JP2014069977A - Nitric oxide purification method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a nitric oxide purification method which inhibits the production of by-products and is suitable for obtaining high-purity nitric oxide gas.SOLUTION: A nitric oxide purification method includes an adsorption step in which material gas including nitric oxide and moisture is fed to an adsorption pipe 2 filled with adsorbent for making moisture adsorbed to the adsorbent. As the adsorbent, used is pretreated adsorbent obtained by applying inorganic adsorbent to pretreatment for generating inorganic acid salt.

Description

  The present invention relates to a method for purifying nitric oxide.

  Nitric oxide may be used as a material gas for forming an oxynitride film on a silicon surface in a semiconductor process, for example. Nitric oxide can be produced by various methods such as an ammonia oxidation method, a method of reacting sodium nitrite and ferrous chloride, and a method of reacting nitric acid and sulfurous acid gas. Contains moisture and acidic gases such as nitrogen dioxide and sulfur dioxide as impurities or by-products. In particular, in a method for producing nitric oxide by reducing nitric acid with sulfur dioxide, sulfur dioxide is contained as an impurity in the crude nitric oxide. In forming the above-described oxynitride film in a semiconductor process, it is desired that nitrogen monoxide as a material gas has higher purity.

  As a relatively simple method for purifying or purifying nitric oxide, crude nitric oxide gas (mixed gas) is passed through inorganic adsorbents such as activated alumina, zeolite, and silica gel under specified conditions. There is a known technique to do this. According to this method, mainly water in the mixed gas is adsorbed and removed by the inorganic adsorbent. Such a technique using an inorganic adsorbent is described in, for example, Patent Document 1 and Patent Document 2 below. In Patent Document 1, silica, alumina, zeolites and a mixture thereof containing no metal cation are used as adsorbents to purify nitric oxide containing water, nitrogen dioxide, and sulfur dioxide. In Patent Document 2, zeolite Y dehydrated with silica and containing 8-30% metal cations is used to adsorb and remove moisture and nitrogen oxides from a gas stream.

However, in the above-described conventional nitric oxide purification method using an inorganic adsorbent, moisture in the mixed gas is effectively adsorbed and removed, but an impurity by-product reaction of nitric oxide (for example, 3NO → N ₂ It is known that the amount of nitrogen oxides or nitrogen in the mixed gas may increase due to O + NO ₂ ). Therefore, in the conventional technology, even if a special adsorbent is used, sufficiently high-purity nitric oxide cannot be obtained.

  Further, as another relatively simple method of purifying nitric oxide, a method of bringing a mixed gas into contact with an alkaline aqueous solution has been known for a long time. According to this technique, mainly nitrogen dioxide and sulfur dioxide in the mixed gas are absorbed and removed by the alkaline aqueous solution.

  However, for example, in the conventional nitric oxide purification method using an aqueous sodium hydroxide solution as an alkaline aqueous solution, nitrogen dioxide and sulfur dioxide in the mixed gas are removed, but moisture remains in the purified gas, and moisture removal is necessary. is there.

  Patent Document 3 discloses a method of performing cryogenic separation at −164 ° C. using liquid nitrogen as another nitric oxide purification method. However, in this method, the cooling operation with liquid nitrogen is complicated and the apparatus is expensive, and further, there is a possibility of explosion of liquid nitric oxide, so it cannot be said that it is suitable for industrialization.

JP-A-8-319104 JP 51-141784 A Korean Patent No. 20100007188 (A) Specification

  The present invention has been conceived under the circumstances as described above, and provides a method for purifying nitric oxide suitable for suppressing the production of by-products and obtaining high-purity nitric oxide gas. For the purpose.

  The nitric oxide purification method provided by the present invention includes an adsorption step for allowing a mixed gas containing at least nitric oxide and moisture to flow through the adsorbent and adsorbing moisture on the adsorbent. Then, as the adsorbent, a pretreated adsorbent in which a pretreatment for generating an inorganic acid salt is performed in advance is used for the inorganic adsorbent. The inorganic acid salt referred to here is represented by nitrate, and in addition, there are sulfate, phosphate, hydrochloride and the like.

  When a crude nitrogen monoxide gas (hereinafter referred to as a mixed gas) is passed through the adsorbent and impurities are adsorbed and removed from the mixed gas, the inorganic adsorbent is subjected to a predetermined pretreatment in advance. The present inventors have found that nitrogen by-product reaction is suppressed. Specifically, for example, when a mineral acid aqueous solution having a relatively strong polarity is brought into contact with the inorganic adsorbent as a pretreatment, a part of the mineral acid undergoes chemical adsorption with the inorganic adsorbent to cause an inorganic acid. It is considered that a salt is generated, and the catalytically active sites related to the by-product reaction of nitric oxide as an inorganic adsorbent are blocked. And according to the nitric oxide purification method which concerns on this invention, the production amount of the by-product by the impurity byproduct reaction of nitric oxide can be aimed at in the adsorption | suction process. Therefore, this method is suitable for obtaining higher-purity nitric oxide.

  According to one embodiment, the pretreatment includes an operation of bringing the mineral acid aqueous solution into contact with the inorganic adsorbent. In this case, preferably, the mineral acid aqueous solution is a nitric acid aqueous solution.

  According to another embodiment, the pretreatment includes an operation of bringing the acidic gas into contact with the inorganic adsorbent.

  In a preferred embodiment, the mixed gas contains at least one of nitrogen dioxide and sulfur dioxide as an impurity, and before the mixed gas is subjected to the adsorption step, the mixed gas is brought into contact with an alkaline aqueous solution to form the impurity ( It further includes an alkali cleaning step for absorbing and removing nitrogen dioxide and sulfur dioxide).

  According to this method, even when the mixed gas contains nitrogen dioxide or sulfur dioxide as impurities, nitrogen dioxide and sulfur dioxide can be significantly absorbed and removed by the alkaline aqueous solution in the alkali cleaning step. Therefore, this method is suitable for obtaining higher-purity nitric oxide.

It is a schematic block diagram of the refinement | purification line which can be used to implement the nitric oxide refinement | purification method concerning this invention.

  Next, preferred embodiments of the present invention will be described with reference to the accompanying drawings.

FIG. 1 is a schematic configuration diagram of a purification line X1 that can be used to perform a nitric oxide (hereinafter referred to as NO) purification method according to the present invention. The purification line X1 is configured to purify the crude NO gas supplied from the NO cylinder Y1, and contacts the absorbing liquid for removing nitrogen dioxide (hereinafter referred to as NO ₂ ) and sulfur dioxide (hereinafter referred to as SO ₂ ). Apparatus 1, adsorption pipe 2 for removing moisture (hereinafter referred to as H ₂ O), pressure regulating valve 3, on-off valves 4 </ b> A to 4 </ b> E, product NO outlet 5, purge gas inlet 6, and gas outlet 7, a vacuum pump 8, and piping connecting them.

The NO cylinder Y1 is for supplying crude NO gas as a raw material gas (mixed gas) to the purification line X1, and is a cylinder in which the crude NO gas is sealed under high pressure conditions. The sealed crude NO gas contains NO as a main component and NO ₂ , SO ₂ and H ₂ O as impurities.

The absorbing liquid contact device 1 is for bringing a raw material gas into contact with an alkaline aqueous solution that is an absorbing liquid, and includes an absorbing liquid tank 1A, a gas inlet pipe 1a, a gas outlet 1b, an absorbing liquid supply port 1c, and a liquid outlet. 1d. The absorbing liquid tank 1A has a container shape, and the inside thereof is filled with an alkaline aqueous solution capable of exhibiting NO ₂ absorbing ability and SO ₂ absorbing ability. The gas introduction pipe 1a extends downward in the absorption liquid tank 1A, and its lower end is opened in an alkaline aqueous solution. The absorption liquid supply port 1c is a passage for supplying a new alkaline aqueous solution into the absorption liquid tank 1A, and the liquid discharge port 1d is a passage for discharging the alkaline aqueous solution in the absorption liquid tank 1A to the outside of the absorption liquid tank 1A. is there.

  As the alkaline aqueous solution used in the present embodiment, for example, an aqueous solution of sodium hydroxide, potassium hydroxide, magnesium hydroxide or the like can be employed. Of these, an aqueous sodium hydroxide solution is preferred from the viewpoint of ease of handling.

  In the present embodiment, from the viewpoint of efficiently bringing the raw material gas into contact with the alkaline aqueous solution, the absorbent liquid contact device 1 is preferably configured to discharge the raw material gas into the alkaline aqueous solution in the form of fine bubbles.

The adsorption tube 2 is configured to allow gas to pass through and has a cylindrical shape in the present embodiment. The inside of the adsorption tube 2 is filled with an inorganic adsorbent that can exhibit H ₂ O adsorption ability, NO ₂ adsorption ability and SO ₂ absorption ability. As such an inorganic adsorbent, activated alumina, zeolite, and silica gel can be employed. Of these, activated alumina and A-type zeolite generally used for dehydration are preferred. As will be described later, these adsorbents are subjected to a predetermined pretreatment prior to purification of NO. The adsorption pipe 2 is attached with a temperature adjustment mechanism (not shown) for adjusting the internal temperature.

  The pressure adjusting valve 3 is for reducing the pressure of the raw material gas supplied from the NO cylinder Y1 to a predetermined pressure. Each of the on-off valves 4A to 4E is configured to be selectable between an open state that allows passage of gas and a closed state that blocks passage of gas.

  In the NO purification method according to the present invention, prior to performing the purification of NO using the purification line X1, a predetermined amount for generating an inorganic acid salt for the inorganic adsorbent filled in the adsorption pipe 2 is obtained. Pre-processing is performed.

  The pretreatment according to the present invention is performed by contacting the mineral acid aqueous solution with an inorganic adsorbent. The contact between the mineral acid aqueous solution and the inorganic adsorbent can be performed, for example, by impregnating the inorganic acid adsorbent with the mineral acid aqueous solution. As the mineral acid, for example, hydrochloric acid, sulfuric acid, nitric acid, phosphoric acid and the like can be used. However, the use of nitric acid which is a by-product in NO purification is preferable from the viewpoint of preventing foreign substances from being mixed. When a nitric acid aqueous solution as a mineral acid aqueous solution and an inorganic adsorbent are brought into contact with each other, nitrate is generated, and a pretreated adsorbent can be obtained. Here, although the density | concentration of the mineral acid aqueous solution to be used is not specifically limited, Usually, 0.01-1 normal mineral acid aqueous solution is used. The impregnation of the mineral acid aqueous solution into the inorganic adsorbent may be performed, for example, for about 1 to 20 hours outside the purification line X1. The impregnation method requires 10 hours or more when the mineral acid aqueous solution and the adsorbent are allowed to stand at room temperature without stirring, but when stirring at 40 to 50 ° C., it takes about 2 hours. Thereafter, the adsorbent is filtered and dried. As the drying method, reduced-pressure drying or hot-air drying is efficient, but a stationary drying method or a drying method using a conical dryer may be used. The dried impregnated adsorbent is stored by replacing the storage container with nitrogen so as not to absorb moisture.

The pretreatment according to the present invention is not limited to the above-described method, and may be performed by contact between an acidic gas and an inorganic adsorbent. For example, an inorganic adsorbent and a gas containing an acidic gas such as NO ₂ or SO ₂ are charged in a pressurizable container such as an autoclave and kept at 30 to 60 ° C. for several hours at 0.1 to 1.5 MpaG. You can also do it. Here, an inert gas such as nitrogen, argon (Ar), helium (He), or NO, or a gas that does not cause a problem even if mixed into the product can be used as the dilution gas. And what is necessary is just to fill the adsorption pipe 2 with the obtained pre-processed adsorbent.

In the state in which the pretreated adsorbent is filled in the adsorption pipe 2 by performing the pretreatment described above, NO purification is performed using the purification line X1. First, the on-off valves 4A, 4C are opened and the on-off valves 4B, 4D, 4E are closed. Then, the raw material gas (crude NO gas) is continuously supplied from the NO cylinder Y <b> 1 through the pressure regulating valve 3 to the absorbent contact device 1, and the alkali cleaning process is executed in the absorbent solution contact device 1. The source gas contains NO as a main component and contains NO ₂ , SO ₂ and H ₂ O as impurities as described above, but the NO ₂ concentration and H ₂ O concentration of the source gas supplied from the NO cylinder Y1 are as follows: Each is, for example, 0.1 to 1000 ppm. The pressure set in the pressure regulating valve 3 is, for example, 0.05 to 20 MPa, preferably 0.1 to 2 MPa.

In the alkali cleaning step, the raw material gas is released from the lower end of the gas introduction pipe 1a so that the raw material gas is brought into contact with the alkaline aqueous solution, and impurities NO ₂ and SO ₂ are absorbed into the alkaline aqueous solution. Absorbed gas is led out of the absorbing liquid contact device 1 from the gas outlet 1b. In the alkali cleaning step, a new alkaline aqueous solution is replenished at a constant flow rate through the absorption liquid supply port 1c, and the alkaline aqueous solution received in the absorption liquid tank 1A is replenished from the liquid discharge port 1d at a constant flow rate. Drain outside. The temperature of the alkaline aqueous solution in the absorption liquid tank 1A is, for example, 10 to 50 ° C, and preferably 20 to 40 ° C.

The raw material gas (non-absorbed gas) that has finished the alkali cleaning process in the absorbing liquid contact device 1 then reaches the adsorption pipe 2 via the on-off valve 4A and is subjected to the adsorption process. In the adsorption process, the non-adsorbed gas is passed through the pretreated adsorbent in the adsorption tube 2 to adsorb or retain the impurities H ₂ O or NO ₂ in the pretreated adsorbent, and then the non-adsorbed gas. To the outside of the adsorption tube 2. The internal temperature of the adsorption tube 2 is, for example, −40 to 50 ° C., preferably 0 to 40 ° C. Here, since nitrate is produced in the pretreated adsorbent, it is considered that the catalytic activity point of the inorganic adsorbent is in a blocked state. Therefore, in the adsorption step, the NO by-product reaction of NO is suppressed, and the amount of N ₂ O and NO ₂ produced as a by-product by the impurity by-product reaction is reduced. Such an adsorption process may be performed until, for example, H ₂ O is sufficiently absorbed and breakthrough starts (breakthrough point), or may be terminated before the breakthrough point. The adsorption process can be completed by closing the on-off valve 4C and opening the on-off valve 4D while continuing to supply the raw material gas from the NO cylinder Y1 to the purification line X1. Until the end of the adsorption process, the gas passing through the adsorption pipe 2 can be taken out from the product NO outlet 5 as purified NO gas.

As described above, a high-purity NO gas can be obtained by refining the crude NO gas (raw material gas) containing NO ₂ , SO ₂ and H ₂ O as impurities.

  In order to repeatedly execute the NO gas purification method according to the present invention in the purification line X1, the adsorption tubes 2 or the adsorbents inside them are regenerated or washed after the purification process as described above is completed.

At the time of regeneration or washing of the adsorption pipe 2 or the pretreated adsorbent inside, the on-off valves 4A, 4C, 4E are closed and the on-off valves 4B, 4D are opened. Then, the inert gas is continuously introduced from the purge gas inlet 6 into the purification line X1. The inert gas introduced from the purge gas introduction port 6 is heated to a predetermined temperature in advance by a heater (not shown), and passes through the on-off valve 4B, the adsorption pipe 2, and the on-off valve 4D, and goes out of the line from the gas discharge port 7. To be discharged. As the inert gas, for example, N ₂ , argon (Ar), or helium (He) can be employed. The temperature of the inert gas realized by the heater is, for example, 100 to 300 ° C, and preferably 150 to 250 ° C. In this way, the adsorbing tube 2 or the pretreated adsorbent is regenerated or washed by passing an inert gas of a predetermined amount and a predetermined pressure through the adsorbing tube 2 or the pretreated adsorbent inside thereof. be able to.

According to the NO purification method of the present invention, NO ₂ in the raw material gas can be absorbed and removed by the alkaline aqueous solution in the above-described alkali cleaning step in the absorbent liquid contact device 1. In addition, by using the pretreated adsorbent in the adsorption step in the adsorption tube 2, NO by-product reactions of NO are suppressed, and the production amounts of N ₂ O and NO ₂ as by-products are reduced. Thus, in this method, when it contains NO ₂ or SO _2, H ₂ O as impurities in the raw material gas, effectively NO _2, SO ₂ and H ₂ O at alkaline washing step and the adsorption step The amount of by-products generated in each step can be reduced, and high-purity NO can be obtained.

If the raw material gas supplied from the NO cylinder Y1 is substantially free of NO ₂ and SO ₂ , or if the NO ₂ content and SO ₂ content are small, the present invention can be realized by omitting the alkali cleaning step. It is possible to appropriately execute the NO purification method according to the invention. In this case, as the purification line for executing the NO purification method, the absorbent liquid contact device 1 may not be provided.

  Below, this invention is described further in detail based on Examples 1-4 and a comparative example.

[Example 1]
In this example, the adsorption step in the present NO purification method was executed using the adsorption tube 2 according to the above embodiment. In this example, activated alumina (trade name: KHD-12, manufactured by Sumitomo Chemical Co., Ltd.) was used as the inorganic adsorbent. As a pretreatment of the adsorbent, 18.6 g of the activated alumina was mixed and stirred with a 0.3 N aqueous nitric acid solution at 40 ° C. for 2 hours, and then vacuum dried at 60 ° C. for 6 hours. The pretreated adsorbent thus obtained was filled into an adsorption tube 2 (a stainless steel cylindrical adsorption tube having an inner diameter of 7.5 mm and a length of 500 mm). Thereafter, the temperature of the adsorption tube 2 is increased to 150 ° C., the on-off valves 4A to 4D are closed, the on-off valve 4E is opened, the vacuum pump 8 is operated, and the pressure in the adsorption tube 2 is 270 Pa (abs). Then, vacuum suction was performed for about 2 hours. Next, crude NO gas (H ₂ O content = 100 ppm, NO ₂ content = 500 ppm, N ₂ O content = 50 ppm) as a raw material gas is passed through the adsorption pipe 2 at 25 ° C. under a pressure of 0.15 MPaG. did. The raw material gas flow rate to the adsorption tube 2 was 60 ml / min. In this way, the adsorption step was performed. As a result of analyzing the non-adsorbed gas derived from the adsorption pipe 2 with a dew point meter and FT-IR when 1 hour has passed after the start of flow, H ₂ O content = 1 ppm, NO ₂ content = 30 ppm, N ₂ O content = 65 ppm The increment of N ₂ O content before and after flow was 15 ppm.

[Example 2]
In this example, molecular sieve 4A (trade name: Zeolum A-4, manufactured by Tosoh Corporation) was used as the inorganic adsorbent instead of the activated alumina of Example 1. NO purification (pretreatment and adsorption step) was carried out in the same manner as in Example 1 except that the inorganic adsorbent was changed. As a result of analyzing the non-adsorbed gas derived from the adsorption pipe 2 after 1 hour has passed since the start of the flow of the raw material gas to the adsorption pipe 2, the H ₂ O content = 1 ppm, the NO ₂ content = 25 ppm, and the N ₂ O content = 60 ppm. The increment of N ₂ O content before and after flow was 10 ppm.

[Example 3]
In this example, as a pretreatment of the adsorbent, 150 ml of activated alumina (trade name: KHD-12, manufactured by Sumitomo Chemical Co., Ltd.) was charged into a stainless autoclave having an internal capacity of 300 ml and a pressure resistance of 3 Mpa, and NO / NO ₂ = A 1/1 gas mixture was added until 1.5 MpaG. After holding at 40 ° C. for 4 hours, the gas was purged, and a gas of NO / NO ₂ = 1/1 was added again to 1.5 MPag and held at 30 ° C. for 5 hours. The adsorbent pretreated in this way was filled in the same adsorption tube 2 as in Example 1 and purified in the same raw material gas supply manner as in Example 1. As a result of analyzing the non-adsorbed gas derived from the adsorption pipe 2 when 1 hour has passed since the start of the flow, the H ₂ O content = 1 ppm, the NO ₂ content = 30 ppm, and the N ₂ O content = 63 ppm. The increment of N ₂ O content was 13 ppm.

[Example 4]
In this example, the alkali cleaning step and the adsorption step in the present NO purification method were carried out using the absorbent contact device 1 and the adsorption tube 2 according to the above embodiment. In this embodiment, the absorbent contact device 1 having a volume of 1000 ml was filled with 500 ml of a 1% by weight sodium hydroxide aqueous solution as an alkaline aqueous solution. The adsorption tube 2 was filled with activated alumina that had been pretreated in the same manner as in Example 1, and the temperature was raised and vacuuming was performed. Next, crude NO gas (H ₂ O content = 100 ppm, NO ₂ content = 500 ppm, SO ₂ content = 500 ppm, N ₂ O content = 50 ppm) as a raw material gas at 25 ° C. under a pressure of 0.15 MPaG The absorbent liquid contact device 1 and the adsorption tube 2 were passed through. As a result of analyzing the non-adsorbed gas derived from the adsorption pipe 2 when 1 hour has passed after starting the flow of the raw material gas to the absorption liquid contact device 1 and the adsorption pipe 2, the H ₂ O content = 1 ppm, the NO ₂ content = 10 ppm, The SO ₂ content was less than 1 ppm, the N ₂ O content was 63 ppm, and the increment of N ₂ O content before and after flow was 13 ppm.

[Comparative example]
In this comparative example, NO purification was performed without pretreatment using the same adsorption tube 2 and inorganic adsorbent as in Example 1. The adsorption tube 2 is filled with 18.6 g of activated alumina (trade name: KHD-12, manufactured by Sumitomo Chemical Co., Ltd.), the temperature of the adsorption tube 2 is increased to 150 ° C., and the on-off valves 4A to 4D are closed and opened and closed. The valve 4E was opened, the vacuum pump 8 was operated, and vacuum suction was performed for about 2 hours so that the pressure in the adsorption tube 2 was 270 Pa (abs). Next, crude NO gas (H ₂ O content = 100 ppm, NO ₂ content = 500 ppm, N ₂ O content = 50 ppm) as a raw material gas is passed through the adsorption pipe 2 at 25 ° C. under a pressure of 1.5 MPaG. did. The raw material gas flow rate to the adsorption tube 2 was 60 ml / min. In this way, the adsorption step was performed. That is, in this comparative example, NO purification was performed in the same manner as in Example 1 except that no pretreatment was performed. As a result of analyzing the non-adsorbed gas derived from the adsorption pipe 2 when 1 hour has passed after starting the feed of the raw material gas to the adsorption pipe 2, the H ₂ O content = 1 ppm, the NO ₂ content = 30 ppm, and the N ₂ O content = 250 ppm. The increment of N ₂ O content before and after flow was 200 ppm.

As can be understood when compared with the N ₂ O content increment of comparative example N ₂ O content increment of Examples 1 to 4, in the adsorption step in Example, N ₂ O content increment significantly reduced as compared with the comparative example Was. This is considered to be because the impurity by-product reaction of NO was suppressed and the production amount of N ₂ O was reduced by applying a predetermined pretreatment to the inorganic adsorbent. In addition, the amount of NO ₂ generated is reduced by suppressing the impurity by-product reaction. For this reason, the NO ₂ content increment can be expected to be reduced as compared with the case where the pretreatment is not performed. Thus, in the examples, the amount of by-products (N ₂ O and NO ₂ ) is reduced through the adsorption process using the pretreated adsorbent, and as a result, NO is further purified. We were able to.

X1 Purification line Y1 NO cylinder 1 Absorption contact device 2 Adsorption pipe 3 Pressure regulating valve 4A-4E On-off valve 5 Product NO outlet 6 Purge gas inlet 7 Gas outlet 8 Vacuum pump

Claims (5)

A method for purifying nitric oxide comprising an adsorption step for allowing a mixed gas containing at least nitrogen monoxide and moisture to flow through the adsorbent and adsorbing moisture to the adsorbent,
A method for purifying nitric oxide, comprising using, as the adsorbent, a pretreated adsorbent in which a pretreatment for generating an inorganic acid salt is performed in advance on an inorganic adsorbent.   The nitric oxide purification method according to claim 1, wherein the pretreatment includes an operation of bringing a mineral acid aqueous solution into contact with the inorganic adsorbent.   The nitric oxide purification method according to claim 2, wherein the mineral acid aqueous solution is a nitric acid aqueous solution.   The nitric oxide purification method according to claim 1, wherein the pretreatment includes an operation of bringing an acidic gas into contact with the inorganic adsorbent. The mixed gas contains at least one of nitrogen dioxide and sulfur dioxide as impurities,
The nitric oxide purification according to any one of claims 1 to 4, further comprising an alkali cleaning step for bringing the mixed gas into contact with an alkaline aqueous solution to absorb and remove the impurities before performing the adsorption step. Method.

Images