Method of liquefying and condensing nitrogen trifluoride and a m

Details Method of liquefying and condensing nitrogen trifluoride and a m Method of liquefying and condensing nitrogen trifluoride and a m

1989-06-26

1990-12-04

Russel, Jeffrey E.

Chemistry of inorganic compounds

Nitrogen or compound thereof

Binary compound

55 23, 55 69, 55 71, 62 11, 62 22, B01D 500, C01B 21083, F25J 300

Patent

active

049752596

DESCRIPTION:

BRIEF SUMMARY
TECHNICAL FIELD

The present invention relates to a method of liquefying and condensing nitrogen trifluoride (hereinafter referred to as NF.sub.3) formed in the production of NF.sub.3 and to a method of purifying NF.sub.3 formed in the production of NF.sub.3.


BACKGROUND ART

NF.sub.3 is a colorless gas having such physical properties that the boiling point is -129.degree. C., and the melting point is -208.degree. C.
Since the activity of NF.sub.3 is suitably weak as a fluorine source in comparison with fluorine (F.sub.2), and the toxicity thereof is low, conventionally NF.sub.3 was used as a fluorine source in adjusting fluoroolefins, and was used as an oxidizing agent of high energy fuels. Also, in recent years, NF.sub.3 as a cleaning agent for CVD apparatuses in the field of electronic materials as well as as a dry etching gas for very large-scale integrated circuits has become to be used.
There are many processes for the production of NF.sub.3. As major processes thereof can be mentioned, for example, a molten salt electrolytic process using an ammonium acid fluoride (U.S. Pat. No. 3,235,474), a process wherein an ammonium acid fluoride in a molten state is reacted with gaseous fluorine (Japanese Patent Publication No. 8926/1980), and a process wherein an ammonium complex of a metal fluoride in the solid state is reacted with F.sub.2 in the elemental state (Japanese Patent Publication No. 21724/1987).
In any of these processes, in the reaction step of producing NF.sub.3, it is required that an inert gas is introduced as a carrier gas into the reaction system in order to prevent explosion to improve the safety or in order to control the reaction suitably, and as this inert gas, inexpensive nitrogen (N.sub.2) gas is employed in most cases. Even in the case wherein the reaction is carried out without making a particular introduction of a carrier gas from the outside, in any of the above processes, N.sub.2 gas is formed as a by-product, and remains mixed in the formed NF.sub.3 gas.
Therefore, when NF.sub.3 produced in any of the above processes is to be used for various applications as mentioned above, it is required to eliminate impurities such as N.sub.2 gas, nitrous oxide (N.sub.3 D), and carbon dioxide (CO.sub.2). However, since generally N.sub.2 gas (N.sub.2 gas as a carrier gas, and N.sub.2 gas as a by-product) is contained in a considerable amount in the NF.sub.3 produced in any of the above processes, in order to eliminate the N.sub.2 gas, the so-called NF.sub.3 condensing step wherein the N.sub.2 gas is eliminated after the reaction step is joined to follow the reaction step. (Note that, in some cases, a purifying step of eliminating impurities other than N.sub.2 gas such as N.sub.2 O and CO.sub.2 as mentioned above, and hydrogen fluoride (HF) that has remained unreacted or has been produced as a by-product is also placed between the reaction step and the condensation step.)
In this case, although there are various ways of condensing NF.sub.3, a process of liquefying NF.sub.3 using a refrigerant is commonly used as the most effective process since impurities will not be introduced and the facilities are simple in comparison with other processes wherein compression by using a compressor is effected. As the refrigerant for liquefying, use is made of a liquefied gas having a boiling point lower than that of NF.sub.3 such as liquid nitrogen, liquid air, and liquid argon, and when, of these, liquid nitrogen is used, it is the most preferable because, for example, the condensation of NF.sub.3 becomes easy, N.sub.2 is an inert substance, therefore the use thereof is safe, and N.sub.2 is inexpensive.
However, in the case wherein NF.sub.3 is produced by using apparatus including the production steps as mentioned above, when the NF.sub.3 formed in the condensation step is cooled and liquefied by using liquid nitrogen as a refrigerant, because N.sub.2 gas that is a carrier gas and N.sub.2 gas that has been formed as a by-product are also partly liquefied together with NF.sub.3, the pressure in the condensat

REFERENCES:
patent: 3181305 (1965-05-01), Schoenfelder
patent: 3235474 (1966-02-01), Thompkins et al.
patent: 4543242 (1985-09-01), Aramalli et al.
patent: 4804447 (1989-02-01), Sartori

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