Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
This invention relates to flue gas desulfurization processes and
more particularly to a process of regenerating sulfur dioxide absorbed in
buffered solutions used to scrub sulfur dioxide from gases such as flue gasO
Presently available flue gas desulfurization processes include
so-called thro~away systems, utili7ing limestone or lime scrubbing, as well
as regenerative processes, utilizing buffered aqueous solutions for sulfur
dioxide absorption~ that yield sulfur as a byproduct. One such recgenerative
process, using a buffered citrate solution to scrub sulfur dioxide from a
gas in a co~mtercurrent absorber and reacting the sulfur dioxide-laden liquor
from the absorber with hydrogen sulfide to recover sulfur, is kno~n by the
name, CITRE~, a trade mark o~-Peabody Engineered Systems, Stamford, Connecticut.
A description of the CITREX process and a discussion of its advantages over
throt~-a~ay msthods such as limestone scrubbing appears in "The CITREX Process
~or SO Removal", Chemical Engineerin~ Pro~ress, VolO 71, No. 5, May 1915.
A limestone process of flue gas desulfurization has the drawbacks
of sludge disposal and high raw material cost of lime or limestoneO By con-
trast, the citrate process, especially as modified in the CITREX process,
! provides advan~ages such as sulfur recovery as a byproduct as ~ell as lower
initlal and operating costsO Nevertheless, the use of hydrogen sulfide to
convert the sulfur dioxide absorbed in the citrate solution to yleld sulfur
and water may not be commercially attractive in all si-tuations. Thus, where
it is neccssary to synthesize hydrogen sulfide at a plant site -from hydrogen
produced from natural gas, the increasing una~ailability of natural gas or
similar raw material for the production of hydrogen makes such a process less
desirablc O
In United States Patent No~ l,589,l33 to Eustis there is disclosed
a method of recovering sulfur dioxide from smelter smoke or other gases by
absorbing the sulfur dioxide in a solution of a metallic salt, such as
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aluminun sulfi~0, which will form a relatively uns-table sulfite or bisul~ite
with the sulfur dioxide and which will readily liberate the sulfur dioxide
gas at moderate temperatures. The sulfur dioxide gas is extracted rom the
solution by diluting the ~tmosphere in contact with the solution and conse-
quently reducing the partial pressure of the sulfur dioxide in the gases or
atmosphere contacting with the solutionO The patentee states that this is
done in an extractor in~o the bottom of which is directly fed live steam and
into the top of which is fed the sulfur dioxide containing solution. Ry mak-
ing the extractor very lar$e and prolonging solution dwell time a large
percen~age of the total sulfur dioxide is said to be extracted at each cycleO
The extraction may be carried out under pressuresbelow atmospheric, using a
vacuum pump, although a vacuum is not necessary as dilution of the atmosphere
resulting in reduction of the partial pressure of the sulfur dioxide is said
to work satisfactorily when the extraction is carried on at, or even above,
atmospheric pressureO
The use of live steamJ as in Eustis, is attendant with several dis-
advantagesO The steam used must be produced from water which has been treated
to avoid contaminating the stripping system or the constituents therein,
resulting in added expenseO Also, the live steam condenses in the system and
dilutes the solution absorbing the sulfur dioxide so that either further
separation or waste discharge is requiredO The former is uneconomical while
the latter is impractical under current environmental procedures as well as a
costly use of raw materialO
I haYe ~ound that the advantages of sulfur dioxide absorption by
the citrate and similar bufered aqueous solution processes can be recognized
without the need for reaction with a reducing gas, such as hydrogen sulfide,
or regenerative reactionO This is achieved, according to this invention,
through the provision of a process in which sulfur dioxide is regenerated
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from the sulfur dioxide-laden absorber liquor by steam stripping under
pressureO
The present invention provides in a process for ~he desulurization
of gases wherein sulfur dioxide is removed from the gases by absorption in a
citrate bufered aqueous solution hich undergoes a regenerative process to
yield sulfur-based byproducts, the improvem0nt resulting in the recovery of
sulfur dioxide from the citrate buffered aqueous solution comprising heating
the aqueous solution containing absorbed sulfur dioxide to a temperature above
212F~ and to strip the sulfur dioxide therefrom, maintaining the pressure
substantially greater than atmospheric pressure, and recovering sulfur dioxide
vapor.
The present invention also provides a process for regenerating
sulfur dioxide rom an organic buffered aqueous solution in which it is
absorbed comprising heating the aqueous solution to a temperature substantially
in excess of 215F~ maintaining a pressure of from 15 to 65 psig above the
solution, recovering sulfur dioxide and water vapor, and then condensing the
sulfur dioxide and water vapor in a hea~ exchanger in the absence of drying,
refrigeration or compression and obtaining liquid sulfur dioxide.
In a preerred embodiment there is provided acccrding to the
2Q invention the process of recovering sulfur dioxide gases comprising the steps
of contacting the gases containing sulfur dioxide wi~h an organically buffered
aqueous solution to absorb sulfur dioxide in said solution, then heating said
solution containing sulfur dioxide to a temperature greater than 212Fo and
under pressure substantially greater than atmospheric pressure and then
recovering and condensing the water vapor and sulfur dioxide for separation
and recoveryO
The present invention also provicles the process o~ removing sulfur
di.oxide from gases comprising the steps of contacting the gases containing
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sulfur dioxide with a buffered aqueous solution containing one or more organic
radicals taken from the group consisting of citrate~ glycolate, glyoxalate
and acetate, then heating the solution to a temperature of from 250Fo to
310F~ at a pressure of from 15 to 65 psig, and then removing and condensing
the water vapor and sul~ur dioxide for separation and recovery.
It was unexpectedly discovered, according to this invention, that
steam stripping at super atmospheric pressure~ for example, in the advantage-
ous range of 5 to 65 psig, results in a lowered steam requirement per pound
of sulfur dioxide stripped as compared to atmospheric or slightly above
atmospheric pressure operation. Thus, it has been surprisingly discovered
tha~ apparently the decomposition of a sulfur dioxide citrate complex to
yield sulfur dioxide proceeds faster and more eficientl.y a* high temperatures
corresponding to high pressures than does the contervailing effect of the
high pressure on the solubility o~ sulfur dioxide in the citrate buffered
solutionO Moreover, the pressure stripp.ing process of this invention yields
stripped sulfur dioxide which can be di.rectly condensed wi~h ordinary cooling
water to produce liquid sulfur dioxide as a product without the need for dry-
ing, refrigeration or compression systems~ The overhead stream from the
stripping process is at a sufficiently high temperature to preheat air that
can be subsequently used for direct mixing and reheating of the cold treated
flue gas s-tream from the absorber, resulting in greater overall process
economies. Also, the pressure stripping process results in actual gas flow
volumes which are several times less than those present in low pressure strip-
ping so that the stripping column may be a more compact unit and initial equip-
ment costs can be reducedu
~ccordingly, a feature of this invention is the provision of a
process for the stripping of sulfur dioxide from buf~ered solutions in which
it is absorbed by stripping under pressureO
3~
Advantageous pressures for khe st~am stripping of sulfur dioxide
from citrate liquor loaded-with sulfur dioxide is in the range of 5-65 psig
and preferably 15-65 psig. At pressures of 15-65 psig, the high temperature
of the steam, in the range 250-310F.~ accelerates the rate of sulfur dioxide
release and more than balances any tendency of sulfur dioxide to go into
solution at the high pressure involved. This higher sulfur dioxide release
results in steam requirements in the range of 5 to 8 pounds steam per pound of
sulfur dioxide stripped in contrast to steam requirements which are several
fold higher at low pressure operation, such as at atmospheric or slightly
above. At atmospheric pressure or slightly above, the overhead sulfur dioxide
gas stream requires a dryer and refrigeration system or compression system for
recovery of st~ipped sulfur dioxide as a product liquidO
In the pressure stripping process according to this invention, the
sulfur dioxide stripped off at 15-65 psig can be directly condensed with
ordinary cooling water having a temperature in the range of 50 to 90Fo to
produce liciuid sulfur dioxide and thus avoiding prohibitive dryer and refriger-
ation or compression power consumption requirementsO The overhead stream
leaving the rectification section of the stripping vessel is at a temperature
of about 240 to 290Fo and can be used to preheat air which in turn can be
used for direct mixing with and reheating of the cold treated flue gas stream
from the top of the absorber to heat it from about 120 to 1~0F~
~hile the ~ressure range of 5-65 psig is advantageous and 15-65
psig is preferred, it will be understood that the acl~antages of the invention
are recognized at pressures above atmospheric generallyO The limiting facts
are the decomposition of the bufered solution which can occur, for example
at 310Fu, which corresponds to about 65 psig, and the availability of low
temperature cooling water to condense the sulfur dioxide vapors. For example,
oOFO water would permit operation at 25-30 psigO
The steam economy in the pressure stripping process of this inven-
tion is such that the process requires only about one-~hird to one-half the
fuel needs of comparable currently available fuel desulfurization processesO
The use of the sulfur dioxide pressure stripping process according to this
invention allows the flue gas desulfurization system to be compact, since
regenerative equipment is reduced, and, since this is an all liquid system
with no solids or sulfur to plug or cake equipment, it is a clean and simple
system to operateO Thus, the process can be operated for extended periods
with reduced operational and maintenance laborO Hence~ economies result from
this invention in several areasO Initial equipment costs are lower because
of the compact regenerative system, the high operating availabili~y reduces
costly down-time and the ability to be operated by one operator results in
lower labor costs, all of which are in addition to the fuel economy described
aboveO
Thus, a further feature o~ this invention is the provision of a
process for steam stripping sulfur dioxide from a solution resulting from a
fuel gas desulfurization process which results in increased reliabil.ity and
reduced costO
The foregoing and additional features, objects and advantages of
this invention will be further apparent from the following detailed descrip-
tion of a preferred embodiment thereof taken in conjunction with the accom-
panying drawing~
The Figure is a schematic diagram illustrating the process scheme
for high pressure steam stripplng of sulfur dioxide absorbed in a buffered
solution in a flue gas desulfur.ization process, according to an embodiment of
this inventionO
~ferring to the Figure, there is shown hot flue gas represented
at 2 entering a scrubbing system 4 in which the gas is cleaned and cooledO
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The scrubbing system 4 may advantageously be of ~he venturi type which is
commercially available and a further description of which is unnecessary to
the present invention except to point out that the flue gas is cleaned out
particulates and cooled by water entering the system 4 at 6 and particulate
material such as fly ash is removed as illustrated at 8.
Gas leaving the scrubber system 4 is passed upward through the
absorber 10 countercurrent to a down-flowing buffered citrate solution intro-
duced into the absorber 10 at 12 and distributed in a tray-type or packed-bed
colu~n absorber schematically shownO The absorber 10 may advantageously be a
Peadboy Tray Absorber having trays 14 which provide high efficiencies o~
sulfur dioxide removal and low L/G ratiosO The clean waste gas is tangentially
mixed with heated air, shown at 16, and exits the absorber at 18 while the
sulfur dioxide-laden absorber liquor exits the absorber bottom at 200
~fter leaving the absorber the sulfur dioxide-laden citrate liquor
is pumped, such as by the pump 22 to enter the ~op, at 24, of the pressure
stripping vessel 26, after passing through the economizer 28 where it is heated
: by the hot bottoms ~rom the stripping vessel~ Alternately, the absorberliquor may partially bypass the economizer 28 and be fed to the top of the
stripping vessel 26, .in the rectification sectionO Within the stripping
vessel 26 the absorber liquor flows downward over either a packed bed, or
a~vantageously, over trays schematically represented at 30. Vapor flows up-
ward through openings in the trays as a result of heating of the solutionO
Heating takes place in the stripping vessel bottom by means of circulation
through the reboile-r 320 The reboiler 32 is heated by steam under pressure
entering at 34 ~or .indirect heating of the sul.~ur dioxide laden liquor cir-
culating through the lines 36 and 380
The buffered citrate solution from which the sulfur dioxide has
been stripped exits the stripping vessel 26 at 40 and is pumped, as by the
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~5~313;~
pump 42, to reenter the absorber 10 as feed 12 after passing through the
economizer 28 and the heat exchanger 44 where it is fur~her cooledO The
stripped sulfur dioxide and water vapor or steam leave the top of the strip-
ping vessel at 46 and enter the heat exchanger 4B where they are condensed by
ordinary coolng water, to the liquid state and pass to the tank 50O The
liquid sulfur dioxide and water separate into two phases within the tank 50.
The heavier sulfur dioxide phase is taken off as a product at 52, rom whence
it may be further processed into sulfuric acid, elemental sulfur or converted
to industrial chemicals. The upper liquid phase in the tank 50 containing
water and dissolved sulfur dioxide is returned as a reflux stream 54 to the
rectification portion of ~he stripping vessel 260
As an alternative to passing directly to the heat exchanger 48,~the
sulfur dioxide and steam leaving the stripping vessel 26 in the stream 46 may
.;~
be passed through a fin type heat exchanger 49 over which air, as shown at Sl,
is ~lown in order to preheat the air which can be subsequently used for direct
mixing with and reheat mg;o the~cold treated flue gas stream, as shown at 16,
from the top of the absorber 10. This direct mixing and heating of clean
-.
flue gas avoids the need for a~separate steam coil which is ~ore costly and
subject to corrosion.;
~20 The advan*ageous operating parameters are as followsO ~ S~eam is
introduced into the~reboiler tubes at a pressure greater than atmosphericO A
preferred steam pressure is 15-65 psig which provides a steam temperature
range of 250-310~O in the reboilerO At this pressure and temperature, the
rate of sulfur dioxide release -from the citrate liquor is accelerated and the
sulfur dioxide content in the vapor phase is increased. By contrast, opera-
tion at a s~eam pressure only slightly above atmospheric, for example, 5 psig,
results in a reboiler temperature range of 215 to 22~Fo and steam require-
ments several-fold higher than the 5 to 8 pounds of steam required per pound
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33
of sulfur ~ioxide stripped at the high pressureO
With a steam pressure of 15-65 psig, the cooling water in ~he heat
exchanger 4~ may be at a temperature range oE 50 to 9OF~ to produce liquid
sulfur dioxide as an overhead productO However, opera~ion at atmospheric
pressure of 5 psig requires a drying and refrigeration system to recover the
stripped sulfur dioxide as a product liquid. The overhead stream ~6 leaving
the rectification or top section of the stripping vessel 26 is at a temperature
of 280 to 290FO and can be used to preheat air to subsequently provide a
20~. reheat of treated flue gasO That is, the flue gas stream leaving the
top of the absorber at 1~ is at about 120 to 140FD and can be reheated by
this preheated air to about 140 to 160~FD by judicious use of the heat of the
the overhead stream 46~ Moreover, by operating the stripping vessel at 50-65
psig, the stripping vessel becomes a compact unit because the actual volume
of gas ~low is one-fifth that of a low pressure unit.
The superior steam economy of the high pressure stripping o~ sulfur
dioxide from a citrate buffered solution was confirmed by testing on an 8-inch
diameter stripping column to simulate gas desulfurlzation on a 0.25 megawatt
scaleD The test equipment included a packed tower of 7 7/8-inch inside dia-
meter filled with 12 ~eet of 1~2-inch ceramic berl saddles. The tower was
maintained at 65 psig by nitrogen pressure. A charge of 21 gallons Oe OD5
molar citrate solution was made up to a pH of 4DO by blending 0.5 molar citrate
acid and 0O5 molar sodium citrateO The bottom liquor was cooled and recycled
at 2~25 gallons per minute ~ulder a pressure of 20 PSI above system pressure
through a packed holding tank and a sight glass, After releasing the pressure
to 65 psig~ the recycle was reheated towith~n 5-10F. of the overhead tempera-
ture and fed into the tower three-fourths of the way upO
A metered amount of liquid sulfur dioxide was pumped into the
pressurized recycled stream prior to the holdup sectionO The recycle in the
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last run was analyzed for bottoin cmd recycle compositions. Overhead conden-
sate was collected in a teller and the upper phase pumped into the top o~ the
tower. I'he bottom phase was drained into a receiver maintainecl at a pressure
of 55 psig. Flow rates o both layers were determined by stopping their
flows and timing the build-up in the teller.
The steam economy was determined by hea-t balances around the towerO
Steam condensate was not collected at the bottom since this steam included
condensate in the feed lines and from the still pot~ The column was electri-
cally heated to the overhead temperature to eliminate heat loss from this
sourceO
The results of various representative runs of the fractionation
of 0.5 molar citrate solution having initial pH of 3.95 and carried out at
65 psig are set forth in Table lo Run 1 was carried out on a different day
than run 2. The results show that steam economies of 5.4 to 8O9 pounds o-
stea~l per pound of sulfur dioxide stripped are obtained with the degree of
stripping varying from 86 to 98 percentO Moreover, the second run showed
that there was no difficulty in obtaining a two-phase condensate at condensing
temperatures up to 89~o and a pressure of 65 psigo This sulfur dioxide layer
was ound ~o contain about 5 percent waterO
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In a citrate steam stripping process for -flue gas desulfurization
~or a 25 megawatt plant producing 110,000 ACFM of flue gas at 300F., the
steam consumption is 12,500 pounds per hour with a power consumption of 280
kilowatts and 30 STPD of liquid sulfur dioxide is recoveredO The total energy
need is therefore 362 BTU per kilowatt hour (KW~I~. These calculations are
based on a 25 megawatt boiler fired Wit~l 3.5 percent sulfur coal and assume
the presence of an electrostatic precipitor with 99.5 percent eficiency.
A comparison of incremental energ~ needs for various flue gas
desulfurization processes are set forth in Table 2. The evaluation of Table
2 assumes a 500 megawatt boiler using 3O5 percent sulfur coal with an electro-
static precipitor of 99O5 percent efficiency and includes a 20F. reheat of
flue gas for the citrate pressure-stripping processes, although in most cases
the energy penalty for reheat is of the order of 1 percent at 100 BTU/KWH for
~hich no special deduction has been taken in these figuresO The results
show that even if the steam consumption were increased by 50 percent due to
site conditions, fuel consumption ~ould increase only from 362 to 487 BTU/KWH
as compared to 607 ~o 1038 BTU/KWH or other regenerable flue gas desulfuri-
zation processes.
T~BLE 2
Process Total Incremental
~uel Consumption (BT
Limestone ~390)
~agnesia 1038 (Sulfur~
60~ (Acid)
2~ Wellman-Lord Sulfite Scrubbing 8~0
Citrex (Phosphate) 670
Atomic International Process670
S0 Steam Stripping Process
2 (at 5 lbsO steam per pound S02) 362
S.~33
The foregoing results are both surprising and unexpected in that
one would normally expect that stripping sulfur dioxide under pressure would
require a greater amount of steam per pound of sulfur dioxide stripped than
stripping at atmospheric pressure. Not only is such not shown not to be the
case according to the process of this invention, but in addition, many other
advantages are obtained in the processO For example, when stripping at atmos-
pheric pressure the sul~ur dioxide must be dried in a column with a suitable
material such as concentrated sulfuric acid, silica gel, alumina or the like
and then refrigerated or condensed for liquefactionO Here a liquid sulfur
dioxide product is obtained by the use of cooling water at normal plant cooling
water temperatures. While~the embodiment described is directed to:the removal
o~ sulfur dioxide from a cltrate buffered solution, the process is applicable
to remo~al from other organic buffered solutions of the type such as glycolate,
i
glyoxalate, acetate and the likeO Also, the system is applicable t:o any gas
containing sulfur dloxlde such as smelter gas and the like and the use of the
term flue gas is intended~to apply to suchO
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