United States Patent (19 (11) 3,760,061 Hammond (45) Sept. 18, 1973

(54) HIGH-STRENGTH ACID contAINING Primary Examiner-Oscar R. Vertiz H2O2. TOSCRUB SO2 Assistant Examiner-Gregory A. Heller 75 Inventor: Myers G. Hammond, Wilmington, Attorney-Lynn N. Fisher Del. 73) Assignee: E. I. du Pont de Nemours and 57 ABSTRACT Company, Wilmington, Del. Sulfur dioxide can be oxidized to sulfuric acid by a pro cess that involves contacting the SO, with an aqueous 22 Filed: Mar. 2, 1971 scrubbing solution of sulfuric acid containing hydrogen (21 Appl. No.: 120,337 peroxide and/or the peroxy acids of sulfur in a gas liquid contacting device in which the liquid scrubbing phase is made the continuous phase during the actual (52) U.S. Cl...... 423/242 contacting. In this process the acid concentration of the 51) int. Cl...... C01b 17/00 scrubbing solution can be as high as 99 percent + by 58 Field of Search...... 23/25 O; 423/242 weight HSO. The process is useful in the abatement of sulfur dioxide (56) References Cited from waste gases created during the manufacture of FOREIGN PATENTS OR APPLICATIONS sulfuric acid and also for abating SO, from other stack 670,966 1/1939, Germany...... 23/178 gases, thus alleviating pollution problems. Further, 930,584 7/1963 Great Britain...... 23/78 since the hydrogen peroxide or peroxy acids oxidize the 1,234,912 2/1967 Germany...... 23/178 sulfur dioxide to HSO, the process results in a useful OTHER PUBLICATIONs product. Copson et al., Industrial & Engineering Chemistry, Vol. 25, No. 8, pp. 909-911 (August 1933). s Claims, No Drawings 3,760,061 2 HIGH-STRENGTH ACID CONTAINING HO TO taining hydrogen peroxide and/or the peroxy acids of SCRUB SO, sulfur as oxidizing compounds. Hydrogen peroxide can be added directly to the BACKGROUND OF THE INVENTION aqueous solution and then fed to the gas-liquid contact In recent years the elimination of various contami device, or the peroxy acid products of the electrochem nants from our environment has become very impor ical oxidation of sulfuric acid in an electrolytic cell can tant. One of the most troublesome contaminants is sul be added to the aqueous solution. These products are fur dioxide. Sulfur dioxide is produced in the manufac peroxydisulfuric acid, monoperoxysulfuric acid or ture of sulfuric acid and its abatement has become a Caro's acid, and hydrogen peroxide. problem for sulfuric acid manufacturers. It is also pro 10 It is believed that peroxydisulfuric acid, monoperox duced during the burning of many fuels. ysulfuric acid, or hydrogen peroxide are the reactive Many processes have been proposed for the removal species with the sulfur dioxide. However, other species of SO, from such waste gases. Some of these processes of the peroxy acids of sulfur or their decomposition involve the conversion of SO2 to sulfur. Others involve products could be involved. Thus the process of the in SO2 absorption, dry sorption, catalytic oxidation, or 15 vention is broadly applicable to the use of hydrogen catalytic oxidation with interpass absorption. All of peroxide, the peroxy acids of sulfur or their mixtures in these existing and proposed processes have technologi the aqueous scrubbing solution. cal drawbacks or present economic problems to indus Useful peroxy acids can be generated by the electrol try. ysis of a sulfuric acid solution. In general, this is con 20 ducted by applying a direct electrical current having a German Patents 1,234,912 and 670,966 and British current density ranging from 0.5 amp./cm. to 2 Patent 930,584 disclose processes for scrubbing sulfur amp./cm. to an aqueous solution of sulfuric acid to dioxide from waste gases by contacting the waste gases electrolyze a portion of the acid, i.e., preferably 10 to with an aqueous sulfuric acid solution, 35 percent to 80 40 percent of the acid present. percent H2SO4 by weight, containing the peroxy acids 25 The sulfuric acid in the cell should be maintained be of sulfur, such as H2SOs and H2SOs, and hydrogen per tween 5° C. and 30°C., preferably 5 to 15° C. The con oxide in a scrubbing tower. The processes of these pa centration of the acid, although not critical, should be tents teach spraying the sulfuric acid solution into the about 60 to 80 percent HSO. If the acid is below waste gas stream. Thus, the gas phase is the continuous 40-50 percent HSO, H, and O, would be the products phase and the sulfuric acid solution is the discrete of electrolysis. Usually only a portion of the acid pres phase during actual interphase mass transfer. ent is electrolyzed, i.e., up to 60 percent and preferably Although these prior art processes also produce sul 10 to 40 percent. In a typical embodiment for a sulfuric furic acid from the SO, in the waste gases, the acid pro acid plant, the preferred range would be 10 to 40 per duced is of such low concentration that it has limited cent. The above operating limits are important in order commercial use. 35 to maintain high cell efficiency and to avoid anodic de SUMMARY OF THE INVENTION polarization believed to be caused by the reactions: I have discovered a process for the scrubbing of sul HSOs -- HO - HSO -- HSO fur dioxide from SO2-containing waste gases by the en H,SO + 2 OH - HaSO + O. + HO + 2. ployment of a gas-liquid contacting device in which the 40 The electrolysis cells which can be used are known liquid scrubbing medium is made to be the continuous in the art. For a representative electrolysis cell, the phase during the actual contacting. The scrubbing is electrodes are constructed of platinum or tantalum effected with an aqueous sulfuric acid solution contain platinum, which can be clad over less costly materials ing hydrogen peroxide and/or the peroxy acids of sul of construction. Normal current density is about fur, e.g., peroxydisulfuric acid (HSOs) and monope 45 amp./cm., although current density may be as high as roxysulfuric acid (HSOs), also known as Caro's acid. 2 to 3 amp./cm. to increase the anode potential and, Although any concentration of acid can be used, the in turn, the current efficiency. Cell voltage will nor arrangement of the contacting device so that the liquid mally range from 5 to 6 volts. phase is continuous is especially advantageous with In operation the residence time for the acid mixture acid concentrations greater than 95 weight percent 50 in the cell should not exceed 15 minutes. Up to 60 per HSO, and results in an HSO product of sufficient cent, preferably 10 to 40 percent, of the acid mixture concentration that it has wide-spread commercial use. is electrolyzed as it passes through the cell; improved cell efficiency is obtained by carrying out only this par DETAILED DESCRIPTION OF THE INVENTION tial electrolysis of HSO per pass through the cell. Cell The process of the invention is applicable for treating 55 efficiency can be improved if the acid mixture contains various gas streams containing sulfur dioxide (SO2). a small quantity of dissolved SO. Normally, cell effi The SO, gaseous process stream could be the stack gas ciency is in the range of 50 to 95 percent, depending from an electrical power generation station, the stack upon operating conditions. gas from various industrial heating units, smelter off In another embodiment, preformed hydrogen perox gases, oil refining off gas, the off gas from a sulfuric 60 ide solution is added directly to the aqueous scrubbing acid plant, the process gas in a sulfuric acid plant, the medium containing 0 to 99 percent by weight sulfuric off gas from a coke plant, off gas from refuse burning acid. The rate of addition can be controlled by sensing or other burning generations. the concentration of SO, in the scrubber tail gases and The process of the invention involves bringing the 65 adding sufficient peroxide to oxidize the SO, to the de SO-containing process stream into contact in a gas sired extent. liquid contacting device with an aqueous solution of 0 An important aspect of the process of the invention to 99 percent by weight sulfuric acid, the solution con is that the liquid phase is made the continuous phase, 3,760,061 3 4. during the actual gas-liquid contacting, for example, by temperature of the gaseous effluent from a contact sul sparging the gas into a liquid. With this feature, very furic acid plant. high concentrations of sulfuric acid can be used and ef The hydrogen peroxide/peroxy acid content of the fective SO, absorption and conversion to HSO is still scrubbing acid should be at least 250 ppm. and can achieved. Heretofore, employment of gas-liquid con 5 range up to 5 percent or even higher, calculated as tacting devices in which the gas was made the continu weight percent HO. In the more highly concentrated ous phase during interphase mass transfer, such as solutions of sulfuric acid, hydrogen peroxide reacts spray towers and packed towers, permitted use of acid with the acid to some extent to form Caro's acid, concentrations ranging up to only 80 weight percent HSOs. HSO. The HSO, thus produced by the scrubbing op 10 The SO, concentration in the waste gas to be treated eration was of marginal utility. can range from 500 ppm. to 5000 ppm or even higher. As the acid concentration in the aqueous scrubbing As the SO, is absorbed into the liquid phase and re medium is increased, the solubility of SO, in the aque acted to form H2SO4, the equivalent quantity of acid ous scrubbing medium decreases, and, more impor will normally be tapped off from the liquid phase in the tantly, the reaction rate of dissolved SO2 and hydrogen 15 contacting apparatus, either intermittently or continu peroxide or peroxy acids of sulfur to form HaSO4, de ously. This sidestream can be blended with other acid creases. At sulfuric acid concentrations above about 80 or used as is. In the process embodiment wherein stag percent in the aqueous scrubbing medium, the low re ing is employed, e.g., in a bubble cap tower, a portion of the liquid effluent can be diverted and recycled to action rate of SO, with hydrogen peroxide or peroxy the top of the staging unit for reuse. acids of sulfur makes the use of conventional gas-liquid Addition of hydrogen peroxide or the peroxy acids to contacting equipment, wherein the gas phase is contin the scrubbing liquid will be necessary to balance the uous, impractical for significant removal of SO, from amount consumed in the oxidation of SO2, decomposi gaseous streams. The reason for this is that, in the more tion losses, and physical losses sustained via the side concentrated solutions, the aqueous scrubbing medium 25 stream draw-off and by volatilization. This make-up more quickly becomes saturated with SO2 because of can be done either intermittently or continuously. the slower reaction rate of dissolved SO2 with hydrogen The SO, concentration in the gaseous effluent from peroxide and peroxy acids of sulfur present in the aque the gas-liquid contact device can be as low as 500 ppm. ous scrubbing medium. If, however, the liquid phase is by volume or even lower. The SO, leakage can usually made continuous during the actual gas-liquid contact be reduced by lengthening the length of travel of the ing, significant removal of SO, from the incoming gas gas bubbles through the liquid. A preferred contact is achieved even with very short gas contact times of time is from 1 to 15 seconds and a most preferred time only a few seconds. It is desirable to provide liquid is 2 to 8 seconds. hold-up times of from about 10 seconds with low sulfu The invention is exemplified in the following exam ric acid concentrations to as much as a few hours at 35 high sulfuric acid concentrations in order to give the ples. Parts are by weight or volume unless otherwise in dissolved SO, time to react with the hydrogen peroxide dicated. or peroxy acids of sulfur present in the aqueous scrub EXAMPLE bing medium. The necessary liquid hold-up time can be A dip tube ending in a fritted cylinder is attached to achieved by proper design of the liquid phase continu 40 a resin kettle so that the top of the cylinder is one inch ous contacting device. below the surface of the liquid contained therein. The Examples of gas-liquid contact devices in which the liquid is agitated with a magnetic stirring bar at a mod process of the invention can be practiced, i.e., in which erate rate. When a gas mixture containing 4300 ppm. the gas is the dispersed phase, are a simple gas sparger SO2, 10.3 percent O, and balance N, all by volume, is used in conjunction with a turbine-type impeller in 45 bubbled through the fritted cylinder at a rate of about mersed in the liquid to increase the degree of gas dis 660 ml/min., into 700 gm. concentrated reagent grade persion, or a bubble-cap tray tower. Another device is sulfuric acid containing the equivalent of 840 ppm. the gas entrainment apparatus, in which a jet of liquid HO, by weight (temperature 79 C.) the effluent gas impinging on a pool of liquid entrains gas bubbles from contains approximately 850 ppm SO2 by volume. The the gas space into the pool. These devices can all be 50 gaseous SO, concentration is measured by injecting used singly or in cascades. samples into a gas chromatograph. The distance the gas bubbles travel in the liquid phase can be as little as one inch but preferably, will be EXAMPLE 2 three inches or greater. Obviously, fine gas bubbles are Example 1 was repeated except that the inlet gas flow preferred over large ones for improved gaseous diffu 55 rate was about 600 ml./min. and the peroxy compound SO level in the acid is the equivalent of about 265 ppm. The acid concentration in the liquid phase can range H2O, by weight. The effluent gas contained approxi from 0% to 99 percent, and even contain free SO3. mately 900 ppm. SO, by volume. However, for reasons of ease of marketability, the pre I claim: ferred acid concentration is 95 percent to 99 percent 60 1. A process for oxidizing and removing the SO, HSO by weight. contained in waste gases comprising contacting the SO The operating temperature of the absorption containing waste gas with an acqueous scrubbing solu oxidation reaction is not critical and can range from tion of up to 99 percent by weight sulfuric acid and an near the freeze-point of the acid to near the boiling oxidizing compound of the group consisting of hydro point of the acid. Preferred are temperatures ranging 65 gen peroxide, the peroxy acids of sulfur, or mixtures from 50 to 90° C., since the reaction is favored by thereof at a temperature above the freezing point but higher temperatures. Further this range is the normal below the boiling point of said sulfuric acid solution in 3,760,061 S 6 a gas-liquid contacting device wherein the liquid scrub waste gas is the off-gas stream from sulfuric acid manu bing solution is made the continuous phase during the facture. actual contacting, the time of contact being sufficient 4. The process of claim 2 wherein the SO, containing to allow the dissolved SO2 to react with said oxidizing waste gas is the off-gas stream from sulfuric acid manu agents. facture. 2. The process of claim 1 wherein the acid concentra 5. The process of claim 1 wherein the oxidizing com tion is 95 percent to 99 percent by weight sulfuric acid. pound is hydrogen peroxide. 3. The process of claim 1 wherein the SO, containing k k t sk k

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