EP2013607A1

EP2013607A1 - Method and device for determining the total oxygen content and/or the total carbon content in ammonia

Info

Publication number: EP2013607A1
Application number: EP07727227A
Authority: EP
Inventors: Karl-Heinz Wostbrock; Walther Schmid
Original assignee: BASF SE
Current assignee: BASF SE
Priority date: 2006-03-28
Filing date: 2007-03-22
Publication date: 2009-01-14
Also published as: WO2007110365A1; DE102006014278B3; CN101410707A; KR20080109051A; JP2009531685A; US20090066958A1; TW200806973A; US7907283B2

Abstract

The invention relates to a method for determining the total oxygen content and the total carbon content in ammonia, wherein ammonia is firstly cleaved into nitrogen and hydrogen, then the oxygen still contained in the ammonia reacted with hydrogen to give water and the carbon still contained in the ammonia reacted with hydrogen to give methane, essentially to completion. In a subsequent step, the water content and the methane content of the gas are determined. Finally the determinations of the total oxygen content from the water content and the total carbon content from the methane content are carried out. The invention further relates to a device for carrying out the methods, comprising a cracker for cleaving the ammonia and for reacting the compounds containing oxygen and/or carbon and at least one cavity ring-down spectrometer for recording the water content and/or carbon content. The cracker and the at least one spectrometer and all devices and connector lines between the cracker and the spectrometer are blanketed in an inert gas.

Description

Method and device for determining the total oxygen content and / or the total carbon content in ammonia

description

The invention relates to a method and a device for determining the total oxygen content and the total carbon content of an intended for the qualification as highly pure ammonia, optionally containing oxygen and / or carbon-containing compounds. The method and the device are particularly suitable for the determination of the total oxygen content and / or the total carbon content in ultrapure ammonia.

High-purity ammonia is becoming increasingly important in the field of manufacturing light-emitting diodes (LEDs). Such light emitting diodes contain a luminous body, which is generally produced by an epitaxy process. This means that the luminous body is produced by a layered atomic structure. High power light emitting diodes, i. Light emitting diodes with a high light output are generally made of GaN on SiC. The nitrogen required for this purpose is supplied in the production process from ammonia. Since every foreign atom, in particular oxygen and carbon atom, weakens the light output of the LED, it is necessary to use high-purity ammonia. This need for high-purity ammonia makes it necessary to detect oxygen and carbon even in the slightest traces.

From JP-A 08-201370 a method is known to measure trace moisture in ammonia. For this purpose, the ammonia in a catalyst-containing container at a temperature in the range of 600 to 1000 ° C is split into nitrogen and hydrogen. The determination of the moisture content then takes place by means of a moisture measuring device, preferably a dew-point device or an infrared spectrophotometer. By the method carried out in this way, moisture contents in the ammonia can be determined from 1 to 10 ppm. However, this measurement accuracy is not sufficient to meet the specifications for high purity ammonia.

A measuring method that can be used to determine the water content in a gas stream up to a detection limit of 0.2 ppb and the methane content in a gas stream up to a detection limit of 2 ppb is cavity ring-down spectroscopy. This is known, for example, from US-A 2005/0122523. Corresponding devices are sold, for example, by the company TigerOptics. However, the specification for these devices indicates that they are not intended for the determination of water or Methane in ammonia are suitable. This is due to the similar spectra of water, methane and ammonia.

A further disadvantage of the processes known from the prior art is that in each case only the water content or the methane content can be determined. However, in addition to water and methane, further compounds containing oxygen and / or carbon are present in the industrially produced ammonia. These

Compounds are, for example, longer-chain hydrocarbons, carbon monoxide,

Carbon dioxide as well as molecular oxygen. These compounds also contain oxygen or carbon, which can be introduced into a diode layer, for example, in the production of LEDs as an impurity atom.

It is therefore an object of the present invention to provide a process by which the total oxygen content and the total carbon content can also be determined in small traces of an ammonia intended for qualification as highly pure, which contains, if appropriate, oxygen and / or carbon-containing compounds.

Another object of the invention is to provide a device for carrying out the method.

The object is achieved by a method for determining the total oxygen content and the total carbon content of a highly pure ammonia for the qualification, which optionally contains oxygen and / or carbon-containing compounds, which comprises the following steps:

a) splitting the ammonia into nitrogen and hydrogen; b) substantially complete conversion of the oxygen of all oxygen-containing compounds with hydrogen to water and of the carbon of all carbon-containing compounds with hydrogen to methane; c) determination of the water content and the methane content in the gas; d) Determination of the total oxygen content from the water content and the total carbon content from the methane content.

The ammonia, in which the total oxygen content and / or the total carbon content are determined, may be both liquid and gaseous. The method is suitable for determining the total oxygen content and / or the total carbon content in both liquid and gaseous ammonia. To determine the total oxygen content and / or the total carbon content, a sample is taken, which is split into hydrogen and nitrogen. in this connection The sample may be taken either as a single sample or preferably continuously. Due to the high temperature required for the cleavage of the ammonia into hydrogen and nitrogen, the ammonia which is supplied to the measurement, even in the determination of the total carbon content and / or the total oxygen content in liquid ammonia, before the cleavage is completely gaseous.

By splitting all oxygen-containing compounds and all carbon-containing compounds and their recombination to water and methane can be according to the invention in a simple manner only by determining the water content and the methane content of the total oxygen content and the total carbon content determine.

Another advantage of the method according to the invention, to determine all oxygen or carbon-containing compounds in that they are converted to water or methane, is that in particular CO ₂ with ammonia enters into a large number of compounds and for this reason difficult to isolate and quantified. Since it is generally sufficient that the level of impurities in ammonia is known, regardless of the compound in which they are present, it is sufficient to determine the total carbon content and the total oxygen content and not the presence of individual compounds that contaminate the ammonia. A further advantage of the method according to the invention by the determination of the total oxygen content and the total carbon content is that not every substance has its own detection limit and thus these detection limits add up for the total contaminants, but that by conversion into water or methane respectively Oxygen detection limit and a carbon detection limit is sufficient and therefore the total carbon content or total oxygen content can be detected up to a detection limit lower than the sum of the individual detection limits with separate detection of impurities.

Due to the almost complete cleavage of the ammonia in nitrogen and hydrogen, the ammonia content after cleavage is less than 1 ppm, and because of the compounds only contained in traces containing carbon or oxygen, due to the high hydrogen excess in the gas stream in compounds or Molecularly contained oxygen substantially completely to water and the carbon contained in the gas stream in compounds substantially completely converted to methane. "Essentially completely implemented" in the sense of the present invention means that at least 98% by volume, preferably at least at least 99% by volume and in particular at least 99.5% by volume of the oxygen or carbon contained are converted into water or methane.

In a preferred embodiment, the cleavage of the ammonia into nitrogen and hydrogen and the reaction of the compounds contained in the ammonia contained oxygen and / or carbon are carried out with hydrogen to water and / or methane in the presence of a catalyst. The cleavage of the ammonia and the reaction of the compounds containing oxygen or carbon in the ammonia to form water or methane can be carried out in any reactor known to those skilled in the art. Suitable reactors are, for example, tube reactors, fluidized bed reactors or fluidized bed reactors. However, the operation of the process is independent of the shape of the reactor. Since the main task of the reactor is the cleavage of the ammonia into nitrogen and hydrogen, the reactor is also referred to as a cracker.

The catalyst may be present, for example, as a coating of the reactor wall. In fluid bed or fluidized bed reactors, preferably the fluid bed or fluidized bed granules contain the catalyst. In a further embodiment, for example, catalyst-containing packing or other catalyst-containing internals can be introduced into a tubular reactor. The catalyst can be applied for example as a coating on the packing or internals. But it is also possible to produce the filler or internals completely made of catalyst material. Another possibility is a material for the Wirbelschichtbzw. Fluid bed granules to use the packing or the internals, which contains the catalyst on a support material.

Suitable catalysts for cleaving the ammonia into nitrogen and hydrogen and for reacting the oxygen of the oxygen-containing compounds to water and the carbon-containing compounds to methane are, for example, noble metals. Preferred catalysts are silver and all catalysts known to those skilled in the art, with the aid of which ammonia can be synthesized.

In a preferred embodiment, the water content and / or the methane content are determined by cavity ring-down spectroscopy. The cleavage of the ammonia into hydrogen and nitrogen eliminates the interference from the ammonia in the spectroscopic measurement. Since 2 moles of hydrogen and nitrogen are formed during the cleavage of 1 mol of ammonia, this doubles the volume of the gas stream. In order to determine the proportion of oxygen or carbon atoms in ammonia, therefore, the determined value must also be doubled. the. Thus, for example, with a measured water content of 5 ppb in the gas stream in the ammonia actually 10 ppb water are included.

In a preferred embodiment, the cracking of the ammonia and the reaction of the oxygen and / or carbon-containing compounds to water or methane takes place at a temperature of at least 600 ° C., preferably of at least 800 ° C., more preferably of at least 900 ° C. At this temperature, the oxygen from all compounds containing oxygen to water and the carbon from all carbon-containing compounds in the presence of the above-described catalyst are substantially completely reacted.

In general, the cleavage of ammonia to hydrogen and nitrogen and the reaction of the oxygen of all oxygen-containing compounds and / or the carbon of all carbon-containing compounds to water and methane occur substantially simultaneously. These reactions are generally carried out in a cracker whose walls are coated with silver inside. In order to achieve a larger catalyst area, the cracker can also be filled with catalyst-containing granules, which is flowed through by the ammonia to be cleaved. However, it is also possible first to split the ammonia into nitrogen and hydrogen in a reactor and then to convert the oxygen of the oxygen-containing compounds and the carbon of the carbon-containing compounds into water or methane in a second reactor. This is particularly useful if different catalysts and / or different temperatures are used for cleaving the ammonia and for reacting the oxygen and the carbon. In this case, a reactor contains a catalyst suitable for cleaving the ammonia and is operated at a temperature suitable for cleaving the ammonia, while the second reactor contains a catalyst for reacting the oxygen of the oxygen-containing compounds with hydrogen to form water and the carbon of the Carbon-containing compounds is suitable with hydrogen to methane, and which is operated at a temperature suitable for this purpose.

When using cavity ring-down spectroscopy to detect the water content or the methane content in the gas, different devices or multi-channel devices in which a single measurement can be performed in each channel are required. Thus, a device for determining the water content and a device for determining the methane content are used. The detection limit for water in the gas is 0.2 ppb with cavity ring-down spectroscopy, and 2 ppb for methane, where volume ppb is meant in each case. The devices with which the water content or the methane content in the gas are determined, are connected in a variant in series. In this case, the gas preferably flows first through the spectrometer, in which the water content is determined, and then through the spectrometer, in which the methane content is determined. Subsequently, the gas is released as exhaust gas to the environment. Of course it is also possible to measure the methane content in the first spectrometer and the water content in the second spectrometer.

In a further embodiment, the devices for determining the water content and the methane content are connected in parallel. For this purpose, the gas stream is first divided. Subsequently, a partial flow is supplied to the device for measuring the water content and a partial flow to the device for measuring the methane content. Once the measurement has been completed, the gas is also released to the environment or returned to production.

In order to avoid that oxygen-containing compounds or carbon-containing compounds from the environment in the gas flow, in which the oxygen content or the carbon content is to be determined, the process is preferably operated under an inert gas atmosphere. For this purpose, for example, the cracker, optionally the further reactor, the measuring devices and all connections between these devices are flowed around by an inert gas. Suitable inert gases are all gases which contain at most slight traces of oxygen and carbon. Preferred gases are noble gases, nitrogen, hydrogen and the gas mixture resulting from the splitting of ammonia. The gas mixture resulting from the decomposition of ammonia is, for example, the exhaust gas from the measuring method. Particularly preferred as an inert gas is nitrogen. The substances which disturb the determination of the total carbon content and / or total oxygen content can alternatively also be avoided by operating the process in an evacuated protection system.

When operating the method in an evacuated protection system, the residual gas atmosphere preferably consists of the abovementioned inert gases.

Due to the very small amounts of compounds containing oxygen and / or carbon in the ammonia, for example, even the smallest leaks at connection points, for example between a line and a measuring device or a line and the cracker, suffice for oxygen, water or carbon dioxide from the air the measuring system can penetrate, if it is not lapped by inert gas. Another way to avoid that contaminants containing oxygen or carbon from the environment penetrate into the measuring system is, for example, to gas-tightly weld all connections. Conventional gas-tight releasable connections are generally not sufficient, since these can not be sure that not even small traces of diffuse therethrough. Due to the large volume of gas that surrounds the device for measuring the total carbon content and / or the total oxygen content, a technical gas, which still contains up to 0.2% of foreign gases, is sufficient as an inert gas, since the diffusion flows through the gas seals generally so low are that the impurities do not distort the measurement result.

It is not sufficient to operate the device for determining the total oxygen content and the total carbon content under overpressure in order to avoid that oxygen or carbon-containing compounds from the environment diffuse into the gas to be measured. Even with overpressure operation, a part of the gas always diffuses against the flow direction into the device and thus falsifies the measurement result.

According to the invention, the apparatus for carrying out the process for determining the total oxygen content and / or the total carbon content in ammonia comprises a cracker in which the ammonia is split into hydrogen and nitrogen and in which the oxygen and / or carbon-containing compounds are converted to water and methane and at least one cavity ring-down spectrometer to measure water content and / or carbon content. Here, the cracker, the at least one spectrometer and all devices and connecting lines between the cracker and the spectrometer are enclosed by an inert gas. For this purpose, the cracker, the compounds and the at least one spectrometer are preferably accommodated in a container which is flooded with the inert gas.

In particular, when various catalysts are used for ammonia splitting and for reacting the oxygen and the carbon to water and methane, it is possible that the cracker another reactor is connected downstream. In this case, the ammonia is split into hydrogen and nitrogen in the cracker, in the subsequent reactor, the oxygen of the oxygen-containing compounds and the carbon of the carbon-containing compounds are converted to water and methane.

However, it is also possible to operate the process so that the cleavage of the ammonia into hydrogen and nitrogen and the reaction of the oxygen of the oxygen substance-containing compounds and the carbon of the carbon-containing compounds are carried out together in a cracker. For this purpose, it is possible, for example, to coat the cracker with different catalysts so that the appropriate catalyst required for each reaction is present.

In the following the invention will be described in more detail with reference to a drawing.

It shows:

FIG. 1 shows a process flow diagram of the process according to the invention, in which the water content and the methane content are determined one after the other,

Figure 2 is a process flow diagram of the method according to the invention, in which the

Water content and the methane content are measured in parallel.

FIG. 1 shows a process flow diagram of a method according to the invention in which devices for measuring the water content and the methane content are connected in series.

A cracker 1 is fed with an ammonia stream 2 which still contains traces of oxygen and / or carbon-containing compounds. In Cracker 1, the ammonia is split into nitrogen and hydrogen. At the same time, due to the high excess of hydrogen, the compounds containing oxygen and / or carbon are converted to water or methane. For this purpose, the cracker 1 is preferably provided with a catalytically active coating. Suitable catalysts are, for example, silver and all catalysts which can be used to synthesize ammonia. Particularly preferred is silver. The cracker is preferably operated at a temperature of at least 600 ° C.

The hydrogen, nitrogen, water and methane and small amounts of non-split ammonia-containing gas stream 3 is fed to a first cavity ring-down spectrometer 4. In the first cavity ring-down spectrometer 4, a small partial flow is branched off from the hydrogen, nitrogen, water, methane and small amounts of ammonia-containing gas stream 3, in which the water content is determined by cavity ring-down spectrometry. This partial flow is, as shown by the arrow with reference numeral 5, discharged after the measurement to the environment.

The remaining gas stream is, as shown by the arrow with reference numeral 6, a second cavity ring-down spectrometer 7 supplied. In the second cavity ring-down spectrometer 7, a partial stream 8 is separated from the gas stream 6. In the partial flow 8 the methane content is determined. Subsequently, the partial flow 8 is released to the environment. The remaining hydrogen, nitrogen, water, methane and residues of ammonia-containing gas stream, as shown by the arrow with reference numeral 9, delivered to the environment. However, it is also possible to supply the gas flow again to the ammonia synthesis.

In order to prevent compounds containing oxygen and / or carbon from the environment from being able to diffuse into the cracker 1 or the cavity ring-down spectrometers 4, 7, the cracker 1 and the first cavity ring-down spectrometer 4 and the second cavity ring Down spectrometer 7 enclosed by a housing 10. The housing 10 is flooded with an inert gas. Suitable inert gases are any gases in which the proportion of oxygen and / or carbon-containing compounds is preferably less than 0.2ppb. Preferred inert gases are all edegases or nitrogen, particularly preferably nitrogen. In order to avoid inert gas being diffused out of the housing 10 and being replaced by ambient air, inert gas can be supplied to the housing 10 via an inert gas feed, as shown by arrow 11.

FIG. 2 shows a process flow diagram of the method according to the invention, in which the water content and the methane content in two cavity ring-down spectrometers connected in parallel are determined. For this purpose, as in the embodiment illustrated in FIG. 1, firstly the ammonia stream 2 containing cracking compounds containing oxygen and / or carbon is fed to the cracker 1. In the cracker, the ammonia is split into hydrogen and nitrogen. Due to the high excess of hydrogen, the compounds containing oxygen and / or carbon are converted to water or methane. The resulting, hydrogen, nitrogen, water, methane and trace gas containing ammonia gas stream 3 is in a first partial stream 12, the first cavity ring-down spectrometer 4 is supplied and a second partial stream 13, the second cavity ring-down spectrometer 7 is supplied, divided. To measure the water content in the first cavity ring-down spectrometer 4, a partial flow 5 is separated from the first partial flow 12, in which the water content is determined. The remaining gas is mixed as the first exhaust gas stream 14 with a second exhaust gas stream 15 and discharged as exhaust gas 16, which contains hydrogen, nitrogen, small amounts of water and methane and ammonia, either to the environment or fed again to the ammonia synthesis. The first exhaust gas flow 14 and the second exhaust gas flow 15 can also be separately discharged to the environment or supplied to the ammonia synthesis. Mixing to the exhaust stream 16, as shown in Figure 2, is not required. The second exhaust gas flow 15 is the gas flow which is not needed in the second cavity ring-down spectrometer 7 for determining the carbon content. The carbon content is determined in the partial stream 8, which is separated from the second partial stream 13. After determining the carbon content of the partial stream 8 is released to the environment.

Also in the embodiment shown in FIG. 2, the cracker, the first cavity ring-down spectrometer 4, the second cavity ring-down spectrometer 7 and all the lines connecting the cracker 1 and the cavity ring-down spectrometers 4 and 7 are of one Housing 10 enclosed, which is flooded with an inert gas. In this embodiment too, inert gas escaping from the housing 10 can be replaced by an inert gas feed 11.

LIST OF REFERENCE NUMBERS

1 cracker

2 ammonia stream

3 gas flow

4 first cavity ring-down spectrometer

5 partial flow

6 gas flow

7 second cavity ring-down spectrometer

8 partial flow

9 gas flow

10 housing

1 1 inert gas supply

12 first partial flow

13 second partial flow

14 first exhaust gas flow

15 second exhaust gas flow

16 exhaust

Claims

claims

1. A method for the determination of the total oxygen content and the total carbon content of an ammonia which is suitable for the qualification as highly pure and which contains optionally oxygen and / or carbon-containing compounds, comprising the following steps:

a) splitting the ammonia into nitrogen and hydrogen;

b) substantially complete conversion of the oxygen of all oxygen-containing compounds with hydrogen to water and / or the carbon of all carbon-containing compounds with hydrogen to methane;

c) determination of the water content and the methane content in the gas;

d) Determination of the total oxygen content from the water content and the total carbon content from the methane content.

2. The method according to claim 1, characterized in that the water content and the methane content are determined in each case by cavity ring-down spectroscopy.

3. The method according to claim 1 or 2, characterized in that the splitting of the ammonia and the reaction of the oxygen and / or carbon-containing compounds to water and / or methane at a temperature of at least 600 ° C.

4. The method according to any one of claims 1 to 3, characterized in that the cleavage of ammonia and the oxygen and / or carbon-containing compounds proceeds almost simultaneously with the reaction of hydrogen and oxygen to water.

5. The method according to any one of claims 1 to 4, characterized in that the water content and the methane content are detected separately.

6. The method according to claim 5, characterized in that the water content and the methane content are detected by series-connected measuring devices.

7. The method according to claim 5, characterized in that the water content and the methane content are detected by parallel connected measuring devices.

8. The method according to any one of claims 1 to 7, characterized in that the device for cleaving the ammonia and the oxygen and carbon-containing compounds, the devices for detecting the oxygen content and the carbon content and all connections between these devices under an inert gas atmosphere and / or in an evacuated

Protection system operated.

9. The method according to claim 8, characterized in that the inert gas is substantially free of oxygen and carbon-containing compounds.

10. The method according to claim 8 or 9, characterized in that the inert gas is a noble gas, nitrogen, hydrogen or the gas mixture resulting from the cleavage of ammonia.

1 1. The method according to any one of claims 1 to 10, characterized in that the oxygen and / or carbon-containing compounds include carbon monoxide, carbon dioxide, hydrocarbons, oxygen and water.

12. An apparatus for carrying out the method according to any one of claims 1 to 1 1, comprising a cracker for cleaving the ammonia and at least one

Cavity ring-down spectrometer for detecting the water content and / or carbon content, wherein the cracker and the at least one spectrometer, and all devices and connecting lines between the cracker and the spectrometer are enclosed by an inert gas.

13. The apparatus according to claim 12, characterized in that the cracker is followed by a reactor in which react the carbon of the carbon-containing compounds with hydrogen to methane and the oxygen of the oxygen-containing compounds with hydrogen to water.

14. The apparatus according to claim 12, characterized in that the reaction of hydrogen with the oxygen of the oxygen-containing compounds to water and the reaction of hydrogen with the carbon of the carbon-containing compounds is carried out to methane in the cracker.