CA2095116A1 - Use of chlorous acid for removing sulfur dioxide from combustion gases - Google Patents

Abstract

The present invention describes a novel method of using chlorous acid to remove sulfur dioxide from exhaust combustion gases by injecting into the vessel (15) containing the exhaust gases a mixture of (1) a salt of a hydroxy organic acid, such as, lactic, citric, malic, tartaric, glycolic, oxalic and mandelic acids; and (2) chlorous acid.

Description

WO 92/1~779~ 2 ~ PCI'/lJS~0/06382 . .

USE OF CF~OF~OUS ACID FY)F~ ,()VI~G SULF11R
OIO~IDE FROrl COr~USTION GASES
Description Technical Field This invention generally relates to the use of chlorous acid for the production of chlorine d~oxide in concentrations suitable for use as an oxidizing a~ent in various industrial processes, more particularly, for removing sulfur dioxide from combustion gases from coal and oil fired furnaces or processes. Other exemplary industrial processes where chlorine dioxide may be used as an oxidizing agent include use as a whitening agent in the paper industry, as a water treatment chemical in the oil recovery industry and as a treatment and disinfecting agent for treating water and/or wastewater.
Background Art Methods and/or apparatuses for removing sulfur dioxide from combustion gases have been described in the prior art. Rapson, et al, in U.S. Patent No. 2,481,241, described a method for removing chlorine from mixtures containing chlorine dioxide and chlorine. Shaheen, in U.S. Patent No. 3,962,112, described a composition and process for removing sulfur dioxide from combustion gases. Richter, in U.S. Patent No. 1,~85~754, described a method o~ recovering sulfur dioxide from waste gases.
Cheng, in U.S. Patent No. 4,31~,643, described a method of removing sulfur compounds from combustion products exhaust. Fredette, In. U.S. Patent No. 4,393,035, described a method of chlorine dioxide produc~ion involving a mixture of hydrochloric and sulfuric acid feed stocks. Fredette, in U.S. Patent No. 4,393,036 described a method of producing a mixture of sulfuric ; : ::.. : , . . . .

~V~92/07792 ~U~ PCT/US90/063~2 acid and hydrochloric acid feed involving chlorine dioxide generation therewith. Bergstrom, in U.S. Patent No. 4,508,593, a process for recovering sulfur dioxide was described which involved liberation thereof in the chemical pulping of lignocellulosic materials. Hanisch, et al, in ~.S. Patent No. 376,883~ described a process of o~taining sulfurous acid from furnace gases or similarly composed gaseous mixtures.
However, none o~ the prior art cited above discloses the unique method of preparing chlorine dioxide from chlorous acid for use for the removal of sulfur dioxide from combustion gases as does the present invention.
Disclosure of Invention One of the primary objectives and advantages of the present invention is that it allows for the formation of chlorous acid in an aqueous solution using bulk quantity reactants which allows for the production of a source of the chlorine dioxide off site rather than on-site at the particular industrial installation of interest. Off site production is important because it allows for a much safer~ generating process for chlorine dioxide whereby the risk of fire and explosion are minimized.
It is anticipated that the chlorous acid would be shipped to the plant site by, e.g., tank truck or railway tank car. Furthermore, the present invention allows for the mixing and formation of the ~hlorine dioxide in an aqueous solution involving bulk quantities and mixing ratios which are extremely simple and basic whereby more or less generally lesser-trained personnel could accomplish the production of the chlorine dioxide.
This method allows for the delivery of a source of chlorine dioxide to a plant site by merely transporting an aqueous solution by, for example, tank truck means or similar means which is not now done because chlorine dioxide can not be safely transported and therefore is now generally generated on-site.

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W092/077~2 ~ G PCT/~SgO/06382 , 3 According to the invention there is provided a method of forming chlorous acid in an aqueous solution from the reaction of lactic acid or citric acid with sodium chlorite to yield a salt of the acid and chlorous acid. This reaction is carried out at a pH <7 and at a temperature of < 80 F., in the range of approximately, F to 80 F. The aqueous solution of chlorous acid formed from the aforementioned reaction is stable and can be safely transported by common carrier, e.g., tank truc~ or railway tank car, to the plant site.
The aforementioned~reaction i5 the first of a chain of reactions leading to the production of chlorine dioxide which is eventually used for the removal of sulfur dioxide from the combustion gases. These further reactions, generally, involve the oxidation of various organic and/or inorganic compounds by either chlorous acid, chloric acid, chlorine dioxide or chlorine gas, or, a mixture of these chemical constituents, and will be further described by use of chemical equations in a later section of this specification.
Note that it may be feasible to substitute other al~ali metal or alkaline earth metal chlorites for sodium chlorite referred to in this specification.
Further note, it may be feasible to substitute other organic hydroxy acids and carboxylic acids for the lactic acid previously mentioned in this specification.
For example, acids which may be substituted may include ~; citric, malic, glycolic, oxalic, mandelic and tartaric acid.
Brief Description of Drawings Fig. 1 is a schematic representation of one exemplary process of using the present invention.
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W092/07792 2 ~ Pcr/usso/o63s2 : ` 7 7 H20 + S02 ---~> H2S3 8. 4 H2S3 + 2 C102 ----> 4 H2S04 + C12 9. C12 ~ H20 ----> HCl + HC10 The above reactions are carried out at a pH less than 7.
iIt is believed that the oxidation and~or otherwise removal of sulfur dioxide from the exhaust combustion gases is due to the presence of chlorous acid, chloric acid, chlorine dioxide or chlorine gas, or, a mixture of these chemical constituents. In practice, the reactants and reactions are produced by mixing bulk quantities of ;sodium chlorite and lactic acid. In practice, three parts of sodium chlorrite at a concentration of 26% by :~ .
volume are mixad with one part of lactic acid at a concentration of 88% by volume which is a food grade of lactic acid. The 26% by volume of sodium chlorite and 88% by volume of lactic acid are commonly commercially available bulk quantities of these compounds and are generally provided to industry commercially in either drum lots or bulk quantities for example tank cars or tank trucks. Furthermore, note in the above reaction Number 1. that citric acid, HOC~CH2COOH)2COOH, may be substituted for the lactic acid to produce a salt of citri~ acid and chlorous acid in an agueous solution.
The above reaction Number 1. is accomplished by mixing the reactants together at atmospheric pressure in an aquPous solution with the water temperature being approximately 620 F being in the range of 600 F to 80 F. The higher water temperatures nearing 800 F can be ; ' ~" ' ~. ''`: !

WO~2/07792 8 PCT/U~90/~6382 used if necessary to increase the reaction speed.
Higher water temperatures may be feasible.
The aqueous solution resulting from reaction number 1 above has a density of about 1.~039, a boiling point of about 101.6 C., a freezing point of about -3O C. and pH of approximately 4.7. The solution is completely miscible in water, has a pungent odor resembling chlorine and a color of clear to slightly amber.
As can be seen, the results of the present inYention can be accomplished by easi~y mixing on a part to part basis commonly available commercial products in commonly available commercial quantities so as to produce the desired reactions. Granulated sodium chlorite can also be substituted for aqueous sodium chlorite and thereafter used to make up the bulk guantity of this aqueous solution.
It is expected that the aqueous solution containing the chlorous acid, chlorine dioxide or mixture of these and/or other chemical constituents previously mentioned would normally ~e made in some type of stack, housing or vessel containing the exhaust combustion gases prior to the discharge of the gases to the atmosphere.
The above reactions produces aqueous solutions ultimately containing very high concentrations of chlorine dioxide ranging from generally about 5000 ppm to about 80,000 ppm. Furthermore, the chlorine dioxide produ~ed by th~ present invention appears to have more oxidizing power on a per unit basis than chlorine dioxide produced ~y otheE methods.
The invention will now be described by an example, it being understood, however, that this Pxample is given by way of illustration and not by way of limitation in that many changes may be effected without affecting in anyway the scope and spirit of this invention as recited in the appended claims.

Turning to Fig 1, one exemplary process schematic for using the present invention to remove sulfur dioxide . ~

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W092/077~2 ~ ~ 3 1 1 6 Pcr/~sso/o~3s2 from combustion gases is shown. At 11, the influent of the exhaust combustion gas into the stack is shown with the stack being 13. A mixing vessel, 15, shown being the point where the addition of the a~ueous solution containing the chlorous acid might be introduced into the exhaust combustion gases. The effluent exhaust gas discharge is shown at 17. Normally, it is expected that the aqueous solution containing the chlorous acid would be injected at the mixing vessel 15.

Claims (7)

Claims

1. In a process for removing sulfur dioxide from combustion gases which includes injecting an aqueous solution into a vessel containing exhaust combustion gases, the improvement comprising using as the aqueous solution injected into the vessel containing said gases a mixture of (1) a salt of lactic acid and (2) chlorous acid.

2. The process of claim 1, wherein a salt of citric acid is substituted for a salt of lactic acid.

3. The process of claim 1, wherein a salt of malic acid is substituted for a salt of lactic acid.

4. The process of claim 1, wherein a salt of tartaric acid is substituted for a salt of lactic acid.

5. The process of claim 1, wherein a salt of glycolic acid is substituted for a salt of lactic acid

6. The process of Claim 1, wherein a salt of oxalic acid is substituted for a salt of lactic acid.

7. The process of Claim 1, wherein a salt of mandelic acid is substituted for a salt of lactic acid.