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Патент USA US2134481

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Oct. 25,1938. '
‘
H. F. JOH‘QNSTONE.
PROCESS "OF REMOVING SULPHUR DIOXIDE FROM WASTE GASES
‘
Filed Dec. 23, 1955
‘
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Patented @ch 25,
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PWCESS or annoying ‘antenna moms
Henry F. Johnstcne, wrbanariiiq assignor, by
mesne assignments, to Commonwealth Edison
@ompany, Chicago, lllit, acorporation oi ,Ilii- _
"
nois
Application December 23,l_l9?35, Serial No. 55,713 _' '
1 Claim.
‘(or rem-ire)
ed to removing and recovering sulphur'dioxide
videia process in which the optimum concentra
tioh of ‘the extracting solution is' predetermined
and ‘controlled to produce, the greatest absorption
capacity‘and reducethe cost of liberation of the
absorbed? sulphur dioxide.
ess, although the process is not limited to these
particular types of waste gases.
110?
It has become a problem of increasing impor
tance in recent years to prevent escape of sulphur
diomd’e from ?ue gases, smelter gases and the,
like into the surrounding atmosphere, due to the
formation of sulphuric acid by the oxidation of
Ed the sulphur dioxide followed by combination with
v‘water vapor in the air.
'
~
'
A. number of methods of obviating this problem
have been developed, but the cost of such math-.
ods has usually been prohibitive of their wide
20 spread adoption, and the recovery of lay-products
oi. any substantial commercial value has been
insumcient to cover the operating costs of the
process.
I
'
Onemethod of removing and recovering sol
‘so
' ‘
considerably with respect. to the ‘quantity oi‘. sul
phur‘dioxide recovered ‘from the gases.
from waste gases, such as boiler .andiurnace
gases containing a relatively small‘ percentage
of sulphur dioxide, by a cyclic regenerative proc
25
'
This invention relates to a process of removing
and recovering sulphur dioxidefrom gases con
taining the same, and more particularly is direct
phur/ diomde from waste products has been de—
scribed brie?y in my copending application,
Serial No. 665,337, filed April 10,_~1933,-now Patent
No. 2,082,006, and consists brie?y of a cyclic proc
ess involving the washing of gases containing
relatively small percentages of sulphur dioxide
‘ ‘ One object of the present invention is to pro
' ;’Another‘ advantage of the present invention is
the ‘control of the concentration of the extracting 10
solution in accordance'with variationsin the fac
tors controlling the operating conditions‘ under‘
which the process is carried out, to insure. the
most‘ e?cient concentrationv under any operating
15
conditions- which may be encountered.
- I'have found also that the composition of the
gas bears a direct relationship to the optimum
concentration of solution employed, depending
on the regenerating. temperature of. the solu
tion. i From these relationships and the eiiiciency 20
‘of the scrubber and regenerator, I have been able
to determine the optimum concentration of the ,
solution as these operating conditions vary,
thereby providing for most e?cient operation of
the solution.
‘
25
-
Another feature of the present invention, is
the reduction of these various relationships to a
single mathematical equation relating these oper»
ating conditions to the optimum concentration
of solutions so that for any given set of opera 3o,
in a scrubber or absorber with an extracting solu
ating conditions the desired concentration can
tion capable of ‘absorbing sulphur dioxide from
these gases at a low temperature, and liberating
the absorbed sulphur dioxide in a regeneration
be readily determined.
‘at . step by heating, whereupon the regenerated solu
‘
‘
Other objects and advantages of the present
invention will appear more fully from the follow
ing detailed description which, taken in conjunc
35
tion with the accompanying drawing, will disclose
sulphur dioxide from the gases. The extracting to those skilled in the art the particular man
solution employed may comprise ammonium sul- ‘ (‘ner of carrying out a‘ preferred form of the pres
tion is then returned for further absorption of
phite and ammonium bisulphite.
40
I have found that the e?iciency of the process
> ent inventionr
'
'
_Iri the drawing—
.
>
40
Figure l is a diagrammatic view of the cycle of r
is greatly ‘increased if the concentrationbf the
extracting solution is accurately controlled in ‘ operation of the process disclosed in the present
accordance-with the concentration of the sul
phur dioxide in the gases being treated, the
45 e?lciency' of the scrubber, the temperature of the
invention; ‘
Figure 2 is a graphical illustration of the rela- '
tionship between certain of the factors a?ecting
extracting solution leaving the scrubber, the , the concentration of. the solution; and
temperature of regeneration and the ef?ciency of
Figure 3 is a similar graphical illustration of
the regenerator. This control gives-an optimum additional relationships, occurring according to
.
concentration of solution for various operating the teachings of the present invention.
The cycle of operation of the process is shown
conditions, and results in producing the greatest
> absorption capacity in the solution while requir
diagrammatically in Figure 1, and follows a
ing the least quantityof steam per pound of. - cyclic system comprising absorption at a low
sulphur dioxide recovered in the. regeneration temperature. and regeneration, or removal of the
process, and thus imports to the process an effi
sulphur dioxide from the absorbing‘ solution, by
ciency capable of reducing its operating cost stripping of this ‘solution with steam at a tem 65
2
2,184,481
conduit 22, or it may be generated in an evap
orator 23 which is heated in any suitable manner,
perature near the normal boiling point of- the
solution, depending upon whether a vacuum or
pressure above atmospheric pressure is em
as by the burner 24. .
The regenerated liquorI called the extractor,
leaves the boiling pot through the conduit 2!, and
is. then preferably pumped through the hot side‘
ployed.
The extracting ‘solution in a preferred em
bodiment of the invention is one containing am
of any suitable heat exchanger which may be
monium sulphite and ammonium bisulphite.
However, the method of determining the optimum
concentration is general,- and other equations
connected between the conduits 2| and .25, being
cooled by this heat exchanger, which reduces its
temperature to approximately the temperature of 10
the solution leaving the scrubber through the
pump it. The liquor which has been regenerated
10 could be developed for other systems such as those
using sodium sulphite and sodium bisulphite. In
this latter case, however, the limiting concentra
tion ?xed by the solubility of the salts would be
and passed through the conduit 25 enters the
top of the scrubber through proper distributing
different and much lower than that for the am
15
monia system.
'
Following the ?ow cycle
devices 26.
The vapors which leave the top of the re
generating column I! contain a small percentage
of ammonia, which is removed in an ammonia
scrubber 21. This scrubber preferably contains a
relatively small area of absorbing surface which
is wet by water, or preferably by condensate
obtained in the removal of the water vapor from
the sulphur dioxide. The ammonia scrubber per
se forms no part of the present invention, and for
a description of its operation reference should
-
ticall
shown in Fig. 1, the waste gases from a boiler,
furnace or other gas producing structure are
passed from the stack through the conduit I0
20 into the lower portion of an absorber [2. These
gases may ?rst be precooled before entrance into
the absorber by a spray washer, which effects the
removal of dust particles, although not mate
,rially a?ecting the subsequent treatment of the
gases. A suitable cooler I3 is employed within
the absorber, having control valves ll connected
thereto for controlling the amount of cooling
effected thereby, in order to control the tempera
ture of the absorbing solution leaving the scrub
ber l2, which scrubber contains any suitable type
is
be had to the copending application of myself
and Alamjit D. Sing’h, Serial No. 97,550, filed
August 24, 1936. In either case, the ammonia
reacts rapidly with any dissolved sulphur di
oxide, and, since the partial pressure of sulphur
dioxide in the vapor is much higher than that
of absorbing surfaces which are suitable for
of the ammonia, the solutionv is very acidic and
handling large quantities of gas at low draft loss
and will give the fastest rate of transfer of the
sulphur dioxide under a low diffusion‘ gradient.
The purpose of the cooler i3 is to have the
solution leaving the scrubbing surfaces in contact
the absorption of the ammonia is complete. The
vapors leaving the ammonia scrubber may pass
directly to a suitable'condenser for removing
water from the sulphur dioxide, leaving the latter
in a concentrated and substantially pure state-or, '
as is possible from the standpoint of the cost of _ for economy of steam, they may pass first to a '
with the entering raw gas at as low a temperature
compressor. In either case, the sulphur dioxide
and moisture is passed outwardly of the scrubber
21 through the conduit 20, and the recovery of
sulphur dioxide in a substantially pure state by a
subsequent operation is not believed necessary of
detailed description herein. The liquid removed
from the ammonia scrubber and the water re-_
moved from the sulphur dioxide, or an equiva
lent amount of make-up water is returned to the
construction and operation. For ?ue gases con
taining approximately three-tenths per cent sul
phur dioxide (by volume) at a temperature of
150° C., and having a humidity of 0.045 pound
water per pound of. dry gas, the exit temperature
of the solution should be approximately 45° 0.,
which is, in fact, a few degrees below the wet
bulb temperature of thegases.
Thegasesleavethetopofthescrubberthrough
an eliminator II, which prevents the loss of the
extractor or scrubbing solution therethrough.
'lhe extracting solution leaving the bottom of
the scrubber?rstthroughssettllngtank
which removes most of the
solids, the
tankbeingindicated at llandbdngconnected
with a suitabledrain l'l, andtheliquor err-solution
then
through the pump ll which forces it
through suitable heat exchangers (not shown) to
‘
system.
.
The present invention concerns itself particu
larly with the provision of an optimum concen
tration in the extracting solution which enters
the scrubber i2 through the distributing devices
20. In the use of such an ammonium sulphite
bisulphite process, it has been found that in the
treatment of gases containing approximately 0% 55
sulphur dioxide, where the absorption tempers- '
thetopoftheregeneratingcolumn llwhereitis
ture is maintained approximately 25 degrees 0., I
discharged downwardly from suitable
the concentration should be approximately 100 to
200' grams per liter of ammonium sulphite, and
approximately 700 to 800' grams per liter of am
monium bisulphite. This corresponds to approx
imately 22,4 moles ammonia per 100 molesHsO
and 17.5 moles 50: per .100 moles 8:0. I
I have found, however, that for dilute gases,
containing 0.5% sulphur dioxide or less, espe
cially when the temperature of abmrption is
about 35° C., the quantity of steamrequired for
regeneration is considerably less when a less
concentrated solution is employed. Furthermore,
means v2| connected to'the conduit 2|. " The col
umn II is provided with surfaces which give con
tact between vapors and liquor. In this case,
however, the quantity of vapor is small compared
withthatoftheoriginalgaaandthedraftloss,
or frictional
may be
larger than that permissible for the scrubber.
The regenerator I0 is a device employed for
separating the sulphur dioxide from the liquor,
and should produce as high a concentration of
this gas as ‘possible. The stripping of the‘ liquor
may be accomplished either by steam alone, or
the capacity of the solution, expressed in pounds
the surface of the liquid or below its surface and
of sulphur dioxide recovered per pound of ex
tracting solution employed, is ata maximum
when the concentration of ammonia is below 22
moles per'100 moles of water. The‘ optimum
bubbling .up through it, through suitable steam
concentration for these two important factors in i
_ by steam in combination with another chemical.
When steam is used, it may be introduced directly
into the bottom of the regenerator II, either above
area-tar
is equal to the partial pressure of the sulphur
dioxide in the raw gases entering the scrubber.
‘the operation of the process, I have found, ap
proximately coincides. It has become evident, in
my examination of this phenomenon, that the
value of the optimum concentration varies from
one condition of operation to another. At least
five operating conditions must‘ be considered,
TABLE I
Optimum concentrations of ammonia for gases
'
namely, the temperature of absorption, the tem
perature of regeneration, the concentration of
containing 0.3% so: (by volume)‘
a
Y
t.-35° c.
:.--40
t-ls
:.=-5o
t.-55
I
sulphur dioxide in the raw gases, and the e?l- '
10 ciency of the scrubber and'regenerator.
I have ‘found that it is possible to obtain an
MOLES or AMMONIA PER 100 MOLES or WATER
approximate mathematical equation for the
maximum in the capacity-ammonia concentra
tion curve on the basis 01' two fundamental
15 equations relating the vapor pressures of sulphur
dioxide and ammonia to the composition of the
solution. The equations have‘ been found to be
‘valid for all conditions of operation likely to be ’
encountered. These equations are:
20
as
24.2
20.4
‘ 17.1
2117
22. 4
21.2
1a 5
17.7
15. 2
14.7
1a s
'
12.
10.
10.4
1 15.9
13.0
10.4
a
ma
14.2
13.0
11.3
10.0
9.0
0.0
7.3
7.
a
1o
14.
15
The optimum concentration of ammonia de
creases as the absorption temperature increases,
decreases as the regeneration temperature in
creases, increases as the sulphur dioxide con
centration of the gas increases, and increases as
'_ (25-- C)2
Pam-f M c___ S
the emciency of the scrubber increases the satu
ration of the solution. It should be emphasized, '
25 where P is the vapor pressure of the correspond- _
ing component in millimeters of mercury, S is
the concentration oi’ sulphur dioxide in the solution, expressed as moles-per 100 moles of water,
30 C is the concentration of ammonia in the solu-.
tion, in the same unit, M and N are constants
which depend only on the temperature of the
solution, as follows:
log M= 5.865- 2:22-92
35
If the vapor pressure of sulphur dioxide over
the solution leaving the scrubber is substantially
equal to the partial pressure of sulphur dioxide
45 so that as much S02 is removed from the solu
tion as is possible, the equation for the capacity
.
‘
approximately 0.3% sulphur dioxide, with an
absorption temperature of approximately 45° C’.
35
onia in the solution giving the
economical operation lies “
somewhere within the range of 11 to 14 moles of
ammonia per 100 moles of’ water, whereas Table
l shows that the calculated optimum concentra
as
tion is approximately 13.0 moles.
Finally, it is to be recognized that there must
be an upper'limit of the concentration, deter
mined by the solubility of the ammonium salts.
This has been found to exist at approximately 45
(3:22 moles per 100 moles oi.’ water. Above this
concentration, the calculated optimumrvalue of
C is meaningless. As a general rule, therefore.
capacity
for low temperatures 02 absorption and high
where a is the ratio of the-partial pressure of
sulphur dioxide in equilibrium with the‘ solution
leaving the scrubber to-the constant M at the
temperature of absorption and tr is the tempera
55 ture of regeneration in degrees centigrade. The
parameter a- includes, the ' e?iciency of the
scrubber, the'concentration of the gas and the
temperature of absorption, the latter being desig
nated below as ts.
The maximum in this equation corresponds
closely to the optimum concentration of am
monia desired. By the use of calculus, the maxi
mum may be related to the operating conditions
as follows:
65
1+
70
As a typical example of the application of my 80
invention to the process when the gases contain '
, most emcient and
100% emciency, and the regenerator is operated
'60
optimum.
100° (3., I have found that the optimum con
in the raw gas, 1. e., the scrubber is operated at
50
tically the same results can be obtained with
concentrations within a range of- 10% of the
centration of
and T is the temperature in degrees Kelvin.
of the solution becomes:
most conditions are not sharp and that prac
‘ and a regenerating temperature of approximately
‘log 1v= lasso-5%?!
40
however, thatthe optimum concentrations for 25
8cm!!!
a
The following table shows different values of
Cm; for several conditions for gases containing
0.3% sulphur dioxide, based on the assumption
that the scrubber operates at 100% e?iciency,
i. e., the vapor pressure of sulphur dioxide in
equilibrium with the solution leaving the scrubber
- concentrationsoi sulphur dioxide, such as are 50
likely to obtainln the recovery of sulphur dioxide .7
from dry smelter gases, the preferred value oi‘
C would be apprommately 22. However, for
boiler furnace gases containing up to 5% S111
phur dioxide and for which the absorption tem 55
perature is ?xed at 35‘ C. or above, a definite ad
vantage can be derived by utilizing the optimum
concentration of ammonia, determined as here
inbefore described.
“
InFigure 2 of the drawing I have disclosed a 60
graph of the values which show the e?ect of the
temperature of absorption and of regeneration on
the optimum value’ of C.‘ Since the constant a
is directly proportional to the concentration of
sulphur dioxide in the gases, it is possible to 65
show the e?ect of changing the gas composition.
This is shown in Figure 3 in which the absorp
tion temperature is maintained at45° 0..
I
Referring
1. to Fig. 2, the abscissae of this
graph are the regeneration temperatures in (re, 70
grees centigrade, ranging from 70-degrees'to-120
degrees. The ordinates represent the optimum
concentration of ammonia in moles per 100 moles
of water, ranging from zero to 32 moles. The
values shown in Table I for the various absorption
14
temperatures are plotted in Figure 2 and repre
sent the relationship existing between the opti
mum concentration oi'_ ammonia desired and va
rious absorption temperatures and regeneration
temperatures. For example, when the absorp
ture of about ‘10° C. These various ranges of
optimum concentrations of ammonia as indicated
by these curves are set forth in the followin
a table:
v
.
/
tion temperature is 35 degrees C. and the regen
eration temperature is only 70 degrees‘ 0., a
molecular concentration of ammonia of the order
of 29 moles is indicated, but since the solubility of
10 the salts indicates a de?nite upper limit of 22'
Partial presume in millimeters
,
_
~
' 5
Concentration
of ammonia in
moles per 100
moles of water
in
moles, for all values indicated as above 22 moles,
, the 22 mole concentration is considered optimum.
The concentration decreases rapidly as the re
generation temperature is increased, being ap
'15 proximately 14% moles at a regeneration tem
perature of 120 ‘degrees. It will be apparent that
the lowest molecular concentration is required
when the absorption temperature and regenera
tion temperature are relatively high.
It will be apparent, from the curves shown
tween the various factors a?ecting the operating
conditions, I am~ able to provide an extracting
solution which is capable of recovering the great 20
est quantity of sulphur dioxide per pound of ex
tracting solution employed. This materially aids
_in Figure 2, that the optimmn concentration of
ammonia in the scrubbing solution for scrubbing
raw’gases containing approximately 0.3% sui - in reducing the operating cost of the equipment,
Phur dioxide by volume varies from about 14.5 and also in reducing the cost of recovering the
moles to 22 moles per 100 moles of water for vari
sulphur dioxide. In addition, it provides for
ations in the temperature of regeneration from ' maintaining the concentration of the extracting
70° C. to 120° C. for a temperature of absorption solution at an optimum quantity in accordance
of about 35° C. These various ranges of opti
with the various factors ailfecting the operation
mum concentrations of ammonia as indicated by of the process, resulting in the use of a‘ solution
80 these curves are set forth in the following table: which is most economical under suchioperating
conditions;
1
' _,
It will be readily apparent that in the operation
Temperature of absorption
moles W 100
of a process or this type, in which continuous cy
moles of water
clic operation is effected, certain of the operating
conditions can be controlled so as to remain sub
stantiallyconstant. Thus, by proper control of
cooler It, with a known incoming temperature
case
of the gases, the absorption temperature can be
maintained substantially constant, and conse
quently the optimum concentration of the solu
In Figure 3, the absorption temperature is
maintained at 45 degrees C., and the curves illus
trate the e?ect oi’ the concentration of sulphur
45 dioxide in the original gases at various regenera
tion temperatures running from '70 degrees to
120 degrees, respectively, in steps of 10 degrees
based on the assumption that the scrubber oper
_ ates at 100% emciency. The ordinates again.
represent the maximum or optimum concentraf
tion can be determined in accordance with the
concentration of sulphur dioxide in the gases and
the regenerating temperatures. Similarly, other
of the‘ factors might be maintained constant and
the concentration varied in accordance with
variations in the absorption temperature, as de
termined by the equation giving maximum opti
mum concentration for any set of operating con
ditions;
While I have disclosed a method of determin 50
tions of ammonia in moles‘ per 100 moles of water, ' ing the optimum concentration of the solution
while the abscissa-e represent the partial pressure in/accordance with variations in factors a?ecting
of sulphur dioxide in the gases in millimeters of
mercury. It will thus be seen that with a con» the operating’ conditions, it is to be understood
55
'stant absorption temperature, the optimum con-
that the present inventionalso contemplates a
reversal of that procedure. I have found, for
example, that it may be more e?icient to change
some of the operating conditions to fit the am
centration of ammonia increases as the partial
pressure of sulphur dioxide in the gases increases,
and that for various constant relationships be
concentration. Thus the temperature of
tween the absorption and regenerating tempera ' monia
the
solution
leaving the scrubber may be varied
ture,‘ the concentration of. the solution must be byvaryingtheamountorcoolinge?ected
by coil 80
increased with ‘increases in the percentage-of, > .li, as the concentration of the sulphur vdioxide
sulphur dioxide present in the gases. These rela 'inthegaseschanges. Likewiseitmaybedesir-v
tionships are shown in the formula or equation’,
which gives the optimum concentration 01' am;v - able to vary the temperature of regeneration in
accordance'with variations in the temperature of
65 monia under various conditions of temperature, _ absorption and/or concentration of sulphur di-' 65
eihciency of the apparatus, and percentage of
oxide in the gases. With such control, the con
sulphur dioxide in the raw gases to be treated... ~
The curves shown in Figure 3 indicate that the centration of the solution would still be optimum
optimum concentration of ammonia in the with respect to the operating factors, and great-'
_
scrubbing
solution for absorbing at a temperature est e?iciency thereby produced.
70
70
of about 45° C. sulphur dioxide from raw gases in _ -
'
which the partial pressure of the sulphur dioxide
varies from about 1.4 to 2.5 millimeters of mer
cury. ranges from about 14.5 moles to 22 moles per
76 100 moles of water for a regeneration tempera
‘ I am aware that the present method of ob
taining optimum concentrations of the solution
in accordance with the various factors a?’ecting
operating conditions may be capable of modi?
cation and change, and I do notintend to limit 75
0,1
‘ my disclosure except as dlc?ncd by the scope and
4cm
tiaiiy M“ G. on i‘
spirit of the appended claim.
I
claim:
.
‘
-
v
'
‘The method of recovering sulphur dioxide fro
gases containing approldmotcly 0.3% sulphur di
' oxide which comprises passing sold cases into
‘ contact with an aqueous solution of
. sulphitc and
W
‘ ‘l iii-o; 1% M30198 M =‘1M-‘nin per 1M) moles
onium bisulphitc at
onium ,
of water, suhceqlucu?y heating said solution to
100° 0. to liberate said sulphur dio'mde, and re-é
-
=-
‘-
midnen
with said u.
mm»;
'
solution for contact
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