Note: Descriptions are shown in the official language in which they were submitted.
3~ :
This invention concerns the removal of sulphur dioxide
(SO2) from gases such as industrial effluent gases that are to be
discharged to atmosphere.
Various processes are known or have been proposed for
this purpose. One such process, often cal]ed the "sulphite"
process, uses a solution of an alkali sulphite/bisulphite to
absorb SO2 from gas scrubbed by that solution. The spent
solution is then treated in various ways to recover the SO2:
t~pically the spent solution is acidified with sulphuric or
phosphoric acid to produce the corresponding alkali sulphate or
phosphate as a byproductO
As hitherto practised or proposed, this process has
certain disadvantages; thus it is not usually possible to
reduce the SO2 content of the gas to less than about 200/250ppm,
I while the treated gas tends to become contaminated with
; sulphite and/or bisulphite salts that lead to the formation
of a mist upon discharge of the treated gas to atmosphere, and
environmental pollution.
An object of the present invention is, therefore, to
provide an improvement of such a process so as to enable it to
produce a treated gas with much lower SO2 content than is usually
possible, and to eliminate or substantially reduce the likelihood
of the formation of sulphite and/or bisulphite mists upon
discharge of the treated gas.
The invention accordingly provides a process for
removal of sulphur dioxide from an e-ffluent gas comprising
treating said gas in as least two successive stages with an
absorbing solution selected from the group consisting o~f an
alkali sulphite solution and an alkali sulphite and bisulphite
solution, part of said absorbing solution having been pretreated
with air or with a gas that is inert with respect to the solution
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to reduce the S02 partial pressure of saicl absorbing solutlon,
wherein said pretreatment of part of the absorbing solution is
accomplished as a partial solution regeneration step in a cyclic
process in which the pretreated absorbing solution is repeatedly
;~ circulated through an absorption stage and a regeneration stage,
S2 absorbed by the solution in the absorption stage being par-
tially stripped from the solution in the regeneration stage and
reabsorbed by the solution concurrently with the absorption by
the solution of S02 from the S02 - con-taining gas, whereby the
So2 content in the effluent gas is reduced to 50 ppm or less.
The wording "alkali sulphite/bisulphite solution" as
used in the present specification to indicate the absorbing solu-
tion is to be meant as referring to a solution whi.ch can either
contain both sulphite and bisulphite in changing proportions, or
only the alkali sulphite.
As compared with prior art practices, the pretreatment
of the sulphite/bisulphite solution with air or inert gas, -there-
by to reduce its S02 partial pressure, before treating the S02-
; containing gas with the solution enables the latter to absorb S02
20 very efficiently and to achleve an S02 content of 50 ppm or lessin the treated gas.
, In practical embodiments of the i~vention the pre-
treatment of the solution is accomplished as a solution regener-
a-tion step in a cyclic process in which solution is repeatedly
~; circulated through an absorptlon stage and a regeneration stage,
S2 absorbed by the solution in the absorption stage being par-
:;
tially stripped from the solution in the regeneration stage.
We have found that although pretreatment with air or
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free oxygen-containing gas is effective to lower the S02 partial
pressure of an alkali sulphite/bisulphite solution satisfactorily
for the purposes of the process of the invention, optimum reduc-
tion of the S02 partial pressure is accomplished by the use of
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substantially fxee oxygen-free gases for the pretreatmen-t. Ideally
the gas should have a free oxygen content not more than about
1% by volume: suitable gases are, for instance, nitrogen ~N2)
carbon dioxide (CO2) and mixtures of N2 and CO2, combùstion pro-
duct gases containing about 1% oxygen by volume have been success-
fully employed for the pretreatment.
We canno-t fully explain the reason for the enhanced
lowering of the SO2 partial pressure by pretreatment with a sub-
stantially free oxygen-free gas as compared with the treatment
of the solution under the same conditions with an oxygen-con-
taining gas such as air: however, i-t would appear that the use,
for pretreatment,of gas substantially devoid of free oxygen
avoids or substantially reduces the formation of sulphate (SO~ )
ions, the presence of which :in a sulphite/bisulphite so].ution
perhaps ls effective to raise the S02 partial pressure.
The process of the invention is preferably conducted
as a multi-stage pr~cess, the SO2-containing gas being first
treated with an alkali sulphite/bisulphite solution that has
a relatively high SO2 partial pressure in comparison with the
. 20 pretreated solution that the gas encounters in a subsequent
stage. The flnal treatment of the gas is preferably a treatment
with water, especially when the alkali sulphite/bisulphite solu-
tion is an ammonium sulphite/bisulphite solution.
If there is more than one stage of gas treatment with
pre-treated suLphite/bisulphite solutlon, the pretreatment of the
solution for each such stage may be the same, or different pre-
trea-tments may be employed. For instance, the gas may be treated .: .
in successively encoun-tered stages with sulphite/bisulphite
solution that has been pretreated with air and with substanti-
ally free oxygen-free gas respectively.
Pretreatment of the sulphite/bisulphite solutlon may
also serve to establish a solution pH that is most suitable for
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effective absorption of S02 by the solution. This is particularly
the case when the solution is an ammonium sulphite/bisulphite
solution.
The pretreatment of the solution with air or other
gas ma~ be accomplished in a packed or plate column in which the
air or other gas flows countercurrently to the solution. The
air or gas used for the pretreatmen-t may be at normal tempera-
tures but is preferably at an elevated temperature. However, .
corrosion problems arise if an attempt is made to pretreat with
air or other gas at temperatures significantly in excess of ~0C.
and therefore in preferred practice of the invention the pre-
treatment is accomplished with air or other gas at a t.emperature
not exceedincJ 80C. By observing this limit t~e need for high
cost corrosion-resistant material in the pretreatment equipment
is avoided.
Depending upon the nature and concentration of the
~, pretreated sulphite-bisulphite solution, the formation of sul-
~i
~ phite/bisulphite mists upon discharge of the treated gas to
,' atmosphere is minimized by holding the solution pH at a ~alue
.~. 20 in the range S.5 to 7.5 when in contact with the gas being trea-
,' ted. In the case of a typical ammonium sulphite/bisulphite solu--
tion the pH value should ideally be kept below 6.5. pH control
requires suitable regulation of .the makeup of alkali and in the
.` case of ammonium sulphite/bisulphite solutions ammonia makeup
.~ should be made in the liquid phase to minimize the tendency to
:~ mist formation. It is to be noted that the pretreatment of
ammonium sulphite/bisulphite solution reduces the ammonia partial
, pressure as well as the SO2 partial pressure of the solution,
and thus reduces the tendency of SO2 in the gas to react with
ammonia to form fine particles of ammonium sulphite and bisul-
phite salts that are readily entrained in the gas and conduce to
the forma-tion of mists upon discharge of the gas to atmosphere.
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A typical embodiment of the process of the invention
will be described by way of example with reference to the accom-
panying drawings, in which:
~igure 1 is a block diagram of apparatus for carrying
out the process of the invention in accord-
ance with a preferred embodiment thereof,
and
Figure 2 is a semi-logarithmic plot of S02 partial
pressure against temperature for an ammonium
sulphite/bisulphite solution both without
~ pretreatment and as pretreated under various
; conditions.
Referring first to Figure 1, line 1 represents an
inlet line for gas to be treated by the process of the inverltion.
, In a particular installation of the apparatus to be described, the
gas was the off-gas of a sulphuric acid production plant, having
an S02 content of 0.18% by volume and entering the apparatus
through ~ine 1 at a flow rate o~ 90,000 Nm3/hour.
The illustrated apparatus comprises an absorption col- :
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umn 2 having three stacked sections 2a, 2b and 2c, the gas from
l.ine 1 entering the lowermost section 2a of the column 2 to flow
upwardly through that sec-tion and successivel.y through sections
2b and 2c.
In the lowermost section 2a the gas is scrubbed with an
ammonium sulphite/bisulphite solution that is sprayed into the
. upper part of the section 2a through nozzles 3 fed by a pump ~
drawing solution continually from the bottom of the column. The
pH of the solution sprayed into the section 2a is maintained at :
a value in the range 5.7 - 6.0 by the regulated sparging of
gaseous ammonia (NH3) into a pool of the solution at the bot-tom
of the column 2, from a line 11. The spray nozzles 3 and the
rate of recycling of solution through these nozzles are designed
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and selected so as to provide a solution contact surface in the
range 15,000 - 20,000 m per m of absorption volume in the section
2a.
The gas -that has been subjected to preliminary scrubbing
in the absorption column section 2a rises i.nto and through the
section 2b that, as indicated, is a packed section through which
the gas passes upwardly in countercurrent to a descending pre-
treated arnmonium sulphite/bisulphite solution fed to the section
2b by a pump 5 througha line 6. The pretreated solution fed through
line 6 is in fact a portion of a bled-off sidestream of the solu-
tion being circulated through the lowermost absorption column
section 2a by the pump 4, after the bled-off sidestream portion
has been substantially stripped of S02 (and excess free ammonia)
by treatment with air or, preferably, substantially free oxygen-
free inert gas in a regeneration colurnn 7.
After absorbing S02 from the gas in the section 2b, the
solution falls into the section 2a to join the solution being
circulated through that section.
In the absorption column section 2b, the gas is strip-
ped of S02 down to very low residual levels. Although the gasmight then be safely discharged to atmosphere with little risk
of environmental pollution and the formation of mists of sulphite
and bisulphite salts, the gas leaving the sec-tion 2b, is, however,
given a further treatment in section 2c, again a packed section,
in which the gas is washed with water circulated through the
section 2c by a pump 8, makeup water enters the head of the col-
umn from a line 9 and is also utilized for washing the gas in
the section 2c.
The regeneration column 7 is surmounted by an acidi-
fier 10 comprising a packed column section.
As no-ted, only part of the bled-off sidestream of the
solution being circulated in the lowermost absorption column
section 2a by the purnp 4 is fed, by a line 12, to the head of
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the regeneration column 7 to be pretreated before entering the
absorption column section 2b via the line 60 In the regeneration
column 7, this sidestream portion descends through packings ln
countercurrent to a flow of air or, preferably, free oxygen-
free inert gas that is pumped into the lower part of the column
7 by means of a compressor 13 and vented from the head of column
7 through a line 14 leading to the bottom section 2a of the ab-
sorption column 2 so that the stripped S02 and free ammonia are
reabsorbed in the ammonium sulphite/bisulphite solution in -the
column 2.
The remainder of the bled-off sidestream of solution
is fed to the acidifier 10 through a line 15 to join a ~stream
of acidified solution circulated through the packing in the
acidi:ier 10 ~y means of a pump 17 and external line 18 includ-
ing a heat-exchanyer 19 for cooliny the circulating solu-tion.
Acid, in this case sulphuric acid, is added to the circulating
solution through a line 16 connected to the suction of the pump
17.
The S02 that is liberated by the acidification of the
sulphite/bisulphite solution is vented from the acidifier 10
through a line 20 that in the case considered extends to the
drying tower of the associated sulphuric acid production plant.
The energy required -to overcome the back pressure of the drying
tower is provided by the admission of compressed air to the
lower part of the acidifier 10, the compressed air being fed
from a compressor 21 through a line 22 and a heat exchanger 23
for cooling the air to a suitable temperature. In addition to
providing the energy for the purpose indica-ted, the compressed
air in ascending through the packing of the acidifier in counter- -
current to the circulating acidified solution effects strippingof the liberated S02 from the solution.
The volume of acidified solution in the c:ircuit com-
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prising the acidifier 10 is maintained by bleeding off a portionof the circulating solution through a line 24 that conveys the
bled-off solution to an ammonium sulphate production plant.
The treated gas leaves the absorption column 2 through
a stack 25. In the example considered and using air for pretreat- -
ment of the solution introduced into the absorption column sec-
tion 2_, the treated gas had an average S02 content of 22 ppm
by volume. The hourly consumption of energy and of materials
was as follows:-
Electricity130 Kwh
Cooling water3000 kg
Makeup water5400 ]cg
Ammonia 160 kg
Sulphuric acid 680 kg
The foregoing description of an embodiment of the pro-
cess of the invention refers to the use of an ammonium sulphite/
bisulphite solution for absorption of S02 from the gas to be
treated. It should be understood that the process of the inven-
tion may be performed with other alkali sulphiteJbisulphite
solutions, for instance sodium or potassium sulphite/bisulphite
solutions, using apparatus generally similar to that described
although in such cases the final water-washing of the ~as, as
in the absorption column section 2c, might be omitted without
a significant effect upon the residual contamination of the
treated gas with S02 and mist-producing salts.
Also the acidification of the bled-off spent sulphite/
bisulphite solution may be performed with an acid other than
sulphurîc acid to liberate the absorbed S02 and to produce a
byproduct salt: for instance, phosphoric acid might be used for
acidification of the spent solution introduced into the acidifier
10 in the described apparatus.
As noted in the description, the gas used for pretreat-
ment of the solution introduced into sec-tion 2b of the absorption
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column may be air or another gas that is inert with respect to
the solution, and is preferably a gas containing not more than
1% by volume cf free oxygen. Improved S02 removal is accomplished
by the use of such preferxed pretreatment gases and whilst such
a gas may be used in the apparatus as described, being intro- -duced into the regeneration column 7 by a compressor 13 in the
manner described, the apparatus might in some cases be modified
so as to achieve the benefits of the use of such a pretrea-tment
gas with less extensive u-tilization thereof. .:
Thus, for instance, absorption of S02 from the gas to
be treated might be conducted in several stages in one or more
absorption columns, one or more later stages using an absorbing
solution pretreated with substantially free oxygen-free gas, ::
whilst earlier stage(s) use absorbing solution pretreated with
air or a gas containing free oxygen, it being understood that .
in such case the process may also include an initial stage or `.stages in which the gas is treated with an unpretreated ab~
sorbing solution, and may also include a final water-washing
stage as in the embodiment described.
, Referring to Figure 2 of the drawings, partial pres-
sures of S02 are plotted as a function of temperature for a solu-
tion containing ammonium sulphite and bisulphite. The curve 1
relates to the solution without pretreatment, while the curves
2 and 3 respectively relate to the same solution after its
pretreatment with alr at 40C. and 60C. respectively. Curves .
4 and 5 relate to the same solution when pretreated with nitro-
gen at 40C. and 60C. respectively.
The curves show that pretreatment of -the solution with
air results in a reduction of the S02 partial pressure of the 1l
30 solu-tion at any temperature, the reduction being the greater :
the higher the temperature of the air used for pretre,atment. The i~
curves also show that pretreatment of the solution w:ith nitro-
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gen at any given temperature produced a greater reduction of
the S02 partial pressure than the equivalent pretreatment of
the solution wi-th air of the same -temperature.
Other tests have shown that similar pretreatment of
the same solution by combustion product gases having a free
oxygen content of about 1% by volume gives results almost iden-
tical to those obtained with the use of nitrogen.
The additional reduction of S02 partial pressure
obtained by the use either of nitrogen or of combustion pro-
duct gases containing about 1% free oxygen by volume amountedto about 10% of the S02 partial pressure reduction resulting
from similar pretreatment of the same solution with air or a
gas containing substantial amounts of free oxygen.
It is apparent from the curves of Figure 2 that the
h.igher the temperature of the gas used for pretreatment, the
lower the resuLtant partial pressure of S02 for any given tem-
perature of the solution.
The data for plotting the curves of Figure 2 was
obtained by percolating the ammonium sulphite/bisulphite solu- ~
20 . tion, in countercurrent to the flow of air or the chosen gas, ~:
through a column having a diameter of 60mm packed with glass
rings 15mm x lSmm to a depth of 2800mm.
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