- Email: [email protected]
Dynamic behaviour of CO catalytic afterburner with electric heating
catalysis todai
ELSEVIER
Catalysis Today 20 ( 1994) 449466
Dynamic behaviour of CO catalytic afterburner with electric heating L. Dvo%k, P. Pinkas, M. Marek” Department
of Chemical Engineering. Prague Institute of Chemical Technology, Technickd 5. 166 28 Prague 6, Czech Republic
Abstract An experimental PC-controlled catalytic reactor with additional local electric heating of the packed bed of porous particles or ceramic honeycomb is described. Typical axial temperature ignition profiles resulting from different electric heating patterns observed for CO combustion on a Pt/AI,O, catalyst are presented and discussed from the point of view of an optimal heating pattern (with respect to maximal average conversion) with limited heating power. The experimental results are both qualitatively and quantitatively compared with the results of a simulation of the two-phase quasi-onedimensional model including axial heat and mass transport in the gaseous and catalyst phase.
1. Introduction
The increasing pressure to lower emissions from mobile sources (mostly passenger cars and trucks) and also stationary sources of various origin has focused attention on increasing the efficiency of catalytic converters particularly those operating under transient (starting) conditions. Especially the cold start characteristics of catalytic car afterburners have now become the subject of intensive research. The main reason stems from the well known fact that a significant portion of the unconverted total HC and CO emissions occurs within the first two minutes of the driving cycle while the catalyst is heating up to operating temperature. A number of proposals for improving catalyst efficiency under starting conditions have been made. Thus, Oh et al. [ 1 ] have studied the light-off behaviour of catalyst pellets and found that it can be improved by using the correct subsurface impregnation depth of noble metals. However, most of the methods for rapid light-off of automotive exhaust catalysts are based on the rapid increase of catalyst temperature by additional heating. Three types of catalysts *Corresponding
author.
0920.5861/94/$07.00
0 1994 Elsewer Science B.V. All rights reserved
.SSDIO920-5861(94)00057-9
450
L. DvoMk et al. /Catalysrs Today 20 (1994) 349-466
are used in exhaust control (in decreasing order of the frequency of application): (a) ceramic monolith catalysts, (b) packed beds, (c) metallic monolith catalysts. Monolith catalysts are used most often in Europe. One of the methods which has been suggested to increase the catalyst temperature (i.e., reducing the light-off time) is the exhaust gas ignition. The method is based on the rapid ignition of rich exhaust gases with injected additional air and is critically dependent on the concentration of the hydrogen present [ 21. However, most often electrically-heated extruded metal converters are proposed for use in the faster light-off studies, cf. [ 31. Different configurations of extruded electrically-heated catalysts are being tested [ 41. The tested catalytic afterburners are composed of preheaters and main catalysts. The preheaters are either in the form of foils arranged to form an electrically resistive element and welded to electrical connection points, or in the form of monolithic catalytic preheaters manufactured from extruded and sintered metallic honeycombs. The power input, the size of the heater, the location relative to the engine and main converter and various heating protocols are being evaluated [ 51. The main drawbacks of the above preheaters are their high power demands, their high cost and their short durability. Many technical obstacles wtll have to be overcome before they can be fully commercialized. The aim of the present work is the study of the effects of the local electrical preheating of the two most commercially used types of catalytic afterburners - the catalytic ceramic monolith and the packed bed of catalyst-on the light-off characteristics of the afterburners. These studies were carried out both experimentally and by mathematical modelling. We report in this paper experimental and modelling observations of a possible fast ignition on the standard catalysts by electric heating of a relatively narrow strip of the catalyst. We compare the effects of various heating patterns on the catalyst. The possibility of modelling
6
451
L. Dvoicik et al. /Catalysis Today 20 (1994) 449-466
4
Fig. 1 (a) Scheme of the experimental reactor. 1,thermocouples; 2, outlet analyzers; 3, tubular reactor; 4, analogdigital converter; 5, personal computer; 6, carbon monoxide developer; 7, oxygen; 8, nitrogen. (b) Packed bed reactor with thermocouples and heatmg elements. 1,thermocouples; 2, tubular reactor, 3, heating; 4, catalyst, (c) Monohth reactor with thermocouples and heating elements I, tubular reactor; 2. thermocouples; 3, channel wtth a thermocouple; 4, empty channel, 5, heating.
the ignition phenomena discussed.
by a two-phase model considering
heat and mass dispersion is also
452
L. Dvoidk et al. / Cataiysrs Today 20 (1994) 449466
2. Experimental Commercial types of noble metal catalysts, Degussa ceramic honeycomb with noble metals in the washcoat and Cherox Pt/Al,O, pellet catalyst were used in the study. The overall scheme of the laboratory testing equipment is shown in Fig. la and the details of the heated catalyst showing the location of resistive heating elements and thermocouples are depicted in Fig. lb for the packed bed and in Fig. lc for the monolith, respectively. The length of the reactors was 11 cm, the diameter of the space filled with the catalyst was 3 cm. A mixture of CO, O2 and N2 was used for the study. The thermocouples in the packed bed were located both inside the pellets, at their surface and in the voids of the packed bed. Thus the difference between the gas phase and the catalyst surface temperature could be estimated, cf. [ 61. In the monolith, the thermocouples were placed at the external surface of the catalyst channels. Axial temperature profiles were both displayed and recorded in the database, together with the outlet CO and CO? concentrations measured by infrared analyzers. The inlet concentrations of CO, O2 and N2 and their flow-rates were computer-controlled by mass-flow-meters. Kantal wire elements located as loops within the packed bed and at the external surface of the honeycomb, cf. Fig. lb and Ic, respectively, were used for resistive heating with controlled power input.
3. Heating protocols US-FTP 75 and ECE.R 15.04 test reference cycles correspond to typical driving schedules in the US and Continental Europe. The flow-rate of exhaust gases, temperature and concentration of reactants vary with the engine power. For example, typical flow velocities of exhaust gases for idling and full speed in the above cycles vary in the ratio 1:50. Hence, we have to consider heating applied to the catalyst both without flow of the reactants and also at different intensities of the flow-rates, taking into account overall power limitations given, e.g., by limited capacity of the car battery. The efficiency of the use of resistively generated heat for temperature increase of the catalyst surface decreases also with the heat losses due to convection, which are proportional to the flow-rate of exhaust gases. An example of radial temperature profiles in the monolith catalyst resistively heated without reaction from the external surface according to the arrangement in Fig. lc are presented in Fig. 2. We observe a sharp decrease of stationary temperatures with increasing flow-rate and also an increasing steepness of radial temperature profiles. Hence, heating protocols that may be considered include a distribution of heating power along the catalyst together with its variation in time with and without the flow of reactants.
4. Mathematical
model
The operation of catalytic afterburners under standard conditions reflected in US and European test cycles is from the modelling point of view quite complex. We are in principle dealing with the forced cycling of catalytic reactor, where the inlet flow-rate, temperature and concentration of reactants vary cyclically. From the hydrodynamic point of view the
L. DvoGk et al. / Cataiws
Fig. 2. Temperature min-‘:2,81mn-‘;3.
To&y 20 (1994) 449-466
profiles in a monolith catalyst showing the dependence 121min-‘,4, 161min~‘;5,201min-I.
on the flow-rate 0, 0 1 min-‘.
1, 4
I
is laminar in the monolith reactor and can vary from the laminar to the turbulent regime in the packed bed. Often a non-uniform distribution of flow-rates over the cross-section of the reactor is also observed [ 71. As can be deduced from the Arrhenius plots; transition
flow
heat removal
GAS PHASE cin7 Tin
inlet
axial dispersion
c,T
heat transfer
mass
Gout transfer 4Y
h
c*,
T*
heat conduction
CATALYST heat removal Fig. 3. Reactor model scheme.
7
T3ut
outlet
L. Dvoidk et al. /Catalysis Today 20 (1994) 449-466
454
Table 1 Model dimensionless
parameters
Parameter
Value
Da
0.35 164.688 274.5 5.769 100 100 000 28.9 150°C 0.120
JM Jt”f* cp &“I Pe* Yr Tref Kads
Parametkr
Value
B
‘p* Pe,,
0.005 279 0.12 0.003 200
Ya
- 2.25
7
1
JH Jf
from the kinetically limited regime to mass and heat transport limited regimes with increasing reaction temperature often also occur [ 81. In principle, both longitudinal and lateral effective thermal dispersion mechanisms should be considered, taking also into account differences in effective thermal conductivities under steady state and transient conditions [ 91. Pathological behaviour connected with resonance has been also observed and modelled in the forced cycled catalytic reactors [ 10-121. Thus a comprehensive two-phase model, including axial dispersion in the gas phase, heat conduction in the catalyst phase and heat and mass transfer between the gas and catalyst
Hysterem
for various values of Kads
“Kadsti “K&=0 “Kads=O “K&-O “Kads=O
Fig. 4. Dependence
of hysteresis loop on Kads.
120” 180” 180” 500” 5W
YX-0. -+ --0. -
L. Dvorik et al. /Catalysis
Today 20 (1994) 449-466
455
phase is generally used for the modelling. A model that includes radial distributions studied later. The dimensionless variables are defined as: 1 z= L
y=
will be
axial coordinate
c?g- cc0
gas phase conversion
c% 6I=
c,a”’ - c;o
catalyst conversion
c22
(-+(T-T,) y TO
&T*-To) TO
gas phase temperature
y
catalyst temperature
The reactor model is schematically balances are in the form:
shown in Fig. 3. The dimensionless
mass and heat
(1)
$= -J;(w-y)
+ qDuR( &,w)
a?9 1 a26 -= --+~~~u~(S,w)-J,*(6-0)-cp*(6-8,)+~Q
z=O:~-pe,Y=O,~~-Pe~(*-~i~)=O,~~=O, z=yo,
ay
a9
a6
--g=o, z=o
Two types of kinetics were used in the simulations, first-order kinetics with respect to CO concentration and Langmuir-Hinselwood type kinetics. (a) First-order kinetics: ~(c;~,,T*)
=k(T*)c&
(b) Langmuir-Hinshelwood
kinetics:
L. Dw&ik
456
conversion
I 0.8
0.6
0.4
0.2
0
0.8
I
et al. /Cutalysrs
(a)
1 1000
‘,
Today 20 (1994) 449-466
catalyst
temperature
yOu':pace time
(dimensionless'
(5)
2000
time
(s)
,
(b)
0.6
300 200 1.0
0.4 0
i 0.2
o1000
2000
Cc)
0.4
:
Fig. 5. Time courses of outlet conversion and spatio-temporal patterns of temperature. packed bed, inlet temperature 178°C. mlet CO concentration 2%. and narrow band heating 120 s. (a) Inlet section, (b) mlddle section. (c) outlet section.
r(c&J*) = [
~(~*)c&,co, 1
+K(T*)c&g’
The Crank-Nicholson type of finite difference approximation with the Taylor expansion of non-linearity has been used for numerical integration of the system ( 1)-( 4).
L. DvoMk et al. /Catalysis
conversion
457
Today 20 (1994) 449-466
’
1
(a)
0.8
catalyst
temperature
0.6 0
space
0.4
‘;“\ 0.2
I 0
*
0.0
,
.
(dimensionless)
,we5
time 3
’
’
’
2000
1000
,
.
,
time
,
(s.1
’ (s)
,
0.6
1.0 0.4
0.8 250 0.6
200 1.0
0.4
0
2 0
1200
600
Fig. 6 Time courses of outlet cowerston and spatio-temporal patterns of temperature. packed bed, repeated heating 120s after 600 s. (a) Inlet section. (b) middle section, (c) single heating at all three locations.
5. Parameters
of mathematical
model
The parameters shown in Table I have been evaluated from physical properties. The value of the dimensionless heat transfer coefficient at the wall has been fitted to measure the dependence of the outlet conversion on the inlet temperature.
L. DvoEk et al. / Catalysis Today 20 (1994) 449-466
458 250
0
200
7 %
0
0
150
.!Y ti m 2 100
50
I
0
0
20
0
50
I
40
1
I
0
I
I
60 X0 apphed heating power [Wa]
,
I
I
I
100
170
I
I
b)
45 -
0
-
40 35 z
30 0
E w
25 -
z 8
20 1s 0 10
-
5
-
0 0 0
0 0
I
I
I
I
I
I
10
40
60 apphed heatq
80
100
170
power [W,]
Fig. 7. Dependence of the increase of overall reaction rate r (g mol s _ ’ ) on the heating power applied to the inlet section, Q (W s) : (a) inlet CO concentration 2%; (b) inlet CO concentration 1%.
L. DvoZk
et al. /Catalysis
Today 20 (I 994) 449466
459
6. Role of kinetics First-order and Langmuir-Hinshelwood type kinetics were compared. The dependence of the extinction and ignition temperatures on the value of the adsorption constant was found to be weak in the considered range of values of Kadscorresponding to experimental values. The point of extinction is practically the same, the point of ignition is slightly shifted to higher values of temperatures for larger value of Kads(cf. Fig. 4). Also the shape of the hysteresis loop is relatively independent of the activation energy EA in the range +20% which is approximately its standard value used in the literature and in our calculations.
7. Experimental
results
Examples of typical observed time courses of axial temperature profiles and outlet conversions for typical heating protocols will now be discussed. They represent partial results of continuing research aimed at generalization and quantification of the effects of different heating protocols leading to rapid ignition under limitations of heating power. 7.1. Packed bed reactor Examples of testing the effect of the location of a narrow heating zone on the ignition of the reaction are given in Fig. 5a-c. The heat applied in the entrance region is, under the present conditions, (cf. legend to Fig. 5) most effective, cf. Fig. Sa. The starting conditions correspond to a partially heated catalyst bed with a relatively high CO concentration (2%). Limited heating power ( 1.5 W, 120 s) causes the ignition of a high temperature combustion front which slowly moves to the reactor outlet. Conversion increases very rapidly and stays approximately at the value of 0.8 for more than 2000 s. Heating applied in the middle of the reactor is less effective as can be seen in Fig. 5b. However, it still causes outlet conversion to be higher than 0.7 for more than 1200 s. Heating applied at the end of the reactor (cf. Fig. 5c) does not cause a permanent light-off of the reactor and only a temporary increase of the conversion is observed. Hence, repeated local heating of the front end of the catalyst appears to be most efficient in the packed bed heated relatively close to the light-off condition. When the concentration of the combustible reactant is low ( I % CO) repeated heating may be necessary to maintain a high level of outlet conversion. The example of the effects of repeated heating applied in 600 s intervals are depicted in Fig. 6a (heating at the inlet) and 6b (heating in the middle of the reactor). This heating protocol is more effective than a single heating period applied at all three heating positions simultaneously at the start, cf. Fig. 6c. The dependence of the increase of the overall reaction rate in the bed Ar on the heating power Q is illustrated for the inlet concentration of 2% CO in Fig. 7a and for 1% CO in Fig. 7b. We can observe a typical jump in this dependence, corresponding to an ignition at a certain relatively well defined value of the heating power in Fig. 7a and an exponential increase of Ar with Q in Fig. 7b.
460
L. DvoMk et al. /Catalysis
Today 20 (1994) 449-466
I 1
0.7 5 e Y
0.6
E 0.5
140 temperature [Cl
1
0.9 08
0.7
g
0.6
G :: e
0.5
04
0.3
02
(I I 110
I 30
I40
temperaturr [Cl
I so
160
170
L.. DvoMk
et (11./Catalysis
I
Today 20 (1994) 449-466 I
t-
I
461 -
/_
*-
(C 1
-- ---~.5lmm_decreastng” “2lmm_decreasmg” “2 5lmm_mcrea,lng” “2hnIn_mcreasmg”
----
1
08
0.2
1
0
110
120
140 150 Inlet temperacure [C]
130
Measured and computed hysterests [2 % CO Wet).
160
170
180
2 % 02 (mlet). 2 1 /mm N2 (mlet)]
1
- ’
I
-= )21Aatl;:;
;
“experiment-” 0.8 -
=8
0.6 -
2 Y 5 z J2
0.4 -
02
-
120
110 mler temperature ICI
I60
1x0
200
Fig. 8. Monolith reactor. outlet conversion-inlet temperature plots: (a) Hysteresis. inlet CO concentration 2%. (b) Monotonous dependence, inlet CO concentration 1%. (c) Hysteresis loops for two different gas flow-rates (2.0 I min-I. 2 5 1 min-‘), inlet CO concentration 2%. (d) Comparison between computed and calculated hysteresis, inlet CO concentration 2%. gas flow-rate 2.0 I min- ‘.
462
L. DvoEk et al. /Catalysis Today 20 (1994) 449466
7.2. Monolith catalyst Typical experimental results are presented along with the results of corresponding simulations. Either single or multiple stationary states can be observed in the monolith catalyst, depending on the inlet conditions and the method of approach to the stationary state. The hysteresis loop in the outlet conversion dependence - the inlet temperature observed for the CO inlet concentration (2%) - is depicted in Fig. 8a and the monotonous outlet conversion - inlet temperature curve found for the 1% inlet CO concentration - is shown in Fig. 8b. A comparison between the hysteresis loops for two different flow-rates and the inlet concentration 2% of CO is depicted in Fig. 8c. We see that the ignition point is unchanged for a lower flow-rate but the extinction point is shifted to higher temperatures
EXPERIMENT
SIMULATION ‘I
bmf
i\
-
1
L. Dvoidk et al. /Catalysis
SIMULATION
Today 20 (1994) 449-166
463
EXPERIMENT
Fig. 9. Time courses of outlet conversion and spatio-temporal temperature patterns in a monolith catalyst. Inlet CO concentration 2%. (a) Heating protocol: heating at the inlet, flow of reactants started at 240 s, heating proceeds for the next 240 s. (b) Heating protocol: heating at the middle part, flow of reactants started at 240 s, heating proceeds for the next 240 s.
reflecting the lower reaction heat generated with a decreased flow-rate. Comparison between computed and measured hysteresis is shown in Fig. 8d. A significant radial temperature profile can be established in the monolith catalyst when narrow band heating is applied at the external surface, particularly when heating occurs under flow conditions. Hence heating protocols which consider a combination of heating without and with reactant flow were also studied. Unsuccessful and successful temporary light-off’s of the monolith reactor are depicted in Fig. 9a and 9b, respectively. In both cases
464
L DvoiZk et ul. / Cata1.y.m Today 20 (1994) 449266
narrow band heating was applied in the front or at the end of the monolith in such a way that the initial heating occurred without flow and then the flow of reactants was started and heating continued. No light-off occurred in the former case (Fig. 9a), where only a sharp peak in the conversion can be observed. In the latter case a temporary light-off occurred after starting the flow of reactants. A significant radial temperature profile was established in the reactor. The extinction of the reaction was connected with the effect of the relatively cold inner core of the monolith. The heating protocol consisted of four minutes of preheating without gas flow and four minutes of heating with gas flow. The best heating location differed for pellet and monoiith catalysts. Heating of monolith catalyst was most successful at a location approximately 3 cm from the inlet of the catalyst; heating immediately at the
EXPEFUMENT
L. DvoFbk et al. /Catalysis Today 20 (1994) 449-466
SIMULATION
465
EXPERIMENT
Fig. 10. Time courses of outlet conversion and spatio-temporal temperature patterns in a monolith catalyst. Inlet CO concentration 2%. (a) Heating protocol: heating at the inlet, adaptive heating. (b) Heating protocol: heating at the middle part and at the end part, adaptive heating.
inlet was less successful. Adaptive heating was also studied. Experiments began in the same way as those for measurements described above but when the conversion decreased below a certain value (75% in our case) additional heating (four minutes with the gas flow) was applied. Effects of the heating location were similar. Cases of successful and unsuccessful adaptive heating are shown in the Fig. 10a and lob. 8. Modelling Both qualitative and quantitative agreement between observed and computed temperature and conversion profiles could be expected with a view to the complexity of the modelled
466
L. Dvofcik et al. /Catalysis
Today 20 (1994) 449-466
transient situation. Typical spatiotemporal patterns of conversion and temperature of the catalyst obtained from the experiments and calculated using Eqs. ( l)-(4) are compared in Fig. 9a and 9b and Fig. 10a and lob and the corresponding values of the parameters are given in Table 1. In Fig. 9a the case of an unsuccessful light-off with single heating at the inlet is shown. We observe that after the heating was stopped, temporary light-off occurred but an extinction of the ignited regime was observed relatively quickly (cf. Fig. 5). On the other hand Fig. 9b shows successful light-off by heating at the middle and at the end part of the catalyst. Fig. 10a and lob depicts adaptive heating. Fig. 10a shows unsuccessful heating at the inlet, the conversion dropped rapidly when the heat was turned off; Fig. lob shows successful heating at the middle and at the end part, conversion was kept for a while at 100% after the heating was turned off.
9. Conclusions Both the experimental and modelling studies demonstrated that narrow band heating of both the packed bed and the monolith combustion catalyst can, under a proper heating protocol, lead to either temporary or permanent ignition (light-off). Detailed studies, including investigations of radial temperature profiles are in progress.
References [ 1J S.H. Oh, J.C. Cavendish and L.L. Hedegus: AIChE J., 26 (1980) 935. 12 ] T. Ma, N. Callings and T. Hands, Exhaust Gas Ignition (EGI) - A New Concept for Rapid Light-Off of Automotive Exhaust Catalyst, SAE Technical Paper 920 400, 1992. 131 W.A. Whittenberger and J.E. Kubsh, Recent Developments in Electrically Heated Metal Monoliths, SAE Technical Paper 900 503, 1990. [4] L.S. Socha and D.F. Thompson, Electrically Heated Extruded Metal Converters for Low Emission Vehicles, SAE Technical Paper 920 093, 1992. [ 51 W.A. Whittenberger and D.T. Sheller, Experiences with 20 User Vehicles Equipped with Electrically Heated Catalyst Systems - Part I, SAE Technical Paper 920 722, 1992. [6] J. KapiEka and M. Marek, J Catal., 119 (1989) 508. [7] B H. Engler, Katalysatoren fur den Umweltschutz, Lecture on the Jahrestmffen der Verfahrensingenieure, 3-5 Oktober, Stuttgart, 1990. [ 81 E. Koberstein and B Engler, Entwicklungs tendenzen bei Katalysatoren zur Reinigung der Abgase von Verbrennungskraftmaschinen, Automobil Industne, 1-91, 1991, pp. 744. [9] J. Levee and R.G. Carbonel, AIChE J., 31 (1985) 581,591. [lo] D.T. Lynch, Can. J. Chem., Eng., 61 (1985) 183. [ 111 .J.Orsag and M. Marek, in Y.S. Matros (Editor), Unsteady State Processes in Catalysis, VSP, Zeist. [ 121 J. KapiEka and M. Marek, Surf. Sci., 222 (1989) L885.
ELSEVIER
Catalysis Today 20 ( 1994) 449466
Dynamic behaviour of CO catalytic afterburner with electric heating L. Dvo%k, P. Pinkas, M. Marek” Department
of Chemical Engineering. Prague Institute of Chemical Technology, Technickd 5. 166 28 Prague 6, Czech Republic
Abstract An experimental PC-controlled catalytic reactor with additional local electric heating of the packed bed of porous particles or ceramic honeycomb is described. Typical axial temperature ignition profiles resulting from different electric heating patterns observed for CO combustion on a Pt/AI,O, catalyst are presented and discussed from the point of view of an optimal heating pattern (with respect to maximal average conversion) with limited heating power. The experimental results are both qualitatively and quantitatively compared with the results of a simulation of the two-phase quasi-onedimensional model including axial heat and mass transport in the gaseous and catalyst phase.
1. Introduction
The increasing pressure to lower emissions from mobile sources (mostly passenger cars and trucks) and also stationary sources of various origin has focused attention on increasing the efficiency of catalytic converters particularly those operating under transient (starting) conditions. Especially the cold start characteristics of catalytic car afterburners have now become the subject of intensive research. The main reason stems from the well known fact that a significant portion of the unconverted total HC and CO emissions occurs within the first two minutes of the driving cycle while the catalyst is heating up to operating temperature. A number of proposals for improving catalyst efficiency under starting conditions have been made. Thus, Oh et al. [ 1 ] have studied the light-off behaviour of catalyst pellets and found that it can be improved by using the correct subsurface impregnation depth of noble metals. However, most of the methods for rapid light-off of automotive exhaust catalysts are based on the rapid increase of catalyst temperature by additional heating. Three types of catalysts *Corresponding
author.
0920.5861/94/$07.00
0 1994 Elsewer Science B.V. All rights reserved
.SSDIO920-5861(94)00057-9
450
L. DvoMk et al. /Catalysrs Today 20 (1994) 349-466
are used in exhaust control (in decreasing order of the frequency of application): (a) ceramic monolith catalysts, (b) packed beds, (c) metallic monolith catalysts. Monolith catalysts are used most often in Europe. One of the methods which has been suggested to increase the catalyst temperature (i.e., reducing the light-off time) is the exhaust gas ignition. The method is based on the rapid ignition of rich exhaust gases with injected additional air and is critically dependent on the concentration of the hydrogen present [ 21. However, most often electrically-heated extruded metal converters are proposed for use in the faster light-off studies, cf. [ 31. Different configurations of extruded electrically-heated catalysts are being tested [ 41. The tested catalytic afterburners are composed of preheaters and main catalysts. The preheaters are either in the form of foils arranged to form an electrically resistive element and welded to electrical connection points, or in the form of monolithic catalytic preheaters manufactured from extruded and sintered metallic honeycombs. The power input, the size of the heater, the location relative to the engine and main converter and various heating protocols are being evaluated [ 51. The main drawbacks of the above preheaters are their high power demands, their high cost and their short durability. Many technical obstacles wtll have to be overcome before they can be fully commercialized. The aim of the present work is the study of the effects of the local electrical preheating of the two most commercially used types of catalytic afterburners - the catalytic ceramic monolith and the packed bed of catalyst-on the light-off characteristics of the afterburners. These studies were carried out both experimentally and by mathematical modelling. We report in this paper experimental and modelling observations of a possible fast ignition on the standard catalysts by electric heating of a relatively narrow strip of the catalyst. We compare the effects of various heating patterns on the catalyst. The possibility of modelling
6
451
L. Dvoicik et al. /Catalysis Today 20 (1994) 449-466
4
Fig. 1 (a) Scheme of the experimental reactor. 1,thermocouples; 2, outlet analyzers; 3, tubular reactor; 4, analogdigital converter; 5, personal computer; 6, carbon monoxide developer; 7, oxygen; 8, nitrogen. (b) Packed bed reactor with thermocouples and heatmg elements. 1,thermocouples; 2, tubular reactor, 3, heating; 4, catalyst, (c) Monohth reactor with thermocouples and heating elements I, tubular reactor; 2. thermocouples; 3, channel wtth a thermocouple; 4, empty channel, 5, heating.
the ignition phenomena discussed.
by a two-phase model considering
heat and mass dispersion is also
452
L. Dvoidk et al. / Cataiysrs Today 20 (1994) 449466
2. Experimental Commercial types of noble metal catalysts, Degussa ceramic honeycomb with noble metals in the washcoat and Cherox Pt/Al,O, pellet catalyst were used in the study. The overall scheme of the laboratory testing equipment is shown in Fig. la and the details of the heated catalyst showing the location of resistive heating elements and thermocouples are depicted in Fig. lb for the packed bed and in Fig. lc for the monolith, respectively. The length of the reactors was 11 cm, the diameter of the space filled with the catalyst was 3 cm. A mixture of CO, O2 and N2 was used for the study. The thermocouples in the packed bed were located both inside the pellets, at their surface and in the voids of the packed bed. Thus the difference between the gas phase and the catalyst surface temperature could be estimated, cf. [ 61. In the monolith, the thermocouples were placed at the external surface of the catalyst channels. Axial temperature profiles were both displayed and recorded in the database, together with the outlet CO and CO? concentrations measured by infrared analyzers. The inlet concentrations of CO, O2 and N2 and their flow-rates were computer-controlled by mass-flow-meters. Kantal wire elements located as loops within the packed bed and at the external surface of the honeycomb, cf. Fig. lb and Ic, respectively, were used for resistive heating with controlled power input.
3. Heating protocols US-FTP 75 and ECE.R 15.04 test reference cycles correspond to typical driving schedules in the US and Continental Europe. The flow-rate of exhaust gases, temperature and concentration of reactants vary with the engine power. For example, typical flow velocities of exhaust gases for idling and full speed in the above cycles vary in the ratio 1:50. Hence, we have to consider heating applied to the catalyst both without flow of the reactants and also at different intensities of the flow-rates, taking into account overall power limitations given, e.g., by limited capacity of the car battery. The efficiency of the use of resistively generated heat for temperature increase of the catalyst surface decreases also with the heat losses due to convection, which are proportional to the flow-rate of exhaust gases. An example of radial temperature profiles in the monolith catalyst resistively heated without reaction from the external surface according to the arrangement in Fig. lc are presented in Fig. 2. We observe a sharp decrease of stationary temperatures with increasing flow-rate and also an increasing steepness of radial temperature profiles. Hence, heating protocols that may be considered include a distribution of heating power along the catalyst together with its variation in time with and without the flow of reactants.
4. Mathematical
model
The operation of catalytic afterburners under standard conditions reflected in US and European test cycles is from the modelling point of view quite complex. We are in principle dealing with the forced cycling of catalytic reactor, where the inlet flow-rate, temperature and concentration of reactants vary cyclically. From the hydrodynamic point of view the
L. DvoGk et al. / Cataiws
Fig. 2. Temperature min-‘:2,81mn-‘;3.
To&y 20 (1994) 449-466
profiles in a monolith catalyst showing the dependence 121min-‘,4, 161min~‘;5,201min-I.
on the flow-rate 0, 0 1 min-‘.
1, 4
I
is laminar in the monolith reactor and can vary from the laminar to the turbulent regime in the packed bed. Often a non-uniform distribution of flow-rates over the cross-section of the reactor is also observed [ 71. As can be deduced from the Arrhenius plots; transition
flow
heat removal
GAS PHASE cin7 Tin
inlet
axial dispersion
c,T
heat transfer
mass
Gout transfer 4Y
h
c*,
T*
heat conduction
CATALYST heat removal Fig. 3. Reactor model scheme.
7
T3ut
outlet
L. Dvoidk et al. /Catalysis Today 20 (1994) 449-466
454
Table 1 Model dimensionless
parameters
Parameter
Value
Da
0.35 164.688 274.5 5.769 100 100 000 28.9 150°C 0.120
JM Jt”f* cp &“I Pe* Yr Tref Kads
Parametkr
Value
B
‘p* Pe,,
0.005 279 0.12 0.003 200
Ya
- 2.25
7
1
JH Jf
from the kinetically limited regime to mass and heat transport limited regimes with increasing reaction temperature often also occur [ 81. In principle, both longitudinal and lateral effective thermal dispersion mechanisms should be considered, taking also into account differences in effective thermal conductivities under steady state and transient conditions [ 91. Pathological behaviour connected with resonance has been also observed and modelled in the forced cycled catalytic reactors [ 10-121. Thus a comprehensive two-phase model, including axial dispersion in the gas phase, heat conduction in the catalyst phase and heat and mass transfer between the gas and catalyst
Hysterem
for various values of Kads
“Kadsti “K&=0 “Kads=O “K&-O “Kads=O
Fig. 4. Dependence
of hysteresis loop on Kads.
120” 180” 180” 500” 5W
YX-0. -+ --0. -
L. Dvorik et al. /Catalysis
Today 20 (1994) 449-466
455
phase is generally used for the modelling. A model that includes radial distributions studied later. The dimensionless variables are defined as: 1 z= L
y=
will be
axial coordinate
c?g- cc0
gas phase conversion
c% 6I=
c,a”’ - c;o
catalyst conversion
c22
(-+(T-T,) y TO
&T*-To) TO
gas phase temperature
y
catalyst temperature
The reactor model is schematically balances are in the form:
shown in Fig. 3. The dimensionless
mass and heat
(1)
$= -J;(w-y)
+ qDuR( &,w)
a?9 1 a26 -= --+~~~u~(S,w)-J,*(6-0)-cp*(6-8,)+~Q
z=O:~-pe,Y=O,~~-Pe~(*-~i~)=O,~~=O, z=yo,
ay
a9
a6
--g=o, z=o
Two types of kinetics were used in the simulations, first-order kinetics with respect to CO concentration and Langmuir-Hinselwood type kinetics. (a) First-order kinetics: ~(c;~,,T*)
=k(T*)c&
(b) Langmuir-Hinshelwood
kinetics:
L. Dw&ik
456
conversion
I 0.8
0.6
0.4
0.2
0
0.8
I
et al. /Cutalysrs
(a)
1 1000
‘,
Today 20 (1994) 449-466
catalyst
temperature
yOu':pace time
(dimensionless'
(5)
2000
time
(s)
,
(b)
0.6
300 200 1.0
0.4 0
i 0.2
o1000
2000
Cc)
0.4
:
Fig. 5. Time courses of outlet conversion and spatio-temporal patterns of temperature. packed bed, inlet temperature 178°C. mlet CO concentration 2%. and narrow band heating 120 s. (a) Inlet section, (b) mlddle section. (c) outlet section.
r(c&J*) = [
~(~*)c&,co, 1
+K(T*)c&g’
The Crank-Nicholson type of finite difference approximation with the Taylor expansion of non-linearity has been used for numerical integration of the system ( 1)-( 4).
L. DvoMk et al. /Catalysis
conversion
457
Today 20 (1994) 449-466
’
1
(a)
0.8
catalyst
temperature
0.6 0
space
0.4
‘;“\ 0.2
I 0
*
0.0
,
.
(dimensionless)
,we5
time 3
’
’
’
2000
1000
,
.
,
time
,
(s.1
’ (s)
,
0.6
1.0 0.4
0.8 250 0.6
200 1.0
0.4
0
2 0
1200
600
Fig. 6 Time courses of outlet cowerston and spatio-temporal patterns of temperature. packed bed, repeated heating 120s after 600 s. (a) Inlet section. (b) middle section, (c) single heating at all three locations.
5. Parameters
of mathematical
model
The parameters shown in Table I have been evaluated from physical properties. The value of the dimensionless heat transfer coefficient at the wall has been fitted to measure the dependence of the outlet conversion on the inlet temperature.
L. DvoEk et al. / Catalysis Today 20 (1994) 449-466
458 250
0
200
7 %
0
0
150
.!Y ti m 2 100
50
I
0
0
20
0
50
I
40
1
I
0
I
I
60 X0 apphed heating power [Wa]
,
I
I
I
100
170
I
I
b)
45 -
0
-
40 35 z
30 0
E w
25 -
z 8
20 1s 0 10
-
5
-
0 0 0
0 0
I
I
I
I
I
I
10
40
60 apphed heatq
80
100
170
power [W,]
Fig. 7. Dependence of the increase of overall reaction rate r (g mol s _ ’ ) on the heating power applied to the inlet section, Q (W s) : (a) inlet CO concentration 2%; (b) inlet CO concentration 1%.
L. DvoZk
et al. /Catalysis
Today 20 (I 994) 449466
459
6. Role of kinetics First-order and Langmuir-Hinshelwood type kinetics were compared. The dependence of the extinction and ignition temperatures on the value of the adsorption constant was found to be weak in the considered range of values of Kadscorresponding to experimental values. The point of extinction is practically the same, the point of ignition is slightly shifted to higher values of temperatures for larger value of Kads(cf. Fig. 4). Also the shape of the hysteresis loop is relatively independent of the activation energy EA in the range +20% which is approximately its standard value used in the literature and in our calculations.
7. Experimental
results
Examples of typical observed time courses of axial temperature profiles and outlet conversions for typical heating protocols will now be discussed. They represent partial results of continuing research aimed at generalization and quantification of the effects of different heating protocols leading to rapid ignition under limitations of heating power. 7.1. Packed bed reactor Examples of testing the effect of the location of a narrow heating zone on the ignition of the reaction are given in Fig. 5a-c. The heat applied in the entrance region is, under the present conditions, (cf. legend to Fig. 5) most effective, cf. Fig. Sa. The starting conditions correspond to a partially heated catalyst bed with a relatively high CO concentration (2%). Limited heating power ( 1.5 W, 120 s) causes the ignition of a high temperature combustion front which slowly moves to the reactor outlet. Conversion increases very rapidly and stays approximately at the value of 0.8 for more than 2000 s. Heating applied in the middle of the reactor is less effective as can be seen in Fig. 5b. However, it still causes outlet conversion to be higher than 0.7 for more than 1200 s. Heating applied at the end of the reactor (cf. Fig. 5c) does not cause a permanent light-off of the reactor and only a temporary increase of the conversion is observed. Hence, repeated local heating of the front end of the catalyst appears to be most efficient in the packed bed heated relatively close to the light-off condition. When the concentration of the combustible reactant is low ( I % CO) repeated heating may be necessary to maintain a high level of outlet conversion. The example of the effects of repeated heating applied in 600 s intervals are depicted in Fig. 6a (heating at the inlet) and 6b (heating in the middle of the reactor). This heating protocol is more effective than a single heating period applied at all three heating positions simultaneously at the start, cf. Fig. 6c. The dependence of the increase of the overall reaction rate in the bed Ar on the heating power Q is illustrated for the inlet concentration of 2% CO in Fig. 7a and for 1% CO in Fig. 7b. We can observe a typical jump in this dependence, corresponding to an ignition at a certain relatively well defined value of the heating power in Fig. 7a and an exponential increase of Ar with Q in Fig. 7b.
460
L. DvoMk et al. /Catalysis
Today 20 (1994) 449-466
I 1
0.7 5 e Y
0.6
E 0.5
140 temperature [Cl
1
0.9 08
0.7
g
0.6
G :: e
0.5
04
0.3
02
(I I 110
I 30
I40
temperaturr [Cl
I so
160
170
L.. DvoMk
et (11./Catalysis
I
Today 20 (1994) 449-466 I
t-
I
461 -
/_
*-
(C 1
-- ---~.5lmm_decreastng” “2lmm_decreasmg” “2 5lmm_mcrea,lng” “2hnIn_mcreasmg”
----
1
08
0.2
1
0
110
120
140 150 Inlet temperacure [C]
130
Measured and computed hysterests [2 % CO Wet).
160
170
180
2 % 02 (mlet). 2 1 /mm N2 (mlet)]
1
- ’
I
-= )21Aatl;:;
;
“experiment-” 0.8 -
=8
0.6 -
2 Y 5 z J2
0.4 -
02
-
120
110 mler temperature ICI
I60
1x0
200
Fig. 8. Monolith reactor. outlet conversion-inlet temperature plots: (a) Hysteresis. inlet CO concentration 2%. (b) Monotonous dependence, inlet CO concentration 1%. (c) Hysteresis loops for two different gas flow-rates (2.0 I min-I. 2 5 1 min-‘), inlet CO concentration 2%. (d) Comparison between computed and calculated hysteresis, inlet CO concentration 2%. gas flow-rate 2.0 I min- ‘.
462
L. DvoEk et al. /Catalysis Today 20 (1994) 449466
7.2. Monolith catalyst Typical experimental results are presented along with the results of corresponding simulations. Either single or multiple stationary states can be observed in the monolith catalyst, depending on the inlet conditions and the method of approach to the stationary state. The hysteresis loop in the outlet conversion dependence - the inlet temperature observed for the CO inlet concentration (2%) - is depicted in Fig. 8a and the monotonous outlet conversion - inlet temperature curve found for the 1% inlet CO concentration - is shown in Fig. 8b. A comparison between the hysteresis loops for two different flow-rates and the inlet concentration 2% of CO is depicted in Fig. 8c. We see that the ignition point is unchanged for a lower flow-rate but the extinction point is shifted to higher temperatures
EXPERIMENT
SIMULATION ‘I
bmf
i\
-
1
L. Dvoidk et al. /Catalysis
SIMULATION
Today 20 (1994) 449-166
463
EXPERIMENT
Fig. 9. Time courses of outlet conversion and spatio-temporal temperature patterns in a monolith catalyst. Inlet CO concentration 2%. (a) Heating protocol: heating at the inlet, flow of reactants started at 240 s, heating proceeds for the next 240 s. (b) Heating protocol: heating at the middle part, flow of reactants started at 240 s, heating proceeds for the next 240 s.
reflecting the lower reaction heat generated with a decreased flow-rate. Comparison between computed and measured hysteresis is shown in Fig. 8d. A significant radial temperature profile can be established in the monolith catalyst when narrow band heating is applied at the external surface, particularly when heating occurs under flow conditions. Hence heating protocols which consider a combination of heating without and with reactant flow were also studied. Unsuccessful and successful temporary light-off’s of the monolith reactor are depicted in Fig. 9a and 9b, respectively. In both cases
464
L DvoiZk et ul. / Cata1.y.m Today 20 (1994) 449266
narrow band heating was applied in the front or at the end of the monolith in such a way that the initial heating occurred without flow and then the flow of reactants was started and heating continued. No light-off occurred in the former case (Fig. 9a), where only a sharp peak in the conversion can be observed. In the latter case a temporary light-off occurred after starting the flow of reactants. A significant radial temperature profile was established in the reactor. The extinction of the reaction was connected with the effect of the relatively cold inner core of the monolith. The heating protocol consisted of four minutes of preheating without gas flow and four minutes of heating with gas flow. The best heating location differed for pellet and monoiith catalysts. Heating of monolith catalyst was most successful at a location approximately 3 cm from the inlet of the catalyst; heating immediately at the
EXPEFUMENT
L. DvoFbk et al. /Catalysis Today 20 (1994) 449-466
SIMULATION
465
EXPERIMENT
Fig. 10. Time courses of outlet conversion and spatio-temporal temperature patterns in a monolith catalyst. Inlet CO concentration 2%. (a) Heating protocol: heating at the inlet, adaptive heating. (b) Heating protocol: heating at the middle part and at the end part, adaptive heating.
inlet was less successful. Adaptive heating was also studied. Experiments began in the same way as those for measurements described above but when the conversion decreased below a certain value (75% in our case) additional heating (four minutes with the gas flow) was applied. Effects of the heating location were similar. Cases of successful and unsuccessful adaptive heating are shown in the Fig. 10a and lob. 8. Modelling Both qualitative and quantitative agreement between observed and computed temperature and conversion profiles could be expected with a view to the complexity of the modelled
466
L. Dvofcik et al. /Catalysis
Today 20 (1994) 449-466
transient situation. Typical spatiotemporal patterns of conversion and temperature of the catalyst obtained from the experiments and calculated using Eqs. ( l)-(4) are compared in Fig. 9a and 9b and Fig. 10a and lob and the corresponding values of the parameters are given in Table 1. In Fig. 9a the case of an unsuccessful light-off with single heating at the inlet is shown. We observe that after the heating was stopped, temporary light-off occurred but an extinction of the ignited regime was observed relatively quickly (cf. Fig. 5). On the other hand Fig. 9b shows successful light-off by heating at the middle and at the end part of the catalyst. Fig. 10a and lob depicts adaptive heating. Fig. 10a shows unsuccessful heating at the inlet, the conversion dropped rapidly when the heat was turned off; Fig. lob shows successful heating at the middle and at the end part, conversion was kept for a while at 100% after the heating was turned off.
9. Conclusions Both the experimental and modelling studies demonstrated that narrow band heating of both the packed bed and the monolith combustion catalyst can, under a proper heating protocol, lead to either temporary or permanent ignition (light-off). Detailed studies, including investigations of radial temperature profiles are in progress.
References [ 1J S.H. Oh, J.C. Cavendish and L.L. Hedegus: AIChE J., 26 (1980) 935. 12 ] T. Ma, N. Callings and T. Hands, Exhaust Gas Ignition (EGI) - A New Concept for Rapid Light-Off of Automotive Exhaust Catalyst, SAE Technical Paper 920 400, 1992. 131 W.A. Whittenberger and J.E. Kubsh, Recent Developments in Electrically Heated Metal Monoliths, SAE Technical Paper 900 503, 1990. [4] L.S. Socha and D.F. Thompson, Electrically Heated Extruded Metal Converters for Low Emission Vehicles, SAE Technical Paper 920 093, 1992. [ 51 W.A. Whittenberger and D.T. Sheller, Experiences with 20 User Vehicles Equipped with Electrically Heated Catalyst Systems - Part I, SAE Technical Paper 920 722, 1992. [6] J. KapiEka and M. Marek, J Catal., 119 (1989) 508. [7] B H. Engler, Katalysatoren fur den Umweltschutz, Lecture on the Jahrestmffen der Verfahrensingenieure, 3-5 Oktober, Stuttgart, 1990. [ 81 E. Koberstein and B Engler, Entwicklungs tendenzen bei Katalysatoren zur Reinigung der Abgase von Verbrennungskraftmaschinen, Automobil Industne, 1-91, 1991, pp. 744. [9] J. Levee and R.G. Carbonel, AIChE J., 31 (1985) 581,591. [lo] D.T. Lynch, Can. J. Chem., Eng., 61 (1985) 183. [ 111 .J.Orsag and M. Marek, in Y.S. Matros (Editor), Unsteady State Processes in Catalysis, VSP, Zeist. [ 121 J. KapiEka and M. Marek, Surf. Sci., 222 (1989) L885.