CA1050734A - Process for the removal of so2 from gases of air - Google Patents
Process for the removal of so2 from gases of airInfo
- Publication number
- CA1050734A CA1050734A CA219,046A CA219046A CA1050734A CA 1050734 A CA1050734 A CA 1050734A CA 219046 A CA219046 A CA 219046A CA 1050734 A CA1050734 A CA 1050734A
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- gas
- pretreated
- pretreatment
- air
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Abstract
ABSTRACT OF THE DISCLOSURE
The disclosure describes a process for removal of sulphur dioxide from an effluent gas by treating the gas in at least two successive stages with an absorbing solution selected from the group consisting of an alkali sulphite solution and an alkali sulphite and bisulphite solution. Part of the absorbing solution has been pretreated with air or with a gas that is inert with respect to the solution to reduce the SO2 partial pressure of the absorbing solution. The 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, SO2 absorbed by the solution in the absorption stage being partially stripped from the solution in the regeneration stage and reabsorbed by the solution concurrently with the absorption by the solution of SO2 from the SO2 - containing gas, whereby the SO2 content in the effluent gas is reduced to 50 ppm or less.
The disclosure describes a process for removal of sulphur dioxide from an effluent gas by treating the gas in at least two successive stages with an absorbing solution selected from the group consisting of an alkali sulphite solution and an alkali sulphite and bisulphite solution. Part of the absorbing solution has been pretreated with air or with a gas that is inert with respect to the solution to reduce the SO2 partial pressure of the absorbing solution. The 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, SO2 absorbed by the solution in the absorption stage being partially stripped from the solution in the regeneration stage and reabsorbed by the solution concurrently with the absorption by the solution of SO2 from the SO2 - containing gas, whereby the SO2 content in the effluent gas is reduced to 50 ppm or less.
Description
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 ,'; :"' i:~ r~
... ~ . .. . . ~. . ;
73L~k 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 : ' , 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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. .~ .
.
.. . . . ..
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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 . ~ 3 ~ :
.................................. .. . . : : .
.. - : .. .
73~
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.
,:.
. _ ~ _ .;
.
~ .
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- :
!.
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 . 5 , , .
., , - ~sa 734 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 .
` ~ILa~73g~
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-:
.',. :.
, . . .
73g~
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 -- E3 -- :`
3~L
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
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 ,'; :"' i:~ r~
... ~ . .. . . ~. . ;
73L~k 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 : ' , 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 .:
.. ..
`,';;`
. .~ .
.
.. . . . ..
~L~9S073~
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 . ~ 3 ~ :
.................................. .. . . : : .
.. - : .. .
73~
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.
,:.
. _ ~ _ .;
.
~ .
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- :
!.
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 . 5 , , .
., , - ~sa 734 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 .
` ~ILa~73g~
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-:
.',. :.
, . . .
73g~
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 -- E3 -- :`
3~L
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-g :,. .
' ' ' - ~ '' ' ' `: ' ' ':'. ,
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-g :,. .
' ' ' - ~ '' ' ' `: ' ' ':'. ,
3~
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.
'~ -- 10 :~,
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.
'~ -- 10 :~,
Claims (6)
1. A process for removal of sulphur dioxide from an effluent gas comprising treating said gas in at least two successive stages with an absorbing solution selected from the group consisting of 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 to reduce the SO2 partial pressure of said absorbing solution, 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, SO2 absorbed by the solution in the absorption stage being partially stripped from the solution in the regeneration stage and reabsorbed by the solution con-currently with the absorption by the solution of SO2 from the SO2 - containing gas, whereby the SO2 content in the effluent gas is reduced to 50 ppm or less.
2. The process of claim 1, wherein the pretreatment of said solution is accomplished at least in part with a gas containing less than 1% free oxygen by volume.
3. The process of claim 2 wherein the SO2 - containing gas is treated with a solution pretreated with air or oxygen-containing gas and thereafter said SO2 - containing gas is treated with a solution pretreated with the gas containing not more than 1% free oxygen by volume.
4. The process of claim 1 wherein said pretreatment is accomplished with air or gas at a temperature not higher than 80°C.
5. The process of claim 1, wherein the pretreatment is effective to adjust the solution pH to a value in the range 5.5-7.5.
6. The process of claim 1, wherein the treatment of the SO2 - containing gas with said pretreated solution is followed by treatment of the gas with water.
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| IT4858674A IT1046241B (en) | 1974-02-20 | 1974-02-20 | PROCEDURE FOR THE PURIFICATION OF SO2 FROM GAS OR AIR |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1050734A true CA1050734A (en) | 1979-03-20 |
Family
ID=11267479
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA219,046A Expired CA1050734A (en) | 1974-02-20 | 1975-01-30 | Process for the removal of so2 from gases of air |
Country Status (3)
| Country | Link |
|---|---|
| CA (1) | CA1050734A (en) |
| IT (1) | IT1046241B (en) |
| PL (1) | PL90788B1 (en) |
-
1974
- 1974-02-20 IT IT4858674A patent/IT1046241B/en active
- 1974-06-21 PL PL17210874A patent/PL90788B1/pl unknown
-
1975
- 1975-01-30 CA CA219,046A patent/CA1050734A/en not_active Expired
Also Published As
| Publication number | Publication date |
|---|---|
| PL90788B1 (en) | 1977-01-31 |
| IT1046241B (en) | 1980-06-30 |
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