NZ621235B2 - Improved nitric acid production - Google Patents

Abstract

Disclosed is a method for reducing the levels of nitrogen oxides in the tail gas (15) from a nitric acid production process by adding ozone at the final absorption stages of the absorber column (F) of the production process. Also disclosed is the enhancement of nitric acid production by adding a mixture of secondary air and oxygen (18) to the absorber column. ture of secondary air and oxygen (18) to the absorber column.

Description

ED NITRIC ACID PRODUCTION CROSS REFERENCE TO RELATED APPLICATIONS This application claims priority from US provisional application Serial Number 61/525,899 filed August 22, 2011.

BACKGROUND OF THE INVENTION The invention provides for lower of en oxides emissions from tail gas streams in nitric acid production process whereby nitric acid manufacturing is improved.

Nitric acid is generally manufactured by the high ature ion of ammonia over noble metal catalyst with air. Ammonia oxidation mainly results in the formation of NO as the s gas stream is cooled in the heat recovery equipment. During cooling, substantiai amounts of NO oxidizes to form N02 in the presence of oxygen in the process gas stream while some water vapor also condenses. This NO and N02 containing gas stream is ted with an aqueous medium in a r current fashion in multiple stages of absorption equipment to form an aqueous solution of nitric acid. Many reactions occur in the gas and liquid phase as well as during cooling, condensing and absorption in the equipment involved. Nitric acid absorption is a most complex industrially practiced absorption system. As process gas flows through multipies stages of gas-liquid contact, NOX concentration depletes gradually. In the final stages, NOX concentrations in the process gas stream are very Iow, typically iess than 0.5 % by volume and the scrubbing medium is process water (aqueous feed stream).

Temperature, pressure, and quid velocities are some of the PATENT Atty. Dkt. P11A039 important parameters which impact on the absorption and process in general thereby affecting the strength of nitric acid solution produced and the final concentration of NOx in the tail gas that can be attained. The tail gas from tion process, in plants operated at pressures higher than ambient are heated and energy from pressurized gas stream is recovered in the turbo er prior to exhausting to the atmosphere.

Generally plants operating at higher absorption pressure tend to have tower emissions. With increasing nmental concerns and stricter regulations, recently constructed plants tend to be rated for ions at higher pressures of 13 Bar gauge, while older plants, especially those built several decades ago were designed for operation at near ambient pressure. in industrially developed nations, most of the nitric acid production plants that operate at ambient pressure have been curtailed due to inherently higher NOX emissions.

Generally limits on allowable NOX emissions in the tail gas are defined in pounds of NOx emissions per ton of acid produced. Although some older plants with lower pressure tend to have higher NOX emissions, they are often not required to meet more recent and stricter standards as required for newly l There are a number of technologies and industriai practices for lowering NOx emissions from tail gas. t the more widely suggested are Selective Catalytic Reduction (SCR), Selective Non—Catalytic Reduction (SNCR), Non-Selective Non-Catalytic Reduction, scrubbing with alkaline solution to form nitrate/nitrite and extending aqueous scrubbing at low temperature around 4°C in the absorption column. The ion and practice of NOX emissions control method will vary from case to case depending on operating parameters of the PATENT Atty. Dkt. P11A039 nitric acid production s, regulatory mandates and economic attractiveness.

In recent years, modern nitric acid production plants operating at high re have lower temperature extended absorption section in the absorption equipment to lower nitrogen oxides emissions. The temperature of the absorption process is towered in the tail gas section of the equipment. In this extended absorption section, the tail gas is held for a longer period of time at tower ature compared to the rest of the absorption equipment and have achieved far lower emissions than those ically practiced. This approach is not effective at ng emissions iently with medium and lower pressure ses.

Therefore, low pressure and medium pressure processes tend to choose technoiogies based on reduction processes to achieve lower emissions.

These reduction based technologies (SCR, SNCR, etc.) require adding equipment downstream of the absorption process. The reduction processes require higher operating atures and therefore the tail gas stream must be heated am of the reduction process equipment. Nitric acid manufacturing is tightly heat integrated, i.e., the streams that need cooling provides heating to the streams that need heating and excess heat is used in ting steam that could be exported.

It is easier to accommodate and incorporate reduction processes within the heat envelope of nitric acid manufacturing for a new plant but it becomes costly when retrofitting an existing installation already used in production. The various challenges include re-engineering heat recovery and reconfiguring process design, accommodating additional equipment in the existing iayouts, ishing new process parameters for operations, loss of production during retrofits, startup and handiing of process disturbances. During normal operations and especially during startup and disturbances monitoring secondary ons of ng agent in the stack is an additional commitment for the life of the plant’s ion. The cost burden of retrofitting nitrogen oxides reduction process is even more nced for lower pressure, smaller capacity plants.

In order to reach very low NOX emissions, ozone based NOx oxidation of exhaust is one of the viable approach. US Pat. No. 5,206,002 s that for nitrogen oxides to be effectively removed, ozone is injected into the exhaust gas stream and allowed to mix and react by providing a residence time large enough to convert substantial parts of NO to N205 and then absorbing in an aqueous medium. The process described in this patent can be applied to the exhaust stream leaving the turbo expander prior to exhausting to the atmosphere.

However, that requires separate process equipments at substantial capital investment and will result in scrubber purge stream and wet stack.

Nitric acid is one of the basic low priced chemicai commodities used in process industry with a major share of its consumption in making fertilizers. The demand for fertilizers is cyclical and for smaller capacity plants, it is mes ically attractive to increase the production of nitric acid by enriching secondary air with oxygen. In some industrially deveioped countries, increasing nitric acid production capacity, even by oxygen enrichment, triggers environmental re~permitting process which may require implementing state of the art nitrogen oxides reduction technology or at a minimum keeping the total nitrogen oxides emissions within the permitted quota. [0012a] Any sion of the prior art throughout the cation should in no way be considered as an admission that such prior art is widely known forms part of common general knowledge in the field. [0012b] It is an object of the present invention to overcome or ameliorate at least one of the disadvantages of the prior art, or to provide a useful alternative.

The approach in preferred embodiments of the present invention is to integrate ozone based oxidation within nitric acid absorption system which not only provides flexibility in towering nitrogen oxides ons without making significant changes in the process or modifications to the equipment but also allows the nitric acid producer to focus on maximizing production with or without oxygen enrichment. in contrast all reduction based nitrogen oxides control technologies require alterations in the heat envelope or heat input and major process modification that will affect nitric acid tion.

Y OF THE INVENTION [0013a] According to a first aspect of the invention there is provided a method for removing contaminants from a tail gas stream of a nitric acid production process wherein nitric acid is recovered from an er coiumn comprising a multistage plate absorber column having from 20 to 70 plates, comprising adding ozone to said absorber column wherein nitrogen oxides are oxidized to form N205.

] According to a second aspect of the invention there is provided a method for removing inants from a tail gas stream of a nitric acid production process wherein nitric acid is recovered from an absorber column comprising feeding a process gas stream and an enhanced -containing stream into an absorber column and adding ozone to said absorber column wherein en oxides are oxidized to form N205. [0013c] According to a third aspect of the ion there is provided a method for producing nitric acid comprising the steps of: a) reacting ammonia in an ammonia ter; b) feeding reaction ts from step a) to a waste heat recovery unit; c) feeding the reaction products from step b) to a heat exchanger thereby heating said reaction products; d) feeding the reaction products of step c) to a cooler condenser thereby cooling said reaction products; e) feeding the cooied reaction products of step d) to an absorber column wherein nitric acid is ted from a tail gas; and f) g ozone to said absorber column to react with contaminants in said taii gas. [0013d] According to a fourth aspect of the invention there is provided a method for removing contaminants from a gas stream from an industrial process comprising the steps: a) mixing said gas stream with a stoichiometric excess amount of oxygen; b) feeding said gas stream mixture to a first packed column; 0) contacting said gas stream mixture with an s nitric acid solution; d) feeding the gas stream mixture of step c) to a second packed column and contacting with an aqueous nitric acid solution; e) contacting said gas stream mixture in said second packed column with ozone wherein nitrogen oxides are ed to form N205; and f) recovering said gas stream. [0013a] Unless the context y requires otherwise, throughout the description and the claims, the words “comprise", “comprising", and the like are to be construed in an inclusive sense as opposed to an exclusive or exhaustive sense; that is to say, in the sense of “including, but not d to".

In an embodiment, there is disclosed a method for removing contaminants from a tail gas stream of a nitric acid production process wherein nitric acid is recovered from an absorber column comprising adding ozone to the absorber column.

In another ment, there is disclosed a method for ng contaminants from a tail gas stream of a nitric acid production process wherein nitric acid is recovered from an absorber column comprising feeding a process gas stream and an enhanced -containing stream into an absorber column and adding ozone to the absorber column.

In a further embodiment, there is disclosed a method for ing nitric acid comprising the steps of: a) reacting ammonia in an ammonia converter; b) feeding reaction products from step a) to a waste heat recovery unit; 0) feeding the reaction products from step b) to a heat exchanger thereby heating the reaction products; d) feeding the on products of step c) to a cooler ser thereby cooling the reaction products; e) feeding the cooled reaction products of step d) to an absorber column wherein nitric acid is separated from a tail gas; and f) feeding ozone to the absorber column to react with contaminants in the tail gas.

The contaminants that are treated by the methods of the present ion are typically nitrogen oxides. The absorber column is typically a multistage absorber column that may also be a plate column having between about 20 to about 70 plates. The ozone will contact the nitrogen oxides in between the . The ozone may be added to the final stages of the absorber column after being raised in pressure to be approximately the same as the pressure of the absorber column. Oxygen enrichment may also occur by ucing oxygen into the absorber column.

Accordingly, the red embodiments of the invention addresses these ns by lowering nitrogen oxides emissions from tail gas streams in nitric acid production processes while intensifying the production of nitric acid.

The preferred embodiments of the invention offers advantages to the nitric acid production facility. No significant modifications need to be made to the nitric acid production process itself, or to the equipment used in nitric acid production. No modifications are necessary to heat recovery schemes or d equipment. The nitrogen oxides emissions are not only inhibited but are converted to incrementally increase the production of nitric acid. Any installations are relatively simple and controlled.

The nitrogen oxides emissions of preferred ments of the invention are lower than by other known PATENT Atty. Dkt. P11A039 techniques. These nitrogen oxides emissions can be d in the tail gas incrementaily as required by regulation by increasing the quantity of ozone added. Lastly there are no secondary emissions through the inventive use of ozone.

Nitrogen oxides concentrations in the tail gas g the absorption equipment are lowered by adding ozone in the final absorption stages where the tail gas is exhausted from the nitric acid absorption equipment. As such, very few additional processing equipment and minimal modifications of absorption ent enables reduction of en oxides trations in the tail gas to lower than t environmental regulations.

Nitrogen oxides in the final stages of the absorption equipment are oxidized by ozone to form N205. Oxidation of nitrogen oxides with ozone is several orders of magnitude faster than with . The spaces between plates or final stages provide adequate space for the desired conversion of NO to N205.

The solubility of N205 is high and results in complete dissolution in aqueous medium in the final stages. Absorption or dissolution of N205 forms nitric acid. in the absence of s acid formation there is no decomposition reaction occurring in the final stages and therefore, no desorption of NO. Nitrogen oxides absorbed are retained in the final stages as stable nitric acid.

BRIEF DESCRIPTlON OF THE DRAWlNGS Figure 1 is a schematic of a typical nitric acid production process.

Figure 2 is a schematic of a nitric acid production process integrated with an ozone oxidation system.

PATENT Atty. Dkt. P11A039 Figure 3 is a schematic of a nitric acid production process integrated with ozone ion and oxygen enrichment.

Figure 4 is a tic depicting the oxidation, absorption and desorption all occurring in a given stage.

Figure 5 is a schematic depicting the ion and absorption in final stages (tail gas section).

DETAILED DESCRIPTION OF THE INVENTION Turning to Figure 1, there is disclosed a schematic of a nitric acid production process. Air is fed through line 1 to ssor A which feeds the compressed air through iine 2 into ammonia converter B. Ammonia is fed through tine 4 to premix with air and the ammonia is subjected to oxidation at high temperature on a noble metal st surface present in ammonia converter 8. The oxidation reaction is highiy exothermic and converts ammonia into nitrogen oxides. The s gas stream leaving the ammonia converter B through line 5 essentially consists of nitrogen with the remainder oxygen, water in vapor form and oxides of nitrogen, particularly NO. The heat from the process gas stream leaving the a converter is recovered in waste heat recovery unit C to form a high pressure steam in line 6 and to heat the tail gas in heat exchanger D and further removed in the cooler condenser E. Here, the high temperature heat recovered as steam in line 6 may be exported to generate power or ed elsewhere within the process.

The process gas fed through line 7 through heat exchanger D then passes through line 8 which is further cooled in the cooler/condenser E where PATENT Atty. Dkt. P11A039 some of the water vapor t in the process gas stream condenses due to water feed to E through line 9. In the heat recovery and cooling section D and E en oxides in the process gas which is predominantly in the divalent form (mainly NO) oxidizes to tetravaient form (N02). The formation of N02 triggers formation of various other oxides such as N204, N203 and oxyacids (HN02 and HNOg) in the process gas stream. Water and oxyacids condense in the cooier condenser E and some nitrogen oxides dissolve in the condensate forming oxy acids. The condensate stream consisting of weak nitric and nitrous acid is collected and fed through line 12 to the appropriate stage in the absorption equipment column F.

In a low, medium or high re nitric acid production process, the process gas leaving the cooler is introduced through iine 10 in multistage absorption equipment such as a plate coiumn whereas atmospheric re s has multiple packed columns placed in series as absorption system.

A typical piate column has an excess of 20 and as many as 70 plates as gas—iiquid contacting stages. Air is suppiementaily added through line 11 to tine 10 to the cooied process gas stream to provide additional oxygen required for oxidizing NO(divaient nitrogen oxide) to N02 valent nitrogen oxide). Part of the supplementary air 18 is also bubbled through a bleacher section at the bottom section of the absorber coiumn F that holds product acid. The process gas stream is uced into the absorber column F at the bottom and rises upward progressively through contacting stages whiie aqueous stream of process water is introduced at the top of the column to flow downward. Nitric acid is formed in the s phase due to absorption of NOX. The spaces between plates provide ion reaction time for gas phase oxidation of NO to N02 s the gas-liquid contacting stage (plate) provides necessary surface area for gases to absorb into the aqueous phase.

PATENT Atty. Dkt. P11A039 The product nitric acid is recovered from the absorber column F through line 13 where it is directed to ent for further processing or to storage. The process gas stream entering absorber column F h line 10 undergoes absorption and oxidation reactions noted below and results finally in the tail gas . The tail gas exits the absorber column F through the top through line 15 to the heat exchanger D. The tail gas stream is indirectly heated by exchanging heat with the process gas stream entering through iine 7. The heated tail gas stream 17 is fed to turbo expander G where pressure energy from the gas stream is recovered and then the gas stream 8 is vented through stack.

A number of reactions occur both in the gas as well as the liquid phase. Please refer to publications by Suchak et al (1991, 1994). For sake of brevity, we have simptified the oxidation and absorption reactions as follows Cooling of NO in presence of oxygen s in oxidation of NO in the gas phase 6NO+302—>6N02 ..........................(1) N02 dimerizes to form N204 6 N02 m) 3 N204 .............................(2) When this gas is contacted with the aqueous liquid medium, the absorption of N204 and reaction with water forms HN03 and HN02 in the s liquid phase 3 N204 + 3 H20 ”9 3 HN03 + 3 HN02 .........................(3) PATENT Atty. Dkt. P11A039 HN02 being unstable in the aqueous liquid phase, decomposes into NO and Nitric acid. NO having very poor lity is released back to the gas phase. 3HNOTaHN03+2NO+HfiD ...............................(p Therefore adding reactions i~4 we get 6NO+302+3H20~e4HN03+2NO+H20 ...................... (5) As per reaction 5, oxidation and absorption of each mole of NO rates one third mole of NO again and released back in the gas phase.

Therefore when gas and liquid are contacted in a countercurrent fashion, the gas stream leaving carries NO released from the liquid due to decomposition on. Therefore in the multiple stage absorption equipment such as plate column, oxidation of NO occurs in the gas phase between two stages and decomposition of HN02 occurs in the liquid phase. Both absorption of N204 as well as desorption of NO occurs simultaneously when gas comes in liquid contact on the plate.

Figure 4 represents the oxidation, tion and desorption all occurring in a given stage.

Some of the contacting stages (plates) have cooling capability to remove ive heat released during absorption and to further promote oxidation of NO in the gas phase between the plates.

PATENT Atty. Dkt. P11A039 {0042] As the process gas stream approaches the final stages of the absorption , the concentration of NOx is significantly depleted and oxidation of low concentrations of NO by oxygen present in the process gas is not fast enough to ively convert divalent nitrogen oxides to tetravalent form in the space between the plates in the column. N02 deimerization to N204 is also limited at low concentration to effectively absorb in the s phase.

It is also known from kinetic data that oxidation of NO can be enhanced by lowering the temperature of the gas phase. Therefore, NO oxidation can be ed by lowering temperature of the absorption stage as it reaches low concentration and also by increasing the partial pressure of oxygen.

The concentration of oxygen in the s gas is ed by the total absorption pressure and stoichiometric excess air. For medium, low and t pressure absorption processes, oxygen concentration or partial pressure in the final stage is low and oxidation is slow. By increasing excess air. oxygen concentration increases but also results in an increase in total gas flow which reduces residence time available for NO oxidation to occur between two plates.

On the other hand, oxygen concentration can be elevated by oxygen enrichment by replacing some of the secondary air with gaseous oxygen. Lowering nitrogen oxides in the tail gas by elevating oxygen concentration is an expensive proposition due simply to the costs of producing the oxygen unless accompanied by production intensification. in high pressure processes, the partial pressure of oxygen is far greater than medium or low pressure processes and ng the temperature to 4°C in the final tion stages can extend absorption and further lower levels of nitrogen oxides in the tail gas.

PATENT Atty. Dkt. P1 1A039 Figure 2 represents a nitric acid tion process retrofitted with ozone oxidation. The numbering convention from Figure 1 is used up to the point of the ozone addition. Air is fed through line 1 to compressor A which feeds the compressed air through line 2 into ammonia converter 8. Ammonia is fed through line 4 to premix with air and the ammonia is subjected to oxidation at high temperature on a noble metal catalyst surface present in a converter B. The oxidation reaction is highly exothermic and converts ammonia into en oxides. The process gas stream leaving the ammonia converter B Arm/Mg through line 5 essentially consists of nitrogen with the remainder oxygen, water in vapor form and oxides of en, particularly NO. The heat from the process gas stream leaving the ammonia converter is recovered in waste heat recovery unit C to form a high pressure steam in line 6 and to heat the taii gas in heat exchanger D and to heat boiler feed water in the cooler condenser E. Here, the high temperature heat recovered as steam in line 6 may be exported to generate power or utilized elsewhere within the process. [0047} The s gas fed through line 7 through heat ger D then passes through line 8 to the cooler/condenser E where some of the water vapor present in the process gas stream condenses due to g water feed to E through line 9. In the heat recovery and cooling n D and E nitrogen oxides in the process gas stream which are predominantly in the divalent form (mainly NO) oxidizes to tetravalent form (N02). The formation of N02 triggers formation of various other oxides such as N204, N203 and oxyacids (HN02 and HNOg) in the s gas stream. Water and oxyacids condense in the cooler condenser E and some nitrogen oxides dissolve in the condensate forming oxy acids. The condensate stream consisting of weak nitric and nitrous acid is collected and fed through line 12 to the appropriate stage in the absorption equipment column F. [0048} in a low, medium or high pressure nitric acid production process, the PATENT Atty. Dkt. P11A039 process gas leaving the cooler is uced h line 10 in multistage absorption ent such as a plate column whereas atmospheric pressure process has multiple packed columns placed in series as absorption system.

A typical plate column has an excess of 20 and as many as 70 plates as gas-liquid contacting stages. Supplemental air is added through line 11 to line to the cooled s gas stream to provide additional oxygen required for oxidizing NO(diva|ent nitrogen oxide) to N02 (tetravalent nitrogen oxide). Part of the supplementary air is also bubbled through a bleacher section at the bottom section of the absorber column F that holds product acid. The process gas stream is introduced into the er column F at the bottom and rises upward progressively through contacting stages while aqueous stream of process water is introduced at the top of the column to flow downward. Nitric acid is formed in the aqueous phase due to absorption of NOx. The spaces between plates provide oxidation reaction time for gas phase oxidation of NO to N02 whereas the quid contacting stage (plate) provides necessary surface area for gases to absorb into the aqueous phase.

The product nitric acid is recovered from the absorber column F my. it is directed to equipment for further processing or to _ through line 13 where storage. The process gas stream entering absorber column F through line 10 undergoes absorption and oxidation reactions 1 to 5 summarized above until final tion stages where ozone wiil be introduced h line 16. The tail gas exits the absorber column F through the top through line 15 to the heat exchanger D. The tail gas stream is indirectly heated by ging heat with the process gas stream entering through line 7. The heated tail gas stream 17 is fed to turbo expander G where pressure energy from the gas stream is recovered and then the gas stream 3 is vented through stack.

PATENT Atty. Dkt. P11A039 Ozone is generated from oxygen in a typical ozone generation unit (not shown) and if necessary, the pressure of the ozone containing oxygen is raised by compressor to the pressure of the tion equipment. Ozone containing gas stream is introduced in the final absorption stages of the absorption column F through line 16.

The oxidation of NC with ozone is several orders of ude faster than that when oxygen is employed alone. The space between two plates in the ///// finai stages of the absorption equipment provides the ed residence time for the oxidation of the nitrogen oxides to N205 valent form). Since pentavalent forms are highly soluble, they dissolve almost instantly in water. The pentavalent form selectively forms nitric acid in the aqueous medium. Since absorption of nitrogen oxides is through the alent form, HNOZ formation in the liquid phase by absorption of tetravalent form of NOx and decomposition to evolve NO is completely ted in the final stages of absorption making the absorption extremely effective in lowering nitrogen oxides leaving the tail gas section. In the case where ozone is introduced below the second plate from the top, the ozone oxidation process will occur in spaces between the second and third plate and first and second plate whereas absorption of nitrogen oxides as pentavalent oxides will occur on the second and first plate. Unlike the techniques taught in US Pat. No. 5,206,002, all oxidation of nitrogen oxides need not occur in the gas phase at once but occur into multiple gas spaces between the absorption stages.

This solution further does not require any modification to the HN03 acid absorption equipment except for introducing ozone by line 16 into the vapor space. Ozone can also be introduced in the aqueous medium by either ging line 16 in the liquid pool over the plate(not shown) or removing liquid from the plate in pump around ioop (not shown) so the operator has a choice of PATENT Atty. Dkt. P11A039 means to input ozone into the final stages of the absorption column.

This solution is desirable for smaller capacity low, medium and heric pressure nitric acid production facilities as production can be enhanced while tail gas nitrogen oxides emissions are maintained within environmental limits.

Nitrogen oxides oxidation with ozone follows several reaction paths to arrive at N205 2NO+203—>2N02+202 ..................... (6) N02 + 03 —-> N03 + 02 ...................... (7) N02 + N03 —-> N205 ..................... (8) The absorption of N205 in the aqueous phase will result in the formation of nitric acid: N205 + H20 —-> 2 HNOS ................(9) Summing up equations (5) h (9) 2NO+303+H20—>2HN03+302 ......................... (10) Lowering the nitrogen oxides level from the tail gas wiil result in an incremental increase of the ty of acid in the aqueous stream.

As stated elsewhere, in contrast to oxidation with 02, the oxidation with PATENT Atty. Dkt. P11A039 ozone does not form HNOZ in the aqueous phase and therefore no NO desorption occurs.

Figure 5 represents the oxidation and absorption in finai stages (tail gas section).

Secondary emissions are also inhibited as excess ozone is absorbed in the aqueous phase or destroyed when tail gas is heated prior to entering the expander G. in a different embodiment of the invention, enhanced nitric acid production is possible white continuing to inhibit nitrogen oxides emissions. This is achieved by replacing a portion or up to all of the secondary air with oxygen.

Typically, oxygen enrichment provides the required oxygen for the bulk of the conversion of NO to HNOa; r, higher en oxides emissions in the tail gas results. ing ozone feed in the tail gas section of the absorption unit with oxygen enrichment in the ary air feed provides an intensified tion of nitric acid t increasing nitrogen oxides emissions.

As noted in Figure 3, the secondary air line 11 is replaced with a secondary airline and oxygen teed attachment which allows for feed of a combination of secondary air and oxygen with options to feed up to 100 percent oxygen content. The remainder of the numbering is the same as used in Figure In another embodiment of the invention, lowering of NOx emissions in the tail gas is possible in the near ambient pressure nitric acid s where the process gas is scrubbed in series of packed columns. While gas stream flows through series of packed column, it is contacted with aqueous nitric acid PATENT Atty. Dkt. P11A039 ons progressively of weaker ths. t acid is withdrawn from the sump of first packed coiumn and the aqueous weaker nitric acid from the 2nd packed column replenishes the displaced volume in the sump of the first column.

The sump in the 2nd column is replenished by the aqueous nitric acid stream in the sump of 3'“ column. The sump of the final packed column is replenished with process waterfeed. Ozone is added in the final packed column to remove NOx as bed in equations (6) to (10). The gas stream g the final packed column is directed to the stack since there in atmospheric pressure column there is no pressure energy to be recovered. Oxygen enrichment may be done by feeding oxygen to the secondary air supplied to the first packed column. in another embodiment of the invention, NOx ning stream is arising from industriai process other than nitric acid manufacture such as nitric acid oxidation of c material or processing of substances with nitric acid or processing materials where NOx is formed in the process. NOx emissions from such a process stream can be lowered with effective recovery of nitric acid using gaseous oxygen to oxidize in series of packed columns. Gas stream is admixed with stoichiometric excess amount of oxygen. While gas stream flows through series of packed column, it is contacted with aqueous nitric acid solutions progressively of weaker strengths. The recovered nitric acid is withdrawn from the sump of first packed column and the aqueous weaker nitric acid from the 2nd packed column replenishes the displaced volume in the sump of the first column.

The sump in the 2nd column is replenished by the aqueous nitric acid stream in the sump of 3‘” column. The sump of the final packed column is replenished with process water feed. Ozone is added in the final packed column to remove NOx as described in equations (6) to (10). The gas stream g the final packed column is directed to the stack with significantly reduced level of NOx with most in the form of recovered nitric acid. The number of packed columns in series can be preferably 2 to 6. They may be stacked on top of one another for sake of PATENT Atty. Dkt. P11 A039 simple gravity overflow. instead of packed column, any other gas liquid contacting device can aiso be used.

While this invention has been described with respect to particular ments f, it is apparent that numerous other forms and modifications of the invention will be obvious to those skilled in the art. The appended claims in this invention generally should be construed to cover ail such obvious forms and modifications which are within the true spirit and scope of the invention. «mix

Claims (38)

1. A method for removing contaminants from a tail gas stream of a nitric acid production s wherein nitric acid is recovered from an absorber column comprising a multistage plate absorber column having from 20 to 70 plates, comprising adding ozone to said absorber column wherein nitrogen oxides are oxidized to form N205.

2. The method as claimed in claim 1 wherein said contaminants are selected from the group consisting of nitrogen oxides.

3. The method as claimed in claim 1 or claim 2 wherein said ozone contacts said nitrogen oxides in between said plates.

4. The method as claimed in any one of claims 1 to 3 further comprising ucing oxygen into said absorber column.

5. The method as claimed in any one of claims 1 to 4 wherein said ozone is introduced into a final stage of said absorber column.

6. The method as claimed in any one of claims 1 to 5 wherein said ozone is raised to the pressure of said absorber column.

7. A method for removing contaminants from a tail gas stream of a nitric acid production process n nitric acid is recovered from an absorber column comprising feeding a process gas stream and an ed -containing stream into an er column and adding ozone to said absorber column wherein nitrogen oxides are oxidized to form N205.

8. The method as claimed in claim 7 wherein said contaminants are selected from the group ting of nitrogen oxides.

9. The method as ciaimed in ciaim 7 or claim 8 wherein said absorber column is a multistage absorber column.

10. The method as claimed in any one of claims 7 to 9 wherein said absorber column is a plate column having from 20 to 70 plates.

11. The method as claimed in claim 10 wherein said ozone contacts said nitrogen oxides in n said plates.

12. The method as d in any one of claims 7 to 11 further comprising introducing oxygen into said absorber column.

13. The method as claimed in claim 12 wherein said ozone is introduced into a final stage of said absorber column.

14. The method as claimed in any one of claims 7 to 13 wherein said ozone is raised to the pressure of said absorber column.

15. The method as claimed in claim 14 wherein said er column is a series of packed absorber s.

16. The method as claimed in claim 15 wherein ozone is added to a final packed absorber column in said series of packed absorber columns.

17. A method for producing nitric acid comprising the steps of: a) reacting ammonia in an ammonia converter; b) feeding reaction products from step a) to a waste heat recovery unit; 0) feeding the reaction products from step b) to a heat exchanger thereby heating said on products; d) g the reaction products of step c) to a cooler condenser thereby cooling said reaction products; e) feeding the cooled reaction products of step d) to an absorber column wherein nitric acid is separated from a tail gas; and f) g ozone to said absorber column to react with contaminants in said tail gas.

18. The method as claimed in claim 17 wherein said inants are selected from the group consisting of nitrogen oxides.

19. The method as claimed in claim 17 or claim 18 n said absorber column is a multistage absorber coiumn.

20. The method as claimed in claim 19 wherein said absorber column is a plate column having from 20 to 70 plates.

21. The method as ciaimed in claim 20 wherein said ozone contacts said nitrogen oxides in between said plates.

22. The method as claimed in any one of claims 17 to 21 further comprising introducing oxygen into said absorber column.

23. The method as claimed in any one of claims 17 to 22 wherein said ozone is introduced into a finai stage of said absorber column.

24. The method as claimed in any one of claims 17 to 23 wherein said ozone is raised to the pressure of said absorber column.

25. A method for removing contaminants from a gas stream from an rial process comprising the steps: a) mixing said gas stream with a stoichiometric excess amount of oxygen; b) feeding said gas stream e to a first packed column; 0) contacting said gas stream mixture with an aqueous nitric acid solution; d) feeding the gas stream mixture of step c) to a second packed column and contacting with an aqueous nitric acid solution; e) contacting said gas stream mixture in said second packed column with ozone wherein nitrogen oxides are oxidized to form N205; and f) ring said gas stream.

26. The method as claimed in ciaim 25 wherein said industrial process is selected from the group consisting of nitric acid ion of organic al and processing of substances with nitric acid.

27. The method as claimed in claim 25 or claim 26 wherein said contaminants are selected from the group consisting of nitrogen oxides.

28. The method as claimed in any one of claims 25 to 27 wherein said nitric acid in said second packed column has a lower concentration than said nitric acid in said first packed column.

29. The method as claimed in any one of ciaims 25 to 28 wherein two to six packed columns are used.

30. The method as claimed in any one of claims 25 to 29 wherein a sump in said first packed column is replenished with aqueous nitric acid from said second packed column.

31. The method as claimed in any one of claims 25 to 30 wherein a sump in said second packed column is ished with s feed water.

32. The method as claimed in any one of claims 25 to 31 wherein said packed column are in .

33. The method as claimed in claim 32 wherein said packed columns are stacked ally.

34. A nitric acid produced by the method of any one of claims 17 to 24.

35. A method for removing contaminants from a tail gas stream of a nitric acid production process substantially as herein described with reference to any one of the embodiments of the invention illustrated in the accompanying drawings and/or examples.

36. A method for producing nitric acid substantially as herein described with reference to any one of the embodiments of the ion illustrated in the accompanying drawings and/or examples.

37. A method for ng contaminants from a gas stream from an industrial process ntially as herein described with reference to any one of the embodiments of the invention illustrated in the accompanying drawings and/or examples.

38. A nitric acid produced by the method substantially as herein described with reference to any one of the embodiments of the invention illustrated in the accompanying drawings and/or examples.