to the absorption zone;
(e~ catalytically hydrogenating the sulfur dioxide
separated from the sulfur dioxide rich absorbent in the
stripping zone in the presence of a source of hydrogen to
hydrogen sulfide at a temperature from about 300 to about
lOQ0F in the presence of a catalyst consisting of at least
one supported metal selected from Group Va, VIa, VIII and
the Rare Earth Series of the Periodic Table;
(f) passing the formed hydrogen sulfide to the
sulfur production zone where sulfur is formed from the
introduced hydrogen sulfide and sulfur dioxide.
Also in accordance with the invention there is provided
a process for the production of sulfur from sulfur dioxide
contained in gas streams which comprises:
10~7~34
1 (a) passing a sulfur dioxide containing gas stream
through a sulfur dioxide absorption zone containing an
aqueous absorbent for sulfur dioxide to form a sulfur
dioxide rich absorbent and sulfur dioxide lean gaseous
stream;
(b) separating the sulfur dioxide rich absorbent
from the absorption zone; ~- ;
~c) stripping sulfur dioxide from about two thirds
of the sulfur dioxide rich absorbent to form a sulfur dioxide
lean absorbent for recycle to the absorption zone;
(d~ catalytically hydrogenating the sulfur dioxide,
separated from the sulfur dioxide rich absorbent by stripping
in the presence of a source of hydrogen to hydrogen sulfide
at a temperature from about 300 to about 1000F in the
presence of a catalyst consisting of at least one supported
metal selected from Group Va, VIa, VIII and the Rare Earth
Series of the Periodic Table;
'~ (el combining the formed hydrogen sulfide and the
balance of the sulfur dioxide rich absorbent solution in a
sulfur production zone where sulfur is formed by an aqueous
phase,reaction between hydrogen sulfide and sulfur dioxide
to yield a sulfur dioxide lean absorption solution for
recycle to the absorption zone.
Thus, according to the present invention there is provided
an improvement to processes where sulfur is to be formed from
the ~ulfur dioxide extracted from gas streams by an aqueous
sulfur dioxide ab.sorption ~olution, such as the Citrate process
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1~57~ ~
l which employs a buffered aqueous solution of an alkali metal
citrate, such as sodium citrate, as the absorbent solution.
In accordance with the practice of the present invention
a gas stream containing sulfur dioxide is passed through an
absorption zone where sulfur dioxide is extracted by an
aqueous sulfur dioxide ab~orption solution. Aqueous absorption
solution laden with sulfur dioxide is continuously
separated from the absorption zone and split. Approximately
two thirds of the sulfur dioxide rich absorption solution is
passed to a sulfur dioxide stripping zone where sulfur dioxide
is separated from the aqueous solution. The sulfur dioxide lean
absorption solution is recycled back to the absorption zone.
The sulfur dioxide separated from the absorption solution
is then catalytically hydrogenated in the presence of a source of
lS hydrogen to hydrogen sulfide. This is preferably accomplished
l by forming a reducing gas stream by the combustion of a
i carbonaceous fuel such as methane, in the presence of a source
of oxygen, typically air, in a reducing gas generator. This
forms a gas stream containing as the principal reductants
hydrogen and carbon monoxide. The gaseous products from the
reducing gas generator are combined with the sulfur dioxide
gas and passed to catalytic conversion zone 42 where at a
temperature from about 300 to about 1000F, preferably from abo~
500 to about 800F, the sulfur dioxide reacts with the hydrogen
present and the hydrogen formed as a consequence of the
reaction of carbon monoxide with water to yield hydrogen
sulfide. The cata~ysts employed are those containing the
metals o Group Va, VIa, VIII and the Rare Earth series of
the Periodic Table as defined by Mendeleef and published as
the "Periodic Chart of the Atoms" by W. N. Welch Manufacturing
Company. The metals are preferably supported on conventional
1~579 3~
supports such as silica, alumina, alumina-silica, and the
zeolites. Alumina is preferred. The preferred catalysts
are those containing one or more of the metals cobalt,
molybdenum, iron, chromium, vanadium, thoria, nickel,
tungsten and uranium. A cobalt-molybdate catalyst where
the support is alumina is particularly preferred.
In addition to causing the hydrogenation of sulfur
dioxide to hydrogen sulfide at the temperatures employed, the
catalyst serves to hydrolyze any carbonyl sulfide and carbon
disulfide which may be present or formed to hydrogen sulfide.
The water required for the hydrogenation and hydrolysis
reactionS may be provided in the stripped sulfur dioxide gas
stream. The hydrogen sulfide gas stream after cooling to
remove any water present, is combined with the aqueous
15 solution containing the balance of sulfur dioxide where,in
the aqueous phase,the hydrogen sulfide reacts with the sulfur
dioxide to form elemental sulfur. The elemental sulfur is
collected as product, a gas stream now free of sulfur species
vented to the atmosphere and the aqueous absorption solution
20 recycled back to the absorption zone.
In the practice of the process of this invention all of
the sulfur dioxide removed from the gas stream in the absorption
zone leaves the process as elemental sulfur and no recycle of
product sulfur is required for the production of hydrogen sulfide.
An additional advantage of the process is that by using
sulfur dioxide stripped from two thirds of the absorption solution
as the source of hydrogen sulfide, the proper stoichiometry
~ill always be maintained in the sulfur dioxide production zone
regardless of solution loading. This enables ready control
over the system and prevents unreacted hydrogen sulfide or
sulfur dioxidc from being vcnted to the atmosphere.
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iO ~'79 ~4
1 The Drawinp,
The attached drawing depicts a flow diagr~m for carrying
out the preferred process of the present invention.
Description
According to the present invention there is provided a
process for the production of sulfur from sulfur dioxide
absorbed in aqueous absorption solutions. Typical of the
absorption solutions are aqueous solutions of a~monia,
alkaline salts of inorganic and organic acids and certain
aromatic amines which are capable of absorbing sulfur dioxide
! at ambient temperature and releasing the absorbed sulfur
dioxide upon the application of heat. The preferred absorbent
is an aqueous solution of an alkali metal citrate such as sodium
citrate.
With reference to the Drawing, the sulfur dioxide
containing gas is passed through sulfur dioxide absorption
zone 10, where it is brought into counter current contact
with a lean sulfur dioxide absorption solution entering
absorption tower 10 in line 12 after being cooled in heat
exchanger 14 to maximize the absorption capacity of the
solution. The gas stream free of sulfur dioxide is vented
to the atmosphere.
The sulfur dioxide-rich absorption solution is withdrawn
from the base of absorption tower 10 in line 16 and split. About
two thirds of the solution is passed by line 18 after under-
going indirect heat exchange with recycled lean solution in
heat exchanger 20 to a sulfur dioxide stripping tower 24. In
sulfur dioxide stripper 24 the aqueous absorption solution is
heated to a temperature which w~ll enable the solution to
effectively desorb the contained sulfur dioxide. The heat
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I required to heat the solution may be provided by indirect
heat supplied through reboiler 26.
The lean sulfur dioxide absorption solution is passed
by line 28 through heat exchanger 40 and by pump 30 through
heat exchanger 14 and back to absorption tower 10.
The sulfur dioxide separated from the absorption solution
is passed by line 32 through cooler 34 which enables water
vapor and droplets of vaporized absorption solution to cool and
coalesce in condenser 36 for recycle by pump 38 back to
stripping tower 24.
The sulfur dioxide gas stream which is substantially
free of absorption solution but which still contains some water,
is passed by line 40 to catalytic converter 42.
Simultaneously there is formed a reducing gas in generator
44. The reducing gas is formed by the partial combustion of a
carbonaceous fuel such as methane, propane, butane and the
like, in the presence of a source of oxygen, typically air.
Steam may be introduced to suppress the formation of soot and
carbon-sulfur compounds. The gas stream exiting reducing gas
generator 44 comprises as principal reductants, hydrogen and
carbon monoxide which serves as a hydrogen doner as it will
react with the water in catalysis zone 42 to generate
hydrogen for reaction with sulfur dioxide.
The streams are combined and passed by line 46 to
catalytic generator 42. Catalytic generator 42 contains
one or more beds of a catalyst capable of hydrogenating the
sulfur dioxide to hydrogen sulfide. Preerably, the catalyst
is also capable of causing hydrolysis of any carbonyl sulfide and
carbon disulfide which may be present or formed in the gas
stream to hydrogen sulfide and carbon dioxide.
lU57934
1 Conversion of the sulfur dioxide to hydrogen sulfide
occurs at a temperature of from about 300 to about 1000F,
preferably from about 500 to about 800F. The catalysts employed
are those containing the metals of Group Va, VIa, VIII and
the Rare Earth Series of the Periodic Table as defined by
; Mendeleef and published as the "Periodic Chart of the Atoms"
by W. N. Welch Manufacturing Company. The metals are prefer-
ably supported on conventional supports such as silica, alumina,
alumina-silica, and the zeolites. Alumina is preferred.
The most preferred catalysts are those which contain one
or more of the metals cobalt, molybdenum, iron, chromium,
; vanadium, thorea, nickel, tungsten and uranium. A cobalt-
molybdate catalyst where the support is alumina is particularly
preferred.
Reaction in catalytic con~erter 42 is highly exothermic.
To maintain the temperature within the prescribed range, an
external coolant can be employed to absorb the heat of
reaction. In the alternative, as shown, there may be injected
water as a fine spray into the catalysis zone to ~uench
the reaction and absorb the heat of reaction. Introduced
water also serves to hydrolYze any carbonyl sulfide and
carbon disulfide present to hydrogen sulfide and to react
with carbon monoxide present in the effluent from the
reducing gas generator to form hydxogen for reaction with
the sulfur dioxide. Another alternative is recycling of a
portion of the effluent gas from reactor 56. Under the
temperature conditions employed, complete conversion of the
~ulfur dioxide to hydrogen sulfide ls realized.
The hydrogen sulfide gas stream from catalytic converter
42 is passed in line 48 to waste heat generator 50 where heat
is generated in the form of steam for use in heating the
solution undergoing sulfur dioxide stripping in stripper
I ~S 7~ ~9~
1 ¦ tower 24, or alternately the hot &aS stream may be passed
¦ through reboiler 26. The gas stream is then passed to contact
¦ cooler 52 where the water present is removed.
¦ Since hydrogenation of sulfur dioxide in converter 42
5 ¦ is complete, the condensate is free o corrosive polythionic
¦ and sulfurous acids.
¦ The hydrogen sulfide gas stream i8 then passed by
I line 54 to sulfur production zone 56 where it is brought
¦ into contact with the balance of the sulfur dioxide rich ?
10 ¦ absorption solution from absorber 109 which is passed to
¦ sulfur production zone 56 by line 58.
¦ In sulfur production zone 56, hydrogen sulfide and
¦ sulfur dioxide react in the aqueous phase to form elemental
¦ sulfur which is withdrawn as product, typically as a slurry.
lS ¦ Gases presént which are essentially free of sulfur dioxide
¦ and hydrogen sulfide are vented to the atmosphere. Upon
¦ formation of sulfur, the absorption solution which is rendered
l lean, is pumped by pump 60 through line 62 back for recycle
¦ to absorption tower 10.
201 In the practice of this invention, all sulfur dioxide
; removed from the gas stream in the absorber leaves the plant
as elemental sulfur and no recycle of product sulfur is
required. An additional advantage of the improved process
is the fact that by using the sulfur dioxide stripped from
two-thirds of the absorbent solution as the source of hydrogen
sulfide to react with sulfur dioxide contained in the remain-
ing one-third of the solution, the proper stoichiometry will
always be maintained in the reaction tank, regardless of
solution loading. This feature permits easy control of
the system and prevents unreacted hydrogen sulfide or
sulfur dioxide to the atmosphere.
1~57~ 3 4
l The improved process, with moclifications, can be applied
to other processes embodying thermal regeneration (and
stripping of sulfur dioxide) of a solvent. For example, the
rich solution from the bottom of the absorber can be split
into two streams, in the ratio of 2:1, and stripped in two
separate strippers. The sulfur dioxide obtained from the
larger stream can be catalytically converted to hydrogen
sulfide as described above. The hydrogen sulfide rich gas,
without being cooled, is then combined with the sulfur
dioxide stripped from the smaller liquid stream in a suLfur
production zone and the mixed gases fed to one or more
catalytic Claus stages of the sulfur production zone.
Since~ the source of the hydrogen sulfide and sulfur dioxide is
the rich absorbent and the control of the split of the rich
solution at a ratio of 2:1 is quite simple, the ratio of
hydrogen sulfide to sulfur dioxide entering the Claus stages
will always be near optimum resulting in maximum conversion ~.-
of elemental sulfur.
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