Note: Descriptions are shown in the official language in which they were submitted.
Back~round of the Invention
The invention relates to an improved process for
removing sulfur dioxide from gas streams. In particular, the
invention relates to an improved method for carrying out the
reaction between a sulfur dioxide rich aqueous absorbent and
hydrogen sulfide. More particularly, the invention relates to an
improved method for regenerating the absorbent in a cyclic
process for removing sulfur dioxide from gas streams.
The combustion of sulfur-containing carbonaceous fuel
such as fuel oil, fuel gas, petroleum coke, or coal, the
production of sulfurinc acid, the production of sulfur from
hydrogen sulfide and other processes produce stack gases
containing small amounts of sulfur dioxide. The discharge of
the sulfur-containing stack gases into the atmosphere consti-
tutes a serious hazzard to animal and plant life. The sulfur
dioxide content of the waste gas is quite small, usually below
about 0.5 to 2%, but the volume of gas produced is so large that
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considerable amounts of sulfur dioxide are introduced into the
atmosphere. In view of the large number of processes which
introduce sulfur dioxide into the atmosphere, it is readily
apparent that means for removing a maJor portion of the sulfur
dioxide fr~m the waste gases is necessary.
Many processes have been proposes for removing low
concentrations of sulfur dioxide from gas streams. In one
particularly useful continuous cyclic process, the sulfur
dioxide containing gas stream is contacted with an aqueous liquid
absorbent which selectively absorbs a major portion of the sulfur
dioxide from the gas stream. The more or less sulfur dioxide
free gas stream is discharged to the atmosphere or otherwise
dlsposed of. The sulfur dioxide rich absorbent is passed to
a regeneration zone where the sulfur dioxide rich absorbent is
contacted with hydrogen sulfide. The hydrogen sulfide reacts
with the sulfur dioxide rich absorbent to form sulfur and
regenerate the absorbent. The sulfur is separated from the
regenerated aqueous absorbent and the regenerated absorbent is
returned to the absorption zone to contact additional sulfur
dioxide containing gas to absorb sulfur dioxide.
The process is capable of substantially reducing the
sulfur dioxide concentration in gas streams containing varying
amounts of sulfur dioxide. The process utilizes hydrogen sulfide
which is frequently available as a waste product. A ~ajor portion
of the sulfur dioxide removed from the gas stream is converted to ;
_2-
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039~3() 1
free sulfur. Cyclic processes for removing sulfur dioxide f~ m l ¦
gas streams are disclosed in U.S. l~atents 2,~63,437 and 2~729,5~3. 1 ;
The aqueous absorbents for absorbing sulfur dioxide from
gas streams can be water or aqueous solutions of alkali metal
salts of non-vclatile acids which have at least one dissociation
constant between about 1 X 10 2 and 1 X 10 5 measured at a
dilution of 40 liters per gram mole at a temperature of 25C.
Aqueous solutions of non-volatile salts of acids such as lactic
acid, glycolic acid, citric acid, orthophosphoric acid, maleic
acids, succinic acid, selenic acid, tartaric acid~ oxal~ic acid,
glutaric acid, diglyoxic acid; certain water soluble aluminum
salts, beryllium salts and the like have been found useful in ~ ~ ;
cyclic processes. Absorbents which provide an equilibrium steady
state sulfur dioxide rich absorpent in the pH range of about 2.8
to about 4.5 can be effectively used in the processes.
One difficulty in operating a continuous process for
removing sulfur dioxide from gas streams involves the reaction of
hydrogen sulfide with the sulfur dioxide rich absorbent Hydrogen
sulfide is relatively insoluble in the aqueous absorbents used in
the processes. In addition, the reaction of hydrogen sulfide with
certain species in the sulfur dioxide rich absorbent leads to
fonmation of undesirable byproduct species. Gertain byproduct
species tend to react slowly with hydrogen sulfide, bu1ldup in
the aqu~ous absorbent and affect the absorption of sulfur dioxide.
l . ~
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1~3~(~30
One method for contacting hydrogen sulfide with aqueous
absorbents containing absorbed sulfur dioxide is to utilize
mixers or beaters to rapidly agitate the liquid to insure
d$spersion of the hydrogen sulfide into the aqueous sulfur
dioxide rich absorbent (see: V.S. Patents 2,729,543,and 2,043,084).
The methods which use mechanical rotating elements to disperse
the hydrogen sulfide into the aqueous absorbent are inefficient,
require rather long holding times, high power inputs and are
subject to the deposition of sulfur on the rotating elements ~hich
lo move at relatively high velocities through the aqueous absorbent.
Sulfur also tends to buildup on the walls of the reaction vessel.
The problem with sulfur buildup and the relatively long reaction
times dictate that large volumes of liquid be rapidly agitated to
contact the liquid absorbent with the gaseous hydrogen sulfide.
Other me~hods for contacting sulfur dioxide rich
aqueous absorbent with hydrogen sulfide such as countercurrent or
cocurrent flow through packed columns and bubbling hydrogen
sulfide through large volumes of aqueous absorbent have been
utilized. The methods are characterized by long reaction times to
regenerate the absorbent.
The ob~ect of the present invention i5 to provide a
method whereby an aqueous absorbent rich in~8ulfur dioxide can
be rapidly reacted with gaseous hydrogen sulfide. Another object
of the present invention to provide a continuous method for
reacting a sulfur dioxide rich aqueous absorbent with hydrogen
~4_
10~
sulfide in a manner in which the deposition of sulfur in the
reaction zone is minimi7ed. It is a further object of the
present invention to provide a method wherein sulfur deposition
in the reaction zone can be controlled.
Brief Summary of the Invention
. .
According to the present invention sulfur dioxide rich
aqueous absorbent is contacted with hydrogen sulfide in a
reaction zone in which the aqueous absorbent and hydrogen sul-
fide are cocurrently conveyed undex turbulent flow conditions
in such a manner that the hydrogen sulfide is intimately inter-
....
mixed with the aqueous absorbent for a sufficient length of
time to react a substantial portion of the sulfur dioxide with
hydrogen sulfide. It is necessary that the velocity of the
gas and liquid flowing in the reaction zone be such that the
hydrogen sulfide and aqueous absorbent are intimately contacted.
The reaction zone usually has the form of an elongated
conduit or pipe of such dimension that the velocity of the
liquid gas mixture provides for turbulent flow conditions
and of such length as to provide for substantial reaction of
the hydrogen sulfide with the sulfur dioxide.
Thus, in accordance with the present tea~ings,
an improvement is provided in a process wherein sulfur
dioxide containing gas is contacted with an aqueous absorbent
selected from the group consisting of water and aqueous solutions
of alkali metal salts of non-volatile acids which have at least
one dissociation constant between 1 x 10 and 1 x 10
measured at a dilution of 40 liters per gram mole at 25C, to
form a sulfur dioxide rich aqueous absorbent with a pH in the
range of about 2.8 to about 4.5 and a sulfur dioxide rich
aqueous absorbent being contacted with hydrogen sulfide to
form sulfur. The improvement of the process comprises forming
.
~ _ 5 _
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~03~03V
a mixture of hydrogen sulfide and the sulfur dioxide rich
aqueous absorbent and cocurrently conveying the mixture
to an elongated conduit reaction zone under turbulent flow
conditions in which the hydrogen sulfide is intimately inter-
mixed with the aqueous absorbent 'or a sufficient length of
time to react a substantial portion of the sulfur dioxide
in the aqueous absorbent with hydrogen sulfide to form sulfur.
Description of the Drawings
Figure 1 is an embodiment of the invention in which
the hydrogen sulfide and sulfur dioxide rich absorbent are
contacted on a once through basis. - .
Figure 2 is an embodiment of the invention which is :
designed to independently provide sufficient liquid flow under
all liquid feed conditions.
-~,
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~39~!3~ : -
Figure 3 is an embodiment of the invention arranged in ~;
a ~anner to react substantially al] of the hydrogen sulfide.
Detailed Description of the Invention `
The process of the invention is useful for reacting
hydrogen sulfide with sulfur dioxide rich absorbents. The
process can be effectively utilized to substantially reduce the
sulfur dioxide content of absorbents utilized in a once through
or cyclic process. In a once through proces ~the aqueous
absorbent is utili~ed only one time to contact the sulfur dioxide
containing gas stream. Water is the preferred absorbent in a
once through process. The process of the invention is particularly
useful for regenerating the aqueous absorbent in a continuous
c~clic process for removing sulfur dioxide from gas streams. In
continuous cyclic processes fs~ removing sulfur dioxide ~-frcm gas
streams, the gas stream containing sulfur dioxide is contacted with
an aqueous absorbent in an absorption apparatus. The concen-
tration of sulfur dioxide in the gas~stream is reduced as the gas
stream passes through the absorption apparatus. The concentration
of sulfur dioxide in the aqueous absorbent increases as the -
absorbent passes through the absorption apparatus. As used herein,
the absorbent which leaves the absorpt$on apparatus is noted as
sulfur dioxide rich absorbent. Sulfur dioxide rich absorbents
usually contain from about 0.5 to about 30 grams of sulfur Y
dioxide per liter of solution. The sulfur dioxide rich absorbent
i5 then contacted with hydrogen sulfide to react with the sulfur
dioxide to form sulfur and to regenerate the absorbent. The
~6-
.~. , . , ~. ., --: ` .- -
:1~3~3~
aqueous absorbent which has beRn contacted with hydrogen sulfide
and has had the concentration of sulfur dioxide substantially
reduced is noted herein as regenerated absorbent.
The sulfur formed by the reaction of the hydrogen
sulfide with the sulfur dioxide in the absorbent is separated
from the absorbent. In a cyclic process the regenerated
absorbent is utilized as feed to the abso~ption apparatus.
The continuous cyclic processes usually do not
require that the sulfur dioxide be completely eliminated from the
aqueous absorbent. The processes do~ however, require that the
sulfur dioxide concentration in the regenerated absorbent ~be
sufficiently low that the sulfur dioxide concentration in the
gas stream can be reduced to the required level in the absorption
apparatus. Regenerated absorbents preferably contain less~than
about 0.002% bg weight sulfur dioxide and it is possible to
provide regenerated absorbents containing no detectable sulfur
dioxide. Re8enerated absorbents containing less than about 0.002%
by weight sulfur dioxide permit removal of sulfur dioxide from
gas streams to a level of less than about 250 ppm by weight.
In operation of a continuous cyclic process under more
or less constant liquid flow and gas flow and composition
conditions the re8enerated aqueous absorbent entering the ~-
absorption zone attains an equilibrium composition which is
characteristic of the particular absorbent solution and its
concentration. The sulfur d~oxide rich absorbent leaving the
absorption zone also attains an equilibrium composition.
-7~
1()39030 :~
Substantial regeneration of the aqueous absorbent requires that
the reaction between the hydrogen sulfide and the sulfur dioxide
rich absorbent provide a regenerated absorbent for feeding to the
absorption zone with the equilibrium composition. Ma~or cnanges
in the composition of the regenerated absorbent entering the
absorption zone indica~e that substantial regeneration of the
absorbent has not been achieved. Minor changes in regenerated
absorbent composltions are not unusual, can indicate small
deficiencies of hydrogen sulfide, and minor deficiencies in
process operation.
The process of the present invention is particularly
effective for regenerating aqueous absorbents such as waeer or
solutions of one or more salts of substantially
non-volatile acids with dissociation constants lying between
about 1 X 10 2 and I X 10 5 measured at a dilution of 40 liters
per gram mole at a temperature of 25C. Other acid salts can
also be utilized.
The quantities of acid and basic radical should be in
such proportions that the sulfur dioxide rich aqueous absorbent
is in a pH range of between about 2,8 and 4.5 and preferably
between about 3.0 and 4Ø The basic radical is usually alkali
metal, ammoniu~ or alkaline earth, but certain other basic
radicals which for~ water s~luble salts with;acids can be
utilized. Aqueous solutions of alkali metal or ammonium salts of
orthophosphoric acid, citric acid, glycolllc acid and succinic
acid and aluminum sulfate are particularly effective ~n cyclic
absorption processes~ The amount of basic radical in the
1~39~30
so]utjorl must be acljusted -to maintairl the pll Or the sulfur
dioxide rich absorbent between about 2.~ and about ~.5.
The absorbents are preferably aqueous solutions of the
acid salts containing between about 0.3 to about 2.5 moles of the
acid moiety per liter of solution.
The process of the present invention is an improved
method for reacting hydrogen sulfide in the sulfur dioxide
removal process as described heretofore. In the present process
the sulfur dioxide rich aqueous absorbent and hydrogen sulfide are
reacted by intimately intermixing the hydrogen sulfide with the
aqueous absorbent and cocurrently conveying the mixture under
turbulent flow conditions for a sufficient length of time to
substantially reduce the concentration of sulfur dioxide in the -~
aqueous absorbent. The turbulent flow conditions must be of
such a nature that the hydrogen sulfide is effectively dispersed
throughout the aqueous absorbent. Dispersed bubble or froth flow
is particularly effective in a horizontally disposed reaction zone,
but flow in which slugs of liquid are interspersed with slugs of
gas can be effective. Flow in which a continuous gas phase and
the more or less continuous liquid phase are in contact do not
provide the intimate contact required to provide for rapid
reaction and effective regeneration of the aqueous absorbent. In
a vertically disposed reaction zone in which the reactants are
cocurrently conveyed in an upward direction dispersed bubble flow
with little slip, slug flow and froth flow are particularly
effective. The particular flow patterns and methods for
estimating the flow characteristics of a particular mixture are
:~ - - -: . - . ~ ::
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.
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~391~)30 ~;`
shown in The Flow of Complex Mixtures in Pipes, G.W. Govier,
K. Aziz, Van Nostrand Reinhold Company, 1972. The flow ~.
conditions are critical to achieving rapid reaction and sub-
stantial regeneration of the absorbent Higher flow velocities
promote more rapid reaction in the process. If flow velocities ~ -
are too low, layer flow or bubble flow having high slip results
and regeneration of the absorbent is not satisfactory in short
reaction times.
The amount of hydrogen sulfide intermixed with the ^
aqueous absorbent in the reaction zone decreases as the hydrogen
sulfide reacts with the sulfur dioxide to form sulfur. The flow
. .
characteristics for the mixture can be expected to vary as the
mixture is conveyed through the reaction zone. However, velocity
of the mixture in the reaction zone must be sufficiently high to
maintain turbulent flow conditions during the reaction of the
hydrogen sulfide with the sulfur dioxide rich absorbent.
The turbulent flow conditions which are necessary in .~ ~;
the reaction zone must be such as to effectively provite inter-
facial contact between the gas and the liquid. ,~
The rate of reaction between the hydrogen sulfide and ~ -^
sulfur dl~oxide rich~aquebus absorbent lS extremely rapid. Under `~
hlghly~turbulent~flow conditions the reaction is substantially
complete ;in~as lit~tle as two seconds and the absorbent can be
.
substantially regenerated in as llttle as four seconds contact ~ ~ ~
time.~ Longer periods of contact are not harmful to the process ~ -
and are~usually bullt lnto the zyztem to accommodate variations ~i
.
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I, 1839030 i ~
, in flow rates of the reactants. Reaction zone residence times of
¦ from about 10 to about 300 seiconds are preEerred for operation of
' li the process. Longer contact times are not harmful to the process
¦ and are usually designed into the system to provide ~or small
¦ variations in feed rates of the reactan~s. The requirement for
, ~ slightly longer re~idence times than the minimum required is ¦
desirable when operating at small excesses of hydrogen sulfide
since small deviations in reactant flow can have a serious effect' ,-
, , on the regeneration of the absorbent.
,~ , I The reaction has been observed Ln a glass pipe reactor.
~' j The formation of yellow sulfur can be observed downstream of the I ,~
, ¦ point at which hydrogen sulfide is intermixed with the sulfur I '
¦ dioxide rich absorbent. As the aqueous sulfur dioxide rich ¦
¦ absorbent and hydrogen sulfide flow rates are increased the point
' ¦ at which the formation of yellow sulfur occurs moves nearer the
", point at which the reactants are intermixed. 1
The amount of hydrogen sulfide provided for reaction ,,
with the sulfur dioxide rich absorbent should be su~ficient to !
' regenerate the absorbent. It is preferable to provide an excess i
of hydrogen sulfide above that necessary to regenerate the
~,, ; absorbent. It is pre~erred to provide excess hydrogen sulfide in,
l the range of about 2 to about 10 ~ and most preferably in the ¦
,~ range of about 5 to 50%. Larger amounts of excess hydroyen
sulfide are not harmful to operation of the process.
~5 ~ Deficiencies of hydrogen sulfide, that is, an ainount .. ..
,`'~ j below that necessary to substantiall~ r,egenerate the aqueous
,
..;
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1~39031(~ ,
absorbent leads to the prescn~ of s~s~antial amounts o~ sulfu~:
dioxide and polythionates in the reyenerated absorbent. The
substantial amounts of sulfur dioxide and polythionatas have an
adverse affect on the ability of the absorbent to remove sulfur
dioxide from gas streams.
The process can be operated over a wide range of
temperatures. At a pH in the range of about 2.8 to 4.5 the
reaction between the hydrogen sulfide and the sulfur dioxide rich
absorbent is rapid in the turbulently flowing reaction zone over
a range of about 85 to about 195F. Higher or lower temperatures
can be utilized. At higher temperatures difficulties with the
vapor pressure of water come into play.
The process o~ the present invention is usually
conducted under superatmospheric pressure. The reaction between I
sulfur dioxide rich absorbent and hydrogen sulfide is enhanced if
the reaction is conducted under pressure. At higher pressurcs,
the reaction times are the shortest.
~ he process of the present invention requires that the
amount of hydrogen sulfide provided for contact with the sulfur
dioxide rich absorbent be at least sufficient to substantially
regenerate the aqueous absorbent. Gas streams containing
relatively low concentrations of hydrogen sulfide can be used in , -
the process. It is however preferred to utilize gas streams
containing at least 2 ~ hydrogen sulfide and preferably more than
5 ~ hydrogen sulfide,
~ 2
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10~90~0
Since i~ is prc~crable to utilize cxccss hydrogen
sulfide in the proc~ss, the excess hydrogen sulide can be
recycled to the reaction zone i~ the feed hydrogen sulfide is of
high purity. If the hydrogen sulfide stream is low in hydrogen
sulfide content, the excess hydrogen sulfide leaving the reaction
zone accompanied by the inert gases must be disposed of in some
other manner. Disposal of a hydrogen sulfide containing gas
stream is not difficult in operations in which hydrogen sulfide
is a product or byproduct. The excess hydrogen sulfide can be
readily handled in operations which have Claus process units or
wet process sulfuric acid plants. j ~ I
However, in operations where hydrogen sulfide is not
available, it is often necessary to manufacture hydrogen sulfide I ¦
from the sulfur recovered by the process. In this case, I 1-
disposal of substantial amounts of hydrogen sulfide or the l ¦~
burning of the hydrogen sulfide to sulfur dioxide and subsequent
recovery of the sulfur dioxide from a dilute gas stream merely
add an extra burden on the process. The excess hydrogen sulfide 1
can be recovered from the inerts in the gas stream by absorption i
and stripping methods and reused in the process or the process of !
the present invention can be operated in two stages in such a
manner that the aqueous absorbent is contacted with excess I
hydrogen sulfide with only small amounts of hydrogen sulfide
appearing in the inert gases venting from the process.
The reaction zone is preferably an elongated conduit
or pipe. The conduit can be positioned horizontally or
vertically. In large sizes, i.e , above about 2 or 3 inches in
, 1:3 i ''
,, 11 ~ .. , , . I
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i'. ~03903~ :
diame~er, it is pl-eEerred to dispose thc rcaction zone or
¦, conduit vertically.
¦ It has be~n found th~t buildup of sulfur on the
i surfaces of the reaction zone can be substantially reduced or
prevented if the portion of the conduit or pipe which contacts
the reacting solution is made of a non-sticking material such as
glass. Non-sticking materials are materials which are
¦ preferentially wetted by the water solution rather than molten
¦ sulfur. When a glass or glass lined pipe is utilized as the
reaction zone sulfur buildup can be prevented or substantially
I reduced if bends or turns in the pipe are long radius bends so
that the sulfur does not impinge at high velocity on the
conduit or pipe wall as the liquid stream is negotiating the turn
I or bend. The flow in the straight runs of the pipe or conduit
¦ reactor is of such a pattern that the flowing liquid stream does
not impinge on the wall at high velocity. A reactor fabricated
from glass pipe or a glass lined pipe substantially reduces or
eliminates the need for shutdowns to remove sulfur buildup from
the walls of the reaction zone. Sulfur buildup on the walls
and agitating means of the reactor have been a problem with
operation of stirred reactors. The agitator, reactor walls and
baffles become coated with large deposits of sulfur after short
operating periods. The agitated reactor must be shutdown to
remove the sulfur deposits. In stirred vèssels, due to the high
rates of agitation required, the newly for~led sulfur impinges on
¦i the reactor surfaces and agitation means at high velocity,
~i '
,. I . I .
ll !
1039030
b~l11ds up and lowcrr. the ~[cctiv~ness of th~ apparatus. The
- 1 sulfur is relatively sticky as it for~s or when it is in the
n~wly formed condition and a~leres to th~ agltated app~ratus and
¦ the sulfur already deposited to cause a substantial buildup to
.. 11
~, ~ occur.
It is pre~erred to use a pipe reaction zone in which
the surface in contact with the reacting absorbent is of non-
~7, sticking material such as glass. HoweverJ metal pipes or
conduits can also effectively be utilized. Since the reaction
s lO I times for the process of the present invention are relatively
.,
short, the reaction zones can be fabricated inexpensively and
setup in duplicate. The reaction zone can be steam jacketed
or heated by other means so that the reaction zone can be easily
,.; ¦ cleaned by application of heat to melt the sulfur. Of course,
L5 I the reaction zone should be designed so that the molten sulfur ca
.~ be removed from the system without clogging valves and pumps.
In large sizes the metal pipe reactor has an advantage :
~; ¦ in that the pipe reactor can be inexpensively steam jacketed and
¦ steam can be applied to the jacket continuously or intermittentlyl
?O to cause an~ sulfur which adheres to the walls of the pipe to
; ¦ become loosened and be carried from the reactor by the turbulently
I
7,~.,,, I flowing reactants. Since the surface area of a large diameter
pipe reactor is small in relation to the amount of material
~` I flowing through the reactor, the amount of heat transferred to the
1 liquid absorbent is relatively small. Operating in this manner,
the large diameter pipe can be effectively cleaned without shutting
; 1I down-the operation. -- ~
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~, 1/ Figure 1 illustrates an embodiment of the process of
j, the present invention. Sulfur dioxide rich aqueous absorbent
; I enters the pL-OCeSS through line 1 and flow meter 2. l~e liquid
¦I passes to pump 3 . Pump 3 forces the sulfur dioxide rich absorbent
¦, through reac~ion zone 5 under turbulent flow conditions. Reaction
¦I zone 5 is a pipe preferably glass~ glass lined or steam jacketed.
Reaction zone 5 can be disposed horizontally, vertically or at any
convenient attitude. Hydrogen sulfide is introduced into
~,, ~ reaction zone 5 through injection nozzle 4. The hvdrogen sulfide
- and the sulfur dioxide rich aqueous absorbent are rapidly
¦ intermixed, react and flow under turbulent flow conditions
',' I through reaction zone 5 into vessel 6. The flow in pipe reactor
,, ¦ 5 is maintained in turbulent flow conditions so that the hydrogen
.,, . 8Ul fide and sulfur dioxide rich absorbent are rapidly intermixed
~,5 I and reacted.
,, I The absorbent containing finely divided sulfur is
separated from the excess hydrogen sulfide and inert gases in
~,,` ~ vessel 6 which is provided with agitator 9 which maintains the
sulfur suspended in,the aqueous absorbent.
~i ! Agitator 9 and vessel 6 can also provide for short
~, ¦ interruptions of hydrDgen sulfide flow during which the amount of
~A~'$ I hydrogen sùlfide may drop below the amount necessary to
substantially regenerate the absor~ent. The vessel holds up a
sufficient volume of absorbent to permit the aqueous absorbent to'
~5 I contact excess hydrogen sulfide which subsequently enters the
reaction zone. This is particularly important when the ratio of '
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1~39~3C)
; hydro~en sulfide to aqueo~ls absorben-t liquor is m~intained close
to that stoichiome-trica],ly required to regenerate the aqueous
,, absorbent.
.
~'' I,ine 7 with control valve 8 which is controlled by
flow meter 2 is utilized to maintain a minimum flow in pipe
~, reactor 5. When flow in line 1 drops below the value necessary
.,
to maintain the necessary -turbulent flow condition in pipe
,` reaction 5, control valve 8 opens to permit regenerated aqueous
~,' absorbent to flow through line 7 and mix with the sulfur dioxide
, rich aqueous absorbent. The additional liquid provides
additional liquid volume feed to reaction zone 5 to prevent two
~ layer flow from occurring in the reaction zone. It is necessary for
,', operation of the process of the present invention that the
'' flow rate in reaction zone 5 be maintained at a level at which
''' the liquid and gas phases are intermixed under turbulent flow
, .:
conditions.
... .. .
Sulfur and regenerated absorbent leave vessel 6
through line 11 and can be passed to means for separating the
', sulfur from the regenerated absorbent liquor (not shown). Sulfur
can be separated from the regenerated absorbent liquor by
,~' means such as filters, flotation separators, by heating the
mixture under pressure to a temperature above the melting point ~'
of sulfur and separating molten sulfur from the liquor or other
.
; means or combination of means for separating sulfur from aqueous ';
liquors.
` The excess hydrogen sulfide and inert gases (which can
~'-' be present in the hydrogen sulfide) leave vessel 6 through line 10.
~; ,. . .
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I Line 10 is provided with pressure control means 12 to m~intain
¦l an elevated prcssure in vessel 6 and reactor 5. The excess
hydrogen sulfide can be disposed of by burni~ to sul~ur dioxide
and absorption in the aqueous absorbent, can be recycled through
line 4 for further reaction with the sulfur dioxide rich aqueous
absorbent, or can be separa~ed from the inert gases and recycled
to the reaction zone. In any event, since hydrogen sulfide is
t", ¦ usually not obtained as a 100~ pure material, it is usually
i ¦ necessary to vent a portion of the hydrogen sulfide from the
.; system. The hydrogen sulfide in the vent gases can be burned
or separated from the inert gases and recovered.
A pressure control valve is usually present in line 10
¦ to maintain an elevated pressure on the reaction systems. Ele-
~; ¦ vated pressure provides for faster reactions in the process.
¦ The interior of pipe reactor 5 is preferably made of
¦ non-sticking material such as glass to reduce the tendency for
¦ sulfur to buildup on the walls of the reactor.
~; Since the aqueous absorbent can be substantially
regenerated in pipe reactor 5 the major portion of the sulfur is
.~ ¦ formed under the tur~ulent flow conditions and there is little
¦ tendency for sulfur to buildup on the walls of the vessel 6.
here is little tendency for sulfur to buildup on agitator 9 or I .
walls of vessel 6 and the system can operate for extended
¦ periods of time without need for shut downs to remove sulfur
,~ ~ I buildup . - ~ 18
.. ,.. ~ I ...
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~039030 1 `:
l~ Fi~ure ~ illustrates an apparatus suitable for I ;
~I carrying out the process of the invention in a vertically
; i disposed reaction zone.
SeJ~arator vessel 15 is mounted at the ends of flow loop
~4 which com~rises a vertical do~flow leg 25 and steam jacketed
reaction zone 16. The reaction zone 16 is a ~team jacketed pipe
¦ sufficiently long to permit reaction of a substantial portivn of
¦ the sulfur dioxide in the sulfur dioxide rich absorbent, with the
hydrogen sulfide. The diameter of the reaction zone 16 is such
~ that the flow in the reaction zone lS turbulent and the gas and I ~
i' liquid form an intimate mixture or a froth in the lower section. , I -
; Downflow leg 25 is a pipe which can be of larger
:~ ¦ di~meter than reaction zone 16. The downflow leg can be of
,1, ¦ larger diameter than reaction zone 16 since t~e flow in down- ~ -
flow leg 25 does not have to be maintained under turbulent
¦ conditions. Downflow leg 25 can be steam jacketed to provide forj
ease of c1eaning but the sulfur buildup in this area is low.
¦ Reaction zone 16 is steam jacketed with steam entering
~: I through line 27 and condensate leaving through steam trap 28.
, p Valve 26 is also steam jacketed to permit removal of melted sulfur
from the reaction zone if the reactor is cleaned while the
, reactor is not in operation.
In operation sulfur dioxide rich absorbent enters
~i ~ reaction zone 16 through inlet line 17 which is pointed in an
I upwardly direction to utilize the velocity of the entering liquid
to encourage upward flow in reaction æone 16. Hydrogen sulfide
, enters reaction zone 16 throug~l-inlet line 18 which is pointed in
o~ 9
; -
c~
' ~L03gO3
~! an upwardly direction. In continuous operation, sulfur dioxide
ri~h absorbent and hydrogen sulfide enter the reaction zone
simult~neously. ~le velocity of the entering reactants and the
1, diference in density in leg 25 and reaction zone 16 caused by
; ¦', the gas dispersed in the liquid causes the liquid 22 in
¦ separator 15 and flow loop 2~ to flow down in leg 25 and up in
, I reaction zone 16. The sulfur dioxide content of the sulfur
dioxide rich absorbent is substantially reduced in the time the
reactants are in reaction zone 16.
Any excess hydrogen sulfide or inert gas l S separated
~; ! from the liquid in separator 15. The excess hydrogen sulfide or
I inert gas passes through entrainment separation means 23, vent
~' ¦ line l9 and pressure control valve 20 which maintains the reaction
zone under an elevated pressure. Entrainment separation means 2~J
I vent line 19 and pressure control valve 20 can be steam jacketed
¦ or heated (not shown) to prevent sulfur carried with liquid
entrainment from blocking this vent line. The gas passing through
pressure control valve 20 can be treated to remove any hydrogen
I sulfide present before being vented to the atmosphere.
~j ~ A slurry of sulfur and substantially regenerated
i.~, I absorbent is removed from separator 15 through line 21. The
- j regenerated absorbent flow in line 21 must be controlled to
, maintain a liquid level in separator 15.
$.~ , Steam can be applied to the jacket of reaction zone 16
~ continuously or intermittently to remove any sulfur deposits
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which may tend to accumulate in the reaction zone. The portion of
reaction zone 16 which is in contact with the reactants can be
made of non-sticking ma~erial such as glass and the steam jacketing
1, or heating eliminated. Non-sticking materials are those materials
j I which are substantially more readily wetted by water than by
molten sulfur.
Reaction zone 16 is a long conduit in relation to the
diameter. Reaction zone lengths from about 1~ to about 80
feet are not unusual depending upon the flow rate and regeneration
¦ requirements.
The vertical flow loop reactor is useful in t~at the
reactor can be designed to maintain sufficient liquid flow in
¦I reaction zone 16 under variation in flow of sulfur dioxide rich
absorbent. The vertical flow loop reactor has the disadvantage
¦ that the sulfur dioxide concentration in the liquid is reduced
by dilution with the recirculating material.
, I Figure ~ is an embodiment of a reaction system which is
¦, useful for reacting substantially all of the hydrogen sulfide
from gas streams containing inert gases. The embodiment
illustrates a combination of the reactor illustrated in Figure 1
with the vertical flow loop reactor illustrated in Figure 2. ~he
i vertical flow loop reactor can be replaced with a stirred kettle
-~ , reactor or other reaction means since a large excess of hydrogen , ;
¦ sulfide can be utilized in the system. The holdup of absorbent
` 5 1l liquid in the second reaction zone,along with the large excess
of hydrogen sulfide ensures substantial regeneration of the 1,
~" ¦l absorbent. _ 21
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, In op~ration sulfur dioxide ri.ch absorhent enters the
,! process through line 30 and flow meter controller 3~. Flow
meter controller 33 measures the flow in line 30 and admits
I' regenerated ahsorbent through line ~1 and control valve 32
¦, should the flow of sulfur dioxide rich absorbent fall below the
I mini.mum required to maintain the necessary flow condition in
, . i
reaction zone 3'7. The sulfur dioxide rich absorbent and regenerated
absorbent if required are passed through line 30 to pump 34 and
: I enter reaction zone 37. If the sulfur dioxide rich absorbent and
,. regenerated absorbent are at a sufficiently high pressure, pump 34
,; I may not be necessary.
Hydrogen sulfide enters reaction zone 37 through line 56
~, I and control valve 36. Control valve 36 is actuated by hydrogen
.~ , sulfide monitor 42 in vent line 55. The amount of hydrogen
~,5 sulfide in the vent gas can be monitored to control the amount of .
; i unreacted hydrogen sulfide leaving the system to a relatively
low level by controlling the amount of hydrogen sulfide entering
. ~ the reaction zone 37. .¦
Reaction zone 37 is a conduit which can be disposed
~: ¦ vertically, hor.izontally or at any convenient attitude and which is
designed to provide for turbulent flow of the liquid and gas and
. I for intimate intermixing of the materials. The length of reaction
. I zone 37 is such as to provide for a su~stantial reduction in the
sulfur dioxide concentration in the sulfur dioxide rich absorbent
~''5 and to react substantially all of the hydrogen sulfide from the
~$.,' gas stream entering reaction zone 37.
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, ~039030
The liquid and gas are separated in separa~or 3~' w]lich
has agitating means ~ to maintain the sulfur in suspension.
~ ~ Unreacted gases pass out of the system through line )~o and
- I pressure control valve 41 which maintains an elevated pressure
3 I in reaction zone 37. The unreacted gases pass through hydrogen
¦ sulfide monitor 42 and vent line 55 to disposal means or a flare.
The slurry of sulfur in partially regenerated sulfur
dioxide rich absorbent passes out of vessel 38 through line 43
1l and is pumped b~ pump 44 through line 45 into reaction zone ~6 in !
D I vertical flow loop reactor 54.
Hydrogen sulfide feed to the process enters the reaction'
system through line 51 and control valve 52 which is operated to
maintain a predetermined pressure on separator 47 and reaction
zone 46. The hydrogen sulfide enters line 57 from control valve
52, mixes with recirculating hydrogen sulfide in line 58 and
! enters reaction zone 46 through line 53. Hydrogen sulfide is
I recirculated through the reaction zone at a high rate by ¦
.' pumping hydrogen sulfide from gas liquid separator 47 through line
49 and gas mover 50. A portion of the gas from gas mover 50 is
passed to reaction zone ~7 through line 35 and control valve 36.
The reaction æone 46 is vertically disposed and designed
to provide intimate intermixing between the hydrogen sulfide and
: aqueous absorbent. The reaction zone is of su~ficient length to ! :
provide a substantially regenerated absorbent.
5; A slurry of sulfur and regenerated absorbent is removed
from the system through line 48. The sulfur is separated from the
- 2~ i -
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1 i~39030
re~enerated ~bsorbcnt and the re(~enerated ahsorbent is utilized
~, aS liquid feed to th~ absorptic)n zone.
¦~ ~le vertical flow loop reactor can be replaced by
, othcr forms of reactor since the amount of unreacted sulfur dioxide
in the absorbent entering the second rcaction zone 46 is low and
, the amount of excess hydrogen sulfide being circulated through
¦I the reaction zone can be high. However, the vertical flow loop
reactor due to its simplicity is preferred for this application.
, ¦ The embodiment illustrated in Figure 3 is particularly
useful where gas streams low in hydrogen sulfide content are
¦ utilized to regenerate the aqueous absor~ent. In the first
reaction zone ~7 the sulfur dioxide rich absorbent having the
, highest concentration of sulfur dioxide is contacted with the gas , i
'~''; i
stream having the lowest concentration of hydrogen sulfide to
provide a gas stream containing only sma]l amounts of hydrogen I
sulfide. The reaction zone ~7 is usually operated with a I If
deficiency or up to a stoichiometric amount of hydrogen sulfide I ,
to ensure that the hydrogen sulfide has been reacted to a low
level in the gas stream leaving the system.
The aqueous absorbent is further contacted with a large
excess of hydrogen sulfide provided to reaction zone 46 by
recirculation of hydrogen sulfide.
The system therefore provides two desirable features,
that is, substantial reaction of hydrogen sulfide from the vent I ¦
gases and contacting of the aqueous absorbent with a large excess
of hydrogen sulfi.de. I
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i. 103903~
~IC ~)~ocesS will bc illustra~ed by refercnce to the
following examples:
!l E~PI E
~ ¦' An apparatus similar to the arrangement shown in
.: 5 ¦ Figure 1 is utilized to re~enerate sulfur dioxide rich aqueous
absor:bcnt from a cyclic absorption process utilizing a packed '
colunn to absorb sulfur dioxide from a gas stream. The reaction
¦¦ zone shown as 5, in the Figure, is a l/2 inch inside diameter ¦
glass pipe, approximately 25 feet long. The pipe is in the
! configuration of a "U" as shown in the figure. The separation
vessel 6 is a glass lined vessel 12 inches diameter by 40 inches
¦ tall having a capacity of about 15 gallons. The sulfur and
regenerated absorbent overflow to maintain about 9 gallons of
a slurry of sulfur and regenerated aqueous absorbent in
~'5 j separation vessel 6. The slurry of sulfur and regenerated
absorbent passes to a means for separating the sulfur from the
,/ I regenerated absorbent and the regenerated absorbent is utilized
as feed to a packed absorption column. Agitator 9 is a 6 bladed
turbine 4 inches in diameter extending to within 12 inches of the
bottom of the vessel. Agitator 9 rotates at 500 rpm. The
¦ reactor 5 discharges into vessel 6 at a point 5 inches from the ' ;;
¦ bottom of the vessel.
¦ Hydrogen sulfide is introduced into reaction zone 5 -
` ¦ through a 1/4 inch diameter nozzle which extends into the liquid ,
¦ stream. The hydrogen sulfide enters the liquid stream at a point
,. I in the straight portion of the glass pipe. The hydrogen sulfide ~ -
! ¦! enters the reaction zone at o.67 cubic feet per minute at 23
psig. This represents an excess of hydrogen sulfide of 133~.
25 -
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Sulf`ur dioxide rich aqueous absorbent is fed to the
reactor a-t a rate oF 3 gallons per minute. The absorbent is a 1
molar sodium phosphate solution having a Na:P ratio of 1.5
exclusive of sodium associated with sodium sulfate which is
present at 60 grams per liter.
The aqueous absorbent is sampled at the inlet to the
reaction zone, at a point 12 feet from the point where the hydrogen
sulfide is admitted to the reaction zone, at the outlet of the
pipe reaction zone and at the feed point to the absorption column.
Table 1 reports the analysis of the liquid stream at
the four sample points during operation of a continuous cyclic
process.
TABLE
S 0 Grams S02 Grams S 06 Grams
Sample Location er31it per liter ~ liter
Pipe reactor inlet 4.59 3.34 1.12
Pipe reactor 12 feet
from H2S inlet 5.78 0
Pipe reactor outlet 5.45 0 0.48
Absorption column - 0 0.45
inlet
,
Temperature at pipe reactor inlet 165 F; pH of aqueous
sulfur dioxide rich absorbent at pipe reactor inlet 3.5.
The liquid has a superficial velocity in the reactor
of 4.9 feet per second. The actual velocity is much higher due
,,~ .
to displacement of the liquid with the bubbles of hydrogen
~ sulfide.
t The example shows that the absorbent solution is
;
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~39~30
substantially regenerated in the short reaction time of less than
5 seconds in the reaction zone. The absence of sulfur dioxide
in the aqueous absorbent and the lowering of the polythionate
le-~el to the same as the polythionate level in the absorbent
entering the absorption column indicates that regeneration is
substantially complete in the short residence time in the
reaction zone.
EXAMPLE 2
The reactor of Example 1 is utilized to regenerate
1 gallon per minute of sulfur dioxide rich absorbent. Valve 8
in line 7 is maintained in the closed position so that no liquid
is recycled,
One gallon per minute of sulfur dioxide rich absorbent
is fed to the reaction system. Hydrogen sulfide enters the
reaction zone at 0.40 cubic feet per minute at 13 psig. The
excess hydrogen sulfide entering the reaction zone is 200%. -~
The aqueous absorbent is of approximately the same
composition as the absorbent of Example 1. Samples are taken at
the same points as in Example 1. Analysis of the samples are as
follows:
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S203 Grams S~2 Grams S 06 Grams S04 Grams
Sample location per 1iter per liter ~liter per liter
Pipe reactor inlet 4.59 3.34 1.12 67
Pipe reactor 12 feet 6.09 0.25 - -
f`rom H2S inlet
Pipe reactor outlet 4.92 0.21 0.63
Temperature at pipe reactor inlet 165F.; pH of aqueous
sulfur dioxide rich absorbent at pipe reactor inlet 3.5.
The sulfur dioxide and polythionate levels in the
absorbent solution at the outlet of the reactor lndicate that
the absorbent has not been substantially regenerated. The
residence time of the liquid in the pipe reactor is approximately
three times that of the previous example. The absorbent solution
is not regenerated because the velocity of the liquid in the
reactor is so low that intimate intermixing of the gas and liquid
is not achieved. At low liquid flow rates the hydrogen sulfide
does not adequately contact liquid absorbent. When the liquid
velocity in the reaction zone is too low to provide for
intimate contact between the hydrogen sulfide and the sulfur
dioxide rich absorbent the aqueous absorbent is not substantially
regenerated even at long residence times in the reaction zone.
A flow rate of between about 2.25 and 3 gallons per minute is
. .
the minimum liquid flow rate for achieving intimate contact with
, horizontal flow in the one-half inch internal diameter pipe
reactor.
:
.
-28-
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~039Q30 ;
EXAMPl.~ 3
'I`he reactor of Example 1 is utilized to regenerate
1 gallon per minute of sulfur dioxide rich absorbent. Valve 8
in line 7 is opened to provide sufficient recycle to maintain a
flow of about 3 gallons per minute in the pipe reactor. One gallon
per minute of a ~ulfur dioxide rich 1.21 molar sodium phosphate
solution having an Na:P ratio of 1.5 exclusive of the sodium
associated with sodium sulfate is mixed with about two gallons
per minute of sulfur and substantially regenerated absorbent from
vessel 6. Hydrogen sulfide of 96 percent purity at the rate of
0.31 actual cubic feet per minute at 25 pounds per square inch
. .
gauge is fed to the reaction zone. The hydrogen sulfide is
present in 197 percent excess. Samples are taken at the same
points as in Example 1. One additional sample is taken before the
,.
; sulfur dioxide rich absorbent is mixed with the regenerated
absorbent. Analysis of the samples is as follows:
~, TABLE 3
.i! = = ~
S20 Grams S02 Grams S O6 Grams S04 Grams
~ Sam~le location per31iter per liter per liter per liter
i Absorbent outlet 14.71 5.2 1.3
; absorption column
,Pipe reactor inlet 15.72 1.25 0.99
(after mixing) -~
Pipe reactor 12' 15.38 0.10 0.40
from H S inlet
Pipe reactor outlet 15.27 0.03 0.40
b`
Absorption column ' 14.82 0 0.67 48
inlet
;~ Temperature at pipe reactor inlet 165 F.; pH of aqueous
; sulfur dioxide rich absorbent at pipe reactor inlet 3.4.
In operations over a three day period no substantial
buildup of sulfur was observed in the glass pipe reactor.
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1039~30
'Ihe absorbent is substantially regenerated in the short
residence time in the pipe reactor. The sulfur dioxide concen-
tration is subst~ntially redu~ed and the polytl~ionate level is
reduced to the equilibriurn level f`or the conditions of the
operation of the absorber-reactor system.
,:
EXAMPLE 4
;' .
The apparatus of Example 1 is utilized to conduct the
following experiements.
Sulfur dioxide rich 1.0 molar sodium phosphate absorbent
having a molar ratio of Na:P of 1.5 is introduced into the pipe
reactor at a rate of 4.5 gallons per minute. A gas stream
~; containing 75% hydrogen sulfide enters the reaction zone through
~' jet 4. The hydrogen sulfide is fed to the reaction zone at a
rate of 0.47 cubic feet per minute at a pressure of 28 pounds
per square inch gauge. The hydrogen sulfide flow represented
30% excess. Sulfur dioxide is not detectable in the aqueous
~i; absorbent at a point 12 feet from the hydrogen sulfide inlet
point. Analysis of the aqueous absorbent at the outlet of the
~; reactor indicates that the polythionate level has been reduced to
~::
the same level as the feed to the absorption column. Analysis of
~ the samples is as follows:
,,; TABLE 4
,
S203 Grams S0 Grams S 0 Grams S0 Grams
~ Sample location per liter per liter ~ liter ~_4r liter
; Pipe reactor inlet 12.78 4.37 0.84 64
;;' :'
Pipe reactor outlet 13.04 0 0.43 64
~..
:~ Temperature at pipe reactor inlet 130 F.
~,.
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~39~30
'I'he foregoing examples illustrate clearly that the
flow rate is critical to the process of the present invention in
that it must be sufficiently high to intimately contact aqueous
absorbent with hydrogen sulfide to regenerate the absorbent.
, At high liquid flow rates contact times can be short. When the
. ~ ,
flow rate is too low, substantial regeneration of the aqueous
absorbent is not achieved at relatively long contact times. .
LXAMPLL 5
. .
The apparatus u-tilized to regenerate the sodium
phosphate absorbent in Example 1 is utilized to regenerate a
sodium citrate absorbent. The absorbent is a 1 molar aqueous
solution of sodium citrate with a Na+:citrate ratio of 1.5:1.
stllfur dioxide rich absorbent containing 4.5 grams S02 per liter
is fed to the reaction zone at a rate of 3 gallons per minute.
A 100 percent excess of hydrogen sulfide above that necessary to
substantially regenerate the absorbent simultaneously enters the
reaction zone. Sulfur dioxide is not detectable in the aqueous
absorbent at the outlet of the pipe reaction zone. The foregoing
examples illustrate that the process of the invention is effective
for rapidly reacting hydrogen sulfide dioxide in
aqueous absorbents. The process lends itself to the substantial
reduction and control of sulfur buildup on the reaction apparatus.
.. . .
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