(189e) Modeling and Optimization of Coupled Fe- and Cu- Based Selective Catalytic Reduction Monolithic Catalyst

Modeling and Optimization of Coupled
Fe- and Cu- based Selective Catalytic Reduction Monolithic Catalyst

Bijesh
M. Shakya, Michael P. Harold[1],
and Vemuri Balakotaiah[2]

Department of Chemical & Biomolecular
Engineering

University of Houston
77204-4004

Abstract

Fe- and Cu-exchanged zeolites are
two of the leading catalysts for the selective catalytic reduction (SCR) of NO_x
with NH₃ under lean conditions. It is established that Cu-exchanged
SCR catalysts exhibit higher activity than Fe- based catalysts at lower
temperatures (<350^oC) and are comparatively less sensitive
towards NO/NOx ratio. However, at higher temperatures (>400^oC), NOx
conversion in Cu- catalyst is lower compared to Fe- catalyst due to higher rate
of undesired ammonia oxidation reaction. This observation that Cu- and Fe-
catalysts give higher activity at lower and higher temperatures, respectively, has
led to the concept of combined Fe- and Cu-based SCR catalysts [1].
Particularly, two configurations (dual-brick and dual-layer) have been investigated
experimentally [1], both of which have shown to yield higher NOx conversion
over wider temperature range compared to individual catalyst. However, an
in-depth comparison between dual-brick and dual-layer configuration has not
been analyzed in the literature. In this study, we use recently developed
kinetic models for Fe-ZSM5 and Cu-CHA SCR catalysts [2] to perform detailed
analysis of brick and layered configuration of combined catalysts [3].

 We use conventional 1D+1D mathematical model
of catalytic monolith that accounts for the diffusion of species in the
washcoat phase. The species diffusion in the washcoat plays crucial role in
determining the optimal configuration of the combined system. Simulations were
performed to identify temperature range over which diffusional limitation
becomes important. Fig 1(a) shows the comparison of standard SCR reaction (NH₃+NO+O₂)
on Cu-CHA and Fe-ZSM5 [2]. The dashed lines represent the conversion when the
washcoat diffusivities are increased by a factor 2. It can be seen that
washcoat diffusion for standard SCR reaction is important in the temperature
range of 180-300^oC and 290-470^oC for Cu-CHA and Fe-ZSM5
catalysts respectively. Above these temperature ranges the system is
essentially in external mass transfer controlled regime. To compare the
performance of different configurations of dual-layer and brick under standard
SCR reaction conditions, we define a function ?f? which is the normalized
square of difference between calculated conversion and maximum measured
conversion i.e. maximum of conversion obtained by Cu-CHA and Fe-ZSM5 (circled
data points in Fig 1(a)). Fig 1(b) shows the computed f-values for different
composition of Fe- and Cu- catalyst both dual-layer and dual-brick. In all
cases, either Fe- layer is coated on top of Cu- (dual layer) or Fe- brick is
followed by Cu- brick (dual brick) [2]. It can be seen that dual-brick
performance is always better than that of dual-layer. This behavior can simply
be explained in term of diffusional limitation that is created in the dual-layer
catalyst. As we can see from Fig 1(a) diffusional limitations for standard SCR
reaction arises in the temperature range of 180-350^oC in which Cu-
catalyst outperforms Fe-. As a result, the underlying Cu layer in the dual-layer
catalyst experiences severe diffusional limitation lowering the conversion.

Fig 1
(a) Comparison of model predicted and experimental NO conversion during the
standard SCR reaction on Fe-ZSM5 and Cu-CHA catalyst for different values of
diffusivity ratios (λ=D_f/D_e) (b) Comparison of dual
layer and dual brick configuration of Fe-ZSM5 and Cu-CHA catalyst for different
Fe-ZSM5 content (volume fraction) during standard SCR reaction Condition: 500
ppm NO, 500 ppm NH₃, 5% O₂ and 2% H₂O  

            Finally,
we also analyzed the impact of feed composition on the performance of combined
system. Based on these analyses, an optimal loading of Fe- and Cu- washcoat is
identified for the case of dual-layer and dual-brick configuration.

References:

1.     
P.S. Metkar, M.P. Harold and V. Balakotaiah,
Appl Catal B: Environ, 111?112 (2012) 67

2.     
P.S. Metkar, M.P. Harold and V. Balakotaiah,
Chem Eng Sci, 87 (2013) 51

3.     
B.M. Shakya, M.P. Harold and V. Balakotaiah, in
preparation