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Ind. Eng. Chem. Res. 2006, 45, 5258-5267
Modeling of Monolith Reactor Washcoated with CuZSM5 Catalyst for Removing NO from Diesel Engine by Urea Joon Hyun Baik, Sung Dae Yim,† and In-Sik Nam* 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
Young Sun Mok Department of Chemical Engineering, Cheju National UniVersity, 66 Jejudaehakno, Jeju 690-756, Korea
Jong-Hwan Lee, Byong K. Cho, and Se H. Oh General Motors R&D and Planning Center, Warren, Michigan 48090-9055
Two sets of reaction kinetics based upon the experimental data independently obtained for NH3-SCR and urea decomposition reactions over CuZSM5 catalyst 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 CuZSM5 catalyst 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 containing 20/30 mesh powder type of CuZSM5 catalyst. 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, NH3 and urea slips, and the temperature of the thermal decomposition reactor on the NO removal activity. Introduction Selective catalytic reduction (SCR) by NH3 is generally recognized as the most effective method for reducing the emission of nitrogen oxide (NOx) from stationary sources.1 Recently, urea-SCR, the selective catalytic reduction of NOx 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 NOx emissions originating from heavy-duty diesel engines.2,3 Baik et al. reported that the removal of NOx by urea-SCR over the CuZSM5 catalyst was competitive to that by NH3-SCR, indicating urea can be effectively utilized in an SCR reactor system as a reducing agent for NOx from automotive engine.4 The urea-SCR process over the CuZSM5 catalyst is mainly based upon the following four reactions: (1) urea thermal decomposition, (2) HNCO hydrolysis, (3) NO reduction, and (4) NH3 oxidation reaction. Urea is thermally decomposed into ammonia and isocyanic acid, and then the isocyanic acid formed by reaction 1 is easily hydrolyzed on the catalyst surface, producing an additional mole of ammonia and carbon dioxide by reaction 2.2,5-7 Consequently, the complete decomposition of one mole of urea produces two moles of ammonia and one mole of carbon dioxide. NH3 is a direct reductant for SCR reaction by reaction 3, while urea is an indirect reducing agent to produce NH3 for the overall deNOx reaction. On the other hand, the NH3 oxidation reaction can occur at a reaction * To whom correspondence should be addressed. Tel.: 82-54-2792264. Fax: 82-54-279-8299. E-mail:
[email protected]. † Present address: Fuel Cell Research Center, Korea Institute of Energy Research (KIER), 71-2 Jang-Dong, Daejeon 305-343, Korea.
temperature >350 °C and mainly produce N2, particularly over CuZSM5 catalyst by reaction 4. It should be noted that N2 is the main product by NH3 oxidation reaction over the Cu ionexchanged zeolite.8-10
NH2-CO-NH2 f NH3 +HNCO
(1)
HNCO + H2O f NH3 + CO2
(2)
4NO + 4NH3 + O2 f 4N2 + 6H2O
(3)
4NH3 + 3O2 f 2N2 + 6H2O
(4)
However, unreacted NH3 decomposed 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 minimum NH3 emissions (e.g., urea/NO feed ratio) generally involves a tradeoff between NO removal activity and NH3 slip.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 CuZSM5 catalyst 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
10.1021/ie060199+ CCC: $33.50 © 2006 American Chemical Society Published on Web 06/22/2006
Ind. Eng. Chem. Res., Vol. 45, No. 15, 2006 5259 Table 1. Physicochemical Properties of CuZSM5 Catalyst Prepared CuZSM5 powder metal (Cu) content Si/Al ion exchange level BET surface area a
2.9 wt % 14 97% 337 m2/g
washcoated honeycomb reactor porositya washcoats thickness catalyst weight CPSI
0.71 27 µm 19.7 wt % 200
Washcoats (CuZSM5 + alumina binder, without codierite).
position and NH3-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 incorporated 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 Figure 1. Catalytic thickness of the monolith reactor washcoated with CuZSM5 catalyst by a SEM image.
model based upon the intrinsic reaction kinetics of the catalyst employed.8 A 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 >1100 K has been extensively investigated.12,13 In the present study, a kinetic model for urea-SCR over CuZSM5 catalyst requiring two reaction kinetics, urea decom-
Figure 2. Schematic flow diagram for NH3 (or urea) SCR reactor system.
Catalyst Preparation. The CuZSM5 catalyst was prepared by wet ion-exchange method using 0.01 M of copper acetate solution at room temperature, as extensively described elsewhere.14 The ZSM5 type zeolite catalyst with Si/Al molar ratio of 14 was obtained from Tosoh Co. (HSZ-830NHA). The monolith reactors were prepared by washcoating CuZSM5 catalyst slurry with an alumina binder (NYACOL AL20DW) on the 200 CPSI 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 2.8 µm 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 reasonably uniform throughout the channels of the monolith reactor.
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Listed in Table 1 are the physicochemical properties of the catalyst and the monolith washcoated with the catalyst prepared. The catalysts were dried at 110 °C for 12 h and calcined in air at 500 °C for 5 h before and after washcoating. They were again pretreated in air at 500 °C for 1 h prior to each experiment. Reaction System and Experimental Procedure. For an intrinsic kinetic study of NH3-SCR, the NO removal activity of the CuZSM5 catalyst has been evaluated in a packed-bed flow reactor system with 1 g of 20/30 mesh size catalyst, where the effect of mass transfer and pressure drop may be ignored, as shown in Figure 2.15 A washcoated monolith reactor with 25 mm × 25 mm × 25 mm 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 (500 ppm), NH3 (500 ppm), O2 (5%), H2O (10%), and N2 (balance) was supplied through mass flow controllers (Brooks 5850E). The kinetic data from a packedbed reactor were collected over a wide range of reaction temperatures (150-500 °C) and reactor space velocities (50 000400 000 h-1). For the simultaneous study of urea decomposition and ureaSCR reactions, an additional reactor system for urea decomposition 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 SUS316 for 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 NH3 were analyzed by an online chemiluminescent NO-NOx analyzer (Thermo Electron Co., model 42H) and NDIR-type NH3 analyzer (Rosemount Analytical, model 880A), respectively. To analyze the urea remaining unconverted and the HNCO produced by the decomposition 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 reaction 3, the reaction kinetics of SCR of NO by NH3 has been examined. Two primary reactions, NO reduction reaction 3 and NH3 oxidation reaction 4, are assumed to mainly occur during the SCR process over CuZSM5 catalyst.10 A maximum NO conversion was generally observed as the reaction temperature was varied over the range 200500 °C for the present catalytic system because of the ammonia oxidation reaction.8 Hence, the intrinsic reaction kinetics for the SCR process has been derived on the basis of reactions 3 and 4 to particularly describe the maximum NO conversion and NH3 slip. In the present study, a dual-site catalysis Langmuir-Hinshelwood-Hougen-Watson (LHHW) mechanism, where the reaction occurs between adsorbed NO and NH3 molecules on two distinct reaction sites such as NH3 adsorption 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.17 The reaction rate equations can be derived on the basis of Hougen-Watson formalism assuming the surface reaction is the rate-determining step.
-
k1CNOCNH3 dCNO ) dτ (1 + KNOCNO)(1 + KNH CNH ) 3
3
(k1 ) kNOKNOKNH3CO2) (5) -
dCNH3 dτ
)
k1CNOCNH3 (1 + KNOCNO)(1 + KNH3CNH3) k2CNH3 (1 + KNH3CNH3)
+
(k2 ) kNH3KNH3CO2) (6)
Rearranging in terms of conversions of NO and NH3, 0 (1 - XNO)(1 - XNH3) k1CNH dXNO 3 ) 0 dτ [1 + KNOC0NO(1 - XNO)][1 + KNH3CNH (1 - XNH3)] 3
(7) dXNH3 dτ
) k1C0NO(1 - XNO)(1 - XNH3)
0 [1 + KNOC0NO(1 - XNO)][1 + KNH3CNH (1 - XNH3)} 3
+
k2(1 - XNH3) 0 1 + KNH3CNH (1 - XNH3) 3
(8)
where KNO and KNH3 are the adsorption equilibrium constants and kNO and kNH3 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 NH3 slip 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
dCUrea ) -k3CUrea dτ
(9)
dCHNCO ) k3CUrea - k4CHNCOCH2O dτ
(10)
Equations 9 and 10 were 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 ureaSCR process over the CuZSM5 catalyst, 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 reaction 1, HNCO hydrolysis by reaction 2, and NO reduction by reaction 3, eqs 5, 9, and 10, respectively, have been employed. On the other hand, the mass balance equation for NH3 oxidation by reaction 4 based upon the formation and the consumption of NH3 from the decomposition of urea and the reduction of NO, respectively, has been rewritten:
Ind. Eng. Chem. Res., Vol. 45, No. 15, 2006 5261
-
dCNH3 dτ
)
k1CNOCNH3 (1 + KNOCNO)(1 + KNH3CNH3) k2CNH3
+
-k3CUrea - k4CNHCOCH2O (11)
(1 + KNH3CNH3)
It should be noted that the homogeneous noncatalytic oxidation of NH3 during 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,
dXUrea ) k3(1 - XUrea) dτ
the axial diffusion term, d2Ci/dx2, 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 16.18 The boundary conditions for eq 16 are
dCi ) 0 at y ) 0 dy Ci ) Cis at y ) R A material balance at axial position x of the honeycomb reactor over the external gas film yields
km,i(Cib - Cis) ) De,i
(12)
dXHNCO k3C0Urea(1 - XUrea) )+ k4CH2O(1 - XHNCO) 0 dτ CHNCO (13) - XNO)(1 - XNH3) dXNO ) 0 0 dτ [1 + KNOCNO(1 - XNO)][1 + KNH3CNH (1 - XNH3)] 3 (7)
dτ
) k1C0NO(1 - XNO)(1 - XNH3)
0 [1 + KNOC0NO(1 - XNO)][1 + KNH3CNH (1 - XNH3)] 3
k2(1 - XNH3) 0 1 + KNH3CNH (1 - XNH3) 3
-
k3C0Urea(1 - XUrea) 0 CNH 3
-
k4CH2OCNHCO(1 - XHNCO) 0 CNH 3
+
(14)
where k4CH2O can be treated as a constant (k4′) because of the relatively high feed concentration of H2O, 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
dCib ) km,iAe(Cib - Cis) -u dx
(15)
with the initial conditions Cib ) Cib,0 at x ) 0. At any axial position x along the channel of the monolith, a material balance over the catalyst layer of thickness dy yields the following: 2
De,i
d Ci dy
2
) -ri
(16)
Equation 16 describes the diffusion-reaction interaction for reactants within the pore of washcoats and catalyst. Note that
dCi dy
s
(17)
The external mass-transfer coefficient km can be estimated by the Hawthorn correlation, eq 17, which can well describe the developing laminar flow in rectangular channels.18-20
d 0.45 Sh ) B 1 + 0.095 Pe L
(
0 (1 k1CNH 3
dXNH3
( ) )
(18)
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 CuZSM5 has 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 5 and 6, were estimated by fitting the model predictions of NO and NH3 conversions to the experimental data at each temperature of interest. Two nonlinear ordinary differential equations, eqs 7 and 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 19 from the experimental and calculated NO and NH3 conversions. A regression routine for the minimization uses the Marquardt algorithm.21 n
Minimize
(Xi exp - Xical)2 ∑ i)0
(19)
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 15 with initial values and by partial differential equations and eq 16 with nonlinear boundary conditions. The ordinary differential equations have been solved by the RungeKutta 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 deNOx performance of the powder form of CuZSM5 catalyst (20/30 mesh) by urea-SCR over the fixedbed 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 NH3 and HNCO,5 the
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Figure 3. Effect of the temperature of the thermal urea-decomposition reactor for NO reduction by urea over CuZSM5 catalyst; SV ) 100 000 h-1.
NO removal activity over CuZSM5 catalyst by urea-SCR was close to the activity observed by NH3-SCR. This clearly indicates that HNCO produced in the thermal decomposition reactor successfully further reacts with H2O to produce NH3 in
the catalytic reactor, thereby making urea as effective as NH3 for NO reduction, when the temperature of the thermal decomposition reactor is >350 °C. Again, the hydrolysis reaction is a fast reaction.5 It should be noted that the thermal decomposition of urea and the hydrolysis of HNCO simultaneously occur in the SCR reactor containing CuZSM5 catalyst. However, when the temperature of the thermal urea decomposition reactor is 150 °C, where urea is not completely decomposed, the catalyst exhibits relatively low NOx conversion, particularly at reaction temperatures
