JP3258394B2 - Method for producing nitrous oxide - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for producing nitrous oxide, and more particularly to a method for producing nitrous oxide by oxidizing ammonia with oxygen in the presence of water vapor. Nitrous oxide is a compound useful as an anesthetic gas, a rocket fuel burner or a semiconductor cleaner.

[0002]

2. Description of the Related Art Conventionally, as a method for producing nitrous oxide,
(1) ammonia oxidation method, (2) ammonium nitrate decomposition method,
(3) A method based on a reaction between sulfamic acid and nitric acid is known. Among them, the ammonia oxidation method (1) is an industrially preferable method because the raw materials are inexpensive ammonia and oxygen and a high yield can be obtained.

[0003] This ammonia oxidation method uses ammonia or air to produce 200 to 500 ammonia on a metal oxide catalyst.
It is a method of producing nitrous oxide by oxidizing at a temperature of ° C, and it is known that the catalyst used is deteriorated. As a countermeasure, a method of regenerating a catalyst (Japanese Patent Publication No. 30-1225) has been proposed. Further, there is known a practical catalyst preparation method (industrial chemistry magazine, 64, 11, 1879 (1961)) in which ammonium nitrate at the time of catalyst preparation is completely washed and hardly deteriorated.

In the ammonia oxidation method, the present inventors oxidize ammonia in the coexistence of water vapor to (1) ensure that the activity is not degraded, and (2) reduce the water vapor to 80% by simply condensing the water vapor into water. The high concentration of nitrous oxide can be obtained as described above, (3) The safe area is larger and the operation is safer than the method of avoiding the explosion limit with oxygen or nitrogen, and (4) The heat capacity of steam is higher than that of nitrogen or oxygen. It has been found that it is easy to control the temperature of the reaction because it is large, and a method for producing nitrous oxide in the presence of steam has already been proposed.

[0005]

The reaction is usually carried out by diluting with oxygen so that the concentration of ammonia is 10 vol% or less in order to avoid an explosion region in the ammonia-oxygen system. However, even if this method is carried out as it is, the concentration of nitrous oxide at the outlet of the reactor is only a few% because unreacted oxygen exists. Therefore, a method has been proposed in which the oxygen concentration is 80 vol% or more, the reaction product gas is circulated, and only ammonia is separately supplied (Japanese Patent Publication No. 46-33210). Nitrogen concentration is also 40
not only vol% but also NO
x (mainly NO and NO ₂ ) by-products amount to several percent. This NOx needs to be thoroughly removed because the main use of nitrous oxide is for anesthesia. Usually, the NOx content in nitrous oxide used for the above use is 0.1 ppm or less. For example, when the NOx content in the produced nitrous oxide is 5%, the NOx content is reduced to 1 / 500,000 in a rectification column or the like.
The fact that x must be removed and the amount of NOx by-produced is large is a major factor that impairs economic efficiency.

On the other hand, when the explosion range is avoided by diluting with nitrogen, the molar ratio of oxygen / ammonia can be reduced, but the concentration of nitrous oxide at the outlet of the reactor is at most a few due to the presence of diluent gas, nitrogen gas. %. In this case, economical separation of nitrous oxide is not possible. In such an ammonia oxidation method, there has been a demand for a method for producing a high concentration of nitrous oxide in a high yield with little by-product of NOx.

[0007]

In this method of coexisting water vapor, as a result of diligent studies on a method of suppressing by-product NOx, the reaction was carried out under the condition that the molar ratio of oxygen / ammonia was 1.5 or less. They found that life was rapidly suppressed and completed the present invention. That is, the present invention relates to a method for producing nitrous oxide by oxidizing ammonia in the presence of water vapor, wherein oxygen and ammonia are reacted so that the molar ratio of oxygen / ammonia at the supply port of the reactor becomes 0.5 to 1.5. N to be supplied to a vessel
The present invention relates to a method for producing nitrous oxide having a small amount of by-products of Ox, and the present invention provides a method for supplying oxygen and / or ammonia to a reactor in a divided stage in supplying oxygen and / or ammonia in the above method. Characteristic NOx
This is a method for producing nitrous oxide with less by-products. further,
The present invention is characterized in that the reaction product gas obtained from the reactor in the above method is cooled, and the condensed unreacted ammonia and water vapor are circulated to the reactor as ammonia water. It is a manufacturing method of.

As the catalyst used in the present invention, a known catalyst known as a catalyst for ammonia oxidation can be used. Surprisingly, the addition of water results in no or very little degradation of the catalyst, which has hitherto been found to have deteriorated. It is presumed that this is probably due to the effect of cleaning poisonous substances such as traces of nitric acid on the catalyst or the effect of maintaining the oxidation state of the catalyst. Examples of such catalysts include:
CuO-MnO ₂ system, Bi ₂ O ₃ system, Fe ₂ O ₃ -Bi ₂ O ₃ -MnO ₂ system, MnO-CoO-
NiO system, Ba ₂ O-CuO system, MnO ₂ system, Pr ₂ O ₃ -Nd ₂ O ₃ -CeO ₃ system, Pt
System. Among them, a Mn-containing catalyst is preferable because of its high activity. Further, a CuO—MnO ₂ system that is easy to prepare is particularly preferred. These catalysts are usually charged into a tubular reactor, and a mixed gas such as steam, ammonia and oxygen is supplied to carry out the reaction.

When the ammonia is oxidized with oxygen in the presence of water vapor according to the present invention, the composition at the supply port of the reactor has a water vapor concentration of 50 vol. desirable. Also, in this ammonia oxidation reaction, there is a danger of explosion depending on the concentration of ammonia, and the lower limit of the ammonia explosion is about 15 vol%. To avoid this explosion area, dilute the reaction gas with oxygen or nitrogen, etc. The ammonia concentration in the solution must be about 15 vol% or less.
vol% or less is preferable. When diluted with oxygen or nitrogen, the reaction efficiency is low due to the low ammonia concentration, and it is necessary to separate excess oxygen and nitrogen in the obtained reaction product gas from nitrous oxide.

However, it has also been found that if the water vapor concentration in the present invention is at least 60 vol% or more, an explosion region can be avoided regardless of the molar ratio of ammonia or oxygen. As described above, if the water vapor concentration is 60 vol% or more at the reactor supply port, there is no need for the above-mentioned extra oxygen or nitrogen for dilution, and high-concentration nitrous oxide can be easily separated. Therefore, the preferred amount of water vapor to be used is at least 50 vol%, more preferably at least 60 vol%, in terms of the concentration at the reactor inlet.

The ammonia used in the method of the present invention can be used not only as pure ammonia but also as an aqueous ammonia solution. As described above, the concentration of ammonia at the reactor supply port is preferably 10 vol% or less in order to avoid an explosion region. However, the use of steam at 60 vol% or more is not limited. Is 1 to 30% by volume, preferably 1 to 20% by volume.
% Range.

As the oxygen used as the oxidizing source in the present invention, not only pure oxygen but also oxygen containing nitrogen and air can be used, but as described above, it is diluted with more nitrogen and the like. The use of oxygen should be avoided because the concentration of nitrous oxide in the reaction product gas becomes even lower.
The preferred amount of oxygen is 0.5 to 1 mol of ammonia.
It is 1.5 moles, more preferably in the range of 0.8 to 1.2 moles. If the oxygen concentration exceeds 1.5 times the molar amount of ammonia, the NOx concentration sharply increases. This tendency is clear from the example (FIG. 1). Also, this molar ratio
If the amount is less than 0.5 mol, not only is the reaction rate slowed down, but the apparatus for recovering excess ammonia becomes complicated, which is not preferable.

Thus, according to the present invention, nitrous oxide can be obtained by supplying water vapor, ammonia and oxygen to the reactor, and the oxygen / ammonia molar ratio in the reactor is in the range of 0.5 to 1.5 mol. As far as possible, for example, even if the reaction is carried out with an excess of ammonia with respect to oxygen, the reaction rate is slightly reduced, but harmful NOx by-products are suppressed, and unreacted ammonia condenses the obtained reaction product gas. Thus, it can be easily recovered and reused as it is or as an aqueous solution, so that the ammonia utilization rate is substantially 100%. Further, if the reaction is carried out in excess of oxygen with respect to ammonia in the above range, the reaction rate can be increased and the by-product of NOx can be suppressed to a small amount, so that the utilization rate of oxygen is extremely high.

In the above-mentioned method of reacting oxygen with an excess of ammonia, it is difficult to recover excess unreacted ammonia by a known method that does not dilute with steam. This is an infeasible method.

In the present invention, the divisional supply of oxygen and / or ammonia not only facilitates the temperature control of the reaction but also reduces the cost of steam. As a specific method of dividing and supplying such raw materials,
For example, the supply of water vapor, oxygen and ammonia to the reactor supply port (first stage) is controlled by oxygen / ammonia = 0.5 to 1.5.
(Molar ratio) at a steam concentration of 60 vol% or more. Then, as the ratio of the concentration including the unreacted oxygen and ammonia from the supply port of the second stage, oxygen / ammonia = 0.
It is sufficient to supply only oxygen and / or ammonia so as to be 5 to 1.5 (molar ratio). From the second and subsequent stages, only oxygen and / or ammonia is supplied without substantially supplying steam. In the case of multi-stage supply, the number of stages is 2 to 10, preferably 2 to 5.
As a result, the amount of steam that can be recovered with respect to nitrous oxide is reduced according to the number of divided stages, and the steam cost is significantly reduced.

The supply rate of the mixed gas such as water vapor, oxygen and ammonia does not greatly affect the selectivity of nitrous oxide, but if it is too small, the reactor becomes large and uneconomical. If it is too large, the conversion of ammonia decreases. Therefore, the supply speed of these mixed gases is converted to a space velocity of 100 to 100,
000 / hr, preferably 1,000 to 50,000 / hr.

The reaction temperature is 200-500 ° C., and if it exceeds 500 ° C., the amount of by-products of NOx increases, which is not preferable. Therefore, the temperature is preferably from 250 to 450 ° C. The reaction pressure is higher at a higher pressure, but the reaction speed is higher, but the reactor is expensive and uneconomical, preferably at 0 to 20 kg / cm ² -G, more preferably at 0.3 to 5 kg / cm ² -G. is there.

The reaction product gas obtained by carrying out the reaction at 250 to 450 ° C. in this manner is cooled to, for example, the boiling point of water of 0 to 80 ° C. or lower, and is a non-condensable gas such as nitrous oxide, oxygen and nitrogen. And water vapor. At this time, the unreacted ammonia is condensed with the water vapor, and the
It is recovered in the range of 0.01 to 5 wt%. This ammonia water is partially purged as necessary, and the remainder is circulated to the reactor. Here, in the case of purging a part of the ammonia water, a method of heating the ammonia water to vaporize most of the ammonia, and then purging the same amount of water generated by the reaction is preferable. As a result, the conversion of ammonia is substantially 100%.

On the other hand, the separated non-condensable gas such as nitrous oxide, oxygen and nitrogen is subjected to a purification step to remove a trace amount of NOx as required. As a method of removing a trace amount of NOx, for example, a method of washing these non-condensable gases with an aqueous sodium hydroxide solution of potassium permanganate and an aqueous solution of sulfuric acid can be mentioned. Further, oxygen and nitrogen are separated to produce high-purity nitrous oxide.

[0020]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail with reference to embodiments.

Example 1 A tubular reactor having an inner diameter of 2.8 cm filled with 500 g of CuO-MnO ₂ catalyst was charged with 4.5 vol% of ammonia, 4.65 vol% of oxygen, and 90.8 of steam.
Each gas was supplied at a rate of 5 vol%. The oxygen / ammonia molar ratio at this time is 1.033. The reaction temperature was 300 ° C., the space velocity was 3,000 / hr, and the reaction pressure was 0.5 kg / cm ² -G.
The reaction product gas at the outlet of the reactor was cooled to 30 ° C., and the gas phase was analyzed. As a result, 74.6 vol% of nitrous oxide and 13.2 vol.
l%, oxygen 12.2 vol%, NOx 17 ppm (23 ppm based on nitrous oxide), and ammonia was not detected. On the other hand, when the liquid phase was analyzed, the amount of ammonia was trace and the conversion of ammonia was 99% or more.

The reaction product gas is passed through an alkaline aqueous solution containing potassium permanganate to remove NOx, and
At 10 kg / cm ² -G, the mixture was cooled to about −80 ° C. to liquefy nitrous oxide and separated from oxygen and nitrogen. The purity of the nitrous oxide thus obtained was 99% or more, which was satisfactory quality.

Example 2 The reaction was carried out in the same manner as in Example 1 except that the oxygen supply amount was changed so that the molar ratio of oxygen / ammonia was 0.5 to 1.5. There was no significant difference in the selectivity, and no significant change was observed in the amount of by-product NOx. The results are shown in FIG.

COMPARATIVE EXAMPLE 1 The proportion of the supplied gas was 2 vol% of ammonia, 38 vol% of oxygen,
The procedure was performed in the same manner as in Example 1 except that the steam was changed to 60 vol%. The molar ratio of oxygen / ammonia at this time is 19. As a result of analyzing the reaction product gas, nitrous oxide 2.4vol%, nitrogen
It was 0.4 vol%, oxygen 97.1 vol%, NOx 0.11% (corresponding to 4.6% with respect to nitrous oxide, 2000 times as compared with Example 1), and ammonia was not detected. On the other hand, when the liquid phase was analyzed, the amount of ammonia was trace and the conversion of ammonia was 99% or more. The conversion rate of oxygen is 5
%. The results are shown in FIG.

Example 3 Two reactors used in Example 1 were connected in series, and the first stage was 4.5 vol% of ammonia and 4.65 vol.
1% and 90.8 vol% of steam were supplied to the second stage reactor at a space velocity of 270 / hr so that 50 vol% of ammonia and 50 vol% of oxygen were not directly mixed. Example 1
Was performed in the same manner as described above. At this time, the molar ratio of oxygen / ammonia in the first stage is 1.033. Ammonia was not detected in the gas at the outlet of the first reactor, and the oxygen concentration was 0.16 vol%.
Equivalent to 1.035. The reaction product gas at the outlet of the second reactor was cooled to 30 ° C., and the gas phase thereof was analyzed. As a result, 74.7 vol% of nitrous oxide, 13.2 vol% of nitrogen, 12.1 vol% of oxygen, and NO
x 19 ppm (25 ppm based on nitrous oxide), and no ammonia was detected.

On the other hand, when the liquid phase was analyzed, the amount of ammonia was trace and the conversion of ammonia was 99% or more. Although the reactor was twice as large and the amount of supplied ammonia was doubled as compared with Example 1, the amount of water vapor was the same. Therefore, the amount of water vapor per nitrous oxide was halved, further improving the economic efficiency.

Example 4 To a tubular reactor having an inner diameter of 2.8 cm filled with 500 g of a CuO-MnO ₂ catalyst, 1.5% aqueous ammonia, 100% ammonia and oxygen described later were supplied to the reactor. The reactor inlet composition was 6 vol% ammonia, 4.5 vol% oxygen, and 89.5 vol% steam. The reaction was performed at a space velocity of 3,500 / hr, a reaction pressure of 0.9 kg / cm ² -G, and a reaction temperature of 300 ° C. The reaction product gas at the reactor outlet was cooled to 5 ° C., and excess ammonia was recovered as a liquid phase aqueous ammonia (1.5%). The aqueous ammonia was preheated to 230 ° C. with steam and circulated to the reactor.

On the other hand, as a result of analyzing the gas phase, 80.1 vol% of nitrous oxide, 14.3 vo% of nitrogen, 5.6 vol% of oxygen, NO
x 10 ppm and traces of ammonia. The result is that the conversion of ammonia is 99% or more and the conversion of oxygen is 97%. The obtained reaction product gas is passed through an alkaline aqueous solution containing potassium permanganate to remove NOx, and further, at 10 kg / cm ² -G, at about −80 ° C.
To liquefy the nitrous oxide and separate it from oxygen and nitrogen. The purity of the nitrous oxide thus obtained is 99
%, Which was satisfactory quality.

[0029]

In the ammonia oxidation method, by limiting the molar ratio of oxygen / ammonia to a specific range by coexisting water vapor, by-products of NOx are extremely small, and unreacted ammonia can be easily recovered and reused. This is a method by which high concentrations of nitrous oxide can be industrially advantageously produced.

[Brief description of the drawings]

FIG. 1 is a graph showing the by-product amount of NOx in a reaction product gas at a molar ratio of oxygen / ammonia.

──────────────────────────────────────────────────続 き Continuation of the front page (72) Inventor Susumu Yoshinaga 1-6-6 Takasago, Takaishi-shi, Osaka Investigator, Mitsui East Pressure Chemical Co., Ltd. Daiji Daikohara (56) References JP-A-47-34092 (JP, A Japanese Patent Publication 46-33210 (JP, B1) Japanese Patent Publication 31-10771 (JP, B1) (58) Fields investigated (Int. Cl. ⁷ , DB name) C01B 21/22 CA (STN)

Claims (3)

(57) [Claims] 1. A method for producing nitrous oxide by oxidizing ammonia in the presence of water vapor, wherein oxygen and ammonia are added to the reactor such that the molar ratio of oxygen / ammonia at the feed port of the reactor is 0.5 to 1.5. A method for producing nitrous oxide with less by-product of NOx, characterized in that the nitrous oxide is supplied to a nitric oxide. 2. A process for producing nitrous oxide having a small amount of NOx by-products, wherein oxygen and / or ammonia are divided and supplied in multiple stages when oxygen and ammonia are supplied to a reactor. Method. 3. A by-product of NOx characterized by cooling a reaction product gas obtained from the reactor according to claim 1 or 2, and circulating condensed unreacted ammonia and steam as ammonia water to the reactor. A method for producing nitrous oxide with low content.