Selective Catalytic Reduction for NO Removal: Comparison of

ARTICLE pubs.acs.org/IECR

Selective Catalytic Reduction for NO Removal: Comparison of Transfer and Reaction Performances among Monolith Catalysts Zhigang Lei,* Cuiping Wen, Jie Zhang, and Biaohua Chen State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Box 266, Beijing, 100029, China

bS Supporting Information ABSTRACT: One of the available technologies that can effectively control the emission of NOx is the selective catalytic reduction (SCR) of NOx with ammonia (NH3). Monolith catalysts are widely used in this technology due to their unique advantages that they offer, like low pressure drop, high external geometric surface area, and resistance to deposition of carbon, dust, and metals from combustion process. But the configuration of monolith catalysts has prominent influence on transfer and reaction performances of SCR for NO removal. This work tries to provide an easy-to-read and comprehensive comparison of momentum transfer, heat transfer, mass transfer, and reaction performance between two types of monolith catalysts with five kinds of channel shapes for SCR for NO removal, and to address the issues specific to SCR applications as to (i) whether or not monolith catalysts can improve the transfer and reaction performances compared to traditional pellet packed-bed reactors; (ii) which type of monolith catalysts and (iii) which kind of channel shapes for each type are optimum from the viewpoint of chemical reaction engineering. It was found that monolith catalysts have a much lower pressure drop and higher effectiveness factor than traditional pellet packed-bed reactors, and a coating catalyst seems more suitable than an extruded catalyst for SCR for NO removal. Although a round channel brings the best heat and mass transfer, a triangle-shaped channel of coating catalyst possesses the highest NOx conversion due to the chemical reaction being the controlling step.

1. INTRODUCTION As the regulation for the nitrogen oxides (NOx) emission becomes strict, much effort has been focused on the development of more efficient NOx removal technology. Selective catalytic reduction (SCR) of NOx with ammonia (NH3) is the most effective and commercially proven technology to remove NOx from stationary sources.1
5 For this purpose, structured catalysts and reactors are often employed, which are divided into monoliths, open cross-flow structures, foams, catalytic membranes, and many others and are the key technology of the SCR process.6
8 Recent investigators9
13 reported that monolith catalysts are becoming increasingly significant as catalyzed gas
solid or multiphase reactions in view of the advantages that they offer in comparison to conventionally used packed-bed reactors, including low pressure drop, much higher surface area per unit reactor volume, and minimum axial dispersion stemming from the uniquely structured multichannel configuration of monoliths. In general, a monolith block is composed of an array of uniformly structured parallel channels, typically having hydraulic diameters between 1 and 5 mm, and support materials such as ceramic,14
17 metal,18
21 or cordierite22
26 are often selected. In fact, the transfer and reaction performances of monolith catalysts considerably depend on the material and geometry characteristics of their support. There are two types of monolith catalysts: one is the coating catalyst in which the catalytically active species are often dispersed within a washcoat (porous medium) that is coated onto the substrate surface, and the other is an extruded catalyst in which the catalytically active species are contained within the whole porous medium. For each type, there are also different channel shapes like round, regular triangle, rectangle, square, and hexagon. However, by far no systematic and critical research on gas
solid catalytic reactions using monolith catalysts has been r 2011 American Chemical Society

done to identify the mechanism of process intensification and thus to guide the selection of suitable monolith catalysts from the viewpoint of chemical reaction engineering. The SCR for DeNOx is selected as the reaction system because it is commonly encountered in industry using monolithic catalysts. Therefore, the focus of this work is on addressing the cogently interesting issues as to (i) whether or not monolith catalysts can improve the transfer and reaction performances compared to traditional pellet packed bed; (ii) which type of monolith catalysts and (iii) which kind of channel shape for each type are optimum. In addition, it is beyond our scope to discuss how to synthesize and characterize the monolith catalysts which may also influence the transfer and reaction performances by means of changing reaction kinetics. One significance of this work is helping guide the selection of a suitable configuration of monolith catalysts for industrial application. Another significance is for catalytic chemists to explore new catalytically active species supported over the well-established optimum monolith configuration in order to bring about good transfer and reaction performances.

2. MODEL DESCRIPTION 2.1. Governing Equations. The main reaction27 is considered and written as follows:

4NO þ 4NH3 þ O2 f 4N2 þ 6H2 O

ð1Þ

Received: October 31, 2010 Accepted: April 2, 2011 Revised: March 22, 2011 Published: April 02, 2011 5942

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Table 1. Parameters and Source Terms for the Governing Equations Given in Equation 2 φ

equations

Γ

Sφ

Bulk Gas Phase continuity

1

0

0

momentum balance energy balance

u CpgT

μ λg


rP 0

mass balance

wi

Di,N2

0

Substrate Phase (Cordierite) continuity

1

0

0

momentum balance

u (= 0)

μ


rP (=0)

energy balance

CpsT

λs

0

mass balance

wi (=0)

Di,N2

0

Porous Medium Phase (γ-Al2O3) continuity momentum balance

1 u

0 μ

0
rP
(μ/a)u

energy balance

CpgT

λs(1-ε) þλgε

(
rNO)(
ΔH)

mass balance

wi

Deff,i


(
rNO)

A uniform gas distribution can be ensured before entering the SCR reactor by installing such internals as ammonia injection grid (AIG), static mixer, and so on in the flow design. Monolith catalysts can be viewed as consisting of a number of repeated building blocks where the basic building block is a single channel with symmetrical peripheral walls. Therefore, in this work, a 3D mathematical model on a single channel is employed in scaling up a monolith reactor to replace the one- and two-dimensional steady-state isotherm description as proposed by Tronconi.28 The governing equations for describing mass, momentum, and energy balances in monolith catalysts for the bulk gas phase, substrate phase, and porous medium phase at steady state have the following conservative form: r 3 ðFuφÞ ¼ r 3 ðΓrφÞ þ Sφ

ð2Þ

where φ represents generic transport properties, Γ is the corresponding transport coefficient, and Sφ is source terms (see Table 1 for the parameters and source terms of eq 2). For extruded catalysts (Figure 1a) or coating catalysts with the washcoat thickness above 0.1 mm (Figure 1b), the internal diffusion effect should be considered in the catalytic domain. Therefore, the porous media model was used by the addition of a momentum source term (
(μ/a)u) to the standard fluid flow equations. For coating catalysts with the washcoat thickness less than 0.1 mm (Figure 1c), the catalyst layer and inner wall of monolith channels can be taken on as overlapped because the effectiveness factor of internal diffusion is close to unity (see section 3.5, Effectiveness Factor).29 Therefore, in this case the surface reaction as boundary condition was selected, and the net deposition rate of a species on the surface is not considered at steady-sate. Fwall Di, N2

D2 wi, wall ¼ Mw, i ri, gas Dn2

Figure 1. Schematic representation of monolith catalysts with square channel in the cross-section direction: (a) extruded catalysts in which the porous media model was used in the catalytic domain; (b) coating catalysts with washcoat thickness above 0.1 mm in which the porous media model was used in the catalytic domain; (c) coating catalysts with washcoat thickness less than 0.1 mm in which the surface reaction as boundary condition was used.

out between coating (where effectiveness factor was assumed to be unity) and extruded catalysts. 2.2. Model Parameters. In this work, the honeycomb-like cordierite was selected as substrate material due to its high mechanical strength, high thermal stability, matching of thermal expansion with catalytically active species, and low cost. Besides, γ-Al2O3 was used as porous medium because it can improve the porosity to permit the active metal (e.g., CuO) dispersion. The physical properties of cordierite and γ-Al2O3 that are required for determining the parameters and source terms in Table 1 are listed in Table 2, which come from the literatures30
32 and some relations.33,34 In the simulation it was assumed that they are constant and independent of temperature and pressure. All physical properties of pure gas components were taken from the process simulation software PROII at operating temperatures and pressures. The physical properties of gas mixture were derived by using the idea gas mixing law, except that the binary diffusion coefficients Di,N2 in the bulk gas phase can be obtained as follows: Di, N2 ¼

1=3

PðVi

1=3

þ V N2 Þ 2

ðm2 s
1 Þ

ð4Þ

where i = NO, NH3, O2, and H2O, since a large amount of the flue gas is N2. But in the porous medium phase, the effective diffusion coefficients Deff,i are calculated by ε Def f , i ¼ Di, N2 τ

ð3Þ

where wi,wall is the mass fraction of gas on the wall, and ri,gas is the net molar reaction rate for species i based on unit volume. It should be noted that for SCR for NO removal, the washcoat thickness of coating catalysts is normally very small. So the comparison of transfer and reaction performances was carried

4:36  10
5 T 1:5 ð1=Mi þ 1=MN2 Þ0:5

ð5Þ

The rate equation for the main reaction needs to be determined beforehand. In line with mechanistic and kinetic evidence that either weakly adsorbed or gaseous NO reacts with NH3 strongly adsorbed on the catalyst surface, we adopted a Rideal-type rate equation with the same form as 5943

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Table 2. Physical Properties of Substrate Phase (Cordierite) and Porous Medium Phase (γ-Al2O3) porous medium physical

substrate phase

phase

properties

(cordierite)

(γ-Al2O3)

density (kg 3 m
3)

thermal capacity (J 3 kg
1 3 K
1)

thermal conductivity (W 3 m
1 3 k
1)

2730

3300

1200

858

3.2

8.4

porosity, ε

0.2

tortuosity, τ

4.0

permeability, a (m2)

a = L2c ε3/ (150(1
ε)2)

reported by Tronconi et al.35 rNO ¼ kNO CNO

kNO ¼ 2:94  109 exp

KNH3 CNH3 1 þ KNH3 CNH3



105790 RT

KNH3 ¼ 9:24 exp


 87900 RT

ð6Þ

ðcm3 g
1 s
1 Þ ðcm3 mol
1 Þ

ð7Þ

ð8Þ

where k is reaction rate constant, K is adsorption equilibrium constant, CNO and CNH3 are the molar concentrations of NO and NH3, respectively. The reaction order of NO is 1. If full coverage of the surface with NH3 can be guaranteed under all conditions, the reaction order of NH3 is zero. The reaction rate is of the firstorder form: 0

rNO ¼ kNO CNO 0

kNO





94010 ¼ 1:3221  10 exp RT 9

ð9Þ 

ðs
1 Þ

ð10Þ

where k0 is the specific reaction rate constant, which comes from our kinetic measurement over sulfated CuO/γ-Al2O3 catalyst. The kinetic experimental results are within the range of pre-exponential factors between 3.10  104 and 54  1017 s
1 3 (mol 3 cm
3)
c, and activation energy between 24 and 115 kJ 3 mol
1, as reported by Marangozis36 after investigating 27 different catalysts. However, the value of source term ΔH in Table 1 does not need to be input because the computed gas enthalpies include the heats of formation based on the same reference temperature (298.15 K). 2.3. Boundary Conditions and Numerical Method. The appropriate boundary conditions were specified at all external boundaries based on the following assumptions: (1) uniform gas velocity, temperature and concentration at the entrance (2) normal pressure at the outlet (3) symmetrical boundary on the peripheral wall of the channel, and no slip condition on the inner wall of the channel (4) axially adiabatic solid boundary at the entrance and outlet (5) homogeneous reaction and heat radiation in the bulk phase were ignored (6) the porous medium phase, if possible, was homogeneous, isotropic and saturated with a single phase fluid

(7) in the porous medium phase, only viscous flow (Darcy flow) was allowed The Gambit software (version 2.3.16) was used to mesh the monolith channel with hexagonal elements by means of the Cooper method. Under a typical operating condition, it was found that as the grid number increases, NO conversion first increases and then tends to be stable after N > Nmin = 0.9 million. Therefore, in our later calculation the total grid number is higher than Nmin. Then, the mesh file was input into the FLUENT software (version 6.3.26) in which pressure based solver, implicit formulation, and laminar model were selected and the SIMPLE (semi-implicit method for pressure-linked equations) method was used to solve the governing equations. The first-order upwind spatial discretization scheme was used for all differential equations, and all residuals were 10
5. The mathematic model was applied to obtain the temperature and concentration distribution and thus to understand the transfer and reaction performances inside the monolith channel. 2.4. Experimental Validation. The experimental validation on the mathematical model of monolith catalysts established in this work that could be used for evaluation of transfer and reaction performances of monolith catalysts, as well as the experimental details on preparation, characterization, and SCR of coating catalysts, is provided in the Supporting Information, to which the interested readers can refer.

3. RESULTS AND DISCUSSION Figure S6 in the Supporting Information shows five kinds of channel shapes of monolith catalysts, that is, round, regular triangle, rectangle, square, and hexagon, and the corresponding structural and operating parameters are listed in Table 3, where their values are within the general range of current SCR reactors. In this work, all comparison of transfer and reaction performances among them was carried out with the same repeated unit area, wall thickness, and height, as well as with the inlet gas flux as abscissa in the following figures which can reflect the production capacity of industrial SCR reactors. 3.1. Pressure Drop. Pressure drop represents the energy dissipated caused by fluid flow through the reactor, which represents the momentum transfer and is important in determining the energy losses.37
40 Figure 2 shows the change of pressure drop between the inlet and outlet of monolith channels, as well as the traditional pellet packed-bed reactor with a spherical catalyst particle of 2.54 mm under the same operating condition and reactor volume. Evidently, pressure drop in the packed-bed reactor is much greater than in the monolith catalysts by 3 orders of magnitude. This is consistent with the conclusion from previous studies, and pressure drop of monolith catalysts obtained in this work has the same order of magnitude as in the literatures.7,41 It can be seen that at a given inlet gas flux, the order of pressure drop for five shapes of monolith catalysts (coating and extruded catalysts) is round > regular triangle > rectangle > square > hexagon, which, however, corresponds reversely to the order of flux areas of monolith channels. That is to say, the smaller the flux area of monolith channels, the greater pressure drop. Moreover, for the same monolith channel, pressure drop in coating monolith catalysts is lower than in extruded monolith catalysts by comparison of Figure 2a with Figure 2b. 3.2. Radial Effective Heat Conductivity. The radial effective heat conductivity λer is an important physical quantity to evaluate 5944

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Table 3. Structural and Operating Parameters for Five Kinds of Channel Shapes of Monolith Catalysts objects

round

regular triangle

square

rectangle

hexagon

height of bed (m)

0.8

0.8

0.8

0.8

0.8

repeated unit area (mm2)

49

49

49

49

49

wall thickness (mm)

1.0

1.0

1.0

1.0

1.0

equivalent diameter of the

6

5.14

6

5.81

6.52

6

8.91

6

7.57  4.72

3.77

flux area of channel (mm2) operating parameters

28.27

34.34

36

35.71

36.84

fluid medium

N2

N2

N2

N2

N2

channel (mm) side length of empty channel (mm)

inlet gas composition (vol %)

H2O 10%, O2 2%, NH3 0.045%, NO 0.05%, N2 87.905%

flux range (mL s
1)

113
737

113
737

113
737

113
737

inlet gas temperature (K)

650

650

650

650

650

gas velocity (m s
1)

4.00
26.07

3.29
21.46

3.14
20.47

3.16
20.64

3.07
20.01

Reynolds number

3.92  10þ2


2.76  10þ2

3.08  10þ2

3.00  10þ2

3.27  10þ2


1.80  10þ3


2.01  10þ3


1.96  10þ3


2.13  10þ3

2.56  10þ3

113
737

Re and Prandtl number Pr. The equations are given as below: Fde u ð11Þ Re ¼ μ Pr ¼

cp μ λ

λer ¼ a þ bðRe 3 PrÞ λg

Figure 2. Pressure drop for five kinds of monolith channel shapes and pellet packed-bed reactor for (a) coating catalysts and (b) extruded catalysts: (9) pellet packed-bed reactor with dp = 2.54 mm, εB = 0.33 and j = 1; (O) round; (Δ) regular triangle; (0) square; ()) rectangle; (/) hexagon.

the heat transfer performance of reactors along the radial direction and is determined by fitting the temperature profiles obtained from the mathematic model to the pseudohomogeneous model in a fixed-bed reactor.42 The ratio of radial effective heat conductivity λer to the heat conductivity of gas phase λg would be expressed as a function of the products between Reynolds number

ð12Þ ð13Þ

where de is equivalent diameter of the channel, cp is constant pressure heat capacity, and a and b are constants. The first term on the right side of eq 13 represents the contribution from static heat transfer such as conduction and radiation, while the other term reflects the contribution from convection. A series of values of λer were obtained for specified monolith catalysts and then were linearly correlated in terms of λer/λg versus Re 3 Pr. Figure 3 plots how λer/λg changes with Re 3 Pr for monolith catalysts, as well as the relationship between λer/λg and Re 3 Pr deduced from the experimental data for a pellet packed-bed reactor.43 It is obvious that the dimensionless radial effective conductivity of a pellet packed-bed reactor is much greater than that of monolith catalysts under the same inlet gas flux. This may be attributed to the pseudohomogeneous mixture of pellet and fluid whose conductivity is larger than a single fluid’s. Besides, λer/λg for a pellet packed-bed reactor increases more rapidly as Re 3 Pr increases. This indicates that the radial heat transfer for a pellet packed-bed reactor proceeds in the manner of both conduction and convection. On the other hand, the dimensionless radial effective heat conductivity of monolith catalysts is almost the same order of magnitude as the fluid heat conductivity. Therefore, the radial heat transfer for monolith catalysts proceeds mainly in the manner of conduction. The order of dimensionless radial effective heat conductivity for five shapes of monolith catalysts (coating and extruded catalysts) is round > regular triangle > rectangle > square > hexagon under the same inlet gas flux. The dimensionless radial effective heat conductivity for extruded catalysts increases quickly with the increase of Re 3 Pr, and thus λer/λg (round channel in extruded catalyst) < λer/λg (round channel in coating catalyst) at low Re 3 Pr, while it is converse at high Re 3 Pr. 5945

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Figure 3. Dimensionless radial effective heat conductivities for five kinds of monolith channel shapes and pellet packed-bed reactor for (a) coating catalysts and (b) extruded catalysts: (9) pellet packed-bed reactor with dp = 2.54 mm, εB = 0.33, and j = 1; (O) round; (Δ) regular triangle; (0) square; ()) rectangle; (/) hexagon.

3.3. Nusselt Number. The overall Nusselt number Nu is used to evaluate the heat transfer performance of the whole monolith catalysts at the gas
solid interface and is defined as follows32,44,45  de DT  Nu ¼ ð14Þ  ðTW
ÆTæÞ Dr  r f gas
solid interface

where TW is the interface temperature between bulk phase and catalyst layer (for both coating and extruded catalysts), and the bulk average temperature ÆTæ is ! Z z Z gas
solid interface . uðr, zÞ FCV Tðr, zÞr dr dz ÆTæ ¼ 0

0

Z zZ 0

gas
solid interface

! uðr, zÞ FCV r dr dz

ð15Þ

0

where CV is constant volume heat capacity of gas. A local Nusselt number Nu along the axial direction of monolith catalysts also can be obtained by analogous definition. However, Nu for the conventional pellet packed-bed reactor was welldefined and derived by means of the JH-factor method found in the standard text.46 Figure 4 shows that Nu increases slowly with the increase of inlet gas flux. But Nu of monolith catalysts is much smaller than that of the traditional pellet packed-bed reactor by 1 order of magnitude under the same inlet gas flux, indicating that the heat

Figure 4. Overall Nusselt number Nu for five kinds of monolith channel shapes and pellet packed-bed reactor for (a) coating catalysts and (b) extruded catalysts: (9) pellet packed-bed reactor with dp = 2.54 mm, εB = 0.33, and j = 1; (O) round; (Δ) regular triangle; (0) square; ()) rectangle; (/) hexagon.

transfer in the traditional packed-bed reactor is better than in cordierite
based monolith catalysts. However, it should be mentioned that in some cases heat transfer may be enhanced by substituting with the metallic substrates supported and even better than in the packed-bed reactor.18 For coating and extruded catalysts, Nu is in the order of round > rectangle >square > regular triangle > hexagon under the same inlet gas flux, which is consistent with the same order of round > square > regular triangle as reported in previous studies,47 and represents a general conclusion for SCR of NO with NH3. The regular triangle and hexagon channels exhibit worse heat transfer than other channel shapes, which may be caused by the acute corners of regular triangle channel and the more corners of hexagon channel, respectively. Moreover, Nu for extruded catalysts increases more quickly as the inlet gas flux increases since in this case both external and internal diffusions are promoted each other. Similarly, Nu (round channel in extruded catalyst) is smaller than Nu (round channel in coating catalyst) at low inlet gas flux, while it is converse at high inlet gas flux. On the other hand, Supporting Information Figure S7 shows the change of local Nu for a square channel of coating and extruded catalysts along the axial direction (note that other channel shapes exhibit the similar trend as square channel). Nu decreases drastically near the entrance, but tends to have an asymptotic value in the end when there is almost no difference of Nu between coating and extruded catalysts. Therefore, the DeNOx reaction enhances the heat transfer on the first part of monolith channel. 5946

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round > rectangle >square > regular triangle ≈ hexagon under the same inlet gas flux. This can be attributed to the same reason as that of Nu by the analogue between mass and heat transfer.47,52,53 Moreover, the maximum Sh of coating catalysts (round channel) is higher than that of extruded catalysts (round channel) at low inlet gas flux, while it is converse at high inlet gas flux. Anyway, round channel brings about the maximum Sh and Nu among all channel shapes investigated. Therefore, it is recommended to select round channel for strongly endothermic or exothermic catalytic reactions in order to improve mass and heat transfer. Figure S8 shows the change of local Sh for a square channel of coating and extruded catalysts along the axial direction (note that other channel shapes exhibit the similar trend as square channel), which is similar to the change of local Nu. Therefore, it demonstrates that the DeNOx reaction not only promotes heat transfer, but also promotes mass transfer. However, it should be mentioned that if the convective heat and mass transfer coefficients rather than Sh and Nu are used to evaluate the heat and mass transfer, the conclusions remain almost constant except for regular triangle although the equivalent diameters for different channel shapes are not the same. 3.5. Effectiveness Factor. For coating catalysts in which the coating thickness is not too thin or for extruded catalysts, the reaction occurs within the porous catalyst layer not on the surface. In this case, the effectiveness factor η is defined as follows:49,54 η¼ Figure 5. Overall Sherwood number Sh for five kinds of monolith channel shapes and pellet packed-bed reactor for (a) coating catalysts and (b) extruded catalysts: (9) pellet packed-bed reactor with dp = 2.54 mm, εB = 0.33, and j = 1; (O) round; (Δ) regular triangle; (0) square; ()) rectangle; (/) hexagon.

3.4. Sherwood Number. To evaluate the mass transfer of external diffusion of the whole monolith catalysts at the gas
solid interface, an overall Sherwood number Sh is defined as follows31,48
51  de DCNO  Sh ¼ ð16Þ  ðÆCNO æW
ÆCNO æb Þ Dr  r f gas
solid interface

where ÆCNOæW is the interface concentration between bulk phase and catalyst layer (for both coating and extruded catalysts), and the bulk average concentration ÆCNOæb is ! Z z Z gas
solid interface . ÆCNO æb ¼ uðr, zÞ CNO ðr, zÞr dr dz 0

0

Z zZ

gas
solid interface

! uðr, zÞr dr dz

0

ð17Þ

0

A local Sherwood number Sh along the axial direction of monolith catalysts also can be obtained by analogous definition. However, Sh for the conventional pellet packed-bed reactor was well-defined and derived by means of the JD-factor method found in the standard text.46 As shown in Figure 5, the overall Sh exhibits the similar trend as Nu. The monolith catalysts are not superior to the traditional pellet packed-bed reactor in mass transfer for SCR for NO removal. For both coating and extruded catalysts, Sh increases slowly with the increase of inlet gas flux, and follows the order of

ðRV Þact ðRV Þsurf

ð18Þ

where (RV)act is the average value of the reaction rate within the washcoat for coating catalysts or the whole wall for extruded catalysts, and (RV)surf is the reaction rate evaluated at the temperature and concentration on the external surface of washcoat/wall (i.e., at the interface between washcoat/wall and bulk phase). It is wellknown that the effectiveness factor η for the conventional pellet packed-bed reactor can be deduced by means of the generalized Thiele modulus method. For instance, if the pellet diameter dp = 2.54 mm, then η = 0.18, indicating that the internal diffusion resistance inside the pellet is very serious. For monolith catalysts, since the reaction takes place inside the monolith washcoat/wall, the reactants have to diffuse from the monolith washcoat/wall surface into the porous structure of catalytic domain.55 In some cases, the internal diffusion inside the porous structure could affect the reaction process or even become the controlling step. For coating catalysts, it can be seen from Figure 6a that the effectiveness factor decreases first slowly when washcoat thickness is less than 100 μm, and then decreases substantially with the increase of washcoat thickness. Finally, it begins to decrease slowly again and approaches to a stable value about 0.20 when washcoat thickness is above 600 μm. Therefore, the internal diffusion has a significant effect on SCR reaction rate over a wide range of washcoat thickness. If the washcoat thickness is less than 0.1 mm as usual, it is reasonable to assume that the effectiveness factor approaches to unity. In this case the reaction can be considered to occur only on the wall surface without any internal molecular diffusion effect. In addition, a thin washcoat thickness with higher effectiveness factor and the corresponding lower pressure drop (see Supporting Information, Figure S9) is favorable for SCR reaction. Extruded catalysts exhibit the similar trend of washcoat thickness versus effectiveness factor as that of coating catalysts, as shown in Figure 6b. However, there is almost no difference of effectiveness factors for all channel shapes of 5947

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Figure 6. Influence of washcoat thickness on effectiveness factor η for five kinds of channel shapes of monolith catalysts at gas inlet velocity 8.0 m s
1 for (a) coating catalysts and (b) extruded catalysts: (O) round; (Δ) regular triangle; (0) square; ()) rectangle; (/) hexagon.

coating and extruded catalysts except for a round channel in extruded catalysts. This is due to the irregular wall thickness of a round channel from its circumference to symmetrical square boundary to form a monolith block in extruded catalysts. 3.6. Reaction Performance. Figure 7 shows that NO conversion in a traditional pellet packed-bed reactor is much higher than in monolith catalysts under the same inlet gas flux. This may be attributed to the large amount of pellet catalysts loaded into the reactor compared to monolith catalysts. For coating catalysts, NO conversion is almost identical for all channel shapes at low inlet gas flux, while it is in the order of regular triangle > rectangular > square > hexagon > round at high inlet gas flux. However, NO conversion is in the order of round > regular triangle > rectangular > square > hexagon for extruded catalysts. It was found that the order of NO conversion for coating and extruded catalysts corresponds to the order of catalytically geometrical areas and volumes among these channel shapes. That is to say, the more the amount of catalysts loaded, the higher NO conversion. Furthermore, NO conversion of the coating catalyst is far higher than that of extruded catalyst at the same inlet gas flux. Whether the DeNOx process is controlled by chemical reaction or external diffusion is dependent on Da (Damkohler) number, which is defined as follows:47 Da ¼

chemical reaction rate external diffusion rate

ð19Þ

It was found that Da decreases with the increase of inlet gas flux for both coating and extruded catalysts, and is far smaller than

Figure 7. NO conversion for five kinds of monolith channel shapes and pellet packed-bed reactor for (a) coating catalysts and (b) extruded catalysts: (9) pellet packed-bed reactor with dp = 2.54 mm, εB = 0.33 and j = 1; (O) round; (Δ) regular triangle; (0) square; ()) rectangle; (/) hexagon.

unity especially at high inlet gas flux (see Supporting Information, Figure S10). This means that the chemical reaction rate has a more strong influence on the DeNOx process than external diffusion. Therefore, the regular triangle channel of the coating catalyst and the round channel of the extruded catalyst possess the maximum NO conversion due to those having the largest amount of catalyst loaded, and thus they should be an important consideration in the design and optimization of SCR reactors aimed at improving reaction performance.

4. CONCLUSIONS It is critical to fully take into account the transfer and reaction performances of monolith catalysts which are being recognized as a new process intensification technology. The comparison of transfer and reaction performances of SCR for NO removal among monolith catalysts, as well as traditional packed-bed reactor, was done in this work. Although a traditional packed-bed reactor is superior to monolith catalysts except for a pressure drop and effectiveness factor, it may not be suitable for the processes where low pressure drop is required (e.g., SCR for NO removal, reaction and separation coupled, etc.), and catalyst price is high (e.g., supported noble metal catalysts, molecular sieves, etc.) because a large amount of catalyst particles with small effectiveness factor, as compared to monolith catalysts, have to be installed . It is interesting for us to take statistics on all the papers published in the special issues of Catalysis Today for ICOSCAR (International Conference on Structured Catalysts and Reactors)-1 (2001), ICOSCAR-2 (2005) and ICOSCAR-3 (2009). We found 5948

dx.doi.org/10.1021/ie102206x |Ind. Eng. Chem. Res. 2011, 50, 5942–5951

Industrial & Engineering Chemistry Research that the number of papers concerning coating catalysts far exceeds that concerning extruded catalysts (see Supporting Information, Figure S11). We relate this phenomenon with lower pressure drop and higher conversion brought on by coating catalysts compared to extruded catalysts. For coating catalysts, round channel brings about the highest Reynolds number and thus the maximum Sh and Nu among all channel shapes investigated. Therefore, it is recommended that the round channel be selected for strongly endothermic or exothermic catalytic reactions in order to improve mass and heat transfer. The transfer and reaction performances of a square channel lie in-between. However, the regular triangle channel of coating catalyst unexpectedly exhibits the maximum NO conversion due to its having the highest catalytically active area, and thus should be paid more attention in the design and optimization of SCR reactors aiming at improving reaction performance, although round and square channels are commonly encountered.

’ ASSOCIATED CONTENT

b S

Experimental details on preparation, characterization of coating catalysts, as well as the comparison of the number of papers concerning coating and extruded catalysts. This material is available free of charge via the Internet at http://pubs.acs.org. Supporting Information.

’ AUTHOR INFORMATION Corresponding Author

*Tel.: þ86 10 64433695. Fax: þ86 10 64419619. E-mail: leizhg@ mail.buct.edu.cn.

’ ACKNOWLEDGMENT This work was financially supported by the National Nature Science Foundation of China under Grant Nos. 20821004, 20736001 and 21076008, and the Research Fund for the Doctoral Program of Higher Education of China (No. 20090010110002). ’ NOMENCLATURE a = permeability, m2 C = mole concentration, kmol 3 m
3 ÆCNOæW = interface concentration between bulk phase and catalyst layer, kmol 3 m
3 ÆCNOæb = bulk average concentration, kmol 3 m
3 Cp = constant pressure heat capacity of gas, J 3 kg
1 3 K
1 CV = constant volume heat capacity of gas, J 3 kg
1 3 K
1 Da = Damkohler number, dimensionless Di,N2 = binary diffusion coefficient, m2 3 s
1 Deff,i = effective diffusion coefficient, m2 3 s
1 de = equivalent diameter of the channel, m dp = diameter of the spherical pellet catalyst, m JH = heat transfer factor, dimensionless JD = mass transfer factor, dimensionless k = reaction rate constant, s
1 k0 = specific reaction rate constant, s
1 Lc = characteristic length (i.e., the ratio of washcoat volume to fluid/solid interfacial area), m M = molecular mass, kg 3 mol
1 n = length in normal direction, m Nu = Nusselt number, dimensionless

ARTICLE

P = Pressure, Pa Pr = Prandtl number, dimensionless R = gas constant, J 3 mol
1 3 K
1 r = radial coordinate, m rNO = reaction rate, kmol 3 m3 3 s
1 Re = Reynolds number, dimensionless (RV)act = average value of reaction rate in the washcoat, kmol 3 m3 3 s
1 (RV)s = reaction rate evaluated at the temperature and concentration at the washcoat external surface, kmol 3 m3 3 s
1 Sh = Sherwood number, dimensionless T = temperature, K TW = interface temperature between bulk phase and catalyst layer, K ÆTæ = bulk average temperature, K u = gas velocity, m 3 s
1 Vi = molecular diffusion volume of gas component, m3 3 mol
1 wi,wall = mass fraction of gas on the wall, dimensionless z = axial coordinate, m Greek Symbols

O = generic transport property defined in eq 2 Γ = transport coefficient defined in eq 2 Sφ = source term defined in eq 2 F = gas density, kg 3 m
3 τ = tortuosity of catalyst, dimensionless μ = molecular viscosity, kg 3 m
1 3 s
1 λer = radial effective heat conductivity, W 3 m
1 3 K
1 λg = gas thermal conductivity, W 3 m
1 3 K
1 λs = solid thermal conductivity, W 3 m
1 3 K
1 ε = porosity of porous medium, dimensionless εB = void fraction of fixed-bed reactor, dimensionless j = equivalent spherical coefficient, dimensionless η = effectiveness factor, dimensionless ΔH = reaction heat, J 3 mol
1 ΔP = pressure drop, Pa Subscripts

g = gas phase i = species (i = NO, NH3, O2, H2O, N2) s = solid phase

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