FIELD OF THE INVENTION

The present invention relates to a method for breaking the NO to N0 ₂ cycle in the production of nitric acid and the recovery of nitric oxide, and more particularly to apparatus and methods for the recycle of N0 _X gas, recovery and the production of nitric acid.

BACKGROUND OF THE INVENTION

The mathematically infinite cycle of gas phase oxidation of nitric oxide (NO) to nitric dioxide (N0 ₂ ) has dominated nitric acid production and the recovery of nitric oxide

The nitric oxide to nitric dioxide cycle, which has been the limiting reaction for all nitric acid processes since processes based upon the catalytic oxidation of ammonia began, is described in the following equations:

(1) 4NH ₃ (g) + 50 ₂ (g) ---> 4NO(g) + 6H ₂ 0(g) (2) 2NO(g) + 0 ₂ (g) ---> 2N0 ₂ <g)

(3) 3N0 ₂ (g) + H,0(1) - - - > 2HN0 ₃ (1) + N0(g)

Equations (2) and (3) describe the cycle eliminated by the present invention As can be seen from equations (2) and (3) , the reaction of every three moles of N0 ₂ with water to form two moles of nitric acid return one of the three moles of oxidized nitrogen as

NO This, of course, requires a reoxidation of the NO with an additional 0 ₂ from air or other source.

Mathematically, one-third of the oxidized nitrogen has to be reox dized with oxygen every time that two-thirds are reacted with water to form nitric acid.

The economic and environmental problems created by the currently practiced gas phase oxidation of nitric oxide (NO) have dominated and shaped the nitric acid industry since its inception

in 1908 when Professor Ostwald piloted the first production of nitric acid based on the catalytic oxidation of ammonia (NH ₃ ) to nitric oxide (NO)

There are three variables controlling the gas-phase oxidation or reoxidation of NO to N0 ₂ , Eq. (2) They are:

a. NO concentration vs. 0 ₂ concentration; b. Temperature: the reaction time decreases with lower temperatures; c Pressure the need to achieve a termolar reaction requiring two molecules of NO and one molecule of 0 ₂ decreases the time required for the oxidation of NO w th 0 ₂ to the third power of the pressure in which the reaction occurs.

This has resulted in the industry' s development and use of expensive high pressure plants

As currently practiced, these reactions require the return of one-third of the nitric dioxide to the gas phase as nitric oxide (NO) , Eq. (3) , which then re-requires the termolar reaction to nitric dioxide (N0 ₂ ) , Eq. (2), and then an additional liquid phase reaction to convert two-thirds of this N0 ₂ to nitric acid (HN0 ₃ ) , with one-third of the entering N0 ₂ again being returned to the gas phase as NO for an additional gas phase reaction, and so on

This currently used series of reactions is an exercise in commercially striving to reach infinity. Of course, in the current practice of the art of nitric acid manufacture, economics dictate that at some point in this infinite series of reactions, whenever enough of the nitric oxides have been converted to nitric acid so that their further recycle adversely affects the economies of the further recycle process, they are wasted This results m atmospheric discharge of NO _x .

In US 3,991,167, Depom ier et al of the firm Produits Chimiques Ugine Kuhlmann point out that current nitric acid from ammonia produces exhaust gases containing from 1,000 to 2,000 cm ³ of nitrogen oxides per cubic meter of effluent while recent legislation

seeks to impose a limit of about 200 cm ³ of nitrogen oxides per cubic meter of ta l gases

They further add that the progressive process, Equation (3), previously described makes it "extremely difficult to absorb the last traces of nitrogen oxides in the absorption system conventionally used in manufacturing nitric acid" (column 1, lines

61-63) .

Attempts to lower the amounts of released nitrogen oxides by mere extension of the absorption system are fraught w th difficult technical problems. Also, the additional installations would entail considerably increased investments (Column 2, lines 1-6) .

Depommier continues (Column 2, lines 15-48) to outline many of the difficulties existing with current nitric oxide emission control processes

Typical discharge rates from such plants are about 3.9 MT (metric ton) of gases being discharged per each MT of 100% HN0 ₃ produced The discharged gases contain a typical concentration of 0.02% to 0.20% NO _x .

With worldwide nitric acid production for 1985 estimated at greater than 30 million MT per year, this represents an atmospheric discharge of 23,400 to 234,000 MT of NO _x per year

Because of environmental regulations and the fact that such discharges are often marked with a disturbing reddish-brown color, most nitric acid plant discharges in the U.S.A. and m other environmentally conscious parts of the world are being treated either to obscure the discharge or to react the discharged NO _x into another chemical form.

Earlier, the most common treatment was catalytic reaction with excess natural gas which served to reduce, dilute, and disperse the discharged nitric oxides, which made the reddish-brown fume invisible and added unreacted natural gas to the atmosphere

Current technology is an expensive add-on consisting of ammonia gas, which is added in excess and catalytically reacted with the N0 _X to form nitrogen and ammonium nitrate, which is kept at a sufficiently high temperature to prevent a visible white fume of ammonia nitrate, as it is being discharged into the atmosphere An excess of ammonia is usually required for this reaction which also discharges chemicals into the atmosphere

In addition to the above economic and environmental losses caused by the nitrogen dioxide to nitric oxide cycle, there is also the economic burden caused by Eq. (2) in which two moles of NO are required to contact one mole of oxygen in order to form two moles of N0 ₂ . This also is repeated again and again by the cycle.

In applying Eq. (2) to obtain a sufficiently close contact to effect a reasonably economic rate of molecular reactions between nitric oxide and oxygen, plants are now designed and operated at elevated pressures The attainment of these pressures requires expensive turbo compressor sets and expensive high pressure stainless steel construction of all equipment under pressure.

Freitag and E. Scheibler, who are experts from the Uhde Co., one of the largest and the oldest suppliers of nitric acid plants and processes in the world, state in Handbook of Chemical Production Processes. Robert A. Meyers, editor, p. 3.6-24, 1986, under their description of "Uhde Nitric Acid Processes" that such turbo compressor sets represent 25-30% of the total cost of a conventional nitric acid plant

The elevated pressures used to increase the reaction rate also causes greater losses of the expensive platinum catalyst and lower efficiencies in ammonia oxidation and conversion to nitric acid.

Typical Performance Figures for NH ₃ Combustion, provided by W Freitag and E. Scheibler, in their report on Uhde Nitric Acid

Processes, previously mentioned, g ve platinum catalyst consumption at 55 mg/MT HN0 ₃ produced at one atmosphere pressure (0.1 MPa abs. ) and ₂ 80 mg/MT HN0 ₃ produced at ten atmospheres of pressure (1.0 MPa abs.) Some of this platinum can be recovered in downstream filters but in all cases platinum costs are substantially higher at the

higher operating pressures which are used in current nitric acid production.

Ammonia conversion to nitric oxide and subsequently to nitric acid suffers also. In the reference cited above, the conversion of ammonia to nitric oxide (NH ₃ → NO) percent drops from 97.5% at 0.1

MPa abs. to 94.0% at 1.0 MPa abs., a loss of 3.5%.

Experts and theory teach that dissolved trivalent nitrogen dissolved in nitric acid cannot be practically oxidized by oxygen alone or in air to additional nitric acid.

This is cogently expressed by Bernard J. Lerner of the Beco

Engineering Company in US Patent N ⁹ 5,017,348, column 4, lines 49-55:

The singular and consistent deficiency of the prior art discussed above is the failure to recognize that treating HN0 ₂ in solution with air or oxygen in an open system will remove from solution the very reactants, HN0 ₂ and NO, that require liquid-phase oxidation. This causes continuous, open-ended removal of NO from the solution, whether air or pure oxygen is used.

SUMMARY OF THE INVENTION

The invention provides a method for rapidly removing nitric oxide (NO) from the gas phase by reaction with catalyzed nitric acid. The reaction forms nitric trioxide (N ₂ 0 ₃ ) in solution.

4NO(g) + 2HN0 ₃ (aq) > 3N ₂ 0 ₃ (aq) + H ₂ 0

The homogeneous catalyst required is dissolved nitrite (N0 ₂ ^" ) ion. When nitrite ion is present, the reaction is both rapid and quantitative.

Although most texts on nitric acids and nitric oxides state that N ₂ 0 ₃ is rare or difficult to form, the present invention produces it rapidly and at such high concentrations that, if desired, one can form a concentrated dark blue, ink-like solution of N ₂ 0 ₃ in nitric acid and water.

The other part of the present invention which breaks the NO to N0 ₂ gas phase cycle is the discovery that, in the above solution, one can achieve rapid, quantitative oxidation of the N ₂ 0 ₃ m the liquid phase rather than the conventional gas phase

(4) N ₂ 0 ₃ (aq) + 0- (g) + H ₂ 0 > 2HN0 ₃ (aq)

Unlike current practice, there is no return of NO or N0 ₂ back to the gas phase The cycle is broken

It is the ability of the present invention to obtain high N ₂ 0 ₃ in solution and effect a rapid liquid phase reaction of N ₂ 0 ₃ with 0 ₂ , that allows the process to be operated so that 0 ₂ entering the N ₂ 0 ₃ aqueous solution is completely reacted, so that no 0 ₂ (g) escapes the liquid surface

Absorption and reaction rates are so efficient and rapid that the process can be effected with equipment as simple as a spray or inductor to capture the NO, with subsequent or cocurrent dispersion of 0 ₂ gas into the resulting liquid, resulting in nitric acid from

NO, 0 ₂ , and H ₂ 0

The expensive pressure and multi-mass-transfer equipment, such as absorption columns, packed towers, bubble caps, etc that is currently required for nitric acid production and nitric oxide recovery, is not required when practicing this invention

OBJECTS OF THE INVENTION

The principal object of the invention is to provide an improved method of effectively and economically recycling N0 _X nto nitric acid.

A further object of this invention is to provide a method of breaking the mathematically infinite cycle of gas phase oxidation of nitric oxide (NO) to nitric dioxide (N0 ₂ )

Another ob _j ect of the invention is to provide a process for the recovery of nitric acid without the evolution of the gaseous nitrogen oxides to the environment.

Another object of the invention is to provide a process for increasing the concentration of nitric ac d without the need for evaporation.

Another object of the invention is to provide a process for forming a trivalent nitrogen in solution which can be rapidly reacted with gaseous oxygen.

Still another object of the invention is a process for the economical production and manufacture of nitric acid at atmospheric pressure while reducing or eliminating harmful environmental emissions

DETAILED DESCRIPTION

Part of the invention is the discovery that one can rapidly remove nitric oxide (NO) from the gas phase by reaction with catalyzed nitric ac d. The reaction forms trivalent nitrogen in solution.

4N0(g) + 2HN0 ₃ (aq) ---> 3N ₂ 0 ₃ (aq) + H ₂ 0

The homogeneous catalyst is dissolved trivalent nitrogen (N0 ₂ ) ion When sufficient trivalent ion is present, the reaction is both rapid and quantitative

Although most texts on nitric and nitric oxides state that N ₂ 0 ₃ is rare or difficult to form, the present invention produces it rapidly and at such high concentrations that, if desired, one can form a dark blue, ink-like solution of N ₂ 0 ₃ in nitric acid and water

The other part of the present invention which breaks the NO to N0 ₂ gas phase cycle is the discovery that in the above solution, one can achieve rapid, quantitative oxidation of the N ₂ 0 ₃ n the liquid phase with dissolved oxygen rather than the conventional gas phase

( 4 ) N ₂ 0 ₃ + 0 ₂ + H ₂ 0 - - - > 2HN0 ₃

The maintenance of excess to stoichiometπc trivalent nitrogen oxides in solution allows rapid oxygen addition with no need to release oxygen into the gas phase

Unlike current practice, there is no return of NO or N0 ₂ back to the gas phase. The cycle is broken.

It is the present invention's ability to obtain high N ₂ 0 ₃ m solution and effect a rapid liquid phase reaction of N ₂ 0 ₃ with 0 ₂ The reaction allows the process to be operated so that no 0 ₂ enters the gas phase and none is required in the gas phase.

As is apparent, the continued supply of N0 _X (NO + N0 ₂ ) into the scrubbing nitric acid and its oxidation to additional nitric acid raised the concentration of the combined nitric acids.

Absorption and reaction rates are so efficient and rapid that the process can be effected with equipment as simple as a spray or inductor to capture the NO, followed by dispersion of 0 ₂ gas into the resulting liquid, resulting in nitric acid from NO, 0 ₂ , and H ₂ 0.

The expensive pressure and multi-mass-transfer equipment, such as absorption columns, packed towers, bubble caps, etc. that is currently required for nitric acid production and nitric oxide recovery, is not required when practicing this invention. Of course, absorption columns, packed towers, bubble caps, and other mass transit equipment can be used but when applied using the present invention, they can significantly be reduced in both cost and complexity.

The invention is a process which consists of the following steps :

1) Establishment and maintenance of a proper dissolved N ₂ 0 ₃ /N0 ₂ ^" level in a nitric acid scrubbing solution.

NO Option: 1.0 to 20.0 g/1 as reacted NO NO Range: 0.1 to 40 0 g/1 as reacted NO

HN0 ₃ Option: 15 to 55% HN0 ₃ HN0 ₃ Range: 5 to 65% HN0 ₃

2) Removal of NO from gas phase by reaction with HN0 ₃ .

3) Maintenance of NO in optimum range by reaction with 0 ₂ .

N ₂ 0 ₃ + 0 ₂ + H ₂ 0 → 2 HN0 ₃

If the N ₂ 0 ₃ (dissolved NO) is maintained in the optimum range, the reaction of NO with HN0 ₃ is rapid and quantitative and only a single or at most several mass transfer stages are required. The same is true of the 0 ₂ reaction.

When practiced as described, NO gas is immediately and quantitatively reacted into the liquid phase. The resulting solution is maintained with an excess of reacted NO.

When oxygen contacts this solution, it is immediately and quantitatively reacted with the dissolved nitric oxide (NO) to form nitric acid (HN0 ₃ ) . As practiced, no 0 ₂ (g) entering the N ₂ 0 ₃ solution is discharged from the solution into the gas phase. The ultimate oxidation of NO to HN0 ₃ totally occurs in the liquid phase.

If N0 ₂ is present either from the process which is evolving the N0 _X or by gas phase oxidation where oxygen is present in the gases being supplied to the present invention, it is converted to HN0 ₃ and NO by the water present in the scrubbing HN0 ₃ /N0 ₂ solution as in Eq. (3) . The resulting NO is reacted with the nitric and is oxidized in the liquid phase by the oxygen supplied. If desired, oxygen can be withheld and the resulting N ₂ 0 ₃ /HN0 ₃ solution can be stored or transferred to another site for oxidation to nitric acid at any convenient time.

ALTERNATIVE EMBODIMENTS

Alternatively this process for generating nitric acid (HN0 ₃ ) can be described as comprising the steps of: one, reacting a +2 oxidation gaseous nitrogen (NO) , as shown in Table l, with a +5 oxidation nitrogen in solution (HN0 ₃ ) to obtain a +3 oxidation nitrogen in solution (N ₂ 0 ₃ ) ; and two, reacting the +3 oxidation nitrogen (N ₂ 0 ₃ ) , in the solution, with oxygen (0 ₂ ) and water (H ₂ 0) to produce a ₊ 5 oxidation nitrogen (HN0 ₃ ) while maintaining a level of a ₊ 3 oxidation nitrogen (N ₂ 0 ₃ ) in excess of added oxygen (0 ₂ ) . The oxidation state also can be referred to as the valence state.

Table l

EXAMPLES

Apparatus is set up with a source of nitric oxides and two or three nitric absorption vessels consisting of one-liter reactors containing nitric acid containing trivalent nitrogen.

The scrubbing or reactant nitric acid is continuously circulated through a laboratory inductor. This supplies the energy required to circulate the nitric oxides through the vessels.

The vessels are connected in a continuous loop.

EXAMPLE 1

An excess of arsenic trioxide (As-0 ₃ ) is added to 4.536 Kg of 50% nitric acid HN0 ₃ . The resulting nitric oxide (NO) is drawn through three spray scrubbers, the spray being a trivalent nitrogen containing solution of 20% HNO, .

As the trivalent nitrogen content of the scrubbing nitric builds from its initial level of 3.2 g/1 expressed as NO, gaseous oxygen is reacted with the scrubbing solution by adding it directly to the scrubbing solution.

Oxygen addition is adjusted so as to maintain an excess of trivalent nitrogen in solution. No oxygen escapes into the head space .

As this cycle continues, the collected and oxidized nitric oxide increases the concentration of the scrubbing nitric.

Recovered nitric is 2.268 Kg as 100% HN0 ₃ or 99.17% of the supplied NO has been converted to nitric acid by the invention. The addition of this 2.268 Kg to the initial scrubbing nitric raises its concentration to 47.8%.

EXAMPLE 2

A gaseous mixture of NO _x with stoichiometric oxygen was passed through two reactors as described above.

The total amount of nitric oxides fed into the system was

63.86 grams expressed as nitric acid.

The amount of nitric oxide recovered as nitric acid was 65.63 g., which was a recovery of 102.78%, essentially full recovery.

In the first absorber, 97.8% of the nitric oxide was recovered and 2.2% was recovered in the second absorber.

SUMMARY OF THE ACHIEVEMENT OF THE OBJECTS OF THE INVENTION

From the foregoing, it is readily apparent that I have invented an improved method that is both environmentally and economically effective for nitric acid production and for recycling

NO _x into nitric acid. This method breaks the mathematically infinite cycle of gas phase oxidation of nitric oxide (NO) to nitric dioxide (N0 ₂ ) and back to nitric oxide when nitric dioxide (N0 ₂ ) is reacted with water to nitric acid. Further, this invention provides a process for the recovery and production of nitric acid without the evolution of the gaseous nitrogen oxides into the environment.

It is to be understood that the foregoing description and specific embodiments are merely illustrative of the best mode of the invention and the principles thereof, and that various modifications and additions may be made to the apparatus by those skilled in the art, without departing from the spirit and scope of this invention.