Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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PROCESS FOR REMOVAL OF SULFUR OXIDES FROM WASTE GASES
B_ck'ground'of the'Inve'nti'on
This invention relates to a process for absorbing
sulfur o~ides from industrial waste gases with a solid sorbent
and regenerating the solid sorbent for reuse.
In the combustion of fossil fuels, and in many
industrial processes, a serious problem is presented by the
combustion of the sulfur-containing components therein. The
noxious sulfur oxides produced are an environmental pollutant
and in recent years considerable effort has been made to
remove the sulfur oxides from the combustion gases exhausted
to the atmosphere. Several methods for removing such oxides
are known. For example, U.S. Patent 3,852,~10 issued to
Rivers et al,, and U.S. Patent 3,846,535 issued to Fonseca,
are illustrative. To applicant's knowledge, however, all
prior art processes have certain disadvantages and, consequent-
ly, an improved method for economically and reliably removing
sulfur oxides from gaseous mixtures would be desirable, and is
herein provided.
~0 Brief Su _a`ry of the Invention
.
Briefly, the process of the invention comprises
treating the waste gas containing sulfur oxides (which is
principally and hereinafter for convenience referred to as
sulfur dioxide) with a solid sorbent selected from the class
consisting of activated sodium carbonate, sodium bicarbonate,
trona and mixtures thereof which can remove 90 percent or more
of the sulfur dioxide. Trona is the mineral name for
Na2CO3 NaHCO3~2H2O. Activated sodium carbonate can be formed
from sodium bicarbonate, trona or a mixture of the two, by
calcining at a temperature between about 70C and about 200 C.
For sodium bicarbonate having a characteristic particle
dimension of abbut 50 microns, a
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calcination period of abollt 10 to about 30 minutes, at a tempera-ture of about
150C will suffice. The clean gas is vented and the resultant unreacted
solids, sodium sulfites, sulfates and mixtures ~hereof, are dissolved in a
basic liquor that is alkaline enough to convert carbonic acid to bicarbonate,
said liquor containing borate ion, ammonia or pre~erably both, to form soluble
sodium compounds. Carbonation of the resultant liquor forrns a sodium precipi-
tate containin(J bicarbonate, trona, or mixtures ~hereo~.
The precipitate is separated from the carbonated liquor and the
liquor treated with a precipitant compound selected from the class consisting
of alkaline earth metal hydroxides, oxides and mixtures thereof to fonn in-
soluble alkaline earth metal sulfates, sulfites and mixtures thereof. Suit-
able alkaline earth metals include calcium, barium and strontium. After
removing the solids, the liquor is recycled to treat spent sorbent.
The presence of borate ion (such as supplied by boric acid) in the
process provides several distinct advantages. For one, it permits the use of
lower flow rates in the regeneration loop of the process. Another important
advantage is that, since borate ion does not degrade chemically or biologi-
cally to any significant extent there is little loss of borate ion in the
system, which accordingly reduces the amount of materials utilized in the
20 process. Moreover, since the borate ion does not act as a reducing agent in
the regeneration loop or in the solid waste disposal area the sulfites and/or
sulfates present are not reduced to noxious sulfur compounds as, for example,
hydrogen sulfide ~hich can present serious health and disposal problems. The
use of ammonia provides similar advantages.
Brief Description of the Drawing
The drawing is a schematic flow diagram of the process of -~he inven-
tion.
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Detailed;Descr;iption of t;he Invention
Referring now to the drawing, a flue gas containing
sulfur dioxide is fed via conduit 2 to a gas-solid contactor 4.
Contactor 4, which can take many forms (e.g., fixed bed, moving
bed, fluidized bed, etc.), is suitably a baghouse collector
employing tube type fabric filter dust collecting surfaces
preloaded with a suitable sorbent, which is introduced into
contactor 4 via conduit 6. Alternately, the sorbent may be
introduced into the gas stream upstream of the contactor. On
passing through the contactor 4, the sulfur dioxide in the
flue gas reacts with the sodium containing sorbent to produce
sodium sulfite and sulfate, leaving a flue gas substantially
free of any sulfur dioxide and which is vented from contactor 4
via conduit 8.
A solids product is removed from contactor 4 via
conduit 10 and transferred to a spent sorbent storage vessel
12. At this point, the solids product will comprise unused
sorbent initially in gas-solid contactor 4 plus soluble (sodium)
sulfite and/or sulfate resulting from the reaction of the
sorbent with the sulfur dioxide in the flue gas and any partic-
ulate matter originally contained in the flue gas such as fly
ash. The solids product is transferred to mixing tank 16 via
conduit 14 where it is admixed with an alkaline recycle liquor
containing borate ion from line 18 and makeup chemicals which
can include Na2B4O7, Na2CO3, Na2SO4, NaCl or various mixtures
of the above. In mixing tank 16, the soluble sulfite and/or
sulfate which were formed by the reaction of the sulfur dioxide
with the sorbent are dissolved. The liquor from mixing tank 16
is transferred via conduit 22 to a fly ash filter 24 where any
fly ash is removed and disposed o via conduit 26. Conduit 26
may go to reaction tank 64 when not all of the sodium sulfite
or sulfate from spent sorbent vessel 12 dissolved in mixing
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tank 16. The fly ash free liquor leaves filter 24 via conduit
28, is introduced into carbonator 30, and is reacted with CO2,
introduced into carbonator 30 via conduit 32. Clean flue gas
is a convenient CO2 source. Excess CO2 leaves carbonator 30
via conduit 34 for venting to the atmosphere or, if preferred,
to the clean gas stack via conduit 8. Bicarbonate ions are
formed which are transferred via conduit 36 to crystallizer 38
and converted to solid sodium bicarbonate, trona, or mixtures
thereof which crystallizes out of solution. Carbon dioxide
~0 may also be added to the crystallizer 38 to drive the crystal-
lization of sodium bicarbonate toward completion. The sodium
bicarbonate crystallized in crystallizer 38 is transferred via
conduit 40 to sodium bicarbonate filter 42. The sodium bi-
carbonate and/or trona recovered from filter 42 is transferred
via conduit 46 to dryer/calciner 48 where it is `either dried
and transferred via conduit 52 to regenerated sorbent storage
vessel 54 or is dried and calcined to form an activated sodium
carbonate which is likewise transferred via conduit 52 to
regenerated sorbent storage vessel 54.
~0 The liquid from filter 42 passes via conduit 44 into
carbon dioxide stripper 56 where it is contacted countercur-
rently with a stripping gas (e.g., steam) introduced in the
lower portion of stripper 56 via conduit 58. A portion of the
CO2, other undissolved gases and any remaining stripping gas is
vented from stripper 56 via conduit 60. This CO2 containing
gas may be added to carbonator 30 or crystallizer 33. The CO2
stripped liquor from stripper 56 is introduced, via conduit 62,
into a reaction vessel 64 where it is contacted with an alkaline
precipitant preferably lime introduced via line 66. In reaction
tank 64, the precipitant, e.g. lime., reacts with the soluble
sodium sulfite and/or sulfate to produce insoluble calcium
sulfate and/or calcium sulfite and regenerate the alkaline
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liquor. The mixture in reaction tank 64 is transferred via
conduit 68 to a sludge dewatering vessel 70 where the insoluble
calcium sulfate and/or sulfite is disposed of via conduit 72,
the liquid from vessel 70 being ~ecycled, as noted above, to
mixing tank 16 via conduit 18.
As can be seen from the drawing, the process is
comprised of two basic steps, a sorption step and a regenera-
tion step. In the sorption step,
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the sulfur dioxide in the flue gas is contacted with the sorbent and converted
into soluble sulfate and/or sulfite cornpounds. In the regeneration step or
loop, the sulfur species is ultimately purge~ ~rom the process as an insoluble
sulfur compound and the sorbent is regenerated ~or reuse in the sorption step.
The sorbent is preferably a sodium carbondte obtained by calcining a
sodium-contdining compound such as sodium bicarbonate, trona, or a mixture
thereof at a temperature of from about 70 to about 200C. It has been found
that while sodium carbonate which has been produced by crystallizing directly
from solution does not act as an effective sorbent in the process of the
present invention, sodium carbonate produced by calcining sodium bicarbonate
or trona makes an excellent sorbent and is easily o~tained by calcining the
precipitated sodium bicarbonate produced in crystallizer 3~.
To remove the soluble sulfites and/or sulfates from -the system, a
precipitant of an alkàline earth metal hydroxide, oxide or mixture thereof is
employed. Thus, for example, the process can employ an oxide or hydroxide of
calcium, barium or strontium or mixtures. The preferred alkaline earth metal
is calcium.
As noted above with regard to the description of the drawing, the
process, with advan~age, employs a carbon dioxide stripper. The stripper,
20 which can be any gas-liquid countercurrent contactor, serves to remove excessC2 from the process which would otherwise be precipitated as calcium carbon-
ate in vessel 6~, thereby increasing the use of lime in the process. The C02
stripper gas can include steam or an oxygen-containing gaseous medium such as~
for example, air.
As pointed out above, the process of the present invention utilizes
borate~ ion in the liquor in mixing tank 16. The ultimate source of alkalinity
in the process is supplied by the precipitant (hereinafter for convenience
referred to as lime) added to the reaction tank 64. ~owever, without the use
of some medium to transfer alkalinity from the solid phase (lime) to the
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1 liquid phase, the alkalinity of the solution would be rapidly depleked during
the carbonation step. Accordingly, for a given circulation rate in the
system, production of sodium bicarbonate in the carbonator would be greatly
reduced. This would necessitate an increased pumping or circulation rate in
the system to the point where the process could become economically not
feasible. The borate ion serves the function of effecting the alkalinity
transfer from the lime to the liquid phase and can thus be considered an
"alkalinity carrier". This alkalinity carrier has an acid Form (boric acid)
and a base form (borate ion), being in the base form as it leaves reaction
;0 tank 64. The clear liquid which is removed from ash filter 24 and which is
used to dissolve the gas-solid contactor solids ~rom contactor 4 is pumped to
the carbonator 30 where the liquid phase alkalinity of the carrier is now
exchanged for liquid phase bicarbonate alkalinity. This liquid phase bicar-
bonate alkalinity is now converted to the solid phase alkalinity of the sodium
bicarbonate in the crystallizer. The alkalinity carrier in the clear liquid
from the crystallizer 38 is now in the acid form, i.e., boric acid. Upon
entering reaction tank 64, the boric acid once again contacts the solid phase
alkalinity provided by the lime, and is converted into the basic form and the
cycle repeated.
Preferred alkalinity carriers include boric acid and, when activated
sodium carbonate is the sorbent, ammonia. It will also be apparent that the
alkalinity carrier can be added in its acid or base form. Thus, for example,
the borate ion may be added in the form of boric acid (acid form) or sodium
tetraborate (base fonn), while the ammonia can be added in the fonn of
ammoniuln sulfate or chloride or the like (acid forrn) or as ammonia gas (base
form). Advantageously, both borlc acid and ammonia are employed to minimize
the possibility of boric acid precipitation in the crystallizer.
The limits for borate and ammonia may be determined from the follow-
ing considerations. As alkalinity carriers, it is desirable to maximize their
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1 concentration. Boric acid solubility will limit the amount of borate that may
be circulated. The solubility oF boric acid decreases wi~h a decrease in
temperature. The coolest part of the circulating loop is the crystallizer
effluent. In the crystallizer effluent alrnost all of the borate in the solu-
tion exists as un-ionized boric acid, 113B03 [or ~(01-1)3~. The solubility of
boric acid is dependent on solution composition as well as -temperd-Cure. Solu-tion c~nposition is determined by both site speci~ic factors (e.g., how rnuch
HCl or NOx are removed from the flue gas) and operating conditions. It is
thus not possible to fix the maximum workable borate concentration without
knowledge of these factors. The order of magnitude oF the borate solubility
may be obtained from solubility data reported by Linke (Linke, William F.
Solubilities: Inorganic and Metal-Organic Compounds. K-Z. Volume 11, 4th
Ed., American Chemical -Society, Washington, D.C. 1965).
Temp, concentration, ~/100_~ H20 _ mole H3B03
C NaCl KCl Na2SO~H3B03 (saturated) K9H20
0 0 0 5.43 0.88
0 0 0 7.19 1.16
36.8 0 0 8.2 1.33
33.2 0 11.9 9.6 1.55
0 41.0 0 11.6 1.88
0 0 53 13.1 2.12
The limitation on ammonia concentration is the vapor pressure of
ammonia. This is greatest at high temperature and pH. An upper limit is the
total solution vapor pressure (water, ammonia, and C02) of the solution equal
to five atmosphere absolute (60 psig). For both borate and ammonia these
general considerations apply anywhere in the system.
The following examples will serve to illustrate the preferred embodi-
ments of the invention. r
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Example I
Flue gas containing 700 lb. mole/hr. of S02 is treated with 760 Ib.
mole/hr. o~ activated sodium carbonate and reacts with 90 percent of the sul-
fur dioxide in the flue gas. The resulting solids are collected in a bag-
house. The solids fr~n the baghouse are dissolved using 1350 gal./min. of d
recirculated liquor containing 2.6 m borate, 6.5 m sodiurn and other dissolved
species such as chlorides, sulfites, sulfates, carbonates, calcium, etc.
Also, makeup soda ash and borate are dissolved into the liquor at the rate o~
28.6 lb. mole/hr. and 6.4 lb. mole/hr~, respectively. The resulting liquor is
then carbonated with 760 lb. mole/hr. of C02 from a combination of clean flue
gas, and C02 recycled from other parts of the process in the carbonator an~
crystallizer. The carbonated liquor is cooled to 95F in the crystallizer to
precipitate 1520 lb. mole/hr. of sodium bicarbonate. The sodium bicarbonate
solids are separated from the liquor, dried and calcined at 300F to form an
activated sodium carbonate which is recycled to the baghouse to treat the flue
gas. The separated liquor is passed through a C02 stripping column to remove
90 lb. mole/hr. of carbon dioxide from the liquor. The liquor leaving the C02
stripping column is treated with 660 lb. mole/hr. of lime in a reaction tank
to precipitate calcium sulfite and/or calcium sul~ate solids. These solids
are separated from the slurry leaving the reaction tank and constitute the
waste product. The separated liquor is recycled as noted above to dissolve
the baghouse solids.
The following Examples II-III, when run in accordance with the
general procedure of Example I, but for the presence or absence of an
alkalinity carrier as indicated in the following table, demonstrate thdt the
circulation rate would be increased at least a thousand~old (if even
technically feasible) without an alkalinity carrier.
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molality, moles/KgH 0 Circulation Rate
ExampleAmmonia Bora~e gpm
I .0 2.6 1,350
II 1.3 1.3 1,350
III 0 0 1,350,000~
While the foregoing description is illustrative of the preferred
embodilllerlts of the process of the invention9 numerous obvious variations and
modifications will be apparent to one of ordinary skill, and accordingly, it
is intended that the invention be limited only by the appended claims.
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