GB1591822A

GB1591822A - Removal and recovery of nitrogen oxides and sulphur dioxide from gaseous mixtures containing them

Info

Publication number: GB1591822A
Application number: GB34034/77A
Authority: GB
Original assignee: Individual
Current assignee: Individual
Priority date: 1976-08-16
Filing date: 1977-08-12
Publication date: 1981-06-24
Also published as: DE2736797A1; AU2794577A; AU515741B2; CA1101192A

Description

(54) REMOVAL AND RECOVERY OF NITROGEN OXIDES AND SULFUR DIOXIDE FROM GASEOUS MIXTURES CONTAINING THEM (71) I, HAL B. H. COOPER, of 4234 Chevy Chase Drive, Flinuridge, California 91011, United States of America, a citizen of the United States of America, do hereby declare the invention, for which I pray that a patent may be granted to me, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to processes for removing and recovering nitrogen oxides from gas mixtures containing them and especially from mixtures having low concentrations of these substances. Such mixtures include combustion gases produced in burning coal, oil, and other low molecular weight hydrocarbons in power plants, industrial furnaces and the like. Nitrogen oxides such as nitric oxide (NO) are extremely difficult to remove from such mixtures. However, nitrogen oxides are regarded as air pollutants and many nations now mandate reducing their emission into the atmosphere. This invention not only permits compliance with such mandates, but permits recovery of these pollutants as commercially valuable products in such quantity and quality that air pollution control may become profitable at best, or at least far less costly at worst.

This invention also relates to processes for the removal of sulfur dioxide and nitrogen oxides from gas mixtures including both. Preferably, sulfur dioxide is removed by processes that achieve significantly higher removal percentages than such widely-adopted processes as the Wellman-Lord, magnesia and lime-limestone scrubbing methods, and permit flexibility in recovering the sulfur values so removed. In particular, recovery of the sulfur in sulfate form, as in the process of U.S.S.N. 804,620, filed June 8, 1977, in which the sulfur dioxide is removed with an alkali metal carbonate/bicarbonate-containing aqueous medium wherein the ratio of alkali metal to sulfur dioxide (K:SO2) is at least about 2 results in very high removals, about 990/c, following which the alkali metal sulfite may be oxidized to alkali metal sulfate with oxygen (air) at low cost. The very high removal and recovery of sulfur values as useful products of commerce through this process makes possible a significant improvement in the economics of the heretofore costly removal of sulfur dioxide pollutants from such gas mixtures as combustion gasses.

The conversion of lower valence nitrogen oxides to high valence nitrogen oxides to facilitate their removal from a gas mixture is, broadly, not an entirely new concept. Thus, for example, U.S. Patents 1,420,477; 3,733,393; 3,991,167, and 3,927,177 propose oxidizing oxides of nitrogen to remove them more easily from gaseous streams. U.S. Patent 3,873,672 employs a related method for removing sulfur dioxide from gas mixtures containing it. All of these primarily seek simply removal of the pollutants and are costly to operate. None discloses removing pollutant nitrogen oxides or sulfur dioxide or both from combustion gases and recovering them as useful products in a practical and economic manner.

Combustion gases from the burning of carbonaceous fuels include substantial quantities of carbon dioxide. These quantities may be in the range of about 5% to about 20% by volume. Although emission of carbon dioxide to the atmosphere is not presently under severe attack as an environmental problem, capture and recovery of carbon dioxide may be highly desirable to provide raw materials for other commercially valuable products. The recovery techniques that form important parts of the processes of this invention permit recovery of large volumes of carbon dioxide in high purity at low cost, thus contributing to the overall efficiency and economy of these processes.

This invention provides a process for removing oxides of nitrogen from a gas stream including carbon dioxide and at least one oxide of nitrogen comprising: (a) treating said gas stream at a temperature less than 100"C in a first zone with an aqueous medium including sulfuric and/or nitric acid, and, as oxidant, hydrogen peroxide or persulfuric acid; (b) maintaining said oxidant in excess such that the ratio of said oxidant to dinitrogen trioxide and nitrogen dioxide exceeds the amount needed to convert said trioxide and dioxide to nitric acid; (c) forming a product aqueous medium which includes nitric acid; and (d) treating said gas stream in a second zone with an alkaline aqueous medium to form a second zone product aqueous medium which includes at least one nitrite or nitrate salt of an alkaline earth or alkali metal.

The peroxy compound is preferably hydrogen peroxide, but may be persulfuric acid.

In the preferred embodiment of this invention the acid is nitric, and the peroxy compound is hydrogen peroxide. Alternatively, the acid may be sulfuric, and the peroxy compound, persulfuric acid. In a practical sense, however, since many carbonaceous fuels contain sulfur, some sulfur dioxide will be present along with the oxides of nitrogen, and the presence of some sulfuric acid is to be expected as part of the acid component. The process preferably includes two steps. First, the gas stream with lower valence nitrogen oxides is treated with aqueous medium that includes an acid and a peroxy compound to form nitric acid and other higher valence nitrogen oxides. Second, the nitric acid and higher valence nitrogen oxides are treated with an aqueous alkaline carbonatelbicarbonate medium to remove the higher valence oxides of nitrogen as nitrites and nitrates. A plurality of zones may be employed, with the number depending upon the flow rate of the gas, the oxidizing efficiency of the acid/peroxy compound system employed, and the extent to which the level of nitrogen oxides is to be reduced. Alternatively, or concomitantly, the process may be conducted so as to also produce nitric acid and sulfuric acid as products by withdrawal from the primary oxidation step.

The acid acts as an oxidation catalyst in this process. The peroxy compound provides the oxygen needed to oxidize the lower valence nitrogen oxides to higher valence forms. The combination of peroxy compound and acid is much more active and effective than the peroxy compound alone. Though the acid may itself also act as an oxidant, such as with nitric acid, any such oxidation reduces the acid to a lower valence form, which in turn is reoxidized by the peroxygen compound in the aqueous media. The dual reagent oxidizing system of this invention permits a major reduction in the size of the scrubbing equipment necessary with other systems and, accordingly, produces a significant reduction in the capital investment requirements. Because the quantities of combustion gas to be treated are exceedingly large and their flow rate rapid, the capital cost of scrubbing equipment is an extremely important consideration.

Hydrogen peroxide alone oxidizes nitric oxide very slowly when it is diluted with inert gases such as nitrogen and carbon dioxide and its removal from such gas mixtures is very slow. On the other hand, when a strong mineral acid such as sulfuric or nitric acid is combined with the hydrogen peroxide, particularly nitric acid, the rate of oxidation and removal is increased markedly. The acid promotes the solubility of the nitric oxide in the aqueous medium by solvation and catalyzes the oxidation of intermediate compounds to nitric acid. It is known that hydrogen peroxide and nitrous acid form an unstable material, peroxynitrous acid, which rearranges to nitric acid at a very rapid rate in the presence of a strong mineral acid.

In the practice of the process, the acidic scrubbing medium is preferably recycled to build up the nitric acid content as the lower valence oxides of nitrogen are oxidized. Hydrogen peroxide is added to replace that consumed by the oxidation reaction and thus the nitric acid content can be raised to a high level despite the low concentration of nitrogen oxides in the gas stream.

Unlike the process disclosed in U.S. Patent 3,733,393, the process of this invention does not rely upon operating at an elevated temperature above the boiling point of water. This is particularly important in treating combustion gases such as those emitted from a power plant where enormous volumes must pass through the process at extremely high flow rates and where the effluent gases will fall to a temperature in the range of about 40 to about 60"C upon contact with aqueous scrubbing solutions. The process of U.S. Patent 3,733,393 is wholly impracticable commercially for removing and recovering nitrogen oxides from such gas mixtures for this reason. Otherwise, expensive hydrogen peroxide is required to oxidize the sulfur dioxide as well as the oxides of nitrogen. Further, the process is based on a one-pass concept and does not permit building up the concentration of nitric acid to a high level where it becomes more practical economically to recover nitric acid or alkali metal nitrates as commercially useful products.

Unlike the process disclosed in U.S. Patent 3,991,167, the process of this invention does not require the conversion of nitric oxide to N2O3, dinitrogen trioxide, and NO2, nitrogen dioxide, followed by reaction of the N2O3 and NO2 with stoichiometric amount of hydrogen peroxide. That process is also impracticable for treating combustion gases. The new process claimed here utilizes the acid as a catalyst and a solvating agent to effect the direct oxidation of nitric oxide (NO) to nitric acid with hydrogen peroxide which is present in the aqueous scrubbing medium. To minimize carryover of hydrogen peroxide in the gas stream due to its vapor pressure and the huge amount of gas to be treated, the process of this invention is preferably effected with the flow of gas mixture cocurrent with the aqueous medium that includes the acid and peroxy compound, although this is not necessary, and counter-current flow may be employed advantageously.

The most common oxide of nitrogen in combustion gases is the nitric oxide (NO), which generally exceeds the concentration of nitrogen dioxide by a factor of about 10. In these combustion gas mixtures, lower valence oxides of nitrogen may be present in concentrations in the range of about 200 to about 20,000. parts per million. But since these mixtures are evolved at such rapid rates, for example, from about 1,000 cubic feet per minute to about 1,000,000 cubic feet per minute from a 500 megawatt power plant, the quantity emitted to the atmosphere is very large and creates a serious air pollution problem. The processes of this invention are especially applicable in minimizing these emissions, or at least lowering them to, say, about 50 parts per million.

Combustion gas mixtures amenable to the new processes also include carbon dioxide, oxygen and nitrogen and may include some carbon monoxide. The carbon dioxide concentration in combustion and similar gases is normally in the range of about 5% to about 20%, more commonly about 10% to about 16% by volume.

Both nitric acid and peroxygen compounds such as hydrogen peroxide are volatile liquids and have significant vapor pressures. Sulfuric acid has a markedly lower vapor pressure. Where nitric acid and hydrogen peroxide are used as the dual reagent system, the vapor pressure of each is determined by the temperature and by their concentration in the aqueous medium. In the temperature range where this process preferably takes place, there can be an appreciable carryover of nitric acid and hydrogen peroxide from the oxidizing step because of their vapor pressures unless their concentration is reduced, as by dilution. Nitric acid may be readily captured in the second alkaline scrubbing stage, but hydrogen peroxide is not so easily removed. For that reason, the hydrogen peroxide concentration is desirably held to less than about 5% in the first step.

The concentration of the acid may vary widely, with increasing acid concentration generally inncreasing the rate of reaction. Preferably, the acid concentration in the first step is in the range of about 1% to about 60 /,, more preferably about 20% to about 45%. The acid concentration also increases with increasing concentration of nitrogen oxides in the gaseous mixture.

A high nitric acid concentration speeds the rate of oxidation and facilitates recovery of nitrate products, either as nitric acid or as nitrate salts. Nitric acid formed during oxidation may either be accumulated and withdrawn as a nitric acidcontaining aqueous medium, or may be allowed to rise to reach an equilibrium level and then be carried over in vapor form with the combustion gas undergoing treatment in the alkaline scrubbing stage. There, the nitric acid is captured and removed as a salt such as an alkali metal or alkaline earth metal nitrate.

In the first stage oxidation, if no nitric acid aqueous media is withdrawn, the nitric acid tends to build up and equilibrate with the gas stream and balance with the amount of nitrogen oxides being oxidized to form nitric acid. For each mole of nitric oxide or nitrogen dioxide oxidized, one mole of nitric acid is formed, as by the following reactions: NO2+1/2 H202eHNO3 NO+3/2 H202eHNO3+H2O In addition to the primary oxidation reactions to produce nitric acid, a small amount of nitrogen dioxide may also be produced which passes to the second stage absorber in the gas stream.

For example, where the nitrogen oxides consist of about 1,000 parts per million in the entering combustion gas stream, and with the process operating at about 55"C, the aqueous medium in the oxidation step will equilibrate at about 4045% nitric acid. Where the nitric oxide concentration is at the 500 parts per million level, the solution will equilibrate in the 350% nitric acid range. An equivalent amount of nitric acid is thus carried from the first step to the second with the treated gas stream under these conditions. When the concentration of nitric acid is lower than the equilibrium value, as when nitric acid is being removed as a product, less nitric acid passes to the second step stage.

Where nitric acid formed during oxidation passes from the first step to the second with other higher valence nitrogen oxides, along with the carbon dioxide, nitrogen and oxygen in the combustion gases, the nitric acid may be converted to a nitrate salt, preferably an alkali metal nitrate, and more preferably potassium nitrate. This is preferably effected by treating the gas stream that contains the nitric acid with an alkali metal (e.g., potassium) carbonate/bicarbonate aqueous medium, preferably formed by reaction of potassium hydroxide with carbon dioxide in the gas stream.

Utilizing an alkali metal carbonate/bicarbonate aqueous medium to convert nitric acid and other higher valence nitrogen oxides to alkali metal salts permits the concentration of these salts to rise to a level where recovery is commercially practicable because the carbonate/bicarbonate acceptor system captures and holds higher valence nitrogen oxides in aqueous medium in non-volatile form. This permits recycling of the nitrate-containing aqueous medium while retaining the higher valence nitrogen oxides in solution as nitrite and nitrate salts of alkali metal.

Further, the acceptor system promotes highly selective recovery of alkali metal nitrates and nitrites in commercial quantities. Crystallization from aqueous media takes place at higher temperatures than would be expected, thus increasing the yield and minimizing the energy needed to evaporate and cool the product aqueous medium to effect crystallization of the nitrate and nitrite products.

The alkali metal carbonate/bicarbonate acceptor system in aqueous medium has additional benefits. For example, the aqueous medium containing this system may be treated to recover some or all of the carbon dioxide absorbed from the gas stream. The presence of carbonates in the aqueous medium facilitates crystallization and recovery of such alkali metal salts as potassium nitrate and potassium nitrite by lowering their solubilities to a major degree. In the aqueous medium, conversion of carbonate to bicarbonate, as by the addition of carbon dioxide or nitric or sulfuric acids, lowers the pH of the product aqueous medium.

That in turn facilitates and expedites oxidation of nitrite to nitrate by oxidants such as hydrogen peroxide. Conversely, conversion of bicarbonate to carbonate by heating permits not only the recovery of relatively pure carbon dioxide in large quantities, but also facilitates the recovery of alkali metal nitrate by crystallization in high purity and yield.

The alkali metal of the carbonate and bicarbonate used in the aqueous treating medium may be any of the alkali metals, but potassium is preferred. Potassium carbonate, in particular, is very soluble in aqueous medium, and has the surprising effect of reducing the solubility and raising substantially the temperature at which the nitrate and the nitrite may be crystallized. This means less energy is needed for concentration and refrigeration to cool the product aqueous medium to crystallize the nitrate, and leads to a higher potassium nitrate selectivity and yield during crystallization. Potassium nitrate is a valuable commercial product and is especially valuable as a fertilizer, both for its fixed nitrogen and its potassium.

The alkali metal makeup for that removed as alkali metal nitrate is preferably introduced to the treating step of the processes of this invention as alkali metal hydroxide, although it can be supplied as alkali metal carbonate. Preferably, the alkali metal hydroxide is electrolytically derived from alkali metal halide. Thus, for example, electrolysis of potassium chloride produces potassium hydroxide in aqueous medium, as well as hydrogen and chlorine gases.

The alkali metal carbonate and bicarbonate used in the treating step of the processes of this invention forms during the treating process by reaction of carbon dioxide from the gas mixture with the alkali metal hydroxide fed thereto.

Oxidation of nitrogen oxides in the first stage of the process of this invention produces some partially oxidized nitrogen dioxide as well as fully oxidized nitric acid. When absorbed by the aqueous medium of the second step of this process, nitrogen dioxide forms a nitrite salt, as by the following reaction: 2 NO2+2 K2CO3+H2OeKNO2+KNO3+2 KHCO3 However, because nitrates are more easily removed from such media than are nitrites, the nitrites are desirably oxidized to nitrates.

KNO2+H2O2KNO2+H2O At the carbon dioxide concentration normally prevailing in combustion gasses, e.g., around 14%, both carbonate and bicarbonate are present in the aqueous medium and the pH is likely to be 9 or greater. An excess of alkali metal carbonate/bicarbonate to acidic nitrate/nitrite is desirable in order to facilitate the absorption of the higher valence oxides of nitrogen from the gas stream. While a stoichiometric ratio can be used, the lower ratio requires a substantially larger and more costly gas-liquid contactor and the risk of incomplete removal. Surprisingly, lowering the pH to less than about 9 facilitates oxidation of nitrites to nitrates in the aqueous medium. This lowering of the pH may be effected by adding sufficient acid to neutralize the carbonate and convert it to bicarbonate, or, preferably, by adding carbon dioxide to the medium.

In these processes, this lowering of pH and conversion of nitrite to nitrate is best effected at the outset to utilize any hydrogen peroxide oxidant carried over in the gas stream from the primary oxidation stage and to raise the nitrate concentration before its recovery by crystallization. If the amount carried over and scrubbed out is insufficient, additional hydrogen peroxide can be added. However, this conversion may be postponed in the recycling of the aqueous medium and may even follow the recovery of nitrate, provided sufficient oxidant is present or is added to the recycling medium to effect the oxidation.

Another important step in recovering products from the aqueous medium is decarbonation, where the bicarbonate formed in the treating step of the processes is converted to carbonate and carbon dioxide by heating the aqueous media and driving off the carbon dioxide. This heating may be effected by steam stripping or by evaporation. Generally, some evaporation of water is required to facilitate the crystallization of the product. The carbon dioxide so made is of high purity and may be captured for use in other processes, used for lowering the pH of the product aqueous medium to facilitate oxidation of nitrite to nitrate, or both. Although -decarbonation preferably follows conversion of nitrite to nitrate, decarbonation may follow immediately after the second step of the process. Following conversion of the bicarbonate to carbonate, the aqueous medium is cooled to recover alkali metal nitrates, preferably by crystallization.

Since the carbonate/bicarbonate-containing product aqueous medium employed holds alkali metal nitrates and nitrites strongly in solution, the aqueous media may be recycled through the scrubbing steps many times to raise the nitrate concentration sufficiently high to make product recovery practicable and economic. Thus, when the nitrate concentration rises into the range of about 2% to about 45% based on the weight of the aqueous medium, recovery may then be effected efficiently. Surprisingly, the presence of potassium carbonate in particular in the aqueous medium has a pronounced effect in reducing the solubility of the potassium nitrate and thus makes its recovery much easier, and more economical.

Not only is the temperature at which crystallization takes place significantly higher than where carbonate is not present, but the yield per pass is also improved.

Where the peroxy compound is hydrogen peroxide, it is preferably produced at the site of the treating process by one of the known processes for its production.

Incorporating or associating the peroxide supply process directly into the treating process effects considerable economy because a separate peroxide source would require the movement of large quantities of water and a supply of costly hydrogen.

For example, where hydrogen peroxide is made by the auto-oxidation of 2alkylanthraquinone, hydrogen peroxide is produced in an anthraquinonecontaining organic solvent phase, which is then extracted with water. The 2alkylanthraquinone moiety is alternately oxidized with air and reduced with hydrogen. The availability of hydrogen gas produced in the electrolytic formation of potassium hydroxide or other alkali metal hydroxide may be used to advantage in the anthraquinone reduction step of this process, thereby lowering substantially the cost of producing this oxidant. Alternatively, the peroxygen material may be produced electrochemically by anodic oxidation as for the case where persulfuric acid is used as the oxidant.

The chlorine produced in the electrolytic formation of potassium hydroxide from potassium chloride may advantageously be reacted with ethylene to form ethylene dichloride, a precursor to polyvinyl chloride. This not only eliminates the problem of liquefying, storing and transporting highly volatile and hazardous chlorine, but produces at low cost a highly valuable, low-volatility highly stable chemical.

Where nitric acid is desired as a product, the acidic aqueous scrubbing medium from the primary oxidation step can be withdrawn and the nitric acid and sulfuric acid therein separated and recovered as products. In this case, the nitric acid concentration does not build up to as high a level as where the nitric.acid is transferred in the gas stream to the second stage for recovery as alkali metal nitrate.

The process of this invention also permits the recovery of sulfur dioxide from gas mixtures where sulfur dioxide and nitrogen oxides are present together in such mixtures. Sulfur dioxide removal may be effected in a number of ways. Preferably, sulfur dioxide is removed separately from, and before the nitrogen oxides are oxidized and removed. To do this, the gas mixture, including carbon dioxide, sulfur dioxide, and lower valence nitrogen oxides, is treated with aqueous medium comprising the carbonate and bicarbonate of an alkali metal where the ratio of the alkali metal to sulfur dioxide is at least about 2. A product aqueous medium is recovered that includes alkali metal carbonate, alkali metal bicarbonate and alkali metal sulfite formed from the sulfur dioxide. Sulfur dioxide removal efficiency of this process is very high, about 99%.

By contrast, the widely adopted Wellman-Lord magnesia and lime-limestone processes for removal of sulfur dioxide from combustion gases achieve removals of only about 90% of the sulfur dioxide present from these mixtures. As with the processes of this invention for removing nitrogen oxides from gas mixtures, this process for the removal of sulfur dioxide from gas mixtures is particularly useful where the gas stream contains low concentrations of sulfur dioxide and where the gas stream is in large volume and must flow at a high velocity, such as where the gas mixture is a combustion gas from a power plant or other high fuel-consuming plant.

Although this new sulfur dioxide removal process is superior to the Wellman Lord process, the Wellman-Lord process may alternatively be combined with the processes of this invention for removing nitrogen oxides from gas mixtures including both sulfur dioxide and nitrogen oxides where there is a direct need for sulfur dioxide as a product in preference to alkali metal sulfate or sulfite.

Moreover, the Wellman-Lord process and others capturing sulfur dioxide as bisulfite may be combined with the sulfur recovery methods of this invention. Thus, where the product aqueous medium from gas scrubbing includes such bisulfite, the alkali metal or alkaline earth hydroxide or carbonate may be added to the medium in an amount sufficient to convert the bisulfite to sulfite. Air oxidation of sulfite to sulfate, and recovery of sulfate by crystallization may then follow.

As in the process for removing and capturing nitrogen oxides from gas mixtures, the preferred alkali metal is potassium and the preferred source for that potassium is electrolytically derived potassium hydroxide made, for example, from the electrolysis of potassium chloride. However, potassium carbonate is also acceptable as a source of potassium.

The preferred sulfur dioxide removal process not only achieves very high removals of sulfur dioxide, but also can recover large quantities of carbon dioxide at low cost. The employment of an excess of alkali metal beyond that needed for reaction with the sulfur dioxide to produce alkali metal sulfite not only ensures a high sulfur dioxide removal efficiency, but, depending upon the extent of the excess and the degree of recycling, may also absorb large amounts of carbon dioxide from the gas mixture which can then be recovered in the form of highly pure carbon dioxide upon decarbonation of the alkali metal bicarbonate. The concentration of alkali metal sulfite may be raised to a high level by recycling in a manner similar to that employed with the nitrogen oxides removal. The sulfur values in the product aqueous medium are preferably recovered as alkali metal sulfate, although alkali metal sulfite may be crystallized and recovered as such and may be preferable where the alkali metal is sodium. The sulfate is preferably formed by oxidation with a low-cost oxidant such as oxygen, or an oxygencontaining gas such as air. The by-products of such oxidation are gaseous and consist of evaporated water and air. Much water can be evaporated in this step, as the oxidation is exothermic and the heat of reaction can be used directly Decarbonation and oxidation may also be effected simultaneously if recovery of carbon dioxide is unnecessary or undesirable. The alkali metal sulfate is recovered from the aqueous media by crystallization. Crystallization of potassium sulfate from the potassium carbonate-containing product aqueous medium is particularly 'efficient. Surprisingly, the solubility of potassium sulfate in potassium carbonate solutions is reduced to very low levels, even where the solution is hot. This is one of the important features of the process. For example, the solubility of potassium sulfate in water at 700C is 19.5 grams/100 milliliters of water, but drops to 0.11 grams/100 milliliters of aqueous solution containing 67 grams of potassium carbonate, a remarkable reduction in solubility.

Alkali metal values removed from the product aqueous medium with the sulfate or sulfite salts may be replenished with alkali metal hydroxide or carbonate and fed directly to the recycling product aqueous medium. Where the product aqueous medium includes sulfite, nitrite and nitrate salts in solution, these may be separated from one another, as by first converting the nitrites to nitrates, then oxidizing the sulfite to sulfate, and finally by separating the sulfate and nitrate by selective crystallization is particularly amenable to treatment for removal and recovery of commercially valuable products in commercial quantities. Alkali metal nitrites and nitrates formed in the alkaline scrubbing are strongly held by the carbonate/bicarbonate acceptor system and may be recycled time and time again through the treating process until the concentration of each rises to a level where recovery is commercially practicable.

Carbonates in the product aqueous medium strongly reduce the solubility of the nitrates, and permit them to be crystallized as the temperature of the medium is lowered. Nitrites in the product aqueous medium are readily converted to nitrates by lowering the pH of the product aqueous medium to less than about 9, which expedites the oxidation of nitrite to nitrate.

The invention is illustrated by the accompanying drawings, Figures 1, 2 and 3, which show the application of this process to a mixture of nitrogen oxides, sulfur dioxide, and carbon dioxide, which are normally found in emissions from the combustion of coal, oil and lower molecular weight hydrocarbons. This mixture is sometimes called stack gas or combustion gas.

Figure 1 illustrates the two-stage oxidation/scrubbing process of this invention for treating a gas stream that includes nitrogen oxides, sulfur dioxide, carbon dioxide, and accompanying oxygen and nitrogen. Typically, the process illustrated in Figure 1 follows the removal of most of the sulfur dioxide from the gas mixture.

In this illustration, the acid is primarily nitric acid, and the peroxy compound is hydrogen peroxide.

Figure 2 illustrates the overall removal and product recovery process of the invention as applied to the scrubbing of a combustion gas from which sulfur dioxide has been previously removed. The acid is primarily sulfuric acid, and the peroxy compound is persulfuric acid.

Figure 3 illustrates the overall removal and product recovery process of this invention as applied to a combustion gas stream from which sulfur dioxide has been previously removed. The acid is primarily nitric acid, and the peroxy compound is hydrogen peroxide.

Referring now to Figure 1, a combustion gas stream, comprising nitrogen oxides, and particularly nitric oxide, in low concentration and at a temperature in the range of about 50"C to about 60"C passes via line 101 to oxidation tower 102 where the stream is treated with an aqueous medium entering via line 103. The aqueous medium includes nitric acid at a concentration usually greater than about 20% by weight, and hydrogen peroxide in a concentration usually less than 5% by weight, together with some sulfuric acid. In tower 102, the following primary reactions take place:

Depending upon the products desired, nitric acid or alkali metal nitrate, the nitric acid, and other high valence nitrogen oxides, produced in tower 102 pass either with the gas stream via line 104 to scrubber tower 105, or in the aqueous medium with sulfuric acid via line 106, or both.

The nitric acid and other valence nitrogen oxides in the gas stream passing from scrubber tower 102 via line 104 to tower 105, are contacted with alkaline aqueous medium entering tower 105 via line 108. The alkaline aqueous medium preferably includes a carbonate/bicarbonate acceptor system such as potassium carbonate/bicarbonate, and some potassium nitrate recycling with aqueous medium from previous scrubbing passes through tower 105. Makeup potassium hydroxide enters line 108 via line 114, and reacts with the carbon dioxide in the gas stream to form potassium carbonate and potassium bicarbonate with aqueous medium. Within tower 105, the following reactions would then take place: HNO3+K2CO3oKNO3+KHCO3 2 NO2+2 K2CO2+H2OKNO2+KNO2+2 KHCO3 The aqueous medium passing from tower 105 via line 110 is in part, recycled via line 111 to line 108 for return to tower 105. The balance of the aqueous medium, which includes potassium nitrate and potassium nitrite, together with potassium carbonate and bicarbonate, passes to recovery of potassium nitrite and nitrate via line 112.

Where the nitric acid and sulfuric acid pass primarily from tower 102 via lines 106 and 109, the aqueous medium is treated, as by distillation, to separate nitric and sulfuric acids from one another in substantial quantity and purity, and are recovered as such, treated further to produce salts of these acids, or the nitric or sulfuric acid recycled to maintain the desired balance. A portion of the aqueous effluent passing from tower 102 via line 106 is recycled via line 107 to line 103, to which incoming hydrogen peroxide, peroxysulfuric acid and nitric acid are added as necessary via line 113.

Scrubbed combustion gas containing little sulfur dioxide and low concentration of nitrogen oxides passes to the atmosphere via line 115.

Referring now to Figure 2, which illustrates the overall oxides of nitrogen removal and product recovery process, combustion gas comprising sulfur dioxide and lower valence nitrogen oxides enters scrubber 202 via line 201 after most of the sulfur 'dioxide has been removed from the gas stream. Aqueous scrubbing medium enters oxidation scrubber 202 via line 206, which includes persulfuric acid from cell 221 added through line 226, recycled aqueous medium from line 207 that includes hydrogen peroxide, sulfuric and nitric acids. In Tower 202, the following reactions take place: H2S208+H20oH202+2 H2SO4 NO+3/2 H2O2HNQ+H2O Aqueous medium passes from tower 202 via lines 208 and 209 to distillation tower 210, from which nitric acid is taken overhead with water via line 211 and sulfuric acid is taken below via line 212. The nitric acid taken via line 211 passes to stripper 213, from which excess water is taken overhead via line 214, and nitric acid is taken below as an aqueous solution that includes (say about 68%) nitric acid.

Nitric acid forms a maximum boiling point azeotrope with water which boils at 122"C. The nitric acid may be removed as a product through line 215, or may be recycled back to scrubber 202 through line 226, or may be transferred to the alkali metal nitrate recovery circuit and reacted with the alkali metal carbonate/bicarbonate aqueous medium to produce alkali metal nitrate and recovered via line 265. This may be added at several points prior to or at crystallizer-separator 263.

The sulfuric acid in aqueous medium passing from distillation tower 210 via line 212 passes to stripper 217, from which water and residual hydrogen peroxide are taken overhead via line 218, and fed into line 226. Sulfuric acid in the range of 60-930/,, passes below from stripper 217 via line 219. Any excess sulfuric acid taken from stripper 217 via line 219 may be removed as product via line 227 with the remainder passing to electrolytic cell 221 via line 216 for conversion to peroxysulfuric acid.

In electrolytic cell 221, persulfuric acid is produced by anodic oxidation with hydrogen gas discharged at the cathode 223. The reaction in electrolytic cell 221 is as follows: 2 H2SO4eH2S208+H2 Optimum conditions for conversion of sulfuric acid to persulfuric acid occur in the range of 60--800/, H2SO4, a temperature of about 100C, with about 40--500/, of the sulfuric acid being converted.

The gas stream passes from scrubber 202 via line 203 to alkaline scrubber 204, where the nitric acid and other higher valence oxides produced in tower 202 and carried with the gas stream are treated with incoming aqueous medium from line 253 that includes potassium carbonate/bicarbonate and potassium nitrate.

Recycling aqueous medium also entering scrubber 204 via lines 252 and 253 typically includes potassium nitrate and nitrite, together with potassium carbonate and potassium bicarbonate. The gas stream essentially free of sulfur dioxide and containing low concentrations of nitrogen oxides, passes from scrubber 204 via line 205. Alkaline aqueous product media, including potassium nitrate, potassium nitrite, potassium carbonate and potassium bicarbonate, and having a pH greater than about 9, passes from scrubber 204 via line 251 and 254 to nitrite converter 255.

There, the aqueous medium is treated with carbon dioxide entering via line 258 and with an oxidizing agent such as hydrogen peroxide to lower the pH to less than about 9, and to convert any potassium nitrite present to potassium nitrate. The potassium bicarbonate-predominating aqueous product medium then passes via line 256 to decarbonator 261 where steam stripping or other applied heat converts potassium bicarbonate to potassium carbonate and carbon dioxide. Carbon dioxide so produced is taken overhead via line 257, and recovered as such from line 259 or a portion is cycled via line 258 to nitrite converter 255 to lower the pH.

The alkaline carbonate aqueous product medium passes from decarbonator 261 to crystallizer/separator 263 via line 262, and is there cooled to crystallize potassium nitrate as a solid. Potassium nitrate is centrifuged and dried and removed via line 265 as relatively pure product. The alkaline mother liquor aqueous medium consisting principally of potassium carbonate and soluble potassium nitrate passes via line 264 into line 269 for recycle to scrubber 204 via line 253.

Makeup potassium for the alkaline aqueous scrubbing medium to replace that removed with the potassium nitrate product may be furnished as aqueous potassium hydroxide solution by the electrolysis of potassium chloride, which enters cell 240 via line 238. Chlorine gas is produced at the anode, and exits the cell via line 236. This gas may be combined with ethylene to produce ethylene chloride, a stable, much less volatile and valuable chemical that is a precursor to polyvinylchloride. Hydrogen, taken overhead via line 235 from the cathode compartment may be used to produce hydrogen peroxide for use in oxidizing nitrites to nitrates in converter 255 or other purposes. Potassium hydroxide is produced at the cathode of cell 240 as a chloride-free product which passes from the cell via line 237 in aqueous medium to lines 269 and 253.

In Figure 3, which illustrates the dverall oxides of nitrogen removal and product recovery process, combustion gas enters scrubber 302 via line 301 after substantially all of the sulfur dioxide in the combustion gas has been removed by one of the processes described previously. Aqueous medium that includes hydrogen peroxide enters scrubber 302 via lines 321 and 306, together with recycling aqueous medium that includes nitric and sulfuric acids and hydrogen peroxide entering line 306 via line 307. In scrubber 302, the primary reactions take place:

Aqueous product medium passing from scrubber 302 via line 308 and not recycled through line 307 passes via line 309 to distillation tower 310, where nitric and sulfuric acids are separated from one another by distillation. Nitric acid is taken overhead from distillation tower 310 via line 311, and the aqueous nitric acid passes to distillation tower 313. There, excess water is removed overhead via line 314 and the 68% aqueous azeotrope of nitric acid as bottoms via line 315. Sulfuric acid aqueous medium passing from distillation tower 310 passes via line 312 to tower 317, from which water and any residual hydrogen peroxide are taken overhead via line 318, and sulfuric acid as bottoms via line 319.

Hydrogen peroxide is preferably made on site in plant 322 by the alternate reduction and oxidation of an alkylanthraquinone, generally, 2ethylanthraquinone, with hydrogen and oxygen. The hydrogen is preferably provided from the electrolysis of potassium chloride, conducted in electrolytic cell 331, which is used as source of the potassium hydroxide makeup for the alkaline scrubbing step of this process.

Combustion gas passing from oxidation scrubber 302 includes nitric acid and other higher valence nitrogen oxides, together with carbon dioxide, oxygen and nitrogen. This combustion gas enters alkaline scrubber 304 via line 303, wherein a substantial removal of the higher valence nitrogen oxides and nitric acid is effected.

The treated exit gas with little nitrogen oxides and sulfur dioxide remaining passes from scrubber 304 via line 305 to the atmosphere.

Aqueous alkaline medium, including typically potassium carbonate, potassium nitrate, and potassium bicarbonate, enters scrubber 304 via line 353. In scrubber 304, contact between the gas stream and the alkaline aqueous medium produces the following reactions: HNO3+K2CO3oKNO3+KHCO3 2 NO2+2 K2CO2+ H2OKNO2+KNO2+2 KHCO2 Product aqueous medium passes from scrubber 304 via line 351. This medium includes potassium nitrite and nitrate, potassium carbonate and bicarbonate, and may contain some hydrogen peroxide. A portion of this aqueous medium is recycled to scrubber 304 via lines 352 and 353. When the potassium nitrite/nitrate concentration reaches a sufficient level to warrant product recovery, a portion of the aqueous medium passes via line 354 to nitrite converter 355. Carbon dioxide or nitric acid from 315 enters converter 355 via line 358 together with, as necessary, sufficient hydrogen peroxide to oxidize nitrites to nitrates. The carbon dioxide acts to convert potassium carbonate to bicarbonate, thus lowering the pH below 9, which facilitates oxidation of nitrite to nitrate.

The alkaline aqueous product medium passes from nitrite converter 355 via line 356 to decarbonator 361. There, by the application of heat, as by steam stripping, potassium bicarbonate is converted to potassium carbonate and carbon dioxide. Carbon dioxide so produced is taken overhead via line 357 and recovered via line 359, or cycled to converter 355 via lines 357 and 358.

Product aqueous medium including primarily potassium carbonate and nitrate passes from decarbonator 361 via line 362 to crystallizer 363. There, the alkaline aqueous medium is cooled to crystallize potassium nitrate therefrom. The potassium nitrate is then centrifuged, dried and removed via line 365, and the alkaline aqueous medium recycled. back via line 364 to line 369 to join with the recycling aqueous scrubbing medium in lines 352 and 353.

Makeup potassium hydroxide is preferable produced in cell 331 by the electrolysis of potassium chloride, which enters the anode compartment of the cell via line 338. In the cell, chlorine is produced at the anode 334, and taken overhead via line 336 for reaction with substances such as ethylene to produce stable, lowvolatility, commercially valuable compounds such as ethylene chloride.

Permselective cationic exchange membrane 332 prevents the migration of chloride ions to the cathode region, thereby producing a chloride-free potassium hydroxide solution. In the cathode zone, hydrogen is discharged at cathode 333 and taken overhead via line 335 to on-site plant 322 for use in producing hydrogen peroxide via the oxidation/reduction of an alkylanthraquinone. Potassium hydroxide produced in the cathode compartment is taken via line 337 and combined with the alkaline aqueous scrubbing medium entering scrubber 304 via lines 369 and 352.

The following Examples I and III, illustrate the process of this invention.

Example II shows the importance of using an acid with the peroxygen substance in the new process.

EXAMPLE I A gas mixture consisting of 400 parts per million of nitric oxide and 99.6 nitrogen was passed countercurrently to an aqueous scrubbing solution containing 3.0% hydrogen peroxide and 35 /O nitric acid at 45"C through a three-foot glass column having a two-inch diameter and packed with 1/2 inch Pall-type rings. The gas stream was fed at a rate corresponding to about 0.1 feet per second superficial gas velocity and the liquid flow rate was adjusted to just below the flooding point.

The concentration of nitrogen oxides in the exit gas following passage through a bubbler filled with 20% potassium carbonate solution was reduced to about 3035 parts per million of nitrogen oxides, or a removal efficiency of 92%.

A hydrogen peroxide usage of 1.01 times the theoretical requirement was required. Essentially all of the nitric oxide removed appeared as nitric acid.

This example proves the effectiveness and practicability of the dual reagent process of this invention.

EXAMPLE II The same gas mixture used in Example I was treated in the same equipment under the same conditions, except that the scrubbing solution was a 20% solution of hydrogen peroxide, which had a pH of about 6.0. The concentration of nitric oxide in the exit gas was reduced to about 340 parts per million, or a removal efficiency of 15%. The hydrogen peroxide usage was 1.05 times the theoretical. Essentially, all of the nitric oxide removed appeared as nitric acid. This example shows that hydrogen peroxide alone, even at a higher concentration than in Examples I and III, is far less effective than the dual reagent method of this invention.

EXAMPLE III The same gas mixture treated in Example I was treated in the same equipment under the same conditions with a scrubbing solution containing 3% hydrogen peroxide and 25% sulfuric acid. The concentration of nitrogen oxides was reduced to about 80 parts per million, or a removal efficiency of 80%. The hydrogen peroxide usage was 1.02 times the theoretical. All of the nitric oxide removed appeared as nitric acid. Again, the dual reagent process of this invention proved highly practicable and effective.

This invention thus provides a cyclic process for removing nitrogen oxides from gaseous mixtures includes treating the mixtures with an aqueous medium including sulfuric and/or nitric acid and a peroxy compound to form nitric acid and other higher valence nitrogen oxides wherein the ratio of the peroxy compound to dinitrogen trioxide (N2O3) and nitrogen dioxide (NO2) is greater than the stoichiometric amount and the treating temperature is not more than about 100"C.

Highly selective recovery of nitric acid and/or nitrates in high purity and yield may then follow.

If present in such gaseous mixtures, sulfur dioxide is best removed first, as in a cyclic process including treating these mixtures with aqueous medium including alkali metal carbonate/bicarbonate where the ratio of alkali metal to sulfur dioxide is not less than about 2.

The sulfur values may be recovered from the resulting carbonate/bicarbonate/sulfite-containing product aqueous medium as alkali metal sulfate or sulfite salts which are removed by crystallization from the carbonatecontaining product aqueous medium. Where the sulfur dioxide is not removed in this manner it may alternatively be recovered as sulfur dioxide, sulfuric acid, or as an alkali metal or alkaline earth metal sulfate.

Where the gas mixture includes both sulfur dioxide and lower valence nitrogen oxides, the processes for removing and recovering lower valence nitrogen oxides and sulfur dioxide may be combined into a single removal/recovery system, or may be advantageously effected in sequence.

WHAT WE CLAIM IS: 1. A process for removing oxides of nitrogen from a gas stream including carbon dioxide and at least one oxide of nitrogen comprising: (a) treating said gas stream at a temperature less than 100 C in a first zone with an aqueous medium including sulfuric and/or nitric acid, and, as oxidant, hydrogen peroxide or persulfuric acid (b) maintaining said oxidant in excess such that the ratio of said oxidant to dinitrogen trioxide and nitrogen dioxide exceeds the amount needed to convert said trioxide and dioxide to nitric acid; (c) forming a product aqueous medium which includes nitric acid; and (d) treating said gas stream in a second zone with an alkaline aqueous medium to form a second zone product aqueous medium which includes at least one nitrite or nitrate salt of an alkaline earth or alkali metal.

2. A process according to Claim I, wherein said treating in the first zone is carried out at a temperature which is less than 80"C.

3. A process according to Claim 1 or Claim 2, wherein said acid is nitric and the oxidant is hydrogen peroxide.

4. A process according to any one of Claims 1 to 3, wherein the gas stream flow is concurrent with the aqueous medium flow.

5. A process according to any one of Claims 1 to 4, wherein said gas stream includes 200 to 20,000 parts per million of said oxides of nitrogen.

6. A process according to any one of Claims 1 to 5, wherein the gas stream flows to said first treating zone at a rate of at least 1,000 cubic feet per minute.

7. A process according to any one of Claims 1 to 6, wherein said product aqueous medium is recycled until the nitric acid formed during said treating reaches a predetermined concentration, then recovering said nitric acid.

8. A process according to any one of Claims I to 7, wherein in the first stage nitric oxide is oxidised to nitrogen dioxide.

9. A process according to any one of Claims 1 to 8, wherein in the first stage nitrogen dioxide is oxidised to nitric acid.

10. A process according to any one of Claims 1 to 9, wherein the nitric acid formed in said first treating zone is recovered.

Il. A process according to any one of Claims 1, 2 and 4 to 10, wherein the oxidant is persulfuric acid.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. EXAMPLE III The same gas mixture treated in Example I was treated in the same equipment under the same conditions with a scrubbing solution containing 3% hydrogen peroxide and 25% sulfuric acid. The concentration of nitrogen oxides was reduced to about 80 parts per million, or a removal efficiency of 80%. The hydrogen peroxide usage was 1.02 times the theoretical. All of the nitric oxide removed appeared as nitric acid. Again, the dual reagent process of this invention proved highly practicable and effective. This invention thus provides a cyclic process for removing nitrogen oxides from gaseous mixtures includes treating the mixtures with an aqueous medium including sulfuric and/or nitric acid and a peroxy compound to form nitric acid and other higher valence nitrogen oxides wherein the ratio of the peroxy compound to dinitrogen trioxide (N2O3) and nitrogen dioxide (NO2) is greater than the stoichiometric amount and the treating temperature is not more than about 100"C. Highly selective recovery of nitric acid and/or nitrates in high purity and yield may then follow. If present in such gaseous mixtures, sulfur dioxide is best removed first, as in a cyclic process including treating these mixtures with aqueous medium including alkali metal carbonate/bicarbonate where the ratio of alkali metal to sulfur dioxide is not less than about 2. The sulfur values may be recovered from the resulting carbonate/bicarbonate/sulfite-containing product aqueous medium as alkali metal sulfate or sulfite salts which are removed by crystallization from the carbonatecontaining product aqueous medium. Where the sulfur dioxide is not removed in this manner it may alternatively be recovered as sulfur dioxide, sulfuric acid, or as an alkali metal or alkaline earth metal sulfate. Where the gas mixture includes both sulfur dioxide and lower valence nitrogen oxides, the processes for removing and recovering lower valence nitrogen oxides and sulfur dioxide may be combined into a single removal/recovery system, or may be advantageously effected in sequence. WHAT WE CLAIM IS:

1. A process for removing oxides of nitrogen from a gas stream including carbon dioxide and at least one oxide of nitrogen comprising: (a) treating said gas stream at a temperature less than 100 C in a first zone with an aqueous medium including sulfuric and/or nitric acid, and, as oxidant, hydrogen peroxide or persulfuric acid (b) maintaining said oxidant in excess such that the ratio of said oxidant to dinitrogen trioxide and nitrogen dioxide exceeds the amount needed to convert said trioxide and dioxide to nitric acid; (c) forming a product aqueous medium which includes nitric acid; and (d) treating said gas stream in a second zone with an alkaline aqueous medium to form a second zone product aqueous medium which includes at least one nitrite or nitrate salt of an alkaline earth or alkali metal.

12. A process according to Claim 11, wherein the product aqueous medium

also includes sulfuric acid and said sulfuric and nitric acids are separated from one another by distillation.

13. A process according to any one of Claims I to 12, wherein in the second zone the nitric acid is reacted with alkali metal carbonate or alkali metal hydroxide to form alkali metal nitrate which is recovered by crystallization.

14. A process according to Claim 12, wherein the sulfuric acid is converted to persulfuric acid by electrolytic anodic oxidation.

15. A process as claimed in Claim 14, wherein the persulfuric acid is recycled to said first treating zone.

16. A process according to any one of Claims 1 to 15, wherein the nitrite/nitrate salts are recovered.

17. A process according to Claim 16, wherein said alkaline aqueous medium is recycled to raise the concentration of said nitrite/nitrate salts to a predetermined minimum and then the nitrite/nitrate salts are recovered from said second zone product aqueous medium.

18. A process according to Claim 17, wherein said predetermined minimum is, for alkali metal nitrate, in the range of 2% to 45% based on the weight of the aqueous medium.

19. A process according to any one of Claims 1 to 18, wherein the alkali metal is potassium.

20. A process according to any one of Claims 1 to 19, wherein the alkali metal is provided in the second zone alkaline aqueous medium as an alkali metai hydroxide.

21. A process. according to Claim 20, wherein the alkali metal hydroxide is made by electrolysis of an alkali metal chloride.

22. A process according to Claim 21, wherein the hydrogen derived from the electrolysis of said alkali metal chloride is used to produce hydrogen peroxide.

23. A process according to Claim 22, wherein the hydrogen peroxide is made by the auto-oxidation of an alkylanthraquinone.

24. A process according to Claim 21, wherein said electrolysis also produces chlorine.

25. A process according to Claim 24, wherein said chlorine is converted to ethylene chloride.

26. A process according to any one of Claims 1 to 25, wherein the second zone product aqueous medium includes alkali metal nitrate, alkali metal nitrite, alkali metal carbonate, and alkali inetal bicarbonate.

27. A process according to Claim 26, wherein the pH of said second zone product aqueous medium is reduced to less than 9 so as to convert at least part of the alkali metal nitrite to alkali metal nitrate.

28. A process according to Claim 27, wherein the pH is reduced by adding carbon dioxide.

29. A process according to Claim 27 or Claim 28, wherein additional oxidant is added to said alkaline product aqueous medium.

30. A process according to any one of Claims 27 to 29, wherein the alkali metal nitrate is recovered from said second zone product aqueous medium.

31. A process according to any one of Claims 27 to 30, wherein alkali metal bicarbonate is converted to alkali metal carbonate and carbon dioxide after converting alkali metal nitrite to alkali metal nitrate.

32. A process according to Claim 31, wherein the carbon dioxide is recovered.

33. A process according to any one of Claims 30 to 32, wherein the second zone product aqueous medium is cooled to crystallize and recover the alkali metal nitrate therefrom.

34. A process according to Claim 33, wherein said second product aqueous medium is cooled to crystallize and recover alkali metal nitrite therefrom after crystallizing said alkali metal nitrate.

35. A process according to any one of Claims 1 to 12, wherein the second zone alkaline aqueous medium includes a carbonate or bicarbonate of an alkaline earth.

36. A process according to any one of Claims 1 to 35, wherein said gas stream also includes sulfur dioxide and sulfuric acid and nitric acid are formed in the product aqueous medium from said first zone.

37. A process according to Claim 36, wherein the first zone product aqueous medium is recycled until the concentration of nitric and sulfuric acids formed during said treating reaches a predetermined minimum.

38. A process according to Claim 36 or Claim 37, wherein alkali metal or alkaline earth metal sulfate is formed in said second zone is crystallized from said second zone product aqueous medium.

39. A process according to any one of Claims 1 to 35, wherein said gas stream also includes sulfur dioxide and wherein said gas stream is treated for removal of sulfur dioxide before treatment for removal of oxides of nitrogen.

40. A process according to Claim 1 substantially as hereinbefore described with reference to the examples given.

41. A process according to Claim 1 substantially as described herein with reference to the drawings.