CA1079030A - Sulfur dioxide scrubbing system - Google Patents

Abstract

AN IMPROVED
SULFUR DIOXIDE SCRUBBING SYSTEM

ABSTRACT OF THE DISCLOSURE

The refractory, non-regenerable alkali metal sulfate produced in a regenerative SO2 scrubbing system which uses thiosulfate-rich solutions of the alkali metal SO2 absorbent is removed from the system by precipitation after regeneration of the alkali metal absorbent but before re-entry into the scrubbing circuit, based upon the discovery that the alkali metal sulfate is least soluble in the multi-component solution leaving the regenerator than anywhere else in the entire system. An additional feature of the invention is the recovery of the precipitated sulfate in an odor-free state by washing with a slip-stream of the spent absorbent solution.

Description

1~79030 BACKGROUND OF THE INVENTION

1. Field of the Invention The present invention relates to aqueous regenerative S2 scrubbing systems which use a sulfite-forming SO2 absorbent.

1 2. Description of the Prior Art ?l ----- ----- ....
!' The aqueous regenerative SO2 scrubbing system of ~ which the present invention is an improvement comprises a ~ -¦I scrubbing circuit and a regenerative section. In the scrubbing ¦ -¦i circuit, there is a scrubber through which an aqueous scrubbing ~0 1~ solution continuosly circulates in contact with the SO2-containing' gas. A substantially constant high concentration, i.e. at -` ~ least ten percent (10%) by weight of either sodium or potassium ¦~ thiosulfate is maintained in the aqueous scrubbing solution ~¦ throughout its traverse around the scrubbing circuit. The S I effective SO2 absorbent is not tbiosulfate but rather is ':'' I ~' '- - ', i '' ~: l C' :
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1~79030 either sodium or potassium carbonate dissolved in the aqueous ; scrubbing solution. The term "carbonate" as used herein includes both carbonate (C03) and bicarbonate (HC03). The - ^ ~-, carbonate is converted by reaction with S02 to sulfite in the ~scrubber under sulfite-forming conditions. Similarly, the ~'term "sulfite" as used herein includes both sulfite (S03) and }
¦~bisulfite (HS03). The letter "M" as used hereinafter means I~either sodium or potassium. s ¦l A second very rapid reaction occurs in the scrubbing ~LO ¦¦circuit but external to the scrubber. That reaction is the ~conversion of sulfite by ~S to thiosulfate (M2S203) in a sulfite conversion zone whereby the sulfite concentration in the scrubbing solution is reduced to very low levels.
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The required carbonate and the required MHS are produced in the regenerative section by reacting the M2S2O3 which is withdrawn from the scrubbing circuit in a slipstream with a reducing gas containing CO at an elevated temperature and pressure. The regeneration reaction may be expressed as 1) 3 M2S2O3 + 12 CO + 5 H2O ~

2 M2CO3 + 2 MHS + 4 H2S + 10 CO2 However, in reality, there appear to be several side reactions in which products may react with the reactants and with each other. Accordingly, careful control of the conditions under which reaction (1) is conducted is essential to form the required quantities of carbonate and MHS for return together in the regenerated aqueous solu-tion to the scrubbing circuit to repeat the cycle.
The maintenance of the proper ratio of MHS to carbonate in the regenerated solution which is returned to the sulfite conversion zone in the scrubbing circuit is essential to satisfactory operation of the scrubbing and regenerative cycle. That ratio is maintained provided the following ratio, R, is maintained in the regenerated solution at or below 1, preferably between 0.75 and 0.98:

, ~79030 R = 2(S0) + 3(S-2) ~ M

where:

(S0) = gram atoms sulfur with valence number zero/100 grams solution.
(S-2) = gram atoms sulfur with valence equal to -2/100 grams solution.
M = gram atoms of M/100 grams solution present in said aqueous effluent as MHS, M2S, M2SX, M2CO3, MHcO3, and MOH (where M is Na or K).
When R is greater than 1, there will be excess MHS and associated sulfides in the regenerated solution. Such excess will reach the scrubbing zone of the scrubbing -~ circuit where it is converted to H2S by hydrolysis and/or CO2 stripping. When R is less than 0.75, or as -it approaches 0.75, there will be insufficient MHS and associated sulfides to maintain the sulfite concentration ~ --, of the recirculating solution in the scrubbing circuit at the desired predetermined low level.
Prior art patents which describe processes similar in some respects to the above-described process include the following:

U.S. Patent 1,937,196 H. A. Gollmar November 28, 1933 2,729,543 J. L. Keller January 3, 1956

3,431,070 J. L. Keller March 4, 1969 ( 3,574,097 P. UrbanApril 6, 1971 ', 3,635,820 P. UrbanJanuary 18, 1972 3,644,087 P. UrbanFebruary 22, 1972 3,714,338 P. UrbanJanuary 30, 1973 3,859,416 P. UrbanJanuary 7, 1975 .

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, 3. The Problem One of the problems which the above-described process, as well as other similar processes present, is the formation of M2SO4. Although its formation may be minimized as by maintenance of low sulfite concentrations, some sulfate will generally form. Since it is refractory and resistant to ready regeneration by any commercially feasible means, its concentration will build up to the point of saturation very quickly in view of its low solubility in water. It is the least soluble of all the solutes in the recirculating absorbent solution and, unless something is done to prevent it, will precipitate out of solution in the scrubber as finely divided crystals that are very difficult to filter. The precipitated sulfate represents a loss of the cation M associated therewith; and its make-up by addition of fresh cation adds to the cost of the operation.
Accordingly, the primary object of this inven-tion is to provide for the removal of the by-product sulfate, M2SO4, so that a clear solution or substan-tially clear solution may be maintained in the scrubbing circuit at all times.

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~)79030 SUMMARY OF THE INVENTION
This invention is an improvement in the above-described aqueous regenerative scrubbing system for the removal of S02 from a gas which contains both S02 and 2- The improvement is in the removal of the sulfate, M2S04, which inevitably forms in the scrubbing zone.
In accordance with my invention, I have discovered that the sulfate may best be removed after regeneration and before return of the regenerated solution to the scrubbing circuit. By choosing such location, many advantages result. The sulfate has a lower solubility at this location than in any other part of the system. Conse-quently, removal of the sulfate at this location not only assures that a clear solution is returned to the scrubbing circuit, but also assures that a clear solution is more readily maintained in the scrubbing zone. Another advan-tage of this location, I have found, is that the sulfate crystallizes in much coarser form than anywhere else so that filtration is more effective.
Another aspect of my invention is the use of a small slipstream of the spent scrubbing solution from the scrubbing circuit to wash the sulfate filter cake. The latter inevitably contains some sulfides which if left alone will, on storage, evolve H2S due to hydrolysis by moisture. The sulfite in the spent solution reacts with the residual sulfides to form the stable and odorless thiosulfate which, if desired, may be removed by water washing.

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1~79030 The objects and advantages of the present invention will become more apparent upon reference to the following description of the preferred embodiment and to the accompanying drawing which is a schematic flowsheet of the preferred embodiment of my invention.

PREFERRED EMBODIMENT
The schematic flowsheet of the accompanying drawing incorporates the preferred embodiment of this invention. The sulfite-forming agent which is preferred for use in the scrubbing zone is potassium carbonate because of the high solubility of potassium thiosulfate in water.

, Scrubbing Circuit ReEerring to the drawing, an SO2- and 2-containing gas, e.g. a flue gas, is introduced into the bottom of a scrubber 10 through an inlet pipe 12. The composition of a typical flue gas from a coal-fired power station using coal with a sulfur content of 2.46 weight percent of the moisture-free coal is as follows, in volume percent: 74.63% N2; 13.98% CO2; 3-30% 2; 0-17%
SO2; and 7.92% of H20. The scrubber 10 may be, for example, a conventional countercurrent or cocurrent packed ---tower, spray tower, or other conventional scrubbing apparatus, but the conventional countercurrent packed ', . . .

1~79030 tower is preferred. The flue gas entering the scrubber may be at a higher temperature than the scrubber and may contain some residual fly ash. Water is fed through a pipe 14 to a washing spray 16 which serves to quench the gas to humidify it and reduce its temperature. Simul-taneously, it cleanses the flue gas of solids. The latter are rejected as a slurry through a discharge pipe 18.
An aqueous scrubbing solution containing, in solution, at least ten percent (10%) and preferably more than twenty-five percent (25%) by weight of potassium thiosulfate is continuously fed through a conduit 20 into the top of the scrubber 10. The solution contains at least sufficient potassium carbonate to react with all the S2 in the flue gas. By the term "carbonate" is meant either K2C03 or KHC03, or mixtures thereof unless otherwise expressly indicated. The scrubbing solution also contains potassium sulfate, potassium formate and potassium sulfite. Similarly, by the term "sulfite" is meant either K2S03 or KHS03, or mixtures thereof, unless otherwise expressly indicated. A typical compo-sition of absorbent solution introduced into the top of the scrubber through conduit 20 is approximately as follows:

2S23 - 50.0 wt. %
K2C03 and KHC03 - 0.5 KOOCH - 5.0 K2S03 and KHS03 - 1.5 "
K2S4 - 1 . O "
H20 - balance ~-1079~3~

The flue gas is passed upwardly in counter-current flow to the aqueous absorbent which enters the top of the scrubber. The temperature within the scrubber is maintained generally between about 120 and 180F., e.g.
about 135F. The principal reactions occurring in the scrubber may be expressed by the following equations:
(2a) K2C3 + SO2 = K2SO3 + CO2 (2b) K2S3 + S2 + H2O = 2 KHSO3 (2c) KHCO3 + SO2 = KHSO3 + C2 The pH of the spent effluent aqueous absorbent leaving the scrubber through conduit 22 is maintained between about 6.0 and 7.8 by regulating the amount of carbonate in excess of that re~uired to react with the SO2 in the flue gas. Generally, the fresh absorbent solution enter-ing the scrubber through conduit 20 will be from 0.2 to 0.8 units higher in pH than the spent effluent absorbent solution. The range of liquid circulation rates through conduit 22 is suitably between 2 and 20 gallons/1000 CF, --~
e.g. 10 gallons/1000 CF of gas entering the scrubber through line 12.
The effluent stream leaving the scrubber contains a mixture of K2SO3 and KHSO3. The ratio of K2SO3 to KHSO3 increases with pH. At a pH
of 7.0, the molar ratio of K2SO3 to KHSO3 is approx-imately one. The K2S2O3 concentration remains essentially unchanged from that of the fresh absorbent :~

-1~79030 solution, as does that of the formate. The carbonate concentration has dropped close to ~ero, while the sulfite concentration has increased. There is also a small amount of K2SO4 in the spent effluent absorbent.
The efficiency of absorption of SO2 and the composition of the effluent aqueous solution are dictated by the feed rate and composition of the regenerated solution entering the scrubbing circuit through conduit 24 ~
which connects with conduit 22. This regenerated solution -contains principally KHS, K2CO3, KHCO3, and KOOCH, along with minor amounts of K2S, K2SX, KOH, K2SO4 and K2S203.
The KHS along with the other potassium sulfides and potassium polysulfides reacts rapidly in a sulfite converter 26 with the sulfite in the spent effluent absorbent. The reaction in the case of KHS and KHSO3, for example, is:
t3) 2 KHSO3 + KHS = 3/2 K2S23 + 3/2 H20 This reacticn occurs rapidly at the same or slightly higher temperature than that maintained in the scrubber. A suffi-cient residence time is provided in the sulfite converter 26 to assure complete consumption of the KHS and the other sulfides.
Such complete consumption is essential to avoid evolution of H2S into the treated flue gas, and is assured by maintaining an excess of KHSO3 in the recirculating solution. The reaction rate of the sulfite conversion decreases with ~, . .
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1~79~130 increasing pH. For this reason, the operating pH should be maintained below 8 in the sulfite converter 26, pre-ferably between 6.7 and 7.8, e.g. 7.5. A residence time of 0.2 to 5 minutes is usually sufficient to provide for complete consumption of the KHS, and its associated sulfides, in the sulfite converter. At the same time there is, as stated, unreacted sulfite leaving the sulfite converter. The carbonate and formate pass through the converter essentially unchanged. The concentration of thiosulfate increases. It is this increase in thiosulfate content that provides the feedstock for producing fresh -carbonate and sulfite-reducing agent. The effluent solution is withdrawn from the converter 26 through a conduit 28. The major portion of this stream is recycled to the scrubber 10 through conduit 20. The minor portion, generally less than ten percent (10%) by volume, is pumped into the regenerative section whose operation will now be described.
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Reqenerative Section The general purpose of the regenerative section is to convert the incremental increase in thiosulfate which corresponds to the sulfur absorbed in the scrubber to H2S
while regenerating the fresh carbonate required as absorbent and the KHS required for converting sulfite to thiosulfate in the sulfite converter 26. The regenerative section must also , . . .

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1~79~30 produce these two essential reagents, carbonate and KHS in an aqueous solution with the desired value of the ratio, R, for return through conduit 24 to the scrubbing circuit.
A further essential objective of the regenerative section in accordance with the present invention is the removal of K2S04 in order to assure the return of a clear solution to the scrubbing circuit and the maintenance of a clear solution in the scrubbing zone.
Adequate conversion of the S02 to K2S203 via sulfite reduction in the converter 26, without evolution of H2S into the treated flue gas, requires that the carbon-~; ates (K2C03 and KHC03) and sulfite KHS (together with minor amounts of other sulfides as previously described) in the regenerated solution fed through conduit 24 have the ratio, sometimes called the "acceptability" ratio, R, which has been previously generally defined but is defined again for potassium as follows:

R = 2(SO) + 3(S-where (SO) = gram atoms sulfur with valence number zero/100 grams solution.
- (S-2) = gram atoms sulfur with valence number equal to -2/100 grams solution.

EK = gram atoms of K/100 grams solution present in the regenerated solution j, fed to the scrubber circuit as KHS, K2S, K2SX~ K2C3~ KHC03, and KOH.
K present in other compounds such as - K2S04, KOOCH and K2S203 is excluded.

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:~79~)30 The acceptability ratio, R, of the regenerated solution in return conduit 24 should not exceed 1.0 nor be below 0.75; and is preferably within the range 0.75 to 0.98. If the value of R is above 1, KHS will appear in the scrubber with consequent evolution of H2S. On the other hand, R must not be below 0.75, for in this case the sulfite concentration will increase with resultant forma-tion of more K2SO4 in the scrubber as a result of oxidation of the sulfite by the oxygen in the flue gas. Since K2SO4 has such a low solubility it will precipitate as very fine crystals which are difficult to filter. By maintaining R in the range of 0.75 to 1.0 inclusive, the sulfite concentration in the scrubbing circuit can be maintained below three weight percent of the scrubbing solution and thereby avoid a high rate of formation of K2SO4 and its subsequent crystallization from solution.
Referring to the drawing, a small portion, e.g.
about one percent by volume of the recirculating solution, ~-is drawn off the scrubbing circuit through a conduit 30 to a surge tank 32 and thence pumped through a conduit 34 to a reductor 36. This reductor is adapted to withstand elevated temperatures and pressures and to confine a liquid phase reaction zone. The reductor vessel may be a stirred tank, a bubble column or a packed bed absorber.
The withdrawn scrubber solution is subjected to reduction by a reducing gas containing CO (for example synthesis gas, CO plus H2) as the principal reductant under - . . - :- . .
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1~79030 conditions selected to produce an effluent gas containing H2S and an effluent aqueous solution whose components have an acceptability ratio, R, greater than one and less than 1.5, e.g. about 1.1. The major portion of the H2S
(which corresponds to the sulfur absorbed in the scrubber) is produced in the reductor. The reaction is conducted in the reduction zone at a temperature between 300 and 600F., and is preferably non-catalytic in which case a temperature between 400 and 500F., e.g. 450F. is prefer-red. The pressure is sufficient to maintain a liquid phase reaction and is generally between 10 and 100 atmo-spheres, e.g. 500 psi. The input of gas and the input of feed solution are controlled to provide a mol ratio of CO
consumed to K2S2O3 consumed at a value of about 4 to 1, although the actual amount of carbon monoxide fed to the reductor is generally in excess of the reaction requirements.
The reducing gas is introduced into the reductor , s 36 through a conduit 37. It usually consists of a mixture of carbon monoxide, carbon dioxide and hydrogen since such a mixture is readily obtainable by partial oxidation of . .
hydrocarbonaceous fuel. Such a partial oxidation plant is designated in the drawing by the numeral 38. The hydro-carbonaceous fuel, a suitable fuel oil, is introduced into the partial oxidation plant through a conduit 40 while the required oxygen is transferred through a conduit 42 from 4 an oxygen source 44~

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~9~30 Pilot plant kinetic studies indicate that the reduction reaction is liquid-film mass-transfer controlled at the selected reductor operating pressure and temperature.
This reaction requires large interfacial area between the gas and the liquid to achieve high transfer rates. The water content of the aqueous solution in the reductor is in the range of 30 to 70 weight percent. The reductor may incorporate mechanical agitation as well as a gas sparger.
As mentioned, the reaction in the reductor 36 is preferably carried out in the absence of a catalyst in which case the essential reducing agent is carbon monoxide since hydrogen is relatively inert under non-catalytic conditions. Reduction of the K2S203 in this fashion produces the desired carbonates and sulfides in accordance with the following simplified reactions.

(4) K2523 + 4 CO + 2 H20 = K2CO3 + 2 H2S + 3 CO2

(5) K2S203 + 4 CO + 3 H20 = 2 KHCO3 + 2 H2S + 2 CO2

(6) K2S23 + 4 CO + H20 = 2 KHS + 4 CO2 In addition to the foregoing products, some potassium formate is produced according to the following reactions:

(7) K2CO3 + 2 CO + H20 = 2 KOOCH + CO2 (8~ K3CO3 + CO = KOOC3 + C2 , .

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1~79030 The amount of potassium formate (KOOCH) formed reaches an equilibrium value which is controlled by operating with incomplete conversion of the thiosulfate, preferably between about 95 and 99 percent. The equili-brium concentration of formate increases sharply as the thiosulfate conversion approaches 100 percent to reach very high values in the aqueous effluent from the reductor.
It may be desirable to have some formate present since its presence decreases the water vapor present over the scrubbing solution, and thus permits higher operating temperatures in the scrubber.
The effluent gaseous product comprising H2S, C2 and unreacted CO and H2 is withdrawn through conduit 46. The H2S may be converted to sulfur, for instance in a Claus plant. The other gases may be separately recovered for recycle as in the case of CO and CO2, or, after removal of the H2S, may serve as a low Btu fuel gas for reheating the SO2-free flue gas.
The desired value of the acceptability ratio, R, i.e. between 1 and 1.5, preferably about 1.1, is readily achieved by proper selection of the conditions maintained in the reductor 36. The ratio of CO2 to CO in the reducing gas fed to the reductor has a significant effect on the value of R. The lower the ratio, the higher is the value of R. A preferred ratio is about 0.5, and may be readily obtained by appropriate recycle of CO2.

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The residence time in the reductor is sufficient to assure conversion of at least 80 percent by weight of the thiosulfate, preferably, as stated before, between about 95 percent and 99 percent. A suitable residence time for the preferred conversion is about one hour.
The effluent aqueous solution having a value of R between 1 and 1.5 is conducted from the reductor 36 through a conduit 48 to a flash decomposition zone suitably confined in a vessel 50. The final adjustment of the acceptability ratio, R, to the preferred value between 0.75 and 0.98 is achieved in the flash decomposition zone through reduction in pressure by flash decomposition in this zone. The pressure is reduced to about that main-tained in the scrubbing circuit. In addition to the evolution of dissolved H2S and C2 by the reduction of pressure, H2S is released and K2CO3 formed by the follow-ing decomposition reaction:
(9) KHS + XHCO3 -~ K2C3 + H2S
An overhead condenser (not shown) maintains water reflux and overall water balance. The evolved gases may be directed through a conduit 52 to a Claus plant for conversion to sulfur.
The a~ueous solution in the flash decomposition zone 50 is pumped through a conduit 54 to a stirred K2SO4 crystallization zone 56. The cooling necessary to cause crystallization of the K2SO4 may occur to some extent in the ~ ' .
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~079030 flash decomposition zone 50. Generally, however, the desired cooling is effected by cooling the crystallization zone itself. A temperature between 130 and 200F. is low enough to precipitate sufficient K2SO4 to keep the scrub-bing solution clear, without at the same time precipitating KHCO3, which is the next least soluble of the several solutes.
The present invention is based upon the discovery that at similar total concentrations, K2SO4 is less soluble in the regenerated solution than in the spent scrubbing solution. For example, at a total concentration of salts of 48 to 52 percent by weight, the solubility of K2SO4 in regenerated solutions at temperatures between 130 and 150F. is within the range of about 0.1 to 0.5 weight percent, in contrast to its solubility in spent scrubbing solutions where its solubility is in the range of 1.1 to 1.6 weight percent of the spent scrubbing solution. While I have not demonstrated the same dif-ference in solubilities for Na2SO4, I think it reasonable to expect the same behavior.
A further unexpected benefit resulting from effecting crystallization of the K2SO4 from the regener-ated solution is based upon the fact that K2SO4 is more soluble in KHCO3 solution than in K2CO3 solutions.
This means higher solubility of the K2SO4 in the reduction zone 36 where KHCO3 is present in relatively higher concen-tration than in the flash decomposition zone 50 where the main reaction is that expressed by equation 9. Conversion 1 9 _ :1~7~30 of KHCO3 to K2CO3 reduces the chances of precipitating KHCO3. Increasing the amount of K2CO3 at the expense of KHCO3 makes it possible to reduce K2SO4 to a lower level of concentration.
The stirred slurry is conducted from the crystal-lization zone 56 through a conduit 58 to a filter system generally designated by the numeral 60. The filter cake is washed with spent scrubbing solution withdrawn from the scrubbing circuit through a conduit 62. The sulfite in this spent scrubbing solution reacts with any KHS and other sulfur compounds in the filter cake to form thiosul-fate. Hydrogen sulfide emission from the filter system is thereby avoided. If desired, the wash liquor may be returned to the regenerated solution. The filter cake is recovered by any suitable means in an odor-free state due to the washing with the spent scrubbing solution. Washing with water will produce an essentially pure K2SO4.
The filtered solution is pumped through a conduit 64 into a regenerated solution storage tank 66 where it is maintained under an inert atmosphere. From -this tank, it is withdrawn through conduit 24 at the required rate and fed into the sulfite converter 26 as fresh regenerated scrubber make-up solution. ~-~079~30 According to the provisions of the patent statutes, the principle, preferred construction and mode of operation of the invention have been explained and what is considered to represent its best embodiment has been illustrated and described. However, it should be understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifi-cally illustrated and described.

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Claims (2)

I claim:

1. In a regenerative system for the removal of SO2 from an SO2-containing gas having a scrubbing circuit which includes a scrubbing zone and a separate sulfite conversion zone, and a regeneration section and where, in the scrubbing circuit there is maintained a continuously recirculating aqueous solution which contains in solution either sodium or potassium thiosulfate in a concentration of at least ten percent by weight of said recirculating solution, and where in the scrubbing zone the SO2-containing gas is contacted with said aqueous solution which contains sodium or potassium carbonate as the effective absorbent for the SO2 to convert said effective absorbent to the corresponding sulfite and a relatively minor amount of the corresponding sulfate, and where in the separate sulfite conversion zone said sulfite is converted to the corresponding thiosulfate by reaction with a reducing agent containing principally the corresponding hydrosulfide as the effective reducing agent, and where in the regeneration section a slipstream of the recirculating solution from the scrubbing circuit is subjected to reaction with a reducing gas containing CO as effective reducing agent under conditions selected to convert thiosulfate to H2S and a regenerated aqueous solution for recycle to the scrubbing circuit, and where said regenerated aqueous solution contains sodium or potassium carbonate and sodium or potassium hydrosulfide in admixture with other by-products in the following ratio, R:

where (S0) = gram atoms sulfur with valence number zero/100 grams solution, (S-2) = gram atoms sulfur with valence number equal to -2/100 grams solution, and .SIGMA.M = gram atoms of M/100 grams solution present in said aqueous effluent as MHS, M2S, M2SX, M2CO3, MHCO3, and MOH (M is either Na or K), and where R has a value in the range of 0.75 to 1, inclusive, the IMPROVEMENT which comprises removing the sulfate M2SO4 formed in the system by precipitation of the sulfate from said regenerated aqueous solution before its return to said scrubbing circuit.

2. The process according to Claim 1 wherein the precipitated sulfate is substantially freed of entrained sulfides by washing the precipitated sulfate with a sulfite-containing slipstream withdrawn from the scrubbing circuit.