Simulation of a Nonisothermal Modern Three-Way Catalyst Converter

Ind. Eng. Chem. Res. 2010, 49, 7039–7051

7039

Simulation of a Nonisothermal Modern Three-Way Catalyst Converter Hyuk Jae Kwon, Joon Hyun Baik,† Sung Bong Kang, In-Sik Nam,* and Byung Jun Yoon Department of Chemical Engineering/School of EnVironmental Science and Engineering, Pohang UniVersity of Science and Technology (POSTECH), San 31 Hyoja-dong, Pohang 790-784, Korea

Se H. Oh General Motors R&D Planning Center, Warren, Michigan 48090-9055

A two-dimensional (2D) nonisothermal monolith reactor model based upon intrinsic detailed reaction kinetics has been developed to simulate the performance of a commercial modern three-way catalytic converter. The model directly employed the reliable kinetic parameters estimated from the detailed reaction kinetics determined over the powder-type three-way catalysts (TWCs). The TWC activity of the monolith reactor containing each Pd and Pt/Rh/Ce catalyst with respect to the catalyst mileages, 4k miles (stabilized) and 100k miles (aged) equivalent aged by engine-dynamometer, has been examined in a molten-salt bath under the steady-state condition. To simulate the commercial performance of a modern TWC converter, both reactor models specifically developed for the Pd (front) and Pt/Rh/Ce (rear) monoliths have been sequentially integrated on the basis of the commercial configuration of the monolith reactors in a dual-bed mode. The 2D nonisothermal monolith reactor model developed in the present study well predicts the TWC performance, including the gas compositions and the temperature distribution with respect to both axial and radial positions of the single-bed containing each individual catalyst monolith as well as of the dual-bed monolith reactor system including both Pd (front) and Pt/Rh/Ce (rear) monolith bricks. The reactor model was further validated by predicting the TWC performance of the dual-bed reactor under the steady-state sweep test (st-ST) condition varying the A/F ratios from 14.23 to 15.03 with respect to the reaction temperature. Introduction The automotive catalytic converter containing multiple monolith reactors washcoated with three-way catalysts (TWCs) including Pd and Pt/Rh/Ce has been commonly installed into recent gasoline-driven engines to meet the ever-tightening emission regulations worldwide.1,2 For designing a modern commercial catalytic converter simultaneously containing two monolith bricks, Pd in a front brick and Pt/Rh/Ce in a rear brick, the development of an appropriate monolith reactor model is critical to predict the reactor performance with respect to the reactor operating conditions, including feed gas composition and reaction temperature. A question may be how the complexity of the physicochemical configuration and process within the monolith channels and walls washcoated by TWCs can be properly included into the model to simulate the commercial performance of the modern automotive catalytic converter containing multiple monoliths. A variety of reactor models have been developed and reported in view of the physical dimension of the monolith, one-, two-, or three-dimensional (1D, 2D, or 3D) model by assuming uniform channels of the monolith, regardless of the number of channels involved.3-5 To further simplify the complexity of the automotive catalytic converter, a one-dimensional (1D) approach has been employed for developing a monolith reactor model.6,7 In addition, based upon the assumption of uniform monolith geometry and adiabatic boundary conditions, a conventional adiabatic monolith reactor model recognized as a classical “single channel” model has been commonly derived by Kuo et al., Vortuba et al., Heck et al., and Oh and Cavendish (to name * To whom correspondence should be addressed. Tel.: 82-54-2792264. Fax: 82-54-279-8299. E-mail: [email protected]. † Present address: Environmental Research Department, Research Institute of Industrial Science & Technology (RIST), San 32, Hyojadong, Pohang 790-330, Korea.

a few).8-14 The simpler 1D single channel model developed only predicts the fundamental axial performance of the monolith along with the length of the monolith reactor without any information on the change of the gas composition and the reaction temperature with respect to the radial position of the monolith. To resolve the drawbacks of the 1D model, a two-dimensional (2D) single channel model was proposed by Young and Finlayson,15 Lee and Aris,16 Otto and LeGray,17 and Hayes et al.18 However, they also simply assumed a single channel of monolith representing a whole piece of the monolith and developed a model with laminar flow distribution and asymptotic dimensionless groups including Nu and Sh numbers determined at the constant wall temperature and heat flux over the monolith channels. The assumptions may be relevant for describing the performance of a single channel. However, they may differ from the actual flow and temperature distribution over an entire piece of a monolith. It may not be fair to predict the performance of a whole piece of monolith by using assumptions only valid over a single channel of monolith. Consequently, a multi channel model has been derived by using a two- or three-dimensional approach to resolve the arguments raised for a single channel model. Flytzani-Stephanopoulos et al. developed a 2D nonadiabatic metal honeycomb monolith model for describing the heat transfer without any chemical reaction.19 Zygourakis and Chen et al. developed 2D and 3D models to describe the transient alteration of the axial or radial temperature and conversion distribution for the oxidation reactions, respectively.20,21 Recently, Chakravarthy et al. reported a multi channel model for simulating the axial or radial temperature distribution during the cold-start transient period of engine operation.22 Indeed, a 2D or 3D multi channel model is relevant for simulating the commercial and practical performance of an

10.1021/ie1007486  2010 American Chemical Society Published on Web 06/29/2010

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Ind. Eng. Chem. Res., Vol. 49, No. 15, 2010

Figure 1. Configuration of dual-bed monolith reactor simulating the realistic automotive catalytic converter.

automotive catalytic converter, as compared to a simple 1D single channel model. However, the multidimensional models reported in the literature have been simply simulated to describe the axial or radial temperature distribution with a variety of the flow patterns, uniform or nonuniform distribution, and the catalytic activity of the oxidation reactions of CO and HC, without any experimental validation to evaluate the appropriateness of the model developed. In addition, they still utilized the simple and classical TWC reaction kinetics developed by Votlz et al.23 and Subramaniam and Varma,24 which can hardly describe the actual performance of the modern TWC under realistic operating conditions. None of the previous reaction kinetics and reactor models developed is capable of simultaneously predicting the changes of the realistic feed and product gas compositions including H2O, H2, O2, CO2, CO, C3H6, NO, NH3, and N2O. Note that the reaction kinetics to be included into the monolith reactor model is critical for identifying the complex reaction system involved in a modern catalytic converter. In the present study, a two-dimensional (2D) nonisothermal monolith reactor model has been developed to describe the performance of the modern catalytic converter simultaneously, including Pd in the front brick and Pt/Rh/Ce in the rear brick with a detailed intrinsic reaction kinetics derived over the powder-type TWCs under the isothermal reaction condition.25 The monolith reactor model has been incorporated with the reaction kinetics to predict the axial and radial performance of a modern catalytic converter. The heat and mass effects on the performance of the monolith were included in the reactor model with respect to the physical configuration of the monolith, including the wall thickness of the washcoat, the cell number and size, and the reaction condition. To further determine the validity of the reactor model developed, the TWC performance over the dual-bed reactor system has been also examined by the steady-state sweep test (st-ST) under realistic lean-rich gas feed compositions.26 Experimental Section Monolith. A set of the commercial monolith bricks (600 cpsi), including Pd in a front brick followed by Pt/Rh/Ce rear

brick, was obtained from General Motors with respect to the catalyst mileages. They were aged in an engine-dynamometer: stabilized (4k miles equivalent; 4k Pd and Pt/Rh/Ce) and aged (100k miles equivalent; 100k Pd and Pt/Rh/Ce) samples. The monolith was cored out in a dimension of 3/4 in. diameter and 0.5 or 1 in. length in the present study. Reaction System. TWC activities of the monoliths have been examined using a U-tube type integral flow reactor immersed into a molten-salt bath under the steady-state reaction condition. Details of the reactor system have been described elsewhere.27 Four thermocouples to determine the gas temperature, one in the upstream and three in the downstream of the monolith reactor, have been installed to observe the distribution of the reaction temperature with respect to the axial and radial positions of the monoliths during the course of the reaction, particularly at the center and two edges in the radial positions of the monolith, as depicted in Figure 1. No temperature distribution of the front side of the monolith in the upstream of the reactor has been observed. The catalytic activity has been examined over both single(each Pd or Pt/Rh/Ce monolith) and dual-bed (both Pd and Pt/ Rh/Ce monoliths) systems with respect to the temperature in the upstream of the monolith reactor identical to that of moltensalt varied from 423 to 723 K under steady-state. The reactor space velocity has been varied from 10 000 to 50 000 h-1 for the single-bed system and from 5000 to 20 000 h-1 for the dualbed. A full feed gas composition including 1% CO, 0.3% H2, 1% O2, 500 ppm NO, 500 ppm C3H6, 10% CO2, 10% H2O, and Ar balance (A/F ) 14.76; λ ) 1.009) has been employed for the TWC activity test. In addition, a steady-state sweep test (st-ST) was performed with respect to the air to fuel ratios (A/ F) at given temperature under the steady-state without any real time dynamic oscillation of the feed gas steam.27 It was conducted under the steady-state lean-rich feed gas conditions: air to fuel ratios (A/F) from 14.23 (λ ) 0.973) to 15.03 (λ ) 1.028) by changing O2 feed concentration from 0.4% to 1.3% over the dual-bed catalytic monolith system.26,28 The variation of the conversions of CO, C3H6, and NO (three major pollutants)


was particularly determined with respect to the A/F ratios at 523, 603, and 673 K. The concentrations of CO, H2, C3H6, CO2, and O2 were analyzed by gas chromatography (GC) equipped with TCD and FID (Agilent, model 6890N). Also, those of NO and products including N2O and NH3 were measured by FTIR analyzer with a 2 m gas cell and DTGS/KBr detector (Nicolet 6700, Thermo Electron Co.) in the range of 500-4000 cm-1 with a resolution of 0.5 cm-1. All of the results reported were obtained over a monolith precalcined under a stoichiometric mixture containing 0.9% CO, 0.6% O2, 0.3% H2, 10% CO2, 10% H2O, and Ar balance (A/F ) 14.63; λ ) 1) at 723 K for 2 h and then cooled in Ar atmosphere to the reaction temperature prior to each test.

of the catalyst washcoated onto the walls of the monolith channels, 35.9 and 58.4 µm, in Pd and Pt/Rh/Ce monolith bricks, respectively. Commonly, the diffusion resistance of the gas composition may not be significant when the thickness of the washcoats is less than 100 µm.3,11,31 Equations 1-4 can be solved simultaneously with the following initial and boundary conditions: Cgi ) C0,i, Tg ) T0, and kez

-u

∂Cgi + km,iAe(Cgi - Csi ) ) 0 ∂z

(1)

For the solid phase: km,iAe(Cgi - Csi ) ) Ri

(2)

Energy balances for the gas phase: FCpu

∂Tg ) hAe(Ts - Tg) ∂z

(3)

ker

[( )

(

∂Ts ∂Ts ∂ 1∂ k + rk ∂z ez ∂z r ∂r er ∂z

)]

(5)

∂Ts ) 0 at z ) L ∂z

(6)

ker

∂Ts ) 0 at r ) 0 ∂r

(7)

∂Ts ) U(Ta - Ts) at r ) R ∂r

(8)

Equations 5-8 are the initial and boundary conditions with respect to the axial (monolith length, z) and the radial positions (monolith radius, r) over a whole piece of monolith. No variation of the concentrations of the gas compositions and the reaction temperature can be expected with respect to the angular position of monolith (2D axisymmetry).22 The inlet concentration and temperature of the feed gas, C0,i and T0, are identical, regardless of the radial position of monolith, r. The heat effect of the nonisothermal monolith reactor employed in the present study has been predominantly described by the convection of a plug flow of the feed gas as well as by the conduction due to the isothermal condition of a molten-salt bath. U is the overall heat transfer coefficient determined by eq 9 based upon the heat resistances by the surroundings around a monolith including its wrapping mat, stainless steel tube reactor, and molten-salt.32 It should be noted that the nonisothermal condition of the monolith reactor discussed may be specific for the reactor system submerged in the molten-salt bath employed in the present study.33,34 ttube tmat 1 1 d ) + + + U hsalt ktube kmat ker

For the solid phase: (1 - ε)

∂Ts ) 0 at z ) 0 ∂z

kez

Monolith Reactor Model 2D Nonisothermal Reactor Model. A 2D nonisothermal monolith reactor model for simulating the TWC activity over a catalytic converter has been developed with the following assumptions: (i) steady-state, (ii) axisymmetric geometry, (iii) the detailed reaction kinetics based upon the LangmuirHinshelwood mechanism,25 (iv) plug flow over a whole piece of monolith,29,30 and (v) heat transfer by conduction and convection between solid and gas phases.19-22 With these assumptions, the material and energy balances in the gas and solid phases to the axial (z) and radial (r) directions as depicted in Figure 1 become.20,21 Material balance for the gas phase:

+ hAe(Tg - Ts) +

∑ (-∆H) R

i i

) 0 (4)

where Cig and Cis and Tg and Ts are the concentrations of the gas compositions and the temperatures in the gas phase and of the monolith surface, respectively; Ae is the geometric surface area; ε is the open frontal area of the monolith; and kez and ker are effective thermal conductivities of axial and radial directions, respectively. Also, Ri is the detailed reaction kinetics determined in the previous study;25 u is the superficial linear velocity of the feed gas in a plug flow mode as assumed. The flow distribution of each channel consisting of a monolith is commonly recognized as fully developed laminar flow, particularly for a single channel 2D monolith reactor model.3,13,22 However, the macroscopic flow distribution over an entire piece of monolith installed in a catalytic converter may be assumed as a uniform plug flow, mainly due to the identical pressure drop over each channel of the monolith, as depicted in Figure 1.29,30 The effect of the internal diffusion resistance on the reactor performance has not been included for deriving the material balance in the gas phase, eq 1, mainly due to the thin thickness

7041

(9)

where hsalt is the heat transfer coefficient for the molten salt; ktube and kmat are the thermal conductivity of tube and mat (insulator); and ttube and tmat are also the thickness of tube wall and mat, respectively. The heat and mass transfer coefficients, h and km, are calculated from the following correlations:18,35,36 h)

Nu · kg Sh · Dg,i and km,i ) dh dh

d Nu ) 2.98 1 + 0.095 PeH L

(

)

0.45

(10)

and

d Sh ) 2.98 1 + 0.095 PeM L

(

)

0.45

(11)

Reaction Kinetics. The majority of the reaction kinetics reported in the literature have been developed by the LangmuirHinshelwood or power law formalisms.23,24,37 They are based on the reaction rate data obtained over a single-component fresh catalyst under the simplified feed gas streams, even though a large number of complex reactions occur in an engine exhaust stream containing O2, H2, H2O, and CO2 besides the three major pollutants, CO, HCs, and NOx. Moreover, the lumped kinetic

7042


parameters estimated from their power law or mechanistic reaction kinetics determined under simple feed condition have been frequently used to simulate the reactor performance of the monolith. Recently, a detailed reaction kinetics based upon the 18 reliable and possible reactions was developed to describe the TWC activity under “real-world” conditions.25 The reaction kinetics had been derived on the basis of the following assumptions: (i) all reaction steps are assumed to be a firstorder reaction, (ii) all reactants except CO2 adsorb on the catalyst surface, (iii) oxygen and hydrogen adsorb dissociatively on the catalyst surface, and (iv) surface reaction is a rate-determining step and described by a dual-site Langmuir-Hinshelwood mechanism. Table 1 shows the surface reaction mechanism for the reaction kinetics employed for the development of the reaction kinetic model. Based upon the reaction mechanism including the 18 reliable surface reactions determined, the seven steady-state rate equations can be obtained as follows:

(

[

1 k12K1K4CCOCO2 + k15K1CCO × D2 k9K′6 + 2k10K4K′6CO2 + k11K3K4CH2CO2 + k19K3K5CH2CNO k14K3CH2 + 2k15K1CCO

RCO )

)]

(13)

[

1 -0.5k9K′6 + 0.5k11K3K4CH2CO2 + 0.5k14K3CH2 × D2 k9K′6 + 2k10K4K′6CO2 + k11K3K4CH2CO2 + k19K3K5CH2CNO + k14K3CH2 + 2k15K1CCO

RH2 )

(

1 [k13K2K4CC3H6CO2] D2

)

0.5k19K3K5CH2CNO + 1.5k21K3CH2

(k16 + k19K3CH2)K5CNO (k17 + k18)K5CNO + k21K3CH2

]

(14)

RNO )

[

2 (k17 + k18)(k16 + k19K3CH2)K52CNO 1 k K C + + 16 5 NO (k17 + k18)K5CNO + k21K3CH2 D2

]

k19K3K5CH2CNO - 2k23K4K7CO2CNH3 + 2 [k25K4K5K7CO2CNOCNH3] (15) D3

RNH3 ) -

[

1 k21(k16 + k19K3CH2)K3K5CH2CNO D2 (k17 + k18)K5CNO + k21K3CH2

]

2(k22 + k23 + k24)K4K7CO2CNH3 + (16) 2 [k25K4K5K7CO2CNOCNH3] D3

RN2O ) -

[

2 k17(k16 + k19K3CH2)K25CNO 1 + D2 (k17 + k18)K5CNO + k21K3CH2

]

k24K4K7CNH3CO2 +

1 [(0.5k10K′6 + 0.5k11K3CH2 + D2

0.5k12K1CCO + 4.5k13K2CC3H6)K4CO2 - 0.5k16K5CNO - 0.5

2 k18(k16 + k19K3CH2)K52CNO + (18) (k17 + k18)K5CNO + k21K3CH2

(1.5k22 + 2.5k23 + 2k24)K4K7CO2CNH3] + 0.5 0.5 [k K C ] [k25K4K5K7CO2CNOCNH3] D 26 8 N2O D3

where D ) 1 + K1CCO + K2CC3H6 + K3CH2 + K4CO2 + K5CNO + K′6 + K7CNH3 + K8CN2O + k9K′6 + 2k10K4K′6CO2 + k11K3K4CH2CO2 + k14K3CH2

(12) RC3H6 )

RO2 )

1 [k K C ] (17) D 26 8 N2O

k19K3K5CH2CNO + + 2k15K1CCO

(19)

(k16 + k19K3CH2)K5CNO (k17 + k18)K5CNO + k21K3CH2 It was assumed that the content of water CH2O is constant due to the high feed concentration of water (10%) employed (K′6 ) K6CH2O). In addition, the reaction kinetics developed over a commercial TWC, Pd, and Pt/Rh/Ce catalysts with respect to the catalyst mileages, 4k (stabilized) and 100k (aged), well described the TWC activity under the realistic conditions. Note that the recent commercial catalytic converter simultaneously employs two monoliths, Pd as a front brick and Pt/Rh as a rear brick. The kinetic parameters, mainly the frequency factor of the reaction rate constant, are altered with respect to the catalyst mileages,38 while keeping the adsorption constants and activation energies unchanged, as listed in Table 1. The physicochemical properties of the monolith (solid) and gas composition for simulating the reactor model have been measured and obtained from the literature as listed in Table 2.11,21,39,40 Numerical Method. The reactor model, a set of simultaneous PDEs, has been solved by using a commercial library program, COMSOL MULTIPHYSICS (version 3.3, COMSOL, Inc.) based upon the finite element method (FEM) with more than 1000 random triangular elements for appropriately describing the geometry of the monolith in 2D axisymmetric domain in a cylindrical coordinate.41 Results and Discussion TWC Activity of the Monolith Reactor. The TWC activity of each monolith, 4k and 100k Pd and Pt/Rh/Ce catalysts, has been examined over a single-bed reactor system with respect to the reactor space velocity defined as the ratio of the gas flow rate to the volume occupied by the monolith, 10 000, 30 000, and 50 000 h-1, and the reactor inlet temperatures of 423-703 K, as shown in Figures 2-4. The higher are the reactor space velocity and the catalyst mileages, the higher are the light-off temperatures (LOTs; T50) of the TWC reactions that have been observed, regardless of the catalysts, Pd and Pt/Rh/Ce, as shown in Figures 2-4. For example, the LOTs of the TWC reaction for H2, NO, CO, and C3H6 increased from 479, 493, 506, and 511 K to 543, 559, 578, and 601 K over the 4k Pt/Rh/Ce catalyst, respectively, as the reactor space velocity increased from 10 000 to 50 000 h-1, as shown in Figure 3. Also, the LOTs of H2, NO, CO, and C3H6 over the Pt/Rh/Ce catalyst at a reactor space velocity of 30 000 h-1 shifted from 517, 539,


7043

25

Table 1. Kinetic Parameters Estimated from the Experimental Data Observed over the Packed-Bed Reactor kinetic parametersa Pd adsorption constants, Ki and surface reactions rate constants,25 ki

4k

CO + S T CO · S C3H6 + S T C3H6 · S H2 + S T 2H · S O2 + 2S T 2O · S NO + S T NO · S H2O + S T H2O · S NH3 + S T NH3 · S N2O + S T N2O · S H2O · S + S f OH · S + H · S H2O · S + O · S f 2OH · S H · S + O · S f OH · S + S CO · S + O · S f CO2 + 2S C3H6 · S + 9O · S f 3CO2 + 3H2O · S + 7S H · S + OH · S f H2O · S + S CO · S + 2OH · S f CO2 + H2O + 2S NO · S + S f N · S + O · S NO · S + N · S f N2O · S + S NO · S + N · S f N2 + O · S + S NO · S + N · S f N · S + OH · S N · S + 3H · S f NH3 · S + 3S 2NH3 · S + 3O · S f N2 + 3H2O · S + 2S 2NH3 · S + 5O · S f 2NO · S + 3H2O · S + 2S 2NH3 · S + 4O · S f N2O · S + 3H2O · S + 2S 2NH3 · S + 2NO · S + O · S f 2N2 + 3H2O · S + 2S N2O · S f N2 + O · S

1.9 × 10 1.0 × 101 2.5 × 102 3.0 × 10-1 1.0 × 10° 1.0 × 10-7 2.0 × 10-1 1.5 × 102 2.9 × 103 1.6 × 104 3.3 × 109 7.6 × 106 5.5 × 109 1.0 × 109 2.0 × 103 5.2 × 10-4 1.5 × 107 5.6 × 108 1.7 × 105 2.8 × 104 2.0 × 104 2.0 × 104 2.5 × 104 2.5 × 104 2.0 × 104

Pt/Rh/Ce

Pd

Pt/Rh/Ce

25

a

K0,1 K0,2 K0,3 K0,4 K0,5 K0,6′ K0,7 K0,8 k0,9 k0,10 k0,11 k0,12 k0,13 k0,14 k0,15 k0,16 k0,17 k0,18 k0,19 k0,21 k0,22 k0,23 k0,24 k0,25 k0,26

100k 1

4k

1.9 × 10 1.0 × 101 2.5 × 102 3.0 × 10-1 1.0 × 10° 1.0 × 10-7 2.0 × 10-1 1.5 × 102 1.9 × 103 1.4 × 104 7.9 × 108 3.4 × 106 9.2 × 108 5.0 × 108 1.1 × 102 1.2 × 10-4 2.4 × 106 2.0 × 108 4.6 × 104 4.5 × 103 2.0 × 104 2.0 × 104 1.4 × 104 2.0 × 104 1.0 × 104 1

100k

2.3 × 10 1.1 × 101 6.6 × 103 9.1 × 10-2 5.0 × 102 4.4 × 10-7 7.2 × 101 5.2 × 102 3.9 × 103 3.6 × 104 7.4 × 108 6.0 × 107 4.1 × 1010 1.2 × 109 1.4 × 105 8.5 × 10-4 4.8 × 1012 5.6 × 1011 1.2 × 104 1.3 × 1013 5.9 × 109 8.0 × 108 6.8 × 107 5.0 × 107 5.6 × 105 1

4k and 100k

2.3 × 10 1.1 × 101 6.6 × 103 9.1 × 10-2 5.0 × 102 4.4 × 10-7 7.2 × 101 5.5 × 102 1.9 × 103 1.6 × 104 2.7 × 108 2.5 × 107 1.0 × 1010 6.9 × 108 1.4 × 105 2.0 × 10-4 1.0 × 1012 5.4 × 1011 4.2 × 103 2.3 × 1012 4.8 × 109 1.7 × 108 4.8 × 107 3.8 × 107 2.8 × 105 1

∆H1 ∆H2 ∆H3 ∆H4 ∆H5 ∆H6 ∆H7 ∆H8 Ea,9 Ea,10 Ea,11 Ea,12 Ea,13 Ea,14 Ea,15 Ea,16 Ea,17 Ea,18 Ea,19 Ea,21 Ea,22 Ea,23 Ea,24 Ea,25 Ea,26

48.7 32.8 74.3 100.0 84.4 94.1 13.4 42.8 146.2 141.1 134.4 76.0 88.6 48.7 43.3 106.7 58.8 83.2 83.2 51.2 63.8 63.8 81.1 116. 8 117.6

60.1 32.3 44.9 58.4 76.0 96.6 11.8 39.9 145.7 141.1 75.6 79.0 88.6 65.5 43.3 52.9 53.3 63.8 76.0 34.4 42.8 76.0 68.5 36.5 115.9

k0,i [cm3/mol], ∆Hi [kJ/mol], k0,i [cm3/mol · s], and Ea,i [kJ/mol].

Table 2. Solid- and Gas-Phase Properties for the Model Simulation solid-phase properties 39

Ae d dH39 ker21 kez21 L U ε39

2

gas-phase properties 3

35.3 cm /cm 1.45 cm 0.095 cm 2.65 × 10-3 J/cm · s · K 5.3 × 10-3 J/cm · s · K 2.54 cm 4.5 × 10-2 J/cm2 · s · K 0.80 (Pd) 0.76 (Pt/Rh/Ce) (with washcoat)

555, and 567 K to 580, 567, 593, and 623 K, respectively, as the catalyst mileage increased. The shift of the LOT to the higher temperature region as the catalyst mileage increased is primarily due to the thermal aging of the catalyst, decreasing the BET surface area of the catalyst and the dispersion of active noble metals on the catalyst surface as reported in the previous study.25 The oxidation activity of CO and C3H6 over the Pd catalyst is higher than that over the Pt/Rh/Ce catalyst, probably due to the stronger intrinsic oxidation activity of Pd.25,42 However, the NO removal activity is significantly low, because no Rh is included in the front brick, widely recognized as an effective catalytic component for the NO reduction reaction.25,43 The maximum reaction temperature difference between the inlet and outlet of the reactor (thermocouples 1 and 2 in Figure 1) varies as much as 60 K with respect to the reactor space velocity as listed in Table 3, mainly due to the exothermic oxidation reaction occurring over the TWC. In addition, the higher is the reactor space velocity, the broader is the temperature difference, particularly in the region of the LOTs over the present catalytic system. The TWC performance over the dual-bed monolith reactor system, 4k Pd in a front bed and 4k Pt/Rh/Ce in a rear bed, including the conversion of the gas compositions and the temperature distribution was also examined under the reaction

11

DCO DC3H611 DH211 DO211 DNO40 DNH340 DN2O40 Cp21 kg21 F40

0.348 cm2/s at 838 K 0.220 cm2/s at 838 K 1.344 cm2/s at 838 K 0.349 cm2/s at 838 K 0.305 cm2/s at 473 K 0.524 cm2/s at 473 K 0.435 cm2/s at 273 K 1.089 J/g · K 2.269 × 10-6Tg J/cm · s · K 1.66 × 10-3 g/cm3

condition identical to that over the single-bed reactor. The configuration of dual-bed monolith reactor employed in the present study for simulating the modern commercial catalytic converter can be observed in Figure 1. Two monoliths are consecutively set into the reactor system, Pd in front bed and Pt/Rh/Ce in rear bed.42 It should be noted that the dimension of the monolith samples in the dual-bed reactor was 1.45 cm (d) × 1.27 cm (L) for the front Pd monolith and 1.45 cm (d) × 2.54 cm (L) for the rear Pt/Rh/Ce monolith. Figure 5 shows the TWC activity of the dual-bed monolith reactor system. The TWC performance was measured at four different reactor space velocities varying from 5000 to 20 000 h-1. A similar LOT trend was also observed over the dual-bed monolith reactor system simultaneously containing 4k Pd and Pt/Rh/Ce as the reactor space velocity increased. Again, the higher is the reactor space velocity, the higher are the LOTs of the TWC reactions that have been examined. The oxidation performance of CO, C3H6, and H2 is quite similar to that of the Pd single-bed, mainly due to the dominant oxidation activity of the Pd catalyst located in the front bed of the dual-bed reactor system, as discussed in Figures 2 and 3. Although the Pt/Rh/Ce catalyst monolith was included in the rear bed of the reactor system, the NO conversion was lower than that of the Pt/Rh/Ce single-bed reactor. This result simply

7044


Figure 2. Comparison of predicted and measured data over the 4k Pd monolith catalyst as a function of temperature for the reactor space velocities. Feed gas composition: 1% CO, 0.3% H2, 500 ppm C3H6, 1% O2, 500 ppm NO, 10% CO2, 10% H2O, and Ar balance (A/F ) 14.76; λ ) 1.009).

indicated that the possible NO reducing component including CO, C3H6, and H2 was readily oxidized by O2 in the front bed including Pd, mainly due to the reactor operating condition under the slightly lean A/F ratio.44 It may directly reflect the commercial performance of a modern catalytic converter, and it is indeed typical for the performance of commercial converter to simultaneously achieve the optimal activity of the major pollutants including CO, HC, and NOx by its operation under the stoichiometric feed gas condition (A/F ) 14.63; λ ) 1).39 The temperature distribution is also quite similar to that of the single-bed reactor. The maximum axial temperature difference increases from 1 to 25 K as the reactor space velocity increases from 5000 to 20 000 h-1 over the dual-bed system, respectively. Again, the higher is the reactor space velocity, the broader is the temperature difference that has been observed,

particularly in the region of the LOTs of the TWC reactions as listed in Table 4. Model Prediction of TWC Activity and Temperature Distribution of the Single-Bed Reactor. The conversions of CO, C3H6, H2, and NO and the formations of NH3 and N2O over each 4k Pd and Pt/Rh/Ce catalyst monolith sample installed into a single-bed reactor have been well predicted by the 2D nonisothermal monolith reactor model developed, eqs 1-4, as observed in Figures 2 and 3. They particularly show the model predictions of the experimental data observed under the full feed condition (A/F ) 14.76; λ ) 1.009) at the reactor space velocities of 10 000, 30 000, and 50 000 h-1 over the 4k Pd and Pt/Rh/Ce catalyst monolith samples. The solid lines indicate the model prediction, and the symbols are experimental data.


7045

Figure 3. Comparison of predicted and measured data over the 4k Pt/Rh/Ce monolith catalyst as a function of temperature for the reactor space velocities. Feed gas composition: 1% CO, 0.3% H2, 500 ppm C3H6, 1% O2, 500 ppm NO, 10% CO2, 10% H2O, and Ar balance (A/F ) 14.76; λ ) 1.009).

The present 2D reactor model developed with the reliable kinetic parameters estimated over the powder form of the TWCs in a packed-bed flow reactor as listed in Table 1 well describes the general trend of the TWC activity, including the conversions of CO, C3H6, H2, and NO as well as the formations of NH3 and N2O observed over the monolith reactor. The model directly utilizes the kinetic parameters obtained from the detailed reaction kinetics.25 No further adjustment of these parameters included in the monolith reactor model developed has been attempted in the present study. It should be noted that the final conversions and formations of the gas compositions were calculated by using the contour map of the concentrations of the gas compositions based upon the radial distribution of the reactor outlet temperatures in the downstream of the monolith reactor.45 Similarly, the TWC activity, particularly the conversions of CO, C3H6, and NO of the 100k Pd and Pt/Rh/Ce catalyst

monoliths included in the single-bed reactor, was also well predicted by the reactor model developed in the present study as shown in Figure 4, regardless of the reactor space velocities employed. The LOTs of the TWC reaction for CO, C3H6, H2, and NO increase as the catalyst mileage increases from 4k to 100k miles. Similar results have been also observed for the formation of the gas composition including NH3 and N2O. Again, the 2D nonisothermal monolith reactor model developed with the kinetic parameters included in the detailed reaction kinetics well describes the general trend of the gas compositions of the monolith. The measured and simulated temperature distributions over the monoliths containing 4k Pd and Pt/Rh/Ce catalysts by the reactor model developed have been directly contrasted with respect to the reactor space velocity as listed in Table 3. Four thermocouples were installed at the inlet and outlet of the

7046


Figure 4. Comparison of predicted and measured data over the 100k Pd and Pt/Rh/Ce monolith catalysts as a function of temperature for the reactor space velocities. Feed gas composition: 1% CO, 0.3% H2, 500 ppm C3H6, 1% O2, 500 ppm NO, 10% CO2, 10% H2O, and Ar balance (A/F ) 14.76; λ ) 1.009).

monolith reactor to experimentally measure the temperature gradient as a function of the axial and radial positions of the monolith as depicted in Figure 1. The reactor inlet temperature was varied to include the region of LOT of the TWC reaction, mainly 423-723 K in the present study. Note that the thermocouples installed mainly measured the gas temperatures with respect to their locations. As listed in Table 3, the temperature difference of the axial position of the monolith increases as the reactor space velocity increases, mainly due to the exothermic oxidation reactions of CO, C3H6, and H2 and their fast heat transfer at the high reactor space velocity. For the radial temperature distribution, the temperature of the center position of monolith is higher than

that of the edge one, primarily due to the transport of the heat of the exothermic reactions through the channels of the monolith to the reactor wall.20 The temperatures of both edges are identical. It should be noted that the nonuniform flow distribution over a monolith was also recognized as another cause for the radial temperature distribution due to the configuration of the short and expanding conical inlet section of a commercial catalytic converter.20,21 The radial temperature distribution determined in the present study, however, might be essentially due to the exothermal reactions occurring over a monolith, because no expanding section of the reactor configuration exists in the present reactor system as shown in Figure 1.


7047

Table 3. Temperature Distribution Predicted and Measured over the 4k Pd and Pt/Rh/Ce Catalyst, Respectively 4k Pd catalyst (single-bed) experimental temp (K) outlet (gas) inlet (gas)

center

edge

4k Pt/Rh/Ce catalyst (single-bed)

simulation temp (K) outlet (gas) center

edge

experimental temp (K)

outlet (solid) center

edge

outlet (gas) inlet (gas)

SV: 10 000 h 455 480 511 604 645

457 484 512 604 645

456 482 511 603 644

456 483 512 604 645

456 482 511 604 645

456 484 512 604 646

456 483 511 604 645

simulation temp (K) outlet (gas)

outlet (solid)

center

edge

center

edge

center

edge

474 507 535 586 636

474 505 534 586 636

474 507 535 586 636

474 506 535 586 636

474 507 535 587 637

474 506 535 586 636

492 546 587 607 657

491 537 571 599 649

492 545 585 605 655

492 540 575 600 650

492 546 589 609 661

492 541 578 602 656

474 530 566 626 641

473 527 560 600 622

473 530 564 623 637

473 528 562 603 626

473 531 565 630 642

473 529 563 618 635

-1

474 504 535 586 636

SV: 30 000 h-1 461 482 494 547 621

462 503 532 575 643

461 501 526 570 639

462 503 533 574 642

462 501 527 570 639

462 505 537 575 645

462 503 530 571 642

492 525 530 585 640

SV: 50 000 h-1 478 495 505 531 603

479 496 528 585 644

476 494 522 578 638

478 498 528 586 642

478 497 523 580 638

478 498 530 589 645

478 497 524 583 642

The model developed in the present study well predicts the temperature distribution over the 4k Pd and Pt/Rh/Ce monolith reactors, regardless of their axial and radial positions of monolith and the reactor space velocity as listed in Table 3. Similar simulation results for the temperature distribution over the 100k counterpart monolith reactor have been also observed by the model. Model Prediction of TWC Activity and Temperature Distribution of the Dual-Bed Reactor. Figure 5 reveals the model predictions over the dual-bed reactor containing 4k Pd in the front bed and Pt/Rh/Ce in a rear bed under the full feed condition at the reactor space velocities of 5000, 10 000, 15 000, and 20 000 h-1. Note that a modern commercial catalytic converter is commonly fabricated in the mode of a dual-bed reactor system containing the oxidation catalyst (Pd) in the front bed and the conventional TWC catalyst (Pt/Rh/Ce) in the rear bed.25,39,42 Similar to the single-bed reactor, the reactor model developed in the present study directly using the identical kinetic parameters employed for the single-bed reactor well predicts the general trend of the conversions of CO, C3H6, H2, and NO as well as the formations of NH3 and N2O. The TWC performance of the dual-bed monolith reactor has been simulated by sequentially integrating each individual reactor model developed for Pd and Pt/Rh/Ce monoliths. The temperature distribution in both axial and radial positions of monoliths included in the dual-bed reactor is also well described by the model developed as listed in Table 4, regardless of the monolith radius and length, and the reactor space velocity. Again, the final conversions and formations of the gas compositions in the downstream of the rear bed monolith were determined by using the contour map of the gas compositions based upon the radial distribution of the reactor outlet temperatures determined by the model developed.45 Figure 6 typically shows the temperature distribution in both axial and radial positions over the dual-bed monoliths including both 4k Pd and Pt/Rh/Ce catalysts simulated by the model developed when the upstream temperature of the monolith is 563 K at 20 000 h-1. The axial temperature distribution of the dual-bed reactor sharply increased from 563 to 577 K, nearly 60% of the maximum temperature at the 1/3 position of the monolith reactor length, 1.27 cm. It reveals that the temperature

471 525 544 569 598

rise of the monolith mainly occurs in the front bed including the monolith with Pd catalyst. Note that the length of the front bed is 1.27 cm (0.5 in.) as mentioned. The model again well describes the axial and radial temperature distribution in the downstream of the reactor within 2 K of the maximum temperature difference as shown in Figure 6 and Table 4. Indeed, the 1D model including both mass and heat balances based upon both internal and external diffusion limitations with the reaction kinetics has been initially developed to predict the TWC activity trend of the monolith reactor as a function of the axial temperature of the monolith reactor. The model could hardly describe the TWC performance of the monolith.46 Note that the radial temperature distribution of the monolith reactor cannot be predicted by the 1D model. However, the 2D multi channel reactor model developed in the present study well predicted the TWC performance including the gas compositions and the temperature distribution over the single- and dual-bed monolith reactors simulating the commercial catalytic converter as discussed. Indeed, it has been commonly recognized that the simulation results by 2D or 3D multi channel models are more detailed, accurate, and reasonable than those by the 1D single channel model.3,47,48 Model Prediction of TWC Activity of the Dual-Bed Reactor by the Steady-State Sweep Test (st-ST). To further validate the 2D nonisothermal monolith reactor model developed in the present study, the variation of TWC activity by the stST was examined over the dual-bed reactor containing 4k Pd in the front bed and Pt/Rh/Ce in the rear bed at the reactor space velocity of 10 000 h-1 with respect to the A/F ratios from 14.23 to 15.03, the typical range of lean-rich operation of gasoline engine as shown in Figure 7.26,28 Note that the st-ST was conducted under the steady-state condition as discussed.27,49 The conversions of CO, C3H6, and NO were specifically determined with respect to the feed gas condition by changing O2 feed concentration from 0.4% to 1.3% at the three reactor inlet temperatures of interest for the exhaust gas stream from gasoline engine, 523, 603, and 673 K. In particular, the reactor inlet temperature of 673 K may be a typical warm-up temperature of the gasoline engine.39 The reactor model developed also well predicts and describes the general trend of the conversions of CO, C3H6, and NO within

7048


Figure 5. The prediction of TWC performance over the dual-bed monolith catalyst (4k Pd-Pt/Rh/Ce) as a function of temperature for the reactor space velocities. Feed gas composition: 1% CO, 0.3% H2, 500 ppm C3H6, 1% O2, 500 ppm NO, 10% CO2, 10% H2O, and Ar balance (A/F ) 14.76; λ ) 1.009).

Table 4. Temperature Distribution Predicted and Measured over the Dual-Bed Catalysts (4k Pd-Pt/Rh/Ce) 4k Pd-Pt/Rh/Ce catalyst (dual-bed) experimental temp (K)

simulation temp (K)


edge1

center

outlet (gas) edge2

center

edge

experimental temp (K)

outlet (solid) center

edge


edge1

center

-1

417 436 486 541 593

417 437 486 541 594

417 437 487 542 594

471 495 512 635 677

471 504 528 641 682

471 505 528 643 683

SV: 5000 h 417 417 437 437 486 486 541 541 593 594 -1 SV: 15 000 h 471 471 505 504 529 527 642 642 683 684

simulation temp (K) outlet (gas) edge2

center

outlet (solid)

edge

center

edge

458 468 493 519 655

458 469 496 522 658

458 469 495 520 657

484 518 538 584 660

484 520 546 588 664

484 519 541 586 662

-1

417 437 486 541 594

417 437 486 542 594

417 437 486 541 594

456 463 487 516 651

458 468 493 519 655

458 468 495 521 657

471 503 526 641 683

471 504 529 644 686

471 503 528 643 684

484 510 523 563 643

484 519 536 583 658

485 520 543 588 661

SV: 10 000 h 458 458 468 469 493 494 519 520 655 656 -1 SV: 20 000 h 485 484 519 519 536 543 584 586 659 662


7049

Figure 6. Temperature distribution of the predicted and measured data over the dual-bed monolith catalyst (4k Pd-Pt/Rh/Ce). Reactor SV: 20 000 h-1, at 563 K (upstream temperature) over the 4k Pd-Pt/Rh/Ce catalyst monolith.

Figure 7. Comparison of predicted and measured data by steady-state sweep test (st-ST) under the A/F ratio from 14.23 (λ ) 0.973) to 15.03 (λ ) 1.028) over the dual-bed monolith catalyst (4k Pd-Pt/Rh/Ce): (a) at 523 K, (b) at 603 K, and (c) at 673 K.

the wide range of lean-rich operating condition, regardless of the reactor inlet temperatures employed. This result reveals again that the 2D nonisothermal monolith reactor model specifically

developed in the present study is capable of predicting and/or describing the commercial TWC performance, regardless of the lean-rich feed gas condition as well as the reaction temperature.

7050


However, the steady-state model developed in the present study may not be directly utilized to predict the transient performance of the catalytic converter. For describing the performance of the transient operation of the commercial converter, the unsteady-state reactor model should be developed as a function of time based upon the transient experimental results. Of course, the identical reaction kinetics and modeling approach may be employed even for developing the unsteady-state reactor model. However, the model developed in the present study will definitely provide useful information for the practical design of the commercial converter with less effort and fewer resources. Conclusions A two-dimensional (2D) nonisothermal monolith reactor model has been developed to describe the TWC performance of a monolith reactor by the close simulation of the commercial operation of an automotive catalytic converter. The monolith reactor model developed on the basis of detailed reaction kinetics and both heat and mass transfers well predicts the general trends of the conversion and formation of the gas compositions and the temperature distribution in the axial and radial positions of the monolith reactor in the single-bed including each Pd and Pt/Rh/Ce catalyst, as well as in the dual-bed including both Pd and Pt/Rh/Ce. The model can particularly describe the typical TWC performance even under simulated steady-state lean-rich feed gas conditions (A/F ) 14.23-15.03). The model directly utilizes the kinetic parameter determined by the detailed kinetic study, and no parameter adjustment has been made for the simulation of the 2D monolith reactor model developed in the present study. The superb prediction of the TWC performance by the monolith reactor model developed in the present study may also directly reflect the quality of the detailed reaction kinetics employed for simulating the TWC activity over a modern commercial catalytic converter installed on a gasoline engine. The present approach will save much effort and time for predicting the performance of the commercial monolith reactor and for designing the catalytic converter. Consequently, the 2D nonisothermal model developed in the present study can predict the reactor performance of the TWC monolith reactor employed for a gasoline engine and may provide a practical guideline for the design of a modern commercial automotive catalytic converter. Acknowledgment Financial support of this work was provided by General Motors (BK21 program) and the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (No. 20090092793). Nomenclature Ae ) geometric surface area, cm2/cm3 Cig ) concentration of i species, mol/cm3 Cis ) concentration of i species in the solid phase, mol/cm3 C0,i ) initial concentration of i species, mol/cm3 Cp ) specific heat of gas, J/(g · K) d ) reactor diameter, cm dh ) hydraulic diameter, cm Dg,i ) diffusivity of i species, cm2/s Ea ) activation energy, kJ/mol h ) heat transfer coefficient, J/(cm2 · s · K) hsalt ) heat transfer coefficient from tube to molten salt, J/(cm2 · s · K) ∆Hi ) heat of adsorption of i species, kJ/mol (-∆H)i ) heat of reaction of i species, kJ/mol

km,i ) mass transfer coefficient of i species, cm/s ker ) thermal conductivity of radial direction, J/(cm · s · K) kez ) thermal conductivity of axial direction, J/(cm · s · K) kg ) thermal conductivity of gas, J/(cm · s · K) ktube ) thermal conductivity of tube, J/(cm · s · K) kmat ) thermal conductivity of mat (insulator), J/(cm · s · K) ki ) rate constant for a reaction i, cm3/(mol · s) k0,i ) frequency factor of rate constant for a reaction i, cm3/(mol · s) Ki ) adsorption equilibrium constants, cm3/mol K0,i ) pre-exponential factor of adsorption equilibrium constant, cm3/mol L ) reactor length, cm Ri ) reaction rate for i species based on catalytic volume, mol/ (cm3 · s) R ) reactor radius, cm S ) active surface reaction site ttube ) thickness of tube wall, cm tmat ) thickness of mat, cm Tg ) gas temperature, K Ts ) solid temperature, K T0 ) reactor inlet temperature of feed gas, K Ta ) temperature of molten salt, K u ) linear superficial velocity of gas, cm/s U ) overall heat transfer coefficient between outer shell and molten salt, J/(cm2 · s · K) Greek Letters ε ) void fraction (open frontal area) F ) gas density, g/cm3 Dimensionless Groups Nu ) Nusselt number, hdh/kg PeH ) Peclet number in the heat transfer, FCpudh/kg PeM ) Peclet number in the mass transfer, udh/Dg Sh ) Sherwood number, kmdh/Dg

Literature Cited (1) Heck, R. M.; Gulati, S.; Farrauto, R. J. The application of monoliths for gas phase catalytic reactions. Chem. Eng. J. 2001, 82, 149–156. (2) Santos, H.; Costa, M. Modelling transport phenomena and chemical reactions in automotive three-way catalytic converters. Chem. Eng. J. 2009, 148, 173–183. (3) Chen, J.; Yang, H.; Wang, N.; Ring, Z.; Dabros, T. Mathematical modeling of monolith catalysts and reactors for gas phase reactions. Appl. Catal., A 2008, 345, 1–11. (4) Hoebink, J. H. B. J.; Harmsen, J. M. A.; Scholz, C. M. L.; Marin, G. B.; Schouten, J. C. Modeling of Automotive Exhaust Gas Converters. In Structured Catalysts and Reactors, 2nd ed.; Cybulski, A., Moulijin, J. A., Eds.; Taylor & Francis: Florida, 2006; Chapter 9, pp 311-354. (5) Ahmadinejad, M.; Desai, M. R.; Watling, T. C.; York, A. P. E. Simulation of Automotive Emission Control Systems. In AutomotiVe Emission Control, AdV. Chem. Eng.; Marin, G. B., Ed.; Elsevier/Academic Press: Amsterdam, 2007; Vol. 33, pp 47-101. (6) Kolsakis, G. C.; Konstantinidis, P. A.; Stamatelos, A. M. Development and application range of mathematical models for 3-way catalytic converters. Appl. Catal., B 1997, 12, 161–191. (7) Depcik, C.; Assanis, D. One-dimensional automotive catalyst modeling. Prog. Eng. Comb. Sci. 2005, 31, 308–369. (8) Kuo, J. C. W.; Morgan, C. R.; Lassen, H. G. Mathematical modeling of CO and HC catalytic converter systems. SAE paper no. 710289. (9) Votruba, J.; Sinkule, J.; Hlavacek, V.; Skrivanek, J. Heat and mass transfer in monolithic honeycomb catalysts-I. Chem. Eng. Sci. 1975, 30, 117–123. (10) Heck, R. H.; Wei, J.; Katzer, J. R. Mathematical modeling of monolithic catalysts. AIChE J. 1976, 22, 477–474. (11) Oh, S. H.; Cavendish, J. C. Transients of monolithic catalytic converters: Response to step changes in feedstream temperatures as related to controlling automobile emissions. Ind. Eng. Chem. Prod. Res. DeV. 1982, 21, 29–37. (12) Pattas, K. N.; Stamatelos, A. M.; Pistikopoulos, P. K.; Koltsakis, G. C.; Konstantinidis, P. A. Transient modeling of 3way catalytic converters. SAE paper no. 940934.

Ind. Eng. Chem. Res., Vol. 49, No. 15, 2010 (13) Siemund, S.; Leclerc, J. P.; Schweich, D.; Progent, M.; Castagna, F. Three-way monolithic converter: Simulations versus experiments. Chem. Eng. Sci. 1996, 51, 3709–3720. (14) Holder, R.; Bollig, M.; Anderson, D. R.; Hochmuth, J. K. A discussion on transport phenomena and three-way kinetics of monolithic converters. Chem. Eng. Sci. 2006, 61, 8010–8027. (15) Young, L. C.; Finlayson, B. A. Mathematical models of the monolithic catalytic converter. Part 1. Development of model and application of orthogonal collocation. AIChE J. 1976, 22, 331–343. (16) Lee, S. T.; Aris, R. On the effect of radiative heat transfer in monoliths. Chem. Eng. Sci. 1977, 32, 827–837. (17) Otto, N. O.; LeGray, W. J. Mathematical models for catalytic converter performance. SAE paper no. 800841. (18) Hayes, R. E.; Kolaczkowski, S. T.; Thomas, W. J. Finite-element model for a catalytic monolith reactor. Comput. Chem. Eng. 1992, 16, 645– 657. (19) Flytzani-Stephanopoulos, M.; Voecks, G. E.; Charng, T. Modelling of heat transfer in non-adiabatic monolith reactors and experimental comparisons of metal monoliths with packed beds. Chem. Eng. Sci. 1986, 41, 1203–1212. (20) Zygourakis, K. Transient operation of monolith catalytic converters: A two-dimensional reactor model and the effects of radially nonuniform flow distributions. Chem. Eng. Sci. 1989, 44, 2075–2086. (21) Chen, D. K. S.; Bissett, E. J.; Oh, S. H.; van Ostrom, D. L. A three-dimensional model for the analysis of transient thermal and conversion characteristics of monolith catalytic converters. SAE paper no. 880282. (22) Chakravarthy, V. K.; Conklin, J. C.; Daw, C. S.; D’Azevedo, E. F. Multi-dimensional simulations of cold-start transients in a catalytic converter under steady inflow conditions. Appl. Catal., A 2003, 241, 289–306. (23) Voltz, S. E.; Morgan, C. R.; Liederman, D.; Jacob, S. M. Kinetic study of carbon monoxide and propylene oxidation on platinum catalysts. Ind. Eng. Chem. Prod. Res. DeV. 1973, 12, 294–301. (24) Subramanian, B.; Varma, A. Reaction kinetics on a commercial three-way catalyst; the CO-NO-O2-H2O system. Ind. Eng. Chem. Prod. Res. DeV. 1985, 24, 512–516. (25) Kwon, H. J.; Baik, J. H.; Kwon, Y. T.; Nam, I.-S.; Oh, S. H. Detailed reaction kinetics over commercial three-way catalysts. Chem. Eng. Sci. 2007, 62, 5042–5047. (26) Koltsakis, G. C.; Stamatelos, A. M. Catalytic automotive exhaust aftertreatment. Prog. Energy Combust. Sci. 1997, 23, 1–39. (27) Heo, I.; Choung, J. W.; Kim, P. S.; Nam, I.-S.; Song, Y. I.; In, C. B.; Yeo, G. K. The alteration of the performance of field-aged Pd-based TWCs towards CO and C3H6 oxidation. Appl. Catal., B 2009, 92, 114– 125. (28) Gonza´lez-Velasco, J. R.; Botas, J. A.; Gonza´lez-Marcos, J. A.; Gutie´rez-Ortiz, M. A. Influence of water and hydrogen processed in feedstream on the three-way behaviour of platium-alumina catalysts. Appl. Catal., B 1997, 12, 61–79. (29) James, A.; Brindley, J.; McIntosh, A. C. Multi-channel monolith reactors as dynamical systems. Combust. Flame 2003, 134, 193–205. (30) Koci, P.; Kubicek, M.; Marek, M. Modeling of three-way catalyst monolith converters with microkinetics and diffusion in the washcoat. Ind. Eng. Chem. Res. 2004, 43, 4503–4510. (31) Hayes, R. E.; Kolaczkowski, S. T. Mass and heat transfer effects in catalytic monolith reactors. Chem. Eng. Sci. 1994, 49, 3587–3599.

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(32) Boger, T.; Menegola, M. Monolith catalysts with high thermal conductivity for improved operation and economics in the production of phthalic anhydride. Ind. Eng. Chem. Res. 2005, 44, 30–40. (33) Tronconi, E.; Groppi, G.; Boger, T.; Heibel, A. Monolithic catalysts with high conductivity honeycomb supports for gas/solid exothermal reactions: characterization of the heat-transfer properties. Chem. Eng. Sci. 2004, 59, 4941–4949. (34) Kolaczkowski, S. T.; Worth, D. J. Modelling channel interactions in a non-adiabatic multichannel catalytic combustion reactor. Catal. Today 1995, 26, 275–282. (35) Hawthorn, R. D. Afterburner catalysts effects of heat and mass transfer between gas and catalyst surface: Recent advances in air pollution control. AIChE Symp. Ser. 1974, 70, 428–438. (36) Reid, R. C.; Prausnitz, J. M.; Sherwood, T. K. The Properties of Gases and Liquids, 3rd ed.; McGraw-Hill: New York, 1977. (37) Manuel, I.; Thomas, C.; Colas, H.; Matthess, N.; Djega-Mariadassou, D. A new approach in the kinetic modelling of three-way catalytic reactions. Top. Catal. 2004, 30/31, 311–317. (38) Baba, N.; Yokota, K.; Matsunaga, S.; Kojima, S.; Ohsawa, K.; Ito, T.; Domyo, H. Numerical Simulation of Deactivation Process of Threeway Catalytic Converters. SAE paper no. 2000-01-0214. (39) Heck, R. M.; Farrauto, R. J.; Gulati, S. T. Catalytic Air Pollution Control: Commercial Technology, 2nd ed.; John Wiley & Sons, Inc.: New York, 2002. (40) Perry, R. H.; Green, D. W. Perry’s Chemical Engineers’ Handbook, 7th ed.; McGraw-Hill: Singapore, 1998. (41) Chemical Engineering Module Model Library, COMSOL AB, Version 3.2, 2005. (42) Kwon, H. J.; Baik, J. H.; Kwon, Y. T.; Nam, I.-S.; Oh, S. H. Enhancement effect of water on oxidation reactions over commercial threeway catalyst. Chem. Eng. J. 2008, 141, 194–203. (43) Taylor, K. C. Nitric oxide catalysis in automotive exhaust systems. Catal. ReV.-Sci. Eng. 1993, 35, 457–481. (44) Tischer, S.; Jiang, Y.; Hughes, K. W.; Patil, M. D.; Murtagh, M. Three-Way-Catalyst Modeling-A Comparison of 1D and 2D Simulations. SAE paper no. 2007-01-1071. (45) Chae, H. J.; Choo, S. T.; Choi, H.; Nam, I.-S.; Yang, H. S.; Song, S. L. Direct use of kinetic parameters for modeling and simulation of a selective catalytic reduction process. Ind. Eng. Chem. Res. 2000, 39, 1159– 1170. (46) Unpublished result, POSTECH; Final report for GM R&D, Sep. 14, 2006. (47) Jahn, R.; Sˇnita, D.; Kubı´cˇek, M.; Marek, M. 3-D modeling of monolith reactors. Catal. Today 1997, 38, 39–46. (48) Jeong, S.-J.; Kim, W.-S.; Kim, T. An application of CFD to improve warm-up performance of the 3-way auto-catalyst by high surface area and low thermal mass. Int. J. Vehicle Des. 2002, 29, 243–268. (49) Hu, Z.; Wan, C. Z.; Lui, Y. K.; Dettling, J.; Steger, J. J. Design of a novel Pd three-way catalyst: Integration of catalytic functions in three dimensions. Catal. Today 1996, 30, 83–89.

ReceiVed for reView March 29, 2010 ReVised manuscript receiVed June 4, 2010 Accepted June 15, 2010 IE1007486

Simulation of a Nonisothermal Modern Three-Way Catalyst Converter

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