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Hydrocarbons catalytic combustion in membrane reactors
NATURAL GAS CONVERSIONV Studies in Surface Science and Catalysis, Vol. 119 A. Parmalianaet al. (Editors) o 1998 Elsevier Science B.V. All rights reserved.
435
Hydrocarbons catalytic combustion in membrane reactors A. Bottino a, G. Capannelli a, A. Comite a, F. Ferrari a, O. Monticelli a, D. Romano", A. Servidaa, F. Cavani b and V. Chiappa ~ aDipartimento di Chimica e Chimica Industriale, Universit/l di Genova, Via Dodecaneso 3 l, 16146 Genova, Italy bDipartimento di Chimica Industriale e dei Materiali, Universit/l di Bologna, Viale Risorgimento 4, 40136 Bologna, Italy ~ SpA, Via C. Navone 3/b, 16017 Busalla (Genova), Italy
This paper reports preliminary results concerning the emission control of volatile organic compounds (VOC) by combustion in catalytic ceramic membrane tubes. Toluene was used as model species of VOC. The performance of the innovative catalytic combustor system was assessed as a function of relevant operating variables such as, temperature, residence time, and oxygen-to-hydrocarbon ratio.
1.
INTRODUCTION
Volatile Organic Compounds (VOC) represent a very important class of pollutants that are present in various kind of industrial streams. VOC include a wide variety of chemical compounds such as for example alcohols, aldehydes, ketones, aliphatic and aromatic hydrocarbons, etc. The most common technologies for reducing and controlling VOC are reviewed in Ref. [1] and include: thermal and catalytic oxidation, adsorption, absorption, condensation, flaring, boiler/process heaters, biofiltration, membrane processes, and UV oxidation. Recently, an innovative reactor, based on the forced permeation of VOC streams through a Pt/y-AI203 catalytic membrane, has been proposed [2,3] for the control of VOC emissions. The results show that complete VOC (toluene and methyl ethyl ketone) combustion is achieved at temperatures much lower than those required in conventional monolith reactors. However, the catalytic system exhibits the disadvantage of a noticeable pressure drop due to the membrane permeation resistance. In this paper we report preliminary results on the VOC emission control through a catalytic membrane combustor operating in a monolith-like flow configuration. The performance of the catalytic combustor was evaluated with respect to the following process variables: temperature, residence time, and oxygen-to-hydrocarbon ratio.
436 2.
EXPERIMENTAL
Catalytic membranes were obtained by depositing the catalyst (Pt) onto the internal active layer of A1203 multilayered porous tubes supplied by SCT (France) and Schumacher (Germany). Both tubes had a length of 150 mm, an outer diameter of 10 mm, and a thickness of 1.5 mm. The tubes were sealed by a vitrification process for an extent of 25 mm at both ends. The active layer of SCT tubes was made of 7-A1203 (average pore diameter 5 nm) while that of Schumacher tubes was made of TiO2 (average pore diameter 5 nm). A scanning electron microscope (SEM, Leica Stereoscan 440) was used for the morphological characterization of the membrane tubes. The catalyst (Pt) was deposited on the active layers by the impregnation method described elsewhere [4]. The catalyst loading was measured by atomic adsorption after the chemical attack of the sample with aqua regia. The specific surface area of the active layer was measured by N2 adsorption/desorption measurements (Micromeritics ASAP 2000). The catalytic activity of the membranes was evaluated in the 100-350 ~ temperature range with a toluene concentration between 800-5000 ppm. Toluene was selected as model species of VOC. A schematic of the experimental set-up used for the catalytic combustion tests is shown in Fig. 1. The carrier gas (N2 or air) was saturated with toluene by using two saturators (A) arranged in series that were maintained at constant temperature. The saturated gas stream was mixed with 02 and/or air in order to reach the desired Oz/toluene ratio. The pre-mixed reactants stream was fed to the reactor (B) located within the oven (C) in order to operate at isothermal conditions. The gas flowrates were controlled through the mass flow meters (D). The feed flowed tangentially to the membrane surface (i.e. without permeating through the membrane) and the combustion products were analyzed by using an on line gas chromatograph (E).
AIR,N2,02 ~, ~
AIR,~
N2"--P~ D
'l
1lL T Ir
Figure 1. Schematic diagram of the experimental set-up used to perform catalytic combustion tests.
437 3.
RESULTS AND DISCUSSION
As an example of SEM characterizations, Fig. 2 shows a micrograph of the upper part of the cross-section of a Schumacher membrane. The active layer appears to be crack-free and very thin. active ! porous layer 'qll~'support
Figure 2. Scanning electron micrograph of the upper part of the cross-section of the Schumacher membrane.
The average thickness of the active layer, evaluated from this and other cross-section micrographs, is reported in Table 1 along with the BET surface area value. For comparison purposes, the characteristics of the SCT membrane are also reported. The results show that the two membranes exhibit comparable BET surface area but different active layer thickness.
Table l BET surface area (m2/g) and thickness (gm) of the active layers of SCT and Schumacher tubular membranes ill
,i
i
Membrane i
Active layer composition
i,
i|
i
,
_ ,
,
,,
, i
BET surface area
,,
,
,
1
Thickness
,,,
SCT
7-A1203
244
2
Schumacher
TiO2
254
0.7
ii
i,
i
,,
i
438
100 --
/
/
/ 80
OQ
>. Z 9 r,.)
/"
/
z
9
/
--
/
60--
/~
40-20--
100
/ []
/ 0
/ ,p
/
D~
f.
I
I
I
I
150
200
250
300
TEMPERATURE, ~ Figure 3. Toluene conversion vs. temperature for the SCT (D) and the Schumacher (~) membranes. Toluene concentration: 4760 ppmv. Residence time: 10 s. OdToluene molar ratio 29.
A first series of combustion experiments was carried to compare the performance of Schumacher and SCT membranes. The results are shown in Fig. 3 where the toluene conversion is reported as a function of temperature. For both the membranes by increasing the operating temperature the conversion increases first slowly, then more rapidly, and finally levels off. The results show that in the Schumacher membrane, complete VOC combustion is achieved at temperatures lower than those required for the SCT one. Indeed, the Schumacher membrane exhibits a light-off temperature, i.e. the temperature measured at a conversion of 50 %, about 30~ lower than that of SCT membrane. On the basis of these preliminary tests the Schumacher membrane was selected for further kinetic studies. The effect of the inlet toluene concentration on the reactor performance is shown in Fig. 4. The results refer to catalytic tests carried out at a residence time of 2.5 s and using air as carrier gas. They show that an increase in toluene inlet concentration decreases the reactor performance by shifting the conversion curve towards higher temperatures. This is in agreement with the results obtained by Pina et al. [2,3] concerning the toluene combustion in catalytic membrane reactors operating in the flow-through mode. The effect of the oxygen-to-toluene molar ratio on the reactor performance is shown in Fig. 5 where the toluene conversion is reported for two different temperatures (169 and 203 ~ The results show that at high temperature the O2-to-toluene ratio does not have any significant effect on the toluene conversion, while at low temperature it improves the reactor performance. In particular, as Fig. 6 clearly indicates, operating at 203~ the catalytic membrane reactor exhibits such a high catalytic activity that complete toluene conversion is attained even at low contact times (less than 1 s).
439
100 --
...O~-CI , . 0
.
'
--
~
"
[]
/ o-e
80
--
z~
9
/
/ 60--
/
9
/ > Z 9
/
/
40--
/
/
20--
I
I
I
150
200
250
0 100
TEMPERATURE, oC Figure 4. Toluene conversion vs. temperature for different toluene concentration in the feed. Toluene concentration: ([]) 890 ppmv, ( - ) 1610 ppmv. Residence time: 2.5 s.
100
--
- - * - - - *
.
.
.
.
.
.
*
.
.
.
.
.
.
.
* -
809 > Z 9
60.~~
4020
--
0 50
I
I
I
I
I
I
100
150
200
250
300
350
O2/TOLUENE MOLAR RATIO Figure 5. Toluene conversion vs. OJtoluene molar ratio. Operating temperature: ([-])169 ~ ( , ) 203 ~ Toluene concentration: 890 ppm. Residence time: 2.5 s
440
100 --
--n
[]
--"
~
--13"--
809
60-
;> Z 9
40200
0
I
I
I
I
I
I
0.5
1
1.5
2
2.5
3
RESIDENCE TIME, s Figure 6. Toluene conversion vs. residence time at 203 ~ operating temperature. Toluene concentration: 890 ppmv.
4.
CONCLUSIONS
Catalytic combustion of toluene has been investigated by using a catalytic membrane reactor operating in a monolith-like flow configuration. The results indicate that in catalytic membrane reactors complete combustion of toluene can be achieved at temperatures lower than those usually found in conventional monolith reactors. This may be due to the surface area of catalytic membrane that is about one order of magnitude higher than that of conventional monoliths. The preliminary results indicate that operating the membrane combustor in the tangential flow configuration, complete combustion is achieved at temperatures slightly higher than those required for the all-through configuration [2, 3]. Work is in progress to identify optimal membrane characteristics and operating conditions in order to improve the performance of the catalytic membrane combustor.
REFERENCES 1. E.C. Moretti and N. Mukhopadhyay, Chem. Eng. Prog., 89 (1993) 20. 2. M.P. Pina, M. Menendez and J. Santamaria, Appl. Catal.B, 11 (1996) 19. 3. M.P. Pina, S. Irusta, J. Santamaria, R. Hughes and N. Boag, Ind. Eng. Chem. Res., 36 (1997) 4557. 4. G. Capannelli, A. Bottino, G. Gao, A. Grosso, A. Servida, G. Vitulli, A. Mastrantuono, L. Lazzaroni and P. Salvadori, Catal. Lett., 20 (1993) 287.
435
Hydrocarbons catalytic combustion in membrane reactors A. Bottino a, G. Capannelli a, A. Comite a, F. Ferrari a, O. Monticelli a, D. Romano", A. Servidaa, F. Cavani b and V. Chiappa ~ aDipartimento di Chimica e Chimica Industriale, Universit/l di Genova, Via Dodecaneso 3 l, 16146 Genova, Italy bDipartimento di Chimica Industriale e dei Materiali, Universit/l di Bologna, Viale Risorgimento 4, 40136 Bologna, Italy ~ SpA, Via C. Navone 3/b, 16017 Busalla (Genova), Italy
This paper reports preliminary results concerning the emission control of volatile organic compounds (VOC) by combustion in catalytic ceramic membrane tubes. Toluene was used as model species of VOC. The performance of the innovative catalytic combustor system was assessed as a function of relevant operating variables such as, temperature, residence time, and oxygen-to-hydrocarbon ratio.
1.
INTRODUCTION
Volatile Organic Compounds (VOC) represent a very important class of pollutants that are present in various kind of industrial streams. VOC include a wide variety of chemical compounds such as for example alcohols, aldehydes, ketones, aliphatic and aromatic hydrocarbons, etc. The most common technologies for reducing and controlling VOC are reviewed in Ref. [1] and include: thermal and catalytic oxidation, adsorption, absorption, condensation, flaring, boiler/process heaters, biofiltration, membrane processes, and UV oxidation. Recently, an innovative reactor, based on the forced permeation of VOC streams through a Pt/y-AI203 catalytic membrane, has been proposed [2,3] for the control of VOC emissions. The results show that complete VOC (toluene and methyl ethyl ketone) combustion is achieved at temperatures much lower than those required in conventional monolith reactors. However, the catalytic system exhibits the disadvantage of a noticeable pressure drop due to the membrane permeation resistance. In this paper we report preliminary results on the VOC emission control through a catalytic membrane combustor operating in a monolith-like flow configuration. The performance of the catalytic combustor was evaluated with respect to the following process variables: temperature, residence time, and oxygen-to-hydrocarbon ratio.
436 2.
EXPERIMENTAL
Catalytic membranes were obtained by depositing the catalyst (Pt) onto the internal active layer of A1203 multilayered porous tubes supplied by SCT (France) and Schumacher (Germany). Both tubes had a length of 150 mm, an outer diameter of 10 mm, and a thickness of 1.5 mm. The tubes were sealed by a vitrification process for an extent of 25 mm at both ends. The active layer of SCT tubes was made of 7-A1203 (average pore diameter 5 nm) while that of Schumacher tubes was made of TiO2 (average pore diameter 5 nm). A scanning electron microscope (SEM, Leica Stereoscan 440) was used for the morphological characterization of the membrane tubes. The catalyst (Pt) was deposited on the active layers by the impregnation method described elsewhere [4]. The catalyst loading was measured by atomic adsorption after the chemical attack of the sample with aqua regia. The specific surface area of the active layer was measured by N2 adsorption/desorption measurements (Micromeritics ASAP 2000). The catalytic activity of the membranes was evaluated in the 100-350 ~ temperature range with a toluene concentration between 800-5000 ppm. Toluene was selected as model species of VOC. A schematic of the experimental set-up used for the catalytic combustion tests is shown in Fig. 1. The carrier gas (N2 or air) was saturated with toluene by using two saturators (A) arranged in series that were maintained at constant temperature. The saturated gas stream was mixed with 02 and/or air in order to reach the desired Oz/toluene ratio. The pre-mixed reactants stream was fed to the reactor (B) located within the oven (C) in order to operate at isothermal conditions. The gas flowrates were controlled through the mass flow meters (D). The feed flowed tangentially to the membrane surface (i.e. without permeating through the membrane) and the combustion products were analyzed by using an on line gas chromatograph (E).
AIR,N2,02 ~, ~
AIR,~
N2"--P~ D
'l
1lL T Ir
Figure 1. Schematic diagram of the experimental set-up used to perform catalytic combustion tests.
437 3.
RESULTS AND DISCUSSION
As an example of SEM characterizations, Fig. 2 shows a micrograph of the upper part of the cross-section of a Schumacher membrane. The active layer appears to be crack-free and very thin. active ! porous layer 'qll~'support
Figure 2. Scanning electron micrograph of the upper part of the cross-section of the Schumacher membrane.
The average thickness of the active layer, evaluated from this and other cross-section micrographs, is reported in Table 1 along with the BET surface area value. For comparison purposes, the characteristics of the SCT membrane are also reported. The results show that the two membranes exhibit comparable BET surface area but different active layer thickness.
Table l BET surface area (m2/g) and thickness (gm) of the active layers of SCT and Schumacher tubular membranes ill
,i
i
Membrane i
Active layer composition
i,
i|
i
,
_ ,
,
,,
, i
BET surface area
,,
,
,
1
Thickness
,,,
SCT
7-A1203
244
2
Schumacher
TiO2
254
0.7
ii
i,
i
,,
i
438
100 --
/
/
/ 80
OQ
>. Z 9 r,.)
/"
/
z
9
/
--
/
60--
/~
40-20--
100
/ []
/ 0
/ ,p
/
D~
f.
I
I
I
I
150
200
250
300
TEMPERATURE, ~ Figure 3. Toluene conversion vs. temperature for the SCT (D) and the Schumacher (~) membranes. Toluene concentration: 4760 ppmv. Residence time: 10 s. OdToluene molar ratio 29.
A first series of combustion experiments was carried to compare the performance of Schumacher and SCT membranes. The results are shown in Fig. 3 where the toluene conversion is reported as a function of temperature. For both the membranes by increasing the operating temperature the conversion increases first slowly, then more rapidly, and finally levels off. The results show that in the Schumacher membrane, complete VOC combustion is achieved at temperatures lower than those required for the SCT one. Indeed, the Schumacher membrane exhibits a light-off temperature, i.e. the temperature measured at a conversion of 50 %, about 30~ lower than that of SCT membrane. On the basis of these preliminary tests the Schumacher membrane was selected for further kinetic studies. The effect of the inlet toluene concentration on the reactor performance is shown in Fig. 4. The results refer to catalytic tests carried out at a residence time of 2.5 s and using air as carrier gas. They show that an increase in toluene inlet concentration decreases the reactor performance by shifting the conversion curve towards higher temperatures. This is in agreement with the results obtained by Pina et al. [2,3] concerning the toluene combustion in catalytic membrane reactors operating in the flow-through mode. The effect of the oxygen-to-toluene molar ratio on the reactor performance is shown in Fig. 5 where the toluene conversion is reported for two different temperatures (169 and 203 ~ The results show that at high temperature the O2-to-toluene ratio does not have any significant effect on the toluene conversion, while at low temperature it improves the reactor performance. In particular, as Fig. 6 clearly indicates, operating at 203~ the catalytic membrane reactor exhibits such a high catalytic activity that complete toluene conversion is attained even at low contact times (less than 1 s).
439
100 --
...O~-CI , . 0
.
'
--
~
"
[]
/ o-e
80
--
z~
9
/
/ 60--
/
9
/ > Z 9
/
/
40--
/
/
20--
I
I
I
150
200
250
0 100
TEMPERATURE, oC Figure 4. Toluene conversion vs. temperature for different toluene concentration in the feed. Toluene concentration: ([]) 890 ppmv, ( - ) 1610 ppmv. Residence time: 2.5 s.
100
--
- - * - - - *
.
.
.
.
.
.
*
.
.
.
.
.
.
.
* -
809 > Z 9
60.~~
4020
--
0 50
I
I
I
I
I
I
100
150
200
250
300
350
O2/TOLUENE MOLAR RATIO Figure 5. Toluene conversion vs. OJtoluene molar ratio. Operating temperature: ([-])169 ~ ( , ) 203 ~ Toluene concentration: 890 ppm. Residence time: 2.5 s
440
100 --
--n
[]
--"
~
--13"--
809
60-
;> Z 9
40200
0
I
I
I
I
I
I
0.5
1
1.5
2
2.5
3
RESIDENCE TIME, s Figure 6. Toluene conversion vs. residence time at 203 ~ operating temperature. Toluene concentration: 890 ppmv.
4.
CONCLUSIONS
Catalytic combustion of toluene has been investigated by using a catalytic membrane reactor operating in a monolith-like flow configuration. The results indicate that in catalytic membrane reactors complete combustion of toluene can be achieved at temperatures lower than those usually found in conventional monolith reactors. This may be due to the surface area of catalytic membrane that is about one order of magnitude higher than that of conventional monoliths. The preliminary results indicate that operating the membrane combustor in the tangential flow configuration, complete combustion is achieved at temperatures slightly higher than those required for the all-through configuration [2, 3]. Work is in progress to identify optimal membrane characteristics and operating conditions in order to improve the performance of the catalytic membrane combustor.
REFERENCES 1. E.C. Moretti and N. Mukhopadhyay, Chem. Eng. Prog., 89 (1993) 20. 2. M.P. Pina, M. Menendez and J. Santamaria, Appl. Catal.B, 11 (1996) 19. 3. M.P. Pina, S. Irusta, J. Santamaria, R. Hughes and N. Boag, Ind. Eng. Chem. Res., 36 (1997) 4557. 4. G. Capannelli, A. Bottino, G. Gao, A. Grosso, A. Servida, G. Vitulli, A. Mastrantuono, L. Lazzaroni and P. Salvadori, Catal. Lett., 20 (1993) 287.