Note: Descriptions are shown in the official language in which they were submitted.
~7~4~
This invention relates to the remov~l of sulfur dioxide
from gas streams by contact with an aqueous sodium sulfite
solution to absorb sulfur dioxide and provide a solution
richer in sodium bisulfite which can be treated to desorb
sulfur dioxide and regenerate the absorbi.ng solution for
reuse, and in which sodium sulfate is formed as a by-product
in the absorption-desorption medium and must be purged from
the system without undue loss of valuable sodium compounds.
More particularly, the invention concerns a process for re-
ducing the loss of desirable and valuable sodium compoundsfrom the cyclic sulfur dioxide removal system by removing
undesirable sodium sulfate from the absorption-desorption
medium as solids containing an increased amount of sodium
sulfate.
Sulfur dioxide is a recognized pollutant of the atmos-
phere and is produced by oxidation of sulfur or sulfur-bearing
materials. Sulfur dioxide is found in significant amounts as
a constituent of various waste gases such as smelter gases,
off-gases from chemical plants, and stack or furnace gases
from coal or oil-burning furnaces such as are used in electric
power plants. Although the concentration of sulfur dioxide
in such gases is generally minor, e.g., from about 0.001 up to
about 5 mole percent, and is frequently less than about 0.5
mole percent (less than about 1% by weight), the emission of
sulfur-dioxide may be substantial, particularly in industrial
applications due to the large amount of sulfur-bearing material
being processed. For instance, a modern electric plant having
a 1,350,000 kw. capacity will burn up to about 15,000 tons of
` coal per day. Despite the fact that the concentration of
sulfur dioxide in the stack gases from the electric plant can
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10~9492
be low, e.g., oE the order of 0.2 to 0.3 mole percent, the
total sulfur dioxide produced mav be in the neighborhood of
1,000 tons per day. Similarl~, significant amounts of sulfur
dioxide are produced in utilization of other fuels which may
bear sulfur.
The removal of sulfur dio~cide from sulfur dioxide-contain-
ing yases may be effected by treatment with an aqueous sodium
sulfite solution. The operation of an efficient and economical
system for removal of sulfur dioxide will be characterized
not only by the efficiency of absorption of sulfur dioxide ~ -
from the sulfur dioxide-containing gases, the efficiency of
desorption of sulfur dioxide from the spent absorbing solution,
and the purity of the sulfur dioxide product, but also by the
minimization of loss of metal values. Sulfur dioxide-containing
gas obtained, for instance, by burning sulfur-bearing mineral
products and the like as fuels, can be contacted with sodium
sulfite in an aqueous solution to form bisulfite, and thereby
substantially reduce the sulfur dioxide content of the gas
to, for instance, less than about 0.02 mole percent when the
sulfur dioxide-containing gas comprises more than about 0.2
mole percent sulfur dioxide. The removal of sulfur dioxide
from the gases is often up to abou~ 95 percent or more. The
spent absorbing solution can be heated to convert the bisulfite
~; to sulfite and sulfur dioxide, and to generate a liquid or
' liquid-solid material which serves as the source of the absorbing
: solution. The sulfur dioxide can be drawn-off and cooled
~ or compressed to provide a liquid product or sent as a gas to
; a sulfuric acid plant or sulfur plant. Regenerated absorbing
solution can be recycled to the absorption zone. For addi-
tional information and further exemplification regarding
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~07g49Z
sulfur dioxide removal systems which can advant~geously employ
the technology disclosed herein, see U. S. Patent Nos. 3,607,037,
3,653,812, and 3,790,660.
The sulfur dioxide-containing gases to be treated usually
contain materials which facilitate oxidation such as sulfur
trioxide, oxygen, elemental iron, and the like, and, particularly
when the gases containing sulfur dioxide are derived from the
combustion of fuel, other materials which may be present
include oxides of nitrogen. At least some of these materials
promote the oxidation of the sodium sulfite or bisulfite to
sulfate. Sodium sulfate is an inert material for purposes -
of the sulfur dioxide absorption-desorption process, because
sulfate cannot be regenerated to sulfite during the desorbing
operation. Sulfate build-up therefore occurs in the sulfur
dioxide absorption-desorption system. A portion of the
absorbing-desorbing medium can be purged from the system to
` prevent unduly large amounts of inert sulfate from accumulating
in the system. This purge may be a portion of the spent
absorption solution or material obtained in the desorption
- of the sulfur dioxide from the absorbing solution. These purge
materials, however, contain substantial amounts of sulfite or
bisulfite, along with the sulfate, and when the purge is
discarded, an undue expense may occur due to the accompanying
loss of useable sodium va~ues from the system which must be
` replaced by the addition of a suitable soluble sodium compound.It has been proposed to purge sulfate from the absorption-
desorption system more selectively with respect to sulfite or
:
bisulfite values, and return the latter to the absorbing-
...
desorbing system. Simple separation of sodium sulfate from
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sul~itc and bisulfite in the spent absorbin~ solution by low
temperature crystallization ~ives a sul~ate-crystallization
product containlng minor amounts of sodium sulfite. A more
selective separ~tion of sulfate is thereby obtained but such
processing involves undue expense because of the cooling
requirements needed to reach the low crystallization tempera-
tures and the necessity to reheat the crystallization mother
liquor returned to the absorption-desorption system. Also the
crystals obtained are in the hydrate form, and drying is
required to facilitate handling if the product is to be further
processed or sold. Large drying facilities are, therefore,
necessary to reduce not only the free water content but also
the water of hydration of the material.
In another system for concentrating sodium sulfate so
that it can be more economically separated from the sulfur
dioxide removal system described above, a purge stream of
spent absorbing solution is contacted with a gas containing
~ sulfur dioxide to convert at least a portion of the sodium
j l sulfite to the more soluble bisulfite. The stream is then
cooled to crystallizing temperature and sodium sulfate preci-
pitates more selectively. This procedure has disadvantages in
that two separate treatments of the purge stream are required,
`~; i.e., sulfur dioxide contact followed by crystallization at a
reduced temperature of, say, about 0 to 10C., which is a
~- considerable expense. The additional sodium bisulfite formed
in the treatment of the purge stream with sulfur dioxide, and
~; contained in the resulting separated liquid stream added to
the absorption-desorption cycle, leads to an additional heat
~`1 requirement to convert the bisulfite to sulfite with the
,
~ 30 formation of sulfur dioxide. Moreover, in this operation the
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~079492
sodium sulfate contains sodium sul~ate hydrates which, ~s
noted above, may nccessitate the provision of relatively large
dryin~ faciliti~s if the product is to be further handled or
sold.
In British Patent No. 489,745 there is described a sulfur
dioxide removal system which although devoted mainly to the
use of an aqueous ammonium sulfite absorbing medium, mentions
the possible use of alkali metal sulfites, and the system is
designed to purge sulfate. In the operation the spent absorption
medium is desorbed of sulfur dioxide without precipitation
of salts. The resulting solution is substantially saturated
with sulfate and relatively unsaturated with respect to sulfite
and bisulfite. The removal of water and cooling of the solution
say from 110 to 50-70C. results in the precipitation of
sulfate. This type of operation is undesirable since the
system is apparently dependent upon the liquid from the sulfur
dioxide desorption zone being relatively unsaturated with
respect to sulfite which means that a large amount of absorption
solution must be supplied for the removal of a given amount of
sulfur dioxide.
In accordance with the present invention, there has been
devised a highly advantageous procedure for reducing the amount
of sodium sulfate in sodium sulfite-bisulfite, absorption-
desorption systems for removing sulfur dioxide from gaseous ?
streams. In the process of this invention it is not
necessary to reach temperatures substantially below ambient,
l and the sodium sulfate removed can be in an essentially non-
`i hydrated or anhydrous form. Accordingly, at least a portion
1 of the aqueous sodium sulfite-bisulfite, absorbing-desorbing
. ~,
medium containing a relatively large quantity of sodium
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1079492
bislllfite is treated to remove water and thereby precipitate a
limited amount of sodium sulEate-containing solids from
sol~ltion which solids have a higher sulfate concentration on a
dr~ basis than the absorption-desorpti~n medium from which
they are ~ormed. The concentration of the feed from the ~b~orp-
tiorl-desorption system by the removal of water is continued for
a sufficient length of time to ensure substantial precipitation
of sodium sulfate, but is concluded before the slurry obtained
from said feed contains svbstantially more than about 10 weight
percent of undissolved solids. The material preferably treated
in accordance with the invention is essentially spent absorption
medium. In one manner of conducting the process of the invention,
the removal of water and precipitation of solids may be effected
without substantial conversion of bisulfite to sulfite and sulfur
dioxide, e.g., there may be less than about 10 weight percent of
the bisulfite so converted. The precipitated solids containing
sodium sulfate can be readily removed from the resulting slurry
by the use of conventional liquid-solid separation equipment to
provide, at least initially, an essentially non-hydrated sulfate
product which is relatively high in sodium sulfate content, ad-
vantageously at least about 50 weight percent, on a dry basis.
The separated solids may contain a major amount of sodium sulfate,
and a minor amount, if any, of sodium sulfite, preferably these
amounts are at least about 70 weight percent sodium sulfate and
less than about 30 weight percent sodium sulfite based on the
total of these materials. The concentration of so~ium sulfate
in the precipitated solids is often at least about 2 times, on a
dry basis, the concentration of sodium sulfate in the absorption-
.:;,
., desorption medium from which the solids are formed by the evapo-
ration of water.
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~07949Z
In the method of the present invention, the aqueous ab-
sorbing-desorbing solution which undergoes water removal and
sulfate precipitation for purging, contains in solution a
major weight amount of sodium bisulfite, a minor amount of
sodium sulfate, and none or a minor amount of sodium sulfite,
based on the total of these components. The solution may
preferably contain this amount of bisulfite at the beginning of
water removal or it may attain this concentration in the liquid
phase during the removal of water. The material from the
absorption-desorption system which is treated may contain in
solution at least about 25 weight percent of total salts, pref-
erably at least about 30 weight percent, and often this amount
may not exceed about 50 weight percent. The sodium sulfate
content of this solution may usually not exceed about l0 weight
percent, and preferably this amount may often not be above about
8 weight percent. The sodium sulfate content in solution is
generally at least about 1 weight percent, and preferably is
about 3 to 7 weight percent. The weight ratio of sodium sulfate
to sodium sulfite in solution in the material treated for limi-
20 ted precipitation of solids usually is, or becomes during the
treatment, at least about 0.7:1, preferably at least about 1:1.
Although in general the higher the sulfate content in the solu-
tion treated the purer the sodium sulfate obtained by the
` method of this invention, an increase in the amount of inert
sodium sulfate material circulating in the absorption-desorp-
` tion system has a detracting factor since there may be less ac-
tive sodium present for a given amount of water in the system.
As a result a greater quantity of circulating material would
` be required to provide a given sulfur dioxide absorption-
~` 30 desorption capacityO Often the absorbing-desorbing medium
treated has in solution about 0.1 to 10 weight percent sodium
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107949Z
sulfite and about 15 to AO ~eight percent sodium bisulfite,
based on the total of these components and the sodium sulfate
and water present. mhe stream mav contai~ minor amounts of
other materials in solution, c.g., sodium thiosulfate.
The solution from the absorption-desorption system which
undergoes water removal for sodium sulfate precipitation and
purging generally contains initially or during the treatment,
a mole ratio of sodium bisulfite to sodium sulfite of at least
about 2:1, often at least about 3:1. The amount of sodium
bisulfite in solution in the material undergoing treatment as
compared to the total active metal in the material may be alter-
natively expressed in terms of "s/c" which is defined as the
number of moles of ac~ive sulfur, e.g., S03= and HS03-, per 100
moles of water divided by the moles of active sodium per 100
moles of water. Thus, a pure sodium bisulfite solution would
have an s/c of 1, and a pure sodium sulfite solution would have
an s/c of 0.5. Sodium sulfate, for instance, does not provide
active sulfur or active base. ~he s/c of the material from the
absorption-desorption system treated in accordance with the
process of this invention is preferably about 0.85 to 0.97.
In the present invention sodium sulfate is removed from
. the system by treating at least a portion of the a~ueous absorp-
tion-desorption medium, preferably spent absorption solution, to
evaporate a sufficient amount of water and precipitate a signi-
ficant, but limited, amount of solids from the solution. Thus,
the evaporation of water is conducted in a manner such that the
slurry obtained from the absorption-desorption system feed solu-
; tion has up to about 10 weight percent crystals, often at least
~` about 1 weight percent, and preferably up to about 5 weight
percent. The water content of the solution from the absorption-
desorption system which is subjeFted to evaporation may often
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~07949Z
be reduced by up to about 75 or more weight percent, prefer-
ably by at least about 10 weight percent, and the material
generally remains sufficiently fluid to be readily pumpable.
The operation is advantageously conducted at somewhat eleva-ted
temperatures which are sufficient to precipitate essentially
non-hydrated crystals without excessive water removal. Gen-
erally, such temperatures are at least about 37 to 38C., and
to be more certain of having temperatures sufficient to form
a non-hydrated product when it is precipitated, a temperature
of at least about 40C. is recommended. Suitable temperatures
for accomplishing the desired evaporation of water thus include
about 40 to 120C., preferably about 40 to 90C. The choice of
temperature may depend on the pressure employed, and the pressure
may be ambient, reduced or elevated. Advantageously, the pres-
sure is about 10 to 20 psia, and preferably, essentially atmos-
pheric pressure is used.
The slurry obtained in the water removal operation is sub-
jected to liquid-solid separation to provide a separate solid
phase which is relatively high in sulfate content. The separa-
tion may be done without reducing the temperature of the slurry,
and the temperature may often be about 40 to 120Co ~ preferably
about 40 to 90C. The separated liquid phase or mother liquor
can be charged to the absorption-desorption system, and prefer-
ably to its desorption zone. The separated solids may, if
desired, be dried and they may undergo self-drying upon standing
by the free water being taken up as water of hydration.
The amount of solids formed in the water evaporation stage
.
` of the process of this invention, and subsequently removed from
~: ,
` the absorption-desorption system is sufficient to prevent undue
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1079492
build-up of sodium sulfate in the system. The amount of sulf~te
purgcd is preferably substantially equal to the amount of sulfate
being formed in the absorption-desorption system while taking
into account any sodium sulfate that is removed from the system
by other means. Also, the amount of solids formation required
may depend on the purity of the sulfate in the precipitated and
separated solid phase, as well as the amount of the total
absorption-desorption medium which is subjected to the sulfate
removal operation. Thus, up to the entire stream of absorption-
desorption medium may be treated for sulfate removal in whichcase the percentage of solids formed may be less than if only a
portion of the medium is subjected to the sulfate precipitation
treatment. Generally, as the percentage of solids formed in
the medium decreases the purity of the precipitated sulfate
increases.
; In one embodiment of the invention substantially the entire
absorption-desorption medium is processed for water removal and
sulfate precipitation~ Alternatively, only a portion of this
medium may be so treated, and in such case frequently about 10
to 90 weight percent of the total medium is treated in this manner,
more often up to about 75 weight percent, say about 20 to 75
weight percent. Preferably, this amount is sufficient so that a
maximum of about 5 weight percent solids need be precipitated in
the slurry formed from the absorption-desorption feed to have an
adequate purge of sodium sulfate. The liquid medium or mother
; liquor separated in the sulfate removal procedure is usually
` passed to the sulfur dioxide desorption stage since the liquid
:
is high in bisulfite content. Depending on the amount of the
mother liquor to be recycled it may be desirable to charge it to
some other part of the absorption-desorption system. Make-up
--11--
1079492
sodium values in the ~or~ of suitable water-soluble sodium com-
pouncls such as so~ium car~onates or hydroxide, may be added to
the system of this invention to replace s~dium loss, including
that removed in the sodium sulfate solids which are precipi-
tated. Advantageously, this adclition is to the lean absorbing
solution to which make-up water may also be added.
In the sulfur dioxide desorption stage of the method of
this invention the spent absorption medium is subjected to ele-
vated temperatures to convert sodium bisulfite into sodium sul-
fite with the concomitant formation of a vapor phase containing
sulfur dioxide and water. Suitable temperatures for this oper-
ation include about 40 to 110C., preferably about 60 to 95C.
The pressure may be about 3 to 21 psia, preferably about 8 to
15 psia. The vapor phase can be treated for the recovery of
purer sulfur dioxide, the manufacture of sulfur, or used, treat-
ed or dis~osed of in any other suitable manner. Various pro-
cedures for sulfur dioxide desorption can be used and a number
are known in the art. It is preferred, however, that the de-
sorption be accomplished with the simultaneous formation of an
undissolved soli~s or crystal phase which enables the desorption
to be accomplished ~ith the use of lesser amounts of heat.
In such operations the amount of undissolved solids in the
desorption zone is generally at least about 15 weight ~ercent
of the slurry undergoing decomposition or sulfur dioxide
desorption. As described in U. S. Patent ~o. 3,790,660 the
amount of such solids is advantageously at least about 25
weight percent in order to alleviate difficulties of tube
fouling, particularly when supplying heat to the desorption
:1 .
æone by passing the slurry through the tubes of an indirect heat
exchanger. Preferably, the amount of undissolved solids is about
30 to 50 weight percent of the slurry undergoing decomposition.
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1079492
Also when the amount of undissolved solids is sufficiently
high, the sodium sulfi-te content of the slurry may be adequate
for a por-tion of the total slurry to be combined with water to
dissolve the solids, and the resultir.g solution can be used as the
le~n solution for absorbing sulfur dioxide from the gas being
treated in the absorption zone of the absorption-desor~tion
system. The lean absorbing solution is usually composed to a
major weight extent of sodium sulfite and minor weight amounts
of sodium bisulfite and sodium sulfate based on the total amount
of these salts present. Often the lean absorbing solution has
about 10 to 35 weight percent sodium sulfite, about 3 to 15 weight
percent sodium bisulfite, and about 1 to 10 weight percent sodium
. sulfate based on these components and water present.
~ The present invention will be further described by reference
- to the drawing which is a schematic flow diagram of a process
employing the present invention in an absorption-desorption system
using sodium sulfite for removal and recovery of sulfur dioxide
from flue gas. Equipment such as valves, pumps, heat exchangers,
surge tanks, and the like, which would be used in a commercial
. . ~`~ 20 embodiment of the invention and in the operation of an absorption-
desorption system, is not shown since it can be of conventional
design and employed in accordance with practices well known in the
art.
;:
Referring to the drawing, sulfur dioxide-containing flue gas,
which may, for example, contain from about 0.05 to about 5 mole
percent sulfur dioxide, enters absorber vessel 10 by way of line
12 near the bottom thereof. Water or other aqueous liquid may be
passed co-currently with the flue gas to a bed of column packing
in the lower portion of vessel 10 to scrub the gas to remove
suspended solids such as fly-ash and the relatively high water-
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1~)79492
soluble components, for instance, sulfur trioxide, hydrogen
chloride and the like from the flue yas.
The flue gas pass~s upwardly through absorber 10 and
liquid-gas contacting means such as sieve trays and throuyh a
descendiny flow of lean absorbing solution which is supplied
to vessel 10 hy line 14. The lean absorbing solution contains
sodium sulfite as the essential sulfur dioxide-absorbing
component. Absorber 10 may employ other types of liquid-gas
contacting structures, such as pac~ing, bubble caps, alternate
- 10 ring and discs or the li]ce. The lean absorbing solution in
line 14 is often at a temperature of at least about 30~.,
~ preferably at least about 40C., up to about 110C., preferably
- up to about 70C. Flow rates of the aqueous absorbing solution
through the absorption zone can be adjusted according to the
sulfur dioxide concentration in the gas being treated, and the
concentration of sodium sulfite in the solution, so that a
major amount, e.g., up to about 95% or more, of the sulfur
dioxide may be removed from the ~as by reaction with the lean
; 20 absorbing solution. The treated gas leaves absorber 10 by way
. ~ . . .
.} - of line 13. Spent absorbing solution is collected on gas- -
..
passing tray 17 in absorber 10 and removed from the latter by
line 16.
The spent absorbing solution is transferred by pump 18
~ and line 54 to heater 52 and thence by line 56 to desorber 50.
- A portion, or even all, of the spent absorbing solution is
withdrawn fro~ line 54 by line 22. The stream in line 22
containing sodium sulfate, as well as sodium sulfite and
bisulfite in solution, is passed to evaporator or dehydrator
24 in accordance with this invention. Heat may be supplied to
j evaporator 24 by steam coil 26, and the added heat serves to
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107949Z
cause the removal of water from the spent absorbing solution
and th~ precipitation o~ s~dium sulEate-containing solids.
The water and any sulfur dioxide in the vapor phase in evapora-
tor 24 is removed by line 36.
A portion of the spent absorbing medium undergoing dehy-
dration in evaporator 24 is withdrawn via line 40 and passed to
crystal separator 42 which may be selected from conventional
processing equipment for effecting separation of solids and
liquids such as filters, including rotary filters, centrifuges,
clarifiers and other sedimentation equipment. Solids rich in
sodium sulfate crystals and containing sodium sulfite can be
removed by line 44, and the resulting liquid stream having
sulfate removed therefrom can be sent via line 46, pump 48, lines
49 and 54, heater 52 and line 56 to desorber 50 for processing
to desorb sulfur dioxide and to regenerate the absorbing
solution.
The desorber section of the system, which can be operated
in the manner, for example, shown in U.S. Patent No. 3,790,660,
is for convenience shown as a single stage desorber, but two
or more stages may be used. The heated solution in line 56
is introduced into the desorber 50. The conditions of
;~ temperature, pressure, and residence time in desorber 50 are
so maintained as to effect the desired decomposition of sodium
bisulfite, evaporation of sulfur dioxide and water, and
precipitation of s~dium sulfite-containing crystals as
described above and in said patent.
To supply heat to desorbing vessel 50 a recycle stream is
~h heated in heat exchanger 58. In order to effect heating in
vessel 50, the slurry in the vessel is withdrawn by line 60,
and passes through pump 62, line 63, metallic tubes of heat
exchanger 58 and back to vessel 50 by way of lines 64 and 56.
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Steam is introduced to exchanger 58 through line 66 as
the prime energy source for the desorption zone. The conden-
sate (water) from heat exchanger 58 is removed through line 68.
The sulfur dioxide and water vapors from desorber 50 are
removed by line 74. A portion of the slurry in desorber 50
is passed to dissolving tank 72 by way of line 78. Since
water has been removed from the absorption solution during
desorption, make up water, for example, from rectification
(not shown), is supplied to tank 72 through line 80. The ~ ;~
solution formed in dissolving tank 72 passes through line 82,
pump 84, and line 14 to absorber 10. Make-up sodium ion,
which may be an aqueous sodium hydroxide or carbonate solution,
is added to line 82 through line 88.
Other ways of conducting the limiied precipitation of
. . :
; solids from the absorption-desorption medium may be used in
conducting the method of this invention. For example, the
medium may be dehydrated while being contacted with a sulfur
dioxide-containing gas charged to the dehydrator.
The following examples will further illustrate the present -
~.,. . i
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EXAMPLE I
This example gives the results of batch evaporation tests
. .:
performed on synthetically prepared, spent absorber solutions
`' as would be obtained in an absorption operation of the type
shown in the drawing. In the tests the spent absorption solu-
tion was placed under vacuum in a flask maintained in a con-
stant temperature bath. The evaporations were conducted at
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1079492
the temperatures noted in l~able I, and were continued until
the indicated amount of solids had been formed in a given
spent absorption solution. The water evaporated in each test
was condensed and collected. The crystals were separated from
the mother liquor at the temperature of evaporation. The
most pertinent data obtained in these tests are in Table I.
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, TABLE I
~ .
Test No. 1 2 3 4 5 6 7
SPENT 2 3 % wt.5.825.74 6.03 6.945.84 5.75 5.87 ,
ABSORBER NaHSO3 (calc. as
SOLUTION Na2S205) " 18.9818.8020.0624.3119.73 19.63 19.73
ANALYSIS Na2SO4 "8.50 8.07 8.767.49 7.91 B.69 8.46 `-
- 2 2 3 " 0.30 0.29 0.33 0.410.30 0.33 0.31
- H20(by diff)" 66.40 67.1064.8260.8566.2265.60 65.63
l Vol.** ccs 500 500 500 500 500 500500
,, Wt. g 649.0649.0 648.0657.4654.8654.0649.4
~;~ Density g/cc 1.34 1.37 1.301.36 1.33 1.35 1.33;
," I i,
-, EVAPORA- WATER REMOVED g 101 122 206 109273 298 351' TION TEMPERATURE C. 86 85 92 86 87 86 86 ~:
~ ~ .. ..
,'l SOLIDS CONTENT ~ wt. 1.1 4.8 11.117.1 22.0 28.9 44.1 -
~ OF SLURRY ~ --
'';I - ,' ~,
~ 2 3 % wt. 8.057.73 7.73 8.606.91 6.38 4.08
;Ji'~ ! ,;, MOTHER NaHSO3 (calc. as
`::!
LIQUOR Na2S2O5) " 22.0123.03 26.3831.1135.8136.6842.58
~' ANALYSIS Na2SO4 " 9.94 6.675.68 5.48 3.78 4.75 3.73 ;~
,~; Na2S23 " 0.38 0.39 0.60 0.480.65 0.73 1.05
H2O (by diff.) " 59.62 62.18 59.61 54.33 52.85 51.46 48.56
DRY Na2S3 % wt. 19.626.90 37.4341.0246.1244.1156.81
~i CRYSTALS NaHSO (calc. as
ANALYSIS* Na2S205) " NIL 1.07 NIL NIL NIL NILNIL
~: Na2S4 " 80.4 72.03 62.5758.9853.8855.8943.19
H20 (by diff.) " NIL NIL NIL NILNIL NILNIL ~,
~ ."
j * Calculated free of mother liquor and expected on a dry basis
~ ** Approximated
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I?rorn the data ir~rrable I il, can be se~en that, as the solids
content Or the mixture undergoing -evaporation increase,~, the
sulfate to sulr:itei ratio ln the crystals decreased. Only ln
t,ests 1 and 2, having respectively, 1.1 and Ll. 8 weight percent
solids f'ormed in the materlal evapora~ed, did the dry crystal
product have greater than about 65 welght percent sulfate.
EXAMPLE 2
Additional batch tests were conducted in a similar manner,
-~ 10 but using spent absorber solutions obtained from an operating
commercial installation in which sulfur dioxide was being ab- ~-
''J. sorbed from a gas in a system of the type shown in the drawing.
In these tests, each spent absorber solution was evaporated at
about 105C. and atmospheric pressure in a stirred, stainless
steel beaker until the stated amount o~' sollds appeared. The
sollds were separated ln a laboratory centrifuge 15 cm. ln
' dlameter, 5 cm. ln helght, operating at approximately 2800
~' rpm. The centrifuge basket was covered with 316 stainless
,.. ; .
,3 steel, 150 mesh. The separated crystals and the mother liquor
Ç~ 20 were weighed and analysed. In almost all cases the crystals
p~ ~ produced could be centrifuged easily. The results of the
" ~ tests are given~in Table II.
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TABLE II
Te st No . 1 2 3 4 5 6
Spent Absorber
Solution Takeng 1300 13161318 13081343 1324
Water Evaporatedg (420)1~350) -- 281286 273
Mother Liquor
Separated g752 867 1009931 962960 ~ -
Wet Crystals
Collected g89.9 64.3 10.137.7 41.741.4
Solids Content
of Slurry% wt.7.9 5.7 0.93.0 3.63.1
' ~
SPENT Na2SO4% wt. 5.47 5.32 -_ 5.20 -- 5.36
; ABSOR~ER Na2SO3 " 0.87 0.91 -- 0.84 -- 0.90
SOLUTION NaHSO3 (calc. as
ANALYSIS Na2S2Os) ~ 31.4831.83 -- 32.67 -- 31.85
" Na2S203 0.14 0.10 -- 0.15 --0.10
H2O (by diff) " 62.0461.84 -- 61.14 -- 61.79
j :
MOTHER Na2SO4 ~ wt.3.80 4.38 5.885.26 4.724.84
LIQUOR Na SO3 " 5 57 5.21 5.315.36 5.355.28
ANALYSIS NaHSO3 (calc. as
Na2S2O5) "39.24 40.24 ~35.16 35.22 ~
Na2S2O3 " 0.30 0.33 388.81 ~54.22 0.30 ~89.88
~ H20 (by diff) " 51.0949.84 ~ ~ 54.41
:
a2So4 % wt.43.44 53.19 65.84 55.09 60.17 56.70
~` CRYSTALS Na2SO3 ~'34.51 31.21 23.47 22.80 24.58 19.48 '
~ ANALYSIS NaHSO3 (calc. as
'- (With Na2S2Os 7.52 6.78 r8.77 5.34
Adhering
Mother Na2S2O3 0.10 0.0810.69 0.08 23.82
Liquor H2O (by diff) " 14.43 8.74 13.34 9.83
.,~
CRYSTALS Na2SO4 % wt. 55.6 63.4 74.0 71.4 71.4 75.4
. ANALYSIS* Na2SO3 " 44.4 36.6 26.0 28.6 28.6 24.6
.
1( ) = Approximation
2Calculated free of mother liquor and expected on a dry basis
-20-
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: ~079492
The results of these ]arger scale tests conf-irm the laboratory
results shown in Example I. The data clearly show that the
dry sollds were high in sulfate content, and generally that ~ -
the ~ulfate content decreased as the amount of ~oli~s forrned
durlng the evaporation increased.
, ' .
EXAMPLE 3
To determine the effect of sulfate and sulfite concentra- :
tion in the spent absorber solution on the sulfate content of -~ -
10 the separated crystal product, batch evaporation tests were
performed on synthetically prepared, spent absorber solutions
as would be obtained in an absorptlon operation of the type
shown in the drawlng. In the tests the spent absorption
solution was placed in a flask maintained in~a constant tem-
perature bath. The evaporations were conducted under vacuum
at~the temperatures noted in Table III, and were contlnued
until the indicated amount of solids had been formed in a
¦ ~ glven spent absorptlon solution. The water evaporated in each
test was condensed and collected. The crystals were separatéd
~rom the mother liquor at the temperature of evaporation.
The~levels~of sulfate in the spent absorber solutions were ~ ;
varied in the~tests but the percentage of the salts in solu-
tion~were kept relatlvely constant. The most pertlnent data
, ~ obtalned in these tests are in Table III.
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-22-
107949~
The data of''rable III :Illustrate the effect of the sulfate and
sulfi~e concentrations in the spent absorber solution on the
concentration Or sulfate ln the separated crystal product.
These data show that an increase of' about 1 percent sulfate
(at constant sulflte concentration) in the spent absorber
solution gave an increase of about 5 to 10 percent sulf'ate in
the dry crystal product. Also a decrease of about 6 percent
sulfite (at constant sulfate concentration) in the spent
absorbing solution gave an increase of about 15 to 20 percent
.
sulfate in the dry crystal product.
Various modifications and equivalents of the process of
this invention will be apparent to one skllled in the art, and
may be made without departing from the spirlt or scope of the
invention.
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