Note: Descriptions are shown in the official language in which they were submitted.
PROCESS FOR REMOVAL OF SULFUR OXIDES FROM WASTE GASES
Background of the Invention
This invention relates to a process for absorbLng sulfur oxldes from
industrial waste gases with a solid sorbent and regenerating the solid sorbent
for reuse.
In the combustlon 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 o.Kides produced are Rn environmental
pollutant and in recent years considerable effort has been made to remove the
sulfur o~ldes from the combustion gasses exhausted to the atmosphere. Several
methods for removing such oxides are known. For example, U.SO Patent
3,852~410 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, consequently, an improved method for
economically and reliably removing sulfur oxides from gaseous mixtures would
be desirable, and is hereln provided.
Brief Su~mary of the Invention
Briefly, the process of the invention comprises treating the waste gas
containing sulfur oxides (which is princlpally and hereinafter for convenience
referred to as sulfur dioxide) with a solid sorbent selected from the class
consisting of activated sodium carbonate1 trona and mixtures thereof which can
remove 90 percent or more of the sulfur dioxide. Trona is the mineral name
for Na2C03.NaHC03;2H20. Activated sodium carbonate can be formed from
sodlum bicarbonate, trona or a mixture of the two, by calcining at a
temperature between about 70C and about 200C. For sodium bicarbonate
having a characteristic partlcle dimension of about 50 microns, a calcination
period
l,~
~3~ 3
of dbout 10 to about 30 minutes, at d temperature o-f about 150C ~lill suf-
fice. rhe clean gas is vented ancl the resul-tant urlreactec~ solids, so~i Utll
sulfites~ sulfa-tes ancl mixtures ~hereof, dre dissolved in a haxic amlllorli~
liquor that is all(dlinc enou(Jh to convert carbonic dCi~ to bicarb()nate, to
fonn soluble sodiulll ccml)ounds. Carbondtiorl o-F lhe resultc~rlt licluor fonlls
socliwn pre~cipitate containin~ bicarbonate, trona, or mixtures thereof.
The precipitate is separated from the carborldted li4uor dnd the
liquor ~reated wi~h d precipitdn~ compouncl selected frorn the class consisting
of alkaline edrth metal hyclroxides, oxides and mix-tures ~hereof to Fo~ in-
soluble alkaline earth metal sulFates, sulfites and rnixtures thereof. Suit-
able alkaline earth metals inclucle calcium, bariurn and strontiurrl. After
removing ~he solids, the liquor is recycled to treat spent sorbent.
The presence of ammonia in the process provides severdl distinct
advantages. For one, it permi~s the use of lower flow rdtes in the regenerd~
tion loop of the process. Another important advantage is that since it does
not degrade chemically o~ biologically to any significant extent~ there is
little loss of ammonia in the system, which accordingly reduces the amount of
materials utilized in the process. Moreover, since the ammonia does not act
as a reducing agent in the regeneratiorl loop or in the solid waste disposal
20 area the sulfites andJor sulfates present are not reduced to noxious sulfur
canpounds as, for example, hydrogen sulfide which can present serious health
and disposal problemsO
Brief Description of the ~rawing
The drawing is a schematic flow diagram of the process of the inven-
tion..
Detailed Descrlptlon of the Inventlon
.
Referring no~ to the drawing, a flue gas contalning sulfur dloxide is
fed v:La 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
] sodlum sul~te and sulfate, leaving a flue gas substantlally free of any
sulfur dixoide and which is vented from contactor 4 via condult 8.
A sollds 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 gase. The solids product is
transferred to mixing tank 16 via conduit 14 where it is admixed with an
alkaline recycle liquor containing ammonia from line 18 and makeup chemicals
which can include NH4Cl, (NH4)2S04, Na2C03, N 2 4~
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 dlsposed of
via conduit 26. Conduit 26 may go to reaction tank 64 when not all of the
sodium sulfite or sulfate from the spent sorbent storage vessel 12 dissolves
in mixing tank 16. The fly ash free liquor leaves ftlter 24 via conduit 28,
ls introduced into carbonator 30, and is reacted with C02, introduced into
carbonator 30 via condult 32. Bicarbonate ions are formed which are
transferred via conduit 36 to crystallizer 38 and converted to solid sodium
bicarbonate which
crystallizes ou-t oF solution. Excess C02 leaves carbonalor 30 Vid conduil ~'~
For ventiny to the atmosphere or, iF preferred, to the c'ledn gas stack via
conduit 8. 'I'he sodium bicarbona-te ancl/or trona crys~a'lli~ecl in cr~stallizer
38 is transferred Vid conduit 40 to sodium bicarbonate filter 42~ Carbon
dioxide may also be addecl to -the crystalli~er 38 ~o drive l;he crystal'liza~ior
of sodium bicarbonate toward completion. The sodium bicarbonate recovered
fronl Filter 42 is transferred via conduit 46 to drier/ca'lciner 4~ ~Ihere it isdried and calcined to an active form oF sodiurn bicarbona~e and transferred via
conduit 52 to regenerated sorbent stordge vessel 54. Alternatively, trona Ina~
be formed in crystalli~er 38, It may be dried or dried and calcined to an
active Form oF sodium carbonate in drier/calciner 4~ and transferred Vid
conduit 52 to regenerated sorben~ storage vessel 54.
The liquid from filter 42 passes via conduit 44 lnto carbon dioxide
stripper 56 where it is contacted countercurrently with a stripping 9dS (eO9.,
steam) introduced in the lower portion of stripper 56 via conduit 58. A por-
tion of the C02, other undissolved gases and any remaining stripping gas is
vented froln stripper 56 via conduit 60. This C02 containing gas may be added
to carbonator 30 or crystallizer 38. The C02 stripped liquor frorn stripper 56
is introduced~ via conduit 62, into a reaction vessel 64 where it is contacted
20 with precipitant preferably lime introduced via line 66. In reaction tank 64,
the precipitant, e.g. lime, reacts with the solub1e sodium sulfite and/or
sulfate to produce insoluble calciùm sulfate and/or calcium sulfite and
regenerate the alkaline 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 recycled~ as noted above, to mixing tank 16 via
conduit 18~ r
As can be seen from the drawing, the process is comprised of two
basic steps, a sorption step and a regeneration step. In the sorption step,
the sulfur dioxide in the f1ue 9dS is contacted with -the sorbent an~ converted
into soluble sulfate and/or sul-fi-te compounds. In the reyenerdtion step or
loop, the sulfur species is ultimately purgecl from the process as an insoluhle
sulFur compound dnd the sorbent is regenera-ted For reuse in the sorption slep.
The sorbent is preFerably a sodium carbona-te obt~ine(i by calcinin(~ d
sodiuln-containing compound such as sodium bicarbonate, trona or a rllixture
thereof at a temperature of from about 70 to about 200C. It has heen Found
that while sodium carbonate which has been produceci by crystalliza~ion
directly from solution does not act as an efFective sorbent in the process of
the present invention, calcined sodium carbonate produced by calcinin~ sodiu
bicarbonate or trona, makes an excellent sorbent and is easily obtained b~
cdlcining the precipitdted sodiurn bicarbonate produced in crystalli~er 3~
To remove the soluble sulFites and/or sulfates frorn the sys~em, d
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, bariurn or strontium or rnix~ures~ The preFerred alkaline earth metal
is cdlcium.
As noted above with regard to the description of the drawing, the
process, with advantagel employs à carbon dioxide stripper. The stripper,
20 which can be any gas-liquid countercurrent contactor, serves to remove excessC0~ from the process which would otherwise be precipitated as calcium carbon-
ate in vessel 64, thereby increasin3 the use o-F lime in the process. The C02
stripper ~as can include steam or an oxygen-containing gaseous rnedium such as,
For example, air.
As pointed out above, the process of the present invention utilizes
ammonia in the liquor in mixing tank 16. The ultimate source of alkalinity in
the process is supplied by the precipitant (hereinaFter For convenienGe
reFerred to as lime~ added to the reaction tank 64. However, without the use
of some medium to transFer alkalinity ~rom the solid phase ~lirne) to the
5 -
liquid phase, the alkalini-ty oF the solution woul~ be rdp-i~ le~'lete~ ~lurinthe carbonation step. Accor~in~lly, for d yiven circul~-ion r~le in lhe
system, pro(luction of so~ium bicdrbondte in ~he carbon~t(Jr ~ould be yrea~
reduce~. -rhis would necessitate dn i ncredsed pUlllp i ny or recircu'lalion rale in
the system -to the point where the process could become economicdlly not ~'C~dSi-
b'le. The arnmonia serves the funccion of effectiny the a'lka'linity trarlsf'erfrom the lime to the liqui(l phase ancl can thus be consiclered an "a'lkdlinity
carrier". This alkalini-ty c~lrrier has an acid Form (ammoniurn ion) an(l a base
~orrn, (amlnonia), being in the base Form as it leaYes reaction tank ~4. The
lo clear liquid which is removed -From ash fi'lter ~4 and which is used l;o ~issolve
the gds-solid contactor so'lids froln contactor 4 is pumped to -the carborlator 30
where -the liquid phase alkalinity oF the carrier is now exchan~ed For li~ui~
phase bicarbonate alkalinity~ This liquid phase bicarbonate alkalirlity is no~
converted to the solid pha$e alkalinity of the sodium bicarbonate in the
crystalli~er. The alkalinity carrier in the clear liquid from crystallizer 3
is now in the acid form, i.e., ammonium ion. Upon entering reaçtion tank 64,
the ammonium ion once again contacts the solid phase alkalinity provided by
the lime, and is converted into the basic form (ammonia) and the cycle
repeated.
It ~ill be apparent that the alkalinity carrier can be added as the
base, as ammonid, or in the acid Form of its respective salts. Thus, For
example, the ammonia can be added in the -Form oF ammonium sulfate, amnlonium
chloride or thP like.
The 'limits for ammonia may be determined from the following consider-
ations. As an alkalinity carrier, it is desirable to maxirnize its concentra~
tion.
The limitation on ammonia concentration is the vapor pressure of am-
monia. This is tlreatest at high ternperature and pH. An upper limit is the
totdl solution vapor pressure (water, dmmonia, and C02) of the solution e4llal
to Five atmosphere absolute (60 psig)~
The following examples will serve to illustrate the preferred emt~odi-
ments oF the invention.
Exclm~
Flue gas containing 700 lb. mo1e/hr. of S02 is treated with 760 lb.
mole/hr. of activated sodium carbonate and reacts ~ith 90 percent of the sul~
fur dioxide in the flue gas. The resulting solids are collected in a bag
house. The solids From the baghouse are dissolved using 1350 gal~/rnin. o-F d
recirculated liquor containing 2.6 m ammonia and 6.5 m sodium, and other
dissolved species such as chlorides, sulfites, sulfates, carbonates, calciuln,
etc. Also, makeup soda ash is dissolved into the liquor at the rate of
35.0 lb. mole/hr. The resulting liquor is ~hen carbonated with 760 lb~
mole/hr. of C02 from a combination of clean flue gas, and C02 recycled from
other parts o~ the process. Excess C0~ is combined with the cleaned flue
gas. Makeup ammonia is also added to the liquor at a rate of 25.4 lb~
mole/hr, in the carbonator tower. The resulting hot, carbonated liquor is
~cooled to 95F ~o precipitate 1520 lb. mole/hr. oF sodium bicarbonate. The
sodium bicarbonate solids are separated from the liquorg dried and calcined at
20 300F to for~ an activated sodium carbonate which is recycled to -the baghouse
to treat the flue gas. The separaced liquor is passed through a C02 stripping
column to remove ~0 lb. mole/hr. of carbon dioxide from the liquor. The
liquor leaving the C02 stripping column is treated with 660 lb. mo1e/hr. of
lime in a reaction tank to precipitate calcium sulfi-te and/or ca1cium sulfate
solids. These solids are separated fr~n the slurry leaving the reaction tank
and constitute the waste productO The separated liquor is recycled dS noted
above to dissolve the baghouse solids.
The fo'llowing Examples II, when run in accordarlce ~ith tne general
procedure of Exdnlple I, but in the absence of an alkalinity cdrrier as indi
cated in ttle following table~ demonstrate that the circu'iation ra-tfl ~lould be
increased at least a thousandfold (iF even technically feasib'le) over the 1350
9pl11 circlllation rate o~ Example I.
molali ~, rnoles/K ~ Circu'lation Rate
rnmonia ~ c~rn
II 0 1,350,000t
While the foregoin~ description is illustrative of -the preferred
embodimenks of the process of the invention, numerous obvious vdrid~ions an(i
modi~'ications will be apparent to one of ordinary skill, and accordingly, i~
is intended that the invenkion be limited only by the appended claims~
