CA1115933A - Process for pressure stripping of sulfur dioxide from buffered solutions - Google Patents

Abstract

Abstract It is known to strip sulphur from aqueous citrate solutions using low pressure steam. This process is disadvantageous in that is requires large volumes of steam. The present invention seeks to overcome this draw-back by providing in a process for the desulfurization of gases wherein sulfur dioxide is removed from the gases by absorption in citrate buffered aqueous solution which undergoes a regenerative process to yield sulfur-based by-products, the improvement resulting in the recovery of sulfur dioxide from the citrate buffered aqueous solution comprising heating the aqueous solution containing absorbed sulfur dioxide to a temperature above 212°F. and to strip the sulfur dioxide therefrom, maintaining the pressure substantially greater than atmospheric pressure, and recovering sulfur dioxide vapor.

Description

This invention relates to flue gas desulfurization processes and more particularly to a process of regenerating sulfur dioxide absorbed in buffered solutions used to scrub sulfur dioxide from gases such as flue gasO
Presently available flue gas desulfurization processes include so-called thro~away systems, utili7ing limestone or lime scrubbing, as well as regenerative processes, utilizing buffered aqueous solutions for sulfur dioxide absorption~ that yield sulfur as a byproduct. One such recgenerative process, using a buffered citrate solution to scrub sulfur dioxide from a gas in a co~mtercurrent absorber and reacting the sulfur dioxide-laden liquor from the absorber with hydrogen sulfide to recover sulfur, is kno~n by the name, CITRE~, a trade mark o~-Peabody Engineered Systems, Stamford, Connecticut.
A description of the CITREX process and a discussion of its advantages over throt~-a~ay msthods such as limestone scrubbing appears in "The CITREX Process ~or SO Removal", Chemical Engineerin~ Pro~ress, VolO 71, No. 5, May 1915.

A limestone process of flue gas desulfurization has the drawbacks of sludge disposal and high raw material cost of lime or limestoneO By con-trast, the citrate process, especially as modified in the CITREX process, ! provides advan~ages such as sulfur recovery as a byproduct as ~ell as lower initlal and operating costsO Nevertheless, the use of hydrogen sulfide to convert the sulfur dioxide absorbed in the citrate solution to yleld sulfur and water may not be commercially attractive in all si-tuations. Thus, where it is neccssary to synthesize hydrogen sulfide at a plant site -from hydrogen produced from natural gas, the increasing una~ailability of natural gas or similar raw material for the production of hydrogen makes such a process less desirablc O
In United States Patent No~ l,589,l33 to Eustis there is disclosed a method of recovering sulfur dioxide from smelter smoke or other gases by absorbing the sulfur dioxide in a solution of a metallic salt, such as ~) .. , ) - ~ .

aluminun sulfi~0, which will form a relatively uns-table sulfite or bisul~ite with the sulfur dioxide and which will readily liberate the sulfur dioxide gas at moderate temperatures. The sulfur dioxide gas is extracted rom the solution by diluting the ~tmosphere in contact with the solution and conse-quently reducing the partial pressure of the sulfur dioxide in the gases or atmosphere contacting with the solutionO The patentee states that this is done in an extractor in~o the bottom of which is directly fed live steam and into the top of which is fed the sulfur dioxide containing solution. Ry mak-ing the extractor very lar$e and prolonging solution dwell time a large percen~age of the total sulfur dioxide is said to be extracted at each cycleO
The extraction may be carried out under pressuresbelow atmospheric, using a vacuum pump, although a vacuum is not necessary as dilution of the atmosphere resulting in reduction of the partial pressure of the sulfur dioxide is said to work satisfactorily when the extraction is carried on at, or even above, atmospheric pressureO
The use of live steamJ as in Eustis, is attendant with several dis-advantagesO The steam used must be produced from water which has been treated to avoid contaminating the stripping system or the constituents therein, resulting in added expenseO Also, the live steam condenses in the system and dilutes the solution absorbing the sulfur dioxide so that either further separation or waste discharge is requiredO The former is uneconomical while the latter is impractical under current environmental procedures as well as a costly use of raw materialO
I haYe ~ound that the advantages of sulfur dioxide absorption by the citrate and similar bufered aqueous solution processes can be recognized without the need for reaction with a reducing gas, such as hydrogen sulfide, or regenerative reactionO This is achieved, according to this invention, through the provision of a process in which sulfur dioxide is regenerated

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from the sulfur dioxide-laden absorber liquor by steam stripping under pressureO
The present invention provides in a process for ~he desulurization of gases wherein sulfur dioxide is removed from the gases by absorption in a citrate bufered aqueous solution hich undergoes a regenerative process to yield sulfur-based byproducts, the improvem0nt resulting in the recovery of sulfur dioxide from the citrate buffered aqueous solution comprising heating the aqueous solution containing absorbed sulfur dioxide to a temperature above 212F~ and to strip the sulfur dioxide therefrom, maintaining the pressure substantially greater than atmospheric pressure, and recovering sulfur dioxide vapor.
The present invention also provides a process for regenerating sulfur dioxide rom an organic buffered aqueous solution in which it is absorbed comprising heating the aqueous solution to a temperature substantially in excess of 215F~ maintaining a pressure of from 15 to 65 psig above the solution, recovering sulfur dioxide and water vapor, and then condensing the sulfur dioxide and water vapor in a hea~ exchanger in the absence of drying, refrigeration or compression and obtaining liquid sulfur dioxide.
In a preerred embodiment there is provided acccrding to the 2Q invention the process of recovering sulfur dioxide gases comprising the steps of contacting the gases containing sulfur dioxide wi~h an organically buffered aqueous solution to absorb sulfur dioxide in said solution, then heating said solution containing sulfur dioxide to a temperature greater than 212Fo and under pressure substantially greater than atmospheric pressure and then recovering and condensing the water vapor and sulfur dioxide for separation and recoveryO
The present invention also provicles the process o~ removing sulfur di.oxide from gases comprising the steps of contacting the gases containing

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sulfur dioxide with a buffered aqueous solution containing one or more organic radicals taken from the group consisting of citrate~ glycolate, glyoxalate and acetate, then heating the solution to a temperature of from 250Fo to 310F~ at a pressure of from 15 to 65 psig, and then removing and condensing the water vapor and sul~ur dioxide for separation and recovery.
It was unexpectedly discovered, according to this invention, that steam stripping at super atmospheric pressure~ for example, in the advantage-ous range of 5 to 65 psig, results in a lowered steam requirement per pound of sulfur dioxide stripped as compared to atmospheric or slightly above atmospheric pressure operation. Thus, it has been surprisingly discovered tha~ apparently the decomposition of a sulfur dioxide citrate complex to yield sulfur dioxide proceeds faster and more eficientl.y a* high temperatures corresponding to high pressures than does the contervailing effect of the high pressure on the solubility o~ sulfur dioxide in the citrate buffered solutionO Moreover, the pressure stripp.ing process of this invention yields stripped sulfur dioxide which can be di.rectly condensed wi~h ordinary cooling water to produce liquid sulfur dioxide as a product without the need for dry-ing, refrigeration or compression systems~ The overhead stream from the stripping process is at a sufficiently high temperature to preheat air that can be subsequently used for direct mixing and reheating of the cold treated flue gas s-tream from the absorber, resulting in greater overall process economies. Also, the pressure stripping process results in actual gas flow volumes which are several times less than those present in low pressure strip-ping so that the stripping column may be a more compact unit and initial equip-ment costs can be reducedu ~ccordingly, a feature of this invention is the provision of a process for the stripping of sulfur dioxide from buf~ered solutions in which it is absorbed by stripping under pressureO

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Advantageous pressures for khe st~am stripping of sulfur dioxide from citrate liquor loaded-with sulfur dioxide is in the range of 5-65 psig and preferably 15-65 psig. At pressures of 15-65 psig, the high temperature of the steam, in the range 250-310F.~ accelerates the rate of sulfur dioxide release and more than balances any tendency of sulfur dioxide to go into solution at the high pressure involved. This higher sulfur dioxide release results in steam requirements in the range of 5 to 8 pounds steam per pound of sulfur dioxide stripped in contrast to steam requirements which are several fold higher at low pressure operation, such as at atmospheric or slightly above. At atmospheric pressure or slightly above, the overhead sulfur dioxide gas stream requires a dryer and refrigeration system or compression system for recovery of st~ipped sulfur dioxide as a product liquidO
In the pressure stripping process according to this invention, the sulfur dioxide stripped off at 15-65 psig can be directly condensed with ordinary cooling water having a temperature in the range of 50 to 90Fo to produce liciuid sulfur dioxide and thus avoiding prohibitive dryer and refriger-ation or compression power consumption requirementsO The overhead stream leaving the rectification section of the stripping vessel is at a temperature of about 240 to 290Fo and can be used to preheat air which in turn can be used for direct mixing with and reheating of the cold treated flue gas stream from the top of the absorber to heat it from about 120 to 1~0F~
~hile the ~ressure range of 5-65 psig is advantageous and 15-65 psig is preferred, it will be understood that the acl~antages of the invention are recognized at pressures above atmospheric generallyO The limiting facts are the decomposition of the bufered solution which can occur, for example at 310Fu, which corresponds to about 65 psig, and the availability of low temperature cooling water to condense the sulfur dioxide vapors. For example, oOFO water would permit operation at 25-30 psigO

The steam economy in the pressure stripping process of this inven-tion is such that the process requires only about one-~hird to one-half the fuel needs of comparable currently available fuel desulfurization processesO
The use of the sulfur dioxide pressure stripping process according to this invention allows the flue gas desulfurization system to be compact, since regenerative equipment is reduced, and, since this is an all liquid system with no solids or sulfur to plug or cake equipment, it is a clean and simple system to operateO Thus, the process can be operated for extended periods with reduced operational and maintenance laborO Hence~ economies result from this invention in several areasO Initial equipment costs are lower because of the compact regenerative system, the high operating availabili~y reduces costly down-time and the ability to be operated by one operator results in lower labor costs, all of which are in addition to the fuel economy described aboveO
Thus, a further feature o~ this invention is the provision of a process for steam stripping sulfur dioxide from a solution resulting from a fuel gas desulfurization process which results in increased reliabil.ity and reduced costO
The foregoing and additional features, objects and advantages of this invention will be further apparent from the following detailed descrip-tion of a preferred embodiment thereof taken in conjunction with the accom-panying drawing~
The Figure is a schematic diagram illustrating the process scheme for high pressure steam stripplng of sulfur dioxide absorbed in a buffered solution in a flue gas desulfur.ization process, according to an embodiment of this inventionO
~ferring to the Figure, there is shown hot flue gas represented at 2 entering a scrubbing system 4 in which the gas is cleaned and cooledO

The scrubbing system 4 may advantageously be of ~he venturi type which is commercially available and a further description of which is unnecessary to the present invention except to point out that the flue gas is cleaned out particulates and cooled by water entering the system 4 at 6 and particulate material such as fly ash is removed as illustrated at 8.
Gas leaving the scrubber system 4 is passed upward through the absorber 10 countercurrent to a down-flowing buffered citrate solution intro-duced into the absorber 10 at 12 and distributed in a tray-type or packed-bed colu~n absorber schematically shownO The absorber 10 may advantageously be a Peadboy Tray Absorber having trays 14 which provide high efficiencies o~
sulfur dioxide removal and low L/G ratiosO The clean waste gas is tangentially mixed with heated air, shown at 16, and exits the absorber at 18 while the sulfur dioxide-laden absorber liquor exits the absorber bottom at 200 ~fter leaving the absorber the sulfur dioxide-laden citrate liquor is pumped, such as by the pump 22 to enter the ~op, at 24, of the pressure stripping vessel 26, after passing through the economizer 28 where it is heated : by the hot bottoms ~rom the stripping vessel~ Alternately, the absorberliquor may partially bypass the economizer 28 and be fed to the top of the stripping vessel 26, .in the rectification sectionO Within the stripping vessel 26 the absorber liquor flows downward over either a packed bed, or a~vantageously, over trays schematically represented at 30. Vapor flows up-ward through openings in the trays as a result of heating of the solutionO
Heating takes place in the stripping vessel bottom by means of circulation through the reboile-r 320 The reboiler 32 is heated by steam under pressure entering at 34 ~or .indirect heating of the sul.~ur dioxide laden liquor cir-culating through the lines 36 and 380 The buffered citrate solution from which the sulfur dioxide has been stripped exits the stripping vessel 26 at 40 and is pumped, as by the ~5~313;~

pump 42, to reenter the absorber 10 as feed 12 after passing through the economizer 28 and the heat exchanger 44 where it is fur~her cooledO The stripped sulfur dioxide and water vapor or steam leave the top of the strip-ping vessel at 46 and enter the heat exchanger 4B where they are condensed by ordinary coolng water, to the liquid state and pass to the tank 50O The liquid sulfur dioxide and water separate into two phases within the tank 50.
The heavier sulfur dioxide phase is taken off as a product at 52, rom whence it may be further processed into sulfuric acid, elemental sulfur or converted to industrial chemicals. The upper liquid phase in the tank 50 containing water and dissolved sulfur dioxide is returned as a reflux stream 54 to the rectification portion of ~he stripping vessel 260 As an alternative to passing directly to the heat exchanger 48,~the sulfur dioxide and steam leaving the stripping vessel 26 in the stream 46 may .;~
be passed through a fin type heat exchanger 49 over which air, as shown at Sl, is ~lown in order to preheat the air which can be subsequently used for direct mixing with and reheat mg;o the~cold treated flue gas stream, as shown at 16, from the top of the absorber 10. This direct mixing and heating of clean -.
flue gas avoids the need for a~separate steam coil which is ~ore costly and subject to corrosion.;
~20 The advan*ageous operating parameters are as followsO ~ S~eam is introduced into the~reboiler tubes at a pressure greater than atmosphericO A
preferred steam pressure is 15-65 psig which provides a steam temperature range of 250-310~O in the reboilerO At this pressure and temperature, the rate of sulfur dioxide release -from the citrate liquor is accelerated and the sulfur dioxide content in the vapor phase is increased. By contrast, opera-tion at a s~eam pressure only slightly above atmospheric, for example, 5 psig, results in a reboiler temperature range of 215 to 22~Fo and steam require-ments several-fold higher than the 5 to 8 pounds of steam required per pound :

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of sulfur ~ioxide stripped at the high pressureO
With a steam pressure of 15-65 psig, the cooling water in ~he heat exchanger 4~ may be at a temperature range oE 50 to 9OF~ to produce liquid sulfur dioxide as an overhead productO However, opera~ion at atmospheric pressure of 5 psig requires a drying and refrigeration system to recover the stripped sulfur dioxide as a product liquid. The overhead stream ~6 leaving the rectification or top section of the stripping vessel 26 is at a temperature of 280 to 290FO and can be used to preheat air to subsequently provide a 20~. reheat of treated flue gasO That is, the flue gas stream leaving the top of the absorber at 1~ is at about 120 to 140FD and can be reheated by this preheated air to about 140 to 160~FD by judicious use of the heat of the the overhead stream 46~ Moreover, by operating the stripping vessel at 50-65 psig, the stripping vessel becomes a compact unit because the actual volume of gas ~low is one-fifth that of a low pressure unit.
The superior steam economy of the high pressure stripping o~ sulfur dioxide from a citrate buffered solution was confirmed by testing on an 8-inch diameter stripping column to simulate gas desulfurlzation on a 0.25 megawatt scaleD The test equipment included a packed tower of 7 7/8-inch inside dia-meter filled with 12 ~eet of 1~2-inch ceramic berl saddles. The tower was maintained at 65 psig by nitrogen pressure. A charge of 21 gallons Oe OD5 molar citrate solution was made up to a pH of 4DO by blending 0.5 molar citrate acid and 0O5 molar sodium citrateO The bottom liquor was cooled and recycled at 2~25 gallons per minute ~ulder a pressure of 20 PSI above system pressure through a packed holding tank and a sight glass, After releasing the pressure to 65 psig~ the recycle was reheated towith~n 5-10F. of the overhead tempera-ture and fed into the tower three-fourths of the way upO

A metered amount of liquid sulfur dioxide was pumped into the pressurized recycled stream prior to the holdup sectionO The recycle in the _ g _ ~ !

last run was analyzed for bottoin cmd recycle compositions. Overhead conden-sate was collected in a teller and the upper phase pumped into the top o~ the tower. I'he bottom phase was drained into a receiver maintainecl at a pressure of 55 psig. Flow rates o both layers were determined by stopping their flows and timing the build-up in the teller.
The steam economy was determined by hea-t balances around the towerO
Steam condensate was not collected at the bottom since this steam included condensate in the feed lines and from the still pot~ The column was electri-cally heated to the overhead temperature to eliminate heat loss from this sourceO
The results of various representative runs of the fractionation of 0.5 molar citrate solution having initial pH of 3.95 and carried out at 65 psig are set forth in Table lo Run 1 was carried out on a different day than run 2. The results show that steam economies of 5.4 to 8O9 pounds o-stea~l per pound of sulfur dioxide stripped are obtained with the degree of stripping varying from 86 to 98 percentO Moreover, the second run showed that there was no difficulty in obtaining a two-phase condensate at condensing temperatures up to 89~o and a pressure of 65 psigo This sulfur dioxide layer was ound ~o contain about 5 percent waterO

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In a citrate steam stripping process for -flue gas desulfurization ~or a 25 megawatt plant producing 110,000 ACFM of flue gas at 300F., the steam consumption is 12,500 pounds per hour with a power consumption of 280 kilowatts and 30 STPD of liquid sulfur dioxide is recoveredO The total energy need is therefore 362 BTU per kilowatt hour (KW~I~. These calculations are based on a 25 megawatt boiler fired Wit~l 3.5 percent sulfur coal and assume the presence of an electrostatic precipitor with 99.5 percent eficiency.
A comparison of incremental energ~ needs for various flue gas desulfurization processes are set forth in Table 2. The evaluation of Table 2 assumes a 500 megawatt boiler using 3O5 percent sulfur coal with an electro-static precipitor of 99O5 percent efficiency and includes a 20F. reheat of flue gas for the citrate pressure-stripping processes, although in most cases the energy penalty for reheat is of the order of 1 percent at 100 BTU/KWH for ~hich no special deduction has been taken in these figuresO The results show that even if the steam consumption were increased by 50 percent due to site conditions, fuel consumption ~ould increase only from 362 to 487 BTU/KWH
as compared to 607 ~o 1038 BTU/KWH or other regenerable flue gas desulfuri-zation processes.
T~BLE 2 Process Total Incremental ~uel Consumption (BT
Limestone ~390) ~agnesia 1038 (Sulfur~
60~ (Acid) 2~ Wellman-Lord Sulfite Scrubbing 8~0 Citrex (Phosphate) 670 Atomic International Process670 S0 Steam Stripping Process 2 (at 5 lbsO steam per pound S02) 362 S.~33 The foregoing results are both surprising and unexpected in that one would normally expect that stripping sulfur dioxide under pressure would require a greater amount of steam per pound of sulfur dioxide stripped than stripping at atmospheric pressure. Not only is such not shown not to be the case according to the process of this invention, but in addition, many other advantages are obtained in the processO For example, when stripping at atmos-pheric pressure the sul~ur dioxide must be dried in a column with a suitable material such as concentrated sulfuric acid, silica gel, alumina or the like and then refrigerated or condensed for liquefactionO Here a liquid sulfur dioxide product is obtained by the use of cooling water at normal plant cooling water temperatures. While~the embodiment described is directed to:the removal o~ sulfur dioxide from a cltrate buffered solution, the process is applicable to remo~al from other organic buffered solutions of the type such as glycolate, i glyoxalate, acetate and the likeO Also, the system is applicable t:o any gas containing sulfur dloxlde such as smelter gas and the like and the use of the term flue gas is intended~to apply to suchO

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Claims (16)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A process for the recovery of sulphur dioxide from gases which comprises removal of the sulphur dioxide from the gases by absorption in an organically buffered aqueous solution; heating the organically buffered aqueous solution to a temperature above 212°F
and at a pressure substantially greater than atmospheric pressure to strip the absorbed sulphur dioxide therefrom; and recovering the sulphur dioxide.

2. In a process for the desulfurization of gases wherein sul-fur dioxide is removed from the gases by absorption in a citrate buffered aqueous solution which undergoes a regenerative process to yield sulfur-based by-products, the improvement resulting in the recovery of sulfur dioxide from the citrate buffered aqueous solu tion comprising heating the aqueous solution containing absorbed sulfur dioxide to a temperature above 212°F, and to strip the sulfur dioxide therefrom, maintaining the pressure substantially greater than atmospheric pressure, and recovering sulfur dioxide vapor.

3. The improved process as claimed in claim 2 wherein the pres-sure is 15-65 psig.

4. The improved process as claimed in claim 2 wherein the heating is carried out with steam at superatmospheric pressure and the amount of steam required per pound of sulfur dioxide removed is less than that required when heating with steam at atmospheric pressure.

5. The improved process as claimed in claim 2 wherein the solution is heated to a temperature above 212°F.

6. The improved process as claimed in claim 2 wherein the solution is heated to a temperature in the range of 250-310°F.

7. The improved process as claimed in claim 2 wherein the amount of steam required per pound of sulfur dioxide recovered is in the range of 5 to 8 pounds.

8. The improved process as claimed in claim 4 wherein the recovered sulfur dioxide vapor is condensed to liquid sulfur dioxide using a cooling medium having a temperature in the range of 50 to 90°F.

9. The improved process as claimed in claim 4 wherein the sulfur dioxide vapor stripped from the solution has a temperature in the range of 240 to 300° F.

10. The improved process as claimed in claim 2 wherein the buffered aqueous solution is a citrate buffered solution.

11. A process as claimed in claim l wherein the sulfur dioxide is recovered from the organic buffered aqueous solution in which it is absorbed by heating the aqueous solution to a temperature sub-stantially in excess of 215°F., maintaining a pressure of from 15 to 65 psig above the solution, recovering sulfur dioxide and water vapor, and then condensing the sulfur dioxide and water vapor in a heat exchanger in the absence of drying, refrigeration or compression and obtaining liquid sulfur dioxide.

12. The process of recovering sulfur dioxide gases comprising the steps of contacting the gases containing sulfur dioxide with an organically buffered aqueous solution to absorb sulfur dioxide in said solution, then heating said solution containing sulfur dioxide to a temperature greater than 212°F. and under pressure substantially greater than atmospheric pressure and then recovering and condensing the water vapor and sulfur dioxide for separation and recovery.

13. The process defined in claim 12 wherein the pressure is from 15 to 65 psig.

14. The process defined in claim 12 wherein the temperature of said solution is from 250° F. to 310° F.

15. The process of removing sulfur dioxide from gases comprising the steps of contacting the gases containing sulfur dioxide with a buffered aqueous solution containing one or more organic radicals taken from the group consisting of citrate, glycolate, glyoxalate and acetate, then heating the solution to a temperature of from 250 F. to 310 F. at a pressure of from 15 to 65 psig, and then removing and condensing the water vapor and sulfur dioxide for separation and recovery.

16. The process defined in claim 15 wherein said water vapor and sulfur dioxide are condensed in a heat exchanger having cooling fluid at a temperature of from about 50 F. to 90°F. to separate sulfur dioxide and water into substantially two liquid phases.