JPH0114164B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for adsorbing and removing trace amounts of nitrogen (N ₂ ) in argon gas (Ar). Since Ar is chemically inert, it is widely used along with nitrogen etc. as an atmosphere for chemical reactions. Particularly in recent years, with the development of the semiconductor industry, high-purity argon has been frequently used for the atmosphere in the synthesis or processing of high-purity crystalline silicon, and the demand for it has increased dramatically. Ar is contained in the air at about 0.9vol%, and usually
This air undergoes two to three stages of low-temperature rectification to obtain Ar with a purity of over 99%. Since Ar is about three times more expensive than N ₂ , it is necessary to recover and reuse used Ar to improve economic efficiency. Used Ar contains various impurities depending on its usage conditions. Contamination with moisture or carbon dioxide gas can be removed by adsorption using silica gel, zeolite, etc., or solidified and separated by low-temperature freezing. Further, the oxygen in Ar can be used to generate water on the deoxo catalyst by hydrogen blowing, and the generated water can be removed by the method described above. In each of the above methods, the impurity concentration is set to 1 vol.
It is widely used as an industrial method to reduce the amount below ppm. Conventionally, when the impurity contained in Ar is N ₂ , a removal method using a high-temperature reaction with a titanium sponge has been used. According to this method,
Purification levels of less than 1 vol.ppm of N ₂ can be easily achieved;
The cost of recovering Ar increases significantly because the reaction temperature requires a temperature of 800°C or higher and the efficiency of using titanium as a raw material is low due to reaction inhibition by the reaction product, titanium nitrogen. On the other hand, several methods are known that use adsorbents to remove _N2 from gas. For example, Nittokuko 52−
Publication No. 20959 describes a method of using natural mordenite or natural clinoptilolite to remove low concentration N ₂ from oxygen obtained from an air liquefaction separation device, and Japanese Patent Publication No. 52-42755 describes a method for removing low concentration N 2 from oxygen obtained from an air liquefaction separation device. Obtained using liquefaction separation equipment and crude Ar rectification equipment
A method is disclosed in which nitrogen impurities in Ar are adsorbed and removed using zeolite at room temperature and at 5 to 35 kg/cm ² G. Although these techniques suggest the possibility of nitrogen removal or Ar purification using zeolite, the obtained gas purity and yield are low, and it cannot be said that they are necessarily sufficient as a recovery technique for high-purity Ar. The present inventors have explored and studied various methods for recovering high-purity Ar from Ar containing _N2 with an extremely high recovery rate using an appropriately sized device. It was discovered that the above object could be achieved by combining the two, and the present invention was completed. According to the present invention, it is not only possible to remove N ₂ mixed in Ar by using a small amount of adsorbent, but also it is possible to drastically reduce the disposal of expensive Ar outside the system, which is necessary for operation, and it is also possible to remove N 2 mixed in Ar. It is possible to obtain product Ar with high purity. The contents will be explained in detail below. The adsorbent used in the present invention preferably has a higher selective adsorption property for N ₂ than Ar and a large N ₂ adsorption capacity. Zeolites with high N ₂ adsorption capacity are known as zeolites such as 5A, 13X, mordenite, etc. Among these adsorbents, mordenite zeolite has a high adsorption capacity compared to other zeolites such as 5A type,
The present inventors discovered that it has superior N ₂ adsorption selectivity to 13X type zeolite, and as a result of further investigation, by ion-exchanging ordinary mordenite with specific ions to a certain exchange rate or higher,
It has been found that particularly excellent adsorption selectivity for N ₂ can be maintained. Mordenite is generally Na ₂ O・Al ₂ O ₃・
It is represented by the composition of XSiO ₂ .nH ₂ O, and X (silica to alumina molar ratio) is about 10-20. n is the amount of water adsorbed in the mordenite crystal, for example, when X = 10, n
=6. Although Na ₂ O in the formula is expressed in the form of an oxide, it exists as Na ions in the crystal, and other cations such as alkali metal ions such as Li ⁺ and K ⁺ , alkaline earths such as Mg ²⁺ and Ca ²⁺ It can be exchanged with similar ions, other metal ions, ammonium ions, hydrogen ions, etc. Japanese Patent Publication No. 46-37162 shows that various ion exchangers of mordenite have a relatively high adsorption capacity, and this can be used to absorb N ₂ in a hydrogen stream.
Methods used for removal have been proposed. In this,
In particular, Mg ²⁺ ion-exchanged mordenite has a large N ₂ adsorption capacity, making it suitable for hydrogen stream purification. In the purification of _N2- containing Ar, which is the target of the present invention, Ar
One of the objectives is to suppress losses and achieve an extremely high recovery rate. Mordenite adsorbents that have adsorbed and captured N ₂ are generally subjected to regeneration operations such as heating, depressurization, or purging in order to restore their N ₂ capture ability, but at this time concentrated N ₂ is released from the adsorbent used. In addition, the co-adsorbed Ar is also desorbed and discharged from the system together. Naturally, the smaller the amount of Ar that is discharged, that is, the amount of co-adsorbed Ar, the higher the Ar recovery rate can be maintained.
Regarding this point, the present inventors further investigated and found that the exchange ion of mordenite is more than Mg ²⁺ .
Even better results were obtained in the case of Ca ²⁺ and/or Sr ²⁺ , and ^the exchange amount of these
It has been found that it is preferable to have an equivalent of 50% or more in Sr ²⁺ . That is, the mordenite with the most suitable composition for Ar purification according to the present invention is one in which more than 50% equivalent of its cations have been exchanged with Ca ²⁺ and/or Sr ²⁺ . It is preferable to use mordenite in a so-called activated state, in which water present in the crystals has been removed by such methods. Next, the operation according to the present invention will be explained in detail. Although the method for contacting the adsorbent with N ₂ -containing Ar (raw material gas) is not particularly limited, it is usually carried out by a fixed bed adsorption method using a packed column filled with an adsorbent. This raw material gas flows in from one end of the adsorption column, the contained N ₂ is selectively adsorbed by the adsorbent, and flows out from the other end as purified Ar containing no N ₂ . The _N2 concentration in the raw material gas is not specified, but it is 10vol.
% or more, preferably 1 vol% or less, is particularly suitable for carrying out the present invention. The adsorption temperature condition is desirably -20°C or lower, and is controlled by the adsorbent temperature and/or the feed gas temperature. If the temperature is higher than that, the N ₂ adsorption ability will decrease, which is not preferable. The lower the adsorption temperature, the greater the N ₂ adsorption capacity, and the temperature can be lowered as long as the raw material gas does not liquefy, but considering the cost of cooling, it is practical to set the temperature to -100°C or higher. It is. In the present invention, it is important that the pressure inside the column during adsorption is equal to or higher than atmospheric pressure. When the raw material gas continues to flow, the adsorbent adsorbs N ₂ from the inflow end, forming a so-called N ₂ adsorption zone, which moves to the outflow end. When a certain amount of raw material gas flows in, the N ₂ adsorption zone reaches the outflow end. Gas inflow is stopped at or before this point. The inside of the adsorption tower with gas inflow stopped contains _N2 .
Filled with Ar. The adsorption layer that has adsorbed N ₂ can be regenerated by heating the same layer from the outside, or by suctioning and extracting the gas remaining in the adsorption tower from one end of the adsorption tower, for example, the outlet end of the purified gas. is heated to 0°C or higher, for example 20°C, using a heater outside the tower, and then fed into the adsorption tower from the raw material gas inlet end of the adsorption tower used for adsorption, and the adsorption tower is heated to a predetermined temperature. The adsorption tower is heated by continuing circulation until the adsorption tower temperature reaches the regeneration temperature. Adsorbed at adsorption temperatures below -20℃
N ₂ and the remaining Ar are desorbed from the adsorbent as the temperature of the adsorption tower increases due to the above operation. When the circulation method is used, when the desorbed gas joins the circulation system, the pressure in the circulation circuit increases, so the gas is extracted from the circuit at any time to control the pressure in the system to avoid excessive pressure. After the temperature of the adsorption tower reaches a predetermined regeneration temperature through gas circulation, the circulation of the heated gas is stopped. At this time, N ₂ that was desorbed during the temperature rise still remains in the adsorption tower, so it is necessary to discharge it to the outside of the adsorption tower. At this time, the method of discharging outside the tower is to use a suction pump, etc. to suck and exhaust, and to use Ar that does not contain _N2 .
Possible methods include a method in which gas is introduced into the column and the gas inside the tower is swept away (purge), and a method in which these two methods are used together. In the method of suction using only a pump or purging with pure Ar after depressurization, the amount of pure Ar consumed is small, and therefore the Ar recovery rate can be kept high.
However, depending on the scale or structure of the device, there is a risk of atmospheric air entering when the adsorption tower becomes vacuum-reduced. In order to avoid this danger, the pressure within the apparatus must be maintained at all times above atmospheric pressure throughout the entire adsorption and regeneration process. In order to discharge the residual N ₂ from the adsorption layer while always maintaining the internal pressure of the adsorption tower at or above atmospheric pressure, the present invention purposely uses a method that consumes a relatively large amount of Ar due to purge.
A purge method using atmospheric pressure Ar is used. This ensures that the Ar purity of the product is maintained at a high purity of 99.9999 or higher. Furthermore, by combining purge exhaust gas recovery, which will be described later, the amount of argon discharged outside the system due to the adoption of normal pressure purge can be kept to a very small increase. The adsorption layer regeneration temperature is preferably 0°C or higher, preferably 20°C or higher. If this temperature is too high, it will cost too much to lower the adsorption layer to the adsorption temperature, so it is practical to set it to 50°C or less. It is possible to desorb (desorb) N ₂ from the adsorbent even at temperatures lower than 0°C, and it has the advantage that the amount of gas desorbed is small, especially in the process of increasing the temperature from the adsorption temperature. Ar
The purge amount increases rapidly, resulting in a decrease in the overall recovery rate of Ar. By pushing out (purging) the N ₂ in the column with pure Ar, the N ₂ concentration in the outflowing gas decreases exponentially. Initially, the outflowing gas contains a high concentration of N ₂ , but as the inflow continues, the outflowing gas reaches a point where the N 2 concentration is lower than the N ₂ concentration in the source gas. By recovering this portion to the source gas line, it is possible to maintain a high Ar yield. After the _N2 concentration in the exhaust gas has decreased sufficiently, pure Ar
The supply of Ar was stopped, and then the Ar in the adsorption layer was sucked out under conditions that the pressure inside the adsorption tower did not drop below atmospheric pressure.
After cooling the Ar to below -20°C, it is fed into the adsorption layer again, and this operation is repeated to perform circulation cooling of the adsorption layer. In addition, regardless of the circulation of cooling Ar,
It is also possible to cool the adsorption layer. During the period of adsorption bed cooling, as the adsorption bed temperature decreases,
As the adsorption of Ar progresses, the pressure inside the circulation circuit decreases, but the pressure inside the circuit must always be above atmospheric pressure.
For example, pure Ar that does not contain N ₂ is supplied into the circuit, such as Ar that has been purified using another adsorption layer. When the temperature of the adsorption layer reaches a predetermined temperature, cooling in the circuit is stopped and new source gas is introduced.
Thereafter, repeat this operation sequentially. Furthermore, by installing a plurality of adsorption layers, purified Ar can be obtained continuously. To summarize the present invention, a mordenite-type zeolite with a specifically high adsorption selectivity for _N2 vs. Ar is used, and Ar containing _N2 is brought into contact with the zeolite at an adsorption temperature of -20°C or lower, which exhibits its specificity. After obtaining high-purity Ar, the temperature of the adsorption layer is raised to a regeneration temperature of 0°C or _higher.N2 remaining in the adsorption layer is then discharged by a temporary flow of pure Ar, and then the adsorption layer is heated. Reduced to adsorption temperature and purified
Ar is introduced and the above operations are repeated one after another while always maintaining the system pressure at or above atmospheric pressure. An example of an apparatus for carrying out the present invention is shown in Figure 1. The adsorption tower 1 is filled with mordenite-type zeolite. The raw material gas to be purified is sucked and discharged by the blower 2 through the pipe line 11 and the flow path switching valve 14.
After being cooled by passing through the switching valve 15 and the cooler 3, it is fed into the adsorption tower 1 from the inlet end 7 via the switching valves 16 and 17. N ₂ in the raw material gas is selectively adsorbed by the packed zeolite, and Ar containing no N ₂ flows out from the outlet end 5 and is sent to the product Ar pipe 8 via the flow path switching valve 18 . After feeding a certain amount of raw material gas, 14, 15, 16,
Switch 18, blower 2, heater 4, adsorption tower 1
The gas discharged by 2 is heated by 4 and then fed into the adsorption tower 1 where it is heated. The gas flowing out from the outlet end 5 passes through the pipe line 6 and the switching valve 14 and circulates again to the suction side of the blower 2. The temperature of the adsorption tower 1 is raised by heating and circulating the gas in the circuit, and the adsorbed N ₂ and Ar are desorbed. This desorbed gas returns to 2 via 5, 18, 6, and 14, and circulates within the circuit. The increase in desorbed gas caused by the temperature rise in step 1 increases the pressure in the circuit, so in order to prevent this pressure from becoming excessive, the pressure in the circuit is sensed and the valve 13 is opened.
The pressurized gas is discharged through tube 12. After the column 1 reaches a predetermined regeneration temperature, the valve 14 is switched and the valve 19 is opened, allowing pure Ar to flow in from the inlet 20, passing through 2, 15, 4, 16, 17, and 1.
sent to. The high concentration N ₂ remaining in the adsorption tower is swept away by the inflowing Ar from 7 and flows out from the outlet end 5.
8 and 13, and is exhausted from the exhaust port 12. The N ₂ concentration in the exhaust gas decreases according to the amount of Ar flowing in from 20, and reaches a time when it becomes equal to the N ₂ concentration in the source gas. At this point, 13 is closed and 14 is switched to send the gas flowing out from 5 to pipe 11 and return to the source gas system. Continue from 20 until it is confirmed that the contained N ₂ concentration has decreased sufficiently at 11.
Deliver Ar. Next, the valves 14, 15, 16, 17, and 18 are switched, and while operating the cooler 3, the air is circulated through 2, the pressure drop in the circuit is sensed, and the air is sent through the valve 19.
Pure Ar is introduced through the pipe 20, and the adsorption tower 1 is circulated and cooled to the adsorption temperature while the pressure inside the circuit is always kept above atmospheric pressure. The adsorption layer may be heated or cooled by external heating or cooling without gas circulation. Moreover, both can also be used together. The present invention can obtain highly purified Ar with a good recovery rate. Example 1 Sodium-type mordenite and mordenite in which 50 and 74 equivalent percent of the sodium ions were replaced with calcium ions, respectively, were prepared. Each sample was packed into three columns with an inner diameter of 2.7 cm and a length of 150 cm, and the temperature was kept at -20°C. Argon gas containing 1000 ppm nitrogen was introduced at a rate of 1 Nl/min, and the volume and nitrogen concentration of the gas flowing out from the column were measured to determine the amount of purified argon gas containing no nitrogen. (Table-
1) When the calcium exchange rate becomes 50% equivalent or more, the purification capacity increases dramatically.

[Table] Example 2 The same test as in Example 1 was conducted on mordenite prepared with strontium to give an exchange rate of 24% equivalent and 79% equivalent, and the amount of purified argon gas containing no nitrogen was determined, and the results are shown in Table 2. Obtained.

[Table] Comparative Example 1 A test was conducted using 5A type zeolite as the adsorbent and keeping the other conditions the same as in Example 1. The amount of purified argon gas that does not contain nitrogen is 24N.
It was hot. Comparative Example 2 A test was conducted using 13X type zeolite as an adsorbent and under the same conditions as in Example 1 except for the following conditions. The amount of purified argon gas that does not contain nitrogen is 11N.
It was hot. Example 3 Mordenite equivalent to a calcium exchange rate of 60% was packed into the column used in Example 1 and heated to -50°C. Next, argon containing 1000 ppm nitrogen is added to the inlet pressure.
It was delivered at 0.18Kg/cm ² G. After collecting the gas flowing out from the column and analyzing the nitrogen concentration, the nitrogen
502N of argon less than 1ppm was obtained. Subsequently, this adsorption column was heated to +20°C and the amount of desorbed gas and its composition were analyzed. During this temperature increase, the pressure inside the column is
It was maintained at 0.02Kg/cm ² G. The amount of desorbed gas released from the column was 10.8N, and the nitrogen concentration was 2.5% on average. Then, while keeping the column at +20℃,
Pure argon is introduced at Kg/cm ² G, and the outflow gas is
The gas was discharged until the nitrogen concentration dropped to 1000 ppm, and as the concentration continued to drop, the effluent gas was collected and continued until the nitrogen concentration reached 1 ppm or less. After supplying pure argon, the amount of exhaust gas released until reaching 1000ppm was 43.6N, and further increased to 1000ppm~
The amount of gas collected during 1 ppm was recovered to the raw material gas tank at 34.3N. Thereafter, the column was heated to −50° C. again, and pure argon was supplied during this time, so that the inside of the column was always 0.1 Kg/cm ² G. After reaching −50°C, argon containing 1000 ppm nitrogen was introduced into the column at 0.18 Kg/cm ² G, and the volume and nitrogen concentration of the gas flowing out from the column were measured.
High purity argon with a nitrogen concentration of 1ppm or less
I was able to get 500N. The ratio of the amount of argon that could be purified to the amount of lost argon that was exhausted was 90:10, and argon could be purified with a high yield of 90%. Comparative Example 3 In exactly the same manner as in Example 3, argon containing 1000 ppm nitrogen was fed into the column at -50°C to obtain 500N of purified argon with a nitrogen concentration of 1 ppm or less, and then the column was heated to -10°C. During this time, 7.7N from the column
There was gas release, and the average nitrogen concentration was 1.3%. Next, pure argon was introduced, the nitrogen concentration in the effluent gas was measured, and the gas was discharged until the nitrogen concentration decreased to 1000 ppm, and the effluent less than 1000 ppm was collected into the raw material tank, and the process was continued until the concentration was 1 ppm or less. The amount of gas discharged to reach 1000 ppm nitrogen was 81N, and the amount of gas discharged from 1000 ppm to 1 ppm was 93N. After cooling to -50°C, argon containing 1000 ppm nitrogen was passed through again to obtain 495N of argon containing less than 1 ppm nitrogen. The argon recovery rate was less than 85%.

[Brief explanation of drawings]

FIG. 1 is a process diagram showing an example of an apparatus for implementing the present invention, in which 1 represents an adsorption layer, 2 represents a blower, 3 represents a cooler, and 4 represents a heater.

Claims (1)

[Claims] 1. In a method for purifying argon gas containing nitrogen by contacting it with zeolite, 1. argon gas to be purified is introduced into a mordenite-type zeolite layer at a temperature of -20°C or lower, and nitrogen is adsorbed thereto; 2. Before the zeolite layer becomes a nitrogen adsorption zone, the introduction of the gas to be purified is stopped; 3. The gas remaining in the adsorption layer is taken out from the layer; 4. The adsorption layer is heated to 0°C or above to remove the nitrogen remaining in the same layer. 5. Then, argon gas not containing nitrogen is introduced into the adsorption layer to push out and remove the remaining nitrogen, and 6. Purification characterized by repeating the above operations one after another at atmospheric pressure or higher. Method. 2. The method according to claim 1, wherein 50% or more of the cations are mordenite-type zeolite exchanged with calcium ions and/or strontium ions.