Process for removing SO.sub.2 from gases

The invention relates to a method for the treatment of industrial effluent gases for the removal of sulphur dioxide, according to which the gases are contacted with an absorbing solution, containing alkali metal sulphite and bisulphite, the solution, before the contact stage, being pre-treated with air or other gas inert to the solution, preferably a gas devoid of oxygen, whereby the SO.sub.2 partial pressure of the solution is reduced, thus enhancing the SO.sub.2 removal from said effluent gases.

This invention concerns the removal of sulphur dioxide (SO.sub.2) from 
gases such as industrial effluent gases that are to be discharged to the 
atmosphere. 
Various processes are known or have been proposed for this purpose. One 
such process, often called the "sulphite" process, uses a solution of an 
alkali sulphite/bisulphite to absorb SO.sub.2 from gas scrubbed by that 
solution. The spent solution is then treated in various ways to recover 
the SO.sub.2 : typically the spent solution is acidified with sulphuric or 
phosphoric acid to produce the corresponding alkali sulphate or phosphate 
as a byproduct. 
As hitherto practised or proposed, this process has certain disadvantages: 
thus it is not usually possible to reduce the SO.sub.2 content of the gas 
to less than about 200/250 ppm, 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 the 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 SO.sub.2 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 the removal of sulphur 
dioxide from gases, such process comprising treating the gas with a 
solution containing at least an alkali sulphite, preferably with an alkali 
sulphite/bisulphite solution that has been pretreated with air or with a 
gas that is inert with respect to the solution in order to reduce the 
SO.sub.2 partial pressure of said solution. 
The wording "alkali sulphite/bisulphite solution" as used in the present 
specification to indicate the absorbing solution is to be construed as 
referring to a solution which 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, thereby to reduce its 
SO.sub.2 partial pressure, before treating the SO.sub.2 -containing gas 
with the solution enables the latter to absorb SO.sub.2 very efficiently 
and to achieve an SO.sub.2 content of 50 ppm or less in the treated gas. 
In practical embodiments of the invention the pretreatment of the solution 
is accomplished as a solution regeneration step in a cyclic process in 
which the solution is repeatedly circulated through an absorption stage 
and a regeneration stage, SO.sub.2 absorbed by the solution in the 
absorption stage being partially stripped from the solution in the 
regeneration stage. 
We have found that although pretreatment with air or free oxygen-containing 
gas is effective to lower the SO.sub.2 partial pressure of an alkali 
sulphite/bisulphite solution satisfactorily for the purposes of the 
process of the invention, optimum reduction of the SO.sub.2 partial 
pressure is accomplished by the use of substantially free oxygen-free 
gases for the pretreatment. Ideally the gas should have a free oxygen 
content not more than about 1% by volume: suitable gases are, for 
instance, nitrogen (N.sub.2) carbon dioxide (CO.sub.2) and mixtures of 
N.sub.2 and CO.sub.2 ; combustion product gases containing about 1% oxygen 
by volume have been successfully employed for the pretreatment. 
We cannot fully explain the reason for the enhanced lowering of the 
SO.sub.2 partial pressure by pretreatment with a substantially free 
oxygen-free gas as compared with the treatment of the solution under the 
same conditions with an oxygen-containing gas such as air: however, it 
would appear that the use, for pretreatment, of gas substantially devoid 
of free oxygen avoids or substantially reduces the formation of sulphate 
(SO.sub.4.sup.=) ions, the presence of which in a sulphite/bisulphite 
solution perhaps is effective to raise the SO.sub.2 partial pressure. 
The process of the invention is preferably conducted as a multi-stage 
process, the SO.sub.2 -containing gas being first treated with an alkali 
sulphite/bisulphite solution that has a relatively high SO.sub.2 partial 
pressure in comparison with the pretreated solution that the gas 
encounters in a subsequent stage. The final treatment of the gas is 
preferably a treatment with water, especially when the alkali 
sulphite/bisulphite solution is an ammonium sulphite/bisulphite solution. 
If there is more than one stage of gas treatment with pretreated 
sulphite/bisulphite solution, the pretreatment of the solution for each 
such stage may be the same, or different pretreatments may be employed. 
For instance, the gas may be treated in successively encountered stages 
with sulphite/bisulphite solution that has been pretreated with air and 
with substantially free oxygen-free gas respectively. 
Pretreatment of the sulphite/bisulphite solution may also serve to 
establish a solution pH that is most suitable for effective absorption of 
SO.sub.2 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 may 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 pretreatment 
may be at normal temperatures 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 
80.degree. C. and therefore in the preferred practice of the invention the 
pretreatment is accomplished with air or other gas at a temperature not 
exceeding 80.degree. C. By observing this limit the 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 sulphite/bisulphite mists 
upon discharge of the treated gas to atmosphere is minimized by holding 
the solution pH at a value in the range 5.5 to 7.5 when in contact with 
the gas being treated. In the case of a typical ammonium 
sulphite/bisulphite solution 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 SO.sub.2 partial pressure of the solution, and thus reduces the 
tendency of SO.sub.2 in the gas to react with ammonia to form fine 
particles of ammonium sulphite and bisulphite salts that are readily 
entrained in the gas and lead to the formation of mists upon discharge of 
the gas to the atmosphere.

Referring first to FIG. 1, line 1 represents an inlet line for gas to be 
treated by the process of the invention. In a particular installation of 
the apparatus to be described, the gas was the off-gas of a sulphuric acid 
production plant, having an SO.sub.2 content of 0.18% by volume and 
entering the apparatus through line 1 at a flow rate of 90,000 Nm.sup.3 
/hour. 
The illustrated apparatus comprises an absorption column 2 having three 
stacked sections 2a, 2b and 2c, the gas from line 1 entering the lowermost 
section 2a of the column 2 to flow upwardly through that section and 
successively 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 
section 2a through nozzles 3 fed by a pump 4 drawing solution continually 
from the bottom of the column. The pH of the solution sprayed into section 
2a is maintained at a value in the range 5.7-6.0 by the regulated sparging 
of gaseous ammonia (NH.sub.3) into a pool of the solution at the bottom of 
column 2, from line 11. The spray nozzles 3 and the rate of recycling of 
solution through these nozzles are designed and selected so as to provide 
a solution contact surface in the range 15,000-20,000 m.sup.2 per m.sup.3 
of absorption volume in section 2a. 
The gas that has been subjected to preliminary scrubbing in the absorption 
column section 2a rises into and through section 2b which, as indicated, 
is a packed section through which the gas passes upwardly in 
countercurrent to a descending pretreated ammonium sulphite/bisulphite 
solution fed to section 2b by a pump 5 through line 6. The pretreated 
solution fed through line 6 is in fact a portion of a bled-off sidestream 
of the solution being circulated through the lowermost absorption column 
section 2a by the pump 4, after the bled-off sidestream portion has been 
substantially stripped of SO.sub.2 (and excess free ammonia) by treatment 
with air or, preferably, substantially free oxygen-free inert gas in a 
regeneration column 7. 
After absorbing SO.sub.2 from the gas in the section 2b, the solution falls 
into section 2a to join the solution being circulated through that 
section. 
In the absorption column section 2b, the gas is stripped of SO.sub.2 down 
to very low residual levels. Although the gas might then be safely 
discharged to the atmosphere with little risk of environmental pollution 
and the formation of mists of sulphite and bisulphite salts, the gas 
leaving section 2b is, however, given a further treatment in section 2c, 
again a packed section, in which the gas is washed with water circulated 
through section 2c by a pump 8; makeup water enters the head of the column 
from a line 9 and is also utilized for washing the gas in section 2c. 
The regeneration column 7 is surmounted by an acidifier 10 comprising a 
packed column section. 
As noted, only part of the bled-off sidestream of the solution being 
circulated in the lowermost absorption column section 2a by the pump 4 is 
fed, by line 12, to the head of the regeneration column 7 to be pretreated 
before entering the absorption column section 2b via line 6. In the 
regeneration column 7, this sidestream portion descends through packings 
in 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 line 14 leading 
to the bottom section 2a of the absorption column 2 so that the stripped 
SO.sub.2 and free ammonia are reabsorbed in the ammonium 
sulphite/bisulphite solution in column 2. 
The remainder of the bled-off sidestream of solution is fed to the 
acidifier 10 through line 15 to join a stream of acidified solution 
circulated through the packing in the acidifier 10 by means of a pump 17 
and external line 18 including a heat-exchanger 19 for cooling the 
circulating solution. Acid, in this case sulphuric acid, is added to the 
circulating solution through line 16 connected to the suction of pump 17. 
The SO.sub.2 that is liberated by the acidification of the 
sulphite/bisulphite solution is vented from the acidifier 10 through 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 line 22 and a heat 
exchanger 23 for cooling the air to a suitable temperature. In addition to 
providing the energy for the purpose indicated, the compressed air in 
ascending through the packing of the acidifier in countercurrent to the 
circulating acidified solution effects stripping of the liberated SO.sub.2 
from the solution. 
The volume of acidified solution in the circuit comprising the acidifier 10 
is maintained by bleeding off a portion of 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 pretreatment of the solution 
introduced into the absorption column section 2b, the treated gas had an 
average SO.sub.2 content of 22 ppm by volume. The hourly consumption of 
energy and of materials was as follows: 
Electricity--130 Kwh 
Cooling water--3000 kg 
Makeup water--5400 kg 
Ammonia--160 kg 
Sulphuric acid--680 kg 
The foregoing description of an embodiment of the process of the invention 
refers to the use of an ammonium sulphite/bisulphite solution for 
absorption of SO.sub.2 from the gas to be treated. It should be understood 
that the process of the invention may be performed with other alkali 
sulphite/bisulphite 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 gas, as in 
the absorption column section 2c, might be omitted without significant 
effect upon the residual contamination of the treated gas with SO.sub.2 
and mist-producing salts. 
Also the acidification of the bled-off spent sulphite/bisulphite solution 
may be performed with an acid other than sulphuric acid to liberate the 
absorbed SO.sub.2 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 pretreatment of the solution 
introduced into section 2b of the absorption 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 of free oxygen. Improved SO.sub.2 
removal accomplished by the use of such preferred pretreatment gases and 
whilst such a gas may be used in the apparatus as described, being 
introduced 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 pretreatment gas with less extensive 
utilization thereof. 
Thus, for instance, absorption of SO.sub.2 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 an 
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 absorbing 
solution, and may also include a final water-washing stage as in the 
embodiment described. 
Referring to FIG. 2 of the drawings, partial pressures of SO.sub.2 are 
plotted as a function of temperature for a solution 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 air at 40.degree. C. and 60.degree. 
C. respectively. Curves 4 and 5 relate to the same solution when 
pretreated with nitrogen at 40.degree. C. and 60.degree. C. respectively. 
The curves show that pretreatment of the solution with air results in a 
reduction of the SO.sub.2 partial pressure of the solution at any 
temperature, the reduction being the greater the higher the temperature of 
the air used for pretreatment. The curves also show that pretreatment of 
the solution with nitrogen at any given temperature produced a greater 
reduction of the SO.sub.2 partial pressure than the equivalent 
pretreatment of the solution with 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 identical to those obtained with the use of 
nitrogen. 
The additional reduction of SO.sub.2 partial pressure obtained by the use 
either of nitrogen or of combustion product gases containing about 1% free 
oxygen by volume amounted to about 10% of the SO.sub.2 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 FIG. 2 that the higher the temperature of 
the gas used for pretreatment, the lower the resultant partial pressure of 
SO.sub.2 for any given temperature of the solution. 
The data for plotting the curves of FIG. 2 was obtained by percolating the 
ammonium sulphite/bisulphite solution, in countercurrent to the flow of 
air or the chosen gas, through a column having a diameter of 60 mm packed 
with glass rings 15 mm.times.15 mm to a depth of 2800 mm.