CA1079492A - Process for removing sulfur dioxide from a gas stream - Google Patents
Process for removing sulfur dioxide from a gas streamInfo
- Publication number
- CA1079492A CA1079492A CA265,157A CA265157A CA1079492A CA 1079492 A CA1079492 A CA 1079492A CA 265157 A CA265157 A CA 265157A CA 1079492 A CA1079492 A CA 1079492A
- Authority
- CA
- Canada
- Prior art keywords
- solution
- absorption
- sulfur dioxide
- weight percent
- sulfite
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
Links
Classifications
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
- B01D—SEPARATION
- B01D53/00—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols
- B01D53/34—Chemical or biological purification of waste gases
- B01D53/46—Removing components of defined structure
- B01D53/48—Sulfur compounds
- B01D53/50—Sulfur oxides
- B01D53/501—Sulfur oxides by treating the gases with a solution or a suspension of an alkali or earth-alkali or ammonium compound
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- C—CHEMISTRY; METALLURGY
- C01—INORGANIC CHEMISTRY
- C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
- C01B17/00—Sulfur; Compounds thereof
- C01B17/48—Sulfur dioxide; Sulfurous acid
- C01B17/50—Preparation of sulfur dioxide
- C01B17/60—Isolation of sulfur dioxide from gases
Abstract
Abstract of the Disclosure Sodium sulfate is purged from a sulfur dioxide removal system involving contact of a sulfur dioxide-containing gas with a solution containing sodium sulfite to absorb sulfur dioxide from the gas. The spent absorbing solution is regenerated by desorbing sulfur dixoide and recycled for further use. To avoid an unduly large build-up of sulfate in the system, at least a portion of the absorbing-desorbing medium, e.g. spent absorbing solution, containing sodium sulfate and a relatively large amount of sodium bisulfite is treated to reduce the amount of water in the medium so that there is precipitated therefrom up to about 10 weight percent undissolved solids containing sodium sulfate in greater concentration than would otherwise be obtained in the absorption-desorption cycle. The insolubles containing sodium sulfate are removed from the liquid, and the liquid can be returned to the sulfur dioxide removal system. In one preferred aspect of the invention, up to about 75 weight percent of the entire stream of spent absorbing solution is treated to form from this solution a slurry having up to about 10 weight percent undissolved solids which are relatively rich in sodium sulfate content. In the invention the sodium sulfate-containing solids can be separated from the liquid which is then subjected to a desorption operation to produce sulfur dioxide, and the latter operation can be conducted while maintaining at least about 25 weight percent undissolved solids in the desorption zone.
Description
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This invention relates to the remov~l of sulfur dioxide from gas streams by contact with an aqueous sodium sulfite solution to absorb sulfur dioxide and provide a solution richer in sodium bisulfite which can be treated to desorb sulfur dioxide and regenerate the absorbi.ng solution for reuse, and in which sodium sulfate is formed as a by-product in the absorption-desorption medium and must be purged from the system without undue loss of valuable sodium compounds.
More particularly, the invention concerns a process for re-ducing the loss of desirable and valuable sodium compoundsfrom the cyclic sulfur dioxide removal system by removing undesirable sodium sulfate from the absorption-desorption medium as solids containing an increased amount of sodium sulfate.
Sulfur dioxide is a recognized pollutant of the atmos-phere and is produced by oxidation of sulfur or sulfur-bearing materials. Sulfur dioxide is found in significant amounts as a constituent of various waste gases such as smelter gases, off-gases from chemical plants, and stack or furnace gases from coal or oil-burning furnaces such as are used in electric power plants. Although the concentration of sulfur dioxide in such gases is generally minor, e.g., from about 0.001 up to about 5 mole percent, and is frequently less than about 0.5 mole percent (less than about 1% by weight), the emission of sulfur-dioxide may be substantial, particularly in industrial applications due to the large amount of sulfur-bearing material being processed. For instance, a modern electric plant having a 1,350,000 kw. capacity will burn up to about 15,000 tons of ` coal per day. Despite the fact that the concentration of sulfur dioxide in the stack gases from the electric plant can , `
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'"'~,: . : . : '' 10~9492 be low, e.g., oE the order of 0.2 to 0.3 mole percent, the total sulfur dioxide produced mav be in the neighborhood of 1,000 tons per day. Similarl~, significant amounts of sulfur dioxide are produced in utilization of other fuels which may bear sulfur.
The removal of sulfur dio~cide from sulfur dioxide-contain-ing yases may be effected by treatment with an aqueous sodium sulfite solution. The operation of an efficient and economical system for removal of sulfur dioxide will be characterized not only by the efficiency of absorption of sulfur dioxide ~ -from the sulfur dioxide-containing gases, the efficiency of desorption of sulfur dioxide from the spent absorbing solution, and the purity of the sulfur dioxide product, but also by the minimization of loss of metal values. Sulfur dioxide-containing gas obtained, for instance, by burning sulfur-bearing mineral products and the like as fuels, can be contacted with sodium sulfite in an aqueous solution to form bisulfite, and thereby substantially reduce the sulfur dioxide content of the gas to, for instance, less than about 0.02 mole percent when the sulfur dioxide-containing gas comprises more than about 0.2 mole percent sulfur dioxide. The removal of sulfur dioxide from the gases is often up to abou~ 95 percent or more. The spent absorbing solution can be heated to convert the bisulfite ~; to sulfite and sulfur dioxide, and to generate a liquid or ' liquid-solid material which serves as the source of the absorbing : solution. The sulfur dioxide can be drawn-off and cooled ~ or compressed to provide a liquid product or sent as a gas to ; a sulfuric acid plant or sulfur plant. Regenerated absorbing solution can be recycled to the absorption zone. For addi-tional information and further exemplification regarding . ~ .
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sulfur dioxide removal systems which can advant~geously employ the technology disclosed herein, see U. S. Patent Nos. 3,607,037, 3,653,812, and 3,790,660.
The sulfur dioxide-containing gases to be treated usually contain materials which facilitate oxidation such as sulfur trioxide, oxygen, elemental iron, and the like, and, particularly when the gases containing sulfur dioxide are derived from the combustion of fuel, other materials which may be present include oxides of nitrogen. At least some of these materials promote the oxidation of the sodium sulfite or bisulfite to sulfate. Sodium sulfate is an inert material for purposes -of the sulfur dioxide absorption-desorption process, because sulfate cannot be regenerated to sulfite during the desorbing operation. Sulfate build-up therefore occurs in the sulfur dioxide absorption-desorption system. A portion of the absorbing-desorbing medium can be purged from the system to ` prevent unduly large amounts of inert sulfate from accumulating in the system. This purge may be a portion of the spent absorption solution or material obtained in the desorption - of the sulfur dioxide from the absorbing solution. These purge materials, however, contain substantial amounts of sulfite or bisulfite, along with the sulfate, and when the purge is discarded, an undue expense may occur due to the accompanying loss of useable sodium va~ues from the system which must be ` replaced by the addition of a suitable soluble sodium compound.It has been proposed to purge sulfate from the absorption-desorption system more selectively with respect to sulfite or :
bisulfite values, and return the latter to the absorbing-...
desorbing system. Simple separation of sodium sulfate from :
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sul~itc and bisulfite in the spent absorbin~ solution by low temperature crystallization ~ives a sul~ate-crystallization product containlng minor amounts of sodium sulfite. A more selective separ~tion of sulfate is thereby obtained but such processing involves undue expense because of the cooling requirements needed to reach the low crystallization tempera-tures and the necessity to reheat the crystallization mother liquor returned to the absorption-desorption system. Also the crystals obtained are in the hydrate form, and drying is required to facilitate handling if the product is to be further processed or sold. Large drying facilities are, therefore, necessary to reduce not only the free water content but also the water of hydration of the material.
In another system for concentrating sodium sulfate so that it can be more economically separated from the sulfur dioxide removal system described above, a purge stream of spent absorbing solution is contacted with a gas containing ~ sulfur dioxide to convert at least a portion of the sodium j l sulfite to the more soluble bisulfite. The stream is then cooled to crystallizing temperature and sodium sulfate preci-pitates more selectively. This procedure has disadvantages in that two separate treatments of the purge stream are required, `~; i.e., sulfur dioxide contact followed by crystallization at a reduced temperature of, say, about 0 to 10C., which is a ~- considerable expense. The additional sodium bisulfite formed in the treatment of the purge stream with sulfur dioxide, and ~; contained in the resulting separated liquid stream added to the absorption-desorption cycle, leads to an additional heat ~`1 requirement to convert the bisulfite to sulfite with the , ~ 30 formation of sulfur dioxide. Moreover, in this operation the ..
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i `-, ~079492 sodium sulfate contains sodium sul~ate hydrates which, ~s noted above, may nccessitate the provision of relatively large dryin~ faciliti~s if the product is to be further handled or sold.
In British Patent No. 489,745 there is described a sulfur dioxide removal system which although devoted mainly to the use of an aqueous ammonium sulfite absorbing medium, mentions the possible use of alkali metal sulfites, and the system is designed to purge sulfate. In the operation the spent absorption medium is desorbed of sulfur dioxide without precipitation of salts. The resulting solution is substantially saturated with sulfate and relatively unsaturated with respect to sulfite and bisulfite. The removal of water and cooling of the solution say from 110 to 50-70C. results in the precipitation of sulfate. This type of operation is undesirable since the system is apparently dependent upon the liquid from the sulfur dioxide desorption zone being relatively unsaturated with respect to sulfite which means that a large amount of absorption solution must be supplied for the removal of a given amount of sulfur dioxide.
In accordance with the present invention, there has been devised a highly advantageous procedure for reducing the amount of sodium sulfate in sodium sulfite-bisulfite, absorption-desorption systems for removing sulfur dioxide from gaseous ?
streams. In the process of this invention it is not necessary to reach temperatures substantially below ambient, l and the sodium sulfate removed can be in an essentially non-`i hydrated or anhydrous form. Accordingly, at least a portion 1 of the aqueous sodium sulfite-bisulfite, absorbing-desorbing . ~, medium containing a relatively large quantity of sodium . , .
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bislllfite is treated to remove water and thereby precipitate a limited amount of sodium sulEate-containing solids from sol~ltion which solids have a higher sulfate concentration on a dr~ basis than the absorption-desorpti~n medium from which they are ~ormed. The concentration of the feed from the ~b~orp-tiorl-desorption system by the removal of water is continued for a sufficient length of time to ensure substantial precipitation of sodium sulfate, but is concluded before the slurry obtained from said feed contains svbstantially more than about 10 weight percent of undissolved solids. The material preferably treated in accordance with the invention is essentially spent absorption medium. In one manner of conducting the process of the invention, the removal of water and precipitation of solids may be effected without substantial conversion of bisulfite to sulfite and sulfur dioxide, e.g., there may be less than about 10 weight percent of the bisulfite so converted. The precipitated solids containing sodium sulfate can be readily removed from the resulting slurry by the use of conventional liquid-solid separation equipment to provide, at least initially, an essentially non-hydrated sulfate product which is relatively high in sodium sulfate content, ad-vantageously at least about 50 weight percent, on a dry basis.
The separated solids may contain a major amount of sodium sulfate, and a minor amount, if any, of sodium sulfite, preferably these amounts are at least about 70 weight percent sodium sulfate and less than about 30 weight percent sodium sulfite based on the total of these materials. The concentration of so~ium sulfate in the precipitated solids is often at least about 2 times, on a dry basis, the concentration of sodium sulfate in the absorption-.:;, ., desorption medium from which the solids are formed by the evapo-ration of water.
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In the method of the present invention, the aqueous ab-sorbing-desorbing solution which undergoes water removal and sulfate precipitation for purging, contains in solution a major weight amount of sodium bisulfite, a minor amount of sodium sulfate, and none or a minor amount of sodium sulfite, based on the total of these components. The solution may preferably contain this amount of bisulfite at the beginning of water removal or it may attain this concentration in the liquid phase during the removal of water. The material from the absorption-desorption system which is treated may contain in solution at least about 25 weight percent of total salts, pref-erably at least about 30 weight percent, and often this amount may not exceed about 50 weight percent. The sodium sulfate content of this solution may usually not exceed about l0 weight percent, and preferably this amount may often not be above about 8 weight percent. The sodium sulfate content in solution is generally at least about 1 weight percent, and preferably is about 3 to 7 weight percent. The weight ratio of sodium sulfate to sodium sulfite in solution in the material treated for limi-20 ted precipitation of solids usually is, or becomes during the treatment, at least about 0.7:1, preferably at least about 1:1.
Although in general the higher the sulfate content in the solu-tion treated the purer the sodium sulfate obtained by the ` method of this invention, an increase in the amount of inert sodium sulfate material circulating in the absorption-desorp-` tion system has a detracting factor since there may be less ac-tive sodium present for a given amount of water in the system.
As a result a greater quantity of circulating material would ` be required to provide a given sulfur dioxide absorption-~` 30 desorption capacityO Often the absorbing-desorbing medium treated has in solution about 0.1 to 10 weight percent sodium .
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sulfite and about 15 to AO ~eight percent sodium bisulfite, based on the total of these components and the sodium sulfate and water present. mhe stream mav contai~ minor amounts of other materials in solution, c.g., sodium thiosulfate.
The solution from the absorption-desorption system which undergoes water removal for sodium sulfate precipitation and purging generally contains initially or during the treatment, a mole ratio of sodium bisulfite to sodium sulfite of at least about 2:1, often at least about 3:1. The amount of sodium bisulfite in solution in the material undergoing treatment as compared to the total active metal in the material may be alter-natively expressed in terms of "s/c" which is defined as the number of moles of ac~ive sulfur, e.g., S03= and HS03-, per 100 moles of water divided by the moles of active sodium per 100 moles of water. Thus, a pure sodium bisulfite solution would have an s/c of 1, and a pure sodium sulfite solution would have an s/c of 0.5. Sodium sulfate, for instance, does not provide active sulfur or active base. ~he s/c of the material from the absorption-desorption system treated in accordance with the process of this invention is preferably about 0.85 to 0.97.
In the present invention sodium sulfate is removed from . the system by treating at least a portion of the a~ueous absorp-tion-desorption medium, preferably spent absorption solution, to evaporate a sufficient amount of water and precipitate a signi-ficant, but limited, amount of solids from the solution. Thus, the evaporation of water is conducted in a manner such that the slurry obtained from the absorption-desorption system feed solu-; tion has up to about 10 weight percent crystals, often at least ~` about 1 weight percent, and preferably up to about 5 weight percent. The water content of the solution from the absorption-desorption system which is subjeFted to evaporation may often , ~ .
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be reduced by up to about 75 or more weight percent, prefer-ably by at least about 10 weight percent, and the material generally remains sufficiently fluid to be readily pumpable.
The operation is advantageously conducted at somewhat eleva-ted temperatures which are sufficient to precipitate essentially non-hydrated crystals without excessive water removal. Gen-erally, such temperatures are at least about 37 to 38C., and to be more certain of having temperatures sufficient to form a non-hydrated product when it is precipitated, a temperature of at least about 40C. is recommended. Suitable temperatures for accomplishing the desired evaporation of water thus include about 40 to 120C., preferably about 40 to 90C. The choice of temperature may depend on the pressure employed, and the pressure may be ambient, reduced or elevated. Advantageously, the pres-sure is about 10 to 20 psia, and preferably, essentially atmos-pheric pressure is used.
The slurry obtained in the water removal operation is sub-jected to liquid-solid separation to provide a separate solid phase which is relatively high in sulfate content. The separa-tion may be done without reducing the temperature of the slurry, and the temperature may often be about 40 to 120Co ~ preferably about 40 to 90C. The separated liquid phase or mother liquor can be charged to the absorption-desorption system, and prefer-ably to its desorption zone. The separated solids may, if desired, be dried and they may undergo self-drying upon standing by the free water being taken up as water of hydration.
The amount of solids formed in the water evaporation stage .
` of the process of this invention, and subsequently removed from ~: , ` the absorption-desorption system is sufficient to prevent undue . , : .
build-up of sodium sulfate in the system. The amount of sulf~te purgcd is preferably substantially equal to the amount of sulfate being formed in the absorption-desorption system while taking into account any sodium sulfate that is removed from the system by other means. Also, the amount of solids formation required may depend on the purity of the sulfate in the precipitated and separated solid phase, as well as the amount of the total absorption-desorption medium which is subjected to the sulfate removal operation. Thus, up to the entire stream of absorption-desorption medium may be treated for sulfate removal in whichcase the percentage of solids formed may be less than if only a portion of the medium is subjected to the sulfate precipitation treatment. Generally, as the percentage of solids formed in the medium decreases the purity of the precipitated sulfate increases.
; In one embodiment of the invention substantially the entire absorption-desorption medium is processed for water removal and sulfate precipitation~ Alternatively, only a portion of this medium may be so treated, and in such case frequently about 10 to 90 weight percent of the total medium is treated in this manner, more often up to about 75 weight percent, say about 20 to 75 weight percent. Preferably, this amount is sufficient so that a maximum of about 5 weight percent solids need be precipitated in the slurry formed from the absorption-desorption feed to have an adequate purge of sodium sulfate. The liquid medium or mother ; liquor separated in the sulfate removal procedure is usually ` passed to the sulfur dioxide desorption stage since the liquid :
is high in bisulfite content. Depending on the amount of the mother liquor to be recycled it may be desirable to charge it to some other part of the absorption-desorption system. Make-up sodium values in the ~or~ of suitable water-soluble sodium com-pouncls such as so~ium car~onates or hydroxide, may be added to the system of this invention to replace s~dium loss, including that removed in the sodium sulfate solids which are precipi-tated. Advantageously, this adclition is to the lean absorbing solution to which make-up water may also be added.
In the sulfur dioxide desorption stage of the method of this invention the spent absorption medium is subjected to ele-vated temperatures to convert sodium bisulfite into sodium sul-fite with the concomitant formation of a vapor phase containing sulfur dioxide and water. Suitable temperatures for this oper-ation include about 40 to 110C., preferably about 60 to 95C.
The pressure may be about 3 to 21 psia, preferably about 8 to 15 psia. The vapor phase can be treated for the recovery of purer sulfur dioxide, the manufacture of sulfur, or used, treat-ed or dis~osed of in any other suitable manner. Various pro-cedures for sulfur dioxide desorption can be used and a number are known in the art. It is preferred, however, that the de-sorption be accomplished with the simultaneous formation of an undissolved soli~s or crystal phase which enables the desorption to be accomplished ~ith the use of lesser amounts of heat.
In such operations the amount of undissolved solids in the desorption zone is generally at least about 15 weight ~ercent of the slurry undergoing decomposition or sulfur dioxide desorption. As described in U. S. Patent ~o. 3,790,660 the amount of such solids is advantageously at least about 25 weight percent in order to alleviate difficulties of tube fouling, particularly when supplying heat to the desorption :1 .
æone by passing the slurry through the tubes of an indirect heat exchanger. Preferably, the amount of undissolved solids is about 30 to 50 weight percent of the slurry undergoing decomposition.
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:~........ . . : -Also when the amount of undissolved solids is sufficiently high, the sodium sulfi-te content of the slurry may be adequate for a por-tion of the total slurry to be combined with water to dissolve the solids, and the resultir.g solution can be used as the le~n solution for absorbing sulfur dioxide from the gas being treated in the absorption zone of the absorption-desor~tion system. The lean absorbing solution is usually composed to a major weight extent of sodium sulfite and minor weight amounts of sodium bisulfite and sodium sulfate based on the total amount of these salts present. Often the lean absorbing solution has about 10 to 35 weight percent sodium sulfite, about 3 to 15 weight percent sodium bisulfite, and about 1 to 10 weight percent sodium . sulfate based on these components and water present.
~ The present invention will be further described by reference - to the drawing which is a schematic flow diagram of a process employing the present invention in an absorption-desorption system using sodium sulfite for removal and recovery of sulfur dioxide from flue gas. Equipment such as valves, pumps, heat exchangers, surge tanks, and the like, which would be used in a commercial . . ~ `~ 20 embodiment of the invention and in the operation of an absorption-desorption system, is not shown since it can be of conventional design and employed in accordance with practices well known in the art.
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Referring to the drawing, sulfur dioxide-containing flue gas, which may, for example, contain from about 0.05 to about 5 mole percent sulfur dioxide, enters absorber vessel 10 by way of line 12 near the bottom thereof. Water or other aqueous liquid may be passed co-currently with the flue gas to a bed of column packing in the lower portion of vessel 10 to scrub the gas to remove suspended solids such as fly-ash and the relatively high water-. , ~,~
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1~)79492 soluble components, for instance, sulfur trioxide, hydrogen chloride and the like from the flue yas.
The flue gas pass~s upwardly through absorber 10 and liquid-gas contacting means such as sieve trays and throuyh a descendiny flow of lean absorbing solution which is supplied to vessel 10 hy line 14. The lean absorbing solution contains sodium sulfite as the essential sulfur dioxide-absorbing component. Absorber 10 may employ other types of liquid-gas contacting structures, such as pac~ing, bubble caps, alternate - 10 ring and discs or the li]ce. The lean absorbing solution in line 14 is often at a temperature of at least about 30~., ~ preferably at least about 40C., up to about 110C., preferably - up to about 70C. Flow rates of the aqueous absorbing solution through the absorption zone can be adjusted according to the sulfur dioxide concentration in the gas being treated, and the concentration of sodium sulfite in the solution, so that a major amount, e.g., up to about 95% or more, of the sulfur dioxide may be removed from the ~as by reaction with the lean ; 20 absorbing solution. The treated gas leaves absorber 10 by way . ~ . . .
.} - of line 13. Spent absorbing solution is collected on gas- -..
passing tray 17 in absorber 10 and removed from the latter by line 16.
The spent absorbing solution is transferred by pump 18 ~ and line 54 to heater 52 and thence by line 56 to desorber 50.
- A portion, or even all, of the spent absorbing solution is withdrawn fro~ line 54 by line 22. The stream in line 22 containing sodium sulfate, as well as sodium sulfite and bisulfite in solution, is passed to evaporator or dehydrator 24 in accordance with this invention. Heat may be supplied to j evaporator 24 by steam coil 26, and the added heat serves to , .
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cause the removal of water from the spent absorbing solution and th~ precipitation o~ s~dium sulEate-containing solids.
The water and any sulfur dioxide in the vapor phase in evapora-tor 24 is removed by line 36.
A portion of the spent absorbing medium undergoing dehy-dration in evaporator 24 is withdrawn via line 40 and passed to crystal separator 42 which may be selected from conventional processing equipment for effecting separation of solids and liquids such as filters, including rotary filters, centrifuges, clarifiers and other sedimentation equipment. Solids rich in sodium sulfate crystals and containing sodium sulfite can be removed by line 44, and the resulting liquid stream having sulfate removed therefrom can be sent via line 46, pump 48, lines 49 and 54, heater 52 and line 56 to desorber 50 for processing to desorb sulfur dioxide and to regenerate the absorbing solution.
The desorber section of the system, which can be operated in the manner, for example, shown in U.S. Patent No. 3,790,660, is for convenience shown as a single stage desorber, but two or more stages may be used. The heated solution in line 56 is introduced into the desorber 50. The conditions of ;~ temperature, pressure, and residence time in desorber 50 are so maintained as to effect the desired decomposition of sodium bisulfite, evaporation of sulfur dioxide and water, and precipitation of s~dium sulfite-containing crystals as described above and in said patent.
To supply heat to desorbing vessel 50 a recycle stream is ~h heated in heat exchanger 58. In order to effect heating in vessel 50, the slurry in the vessel is withdrawn by line 60, and passes through pump 62, line 63, metallic tubes of heat exchanger 58 and back to vessel 50 by way of lines 64 and 56.
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Steam is introduced to exchanger 58 through line 66 as the prime energy source for the desorption zone. The conden-sate (water) from heat exchanger 58 is removed through line 68.
The sulfur dioxide and water vapors from desorber 50 are removed by line 74. A portion of the slurry in desorber 50 is passed to dissolving tank 72 by way of line 78. Since water has been removed from the absorption solution during desorption, make up water, for example, from rectification (not shown), is supplied to tank 72 through line 80. The ~ ;~
solution formed in dissolving tank 72 passes through line 82, pump 84, and line 14 to absorber 10. Make-up sodium ion, which may be an aqueous sodium hydroxide or carbonate solution, is added to line 82 through line 88.
Other ways of conducting the limiied precipitation of . . :
; solids from the absorption-desorption medium may be used in conducting the method of this invention. For example, the medium may be dehydrated while being contacted with a sulfur dioxide-containing gas charged to the dehydrator.
The following examples will further illustrate the present -~.,. . i ~ invention, but do not limit it. ~
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EXAMPLE I
This example gives the results of batch evaporation tests . .:
performed on synthetically prepared, spent absorber solutions `' as would be obtained in an absorption operation of the type shown in the drawing. In the tests the spent absorption solu-tion was placed under vacuum in a flask maintained in a con-stant temperature bath. The evaporations were conducted at ~"~"
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the temperatures noted in l~able I, and were continued until the indicated amount of solids had been formed in a given spent absorption solution. The water evaporated in each test was condensed and collected. The crystals were separated from the mother liquor at the temperature of evaporation. The most pertinent data obtained in these tests are in Table I.
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, TABLE I
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Test No. 1 2 3 4 5 6 7 SPENT 2 3 % wt.5.825.74 6.03 6.945.84 5.75 5.87 , ABSORBER NaHSO3 (calc. as SOLUTION Na2S205) " 18.9818.8020.0624.3119.73 19.63 19.73 ANALYSIS Na2SO4 "8.50 8.07 8.767.49 7.91 B.69 8.46 `-
This invention relates to the remov~l of sulfur dioxide from gas streams by contact with an aqueous sodium sulfite solution to absorb sulfur dioxide and provide a solution richer in sodium bisulfite which can be treated to desorb sulfur dioxide and regenerate the absorbi.ng solution for reuse, and in which sodium sulfate is formed as a by-product in the absorption-desorption medium and must be purged from the system without undue loss of valuable sodium compounds.
More particularly, the invention concerns a process for re-ducing the loss of desirable and valuable sodium compoundsfrom the cyclic sulfur dioxide removal system by removing undesirable sodium sulfate from the absorption-desorption medium as solids containing an increased amount of sodium sulfate.
Sulfur dioxide is a recognized pollutant of the atmos-phere and is produced by oxidation of sulfur or sulfur-bearing materials. Sulfur dioxide is found in significant amounts as a constituent of various waste gases such as smelter gases, off-gases from chemical plants, and stack or furnace gases from coal or oil-burning furnaces such as are used in electric power plants. Although the concentration of sulfur dioxide in such gases is generally minor, e.g., from about 0.001 up to about 5 mole percent, and is frequently less than about 0.5 mole percent (less than about 1% by weight), the emission of sulfur-dioxide may be substantial, particularly in industrial applications due to the large amount of sulfur-bearing material being processed. For instance, a modern electric plant having a 1,350,000 kw. capacity will burn up to about 15,000 tons of ` coal per day. Despite the fact that the concentration of sulfur dioxide in the stack gases from the electric plant can , `
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'"'~,: . : . : '' 10~9492 be low, e.g., oE the order of 0.2 to 0.3 mole percent, the total sulfur dioxide produced mav be in the neighborhood of 1,000 tons per day. Similarl~, significant amounts of sulfur dioxide are produced in utilization of other fuels which may bear sulfur.
The removal of sulfur dio~cide from sulfur dioxide-contain-ing yases may be effected by treatment with an aqueous sodium sulfite solution. The operation of an efficient and economical system for removal of sulfur dioxide will be characterized not only by the efficiency of absorption of sulfur dioxide ~ -from the sulfur dioxide-containing gases, the efficiency of desorption of sulfur dioxide from the spent absorbing solution, and the purity of the sulfur dioxide product, but also by the minimization of loss of metal values. Sulfur dioxide-containing gas obtained, for instance, by burning sulfur-bearing mineral products and the like as fuels, can be contacted with sodium sulfite in an aqueous solution to form bisulfite, and thereby substantially reduce the sulfur dioxide content of the gas to, for instance, less than about 0.02 mole percent when the sulfur dioxide-containing gas comprises more than about 0.2 mole percent sulfur dioxide. The removal of sulfur dioxide from the gases is often up to abou~ 95 percent or more. The spent absorbing solution can be heated to convert the bisulfite ~; to sulfite and sulfur dioxide, and to generate a liquid or ' liquid-solid material which serves as the source of the absorbing : solution. The sulfur dioxide can be drawn-off and cooled ~ or compressed to provide a liquid product or sent as a gas to ; a sulfuric acid plant or sulfur plant. Regenerated absorbing solution can be recycled to the absorption zone. For addi-tional information and further exemplification regarding . ~ .
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sulfur dioxide removal systems which can advant~geously employ the technology disclosed herein, see U. S. Patent Nos. 3,607,037, 3,653,812, and 3,790,660.
The sulfur dioxide-containing gases to be treated usually contain materials which facilitate oxidation such as sulfur trioxide, oxygen, elemental iron, and the like, and, particularly when the gases containing sulfur dioxide are derived from the combustion of fuel, other materials which may be present include oxides of nitrogen. At least some of these materials promote the oxidation of the sodium sulfite or bisulfite to sulfate. Sodium sulfate is an inert material for purposes -of the sulfur dioxide absorption-desorption process, because sulfate cannot be regenerated to sulfite during the desorbing operation. Sulfate build-up therefore occurs in the sulfur dioxide absorption-desorption system. A portion of the absorbing-desorbing medium can be purged from the system to ` prevent unduly large amounts of inert sulfate from accumulating in the system. This purge may be a portion of the spent absorption solution or material obtained in the desorption - of the sulfur dioxide from the absorbing solution. These purge materials, however, contain substantial amounts of sulfite or bisulfite, along with the sulfate, and when the purge is discarded, an undue expense may occur due to the accompanying loss of useable sodium va~ues from the system which must be ` replaced by the addition of a suitable soluble sodium compound.It has been proposed to purge sulfate from the absorption-desorption system more selectively with respect to sulfite or :
bisulfite values, and return the latter to the absorbing-...
desorbing system. Simple separation of sodium sulfate from :
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sul~itc and bisulfite in the spent absorbin~ solution by low temperature crystallization ~ives a sul~ate-crystallization product containlng minor amounts of sodium sulfite. A more selective separ~tion of sulfate is thereby obtained but such processing involves undue expense because of the cooling requirements needed to reach the low crystallization tempera-tures and the necessity to reheat the crystallization mother liquor returned to the absorption-desorption system. Also the crystals obtained are in the hydrate form, and drying is required to facilitate handling if the product is to be further processed or sold. Large drying facilities are, therefore, necessary to reduce not only the free water content but also the water of hydration of the material.
In another system for concentrating sodium sulfate so that it can be more economically separated from the sulfur dioxide removal system described above, a purge stream of spent absorbing solution is contacted with a gas containing ~ sulfur dioxide to convert at least a portion of the sodium j l sulfite to the more soluble bisulfite. The stream is then cooled to crystallizing temperature and sodium sulfate preci-pitates more selectively. This procedure has disadvantages in that two separate treatments of the purge stream are required, `~; i.e., sulfur dioxide contact followed by crystallization at a reduced temperature of, say, about 0 to 10C., which is a ~- considerable expense. The additional sodium bisulfite formed in the treatment of the purge stream with sulfur dioxide, and ~; contained in the resulting separated liquid stream added to the absorption-desorption cycle, leads to an additional heat ~`1 requirement to convert the bisulfite to sulfite with the , ~ 30 formation of sulfur dioxide. Moreover, in this operation the ..
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i `-, ~079492 sodium sulfate contains sodium sul~ate hydrates which, ~s noted above, may nccessitate the provision of relatively large dryin~ faciliti~s if the product is to be further handled or sold.
In British Patent No. 489,745 there is described a sulfur dioxide removal system which although devoted mainly to the use of an aqueous ammonium sulfite absorbing medium, mentions the possible use of alkali metal sulfites, and the system is designed to purge sulfate. In the operation the spent absorption medium is desorbed of sulfur dioxide without precipitation of salts. The resulting solution is substantially saturated with sulfate and relatively unsaturated with respect to sulfite and bisulfite. The removal of water and cooling of the solution say from 110 to 50-70C. results in the precipitation of sulfate. This type of operation is undesirable since the system is apparently dependent upon the liquid from the sulfur dioxide desorption zone being relatively unsaturated with respect to sulfite which means that a large amount of absorption solution must be supplied for the removal of a given amount of sulfur dioxide.
In accordance with the present invention, there has been devised a highly advantageous procedure for reducing the amount of sodium sulfate in sodium sulfite-bisulfite, absorption-desorption systems for removing sulfur dioxide from gaseous ?
streams. In the process of this invention it is not necessary to reach temperatures substantially below ambient, l and the sodium sulfate removed can be in an essentially non-`i hydrated or anhydrous form. Accordingly, at least a portion 1 of the aqueous sodium sulfite-bisulfite, absorbing-desorbing . ~, medium containing a relatively large quantity of sodium . , .
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bislllfite is treated to remove water and thereby precipitate a limited amount of sodium sulEate-containing solids from sol~ltion which solids have a higher sulfate concentration on a dr~ basis than the absorption-desorpti~n medium from which they are ~ormed. The concentration of the feed from the ~b~orp-tiorl-desorption system by the removal of water is continued for a sufficient length of time to ensure substantial precipitation of sodium sulfate, but is concluded before the slurry obtained from said feed contains svbstantially more than about 10 weight percent of undissolved solids. The material preferably treated in accordance with the invention is essentially spent absorption medium. In one manner of conducting the process of the invention, the removal of water and precipitation of solids may be effected without substantial conversion of bisulfite to sulfite and sulfur dioxide, e.g., there may be less than about 10 weight percent of the bisulfite so converted. The precipitated solids containing sodium sulfate can be readily removed from the resulting slurry by the use of conventional liquid-solid separation equipment to provide, at least initially, an essentially non-hydrated sulfate product which is relatively high in sodium sulfate content, ad-vantageously at least about 50 weight percent, on a dry basis.
The separated solids may contain a major amount of sodium sulfate, and a minor amount, if any, of sodium sulfite, preferably these amounts are at least about 70 weight percent sodium sulfate and less than about 30 weight percent sodium sulfite based on the total of these materials. The concentration of so~ium sulfate in the precipitated solids is often at least about 2 times, on a dry basis, the concentration of sodium sulfate in the absorption-.:;, ., desorption medium from which the solids are formed by the evapo-ration of water.
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In the method of the present invention, the aqueous ab-sorbing-desorbing solution which undergoes water removal and sulfate precipitation for purging, contains in solution a major weight amount of sodium bisulfite, a minor amount of sodium sulfate, and none or a minor amount of sodium sulfite, based on the total of these components. The solution may preferably contain this amount of bisulfite at the beginning of water removal or it may attain this concentration in the liquid phase during the removal of water. The material from the absorption-desorption system which is treated may contain in solution at least about 25 weight percent of total salts, pref-erably at least about 30 weight percent, and often this amount may not exceed about 50 weight percent. The sodium sulfate content of this solution may usually not exceed about l0 weight percent, and preferably this amount may often not be above about 8 weight percent. The sodium sulfate content in solution is generally at least about 1 weight percent, and preferably is about 3 to 7 weight percent. The weight ratio of sodium sulfate to sodium sulfite in solution in the material treated for limi-20 ted precipitation of solids usually is, or becomes during the treatment, at least about 0.7:1, preferably at least about 1:1.
Although in general the higher the sulfate content in the solu-tion treated the purer the sodium sulfate obtained by the ` method of this invention, an increase in the amount of inert sodium sulfate material circulating in the absorption-desorp-` tion system has a detracting factor since there may be less ac-tive sodium present for a given amount of water in the system.
As a result a greater quantity of circulating material would ` be required to provide a given sulfur dioxide absorption-~` 30 desorption capacityO Often the absorbing-desorbing medium treated has in solution about 0.1 to 10 weight percent sodium .
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sulfite and about 15 to AO ~eight percent sodium bisulfite, based on the total of these components and the sodium sulfate and water present. mhe stream mav contai~ minor amounts of other materials in solution, c.g., sodium thiosulfate.
The solution from the absorption-desorption system which undergoes water removal for sodium sulfate precipitation and purging generally contains initially or during the treatment, a mole ratio of sodium bisulfite to sodium sulfite of at least about 2:1, often at least about 3:1. The amount of sodium bisulfite in solution in the material undergoing treatment as compared to the total active metal in the material may be alter-natively expressed in terms of "s/c" which is defined as the number of moles of ac~ive sulfur, e.g., S03= and HS03-, per 100 moles of water divided by the moles of active sodium per 100 moles of water. Thus, a pure sodium bisulfite solution would have an s/c of 1, and a pure sodium sulfite solution would have an s/c of 0.5. Sodium sulfate, for instance, does not provide active sulfur or active base. ~he s/c of the material from the absorption-desorption system treated in accordance with the process of this invention is preferably about 0.85 to 0.97.
In the present invention sodium sulfate is removed from . the system by treating at least a portion of the a~ueous absorp-tion-desorption medium, preferably spent absorption solution, to evaporate a sufficient amount of water and precipitate a signi-ficant, but limited, amount of solids from the solution. Thus, the evaporation of water is conducted in a manner such that the slurry obtained from the absorption-desorption system feed solu-; tion has up to about 10 weight percent crystals, often at least ~` about 1 weight percent, and preferably up to about 5 weight percent. The water content of the solution from the absorption-desorption system which is subjeFted to evaporation may often , ~ .
:' ,~ , , : ' ' ' ~07949Z
be reduced by up to about 75 or more weight percent, prefer-ably by at least about 10 weight percent, and the material generally remains sufficiently fluid to be readily pumpable.
The operation is advantageously conducted at somewhat eleva-ted temperatures which are sufficient to precipitate essentially non-hydrated crystals without excessive water removal. Gen-erally, such temperatures are at least about 37 to 38C., and to be more certain of having temperatures sufficient to form a non-hydrated product when it is precipitated, a temperature of at least about 40C. is recommended. Suitable temperatures for accomplishing the desired evaporation of water thus include about 40 to 120C., preferably about 40 to 90C. The choice of temperature may depend on the pressure employed, and the pressure may be ambient, reduced or elevated. Advantageously, the pres-sure is about 10 to 20 psia, and preferably, essentially atmos-pheric pressure is used.
The slurry obtained in the water removal operation is sub-jected to liquid-solid separation to provide a separate solid phase which is relatively high in sulfate content. The separa-tion may be done without reducing the temperature of the slurry, and the temperature may often be about 40 to 120Co ~ preferably about 40 to 90C. The separated liquid phase or mother liquor can be charged to the absorption-desorption system, and prefer-ably to its desorption zone. The separated solids may, if desired, be dried and they may undergo self-drying upon standing by the free water being taken up as water of hydration.
The amount of solids formed in the water evaporation stage .
` of the process of this invention, and subsequently removed from ~: , ` the absorption-desorption system is sufficient to prevent undue . , : .
build-up of sodium sulfate in the system. The amount of sulf~te purgcd is preferably substantially equal to the amount of sulfate being formed in the absorption-desorption system while taking into account any sodium sulfate that is removed from the system by other means. Also, the amount of solids formation required may depend on the purity of the sulfate in the precipitated and separated solid phase, as well as the amount of the total absorption-desorption medium which is subjected to the sulfate removal operation. Thus, up to the entire stream of absorption-desorption medium may be treated for sulfate removal in whichcase the percentage of solids formed may be less than if only a portion of the medium is subjected to the sulfate precipitation treatment. Generally, as the percentage of solids formed in the medium decreases the purity of the precipitated sulfate increases.
; In one embodiment of the invention substantially the entire absorption-desorption medium is processed for water removal and sulfate precipitation~ Alternatively, only a portion of this medium may be so treated, and in such case frequently about 10 to 90 weight percent of the total medium is treated in this manner, more often up to about 75 weight percent, say about 20 to 75 weight percent. Preferably, this amount is sufficient so that a maximum of about 5 weight percent solids need be precipitated in the slurry formed from the absorption-desorption feed to have an adequate purge of sodium sulfate. The liquid medium or mother ; liquor separated in the sulfate removal procedure is usually ` passed to the sulfur dioxide desorption stage since the liquid :
is high in bisulfite content. Depending on the amount of the mother liquor to be recycled it may be desirable to charge it to some other part of the absorption-desorption system. Make-up sodium values in the ~or~ of suitable water-soluble sodium com-pouncls such as so~ium car~onates or hydroxide, may be added to the system of this invention to replace s~dium loss, including that removed in the sodium sulfate solids which are precipi-tated. Advantageously, this adclition is to the lean absorbing solution to which make-up water may also be added.
In the sulfur dioxide desorption stage of the method of this invention the spent absorption medium is subjected to ele-vated temperatures to convert sodium bisulfite into sodium sul-fite with the concomitant formation of a vapor phase containing sulfur dioxide and water. Suitable temperatures for this oper-ation include about 40 to 110C., preferably about 60 to 95C.
The pressure may be about 3 to 21 psia, preferably about 8 to 15 psia. The vapor phase can be treated for the recovery of purer sulfur dioxide, the manufacture of sulfur, or used, treat-ed or dis~osed of in any other suitable manner. Various pro-cedures for sulfur dioxide desorption can be used and a number are known in the art. It is preferred, however, that the de-sorption be accomplished with the simultaneous formation of an undissolved soli~s or crystal phase which enables the desorption to be accomplished ~ith the use of lesser amounts of heat.
In such operations the amount of undissolved solids in the desorption zone is generally at least about 15 weight ~ercent of the slurry undergoing decomposition or sulfur dioxide desorption. As described in U. S. Patent ~o. 3,790,660 the amount of such solids is advantageously at least about 25 weight percent in order to alleviate difficulties of tube fouling, particularly when supplying heat to the desorption :1 .
æone by passing the slurry through the tubes of an indirect heat exchanger. Preferably, the amount of undissolved solids is about 30 to 50 weight percent of the slurry undergoing decomposition.
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:~........ . . : -Also when the amount of undissolved solids is sufficiently high, the sodium sulfi-te content of the slurry may be adequate for a por-tion of the total slurry to be combined with water to dissolve the solids, and the resultir.g solution can be used as the le~n solution for absorbing sulfur dioxide from the gas being treated in the absorption zone of the absorption-desor~tion system. The lean absorbing solution is usually composed to a major weight extent of sodium sulfite and minor weight amounts of sodium bisulfite and sodium sulfate based on the total amount of these salts present. Often the lean absorbing solution has about 10 to 35 weight percent sodium sulfite, about 3 to 15 weight percent sodium bisulfite, and about 1 to 10 weight percent sodium . sulfate based on these components and water present.
~ The present invention will be further described by reference - to the drawing which is a schematic flow diagram of a process employing the present invention in an absorption-desorption system using sodium sulfite for removal and recovery of sulfur dioxide from flue gas. Equipment such as valves, pumps, heat exchangers, surge tanks, and the like, which would be used in a commercial . . ~ `~ 20 embodiment of the invention and in the operation of an absorption-desorption system, is not shown since it can be of conventional design and employed in accordance with practices well known in the art.
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Referring to the drawing, sulfur dioxide-containing flue gas, which may, for example, contain from about 0.05 to about 5 mole percent sulfur dioxide, enters absorber vessel 10 by way of line 12 near the bottom thereof. Water or other aqueous liquid may be passed co-currently with the flue gas to a bed of column packing in the lower portion of vessel 10 to scrub the gas to remove suspended solids such as fly-ash and the relatively high water-. , ~,~
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1~)79492 soluble components, for instance, sulfur trioxide, hydrogen chloride and the like from the flue yas.
The flue gas pass~s upwardly through absorber 10 and liquid-gas contacting means such as sieve trays and throuyh a descendiny flow of lean absorbing solution which is supplied to vessel 10 hy line 14. The lean absorbing solution contains sodium sulfite as the essential sulfur dioxide-absorbing component. Absorber 10 may employ other types of liquid-gas contacting structures, such as pac~ing, bubble caps, alternate - 10 ring and discs or the li]ce. The lean absorbing solution in line 14 is often at a temperature of at least about 30~., ~ preferably at least about 40C., up to about 110C., preferably - up to about 70C. Flow rates of the aqueous absorbing solution through the absorption zone can be adjusted according to the sulfur dioxide concentration in the gas being treated, and the concentration of sodium sulfite in the solution, so that a major amount, e.g., up to about 95% or more, of the sulfur dioxide may be removed from the ~as by reaction with the lean ; 20 absorbing solution. The treated gas leaves absorber 10 by way . ~ . . .
.} - of line 13. Spent absorbing solution is collected on gas- -..
passing tray 17 in absorber 10 and removed from the latter by line 16.
The spent absorbing solution is transferred by pump 18 ~ and line 54 to heater 52 and thence by line 56 to desorber 50.
- A portion, or even all, of the spent absorbing solution is withdrawn fro~ line 54 by line 22. The stream in line 22 containing sodium sulfate, as well as sodium sulfite and bisulfite in solution, is passed to evaporator or dehydrator 24 in accordance with this invention. Heat may be supplied to j evaporator 24 by steam coil 26, and the added heat serves to , .
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cause the removal of water from the spent absorbing solution and th~ precipitation o~ s~dium sulEate-containing solids.
The water and any sulfur dioxide in the vapor phase in evapora-tor 24 is removed by line 36.
A portion of the spent absorbing medium undergoing dehy-dration in evaporator 24 is withdrawn via line 40 and passed to crystal separator 42 which may be selected from conventional processing equipment for effecting separation of solids and liquids such as filters, including rotary filters, centrifuges, clarifiers and other sedimentation equipment. Solids rich in sodium sulfate crystals and containing sodium sulfite can be removed by line 44, and the resulting liquid stream having sulfate removed therefrom can be sent via line 46, pump 48, lines 49 and 54, heater 52 and line 56 to desorber 50 for processing to desorb sulfur dioxide and to regenerate the absorbing solution.
The desorber section of the system, which can be operated in the manner, for example, shown in U.S. Patent No. 3,790,660, is for convenience shown as a single stage desorber, but two or more stages may be used. The heated solution in line 56 is introduced into the desorber 50. The conditions of ;~ temperature, pressure, and residence time in desorber 50 are so maintained as to effect the desired decomposition of sodium bisulfite, evaporation of sulfur dioxide and water, and precipitation of s~dium sulfite-containing crystals as described above and in said patent.
To supply heat to desorbing vessel 50 a recycle stream is ~h heated in heat exchanger 58. In order to effect heating in vessel 50, the slurry in the vessel is withdrawn by line 60, and passes through pump 62, line 63, metallic tubes of heat exchanger 58 and back to vessel 50 by way of lines 64 and 56.
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Steam is introduced to exchanger 58 through line 66 as the prime energy source for the desorption zone. The conden-sate (water) from heat exchanger 58 is removed through line 68.
The sulfur dioxide and water vapors from desorber 50 are removed by line 74. A portion of the slurry in desorber 50 is passed to dissolving tank 72 by way of line 78. Since water has been removed from the absorption solution during desorption, make up water, for example, from rectification (not shown), is supplied to tank 72 through line 80. The ~ ;~
solution formed in dissolving tank 72 passes through line 82, pump 84, and line 14 to absorber 10. Make-up sodium ion, which may be an aqueous sodium hydroxide or carbonate solution, is added to line 82 through line 88.
Other ways of conducting the limiied precipitation of . . :
; solids from the absorption-desorption medium may be used in conducting the method of this invention. For example, the medium may be dehydrated while being contacted with a sulfur dioxide-containing gas charged to the dehydrator.
The following examples will further illustrate the present -~.,. . i ~ invention, but do not limit it. ~
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EXAMPLE I
This example gives the results of batch evaporation tests . .:
performed on synthetically prepared, spent absorber solutions `' as would be obtained in an absorption operation of the type shown in the drawing. In the tests the spent absorption solu-tion was placed under vacuum in a flask maintained in a con-stant temperature bath. The evaporations were conducted at ~"~"
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the temperatures noted in l~able I, and were continued until the indicated amount of solids had been formed in a given spent absorption solution. The water evaporated in each test was condensed and collected. The crystals were separated from the mother liquor at the temperature of evaporation. The most pertinent data obtained in these tests are in Table I.
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1~79~9Z
, TABLE I
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Test No. 1 2 3 4 5 6 7 SPENT 2 3 % wt.5.825.74 6.03 6.945.84 5.75 5.87 , ABSORBER NaHSO3 (calc. as SOLUTION Na2S205) " 18.9818.8020.0624.3119.73 19.63 19.73 ANALYSIS Na2SO4 "8.50 8.07 8.767.49 7.91 B.69 8.46 `-
- 2 2 3 " 0.30 0.29 0.33 0.410.30 0.33 0.31 - H20(by diff)" 66.40 67.1064.8260.8566.2265.60 65.63 l Vol.** ccs 500 500 500 500 500 500500 ,, Wt. g 649.0649.0 648.0657.4654.8654.0649.4 ~;~ Density g/cc 1.34 1.37 1.301.36 1.33 1.35 1.33;
," I i, -, EVAPORA- WATER REMOVED g 101 122 206 109273 298 351' TION TEMPERATURE C. 86 85 92 86 87 86 86 ~:
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,'l SOLIDS CONTENT ~ wt. 1.1 4.8 11.117.1 22.0 28.9 44.1 -~ OF SLURRY ~ --'';I - ,' ~, ~ 2 3 % wt. 8.057.73 7.73 8.606.91 6.38 4.08 ;Ji'~ ! ,;, MOTHER NaHSO3 (calc. as `::!
LIQUOR Na2S2O5) " 22.0123.03 26.3831.1135.8136.6842.58 ~' ANALYSIS Na2SO4 " 9.94 6.675.68 5.48 3.78 4.75 3.73 ;~
,~; Na2S23 " 0.38 0.39 0.60 0.480.65 0.73 1.05 H2O (by diff.) " 59.62 62.18 59.61 54.33 52.85 51.46 48.56 DRY Na2S3 % wt. 19.626.90 37.4341.0246.1244.1156.81 ~i CRYSTALS NaHSO (calc. as ANALYSIS* Na2S205) " NIL 1.07 NIL NIL NIL NILNIL
~: Na2S4 " 80.4 72.03 62.5758.9853.8855.8943.19 H20 (by diff.) " NIL NIL NIL NILNIL NILNIL ~, ~ ."
j * Calculated free of mother liquor and expected on a dry basis ~ ** Approximated . .
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I?rorn the data ir~rrable I il, can be se~en that, as the solids content Or the mixture undergoing -evaporation increase,~, the sulfate to sulr:itei ratio ln the crystals decreased. Only ln t,ests 1 and 2, having respectively, 1.1 and Ll. 8 weight percent solids f'ormed in the materlal evapora~ed, did the dry crystal product have greater than about 65 welght percent sulfate.
Additional batch tests were conducted in a similar manner, -~ 10 but using spent absorber solutions obtained from an operating commercial installation in which sulfur dioxide was being ab- ~-''J. sorbed from a gas in a system of the type shown in the drawing.
In these tests, each spent absorber solution was evaporated at about 105C. and atmospheric pressure in a stirred, stainless steel beaker until the stated amount o~' sollds appeared. The sollds were separated ln a laboratory centrifuge 15 cm. ln ' dlameter, 5 cm. ln helght, operating at approximately 2800 ~' rpm. The centrifuge basket was covered with 316 stainless ,.. ; .
,3 steel, 150 mesh. The separated crystals and the mother liquor Ç~ 20 were weighed and analysed. In almost all cases the crystals p~ ~ produced could be centrifuged easily. The results of the " ~ tests are given~in Table II.
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TABLE II
Te st No . 1 2 3 4 5 6 Spent Absorber Solution Takeng 1300 13161318 13081343 1324 Water Evaporatedg (420)1~350) -- 281286 273 Mother Liquor Separated g752 867 1009931 962960 ~ -Wet Crystals Collected g89.9 64.3 10.137.7 41.741.4 Solids Content of Slurry% wt.7.9 5.7 0.93.0 3.63.1 ' ~
SPENT Na2SO4% wt. 5.47 5.32 -_ 5.20 -- 5.36 ; ABSOR~ER Na2SO3 " 0.87 0.91 -- 0.84 -- 0.90 SOLUTION NaHSO3 (calc. as ANALYSIS Na2S2Os) ~ 31.4831.83 -- 32.67 -- 31.85 " Na2S203 0.14 0.10 -- 0.15 --0.10 H2O (by diff) " 62.0461.84 -- 61.14 -- 61.79 j :
MOTHER Na2SO4 ~ wt.3.80 4.38 5.885.26 4.724.84 LIQUOR Na SO3 " 5 57 5.21 5.315.36 5.355.28 ANALYSIS NaHSO3 (calc. as Na2S2O5) "39.24 40.24 ~35.16 35.22 ~
Na2S2O3 " 0.30 0.33 388.81 ~54.22 0.30 ~89.88 ~ H20 (by diff) " 51.0949.84 ~ ~ 54.41 :
a2So4 % wt.43.44 53.19 65.84 55.09 60.17 56.70 ~` CRYSTALS Na2SO3 ~'34.51 31.21 23.47 22.80 24.58 19.48 ' ~ ANALYSIS NaHSO3 (calc. as '- (With Na2S2Os 7.52 6.78 r8.77 5.34 Adhering Mother Na2S2O3 0.10 0.0810.69 0.08 23.82 Liquor H2O (by diff) " 14.43 8.74 13.34 9.83 .,~
CRYSTALS Na2SO4 % wt. 55.6 63.4 74.0 71.4 71.4 75.4 . ANALYSIS* Na2SO3 " 44.4 36.6 26.0 28.6 28.6 24.6 .
1( ) = Approximation 2Calculated free of mother liquor and expected on a dry basis .... . . . . . .
: ~079492 The results of these ]arger scale tests conf-irm the laboratory results shown in Example I. The data clearly show that the dry sollds were high in sulfate content, and generally that ~ -the ~ulfate content decreased as the amount of ~oli~s forrned durlng the evaporation increased.
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To determine the effect of sulfate and sulfite concentra- :
tion in the spent absorber solution on the sulfate content of -~ -10 the separated crystal product, batch evaporation tests were performed on synthetically prepared, spent absorber solutions as would be obtained in an absorptlon operation of the type shown in the drawlng. In the tests the spent absorption solution was placed in a flask maintained in~a constant tem-perature bath. The evaporations were conducted under vacuum at~the temperatures noted in Table III, and were contlnued until the indicated amount of solids had been formed in a ¦ ~ glven spent absorptlon solution. The water evaporated in each test was condensed and collected. The crystals were separatéd ~rom the mother liquor at the temperature of evaporation.
The~levels~of sulfate in the spent absorber solutions were ~ ;
varied in the~tests but the percentage of the salts in solu-tion~were kept relatlvely constant. The most pertlnent data , ~ obtalned in these tests are in Table III.
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107949~
The data of''rable III :Illustrate the effect of the sulfate and sulfi~e concentrations in the spent absorber solution on the concentration Or sulfate ln the separated crystal product.
These data show that an increase of' about 1 percent sulfate (at constant sulflte concentration) in the spent absorber solution gave an increase of about 5 to 10 percent sulf'ate in the dry crystal product. Also a decrease of about 6 percent sulfite (at constant sulfate concentration) in the spent absorbing solution gave an increase of about 15 to 20 percent .
sulfate in the dry crystal product.
Various modifications and equivalents of the process of this invention will be apparent to one skllled in the art, and may be made without departing from the spirlt or scope of the invention.
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," I i, -, EVAPORA- WATER REMOVED g 101 122 206 109273 298 351' TION TEMPERATURE C. 86 85 92 86 87 86 86 ~:
~ ~ .. ..
,'l SOLIDS CONTENT ~ wt. 1.1 4.8 11.117.1 22.0 28.9 44.1 -~ OF SLURRY ~ --'';I - ,' ~, ~ 2 3 % wt. 8.057.73 7.73 8.606.91 6.38 4.08 ;Ji'~ ! ,;, MOTHER NaHSO3 (calc. as `::!
LIQUOR Na2S2O5) " 22.0123.03 26.3831.1135.8136.6842.58 ~' ANALYSIS Na2SO4 " 9.94 6.675.68 5.48 3.78 4.75 3.73 ;~
,~; Na2S23 " 0.38 0.39 0.60 0.480.65 0.73 1.05 H2O (by diff.) " 59.62 62.18 59.61 54.33 52.85 51.46 48.56 DRY Na2S3 % wt. 19.626.90 37.4341.0246.1244.1156.81 ~i CRYSTALS NaHSO (calc. as ANALYSIS* Na2S205) " NIL 1.07 NIL NIL NIL NILNIL
~: Na2S4 " 80.4 72.03 62.5758.9853.8855.8943.19 H20 (by diff.) " NIL NIL NIL NILNIL NILNIL ~, ~ ."
j * Calculated free of mother liquor and expected on a dry basis ~ ** Approximated . .
^~ -18-,::
I?rorn the data ir~rrable I il, can be se~en that, as the solids content Or the mixture undergoing -evaporation increase,~, the sulfate to sulr:itei ratio ln the crystals decreased. Only ln t,ests 1 and 2, having respectively, 1.1 and Ll. 8 weight percent solids f'ormed in the materlal evapora~ed, did the dry crystal product have greater than about 65 welght percent sulfate.
Additional batch tests were conducted in a similar manner, -~ 10 but using spent absorber solutions obtained from an operating commercial installation in which sulfur dioxide was being ab- ~-''J. sorbed from a gas in a system of the type shown in the drawing.
In these tests, each spent absorber solution was evaporated at about 105C. and atmospheric pressure in a stirred, stainless steel beaker until the stated amount o~' sollds appeared. The sollds were separated ln a laboratory centrifuge 15 cm. ln ' dlameter, 5 cm. ln helght, operating at approximately 2800 ~' rpm. The centrifuge basket was covered with 316 stainless ,.. ; .
,3 steel, 150 mesh. The separated crystals and the mother liquor Ç~ 20 were weighed and analysed. In almost all cases the crystals p~ ~ produced could be centrifuged easily. The results of the " ~ tests are given~in Table II.
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~07949Z
TABLE II
Te st No . 1 2 3 4 5 6 Spent Absorber Solution Takeng 1300 13161318 13081343 1324 Water Evaporatedg (420)1~350) -- 281286 273 Mother Liquor Separated g752 867 1009931 962960 ~ -Wet Crystals Collected g89.9 64.3 10.137.7 41.741.4 Solids Content of Slurry% wt.7.9 5.7 0.93.0 3.63.1 ' ~
SPENT Na2SO4% wt. 5.47 5.32 -_ 5.20 -- 5.36 ; ABSOR~ER Na2SO3 " 0.87 0.91 -- 0.84 -- 0.90 SOLUTION NaHSO3 (calc. as ANALYSIS Na2S2Os) ~ 31.4831.83 -- 32.67 -- 31.85 " Na2S203 0.14 0.10 -- 0.15 --0.10 H2O (by diff) " 62.0461.84 -- 61.14 -- 61.79 j :
MOTHER Na2SO4 ~ wt.3.80 4.38 5.885.26 4.724.84 LIQUOR Na SO3 " 5 57 5.21 5.315.36 5.355.28 ANALYSIS NaHSO3 (calc. as Na2S2O5) "39.24 40.24 ~35.16 35.22 ~
Na2S2O3 " 0.30 0.33 388.81 ~54.22 0.30 ~89.88 ~ H20 (by diff) " 51.0949.84 ~ ~ 54.41 :
a2So4 % wt.43.44 53.19 65.84 55.09 60.17 56.70 ~` CRYSTALS Na2SO3 ~'34.51 31.21 23.47 22.80 24.58 19.48 ' ~ ANALYSIS NaHSO3 (calc. as '- (With Na2S2Os 7.52 6.78 r8.77 5.34 Adhering Mother Na2S2O3 0.10 0.0810.69 0.08 23.82 Liquor H2O (by diff) " 14.43 8.74 13.34 9.83 .,~
CRYSTALS Na2SO4 % wt. 55.6 63.4 74.0 71.4 71.4 75.4 . ANALYSIS* Na2SO3 " 44.4 36.6 26.0 28.6 28.6 24.6 .
1( ) = Approximation 2Calculated free of mother liquor and expected on a dry basis .... . . . . . .
: ~079492 The results of these ]arger scale tests conf-irm the laboratory results shown in Example I. The data clearly show that the dry sollds were high in sulfate content, and generally that ~ -the ~ulfate content decreased as the amount of ~oli~s forrned durlng the evaporation increased.
, ' .
To determine the effect of sulfate and sulfite concentra- :
tion in the spent absorber solution on the sulfate content of -~ -10 the separated crystal product, batch evaporation tests were performed on synthetically prepared, spent absorber solutions as would be obtained in an absorptlon operation of the type shown in the drawlng. In the tests the spent absorption solution was placed in a flask maintained in~a constant tem-perature bath. The evaporations were conducted under vacuum at~the temperatures noted in Table III, and were contlnued until the indicated amount of solids had been formed in a ¦ ~ glven spent absorptlon solution. The water evaporated in each test was condensed and collected. The crystals were separatéd ~rom the mother liquor at the temperature of evaporation.
The~levels~of sulfate in the spent absorber solutions were ~ ;
varied in the~tests but the percentage of the salts in solu-tion~were kept relatlvely constant. The most pertlnent data , ~ obtalned in these tests are in Table III.
.
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107949~
The data of''rable III :Illustrate the effect of the sulfate and sulfi~e concentrations in the spent absorber solution on the concentration Or sulfate ln the separated crystal product.
These data show that an increase of' about 1 percent sulfate (at constant sulflte concentration) in the spent absorber solution gave an increase of about 5 to 10 percent sulf'ate in the dry crystal product. Also a decrease of about 6 percent sulfite (at constant sulfate concentration) in the spent absorbing solution gave an increase of about 15 to 20 percent .
sulfate in the dry crystal product.
Various modifications and equivalents of the process of this invention will be apparent to one skllled in the art, and may be made without departing from the spirlt or scope of the invention.
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Claims (20)
1. In a process for the removal of sulfur dioxide from gas in which sulfur dioxide is absorbed from the gas in an aqueous absorption solution of sodium sulfite to form the corresponding bisulfite, resulting bisulfite-containing absorption solution is desorbed to form sulfur dioxide and crystallize sodium sulfite from solution, and a sulfite-containing absorption solution is regenerated for recycling, and in which sodium sulfate is in the absorption-desorption medium, the improvement for removing sul-fate from the system comprising evaporating water from at least a portion of the aqueous absorption-desorption medium having in solution a major amount of sodium bisulfite, a minor amount of sodium sulfate, and a minor amount of sodium sulfite, based on the total weight of these components, to precipitate up to about 10 weight percent undissolved, sulfite and sulfate-containing solids in the slurry resulting from said medium, separating from the slurry sulfite and sulfate-containing precipitate and bisul-fite-containing solution, and passing said latter solution to the absorption-desorption cycle.
2. The process of claim 1 in which the absorption-de-sorption medium which undergoes said evaporation is spent absorb-ing solution.
3. The process of claim 1 in which the desorption is conducted with a slurry containing dissolved bisulfite and sodium sulfite solids, said slurry containing at least about 25 weight percent of undissolved solids.
4. The process of claim 1, 2 or 3 in which the slurry resulting from said medium contains about 1 to 8 weight percent of undissolved solids which contain at least about 50 weight percent of sodium sulfate.
5. The process of claim 1, 2 or 3 in which the absorp-tion-desorption medium which undergoes said evaporation is only a portion of the spent absorbing solution.
6. In a process for the removal of sulfur dioxide from gas in which sulfur dioxide is absorbed from the gas in an aqueous absorption solution of sodium sulfite to form the corresponding bisulfite, resulting bisulfite-containing absorption solution is desorbed to form sulfur dioxide and crystallize sodium sulfite from solution, and a sulfite-containing absorption solution is re-generated for recycling, and in which sodium sulfate is in the spent solution from said absorption, the improvement for removing sulfate from the system comprising evaporating water from said spent absorption solution at a temperature of about 40 to 120°C
to precipitate about 1 to 8 weight percent of undissolved sulfite and sulfate-containing solids in the slurry resulting from said spent absorption solution, said solids containing a major amount of sodium sulfate, separating from the slurry sulfite and sulfate-containing precipitate and bisulfite-containing solution, and de-sorbing sulfur dioxide from said bisulfite-containing solution.
to precipitate about 1 to 8 weight percent of undissolved sulfite and sulfate-containing solids in the slurry resulting from said spent absorption solution, said solids containing a major amount of sodium sulfate, separating from the slurry sulfite and sulfate-containing precipitate and bisulfite-containing solution, and de-sorbing sulfur dioxide from said bisulfite-containing solution.
7. In a process for the removal of sulfur dioxide from gas in which sulfur dioxide is absorbed from the gas in an aqueous absorption solution of sodium sulfite to form the corres-ponding bisulfite, resulting bisulfite-containing absorption sol-ution is desorbed to form sulfur dioxide and crystallize sodium sulfite from solution, and a sulfite-containing absorption solu-tion is regenerated for recycling, and in which sodium sulfate is in the absorption-desorption medium, the improvement for re-moving sulfate from the system comprising evaporating water from at least a portion of the aqueous absorption-desorption medium having in solution a major amount of sodium bisulfite, a minor amount of sodium sulfate, and a minor amount of sodium sulfite, based. on the total weight of these components, to precipitate up to about 10 weight percent undissolved solids in the slurry re-sulting from said medium, said undissolved solids having a major amount of sulfate and a minor amount of sulfite, separating sul-fate- and sulfite-containing precipitate and bisulfite-containing solution from said slurry, and passing said latter solution to the absorption-desorption cycle.
8. In a process for the removal of sulfur dioxide from gas in which sulfur dioxide is absorbed from the gas in an aqueous absorption solution of sodium sulfite to form the corres-ponding bisulfite in an absorption zone, resulting bisulfite-con-taining spent absorption solution is desorbed in a desorption zone to form sulfur dioxide and crystallize sodium sulfite from solution and provide a slurry in said desorption zone containing at least about 25 weight percent solids, and a sulfite-containing absorption solution is regenerated for recycling by dissolution of crystallized sodium sulfite, and in which sodium sulfate is in the absorption-desorption medium, the improvement for removing sulfate from the system comprising dividing said spent absorption solution into portions, said spent absorption solution containing about 1 to about 10 weight percent of sodium sulfate, passing one of said portions to said desorption zone, evaporating water from another portion of said spent aqueous absorption-desorption medium having a major amount of sodium bisulfite, a minor amount of sodium sulfate, and a minor amount of sodium sulfite, based on the total weight of these components, to precipitate up to about 10 weight percent undissolved solids from said medium, said undissolved solids having a major amount of sulfate and a minor amount of sulfite, separating sulfate and sulfite-contain-ing precipitate and bisulfite-containing solution from the re-sulting slurry, and passing said latter solution to the absorp-tion-desorption cycle.
9. A process of claim 8 in which said slurry contains about 1 to 5 weight percent of undissolved solids precipitated from said medium which solids contain at least about 50 weight percent of sodium sulfate.
10. In a process for the removal of sulfur dioxide from gas in which sulfur dioxide is absorbed from the gas in an aqueous absorption solution of sodium sulfite to form the corresponding bisulfite in an absorption zone, resulting bisulfite-containing spent absorption solution is desorbed in a desorption zone to form sulfur dioxide and crystallize sodium sulfite from solution and provide a slurry in said desorption zone containing at least about 25 weight percent solids, and a sulfite-containing absorp-tion solution is regenerated for recycling by dissolution of cry-stallized sodium sulfite, and in which sulfate is in the spent solution from said absorption, the improvement for removing sul-fate from the system comprising dividing said spent absorption solution into portions, said spent absorption solution containing about 1 to about 10 weight percent of sodium sulfate, passing one of said portions to said desorption zone, evaporating water from another portion of said spent absorption solution at a tem-perature of about 40 to 120°C. to precipitate about 1 to 5 weight percent of undissolved solids from said spent absorption solution containing a major amount of sodium bisulfite and minor amounts of sodium sulfite and sodium sulfate, based on the total weight of these components, said solids containing a major amount of sodium sulfate and a minor amount of sodium sulfite, separating sulfate and sulfite-containing precipitate and bisulfite-contain-ing solution from the resulting slurry, and passing said latter solution to said desorption zone to desorb sulfur dioxide from said bisulfite-containing solution.
11. The process of claim 10 in which said slurry con-tains about 1 to 5 weight percent of undissolved solids precipi-tated from said spent absorption solution which solids contain at least about 70 weight percent of sodium sulfate.
12. The process of claim 8 in which the temperature of said evaporation is about 40 to 120°C.
13. The process of claim 12 in which the desorption is conducted in a slurry containing dissolved bisulfite and sodium sulfite solids, said slurry containing at least about 25 weight percent of undissolved solids.
14. The process of claim 10 in which the spent absorb-ing solution has an s/c of about 0.85 to 0.97.
15. The process of claim 14 in which the desorption is conducted with a slurry containing dissolved bisulfite and sodium sulfite solids, said slurry containing at least about 25 weight percent of undissolved solids.
16. The process of claim 15 in which the absorption-desorption medium which undergoes said evaporation is only a portion of the spent absorbing solution.
17. The process of claim 14, 15 or 16 in which the slurry resulting from said medium contains about 3 to 7 weight percent of undissolved solids which contain at least about 70 weight percent of sodium sulfate.
18. The process of claim 1 in which the temperature of said evaporation is about 40 to 120°C.
19. The process of claim 18 in which the slurry re-sulting from said medium contains about 1 to 8 weight percent of undissolved solids which contain at least about 50 weight percent of sodium sulfate.
20. The process of claim 18 in which the absorption-desorption medium which undergoes said evaporation is only a portion of the spent absorbing solution.
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US63050375A | 1975-11-10 | 1975-11-10 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1079492A true CA1079492A (en) | 1980-06-17 |
Family
ID=24527439
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA265,157A Expired CA1079492A (en) | 1975-11-10 | 1976-11-08 | Process for removing sulfur dioxide from a gas stream |
Country Status (15)
| Country | Link |
|---|---|
| JP (1) | JPS5261173A (en) |
| AT (1) | AT367653B (en) |
| BE (1) | BE848119A (en) |
| BR (1) | BR7607526A (en) |
| CA (1) | CA1079492A (en) |
| DE (1) | DE2651045C2 (en) |
| ES (1) | ES453174A1 (en) |
| FR (1) | FR2330439A1 (en) |
| GB (1) | GB1557294A (en) |
| IT (1) | IT1070093B (en) |
| NL (1) | NL7612494A (en) |
| NO (1) | NO147939C (en) |
| SE (1) | SE421407B (en) |
| SU (1) | SU950187A3 (en) |
| ZA (1) | ZA766140B (en) |
Families Citing this family (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US4177244A (en) * | 1976-07-24 | 1979-12-04 | Ullrich Neumann | Process for purge stream treatment in removal of sulfur dioxide |
| US4708856A (en) * | 1986-03-03 | 1987-11-24 | Norton Company | Liquid-gas absorption process |
| CN105833667A (en) * | 2016-05-27 | 2016-08-10 | 四川益能康生环保科技有限公司 | Renewable adsorbent for removing sulfur dioxide |
Family Cites Families (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US3653812A (en) * | 1969-07-18 | 1972-04-04 | Wellman Lord Inc | Process for removal of sulfur dioxide from gas streams |
| US3971844A (en) * | 1974-02-19 | 1976-07-27 | Davy Powergas Inc. | Method for removing sodium sulfate from aqueous solutions |
-
1976
- 1976-10-14 ZA ZA00766140A patent/ZA766140B/en unknown
- 1976-11-08 BE BE172178A patent/BE848119A/en not_active IP Right Cessation
- 1976-11-08 AT AT0829076A patent/AT367653B/en not_active IP Right Cessation
- 1976-11-08 CA CA265,157A patent/CA1079492A/en not_active Expired
- 1976-11-08 GB GB46358/76A patent/GB1557294A/en not_active Expired
- 1976-11-09 SE SE7612470A patent/SE421407B/en not_active IP Right Cessation
- 1976-11-09 IT IT69677/76A patent/IT1070093B/en active
- 1976-11-09 FR FR7633713A patent/FR2330439A1/en active Granted
- 1976-11-09 NO NO763810A patent/NO147939C/en unknown
- 1976-11-09 DE DE2651045A patent/DE2651045C2/en not_active Expired
- 1976-11-10 JP JP51135131A patent/JPS5261173A/en active Granted
- 1976-11-10 ES ES453174A patent/ES453174A1/en not_active Expired
- 1976-11-10 NL NL7612494A patent/NL7612494A/en not_active Application Discontinuation
- 1976-11-10 BR BR7607526A patent/BR7607526A/en unknown
- 1976-11-10 SU SU762417313A patent/SU950187A3/en active
Also Published As
| Publication number | Publication date |
|---|---|
| DE2651045C2 (en) | 1986-02-20 |
| FR2330439B1 (en) | 1982-10-08 |
| DE2651045A1 (en) | 1977-05-12 |
| BE848119A (en) | 1977-03-01 |
| NO147939C (en) | 1983-07-13 |
| NL7612494A (en) | 1977-05-12 |
| ATA829076A (en) | 1981-12-15 |
| NO763810L (en) | 1977-05-11 |
| SU950187A3 (en) | 1982-08-07 |
| BR7607526A (en) | 1977-09-20 |
| JPS5261173A (en) | 1977-05-20 |
| NO147939B (en) | 1983-04-05 |
| GB1557294A (en) | 1979-12-05 |
| SE421407B (en) | 1981-12-21 |
| ES453174A1 (en) | 1977-11-01 |
| JPS6339284B2 (en) | 1988-08-04 |
| IT1070093B (en) | 1985-03-25 |
| AT367653B (en) | 1982-07-26 |
| SE7612470L (en) | 1977-05-11 |
| ZA766140B (en) | 1978-05-30 |
| FR2330439A1 (en) | 1977-06-03 |
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