Catalytic oxidation of volatile carbon compounds

Process for the conversion of waste gas streams containing volatile carbon compounds to carbon monoxide by contacting the gas stream with an electrically conductive metal or electrically conductive metal oxide catalyst that is heated to reaction temperature by induction heating.

FIELD OF THE INVENTION 
This invention relates to the catalytic oxidation of volatile carbon 
compounds by contacting a gas stream containing said compounds and oxygen, 
with an electrically conductive metal catalyst or an electrically 
conductive metal oxide catalyst that has been heated to reaction 
temperature by electrical induction heating. 
BACKGROUND OF THE INVENTION 
Many industrial processes produce off-gases containing volatile carbon 
compounds. It is desirable that these compounds be oxidized to carbon 
dioxide before release to the atmosphere. 
Catalytic converters in automobile exhaust systems act on a mixture of 
volatile carbon compounds and oxygen to convert the carbon compounds to 
carbon dioxide. Such converters often contain a platinum group metal, for 
example, rhodium. 
Published European Patent Application 0,585,795 (Aug. 25, 1993) notes that 
in DE-AS 22 57 968 a device is described in which metal wool is placed in 
the exhaust line before an oxidation catalyst. This device is designed in 
such a fashion that soot particles and hydrocarbons are precipitated from 
exhaust gases onto the metal wool. The gases, thus, purified and 
stabilized, then flow through the oxidation catalyst. When the engine is 
heated to operating temperature, the precipitated hydrocarbons evaporate 
and are oxidized in the oxidation catalyst. 
Some industrial gas streams, such as the gas stream from the oxidation of 
para-xylene to produce terephthalic acid, contain, in addition to carbon 
monoxide and/or hydrocarbons, other volatile carbon compounds such as 
methylbromide. It is desirable to remove the bromine component from such a 
stream, in addition to oxidizing the other volatile carbon compounds. 
SUMMARY OF THE INVENTION 
The present invention is a process for the oxidation at least one volatile 
carbon compound contained in a gas stream also containing molecular 
oxygen, which comprises heating an electrically conductive metal catalyst 
having a melting point of at least 400 degrees C. and higher than the 
temperature of operation or an electrically conductive metal oxide 
catalyst having a melting point of at least 400 degrees C. and higher than 
the temperature of operation by induction heating to a temperature 
sufficiently high to oxidize said at least one volatile carbon compound 
and contacting said stream with said heated catalyst. The catalyst is 
preferably selected from the group consisting of iron, steel, aluminum, 
stainless steel, copper, monel.TM. (an alloy comprising nickel and 
copper), and their oxides. One of the most preferred catalysts is a steel 
wool. 
Usually, the gas stream contains one or more of the following volatile 
carbon compounds: carbon monoxide, halogenated organic compounds, aromatic 
compounds, alkanes, alcohols, and esters. 
The process is effective when the catalyst is heated to a temperature in 
the range of 200 to 800 degrees C.; preferably, 400 to 600 degrees C. 
The process of the present invention works well on a gas stream from the 
oxidation of para-xylene to terephthalic acid, which contains methyl 
bromide, in which case iron bromide forms on the surface of the catalyst 
if the catalyst is steel wool. Preferably, the catalyst is heated to a 
temperature of at least 400 degrees C. by induction heating. The process 
also works well on a gas stream containing off-gas from the preparation of 
chlorofluorocarbon compounds or from the preparation of titanium dioxide 
in which the off-gas contains carbon monoxide or vapor from a hydrocarbon 
storage tank. If the off-gas does not contain sufficient oxygen to oxidize 
the volatile carbon compound(s) to the desired degree, additional oxygen 
containing gas (air, for example) may be added to the stream prior to the 
stream contacting the catalyst.

DETAILED DESCRIPTION 
An inexpensive process has been discovered which is effective in the 
destruction of halogenated organic compounds, particularly those present 
in gaseous waste streams of processes, such as the manufacture of 
terephthalic acid or chlorofluorocarbons. Gas streams from the manufacture 
of terephthalic acid also contain carbon monoxide, methane, methanol, 
methyl acetate, benzene, toluene, and para-xylene. For such a waste stream 
the preferred catalyst are electrically conductive filamentary metal or 
electrically conductive metal oxide catalysts listed here in decreasing 
order of activity: steel wool, zinc, aluminum, monel, and stainless steel. 
In some cases catalyst, in the form of electrically conductive metal or 
metal oxide shavings and spheres, are as satisfactory as filamentary 
catalysts. Induction heating of the catalyst results in more efficient use 
of energy and improved economics. The catalyst is heated to a higher 
temperature than the exiting gas. 
In order to be effective, the catalyst should have a bulk conductivity in 
the range of 0.01 Siemens/cm to 10 6 Siemens/cm at the frequency of 
operation. Normally, the process will be operated in the range of about 3 
KHz to 30 MHz Radio Frequency (RF) power. Bulk conductivity is affected by 
the composition of the catalyst, the size and shape of the catalyst, and 
the compactness of the catalyst. 
EXAMPLES 
Example 1 
A 12 mm diameter quartz tube is placed inside a copper solenoid which 
generates the necessary alternating magnetic field. The coil with an inner 
diameter of 3 cm comprised of 8 turns over the height of 12 cm. A 
capacitor is connected in parallel with the main coil and resonates the 
coil at the frequency of interest. An additional coil is placed between 
the radio frequency power source and the capacitor/coil circuit in order 
to provide impedance matching for optimal power transmission from the 
source to the reaction. Flowing water is passed through the coil and 
around the capacitor for cooling. The 27 MHz RF power was generated and 
amplified by commercially available equipment. The quartz tube contained 
1.5 grams of degreased steel wool fine grade 0, which has fiber diameter 
of 40 microns +/-14 microns. 
This apparatus was used at near atmospheric pressure to abate a mixture of 
volatile carbon compounds: methanol (MeOH), methyl bromide (MeBr), and 
methyl acetate (MeOAc) found in the off-gas from the oxidation of 
para-xylene to terephthalic acid. The gas stream contained about 5.5 
volume percent oxygen. Table 1, below, shows the results: 
TABLE 1 
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Total Feed Rate = 0.8 SLM*; Power Input = 95 Watts; 
Pressure = 2 psig 
SPECIES FEED PRODUCT % CONVERSION 
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Methane 172 ppm 16 ppm 90.5 
MeOH & MeBr 
43 ppm 2 ppm 94.5 
MeOAc 1195 ppm 6 ppm 99.5 
Benzene 24 ppm none 100 
Toluene 30 ppm trace 99.6 
Para-xylene 
300 ppm 4 ppm 98.7 
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*SLM = standard liters per minute 
Iron bromide formed on the catalyst, thus, reducing the amount of bromine 
released to the atmosphere. 
Example 2 
The same apparatus as described in Example 1 was used to oxidize methanol 
from a gas containing methanol and oxygen. The feed rate was 2.3 SLM. The 
power was 105 watts, and the pressure was 2 psig. The feed contained 1.3 
volume percent methanol and 7.2 volume percent oxygen. The product 
contained 0.004 volume percent methanol. The methanol conversion was 
99.7%. 
Example 3 
The same apparatus as described in Example 1 was used to oxidize a mixture 
of hexane isomers with oxygen. The hexane was added by bubbling nitrogen 
through liquid hexanes and utilizing the vapor pressure of the liquid. 
Oxygen was added as air and additional nitrogen was added to dilute the 
fed to 0.549 volume percent hexane isomers. Greater than 99.9% oxidation 
of the hexane isomers was achieved with 150 watts of power. The feed 
contained 7.2 volume percent oxygen. The product contained 0.00017 volume 
percent hexane isomers. 
Example 4 
The same apparatus as described in Example 1 was used to oxidize a mixture 
of toluene with oxygen. The toluene was added by bubbling nitrogen through 
toluene and utilizing the vapor pressure of the liquid. Oxygen was added 
as air and additional nitrogen was added to dilute the feed to 0.549 
volume percent toluene. Greater than 99% oxidation of the toluene was 
achieved with 130 watts of power. The feed contained 7.2 volume percent 
oxygen. The product contained 0.0012 volume percent toluene. 
Example 5 
The same apparatus as described in Example 1 was used to oxidize carbon 
monoxide and methyl bromide with oxygen. The results for several metals 
are shown in Table 2. The feed in all cases was 2.3 SLM of 0.63% CO, 68 
ppm CH.sub.3 Br, and 5.5% O.sub.2 in nitrogen. 
TABLE 2 
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POWER % CO % CH.sub.3 Br 
CATALYST (W) CONVERSION 
CONVERSION 
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100 mesh aluminum screen 
200 17 92 
.016" wire diameter 
100 mesh copper screen 
150 100 5.7 
.0045" wire diameter 
200 100 15.9 
16 mesh Monel screen 
170 100 58.2 
.011" wire diameter 
200 100 82.4 
60 mesh knitted SS screen 
155 4.8 44.8 
.011" wire diameter 
200 12.7 76.9 
100 mesh steel wool as 
50 100 100 
in Example 1 
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