DE19641644A1 - Nitrous oxide dissociation during air fractionation - Google Patents

Abstract

In the dissociation of N2O enriched with methane during noble gas recovery in air fractionation units having an adjacent deoxygenation stage in which the methane is catalytically dissociated, the N2O is also catalytically dissociated in the deoxygenation stage (6). The same catalyst is used for both N2O and methane dissociation and consists of Pd, Pt, Ag or Au on an Al2O3 support.

Description

The invention relates to a process for the decomposition of N ₂ O, which is enriched with methane in air separation plants during the extraction of noble gas, and the methane being catalytically decomposed in a subsequent deoxo stage.

N ₂ O (laughing gas) is a chemically inert gas that occurs in the atmosphere at a concentration of 310 ppb, with the N ₂ O concentration in the air increasing annually by about 0.2 to 0.3%. N ₂ O has a higher boiling temperature than nitrogen, oxygen, krypton and xenon, and therefore accumulates in an air separation plant when nitrogen and oxygen are separated from the liquid oxygen. To obtain Kr / Xe, part of the liquid oxygen is passed into a krypton enrichment column. A concentrate is then obtained from the krypton enrichment column, which in addition to the main component oxygen also contains krypton, methane, N ₂ O and traces of argon, xenon and impurities.

The further enrichment of Kr and Xe is determined by the lower ignition limit in the Concentrate remaining methane determined. For security reasons, the Do not exceed methane concentration 0.5%, as methane is higher Concentrations with oxygen form an explosive mixture.

In order to enable a further enrichment of the noble gases krypton and xenon, the methane is z. B. catalytically burned in a so-called deoxo stage. In a subsequent further rectification, krypton and xenon are then obtained, but which are still contaminated with N ₂ O.

Various catalysts are generally known for the decomposition of nitrous oxide, which are often not suitable for use in air separation plants, however their effectiveness is greatly reduced in the presence of oxygen. Other Catalysts such as activated carbon, which are particularly useful in nitrous oxide decomposition can prove effective at high oxygen excess because of Risk of explosion not used for safety reasons.

The object of the present invention is therefore to demonstrate a method with which N ₂ O which accumulates during air separation can be decomposed. In particular, a process is to be developed with which the noble gases krypton and xenon can be obtained in higher purity.

According to the invention, this object is achieved in that the N ₂ O is catalytically decomposed in the deoxo stage.

In the deoxo stage, a complete catalytic conversion of the methane into carbon dioxide and water is achieved at a temperature between 300 ° C and 350 ° C. The maximum temperature load of the catalysts used is usually about 600 ° C. In contrast, the thermal decomposition of N ₂ O into nitrogen and oxygen starts at atmospheric pressure at temperatures above 600 ° C to 650 ° C. Surprisingly, it has been shown that N ₂ O can be decomposed in the deoxo stage even at temperatures below 600 ° C.

Under the conditions in the deoxox stage, the catalytic conversion of N ₂ O into N ₂ and O2 begins at 180 ° C. The catalyst is expediently heated to 400 ° C. to 600 ° C., preferably to temperatures above 500 ° C., whereby a complete decomposition of the N ₂ O is achieved.

At higher N ₂ O concentrations, the catalyst is preferably also brought to a higher temperature. It is also advantageous to increase the catalyst temperature as methane concentrations increase. With simultaneous conversion of methane and nitrous oxide, there is competition for the active centers of the catalyst. At higher temperatures, however, both the methane and the nitrous oxide can be completely decomposed.

With increasing concentrations of N ₂ O and / or CH ₄ , the temperature of the catalyst is advantageously increased further, the maximum temperature load of the catalyst of, for example, 600 ° C. being an upper limit. If the gas entering the catalyst is too contaminated, the space velocity, which describes the gas throughput based on the catalyst volume, is expediently reduced. This can be achieved by reducing the amount of gas flowing through the catalyst per hour or by increasing the amount of catalyst. Both effects result in an increased conversion of methane and nitrous oxide in the catalyst.

All materials which act both in the methane and in the nitrous oxide decomposition are suitable as catalysts. Catalysts are preferably used which consist of a noble metal, such as Pt, Pd, Au or Ag, on support materials, such as, for. B. Al ₂ O ₃ exist. Layer catalysts are used with particular advantage, the individual layers being optimized either for their N ₂ O or CH ₄ decomposition both in their composition and in terms of the process conditions.

After the methane and nitrous oxide have been burned in the deoxo stage, the CO ₂ and H ₂ O formed during the methane decomposition are removed from the gas by molecular sieve adsorbers. These adsorbers are expediently designed such that residual traces of N ₂ O remaining in the gas are also adsorbed. It is particularly favorable if the adsorber is provided with an additional layer with a higher capacity for laughing gas.

The invention is particularly suitable for the decomposition of N ₂ O, which occurs in a gas mixture consisting essentially of oxygen. However, the process is just as effective if, instead of oxygen, air is the main component of the gas. The invention is suitable for a wide variety of N ₂ O concentrations, since adaptation to the particular gas composition is easily possible via the process temperature and the space velocity.

The nitrous oxide reduction according to the invention can be independent of the prevailing pressure be performed. Nitrous oxide decomposition has proven to be beneficial Atmospheric pressure proved. It is also particularly expedient to have the decomposition in the pressure range between 10 and 18 bar, which is often present in the deoxo stage to make.

The invention is to be explained in more detail by way of example with reference to the schematic drawings will.

Fig. 1 shows the conversion of N ₂ O on the catalyst in the Deoxostufe at an N ₂ O concentration in the gas stream of 50 vppm and 250 vppm.

Fig. 2 shows schematically part of a plant for krypton / xenon extraction of an air separator.

In the deoxo stage 6 of an air separation plant with noble gas extraction, a catalyst is used which consists of a carrier made of aluminum oxide, which is coated with 0.5% Pd in the outer layer. 0.5% CH _{4 is} completely decomposed on this catalyst at a temperature between 170 and 340 ° C. With reference to Fig. 1 of the N ₂ O conversion is seen on this catalyst. With an N ₂ O concentration of the incoming gas stream of 50 Vppm, a complete decomposition of the N ₂ O is achieved at 400 ° C, but the catalyst is also effective at 250 ° C. At higher N ₂ O concentrations, e.g. B. at 250 Vppm N ₂ O, a temperature of at least 500 ° C is necessary for complete N ₂ O degradation.

FIG. 2 schematically shows part of the krypton / xenon extraction in an air separation plant. From a pre-enrichment column 1 , O ₂ , N ₂ O and CH ₄ as well as traces of Kr and Xe pass through a compressor 2 , via heat exchangers 3 and 4 and a heating device 5 to a catalyst bed 6 , the so-called deoxo stage. For further processing, the gas is passed over the molecular sieve adsorbers 7 and 8 .

The catalyst of deoxo stage 6 is heated to a temperature of 570 ° C. The gas stream supplied to deoxo stage 6 is composed of approximately 98% oxygen, 1% krypton, a maximum of 0.5% methane and traces of argon, xenon and impurities. The nitrous oxide concentration is approximately 100 to 150 Vppm, but can also rise to values of up to 300 Vppm. The gas stream is passed over the catalyst at a space velocity of 2000 h ^-1 . A complete breakdown of methane and nitrous oxide is achieved. The methane decomposition products emerging from the catalyst are then adsorbed in the molecular sieve adsorbers 7 , 8 . These are designed so that, in addition to CO ₂ and H ₂ O, residual traces of N ₂ O are also removed from the gas stream. Molecular sieves of type 13 X or 5 A are suitable for this purpose. A further rectification can be used to obtain high-purity krypton and xenon from the gas purified from N ₂ O and methane.

Claims (7)

1. A process for the decomposition of N ₂ O, which is enriched with methane in air separation plants during the noble gas extraction, and wherein the methane is catalytically decomposed in a subsequent deoxo stage ( 6 ), characterized in that the N ₂ O in the deoxo stage ( 6 ) is catalytically decomposed. 2. The method according to claim 1, characterized in that N ₂ O and methane are decomposed using the same catalyst. 3. The method according to any one of claims 1 or 2, characterized in that Pd, Pt, Ag or Au are used as a catalyst on a support made of Al ₂ O ₃ . 4. The method according to any one of claims 1 to 3, characterized in that the N ₂ O is decomposed at a temperature between 400 ° C and 600 ° C. 5. The method according to claim 4, characterized in that the N ₂ O is decomposed at a temperature of at least 500 ° C. 6. The method according to any one of claims 1 to 5, characterized in that a higher temperature in the deoxo stage ( 6 ) and / or a lower space velocity are set at higher concentrations of N ₂ O and / or methane than at lower concentrations. 7. The method according to any one of claims 1 to 6, characterized in that after the decomposition of the N ₂ O residues of N ₂ O through a molecular sieve ( 7 , 8 ) are removed.