CA1165099A

CA1165099A - Removal of nitrogen oxides from gas

Info

Publication number: CA1165099A
Application number: CA000409978A
Authority: CA
Inventors: Bernard J. Lerner
Original assignee: Individual
Current assignee: Individual
Priority date: 1981-11-04
Filing date: 1982-08-24
Publication date: 1984-04-10
Also published as: FR2546075A1; JPS59206031A; GB2115798B; GB2115798A; AU1407683A; US4400362A; AU555431B2

Abstract

ABSTRACT OF THE DISCLOSURE
Gas containing nitrogen oxides is treated in two separate stages. In the first stage, the nitrogen oxides in the gas are reacted with sulfurous acid. The gaseous product of this reaction is reacted with an alkaline solu-tion, typically caustic alkali. As a result of these treatments, the nitrogen oxides are converted into neutral salts.

Description

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1 B~CKGRO~ND OF T~IE' ~NVE~T.LO~
___ This invention rela-tes to ~he treatment of gases to remove pollutants, and has particular relationship to the removaL of nitrogen oxides from gases containiny these oxides in order to prevent the emission of these oxides to the atmosphere. Nitrogen oxides are kno~n atmospheric pollutants and are belie~ed responsible for the production of photochemical smog and "acid rain" phenomena. Although there are many chemically derivable oxides of nitrogen, the principal oxides implicated in atmospheric pollution are mixtures of the stable oxides, nitric oxide, NOj and nitro-gen dioxide, NO2,. and their equilibrium compounds and these nitrogen oxides will be referred to herein as ''NOX''.
Very high concentrations of NOX may be evolved in processes utilizing concentrated nitric acid in the chemical milling and passivation o~ stainless steels, tungsten, molybdenum, copper and the like, and the dissolution of precious metals. ~ower concentrations of NOX are present in tail-gases from nitric acid manufacture ~0.1 to 0.5% by volume) and in combustion flue gases from..industrial furnaces or internal combustion engines. .Various.types of treatment processes have been proposed which involve either catalytic conversion or catalytic reduction of the nitrogen.oxides, or wet scrubbin~ with various solutions o metal complexes or metallo-organic.compounds. Catalytic methods of NOX
destruction generally require the use of elevated yas tem-peratures and the addition of a reducing agent such as NH3, CO, H2 or hydrocarbons. Catalytic NOX reduction methods become very expensive if the NOX concentrations are higher than normal flue gas NOx concentrations, or if.the bulk gas must be heated from near ambient conditions to the elevated temperatures required for.catalytic reduction.
Therefore, catalytic NOX reduction processes are not normally applicable to processes generating high NOX
concentrations in the exhaust gases; i.e., concentrations higher than 0.5% by volume (.5,000 parts per million), or ~;5~
1 where the exhaust gases are at or near atmospheric or ambient temperatures.
Wet processes proposed for NOX absorption include various forms of aqueous alkaline solutions, transition metal complexes, and reduc-tive reagen-ts, such as catalyzed sulfite solu~ions, or oxidative reagents, such as perman-ganates. Processes employing transition metal complexes(l) and/or organic-compound-containing salts or reductive/
oxidative chemicals, generate liquid waste by-products which are themselves biotoxic water pollutants and cannot be sewered or easily disposed of. Wet process treatment with alkaline-earth oxides, h~droxides and carbonakes, particularly calcium carbonates, has also been proposed, but these treatments have very unfavorable overall absorption kinetics for NOX and are ineffective unless very large and uneconomic sizes of equipment are employed.
It is accordingly an object of this invention to overcome the disadvan-tages and drawbacks of the prior art and to remove nitrogen oxides from a gas effectively and at rapid, economic absorption rates, without the creation of by-product waste liquor pollution problems. It is an object of this invention to provide a method and apparatus for carrying out such removal.

SUMMAR~ OF THE INVENTION
This invention arises from the realization that wet scrubbing processes for NOX removal involving the use of water or a~ueous solutions of salts depend on the absorption from the gas of an oxide of nitrogen as the primary rate-controlling step. The dissolution behavior of the various ~orms of oxides of nitrogen has been reviewed by Sherwood and Pigford~2) and Wilke(3~. The principal conclusions of these authors are:
_________________________________ (1) See Hishinuma US 4,081,509 and Saitoh US ~,087,372.

(2) Sherwood & Pigford, "Absorption & Extraction", pp.377-385, McGraw-~lill, NY lg52.

(3) Sherwood, Pigford & Wilke, "Mass ~r~ns~er", pp. 3~6-361, McGraw-Hill,NY 1975.

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1 l. NO is jnsoluble and unreackive ~o~ard water and aqueous alkaline solutions.
2. NO2 has a very slow rate of solution in wa-ter, and the dissolution rate in caustic soda is slower than in water.
3. There are only two significant mass transfer processes which cause dissolution of gas-phase NO in aqueous solutions: the absorption of nitrogen tetroxide, N2O4, and the absorption of nitrogen trioxide, N2O3.

4. At 25C., N2O3 is only 30 percent as soluble in water as N2O4, but it reacts with w~ter 40 times as fast, so that at equal concentrations of N2O4 and N2O3, the primary dissolution path is by way of N2O3.

5. Nitrogen tetroxide, N2O~, the dimer of NO2, is formed in small equilibrium concentrations from NO2, and the nitrogen trioxide, N2O3, is also formed in small concentrations from nitrogen dioxide, NO2, and nitric oxide, NO:
NO2 + NO ~ N2O3 ~1) In highly concentrated NOX gases the efficiency of nitrogen absorption in high-pressure nitric acid absor-bers is large, primarily because the reaction involvingboth diffusion and reaction of N2O4 is favored. For dilute gàses or gases at atmospheric pressure however, the partial pressure of N2O4 becomes 50 small that the second reaction path involving N2O3 produces HNO3 more efficiently. The basic problem is that the equilibrium concentration of both N2O4 and N2O3 in gases at atmospheric pressure is so small that both efficiency and rates of NOX absorption in aqueous systems, are low.
In the practice of this invention, the disadvantages and drawbacks of the prior art are overcome by a wet scrubbing process including two separate stages. In the ~ti~

1 first staye, the gas is treated with ~n a~ueous solution of sulfurous acid. ~n the second sta~e, the gas leaving the first s-tage is treatecl wlth an alkaline or basic solution.
The two s-tages are indispensible because the first stage conversion-treatment alone provides for li~tle or no removal of the total NOx from the gas phase. Omis-sion of the first stage results in non-absorption of NOx in the basic-solution treatment stage and also causes the formation of HN03 and ~N02 mist in the vapor phase with copious dense white acid mist emissions at high NOx concen-trations. ~hile the chemistry of this treatment has not been positively established, it is believed that the firs-t--stage sulfurous-acid treatment converts the nitrogen oxides to a form that is rapidly and readily absorbed by the basic solution in the second stage. The two-stage system provides for removal of NOx and a final exhaust gas free of color, fume or acid gas content.
It is of interest to note that Collins, in U.S.
Patent No. 3,920,421~ calls for the removal of NOx from gas streams also containing sulfur dioxide by absorption in water of the sulfur dioxide to form sulfurous acid as part of a single-sta~e reduction process. However, Collins reduces the nitrogen oxides to nitrogen and finds it necessary to add a multivalent metal, such as iron, to the solution to achieve this purpose. Similarly, Kudo and Haguvara, in U.S. Patent ~o. 4,288,421, treat an exhaust gas containing both NOx and Sx with an absorbing solution containing an iron chelate salt and potassium sulfite in order to form imidodisulfonates. The present invention avoids the use of expensive or water-polluting catalysts and achieves NOx removal by a unique separation of ln SltU
chemical conversion and absorption steps.
Possible chemical reactions involved in the invention will now be discussed. It is emphasizea that these reactions are formulated based on hindsight and that there is no certainty that they are the reactions which actually occur. They are presented here with the thought that they provide a basis for understandin~ o~ the invention. It is 4 ~

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1 to be understood that this presentation i5 in no way to be regarded as critical to~ or limitin~ oE, this invention and that the existence of other formul~tions which may logically describe the reactions of this invention in no way re1ects negatively on the merit of this invention.
The process chemistry o this invention is believed to be essentially and ideally comprised of two separate steps:
~ a~ converting N0x in the gas under treatment to N203' and (b~ reacting the N203 with a~ueous alkaline solution.
The conversion of N0x to N203 in the fixst-stage contactor is effected by scrubbing wi~h sulfurous acid solution.
In the now-obsolete lead chamber process for the production of H2S04, nitrogen oxides are used as the oxi-dant for S02, which is oxidized to S03. The S03 then reacts with water and/or is absorbed in H2S04 in the Gay-Lussac tower where the nitrogen oxides are regenerated for recycle. -If the process is now reversed, so that S02 or H2S03 solutions are used to control the formation of soluble forms of gaseous nitrogen oxides, and/or liquid-phase com-plexes, the "old" chemistry may be utilized to explain the probable reaction sequence o~ the present invention.
The exact nature of the reactions between S02 and NOX in the lead chamber process was the subject of continuous investigations for some 50 years, ~1895-1935) but the mechanisms were never fully clarified. The most self-consistent mechanistic explanations have been provided by Berl~4~ in 1935 r and earlier by Lunge and Berl~5~ in 1906. Both reaction sequences involve intermediate compounds or complexes of N0, N02, S02 and H20. The com-pound common to both theoretical chain mechanisms is nitrosyisulfuric acid, S05NH, or structurally, as shown by ____________________________ (4) Berl, E., Trans-Am. Inst. Chem. En~rs. 31, 193 ~1935) (5~ Lunge, C., Z. An~ew, Chem. 19, 807, 857, 881 ~1906 Lunge, G., ~. Chem. Soc. 47, 65 ~885) 1 Elliott(-), (IIO-S020NO). Nitrosylsulfuric acid is ~table in concentrated H2SO~, but undercJoes rapid hyd~olysis in H20 as follows:

5 H ~ ~2 ~ 2H2S04 ~ NO ~ N02 (2) This reaction is the generating reaction for the desired equimolar mixture of N02 and NO, or N203. The mechanism of formation of the nitrosylsulfuric acid from S02, NOx and H20 was the subject of experimental and theore-tical investigations of Lunge and Berl, who established that a second, transient complex, sulfonitronic acid, (H2S04~-NO, also called "violet acid" because of its in-tense color, was also involved. Althouyh this compound has been synthesized in pure form, it occurs as a transient intermediate in the chain reaction leading ~in the case of the chamber process) to the formation of H2S04. Oxidation of sulfonitronic acid yields nitrosylsulfuric acid. The chain sequence postulated by Berl and Lunge can be stated as:
2S02 ~ 2~I20 = 2~2S3 2H SO + 2NO = 2NO H2S04 (4~
2N0 H2S4 + 0 5 2 = 2S05NH ~ H20 (5) 2SO NH + H O = 2H2S04 + N203 (6) ____________________________________________ 2S0 ~ 2H ~ 2N02 + 0 5 2 = 2H2S04 2 3 While the individual reactions of the chain se-~uence are of both theoretical and practice interest (for the lead chamber operation) the next overall reaction, Reac-tion ~7~ is the statement of the first-stage chemistry of the present invention. Laboratory tests described below are consistent with the reaction se~uence as written.
___________________________ ~6) Elliott, G.A., Kleist, L.L., Wilkins, F.J., and Webb, H.W , ~. Chem. Soc., 1219 ~1~26~

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1 Laboratory investigation oE th~ SO2-~lOy~~2O
system, confirmed qualitatively ne-t Reaction (7~ but not its stoichiome-try. Of basic i~portance was the determina-tion in the laboratory that less-than-stoichiometric addition f S2 with respect to NOX could be used. Most laboratory runs made were made with a ratio of SO2/NOX of 0.5 or less.
Additionally, although SO2 is converted into H2SO4 in the first-stage, nitrosyl~ulfuric acid is stable only in concentrated H2SO4, and the ver~ dilute aqueous solution of H2SO3 would make the presence of this compound somewhat suspect. Nevertheless, comparative laboratory runs with air and nitrogen verified ~he necessity for oxygen in the reaction sequence. In view of the uncertainty as to the actual formation of N2O3, the reaction of the first stage is frequently referred to in this application and in the clai~s as the conversion of the nitrogen oxides in the gas into a form in which they readily react with the alkaline solution. Based on the experience in arriving at this invention, this certainly occurs.
It should be noted that the transient complex, sulfonitronic acid, NO-~I2SO4, can be considered as an un-stable compound of NO and H2SO4, indicating that NO can participate in the chain sequence producing N2O3. Visual e~idence o the forma-tion of the sulfonitronic acid complex was obtained in a number of runs in which the amount of sulfurous acid solution was limited, and the H2SO4 was allowed to accumulate until a solution of p~I of 0.5 was reached~ At these high concentrations of H2SO4, the solution frequently turned violet on continued addition of nitrogen oxides and SO2. Additionally, globs of violet liquid accumulated in the exhaust line from the conversion scrubber well prior to the change in solution color from colorless to violet. However, the change in solution color occurred well after the exhaustion point where the solution had lost its conversion ability. H2SO4 is both a byproduct and reactant in the reduction sequence, so that its formation l in solution is not detrimental to the con~ersi~n reaction until it reaches a hiyh enou~h concen-tration to inhibit the absorption of SO2 gas (Reaction (3)). The ~I2SO4 con-centration is controlled at -the desired level by recycling the -Eirst-stage absorp-tion liquor and bleeding off a part of the solution, replacing the solution blowndown with fresh water. The solution absorption capacity for S2 may be monitored by means o~ solution oxidation reduction potential (ORP~ and the solution H2SO4 concentration may be monitored by means of a pH indicator.
It is economically desirable to recover the SO2 content of the recycle solution blowdown from the first stage contactor. ~ccordingly, it is preferred that the first stage blowdown liquor be stripped of its SO2 content by air or steam in a conventional desorption de~ice and that the stripper off-gas be fed back to the SO2 absorber for reabsorption of the SO2.
Some gas streams, particularly those generated in combustion processes wherein a sulfur-containing fuel oil, or coal, is used, inherently contains significant concentrations o SO2. In such cases, it may not be necessary to add incremental SO2 to either the gas fed to :
the first-stage scrubber, or to the first-stage recycle liquor. The combustion gas naturally containing both the SO2 and NOX can be scrubbed with (initially) water, which upon continued recycle, becomes the sulfurous acid solution required for the operation o~ this invention. Although combustion gases are hot, the gas will rapidly quench to approximately the wet-bulb temperature upon contact with the first-stage aqueous liquor, or, more preferably, may be precooled prior to entry to the sulfurous acid-con~rersion scrubber. While the SO2 solubility in the solution will decrease at increasing absorber temperatures, the decreased solubility is compensated for by the increase in the reaction rates at the higher temperatures.
There is a signiicant heat of reaction generated in the first-stage sulurous acid conversion scrubber, and 1 the adiabatic tempex~ture rise of the li~quid can b~
considerable at hi~h NQx conversion rates. A theoretical estimate of solution adiabatic temperature rise may be obtained from net Reaction (7). Assuming a feed rate of one lb. mole of SO2 per hour, the total heat liberated is comprised of the heat of solution of gaseous SO2, plus the heat of reaction as represented by ~eaction ~7~. The heat of solution is ~ 15j410 Btu/lb mole of SO2, and the heat of reaction is - 136,764 Btu/lb mole of SO2, gi~ing a total exothermic quantity of -152,174 Btu/lb mole of SO2. At a 1:2 ratio of SO2:NOX, this heat production corresponds to the conversion of 2 mole-s of NOX, or 92 lbs. of NOX as NO2. Assuming this quantity of NO is fed to the first-stage conversion scrubber per hour, the temperature rise for once-through water fed to the scrubber at a rate of 10 GP~
could he 30.3 F under adiabatic conditions. Under recycle conditions, it would therefore be theoretically possible for the aqueous recycle solution to go to its boiling point.
However, even u~der adiabatic conditions, the ma~or heat sink is the evaporation of water, and evaporative cooling will generally prevent the recycle liquid from excessive temperature rise. The actual equilibrium temperature of the recycle absorption liquor depends on the NOX load, the SO2:NOX ratio, the liquid/gas ratio, makeup rate and other factors. When these factors cannot be suitably controllea by design to ensure against excessive liquor temperatures, then a heat exchanger may be placed in the recycle liquor loop of the first-stage scrubber to adequately cool the liquor.
In some-processes, the gaseous NOX stream may be admixed with acid mists or with other acid gases, such as HCl. The latter acid gas would be normally presen-t as a contaminant if aqua regia is used in the process which generates the NO~. HCl is pxeferentially absorbed by water as compared to SO2, and HCl and similarly highly-soluble acid gases will either displace SO2 from a H2SO3 solution, or impede the dissolution of SO2. Accordingly, it is _ g _ -q.-3 1 desirable to prevent the penetration of such acid gases to the first-stage sulfurous acid scrubber, and an upstream pre-absorber is desirable when such acid gases are present.
The pre-absorber may be any conventional contactor such as a spray chamber or packed scrubber, having sufficient ab-sorption capacity for the removal of EICl and similar acid gases. If the concentration of N0x is very high, it is preferred that the pre-absorber aqueous scrubbing solution be neutral or acidic. The contact of alk~line solutions with high gaseous N0x concentrations results in the forma-tion of copious amoun-ts of nitric-acid Eume and mist in the vapor phase. In the practice of this invention, the N0x is converted in th~ first stage scrubber to a form which does not form the nitric-acid fume and mist with alkaline solutions.
Similarly, in applications where there are acid mists contained in the gas to be treated, the presence of such mists in the sulfurous acid scrubber is undesirable because of the possibility of inhibition of S02 dissolution.
Accordingly, it is preferred that acid mists, including nitric acid mists, he removed upstream of the sulfurous acid scrubber by means of a suitable efficient mist eliminator.
The second-stage contactor utilizes an aqueous solution of a basic alkali metal or alkaline-earth or ammonium salt or hydroxide, selected from the group of alkaline salts, or hydro~ides and their mixtures, for the absorption of the reactive nitrogen ~xides generated in the first-stage. Another func-tion of the second-stage alkaline scrubber is to absorb any excess S02 coming over from the first-stage gas-liquid contactor, preventing emission of S02 from the combined system. As long as free alkaline is present in the second-stage liquor, there is no danger of significant S02 emission from the second-stage.
Nevertheless, any excess S02 coming over from the first-stage represents a potential reagent loss and it is pre-ferred to minimize first-stage S02 evolution by avoiding 1 saturation or near-s~turakion ope~atin~ conditlorls for the first-stage scrubbing liquor. Ilowe-ver~ ,it wa~ noted during the course of de~elopment of this invention that alkaline sulfites, which would be formed by carryover of S2 into a second-staye alkaline liquor scrubber, are also fairly effective in the absorption o nitrogen oxides, so that some deg~ee of S02 carryover enhances second-stage NOx removal, rather than inhibi-~ing it, and S02 carryover is not wholl~ detrimental.
~ven without recovery of S02 rom first-stage scrubber ~blowdown, the consumption of reagent for the pro-cess is economically acceptable. For a typical application involving concentrations of N02 of 33 mg/m3 and NO o 0.7 mg/m3 at a total 10w of 1.113 kg/h o NOx, 24 hrs/day, a flow of 0.783 kg/h of S02 would be required i the molar input rate of S02 was half that of the NOx. This amounts to a total consumption of 18.79 Kg/day of S02, or at a unit cost of $0.231/kg, a total cos-t of $4.34/day or the S2 cost. To this cost, the cost o the NaOH, or other alkaline reagent consumed in the second-stage, must be added. However, the NaOH xeaction costs will be incurred in any process scheme or removal o ~x as nitrite or nitrate salt, so that the incremental cost o the two-stage operation is represented by the S02 cost of $4.34/day.
The reactive forms of NOx absorbed in the second-stage scrubber rapidly react with the alkaline solution to form soluble nitrites and nitrates. As in the first-stage scrubber, recycle liquor operation is desirable in the second-stage scrubber to conserve reagent chemicals.
Assuming the use of NaOh as the alkaline reagent in the second-stage scrubber, the theoretical generation of N203 in the H2S03 scxubber should yield the following second-stage reaction:
N O ~ 2 NaOH -> 2NaN02 ~ H20 (81 Assuming the dominance of Reaction ~8) in the second~stage scrubber, the liquor and the blowdown liquor

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1 from the scrubber will contain n:ikrites and sulfites, which would have a ver~ high chemical oxygen demand ~C~D) if allowed to gc~ directly to sewer. While some oxidation will be effected by the contact with the air or gas phase oxygen in the scrubber itself, it is desirable to reduce the COD
of the blowdown from second-stage liquor by oxidation in a separate aeration unit prior to disposal.
The oxidized blowdown liquor from the second-stage scrubber contains residual caustic salts plus neutral salts such as sulfates and nitrates. The stripped liquor blowdown from the first-stage scrubber is acidic. One-stream may be used to Eull~ or partly neutralize the other so that subsequent sewage treatment loads are minimized.
The heat generated in the second-stage scrubber is also significantly exothermic, with an estimated heat o reaction of - 18,180 Btu/lb mole of N203 from Reaction (8).
Additionally, if solid NaOH is used for replenishing the alkaline strength of the second-stage solution, the heat of solution of - 18,360 Btu/lb mole of NaOH is additive to the heat of the chemical reactions. Heat exchange in the scrubber recycle liquor loop is desired to control the temperature o the absorber if the evaporative cooling effects are insufficient.
In certain applications of the process of this invention to gas streams containing very high concentrations of NOx, it may be economically desirable to recover HN03 in an upstream pre-scrubber. This may be done by counter-current pre-scrubbing of the gas with wa-ter or dilute HN03 to effec-t the partial absorption of N02 or N20~, and the conversion of the dissolved nitrogen oxides to HN03 by hydrolysis in the liquid phase. Although such a pre-scrub operation is effective for only partial absorption of NOx, it does provide for some recovery of reusable HN03, and reduces the downstream process load and reagent consumption.

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1 Br~e~ Description o~ the ~rawlngs For a better understanding o~ this invention, ~oth as to its organization and as to its method o~ operation, together with additional objects and advantages thereof, reference is made to the following description, taken in connection with the accompanying drawings, in which:
Fig. 1 is a block diagram showin~ the basic features of the invention:
Figs. 2A and 2B together is a diagrammatic view showing apparatus in accordance with this invention and for practicing the method of this invention on a commercial scale; and Fig. 3 is a diagrammatic view of apparatus with which the effectiven~ss of this invention in removing NO2 from a gas under treatment is demonstrated.
Detailed Description of Invention The apparatus shown in Fig. 1 includes a first scrubber 11 and a second scrubber 13. In the first scrubber 11 the gas to be treated is reacted with H2SO3 and in the second scrubber 13, the gas product of the reaction in the first scrubber is reacted with an alkaline solution, typi-cally NaOH, an aqueous solution, of 2 to 25% by weight, preferably 4-10~ by weight. The gas to be treated is injected into the first scrubber at its gas input 15~ The first scrubber 11 includes a source 17 of ~I2SO3 which is supplied to a second input 19. The H2SO3 reacts with the NOX in scrubber 11. The gaseous product of this reaction is predominately N2O3. This product is supplied to scrubber 13 through input 21. At another input 23 to scrubber 13, an alkaline solution from a source 25 is supplied. This solution is typically caustic soda, NaOH, or potassium hydroxide. The treated gas free of NOX is derived from output 27 of scrubber 13. The liquid product of the reaction of -the N2O3 and the alkaline solution is derived from output 29 of scrubber 13 and is treated as waste.

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1 Fig. 2 shows apparatus 31 for treat:i.ny ~as con-taining NOX. This apparatus includes an upstream water pre-scrubber 33, an ~12SO3 scrubber 35 and an alkalirle-solution scrubber 37. A blowdown s~ripper 39 is cooperatively connected to the H2SO3 scrubber 35 and a blowdown oxidizer 41 is cooperatively connec-ted to the alkaline solution scrubber 37. There is also a source of SO2 which may be one or more bottles or cylinders 43. Ancillary components including pumps, valves and indicators are associated with each of the scrubbers.
The upstream water scrubber 33 includes a scrub-bing tower 45 and a scrubber rec~cle tank 49. The tower 45 has packing which may be of suitable type such as Pall rings or the pac~ing disclosed in U.S. Patent ~,238,386 to Bernard J. Lerner. The tower 45 is vertical and has a gas inlet 51 below the packing and an outlet 53 above the packing. The gas to be treated containing the NO flows through inlet 51 and water or recycle acid is supplied to a distributor 55 through the header 53. The liquor flows through the packing in countercurrent-flow relationship to the gas absorbing a fraction of the NOx and any HCl from the gas to be treated. The resulting liquid flows into tank 49. In its top, the tower 45 has an outlet 57 for the treated gas. This outlet i5 connected to duct 58.
Water or recycle acid for the distributor 55 is derived from the tank 49. The tank 49 has an outlet 59 in its base which is connected to inlet 53 through a line 61 which includes - valve 63, a pump 65, a temperature indicator 67 and a valve 69. The pump 65 drives the liquor from tank 49 to the distributor 55.
The water or dilute HNO3 flowing from the distri-butor reacts with the gas in the packing 47 to produce nitric acid. The nitric acid may be removed batchwise or continuously through a branch line 73 including valve 74 when it reaches a predetermined concentration 74. Pressure of the liquid discharged from -the pump is measured in branch line 75 which includes indicator 77 and ~alve 79. The liquor .~ .

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1 in tank 49 may be replenishe~ through a makeup water line 79 which includes control valve 76 and check valve 80.
The valve 76 is controlled dependent upon the level of the liquid in -tank 49. The valves 63, 69, 71 and 74 may be manually operable by the attendant of the apparatus or all or some of these valves may be responsive auto-matically to conditions in the system such as the concen-tration of the HNO3 in the recycle liquor.
The H2SO3 scrubber 35, like the scrubber 33, in-cludes a vertical scrubbing tower 81 and a recycle tank 83.
The tower 81 has packing 85 similar to the packing in tower 45, an inle-t 87 for the gas from tower 45, an inlet 89 for the recycle liquid H2SO3 solution and an outlet 91 for the treated gas. The outlet 91 is connected to duct 93.
Through inlet 89, the recycle H2SO3 solution is supplied to the distributor 95. The H2SO3 solution i5 distributed over the packing 85 and flows in countercurrent reIation-ship to the gas entering the tower from duct 58 through the gas inlet 87, and drains into recycle tank 83. The ~12SO3 solution reacts with the NOX content of the input gas exo thermically.
The duct 58 is connected to the tower gas inlet - 87 through a junction 60 which may be a T-joint and includes a branch 97 for supplying supplementary air to tower 81 if no oxygen is initially present in the gas in duct 58. In addition, SO2 is supplied to inlet 87 from tank 43 through a valve 99 and a rotameter 101 which measures the flow rate ~of the SO2 into inlet 87.
The recycle tank 83 has an outlet 105 which is connected to the distributor 95 through a line 107 which includes valve 109, pump 111, valve 112, temperature indi-cator 113, heat.exchanger 115, temperature indicator 116, and tower inlet header 89. The pump 111 circulates the liquid from tank 83 to the distributor 95.
A branch line 117 including valve 119 is connected to line 107 for removing liquid from tank 83 under pre-determined conditions. Pump discharge pressure in line 107 i,.~

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1 is measured by a pressure indicato;~ 123. The recycle liquid passin~ thro~lgh hea-t e~changer 115 is cooled by coolant which flows throu~h the primary tubing ~not shown~ of khe exchanger through inle-t line 127, includiny valves 129 and 131 and temperature lnaica-tor 133 and outlet line 135 in-cluding temperature indi~ator 137 and valves 139 and 141.
Valves 129 and 131 may be controlled in accordance with the measurements o~ indicator 133 and ~alves 139 and 141 may be controlled in accordance with the indications of 137.
At the start of an NOX removal operation tank 83 contains water. Initially, the valve 99 responding to the oxygen-reduction-potential controller 103 is fully open so that substantial quantlties of SO2 are fed into tower 81.
The SO2 reacts with the recycle water from tank 83 producing H2SO3 in increasing concentration. As the concentration of H2SO3 increases, the controller 103 causes valve 99 to be throttled reducing the flow of SO2 appropriately. During operation, the liquid in tank 83 will contain HNO2, HNO3, H2SO4 and H2SO3. When the concentrations of these acids reaches limited or predetermined magnitudes, the liquid in tank 83 is drawn off through valve 119 or through branch - line 143 to the blowdown stripper 39. Line 143 includes valve 145. ~he water in tank 83 is replenished through line 147 including control valve 149 and check valve 153.
Valve 149 may be controlled in accordance with the level in tank 83.
Instead of being fed directly into the scrubber column 81, the SO2 may be supplied to tank 83 or to the circulating liquid prior to the operation of scrubber 35.
If this procedure is adopted, half the quantity of SO2 necessary for saturation of the liquid in tank 83 may be supplied to the tank before operation. During operation, the SO2 may be admitted to the scrubber 81 at a reduced rate.
The alkaline-solution scrubber 37, like scrubbers 33 and 35, includes a vertical scrubbing tower 155 and a recycle tank 157 into which the liquid from the tower drains.

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1 The tower 155 includes packiny 159 s~m~lar to the toWers ~S
and 81, an inlet 1~1 ~or the ~as ~n duct 93~ an inlet 163 for alkaline solution and a top outlet 165 for the treated gas. The outlet 165 is connected to the blower 167 which exhausts the treated gas to the atmosphere and maintains a small negative pressure (suction) in the ducts 58 and 93.
The gas ~rom duct 93 ~s supplied to the bottom of packing 159 and the treatlng alkaline liquid is supplled to a distributor 169 on top o~ the packing 159.
The tank 157 contains alkaline solution typically NaOH or KOH. The concentration of the alkali may be between 1 and 20%, but is typically about ~ or 5~. The alkaline solution absorbs ana reacts with the reactive forms of NOX derived from the first-stage 35.
Tank 157 has an outlet 171. Liquor from this outlet is recycled ~o the distributor 169 through line 173 includin~ valve 175, pump 177, temperature indicator 179, valve 181, heat exchanger 183, temperature indicator 185 and inlet 163. The alkaline solution rom distributor 169 flows through the packing 159 in countercurrent flow to the gas from duct 93. The alkaline solution absorbs and reacts with the reactive ~orms of NOX derived ~rom the first-stage 35. The product of this reaction may include nitrites, sulfites, nitrates and sulfates.
A branch line 187 including valve 189 is connec~ed to line 173 ~or draining off the liquid in tank 157 as desired.- This liquid is also discharged to blowdown oxidizer 41 through branch line 191 which includes valve 193 on the pump discharge branch line. Pressure in line 173 is measured by indicator 195 in branch line 197 which includes valve 199 ~
The water in tank 157 is replenished through line 201 which includes control valve 203 and check valve 207. Alkaline solution is supplied to tank 157 through line 209 which includes valve 211. Typically, the makeup alkaline solution supplied has a concentration of about 50%. ~alve 203 is controlled in accordance with the leve~
in tank 157.

1 The blowdown stripper 39 includes kank 213. ~ir is forced through this tank 213 by compressor 21S. The air strips the So2 f~om the liquid which flows into khe tank through line 143 and feeds it back into input 87 of H2S~3 tower 81 through duct 217. The air supplied by compressor 215 also flows lnto tower 81. The liquid from the tank 213 also flows to a waste treatment facility ~not shown) through line 219. This liquid includes H2SO4 and HNO3 and flows into a mixing tank 220.
The primary tubing of heat exchanger 183 is supplied with coolant through inflow duct 200 and outflow duct 202. Inflow duct 200 includes valve 204 and tempera-ture indicator 208 and outflow duct 202 includes valve 210 and temperature indicator 214.
The blowdown oxidizer includes tank 2.21. Tank 221 is supplied with air by compressor 223. The air oxidizes the sulfites and nitrites which flow into tank 191 from tank 157. Where the alkaline solution is ~aO~I, the n.itrites and sulfites are predominately NaNO2.and Na2SO3. These are reducing agents which have a high chemical oxygen demand (COD~ injurious to marine life. They are converted in the tank 221 to harmless or beneficial NaNO3 and Na2SO4. The output of tank 221 ~lows into mixing tank 220. The liquid from tank 157 lncludes alkaline solution, typically NaOH.
This solution reacts with the H2SO4 and HNO3 in mixing tank 220. The.output of tank 221 flows into mixing tank 220 which also receives the output of tank 213. The outputs of tank 213 and 220 react to neutralize each other partially or wholly. The output of mixing tank 220 flows into the waste treatment facility.
Tests to evaluate the efficacy of the invention were conducted with the apparatus.shown in Fig. 3. This apparatus includes a plurality of bubbler flasks 241, 243 and 245. Each flask is provided with a stopper 247. A
long inlet tube 249 r 251 and 2S3 extends into each flask through its stopper 24~. Each inlet tube terminates near .

.

1 the bottom of its 1ask. ~ short outlet -tube 255, ~57 an~
259 extends into each flask -th~ou~h its stopper. Each out let tube terminates near the top of its flask. The apparatus shown includes a compressed air line 261, an NO2 cylinder 263 and an S02 c~linder 265. The compressed air line 261, the NO2 cylinder 263 and the NO2 cylinder 265 are connected in parallel to inlet tube 249. The compressed air line is connected to inlet tube 249 through a valve 267 and a rotameter 269; the NO2 cylinder 263 is connected to the tube through a valve 271, an infra-red heater 273 and a rotameter 275; the SO2 cylinder 265 is likewise connected through valve 277, heater 279 and rotameter 281. The hea~ers 273 and 279 serve to prevent the formation of SO2 or NO2 li~uid when the valves 271 and 277 are opened. The short outlet tube 255 of flask 241 is connected to long tube 251 of flask 243 and the short outlet tube 257 of flask 243 is connected to long inlet tube 253 o~ flask 245. The treated gas is emitted from short tube 259 in flask 245 and is observed against the background of a white paper 283. The gas may also be observed in flask 245.
Prior to a test, water is poured into flask 241.
The level of the water is near the top of the flask but below the end of outlet tube 255. Also, the alkaline solution to be investigated is poured into flask 243 to a level near the top of the flask, but below the end of outlet tube 257.
Flask 245 is empty. The flow of compressed air is then started. If ~lask 241 is to contain H2SO3, the appropriate quantity of SO2 is bled into flask 241 through inlet tube 249 under the action of the compressed air.
During the test, NO2 is fed into flask 241 through inlet tube 249 under the action of the compressed air.
The emission into and through flask 245 is then observed.
When the NO2 breaks through, a reddish-brown color is seen against the white paper 283. The time which elapses between the start of the injection o the N02 and the appearance of the reddish~brown color is measured. ~ short interval indicates that the H2SO3 and/or the solution in ' ~

1 flask 243 are ine~fectl~e ~n absorbiny the NO2; a lorly interval indicates that the ~12So 3 and the alkallne solu-tion are effective in :removiny NO2.
The results of a series of tests are shown in the following Table ~

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~r ~ h l td 01 X
(~,o r1 ,-1 0 01 h Ln ~nl ~ o a O ~ Z Z O ~ Z Z Z Z Z Z Z z ~ o ol o~ o~ O p~ o~ o~ O p~ o`~O o~ O 11, o~ o~'O ~:4 o~ o~O oP o~ o~ o\ o\ o~ O ~q ~ Ln U~l "1~ Ln P~ Ln ~ ,~ p, Ln ~ ~ p, Ln Ln ~ p, Ln Ln Ln Ln Ln Ln Ln ~ Ln E~
~1 .
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. ., 1 The initial portion of the proyram, represented by runs 93 through 120 in Table ~II, was an investiyation of the possibility o;E inhibitincJ the oxidation of single-stage sodium sulfite or caustic/sulfite solutions when used to absorb NOx from air. For the usual case where NOx is admixed with air or oxygen, the sulfite solution is ~apidly oxidized to sulfate, rendering the solution in-e~fective. The uninhibited-solution runs 97-99 show an .average of 16.3 minutes for NOx'color breakthrough time.
The various oxidation inhibitors tried included paraphenylene diamine (PPD~, tributylhydroquinone ~TBHQ~, propyl gallol, triethanolamine, benzyl alcohol, and sugar, none of which significantly improved the breakkhrough ~ime, and most of which depressed the breakthrough time. Experiments omitting thè sul~itè and caustic, and using a single-stage sulfurous acid scrubbing solution, Runs 121-122 and 124-125, showed some interesting solution color change behavior, but no improvement on NOx breakthrough time. The "discovery" run,.
Run 126, used the two stages of scrubbing in series H2S03 followed by 5% NaOH, and gave a breakthrough time of 48.5 minutes. The blue liquid observed in the line between the two bubbler flasks is suspected of being N203, which is a blue liquor, but no positive identification of this material could be obtained because.of its unstable nature.
'25 In Table II~, the numbers in the left-hand column identify the run or test, the N02 breakthrough time in min-utes is in the second column from the left, the content of alkaline solution.in flask 241 is in the third column, the content of ~2S03 in flask 243 is in the fourth column, the flow of compressed air in cubic centimeters per minute is in the fifth column, the flow of N02 is cc per minute in the sixth column, and the flow of additional S02 in cc per minute is in .the seventh column.' The eighth column describes other parameters.of the tests and unusual observations.
The results of tests 126 and 128 through 132 demonstrate the efficacy of the invention in removing N02.

` ~ ~
_ . ~.

1 While preferred embodiments of this invention have been disclosed herein, many modiications thereof are feasible. This invention is not to be restricted except insofar as is necess1tated by the spirit o the prior art.

., .

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. The method of removing nitrogen oxides from a gas comprising reacting the nitrogen oxides with sulfurous acid, thereafter reacting the gaseous product of the reaction of the sulfurous acid and the nitrogen oxides with an alkaline solution to convert the said product into a salt.

2. The method of claim 1 wherein the sulfurous acid is continuously formed by flowing sulfur dioxide into water.

3. The method of claim 2 wherein, prior to the reaction of the nitrogen oxides in the gas with sulfurous acid, the gas is contacted by water to remove any components of the gas which inhibit the formation of sulfurous acid from the sulfur dioxide and water.

4. The method of claim 1 wherein one or more of the acids of the class consisting of nitrous acid, nitric acid, sulfurous acid, and sulfuric acid is produced as a result of the reaction of the gas and the sulfurous acid and an excess of the alkaline-solution is supplied, the said excess reacting with the said one or more acids to produce one or more of the class consisting of nitrite, nitrate, sulfite and sulfate salts.

5. The method of claim 1 wherein the alkaline solution is a solution of sodium hydroxide.

6. The method of claim 4 including the additional step of oxidizing the nitrite and sulfite salts to nitrates and sulfates.
7. The method of claim 1 wherein liquid effluent from the reaction of the gas with sulfurous acid which liquid

Claim 7 continued.....
effluent includes excess sulfur dioxide and nitrogen oxides is added back to the reaction of the sulfurous acid and the gas.

8. The method of claim 1 wherein the product of the reaction of the nitrogen oxides in the gas and sulfurous acid, which is converted into a salt by reaction with alka-line solution, is predominantly compounds which are more soluble in and more reactive with said alkaline solution than nitric oxide and nitrogen dioxide.

9. Apparatus for removing nitrogen oxides from a gas including a first vertical scrubber column having packing, means, connected to said column near the lower surface of said packing, for supplying said gas thereto, means, connected to said column near the upper surface of said packing, for supplying sulfurous acid thereto in counterflow contacting relationship to said gas to react the nitrogen oxides in said gas with said sulfurous acid, a product of said reaction being gaseous and including compounds which are more soluble in and more reactive with alkaline solution than nitric oxide and nitrogen dioxide, a second vertical scrubber column including packing, means, connected to said second column near the lower surface of its packing, for supplying thereto the gaseous product of the reaction of said first column, including the said compounds, means, connected to said second column near the upper surface of said packing, for supplying thereto an alkaline solution in counterflow contacting relationship with said product of said reaction to convert said compounds in said product into salts of one or more of the class of acids consisting of nitric and nitrous acid, and means for conducting away the said salts.
10. The apparatus of claim 9 including a gas-treatment tank, means conducting liquid products of the reaction of the nitrogen oxides in the gas and sulfurous acid to said gas-treatment tank, said liquid products including

Claim 10 continued....
one or more of the class consisting of sulfurous acid, sulfuric acid, nitrous acid and nitric acid, means supplying a treating gas to said gas-treatment tank to remove sulfur dioxide therefrom, gas conducting means, connecting said gas-treatment tank to the first scrubber column near the lower surface of said packing, for con-ducting said sulfur dioxide to said first scrubber column, additional conducting means, connected to said gas treating tank, for conducting away the liquid products of the reaction, and means, connected to said additional conducting away means for neutralizing the liquid products of the reaction.

11. The apparatus of claim 10 including a second gas-treating tank, means, connected to the second scrubber column, for conducting to said second gas-treating tank the liquid products of the reaction in said second scrubber column, means, connected to said second gas-treating tank for supplying thereto a gas including oxygen, said oxygen oxidizing some of said liquid products of reaction, the liquid products of the reaction from said second scrubber column, including said oxidized products, being neutralized by the liquid products from the first gas-treating tank.

12. The method of claim 1 wherein the nitrogen oxides predominantly include nitrogen oxide and nitrogen di-oxide and wherein the reaction of the nitrogen oxides with the sulfurous acid produces nitrogen-oxygen compound which reacts readily with alkaline solution, the said method including reacting the nitrogen-oxygen compound with an alkaline solution to produce a nitrite salt and then reacting the nitrite salt with oxygen to convert the nitrite salt into a nitrate salt.