JPH1129320A - Nitrogen fixing method in closed system and device for the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method safely fixing nitrogen as ammonium nitrate under a low pressure in a closed system. SOLUTION: Hydrogen obtained by electrolysis of water and nitrogen 1 are processed in a low pressure ammonium synthesis at less than 10 barr, the reaction mixture containing unreacted gas is separated into a gas mixture rich in ammonium and the other gas rich in nitrogen and hydrogen by PSA 6. The gas rich in nitrogen and hydrogen is circulated to ammonium synthesis 5, and a part of the gas rich in ammonium is collected as ammonia water, the catalytic oxidation of the rest and the oxygen generated by electrolysis of water to NO and NO2 us carried out at the ammonium oxidizing tower 8, the reaction mixture is quenched in a quenching tower 9 having a choke up inhibiting system for start.up and shut.down of the ammonium oxidizing tower. The NO2 is absorbed by water in the absorbing tower 10 affording nitric acid, a part of the gas containing No discharged from the NO2 absorbing tower 10 is circulated to ammonium oxidizing tower 8, and the rest is discharged to outside of the system, the ammonium nitrate is produced from ammonia water and nitric acid.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for fixing nitrogen in a closed system and an apparatus for the same, and more particularly to a method for fixing nitrogen with a minimum discharge to the outside of the system and an apparatus therefor.

[0002]

2. Description of the Related Art The act of animals including humans ingesting plants as nutrients as food, while excreting excrement, has been repeated historically. This action is in the so-called liberation system.

In a closed system including the ground, the outer space, and the like, since a self-contained process is required, a so-called closed system is required. It goes without saying that in this closed system, nitrogen is also one of the objects. As an example, a treatment method in which nitrogen is converted into ammonia, nitric acid, or the like, or converted into ammonium nitrate and supplied to plants as fertilizer is considered.

In the release system, in an ammonia synthesis process, for example, an iron-based catalyst is used at 300 ° C. and 300 ° C.
It is well known that it is synthesized under high temperature and high pressure of bar. In the process of synthesizing nitric acid, for example, ammonia is converted to NO at a normal pressure of less than 10 bar and a temperature of 800 ° C. to 900 ° C. with a platinum-Rh-based catalyst, and the ammonia is cooled to about 100 ° C. to almost completely reduce NO. N
It is oxidized to O ₂ and NO ₂ is absorbed in water to obtain nitric acid.

[0005] In a closed system requiring self-completion, the process for synthesizing ammonia and the process for synthesizing nitric acid do not necessarily need to be performed at high temperature and high pressure. Rather, they are not regulated by the High Pressure Gas Control Law from the aspect of safety.
It may be desirable to operate at low pressures below bar and at low temperatures.

[0006] In these processes, the above-mentioned release system emits gas or wastewater below the environmental regulation value. It is also possible to organically combine these processes to produce ammonium nitrate on a large scale.

[0007]

However, when used in a closed system, the amount of nitrogen to be treated per day is small, the installation area and the installation volume are limited due to space limitations, and gas and drainage are almost completely eliminated. There are various restrictions in a closed system, such as the need to adopt a closed system, safety, and the absence of trouble during a series of operations such as start-up, steady operation, and shot-down.

From the viewpoint that no trouble occurs during a series of operations, it takes time for the catalyst to exhibit normal activity, and ammonium nitrate accumulates particularly in the upper part of the quench column following the ammonia oxidation, and the quench column is removed. Problems such as blockage sometimes occurred.

From another viewpoint, the pressure in the system is 1 for safety.
0 bar and nitrogen, hydrogen, ammonia,
A mixed gas of nitrogen monoxide, nitrogen dioxide, oxygen and the like was operated under operating conditions outside the explosion limit, and it was necessary to reduce nitrogen monoxide and the like in the mixed gas.

It is an object of the present invention to provide a method and an apparatus for fixing nitrogen as ammonium nitrate safely in a closed system at a low pressure and with a minimum of discharge outside the closed system. .

[0011]

The above object of the present invention is attained by the following method and apparatus for fixing nitrogen in a closed system.

1. Subjecting the nitrogen and hydrogen produced by the electrolysis of water to ammonia synthesis under a pressure of less than 10 bar, and subjecting the resulting reaction mixture containing ammonia, unreacted nitrogen and unreacted hydrogen to pressure swing adsorption. And separating the gas rich in ammonia and the gas rich in nitrogen and hydrogen, circulating the gas rich in nitrogen and hydrogen to the ammonia synthesis process, washing a part of the gas rich in ammonia with water Recovering the ammonia as ammonia water, contacting the remainder of the ammonia-rich gas with a catalyst together with oxygen gas generated by the electrolysis of water to oxidize the ammonia to nitric oxide and nitrogen dioxide, the resulting reaction Quenching the mixture in a quench zone having a clogging prevention function at the start-up of the ammonia oxidation step, The quenched reaction mixture is then washed with water to absorb nitrogen dioxide to form nitric acid, and a portion of the exhaust gas containing nitric oxide from this absorption step is discharged out of the system to remove nitric oxide. A step of circulating the remainder of the exhaust gas to the step of oxidizing ammonia and a step of reacting the aqueous ammonia thus obtained with nitric acid to produce ammonium nitrate.

2. The method of claim 1, wherein the function of preventing clogging at start-up is to spray water in a direction opposite to the flow direction of the reaction mixture to be quenched.

3. Ammonia synthesis means for converting nitrogen and hydrogen produced by the electrolysis of water to ammonia under a pressure of less than 10 bar, the reaction mixture from this ammonia synthesis means being enriched in ammonia-rich gas and nitrogen and hydrogen Pressure swing adsorption means for separating gas and rich gas, means for circulating this nitrogen and hydrogen rich gas to ammonia synthesis means, ammonia by washing a part of the ammonia rich gas with water Means for absorbing ammonia into ammonia water, means for bringing the remainder of the ammonia-rich gas into contact with a catalyst together with oxygen gas generated by electrolysis of water to convert it into nitric oxide and nitrogen dioxide, With a clogging prevention mechanism at the start-up of ammonia oxidation to quench the reaction mixture Means, means for absorbing nitrogen dioxide in the quenched reaction mixture with water to form nitric acid, and treating a part of exhaust gas discharged from the absorbing means with nitric oxide contained therein Means for circulating the remainder of the exhaust gas to the ammonia oxidizing means, and means for producing ammonium nitrate by reacting the ammonia water thus obtained with nitric acid. A nitrogen fixer in a closed system, characterized by:

[0015]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described in detail with reference to FIGS. FIG. 1 is a flow sheet showing one embodiment of the present invention, and FIG. 2 is a schematic view showing a method for preventing clogging of a quench tower during start-up and shut-down in a quench step of the present invention. .

In FIG. 1, nitrogen 1 supplied from outside the closed system is passed through a line 11, and hydrogen from a water electrolyzer 4, which will be described later, is joined through a line 12 and passed through a line 13. And a gas rich in nitrogen and hydrogen from pressure swing adsorption (hereinafter referred to as PSA)
4 and is fed to the compressor 3 through a line 15. Water 2 is supplied to the electrolyzer 4 from outside the closed system through a line 23 and electrolyzed to hydrogen and oxygen.

A mixed gas having a molar ratio of nitrogen to hydrogen of 1: 3, which has been pressurized by the compressor 3 to a pressure of less than 10 bar,
It is introduced into the ammonia synthesis tower 5 through the line 16.
The ammonia synthesis tower 5 is filled with a Ru catalyst, and the gas comes into contact with the catalyst at a temperature of 300 to 400 ° C., and 2 to 4 vol. % Ammonia is synthesized.

The reaction mixture comprising ammonia, unreacted nitrogen and unreacted hydrogen that has exited the ammonia synthesis tower 5 is P
It is introduced into a SA device, where a gas rich in nitrogen and hydrogen and a gas rich in ammonia, preferably 6
0 vol. % Or more, preferably 80 vol. % Or more, particularly preferably 90 vol. % Ammonia. As the PSA method, a pressure swing apparatus for ammonia separation and an ammonia separation method disclosed in JP-A-6-296819, which was previously filed by one of the present inventors, or disclosed in Japanese Patent Application No. Hei 9-11391, In separating the adsorbed component from the mixed gas containing the adsorbed component having a pressure equal to or higher than normal pressure by the PSA method in at least one stage, under the same pressure as the mixed gas to be treated, in the same step, the regeneration process is performed. Any of the improved methods in which the adsorbed component desorbed from one adsorption tower under negative pressure is used for reflux in another adsorption tower can be used.

The nitrogen and hydrogen rich gas from the PSA unit 6 is sent through lines 14, 15 to the income side of the compressor 3, as described above. The circulation ratio is selected from 10 to 20.

On the other hand, a part of the ammonia-rich gas from the PSA device 6 passes through the line 19 and passes through the ammonia absorption tower 7.
Where water supplied to the tower top through lines 24 and 27 absorbs ammonia in the gas, and exhaust gas (nitrogen and hydrogen) containing a small amount of ammonia is taken out from the tower, and , 22, 32 and 3
After passing through 3, it is sent to the ammonia oxidation tower 8.

On the other hand, the remainder of the ammonia-rich gas is sent to the ammonia oxidation column 8 via lines 32 and 33 together with the following exhaust gas from line 31 via the line 20.

The ratio of the gas amount in the line 19 to the gas amount in the line 20 is selected to be 0.5 to 1.5, particularly around 1.0, with respect to the former in terms of capacity.

In the ammonia oxidation tower 8, a platinum-based catalyst, for example, a 90% Pt-10% Rh catalyst is used as a catalyst. In a steady state, the temperature is 650 ° C. to 850 ° C. and the pressure is 1.7 to 4. 0 bar, where ammonia is converted to nitric oxide and nitrogen dioxide by oxygen supplied from electrolyzer 4 through lines 26 and 33.

The reaction mixture from the ammonia oxidation tower 8 is introduced into the quench tower 9 via line 34, where it is quenched by injecting water, where the nitric oxide reacts with oxygen to form nitrogen dioxide, It is further passed through line 35 to the nitrogen dioxide absorption tower 10 where the pressure in the lines 24 and 2 is less than 1.7 bar and less than 10 bar.
Nitrogen dioxide comes into contact with the water 2 fed through 8 and becomes nitric acid.

By the way, in the ammonia oxidation tower, to reach gradually predetermined reaction temperature during start-up, the catalyst, not to exert 100% efficiency, ammonia and NO ₂ gas and NO gas in line 34 May coexist. When the gas supplied from the line 34 to the quench tower 9 is quenched, ammonium nitrate may precipitate at the line 34 and at the inlet of the quench tower 9, and the system may be gradually clogged and operation may become impossible, or the system may be completely exhausted. Even if the system is not closed, there may be a problem that the mass in the system cannot be balanced. In order to prevent this problem, a clogging prevention mechanism for starting up and shutting down is provided. As shown in FIG. 2, this mechanism sprays water so as to reverse the flow of the gas while covering the upper part of the quench tower 9 with a heat insulating material 90 and maintaining the surface temperature of the tower at 230 ° C. to 240 ° C. Is what you do. In FIG. 2, water 2 is supplied from a line 91 and sprayed from a spray 93 as a water droplet 94 so as to go back to the flow of the gas 92. A commercially available hollow cone type spray can be used.

The amount of water used may be 40 cc to 50 cc per liter of the flow rate of the gas 92.

When the steady state is reached, there is almost no ammonia in the line 34, so that it can be used as a normal quench tower. When used as a normal quench tower, it may be sprayed in countercurrent to the gas as shown in FIG. 2 or not shown in FIG. 2, but sprayed in parallel with the gas using a separately provided spray. Is also good.

In short, while ammonia is present, the top of the quench tower is kept at a temperature higher than the decomposition temperature of ammonium nitrate to prevent the precipitation of ammonium nitrate and bring the gas and water into contact with each other in a countercurrent flow with good contact efficiency. I just need to.

Exhaust gas containing nitrogen monoxide, nitrogen and oxygen is discharged from the top of the nitrogen dioxide absorption tower 10, and this exhaust gas is branched into a line 30 and a line 31. The gas in the line 30 is discharged out of the system and is separately processed. The exhaust gas from the line 31 is circulated to the ammonia oxidation tower 8 as described above.

The amount of exhaust gas circulated from the line 31 to the ammonia oxidation tower 8 is set so that the gas supplied to the ammonia oxidation tower 8 does not fall within the explosion limit. Specifically, the amount of exhaust gas circulated is preferably 20 to 40, when the amount of exhaust gas discharged outside the system is set to 1. If it is less than 20, the line 33 determined by the confluence of the oxygen supplied from the above-mentioned line 26 and the oxygen recycled from the line 31 due to the fluctuation of the operation.
Should be avoided because the oxygen concentration at may reach the explosion limit, and the reaction of NOx may not reach equilibrium. Also, even if it exceeds 40, the line 33
Although there is no danger that the oxygen concentration at will approach the explosion limit, the reaction of NOx will not reach equilibrium and change,
It should be avoided from the viewpoint of energy saving.

The ammonia and nitric acid obtained as described above are withdrawn from the bottoms of the ammonia absorption tower 7 and the nitrogen dioxide absorption tower 10 and stored, respectively, and if necessary, reacted to form an aqueous ammonium nitrate solution. It is used as it is or by evaporating water as solid ammonium nitrate outside the system. When the aqueous ammonium nitrate solution is evaporated, the evaporated water can be condensed and used as an absorption medium in an ammonia absorption tower and / or a nitrogen dioxide absorption tower.

[0032]

The present invention will be specifically described below with reference to examples and comparative examples. However, it goes without saying that the invention is not limited only to these examples. First Embodiment A start-up operation will be described first with reference to FIG. The flow rate of the nitrogen 1 was 7.82 liters / hour, and the hydrogen generated by the decomposition of the water 2 by the electrolyzer 4 was 1.
It was 74 liters / hour. These and 337.4 liters / hour of gas recycled from the line 14 were supplied to the compressor 3 together, and supplied to the ammonia synthesis tower 5 filled with a Ru-based catalyst. The reaction conditions of the ammonia synthesis tower 5 were set to a pressure of 3.8 bar and a temperature of 325 ° C. in a steady state. The synthesis rate at this time was 2.95% on a volume basis as the ammonia concentration in the outlet gas of the synthesis tower.

The PSA device 6 separated the gas into a gas rich in nitrogen and hydrogen and a gas rich in ammonia. The gas rich in nitrogen and hydrogen was supplied to the compressor 3 via line 14. The ammonia-rich gas was branched from line 18 to lines 19 and 20. The branch ratio between the line 19 and the line 20 was 1: 1.

The ammonia-rich gas branched into the line 19 contains ammonia in the ammonia absorption tower 7
It was absorbed under conditions of 1.7 bar and 25 ° C. and recovered as a 10% aqueous ammonia solution. Ammonia absorption tower 7
The top gas of the above is that the ammonia-rich gas branched into the line 20 merges with the exhaust gas from the top of the nitrogen dioxide absorption tower 10 described later and the oxygen gas supplied from the line 26,
It was supplied to the ammonia oxidation tower 8. Ammonia oxidation tower 8
Was filled with a 90% Pt-10% Rh-based catalyst, and the reaction conditions were set to a reaction temperature of 750 ° C. and a reaction pressure of 1.7 bar in a steady state. Table 1 shows the intermediate values up to the steady state.

At start-up, the outlet gas of the ammonia oxidation tower 8 was introduced into the quench tower 9, where water was sprayed at 7.8 liters / hour from the line 91 shown in FIG. The spray used was a hollow cone type having a diameter of 0.4 mm.

The top line 28 of the nitrogen dioxide absorption tower 10
, Water was supplied at 0.06 liters / hour, and under conditions of 1.7 bar and 25 ° C., NO ₂ gas was absorbed and converted to nitric acid.

From the top of the nitrogen dioxide absorption tower 10, a gas rich in NO passes through a line 29, a line 30 and a line 3.
Branched to 1. The amount branched to line 31 and line 3
The ratio with the zero amount was set to 33.8.

One of the operating conditions until the above steady state is reached
Examples and operating results are listed in Table 1. Example 2 In Example 1, the set conditions were reached and a steady state was reached. Therefore, the clogging tower 9 clogging prevention function was stopped, and the gas was sprayed in parallel with a separately provided spray. Performed in a similar manner to 1. The results are shown in Table 1. Example 3 Ammonia gas at the outlet of the PSA device of Example 2 was 62.5%.
% To 96.9%, the spray amount in the quench tower was changed from 7.8 Nl / h to 10.3 Nl / h, and line 3
The ratio of the amount branched to 1 to the amount branched to the line 30 was changed to 23.3, and the measurement was performed. The operating conditions and results are also shown in Table 1. Comparative Example 1 The operation was performed in the same manner as in Example 1 except that the clogging tower 9 of Example 1 was started up by spraying the gas in parallel with a separately provided spray while stopping the clogging prevention function of the quench tower 9. Before the steady state was reached, clogging between line 34 and quench tower 9 could occur. At this time, the operation could not be continued, and the operation was stopped.

[0039]

[Table 1]

[0040]

[Table 2]

[0041]

According to the present invention, the following effects can be obtained. (1) The quench tower starts up and shuts down.
With the function to prevent clogging for down, problems at start-up and shutdown are eliminated.
Therefore, no trouble occurs during a series of operations such as start-up, steady operation, and shutdown. (2) The amount of NO gas discharged outside the system by recycling the exhaust gas from the top of the nitric acid absorption tower to the ammonia oxidation tower can be extremely small. (3) Since nitrogen can be fixed in a closed system in the form of ammonia, nitric acid, and ammonium nitrate, it has become possible to separately supply these to places where they are needed in a closed system.

[Brief description of the drawings]

FIG. 1 is a flow sheet showing one embodiment of the present invention.

FIG. 2 is a diagram illustrating the start and start of the gas quenching process of the present invention
It is a schematic diagram showing a mechanism for preventing clogging of a quench tower at the time of up and shut down.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Nitrogen 2 Water 3 Compressor 4 Electrolysis apparatus 5 Ammonia synthesis tower 6 PSA apparatus 7 Ammonia absorption tower 8 Ammonia oxidation tower 9 Quenching tower 10 Nitrogen dioxide absorption tower 90 Heat insulating material 92 Gas 93 Spray 94 Droplets 11 to 35, 91 lines

 ──────────────────────────────────────────────────の Continuing on the front page (72) Inventor Keiji Nitta 25-18 Shimokubo, Misawa-shi, Aomori Prefecture

Claims (3)

[Claims] 1. A step of subjecting nitrogen and hydrogen produced by the electrolysis of water to ammonia synthesis under a pressure of less than 10 bar, a reaction comprising the obtained ammonia, unreacted nitrogen and unreacted hydrogen. Subjecting the mixture to pressure swing adsorption to separate ammonia-rich gas and nitrogen and hydrogen-rich gas, circulating nitrogen and hydrogen-rich gas to the ammonia synthesis process, and removing part of the ammonia-rich gas with water. Recovering the ammonia in the gas as aqueous ammonia by washing with ammonia, and oxidizing the ammonia to nitrogen monoxide and nitrogen dioxide by contacting the remainder of the ammonia-rich gas with a catalyst together with oxygen gas generated by electrolysis of water The process and the resulting reaction mixture are quenched in a quench zone that has A process of washing the quenched reaction mixture with water to absorb nitrogen dioxide to make nitric acid, and discharging a part of the exhaust gas containing nitric oxide from the absorption process to the outside of the system. Nitrogen fixation in a closed system comprising a step of removing nitrogen oxide, a step of circulating the remainder of the exhaust gas to an ammonia oxidation step, and a step of reacting the ammonia water thus obtained with nitric acid to produce ammonium nitrate. Method. 2. The method according to claim 1, wherein the start-up anti-clogging function is to spray water in a direction opposite to the flow direction of the reaction mixture to be quenched. 3. Ammonia synthesis means for converting nitrogen and hydrogen produced by the electrolysis of water to ammonia under a pressure of less than 10 bar, the reaction mixture from the ammonia synthesis means being combined with an ammonia-rich gas. A pressure swing adsorption means for separating into a gas rich in nitrogen and hydrogen, a means for circulating the gas rich in nitrogen and hydrogen to an ammonia synthesizing means, and a part of the gas rich in ammonia with water. A means for absorbing ammonia by washing to make ammonia water, the rest of the ammonia-rich gas being brought into contact with a catalyst together with oxygen gas generated by electrolysis of water to be converted to nitric oxide and nitrogen dioxide And a mechanism to prevent clogging during the start-up of ammonia oxidation to quench the resulting reaction mixture. Quenching means, means for absorbing nitrogen dioxide in a quenched reaction mixture with water to form nitric acid, and treating a part of exhaust gas discharged from the absorbing means with nitric oxide contained therein Means for circulating the remainder of the exhaust gas to the ammonia oxidation means, and means for producing ammonium nitrate by reacting the ammonia water thus obtained with nitric acid. A nitrogen fixing device in a closed system, characterized in that: