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Catalytic purification of waste gases containing chlorinated hydrocarbons with precious metal catalysts
Catalysis Today, 17 (1993) 383-390 Elsevier Science Publishers B.V., Amsterdam
383
CATALYTIC PURIFICATION OF WASTE GASES CONTAINING CHLORINATED HYDROCARBONS PRECIOUS METAL CATALYSTS
WITH
H. MijLLER, K. DELLER, B. DESPEYROUX and E. PELDSZUS Degussa AG, P.O. Box 13 45, D-6450 Hanau 1 P. KAMMERHOFER, W. KUHN, R. SPIELMANNLEITNER Hoechst Aktiengesellschaft, Werk Gendorf, 8269 Burgkirchen
and M. STC)GER
ABSTRACT The development of a two bed catalytic system, consisting of an activated Al203 guard catalyst and a bimetallic Pd,Pt/Al203 oxidation catalyst, is described. The catalysts proved to be highly active and resistant to chlorinated hydrocarbons, which are well-known catalyst poisons. Based on the good results of pilot tests a big scale unit was built for the purification of an off-gas stream (up to 15,000 Nm3/h) coming from an industrial vinyl chloride manufacturing plant. After meanwhile 12,000 hours on stream the catalytic system is still operating with excellent performance. Estimates for the application described show that the catalytic process has a significant economical and technical benefit compared to the conventional thermal incineration. 1. INTRODUCTION Increasing air pollution has become a serious problem in highly developed industrialized countries in recent decades. Besides the pollutants SO, and NO,, the emission of ozone, hydrocarbons and in particular of halogenated hydrocarbons has been the object of considerable public criticism as a possible source of serious environmental damage. The vinyl chloride (VC) production is an example of an industrial process that also produces waste gases containing chlorinated hydrocarbons. About one-half of the worldwide produced 1,2-dichloroethane (EDC) is made using the well-known oxychlorination process from air, ethylene and hydrogen chloride. Hereby a waste gas is produced in large amounts. It contains essentially the pollutants carbon monoxide, ethylene and ethane in the range below 1 vol%, the toxic chlorinated hydrocarbons methyl chloride, vinyl chloride, ethyl chloride and dichloroethane in the range of up to several hundred vpm as well as a solvent that serves as hydrocarbon absorbant (Table 1). In order to comply with the current German regulations for emission control (“TA-L&l), this waste gas makes additional cleaning necessary. 0920~5661/93/$6.00
0 1993 Elsevier Science Publishers B.V. All rights reserved.
384
TABLE 1 Pollutant concentrations in the oxychlorination off-gas Pollutant
Concentration
Oxygen Ethylene
4-7 0.4 - 0.7* 0.2 - 0.9*
Ethane
0.03 - 0.15
Methyl chloride Vinyl chloride
2 - 30 15-60
Ethyl chloride
30 - 400
1,2-Dichloroethane Hydrocarbon absorbant
5- 100 50 - 250
Carbon monoxide
[vol.%] [vol.%] [vol.%] [vol.%]
Wml [vml [wml Ivml [vml
*Depending on oxychlorination catalyst quality A solution of this problem by thermal incineration is practicable [l], but very energy consuming. Our task was thus to check the feasibility of a catalytic purification process, i. e. to develop a catalyst which is resistant to the chlorinated hydrocarbons known as catalyst poisons [2,3].
2. LABORATORY
AND PILOT TRIALS
After initial laboratory trials with a synthetic waste gas mixture a pilot system was constructed and installed in the by-pass stream of an oxychlorination plant in cooperation with our partners Hoechst, Gendorf and Uhde. The results of the pilot plant trial with a conventional Pt,Rh/A1203 pellet catalyst were unsatisfactory. Analysis of the used, deactivated catalyst revealed a high degree of chlorination as well as the nearly complete chemical destruction of the Al203 carrier material used. These rather sobering results initiated an extensive catalyst development program accompanied by pilot plant tests under industrial operating conditions. The steps of that work can be summarized as follows: An Al203 carrier was found with improved chemical resistance to HCl. Additionally a method was developed to stabilize the support material by a special pretreatment. . Numerous combinations of various noble metal compounds were tested out as catalytically active components. A bimetallic Pd,Pt/A1203_catalyst proved to be highly poisoning resistant to the chlorinated hydrocarbons. This effect was already found in earlier studies [4]. The optimization of the noble metal content and the noble metal distribution in the catalyst pellets gave a further improvement in catalyst activity.
.
l
l
385
Figure 1 shows the conversion curves for several waste gas components as measured with the optimized Pd,Pt-catalyst in laboratory tests. Ethane is almost completely oxidized already at approx. 350 “C. Quantitative removal of methyl chloride however still requires high temperatures. Conversion @]
100 t 80 :----60 40 20 0
L 0
100
200
300
400
500
600
700
Inlet temperature [“Cl
Fig. 1. Conversion curves of pollutants; catalyst: Pd,Pt/Al 03; pollutant concentration: 0.1 vol.% each; GHSV: 15,000 h-3 . An activated Al203-catalyst was developed to improve the conversion of the chlorinated hydrocarbons and especially of methyl chloride. Finally the optimized technical solution turned out to be the use of both catalysts arranged in two separate layers. Figure 2 shows the functional type of the two catalysts. The catalyst of the first layer consists of a doped and stabilized aluminium oxide. It catalyses the oxidative decomposition of the aromatic hydrocarbon absorbant (Solvesso) in short-chained intermediate products and CO2 as well as the conversion of the halogenated hydrocarbons. Beyond that the A1203-bed acts as a filter for heavy metals (Cr, Fe, Ni) coming from the materials used and thus protects the noble metal catalyst from poisoning. The catalyst of the second layer contains mainly palladium and platinum as active components. It converts carbon monoxide, ethane, ethylene and the residual halogenated hydrocarbons that pass the fast layer.
386
Concentration
1461 Ethane \I
‘-
A120g-Catalyst
-
a
Pd,Pt-catalyst
-
Fig. 2. Axial concentration profiles of pollutants in the Al2O3- and Pd,Pt-catalyst layer. The two bed catalyst arrangement described was subjected to a durability test under industrial operating conditions in the pilot reactor. Figure 3 shows the measured emitted concentrations of carbon monoxide, the total amount of the organic pollutants of class I to III, classified according “TA-Luft” [5], and the carcinogenic chlorinated hydrocarbons EDC and vinyl chloride. The emission limit for the latter is 5 mg/Nm3. CCL I - IIIorg. and CO [mg/Nrr?]
C VC, EDC [mg/Nm’]
--
0
2
4
6
8
2
12
Time on Stream (hours x 1000)
Fig. 3. Results of the durability test in the pilot reactor; catalyst: Pd,Pt/A1203; GHSV: 15,000 h-l (variation: 10,000 to 20,000 h-1); inlet temperature: 420 “C (variation: 370 to 490 “C).
387
It is evident that all the measured values are well below the emission limits during the whole test period. Occasional fluctuations can be explained by operational variations of the EDC unit or altered test settings. In general the space velocity (GHSV) was adjusted to 15,000 h-l for this trial, based on the volume of the noble metal catalyst, and the inlet temperature was approx. 450 “C. Even after more than 12,000 hours of operation, the activity level of the catalyst had not noticeably decreased. Towards the end of the trial operational trouble was simulated. For that purpose the catalyst system was exposed to an excess of several thousand vpm chlorinated hydrocarbons to investigate the poisoning resistance of the catalysts. Even at chlorinated hydrocarbon concentrations of five times the usual level the catalysts showed no sign of lessening activity during the test period of about 620 hours.
3. INDUSTRIAL
SCALE PLANT AND OPERATIONAL
RESULTS
Based on that positive experience with the pilot system, a technical plant was subsequently designed and built to clean up to 15,000 Nm3/h of waste gas. Figure 4 shows a flow diagram of the industrial purification unit.
Fig. 4. Flow diagram of the industrial off-gas purification plant, Hoechst Gendorf. The waste gas from the oxychlorination process is first cleaned in an absorber to remove EDC. However this purification step is not sufficient to comply with the German emission regulations.
388
The pollutants containing gas is then preheated to reaction temperature in a countercurrent heat exchanger using the hot purified gas as heating medium. The reactor contains the two different catalyst layers in a ring-shaped, conical arrangement. In general, the reactor is operated on supportive tiring, but autothermal operation is possible as well. After having passed the heat exchanger the purified gas is cooled down in two steps. Hereby low-pressure steam and hot water are generated. Finally the HCl is washed out with water in a scrubber. The first operational trials were carried out in December 1990. Meanwhile approx. 12,000 hours of pure operating time have passed. Table 2 contains the operating conditions and the achieved pollutant concentrations in the purified off-gas. TABLE 2 Operating conditions Hoechst, Gendorf
and emission values acieved in the industrial plant of
Operating conditions Inlet temperature: Outlet temperature: Pressure: GHSV:
350 - 380 “C 580 - 680 “C 1 atm approx. 10,000 h-l*
*related to the volume of the precious metal catalyst Emission values Pollutants
Emission limits according German “TA-Luft” (1986)
bi@m31
co C2H4> C2H6 HCl Cl2 NO, C2H3Cl (VC) CH3C1, C2H5CI C2H4Cl2 (EDC) C Subst. Cl. I - III org. C Subst. Cl. I org.
250
Pollutant concentrations in the purified off-gas
[mg/Nm31
30 5 500 5
< 100 < 20 < 10 < 1 < 50 < 0.5
150 20
< ‘z <
5 20 3
389
The emitted CO level is far below 100 mg/Nm3. After the scrubber HCl concentrations of well under 10 mg/Nm3 are usually achieved. The total amount of the chlorinated hydrocarbons are in the range of < 5 mg/Nm3 and the total amount of pollutants of class I to III is under 20 mg/Nm3. Thus all the concentrations of the relevant pollutants remain clearly below the emission limits. Table 3 shows some performance data of the industrial purification plant, based on a balance after 6334 hours of operation. During that time it was possible to reduce the emission of the chlorinated hydrocarbons from 30.04 t down to 0.09 t. The total emission of pollutants coming from the oxychlorination plant was decreased from 691.6 t to 3.07 t. This corresponds to an overall conversion of 99.6 %! TABLE 3 Performance data of the industrial purification plant, Hoechst Gendorf. Balance after 6,334 hours of operation pollutants
co
emission [t] without with catalytic purification purification
overall conversion [%]
C hydrocarbons C chlorohydrocarbons
425.00 236.60 30.04
1.84 1.14 0.09
99.6 99.5 99.7
total emission
691.60
3.07
99.6
4. COMPARISON OF THERMAL AND CATALYTIC WASTE GAS PURIFICATION Table 4 makes a final comparison of relevant technical process parameters and economical figures for thermal and catalytic waste gas purification estimated for the VC/EDC manufacturing process. The thermal incineration of chlorinated hydrocarbons requires temperatures up to 1,200 “C [ 11 involving a high fuel consumption. The catalytic purification operates at much lower temperatures and thus helps to spare natural resources. For thermal purification the off-gas has to be mixed with air to provide sufficient oxygen for the burning process. The burner gases enlarge the waste gas stream additionally. The investment costs and especially the operating costs for the thermal incineration are substantially higher. A large share of the operating costs are caused by the high fuel consumption.
390
TABLE
4
Comparison between thermal and catalytic off-gas purification for the VCYEDC manufacturing process Thermal incineration Temperature Total off-gas stream Investment costs Operating costs Fuel consumption Space requirement Steam generation CO2 emission NO, formation Limitations
rc1 [“A]
WI WI WI
up to 1,200 100 100 100 100 100 100 100 considerable material problems (corrosion)
Catalytic purification 350 - 680 70 75 - 80 50 10 40 15 10 negligible catalyst life tune (poisoning)
The less space requirement of the catalytic system is an advantage when it is necessary to integrate this unit into an already existing VC plant. Of course the amount of energy recovered as steam horn the thermal process is much larger. Steam however cannot always be utilized in an economical manner. The threat of drastic climatic changes caused by the “greenhouse effect” will in the future provide a powerful argument against the use of the thermal process, which emits the tenfold CO2 amount compared to the catalytic purification. Another advantage of the catalytic process is that the formation of NO, by oxidation of air contained nitrogen can be neglected. However the drawbacks involved in catalytic purification should be mentioned as well. The lifetime of the catalyst is decisive in evaluating the economical aspects of the process. Estimates for the application described here revealed that the catalytic purification should operate economical already after one year catalyst lifetime. According to our experience gained from the pilot trials and the performance data of the technical plant a catalyst lifetime of several years can be expected, so that the catalytic process will surely prove to be economical in this application. REFERENCES E. Lundberg, VDI Berichte 525, Katalytische und thermische Verfahren der Abgasremigung, VDI-Verlag GmbH, Dusseldorf, 1985, p 347. G Laidig, Abbau von chlorierten Kohlenwasserstoffen durch katalytische Gasphasenoxidation, Disseration TH Karlsruhe, 198 1. C F. Cullis, D.E. Keene, D. L. Trmun, J. Catal 19 (1970) 378 B. Mendyka, J. D. Rutkowski, Environment Protection Engineermg, 10 (1984) 5. Erste Allgemeine Verwaltungsvorschrift zum Bundes-Immissionsschutzgesetz (Technische Anleitung zur Remhaltung der Luft - TA Lufi), 27. Feb. 1986.
383
CATALYTIC PURIFICATION OF WASTE GASES CONTAINING CHLORINATED HYDROCARBONS PRECIOUS METAL CATALYSTS
WITH
H. MijLLER, K. DELLER, B. DESPEYROUX and E. PELDSZUS Degussa AG, P.O. Box 13 45, D-6450 Hanau 1 P. KAMMERHOFER, W. KUHN, R. SPIELMANNLEITNER Hoechst Aktiengesellschaft, Werk Gendorf, 8269 Burgkirchen
and M. STC)GER
ABSTRACT The development of a two bed catalytic system, consisting of an activated Al203 guard catalyst and a bimetallic Pd,Pt/Al203 oxidation catalyst, is described. The catalysts proved to be highly active and resistant to chlorinated hydrocarbons, which are well-known catalyst poisons. Based on the good results of pilot tests a big scale unit was built for the purification of an off-gas stream (up to 15,000 Nm3/h) coming from an industrial vinyl chloride manufacturing plant. After meanwhile 12,000 hours on stream the catalytic system is still operating with excellent performance. Estimates for the application described show that the catalytic process has a significant economical and technical benefit compared to the conventional thermal incineration. 1. INTRODUCTION Increasing air pollution has become a serious problem in highly developed industrialized countries in recent decades. Besides the pollutants SO, and NO,, the emission of ozone, hydrocarbons and in particular of halogenated hydrocarbons has been the object of considerable public criticism as a possible source of serious environmental damage. The vinyl chloride (VC) production is an example of an industrial process that also produces waste gases containing chlorinated hydrocarbons. About one-half of the worldwide produced 1,2-dichloroethane (EDC) is made using the well-known oxychlorination process from air, ethylene and hydrogen chloride. Hereby a waste gas is produced in large amounts. It contains essentially the pollutants carbon monoxide, ethylene and ethane in the range below 1 vol%, the toxic chlorinated hydrocarbons methyl chloride, vinyl chloride, ethyl chloride and dichloroethane in the range of up to several hundred vpm as well as a solvent that serves as hydrocarbon absorbant (Table 1). In order to comply with the current German regulations for emission control (“TA-L&l), this waste gas makes additional cleaning necessary. 0920~5661/93/$6.00
0 1993 Elsevier Science Publishers B.V. All rights reserved.
384
TABLE 1 Pollutant concentrations in the oxychlorination off-gas Pollutant
Concentration
Oxygen Ethylene
4-7 0.4 - 0.7* 0.2 - 0.9*
Ethane
0.03 - 0.15
Methyl chloride Vinyl chloride
2 - 30 15-60
Ethyl chloride
30 - 400
1,2-Dichloroethane Hydrocarbon absorbant
5- 100 50 - 250
Carbon monoxide
[vol.%] [vol.%] [vol.%] [vol.%]
Wml [vml [wml Ivml [vml
*Depending on oxychlorination catalyst quality A solution of this problem by thermal incineration is practicable [l], but very energy consuming. Our task was thus to check the feasibility of a catalytic purification process, i. e. to develop a catalyst which is resistant to the chlorinated hydrocarbons known as catalyst poisons [2,3].
2. LABORATORY
AND PILOT TRIALS
After initial laboratory trials with a synthetic waste gas mixture a pilot system was constructed and installed in the by-pass stream of an oxychlorination plant in cooperation with our partners Hoechst, Gendorf and Uhde. The results of the pilot plant trial with a conventional Pt,Rh/A1203 pellet catalyst were unsatisfactory. Analysis of the used, deactivated catalyst revealed a high degree of chlorination as well as the nearly complete chemical destruction of the Al203 carrier material used. These rather sobering results initiated an extensive catalyst development program accompanied by pilot plant tests under industrial operating conditions. The steps of that work can be summarized as follows: An Al203 carrier was found with improved chemical resistance to HCl. Additionally a method was developed to stabilize the support material by a special pretreatment. . Numerous combinations of various noble metal compounds were tested out as catalytically active components. A bimetallic Pd,Pt/A1203_catalyst proved to be highly poisoning resistant to the chlorinated hydrocarbons. This effect was already found in earlier studies [4]. The optimization of the noble metal content and the noble metal distribution in the catalyst pellets gave a further improvement in catalyst activity.
.
l
l
385
Figure 1 shows the conversion curves for several waste gas components as measured with the optimized Pd,Pt-catalyst in laboratory tests. Ethane is almost completely oxidized already at approx. 350 “C. Quantitative removal of methyl chloride however still requires high temperatures. Conversion @]
100 t 80 :----60 40 20 0
L 0
100
200
300
400
500
600
700
Inlet temperature [“Cl
Fig. 1. Conversion curves of pollutants; catalyst: Pd,Pt/Al 03; pollutant concentration: 0.1 vol.% each; GHSV: 15,000 h-3 . An activated Al203-catalyst was developed to improve the conversion of the chlorinated hydrocarbons and especially of methyl chloride. Finally the optimized technical solution turned out to be the use of both catalysts arranged in two separate layers. Figure 2 shows the functional type of the two catalysts. The catalyst of the first layer consists of a doped and stabilized aluminium oxide. It catalyses the oxidative decomposition of the aromatic hydrocarbon absorbant (Solvesso) in short-chained intermediate products and CO2 as well as the conversion of the halogenated hydrocarbons. Beyond that the A1203-bed acts as a filter for heavy metals (Cr, Fe, Ni) coming from the materials used and thus protects the noble metal catalyst from poisoning. The catalyst of the second layer contains mainly palladium and platinum as active components. It converts carbon monoxide, ethane, ethylene and the residual halogenated hydrocarbons that pass the fast layer.
386
Concentration
1461 Ethane \I
‘-
A120g-Catalyst
-
a
Pd,Pt-catalyst
-
Fig. 2. Axial concentration profiles of pollutants in the Al2O3- and Pd,Pt-catalyst layer. The two bed catalyst arrangement described was subjected to a durability test under industrial operating conditions in the pilot reactor. Figure 3 shows the measured emitted concentrations of carbon monoxide, the total amount of the organic pollutants of class I to III, classified according “TA-Luft” [5], and the carcinogenic chlorinated hydrocarbons EDC and vinyl chloride. The emission limit for the latter is 5 mg/Nm3. CCL I - IIIorg. and CO [mg/Nrr?]
C VC, EDC [mg/Nm’]
--
0
2
4
6
8
2
12
Time on Stream (hours x 1000)
Fig. 3. Results of the durability test in the pilot reactor; catalyst: Pd,Pt/A1203; GHSV: 15,000 h-l (variation: 10,000 to 20,000 h-1); inlet temperature: 420 “C (variation: 370 to 490 “C).
387
It is evident that all the measured values are well below the emission limits during the whole test period. Occasional fluctuations can be explained by operational variations of the EDC unit or altered test settings. In general the space velocity (GHSV) was adjusted to 15,000 h-l for this trial, based on the volume of the noble metal catalyst, and the inlet temperature was approx. 450 “C. Even after more than 12,000 hours of operation, the activity level of the catalyst had not noticeably decreased. Towards the end of the trial operational trouble was simulated. For that purpose the catalyst system was exposed to an excess of several thousand vpm chlorinated hydrocarbons to investigate the poisoning resistance of the catalysts. Even at chlorinated hydrocarbon concentrations of five times the usual level the catalysts showed no sign of lessening activity during the test period of about 620 hours.
3. INDUSTRIAL
SCALE PLANT AND OPERATIONAL
RESULTS
Based on that positive experience with the pilot system, a technical plant was subsequently designed and built to clean up to 15,000 Nm3/h of waste gas. Figure 4 shows a flow diagram of the industrial purification unit.
Fig. 4. Flow diagram of the industrial off-gas purification plant, Hoechst Gendorf. The waste gas from the oxychlorination process is first cleaned in an absorber to remove EDC. However this purification step is not sufficient to comply with the German emission regulations.
388
The pollutants containing gas is then preheated to reaction temperature in a countercurrent heat exchanger using the hot purified gas as heating medium. The reactor contains the two different catalyst layers in a ring-shaped, conical arrangement. In general, the reactor is operated on supportive tiring, but autothermal operation is possible as well. After having passed the heat exchanger the purified gas is cooled down in two steps. Hereby low-pressure steam and hot water are generated. Finally the HCl is washed out with water in a scrubber. The first operational trials were carried out in December 1990. Meanwhile approx. 12,000 hours of pure operating time have passed. Table 2 contains the operating conditions and the achieved pollutant concentrations in the purified off-gas. TABLE 2 Operating conditions Hoechst, Gendorf
and emission values acieved in the industrial plant of
Operating conditions Inlet temperature: Outlet temperature: Pressure: GHSV:
350 - 380 “C 580 - 680 “C 1 atm approx. 10,000 h-l*
*related to the volume of the precious metal catalyst Emission values Pollutants
Emission limits according German “TA-Luft” (1986)
bi@m31
co C2H4> C2H6 HCl Cl2 NO, C2H3Cl (VC) CH3C1, C2H5CI C2H4Cl2 (EDC) C Subst. Cl. I - III org. C Subst. Cl. I org.
250
Pollutant concentrations in the purified off-gas
[mg/Nm31
30 5 500 5
< 100 < 20 < 10 < 1 < 50 < 0.5
150 20
< ‘z <
5 20 3
389
The emitted CO level is far below 100 mg/Nm3. After the scrubber HCl concentrations of well under 10 mg/Nm3 are usually achieved. The total amount of the chlorinated hydrocarbons are in the range of < 5 mg/Nm3 and the total amount of pollutants of class I to III is under 20 mg/Nm3. Thus all the concentrations of the relevant pollutants remain clearly below the emission limits. Table 3 shows some performance data of the industrial purification plant, based on a balance after 6334 hours of operation. During that time it was possible to reduce the emission of the chlorinated hydrocarbons from 30.04 t down to 0.09 t. The total emission of pollutants coming from the oxychlorination plant was decreased from 691.6 t to 3.07 t. This corresponds to an overall conversion of 99.6 %! TABLE 3 Performance data of the industrial purification plant, Hoechst Gendorf. Balance after 6,334 hours of operation pollutants
co
emission [t] without with catalytic purification purification
overall conversion [%]
C hydrocarbons C chlorohydrocarbons
425.00 236.60 30.04
1.84 1.14 0.09
99.6 99.5 99.7
total emission
691.60
3.07
99.6
4. COMPARISON OF THERMAL AND CATALYTIC WASTE GAS PURIFICATION Table 4 makes a final comparison of relevant technical process parameters and economical figures for thermal and catalytic waste gas purification estimated for the VC/EDC manufacturing process. The thermal incineration of chlorinated hydrocarbons requires temperatures up to 1,200 “C [ 11 involving a high fuel consumption. The catalytic purification operates at much lower temperatures and thus helps to spare natural resources. For thermal purification the off-gas has to be mixed with air to provide sufficient oxygen for the burning process. The burner gases enlarge the waste gas stream additionally. The investment costs and especially the operating costs for the thermal incineration are substantially higher. A large share of the operating costs are caused by the high fuel consumption.
390
TABLE
4
Comparison between thermal and catalytic off-gas purification for the VCYEDC manufacturing process Thermal incineration Temperature Total off-gas stream Investment costs Operating costs Fuel consumption Space requirement Steam generation CO2 emission NO, formation Limitations
rc1 [“A]
WI WI WI
up to 1,200 100 100 100 100 100 100 100 considerable material problems (corrosion)
Catalytic purification 350 - 680 70 75 - 80 50 10 40 15 10 negligible catalyst life tune (poisoning)
The less space requirement of the catalytic system is an advantage when it is necessary to integrate this unit into an already existing VC plant. Of course the amount of energy recovered as steam horn the thermal process is much larger. Steam however cannot always be utilized in an economical manner. The threat of drastic climatic changes caused by the “greenhouse effect” will in the future provide a powerful argument against the use of the thermal process, which emits the tenfold CO2 amount compared to the catalytic purification. Another advantage of the catalytic process is that the formation of NO, by oxidation of air contained nitrogen can be neglected. However the drawbacks involved in catalytic purification should be mentioned as well. The lifetime of the catalyst is decisive in evaluating the economical aspects of the process. Estimates for the application described here revealed that the catalytic purification should operate economical already after one year catalyst lifetime. According to our experience gained from the pilot trials and the performance data of the technical plant a catalyst lifetime of several years can be expected, so that the catalytic process will surely prove to be economical in this application. REFERENCES E. Lundberg, VDI Berichte 525, Katalytische und thermische Verfahren der Abgasremigung, VDI-Verlag GmbH, Dusseldorf, 1985, p 347. G Laidig, Abbau von chlorierten Kohlenwasserstoffen durch katalytische Gasphasenoxidation, Disseration TH Karlsruhe, 198 1. C F. Cullis, D.E. Keene, D. L. Trmun, J. Catal 19 (1970) 378 B. Mendyka, J. D. Rutkowski, Environment Protection Engineermg, 10 (1984) 5. Erste Allgemeine Verwaltungsvorschrift zum Bundes-Immissionsschutzgesetz (Technische Anleitung zur Remhaltung der Luft - TA Lufi), 27. Feb. 1986.