Modeling of Monolith Reactor Washcoated with CuZSM5 Catalyst for Removing

Modeling of Monolith Reactor Washcoated with CuZSM5Catalyst for Removing NO from Diesel Engine by Urea

Joon Hyun Baik,Sung Dae Yim,?and In-Sik Nam*

Department of Chemical Engineering/School of En V ironmental Science and Engineering,

Pohang Uni V ersity of Science and Technology(POSTECH),san31Hyoja-dong,Pohang790-784,Korea

Young Sun Mok

Department of Chemical Engineering,Cheju National Uni V ersity,66Jejudaehakno,Jeju690-756,Korea

Jong-Hwan Lee,Byong K.Cho,and Se H.Oh

General Motors R&D and Planning Center,Warren,Michigan48090-9055

Two sets of reaction kinetics based upon the experimental data independently obtained for NH3-SCR and

urea decomposition reactions over CuZSM5catalyst have been deduced to design the urea-SCR process for

application to heavy-duty diesel engines.They were finally employed simultaneously to simulate the commercial

performance of a monolith reactor for the urea-SCR process.The monolith reactor washcoated with the

CuZSM5catalyst was prepared and its SCR activity was evaluated to confirm the reaction kinetics and the

reactor model developed in the present study for its commercial application.The monolith reactor model

including the mass-transfer resistance directly employs the kinetic parameters obtained from the kinetic study

over a packed-bed flow reactor containing20/30mesh powder type of CuZSM5catalyst.The kinetic and

monolith reactor models developed in the present study well predict the reactor performance of the urea-SCR

process.The model is also capable of describing the effect of the reaction conditions,a critical issue for the

commercial operation of the urea-SCR process to the automotive engine,including the reactor space velocity,

NH3(and/or urea)/NO feed ratio,NH3and urea slips,and the temperature of the thermal decomposition

reactor on the NO removal activity.

Introduction

Selective catalytic reduction(SCR)by NH3is generally recognized as the most effective method for reducing the emission of nitrogen oxide(NO x)from stationary sources.1 Recently,urea-SCR,the selective catalytic reduction of NO x using urea as an alternative reducing agent for its easy transportation and handling,has been reported as one of the most promising way to reduce or control NO x emissions originating from heavy-duty diesel engines.2,3Baik et al.reported that the removal of NO x by urea-SCR over the CuZSM5catalyst was competitive to that by NH3-SCR,indicating urea can be effectively utilized in an SCR reactor system as a reducing agent for NO x from automotive engine.4

The urea-SCR process over the CuZSM5catalyst is mainly based upon the following four reactions:(1)urea thermal decomposition,(2)HNCO hydrolysis,(3)NO reduction,and (4)NH3oxidation reaction.Urea is thermally decomposed into ammonia and isocyanic acid,and then the isocyanic acid formed by reaction1is easily hydrolyzed on the catalyst surface, producing an additional mole of ammonia and carbon dioxide by reaction2.2,5-7Consequently,the complete decomposition of one mole of urea produces two moles of ammonia and one mole of carbon dioxide.NH3is a direct reductant for SCR reaction by reaction3,while urea is an indirect reducing agent to produce NH3for the overall deNO x reaction.On the other hand,the NH3oxidation reaction can occur at a reaction temperature>350°C and mainly produce N2,particularly over CuZSM5catalyst by reaction4.It should be noted that N2is the main product by NH3oxidation reaction over the Cu ion-exchanged zeolite.8-10

However,unreacted NH3decomposed from urea can be a secondary air pollutant because of its toxicity and the formation of ammonium salts inside the reactor as well as in the atmosphere.Searching optimal operating conditions for mini-mum NH3emissions(e.g.,urea/NO feed ratio)generally involves a tradeoff between NO removal activity and NH3slip.11 The development of the kinetic model may resolve this problem more efficiently,and it will also provide critical information for the optimal design and operation of a commercial urea-SCR reactor for its automotive application.

A kinetic model for the simultaneous thermal and catalytic decomposition of urea over CuZSM5catalyst was developed by a unified approach based upon the power-law kinetic model.5 A model containing three main reactions,including the thermal decomposition of urea,the catalytic hydrolysis of HNCO,and the catalytic oxidation of ammonia during the course of the decomposition of urea,adequately described the experimental data.For NH3-SCR,Chae et al.developed a honeycomb reactor

*To whom correspondence should be addressed.Tel.:82-54-279-2264.Fax:82-54-279-8299.E-mail:isnam@postech.ac.kr.

?Present address:Fuel Cell Research Center,Korea Institute of Energy Research(KIER),71-2Jang-Dong,Daejeon305-343,Korea.

NH

2

-CO-NH

2

f NH3+HNCO(1)

HNCO+H

2

O f NH

3

+CO

2

(2)

4NO+4NH

3

+O

2

f4N2+6H2O(3)

4NH

3

+3O

2

f2N2+6H2O(4)

5258Ind.Eng.Chem.Res.2006,45,5258-5267

10.1021/ie060199+CCC:$33.50?2006American Chemical Society

Published on Web06/22/2006

model based upon the intrinsic reaction kinetics of the catalyst employed.8A monolith model directly employing the kinetic parameters estimated from the kinetic study could significantly reduce the modeling effort for the honeycomb reactor.They also considered the diffusion effect on the performance of the catalytic honeycomb reactor for the model with respect to the configuration of monolith,including the catalytic wall thickness and the reactor operating conditions.The urea-SCR process,particularly for automotive applications,however,has been rarely examined.Few works on the reaction kinetics and the reactor modeling of urea-SCR can be found in the literature,while SNCR of NO by urea (selective non-catalytic reduction)at high reaction temperatures >1100K has been extensively investigated.12,13

In the present study,a kinetic model for urea-SCR over CuZSM5catalyst requiring two reaction kinetics,urea decom-

position and NH 3-SCR,has been developed.A monolith reactor has been also prepared by washcoating the powder of the catalyst on cordierite.The reaction kinetics has been incorpo-rated into a monolith reactor model developed in the present study in order to optimize the reactor design and operating parameters for the commercial urea-SCR process to reduce NO from diesel engines.Experimental Section

Catalyst Preparation.The CuZSM5catalyst was prepared by wet ion-exchange method using 0.01M of copper acetate solution at room temperature,as extensively described else-where.14The ZSM5type zeolite catalyst with Si/Al molar ratio of 14was obtained from Tosoh Co.(HSZ-830NHA).The monolith reactors were prepared by washcoating CuZSM5catalyst slurry with an alumina binder (NYACOL AL20DW)on the 200CPSI cordierite monolith (Dongsu Industrial Co.).The catalyst powder was completely suspended in the solution when mixed with water and binder during the course of the washcoating procedure,and the mean particle size of the powder including catalyst and alumina binder for washcoating has been confirmed as 2.8μm by particle size analyzer (Beckman Coulter LS 230).The thickness of the washcoats on the honeycomb reactor was measured by a scanning electron microscope (SEM)image (Hitachi,S2460N)shown in Figure 1,and the catalyst powder deposited on the cordierite surface seems to be reason-ably uniform throughout the channels of the monolith

reactor.

Figure 1.Catalytic thickness of the monolith reactor washcoated with CuZSM5catalyst by a SEM

image.

Figure 2.Schematic flow diagram for NH 3(or urea)SCR reactor system.

Table 1.Physicochemical Properties of CuZSM5Catalyst Prepared

CuZSM5powder

washcoated honeycomb reactor metal (Cu)content 2.9wt %porosity a

0.71Si/Al

14washcoats thickness 27μm ion exchange level 97%catalyst weight 19.7wt %BET surface area

337m 2/g

CPSI

200

a

Washcoats (CuZSM5+alumina binder,without codierite).

Ind.Eng.Chem.Res.,Vol.45,No.15,20065259

Listed in Table1are the physicochemical properties of the catalyst and the monolith washcoated with the catalyst prepared. The catalysts were dried at110°C for12h and calcined in air at500°C for5h before and after washcoating.They were again pretreated in air at500°C for1h prior to each experiment. Reaction System and Experimental Procedure.For an intrinsic kinetic study of NH3-SCR,the NO removal activity of the CuZSM5catalyst has been evaluated in a packed-bed flow reactor system with1g of20/30mesh size catalyst,where the effect of mass transfer and pressure drop may be ignored, as shown in Figure2.15A washcoated monolith reactor with 25mm×25mm×25mm dimensions was prepared for the evaluation of the reactor performance,and the reactor space velocity was defined as the ratio of the total volumetric flow rate to the volume of the monolith reactor.A feed gas mixture containing NO(500ppm),NH3(500ppm),O2(5%),H2O (10%),and N2(balance)was supplied through mass flow controllers(Brooks5850E).The kinetic data from a packed-bed reactor were collected over a wide range of reaction temperatures(150-500°C)and reactor space velocities(50000-400000h-1).

For the simultaneous study of urea decomposition and urea-SCR reactions,an additional reactor system for urea decomposi-tion has been fabricated.The reactor consists of a urea injection part,a reactor for urea thermal decomposition,a catalytic reactor, and an analysis train for remaining NH3,urea,and HNCO.4,5 The temperature of the thermal decomposition reactor can be controlled independently.It should be noted that aluminum and SUS316for packed-bed and honeycomb reactors,respectively, have been specifically employed as the reactor materials to prevent the catalytic effects of the reactor.

The concentrations of NO and NH3were analyzed by an on-line chemiluminescent NO-NO x analyzer(Thermo Electron Co.,model42H)and NDIR-type NH3analyzer(Rosemount Analytical,model880A),respectively.To analyze the urea remaining unconverted and the HNCO produced by the decom-position reaction and urea-SCR,a part of the reacted gas was absorbed into a series of the absorption bottles containing deionized water.The concentrations of urea and HNCO absorbed into the water were then analyzed by high-performance liquid chromatography(HPLC)with a UV detector(Younglin UV730D),as reported previously.5,16

Development of Reaction Kinetics and Monolith Reactor Model

SCR by Ammonia.Since NO selectively reacts with NH3 for the urea-SCR process by reaction3,the reaction kinetics of SCR of NO by NH3has been examined.Two primary reactions, NO reduction reaction3and NH3oxidation reaction4,are assumed to mainly occur during the SCR process over CuZSM5 catalyst.10A maximum NO conversion was generally observed as the reaction temperature was varied over the range200-500°C for the present catalytic system because of the ammonia oxidation reaction.8Hence,the intrinsic reaction kinetics for the SCR process has been derived on the basis of reactions3 and4to particularly describe the maximum NO conversion and NH3slip.

In the present study,a dual-site catalysis Langmuir-Hin-shelwood-Hougen-Watson(LHHW)mechanism,where the reaction occurs between adsorbed NO and NH3molecules on two distinct reaction sites such as NH3adsorption site(acidic site)and NO site(basic site),is assumed by the results of the temperature-programmed desorption(TPD)experiments and the previous study on the identical reaction system over CuHM catalyst.17The reaction rate equations can be derived on the basis of Hougen-Watson formalism assuming the surface reaction is the rate-determining step.

Rearranging in terms of conversions of NO and NH3,

where K NO and K NH

3

are the adsorption equilibrium constants and k NO and k NH

3

are the surface reaction rate constants.Since the kinetic model developed is represented by two mass balances expressed by nonlinear coupled first-order ordinary differential equations with respect to NO and NH3,both NO removal activity and NH3slip can be predicted as a function of reactor space velocity and reaction temperature.

Urea Decomposition.Assuming plug flow,the steady-state mass balances for urea and isocyanic acid over a fixed-bed reactor can be expressed as

Equations9and10were derived under the assumption that they are elementary reactions and obey first-order reaction kinetics. Details for the kinetic study of urea decomposition were extensively described in the previous study.5

SCR by Urea.For predicting the performance of the urea-SCR process over the CuZSM5catalyst,the reaction kinetics of NH3-SCR and urea decomposition as derived above have been simultaneously combined.

As the reaction kinetics for urea thermal decomposition by reaction1,HNCO hydrolysis by reaction2,and NO reduction by reaction3,eqs5,9,and10,respectively,have been employed.On the other hand,the mass balance equation for NH3oxidation by reaction4based upon the formation and the consumption of NH3from the decomposition of urea and the reduction of NO,respectively,has been rewritten:

-

d C

NO

dτ

)

k

1

C

NO

C

NH3

(1+K

NO

C

NO

)(1+K

NH3

C

NH3

)

(k

1

)k

NO

K

NO

K

NH3

C

O2

)(5) -

d C

NH3

dτ

)

k

1

C

NO

C

NH3

(1+K

NO

C

NO

)(1+K

NH3

C

NH3

)

+

k

2

C

NH3

(1+K

NH3

C

NH3

)

(k

2

)k

NH3

K

NH3

C

O2

)(6)

d X

NO

dτ

)

k

1

C

NH3

0(1-X

NO

)(1-X

NH3

)

[1+K

NO

C

NO

0(1-X

NO

)][1+K

NH3

C

NH3

0(1-X

NH3

)]

(7)

d X

NH3

dτ

)

k

1

C

NO

0(1-X

NO

)(1-X

NH3

)

[1+K

NO

C

NO

0(1-X

NO

)][1+K

NH3

C

NH3

0(1-X

NH3

)}

+

k

2

(1-X

NH3

)

1+K

NH3

C

NH3

0(1-X

NH3

)

(8)

d C

Urea

dτ

)-k

3

C

Urea

(9)

d C

HNCO

dτ

)k

3

C

Urea

-k

4

C

HNCO

C

H2O

(10)

5260Ind.Eng.Chem.Res.,Vol.45,No.15,2006

It should be noted that the homogeneous noncatalytic oxidation of NH 3during the thermal decomposition of urea was ignored because of the negligible oxidation reaction activity based upon a nitrogen balance in the down stream of the reactor,as reported previously.5

These equations can be rearranged in terms of conversions as follows,

where k 4C H 2O can be treated as a constant (k 4′)because of the relatively high feed concentration of H 2O,10%both in the present study and in an actual exhaust stream from a diesel engine.

Monolith Reactor Model.A mathematical model for the washcoated honeycomb reactor has been developed on the basis of the kinetic study examined over a packed-bed flow reactor for a urea-SCR system.The effect of the external and internal diffusion on NO removal activity in a washcoated monolith reactor has been also considered and included for developing the model similar to the work by Chae et al.8

A material balance of the gas-phase reactants in a channel is

with the initial conditions C i b )C i b,0at x )0.At any axial position x along the channel of the monolith,a material balance over the catalyst layer of thickness d y yields the following:

Equation 16describes the diffusion-reaction interaction for reactants within the pore of washcoats and catalyst.Note that

the axial diffusion term,d 2C i /d x 2,has been ignored in eq 16;when the spatial variables are transformed into a dimensionless group,its coefficient (～10-7)is found to be negligible compared to that for the transverse diffusion term (～1)in eq 16.18The boundary conditions for eq 16are

A material balance at axial position x of the honeycomb reactor over the external gas film yields

The external mass-transfer coefficient k m can be estimated by the Hawthorn correlation,eq 17,which can well describe the developing laminar flow in rectangular channels.18-20

where B is a shape factor of the channel of the honeycomb.Numerical Method.To estimate the kinetic parameters of the global reaction kinetics derived in the present study,the catalytic activity of CuZSM5has been examined over wide ranges of reactor space velocities and reaction temperatures.Upon the basis of the kinetic data,the parameters of the proposed global reaction kinetic expressions,eqs 5and 6,were estimated by fitting the model predictions of NO and NH 3conversions to the experimental data at each temperature of interest.Two nonlinear ordinary differential equations,eqs 7and 8,were solved by Gear’s method,which can handle any degree of the stiffness of the gradients for the model equations while allowing the desired accuracy of integration with moderate computing time.The parameter estimation was made by nonlinear regression,minimizing the sum of squares calculated by eq 19from the experimental and calculated NO and NH 3conversions.A regression routine for the minimization uses the Marquardt algorithm.21

The computer subroutine program for the estimation of the parameters was prepared by using MATLAB (version 6.1,The MathWorks,Inc.).

A mathematical description of the honeycomb reactor derived from the present study is coupled by ordinary differential equations:eq 15with initial values and by partial differential equations and eq 16with nonlinear boundary conditions.The ordinary differential equations have been solved by the Runge -Kutta method.The nonlinear second-order derivatives were discretized by the finite difference method (FDM)and/or accelerated fixed-point iteration (FPI).Results

SCR by Urea.The deNO x performance of the powder form of CuZSM5catalyst (20/30mesh)by urea-SCR over the fixed-bed reactor can be observed in Figure 3.When the temperature of the thermal decomposition reactor was set at 350°C where urea was completely decomposed into NH 3and HNCO,5the

-

d C NH 3d τ

)

k 1C NO C NH 3

(1+K NO C NO )(1+K NH 3C NH 3)

+

k 2C NH 3

(1+K NH 3C NH 3)

-k 3C Urea -k 4C NHCO C H 2O (11)

d X Urea

d τ

)k 3(1-X Urea )(12)

d X HNCO d τ)-k 3C Urea 0(1-X Urea )

C HNCO

0+k 4C H 2O (1-X HNCO )(13)d X NO

d τ)k 1C NH 3

(1-X NO )(1-X NH 3)[1+K NO C NO 0(1-X NO )][1+K NH 3C NH 30(1-X NH 3)]

(7)d X NH 3d τ

)

k 1C NO 0(1-X NO )(1-X NH 3)

[1+K NO C NO 0(1-X NO )][1+K NH 3C NH 30

(1-X NH 3)]

+

k 2(1-X NH 3)

1+K NH 3C NH 3

(1-X NH 3)-

k 3C Urea 0(1-X Urea )

C NH

3

-

k 4C H 2O C NHCO (1-X HNCO )

C NH

3

0(14)

-u d C i b

d x

)k m,i A e (C i b -C i s )(15)

D e,i

d 2C i d y 2

)-r i

(16)

d C i

d y

)0at y )0C i )C i s at y )R

k m,i (C i b -C i s ))D e,i

(d C i d y

)

s

(17)

Sh )B (1+0.095d L Pe

)

0.45

(18)

Minimize

∑i )0

n

(X i exp -X i cal )2(19)

Ind.Eng.Chem.Res.,Vol.45,No.15,20065261

NO removal activity over CuZSM5catalyst by urea-SCR was close to the activity observed by NH 3-SCR.This clearly indicates that HNCO produced in the thermal decomposition reactor successfully further reacts with H 2O to produce NH 3in

the catalytic reactor,thereby making urea as effective as NH 3for NO reduction,when the temperature of the thermal decomposition reactor is >350°C.Again,the hydrolysis reaction is a fast reaction.5It should be noted that the thermal decomposition of urea and the hydrolysis of HNCO simulta-neously occur in the SCR reactor containing CuZSM5catalyst.However,when the temperature of the thermal urea decom-position reactor is 150°C,where urea is not completely decomposed,the catalyst exhibits relatively low NO x conver-sion,particularly at reaction temperatures <250°C,apparently due to the incomplete decomposition of urea resulting in the deficiency of NH 3during the course of the SCR reaction.Thus,the temperature of the thermal urea decomposition reactor is of critical importance for the high performance of the urea-SCR system.Note that the contribution of the residence time to the decomposition of urea should not be excluded.4,5

Reaction Kinetics.Figure 4compares the conversions of NO and NH 3predicted by the kinetic model developed in the present study,eqs 5,9,and 10,with the experimental data from the packed-bed flow reactor at various reactor space velocities.The model well predicts the NH 3-SCR performance over the powder type of the CuZSM5catalyst throughout the reactor operating conditions covered in the present study.Also,a maximum in the NO conversion versus temperature plots can be predicted by the model.The kinetic parameters estimated by the

numerical

Figure 3.Effect of the temperature of the thermal urea-decomposition reactor for NO reduction by urea over CuZSM5catalyst;SV )100000h -1

.

Figure https://www.360docs.net/doc/8e11035448.html,parisons of predicted and measured NO and NH 3conversions over CuZSM5catalyst for four different reactor space velocities.

5262Ind.Eng.Chem.Res.,Vol.45,No.15,2006

method based upon the experimental data are listed in Table 2.The results have been reconfirmed by Arrhenius temperature dependence,revealing the good linearity of the constants with respect to the temperatures.It should be noted that the temperature dependences of the rate constants for the thermal decomposition of urea and hydrolysis of HNCO were well described by Yim et al.5As listed in Table 2,the activation energy of NH 3oxidation,39.7kcal/mol,is nearly 3times greater than that of NO reduction,12.5kcal/mol.This is a primary cause for observing a maximum in the NO conversion as a function of temperature as shown in Figure 4.

Figure 5shows the prediction of NO and NH 3conversions by the model to describe the experimental data attained at a variety of NH 3/NO feed ratios (0.7,0.85,and 1.0)for a 2-fold check of the validity of the reaction kinetics developed in the present study.The level of the maximum NO conversion strongly depends on the NH 3/NO feed ratio,and the model well predicts the NO conversion and NH 3slip of the catalyst with respect to the NH 3/NO feed ratio and reaction temperature.The present kinetic model is quite adequate in predicting the NO conversion and NH 3slip of the CuZSM5catalytic system at various reaction conditions including the reactor space velocity,NH 3/NO feed ratio,and reaction temperature.

For the urea decomposition reaction,the kinetic parameters were obtained by the previous study as listed in Table 2.5Yim et al.reported that the activation energy for the thermal decomposition of urea in the catalytic reactor,5.4kcal/mol,is quite close to the value observed for the thermal decomposition reactor (5.5kcal/mol).5On the other hand,the CuZSM5catalyst employed in the present study significantly reduced the activa-tion energy for the HNCO hydrolysis reaction from 14.9to 3.8kcal/mol.

Modeling of Washcoated Monolith Reactor.The honey-comb reactor model developed in the present study s eqs 15-

17along with eqs 5,9,and 10s has been employed to predict the deNO x performance by NH 3of the monolith reactor washcoated with the CuZSM5catalyst at three different reactor space velocities,as shown in Figure 6.

The present monolith model with the kinetic parameters directly estimated from the packed-bed flow reactor containing the catalyst powders adequately describes the effects of the reactor space velocity on the NO removal activity and NH 3slip over the washcoated honeycomb reactor.It also reveals that the present model considering both external and internal diffusion can describe the geometric effects of a monolith,such as the thickness of washcoats of the honeycomb reactor measured by SEM image as shown in Figure 1.

Figure 7also compares the predicted and measured NO and NH 3conversions over CuZSM5catalyst washcoated on a honeycomb reactor at various NH 3/NO feed ratios (0.7,0.85,and 1.0).These results reveal that the level of the maximum NO conversion strongly depends on the NH 3/NO feed ratio,and the model well predicts the NO conversion and NH 3slip of the catalyst as function of NH 3/NO feed ratio and the reaction temperatures.Discussion

Modeling of Urea-SCR Technology over the Powder Type of CuZSM5Catalyst.Numerical simulation of the urea-SCR performance over 20/30mesh size of CuZSM5catalyst was performed by combining the reaction kinetics for both urea decomposition and NH 3-SCR,as mentioned.Figure 8shows the model prediction of NO removal activity by urea with respect to the temperature of the thermal decomposition reactor.Both kinetic models independently developed in the present study well predict the reactor performance of the urea-SCR process,as shown in Figure 8.It also compares the

predicted

Figure https://www.360docs.net/doc/8e11035448.html,parisons of predicted and measured NO and NH 3conversions over CuZSM5catalyst for two different NH 3/NO ratios (i.e.,0.7and 0.85);SV )100000h -1.

Table 2.Kinetic Parameters Estimated from the Experimental Data Observed over Packed-Bed Reactor

SCR by NH 3

k 1)3.09×1016exp(-12457/RT )k 2)6.56×1015exp(-39712/RT )K NO )4.13×10-5exp(14969/RT )K NH 3)1.39×10-2exp(21541/RT )

thermal decomposition reactor a

catalytic reactor a

urea decomposition 5

k 3)4.9×103exp(-5505/RT )k 3)4.5×103exp(-5405/RT )k 4′)2.5×105exp(-15861/RT )

k 4′)3.2×104exp(-3780/RT )

a

k 4′)k 4C H 2O .

Ind.Eng.Chem.Res.,Vol.45,No.15,20065263

and measured NO,NH 3,urea,and HNCO conversions at the temperature of thermal decomposition reactor of 350°C,where urea can be completely decomposed into two moles of NH 3and one mole of CO 2,as confirmed in Figure 3.The model well predicts the experimental data and reveals that the HNCO generated in the thermal decomposition reactor has been immediately hydrolyzed in the catalytic reactor,and then the produced NH 3can remove NO.Again,in the catalytic reactor,NH 3produced by both thermal decomposition and catalytic hydrolysis of HNCO can immediately react with NO on the surface of the catalyst.The model is also capable of describing a maximum NO conversion with increasing temperature,which is typical for SCR reaction as well as NH 3slip.

The model developed in the present study can also

describe

Figure https://www.360docs.net/doc/8e11035448.html,parisons of predicted and measured NO and NH 3conversions over the monolith reactor washcoated by CuZSM5catalyst for three different reactor space

velocities.

Figure https://www.360docs.net/doc/8e11035448.html,parisons of predicted and measured NO and NH 3conversions over the monolith reactor washcoated by CuZSM5catalyst for two different NH 3/NO ratios (i.e.,0.7and 0.85);SV )10000h -1.

5264Ind.Eng.Chem.Res.,Vol.45,No.15,2006

the effect of the thermal decomposition reactor temperature on the NO removal activity,as shown in Figure https://www.360docs.net/doc/8e11035448.html,pared with the result in Figure 8a,the conversion of NO is rather low,especially at a reaction temperature <300°C when the tem-perature of thermal decomposition reactor is 150°C.It is quite understandable that the deNO x performance of urea-SCR is strongly influenced by the temperature of the thermal decom-position reactor,as mentioned before.When the urea is completely decomposed into ammonia and isocyanic acid,NO removal activity by urea corresponds to that by ammonia.Therefore,the kinetic parameters estimated for the model developed in the present study were quite appropriate to describe the performance of the urea-SCR process over CuZSM5catalyst.In addition,no direct reaction of NO with urea may be expected in the present work.

Modeling of Urea-SCR Technology over Monolith Reac-tor.The honeycomb reactor model developed in the present study s eqs 15and 16along with the reaction kinetics,eqs 7,12,13,and 14s has been employed to predict the deNO x performance of the honeycomb reactor washcoated with the CuZSM5catalyst by urea-SCR,as shown in Figure 9.Under the operating conditions of Figure 9a,by the calculation via eqs 9and 10,250ppm of the feed concentration of urea into the urea thermal rector will produce 199ppm NH 3and 189ppm HNCO,as well as 56ppm of unreacted urea in the thermal decomposition reactor.Although urea did not decompose completely in the thermal reactor,the NO removal activity over the honeycomb reactor has revealed similar activity trends to that over the packed-bed reactor with the powder type of the catalyst,as described.This result simply indicates that a small amount of unreacted urea cannot affect the NO removal activity,even at low reaction temperature.The remaining urea can be also decomposed as the temperature of the catalytic reactor is increased,which leads the further reaction of NO,as shown in Figure 9a.

Figure 9b shows the result of model prediction when

the

Figure https://www.360docs.net/doc/8e11035448.html,parison of predicted and measured conversions over the CuZSM5catalyst;SV )100000h -1.Conditions of thermal decomposition reactor:1.6L/min (a)at 350°C [feed composition to SCR reactor;urea (0ppm),HNCO (216ppm),NH 3(284ppm)],(b)at 150°C [feed composition to SCR reactor;urea (143ppm),HNCO (107ppm),NH 3(107

ppm)].

Figure https://www.360docs.net/doc/8e11035448.html,parison of predicted and measured conversions over the monolith reactor washcoated by CuZSM5catalyst;SV )10000h -1.Conditions of thermal decomposition reactor: 3.3L/min (a)at 350°C [feed composition to SCR reactor;urea (56ppm),HNCO (189ppm),NH 3(199ppm)],(b)at 150°C [feed composition to SCR reactor;urea (191ppm),HNCO (59ppm),NH 3(59ppm)].

Ind.Eng.Chem.Res.,Vol.45,No.15,20065265

temperature of thermal decomposition was set at 150°C.The model well describes the experimental data with respect to the operating condition of the thermal decomposition reactor for urea.As expected,the deNO x performance of urea-SCR process has been strongly influenced by the amount of urea decomposed in the thermal and catalytic rectors particularly at low reaction temperature.Figure 10also shows that the effect of the urea/NO feed ratio on the NO removal activity is similar to that for the NH 3-SCR process in Figure 7,as mentioned.Obviously,the model is superbly capable of describing the deNO x performance of the urea-SCR process as well as NH 3and urea slips over the monolith reactor washcoated with CuZSM5catalyst in the wide range of operating condition of urea-SCR technology for removing NO from automotive diesel engines.It should be noted that a transient analysis of the kinetic and reactor models developed has to be considered for the practical mobile application of the urea-SCR technology.Conclusions

The removal of NO x by urea-SCR strongly depends on the operating conditions of the reactor,especially the temperature and the residence time of the feed gas stream in the thermal decomposition reactor.A model based on four main reactions,including thermal decomposition of urea,the catalytic hydrolysis of HNCO,reduction of NO,and NH 3oxidation,well describes the experimental data with appropriate activation and adsorption energies over a wide range of experimental conditions.Since the parameters estimated from the kinetic study over a catalyst pellet have been directly employed to anticipate the performance of the monolith reactor washcoated by CuZSM5catalyst,the present approach will save much effort and time for estimating and predicting the deNO x activity of the monolith for the commercial design of the honeycomb reactor for removing NO from a diesel engine by urea.Furthermore,the performance of the monolith reactor washcoated with any other catalyst can be predicted even without the fabrication of the reactor,if there is available reaction kinetics confirmed by the experimental data over the powder type of its counterpart catalyst.Again,the model for urea-SCR developed in the present study can predict the reactor performance of the washcoated monolith reactor

commonly employed for automotive engines and may provide a practical guideline for the design of a commercial urea-SCR process.Nomenclature

A e )honeycomb geometry surface area,cm 2/cm 3

B )shape factor of the channel of a honeycomb (2.976for a square)

C i )concentration of i species,mol/cm 3d )width of honeycomb channel,cm

d h )hydraulic diameter of honeycomb channel,cm D )diffusivity,cm 2/s

D e,i )effective diffusivity of the i th species,cm 2/s

k m,i )gas -solid mass-transfer coefficient of the i th species,cm/s

L )reactor length,cm

r i )reaction rate for the i th species based on catalytic volume,mol/(cm 3?s)

R )washcoat thickness for a washcoated honeycomb,cm u )average velocity at the reactor inlet,cm/s

x )axial coordinate in the honeycomb channel geometry,cm y )transverse coordinate in the honeycomb channel geometry,cm

τ)reactor space time (residence time)of the feed gas stream,s Subscripts and Superscripts b,s )bulk and surface,respectively Dimensionless Groups

Pe )Peclet number,ud h /D

Sh )Sherwood number,k m d h /D Literature Cited

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Figure https://www.360docs.net/doc/8e11035448.html,parison of predicted and measured conversions over the monolith reactor washcoated by CuZSM5catalyst at two urea/NO feed ratios;SV )10000h -1.Conditions of thermal decomposition reactor:350°C (a)(urea ×2)/NO )0.75[feed composition to SCR reactor;urea (27ppm),HNCO (92ppm),NH 3(97ppm)],(b)(urea ×2)/NO )0.85[feed composition to SCR reactor;urea (42ppm),HNCO (142ppm),NH 3(149ppm)].

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Recei V ed for re V iew February17,2006

Re V ised manuscript recei V ed May1,2006

Accepted May11,2006

IE060199+ Ind.Eng.Chem.Res.,Vol.45,No.15,20065267