CATALYSIS, KINETICS AND REACTION ENGINEERING
Chinese Journal of Chemical Engineering, 21(8) 835—843 (2013)
DOI: 10.1016/S1004-9541(13)60559-5
Transfer and Reaction Performances of Selective Catalytic Reduction
of N2O with CO over Monolith Catalysts*
DAI Chengna (代成娜), LEI Zhigang (雷志刚)**, WANG Yuli (王玉丽), ZHANG Runduo
(张润铎) and CHEN Biaohua (陈标华)
State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing
100029, China
Abstract This work tries to identify the relationship between geometric configuration of monolith catalysts, and
transfer and reaction performances for selective catalytic reduction of N2O with CO. Monolith catalysts with five
different channel shapes (circle, regular triangle, rectangle, square and hexagon), was investigated to make a comprehensive comparison of their pressure drop, heat transfer Nu number, mass transfer Sh number and N2O conversion. It was found that monolith catalysts have a much lower pressure drop than that of traditional packed bed, and
for monolith catalysts with different channel shapes, pressure drop decreases in the order of regular triangle > rectangle > square > hexagon > circle. The order of Nu is in regular triangle > rectangle ≈ square > hexagon > circle,
similar to that of Sh. N2O conversion follows the order of regular triangle > rectangular ≈ square ≈ circle > hexagon.
The results indicate that chemical reaction including internal diffusion is the controlling step in the selective catalytic reduction of N2O removal with CO. In addition, channel size and gas velocity also have influence on N2O
conversion and pressure drop.
Keywords selective catalytic reduction, N2O conversion, momentum transfer, heat transfer, mass transfer, monolith catalysts, mathematical modeling
1
INTRODUCTION
As one strong greenhouse gas, N2O can make the
global warming because it can adsorb and emit
long-wave (infrared) radiation in a planetary atmosphere, and has a very long atmospheric lifetime which
may reach up to 120 years [1-3]. The global warming
potential of N2O is as 310 times as that of CO2. N2O
also should be responsible for the destruction of ozone
layer because it breaks down in the stratosphere by
photolysis, affects the content of hydrogen, nitrogen,
chlorine and bromine, and leads to stratospheric ozone
destruction. It is well-known that N2O emission is
caused by both natural and human-related sources, such
as soil fertilization, livestock manure, combustion and
certain other industrial processes including the chemical industry in adipic acid and nitric acid plants [4-6].
As the regulation for the N2O emission becomes
more and more strict, much effort has been focused on
the development of more efficient N2O removal technology. A number of methods have potentially been
applied to the removal of N2O. In addition to thermal
decomposition [7], the selective adsorption [8], application of plasma technology [9], catalytic destruction
[10] or selective catalytic reduction (SCR) [11-13] is
also employed. The selective catalytic reduction (SCR),
using reductants such as light hydrocarbons (C1-C3),
ammonia (NH3) and carbon monoxide (CO), is an
effective way to minimize the impact of N2O on the
environment, and it has been widely investigated over
iron-containing zeolites [14-20]. Nitrogen (N2) and
helium (He) are two kinds of widely used carrier gas
for SCR of N2O with CO.
On the other hand, structured catalysts and reactors as a new sustainable process intensification technology have been investigated in recent years, which
are divided into monoliths, open cross-flow structures,
foams, catalytic membranes and many others. Monolith
structure is becoming widely used in gas-solid or multiphase catalytic reactions because of its unique advantages compared to traditional pellet packed-bed reactor,
such as lower pressure drop, much higher surface area
per unit volume of the bed and minimum axial dispersion stemming from the unique structured multichannel
configuration of monoliths [21-23]. Ceramic [24, 25],
metallic [26, 27], and cordierite [28] are often employed
as the support materials of the monolith honeycomb
reactor consisting of hundreds of parallel channels.
Due to its high mechanical strength, high thermal stability and low cost, cordierite exhibits a number of
advantages over these alternative support materials in
monolith honeycomb reactors for controlling N2O
emission from cars. Tronconi et al. [29] established oneand two-dimensional steady-state isothermal mathematical models of monolith reactors for SCR of NOx
by NH3. Bhattacharya et al. [30, 31] studied the mass
transfer coefficient in washcoated monoliths by 2D
model. A few researches on SCR of NO with NH3
using monolith catalysts have been reported [32-36].
However, the study on SCR of N2O removal with CO
using monolith catalysts has rarely been reported.
Received 2012-01-08, accepted 2012-08-04.
* Supported by the National Natural Science Foundation of China (21121064, 21076008), and the Projects in the National Science & Technology Pillar Program During the 12th Five-Year Plan Period (2011BAC06B04).
** To whom correspondence should be addressed. E-mail: [email protected]
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Chin. J. Chem. Eng., Vol. 21, No. 8, August 2013
In this work, it is assumed that SCR takes place
within the whole cordierite matrix which can be taken
on as a porous medium. Monolith catalysts are the key
technology of SCR process, and the configuration of
monolith catalysts has prominent influence on transfer
and reaction performances. Five channel shapes (circle, regular triangle, rectangle, square and hexagon) of
monolith catalyst are introduced. The aim of this work
is to establish the mathematical models to explicitly
identify the relationship of monolith catalysts between
geometric configuration and performances of transfer
and reaction for SCR of N2O removal with CO by
means of CFD (computational fluid dynamics) method
so as to save a lot of labor and time in designing effective and efficient systems. Therefore, this work may
arouse common interest for the environmentally catalytic chemists who are engaged in the field of poisonous gas removal using monolith catalysts to select the
optimum monolith configuration from the viewpoint
of chemical reaction engineering for further research.
to establish a three-dimensional (3D), rigorous steadystate mathematical model so as to obtain the temperature and concentration distribution and further quantify the transfer and reaction performances inside the
monolith channel based on the following assumptions:
(1) uniform gas velocity, temperature and concentration at the entrance;
(2) normal pressure at the outlet (101325 Pa);
(3) symmetrical boundary on the axis and plane
of symmetry, and no slip condition on the inner wall
of the channel;
(4) homogeneous reaction and heat radiation in
the bulk phase are ignored;
(5) the porous catalytic layer of Fe-ZSM-5 is
homogeneous, isotropic and saturated with a single
phase fluid;
(6) only viscous flow (Darcy flow) is allowed in
the porous medium in case of fluid passing through
porous media.
2.2
2
2.1
MODEL DESCRIPTIONS
Physical model and assumptions
Monolith catalysts with cordierite as the support
material and Fe-ZSM-5 catalyst as catalytic actives
are used in this work for the selective catalytic reduction of N2O with CO. The main reaction is considered
and written as follows [37]:
N 2 O + CO ⎯⎯
→ N 2 + CO 2
(1)
In principle, the whole reaction process involves
internal diffusion, external diffusion and reaction, as
shown in Fig. 1. All channels of the monolith are
equivalent with uniform flow distribution so that monolith catalysts can be viewed as consisting of a number
of the same building block: a single channel with
symmetrical peripheral walls. The CFD tool was used
Figure 1
Governing equations
2.2.1 Gas phase
With the above assumptions, a set of governing
equations including mass, momentum and energy
balances for describing gas phase and porous medium
phase for the extruded cordierite matrix are summarized as below.
continuity equation:
∇ ⋅ ( ρ u) = 0
(2)
momentum balance equation:
∇ ⋅ ( ρ u × u) = −∇p + ∇ ⋅ ( μ (∇u + (∇u)T ))
energy balance equation:
∇ ⋅ ( ρ c p uT ) = ∇ ⋅ (λg ∇T )
(3)
(4)
mass balance for species:
∇ ⋅ ( ρ uwi ) = ∇ ⋅ ( ρ Di ,carrier ∇wi )
(5)
Schematic representation of physical modeling domain for SCR of N2O removal with CO using monolith catalysts
837
Chin. J. Chem. Eng., Vol. 21, No. 8, August 2013
where i = N2O, CO, N2 or He, ρ is gas density, u is gas
velocity, μ is molecular viscosity, p is pressure, T is
temperature, cp is constant pressure heat capacity, λg is
gas thermal conductivity, and wi is mass fraction of
species i. The carrier gas is N2 or He.
2.2.2 Porous medium phase (cordierite matrix)
The porous media model was used for the cordierite zone where selective catalytic reduction of N2O
with CO takes place. The governing equations for porous medium phase are similar as those for gas phase,
except that there is an additional term on the right-hand
side of momentum, energy and mass balance equations
given as follows.
momentum balance equation:
∇ ⋅ ( ρ u × u) = S
(6)
where S is a momentum source term expressed as
[−(μ/a)u], and a ( = d p2ε 3 /[150(1 − ε )2 ] ) is the perme-
⎛ −62 × 106 ⎞
k N 2O = 29050 × T 0.2 exp ⎜
⎟
⎝ RT
⎠
where k is reaction rate constant, cN 2 O and cCO are
the molar concentrations of N2O and CO, respectively.
It is noted that the reaction order of CO is larger than
that of N2O due to the stronger N2O adsorption on the
surface sites. But it is less than unity, indicating that
N2O does not react completely from the gas phase, but
a small part comes from an adsorbed state.
All physical properties of pure gas components
were taken from the process simulation software
PROII (version 8.2) at the operating temperature and
pressure. The physical properties of gas mixture were
derived by using ideal gas mixing law, except that the
binary diffusion coefficients Di,carrier can be obtained
as follows [41]:
ability.
energy balance equation:
Di ,carrier =
∇ ⋅ ( ρ c p uT ) = ∇ ⋅ [λg ε + λs (1 − ε )∇T ] +
(− RN 2O )(−ΔH r )
(7)
where − RN 2O and ΔH r are the reaction rate and reaction heat, respectively.
mass balance for species:
∇ ⋅ ( ρ uwi ) = ∇ ⋅ ( ρ Di ,carrier ∇wi ) + Ri
(8)
where Ri is the reaction or formation rate of specie i.
2.3
Model parameters
The rate equation for main reaction is necessary
to be determined beforehand. It is generally thought
that this reaction follows the Eley-Rideal mechanism
over Fe-ZSM-5 catalyst, which, however, has two
kinds of possible reaction schemes. One consists of
two elementary steps expressed by Eqs. (9) and (10)
[38, 39], where CO from the gas phase induces the fast
removal of surface atomic oxygen (often referred to as
scavenging mechanism), while the other also consists
of two elementary steps expressed by Eqs. (11) and
(12) [39, 40], where the adsorbed CO species reacts
with N2O to form products.
N 2 O + ∗ ⎯⎯
→ N2 + O ∗
(9)
CO + O ∗ ⎯⎯
→ CO 2 + ∗
(10)
CO + ∗ ⎯⎯
→ CO ∗
(11)
N 2 O + CO ∗ ⎯⎯
→ N 2 + CO 2 + ∗
(12)
In this work, for simplification, we adopted a rate
equation over Fe-ZSM-5 in a cordierite support, as
proposed by Debbagh et al. [37]:
0.7
RN 2O = k N 2 O cN0.52O cCO
(13)
(14)
4.36 × 10−5 T 1.5 (1/ M i + 1/ M carrier )
p(
Vi1/ 3
)
2
1/ 3
+ Vcarrier
0.5
(15)
where M is molecular mass and V is molecular diffusion volume. Since a large amount of mixture gas is
N2 or He, the composition dependency of binary diffusion coefficients can be neglected. In addition, the
physical properties of solid phase (cordierite) are assumed to be constant, with effective heat conductivity
λs = 3.2 W·m−1·K−1, porosity ε = 0.2, diameter of particles dp = 165 μm, and permeability a = 2.27×10−12 m2.
2.4
Numerical procedure
The Gambit software (version 2.3.16) was used
to mesh the monolith channel with hexagonal elements with the Cooper method. It was found that as
the grid number increases, N2O conversion firstly increases and then tends to be stable as the cell number
is above 0.9 million. Thus, in our later calculations the
total mesh number always exceeds 0.9 million. Then,
the mesh file was input into the FLUENT software
(version 6.3.21) in which the laminar model was selected (Re < 2000), 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. The residuals of energy, x-velocity,
y-velocity, z-velocity and mass balance for each species were set to be 10−6.
2.5
Modeling validation
For SCR of N2O removal with CO, only the experimental data obtained from the traditional pellet
packed-bed reactor reported in Ref. [37] are available.
Steady-state experiments of N2O reduction by CO
over steam-activated Fe-ZSM-5 catalyst were carried
out in an integral fixed-bed micro-reactor with diameter
of 4 mm, and the details can be found in Ref. [37]. The
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Chin. J. Chem. Eng., Vol. 21, No. 8, August 2013
calculated results of N2O conversion using the porous
medium model are compared with the experimental
data, as shown in Fig. 2, where the average diameter
of catalyst particles (in the range of 125-200 μm, as
in Ref. [37]) is 165 μm, the porosity of the pellet
packed-bed reactor is 0.33, and helium was used as
the carrier gas. The inlet gas velocity is 0.06 m·s−1 at
ambient pressure with a fixed inlet N2O concentration
(partial pressure 150 Pa) but varying the inlet CO
concentrations (from 20 to 300 Pa). In the calculation,
the pellet packed-bed reactor was treated as a whole
porous medium, and the governing equations include
momentum, energy, and mass balance equation as Eqs.
(6)-(8). It can be seen that the stimulation and experimental results agree very well except at high temperature. This may be due to the direct N2O decomposition which takes place during the experiments and
can be promoted at high temperature. Overall, the porous medium model shows good performance, and
thus is adopted in cordierite matrix zone when establishing the mathematical model of monolith catalysts.
3
Figure 2 N2O conversion for pellet packed-bed reactor
using He as carrier gas at a velocity of 0.06 m·s−1 and an
inlet partial pressure of N2O 150 Pa
■ 20 Pa (CO), exp.; ◆ 60 Pa (CO), exp.; ● 150 Pa (CO),
exp.; ▲ 300 Pa (CO), exp.; calculated results: solid lines
Figure 3
Table 1
Object
RESULTS AND DISCUSSION
Structural and operating parameters of monolith
catalysts for five kinds of channel shapes (i.e. circle,
regular triangle, rectangle, square and hexagon, as
shown in Fig. 3) with the same cross-sectional area,
wall thickness and height are listed in Table 1.
Five kinds of channel shapes with the same cavity area and wall thickness of monolith catalysts
Structural and operating parameters for five kinds of channel shapes of cordierite-based monolith catalysts
(substrate: cordierite; catalyst: Fe-ZSM-5; fluid medium: N2)
Effective heat
Height of
conductivity
bed/m
/W·m−1·K−1
Channel
total area
/mm2
Channel
cavity area
/mm2
Wall
thickness
①
tw /mm
Equivalent
Side length of
diameter of the empty channel
channel/mm
/mm
Inlet gas
velocity
/m·s−1
circle
3.2
0.8
49
36
0.23
6.77
6.77
2
regular triangle
3.2
0.8
49
34.34
1.0
5.14
8.91
2
square
3.2
0.8
49
36
1.0
6
6
2
rectangle
3.2
0.8
49
35.71
1.0
5.81
7.57×4.72
2
hexagon
3.2
0.8
49
36.84
1.0
6.52
3.77
2
Inlet gas
composition/%
(by volume)
N2O 0.148,
CO 0.148,
He/N2
99.704
For circle channel the wall thickness (0.23 mm) means that the thickness of the thinnest part between two adjacent channels is
0.23 mm. The corresponding distance is uniformly 1 mm for other geometry.
①
Chin. J. Chem. Eng., Vol. 21, No. 8, August 2013
839
Figure 4 Concentration and temperature profiles on the intersection of z = 0 mm (the middle cross section) of five kinds of
monolith channel shapes with inlet temperature of 975 K
(a) square; (b) circle; (c) regular triangle; (d) rectangle; (e) hexagon
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Chin. J. Chem. Eng., Vol. 21, No. 8, August 2013
As an example, Fig. 4 shows the N2O concentration
and temperature profiles on the cross section at z = 0
mm (z ranged from −400 mm to 400 mm) for the five
kinds of monolith channel shapes with inlet temperature of 975 K. It can be seen that the concentration and
temperature gradients in gas bulk phase become larger
near the channel wall.
3.1
Pressure drop
Pressure drop represents the momentum transfer
and is important in determining the energy losses [35].
Fig. 5 shows the predicted pressure drop using CFD
simulation between the inlet and outlet of monolith
channels, as well as the traditional pellet packed-bed
reactor with spherical Fe-ZSM-5 particles of diameter
dp = 2.54 mm, porosity ε = 0.33 under the same operating conditions. Evidently, pressure drop in the packedbed reactor is much greater than that in the monolith
catalysts by three orders of magnitude, which is consistent with the conclusion from previous studies [35, 42, 43].
It can be seen that at a given inlet temperature pressure
drop is in the order of regular triangle > rectangle >
square > hexagon > circle for monolith catalysts, which,
however, corresponds positively to the circumference
of the flux areas of monolith channels. That is to say,
the larger the circumference of monolith channel, the
greater pressure drop.
Tw =
∫w TdS ,
∫w dS
T =
∫Ω u ρ cvTdV
∫Ω u ρ cv dV
(17)
where cv is constant volume heat capacity of gas, w
represents the wall between bulk phase and catalyst
layer, Ω represents the bulk phase flow section, and S
and V are the corresponding integral surface and volume, respectively.
Figure 6 shows that the overall Nu does not
change significantly with the increase of reaction
temperature. Nu of monolith catalysts is in the order
of regular triangle > rectangle ≈ square > hexagon >
circle under the same operating condition, and regular
trangle channel exhibits the best heat transfer performance. Like the pressure drop, the smaller the circumference of monolith channel, the larger Nu number.
Herein, only the overall Nusselt number was discussed, but in the future work the influence of axial
direction position on the local Nu should also be taken
into account.
Figure 6 Overall Nusselt number Nu for five kinds of
monolith channel shapes
□ square; ○ circle; △ regular triangle; ◇ rectangle;
hexagon
3.3
Figure 5 Pressure drop for five kinds of monolith channel
shapes
■ pellet packed-bed; □ square; ○ circle; △ regular triangle;
◇ rectangle;
hexagon
3.2
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 [47, 48]
Sh =
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 was computed from the temperature profiles as follows [44-46]:
Nu =
de
(Tw − T
∂T
) ∂x
(16)
x →gas-solid interface
where Tw is the average temperature of the interface
between bulk phase and catalyst layer, and T is the
volume-average temperature in the gas bulk phase.
(c
N2O w
where cN2 O
w
de
− cN 2 O
b
)
∂cN 2O
∂x
(18)
x → gas-solid interface
is the interface concentration between
bulk phase and catalyst layer, and cN 2 O
b
is vol-
ume-average concentration in the gas bulk phase.
cN 2 O
w
=
∫w cN O dS ,
∫w cdS
2
cN 2 O
b
=
∫Ω ucN O dΩ
∫Ω udΩ
2
(19)
As expected, the overall Sh exhibits the similar
trend as Nu in terms of analogy between heat and
mass transfers (see Fig. 7). Sh of monolith catalysts
Chin. J. Chem. Eng., Vol. 21, No. 8, August 2013
841
N2O conversion is applied to represent the reaction
performance of monolith catalysts and traditional pellet packed-bed reactor. For traditional pellet packed-bed
reactor, the results are obtained by using FLUENT
software with porous medium model as mentioned
above. As shown in Fig. 8, N2O conversion increases
as the inlet gas temperature increases. For monolith
catalysts, N2O conversion is in the order of regular
triangle > rectangular ≈ square ≈ circle > hexagon, which
is the results of joint contributions of reaction and transfer, and can be attributed to two aspects: one corresponds to the order of heat and mass transfers (i.e. Nu
and Sh) as mentioned above, and the other is that, at a
given reaction kinetics, the more the fraction of catalyst
volume taken up by porous medium region (the order
of which is consistent with that of N2O conversion),
the higher N2O conversion. In addition, it was found
that N2O conversion of monolith catalysts with channel shapes of circle, square, and rectangle, loading the
same amount of catalysts, is almost identical, although
Sh and Nu for circle channel are apparently lower than
those for the other two channel shapes. This indicates
that chemical reaction including internal diffusion is the
controlling step in the SCR of N2O removal with CO.
It is obvious that N2O conversion in traditional
pellet packed-bed reactor is much higher than that in
monolith catalysts under the same operating condition
because a much larger amount of pellet catalysts is
loaded into the reactor than monolith catalysts. Therefore, the distinct advantage of monolith catalysts over
traditional pellet packed-bed reactor lies in their very
low pressure drop. It is noted that the gas mixture
containing N2O from the industry is often near normal
pressure and thus a low pressure drop is important for
the scavenging reactor. In this case it is advisable to
select monolith catalysts for a large scale of N2O removal in industry. Moreover, among monolith catalysts, circular channel possesses the lowest pressure
drop with a slightly lower N2O conversion.
On the other hand, the channel size is another
important structural parameter influencing N2O conversion. As shown in Fig. 9, as the diameter of circle
channel decreases, N2O conversion increases rapidly
provided that other structural parameters are kept constant. But pressure drop also increases at the same
time, and the approximate channel size is in the range
of 3-5 mm under the investigated conditions. Otherwise, pressure drop will increase drastically at less
than 3 mm. Therefore, a compromise between N2O
conversion and pressure drop should be considered in
the design and optimization of monolith catalysts. The
plot of N2O conversion vs. gas velocity for circle
shape with the diameter of 7 mm at 1050 K is shown
in Fig. 10. It can be seen that N2O conversion increases
rapidly as the gas velocity decreases. That is to say,
small channel size and low gas velocity are favorable
for achieving a high N2O conversion and decreasing
the reaction temperature. Anyway, monolith catalysts
Figure 8 N2O conversion for five kinds of monolith channel
shapes
▲ pellet packed-bed; □ square; ○ circle; △ regular triangle;
◇ rectangle;
hexagon
Figure 9 Influence of channel size on N2O conversion and
pressure drop for circular channel at 1050 K
○ N2O conversion; ● pressure drop
Figure 7 Overall Sherwood number Sh for five kinds of
monolith channel shapes
□ square; ○ circle; △ regular triangle; ◇ rectangle;
hexagon
also follows the order of regular triangle > rectangle ≈
square ≈ hexagon > circle under the same operating
condition, and regular triangle channel brings about
the maximum Sh and Nu among all channel shapes
investigated in this work. Similarly, the local Sh along
the axial direction should be further discussed in the
future work.
3.4
N2O conversion
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Chin. J. Chem. Eng., Vol. 21, No. 8, August 2013
cN 2 O
cN 2 O
Figure 10 Influence of gas velocity on N2O conversion for
circular channel with the diameter of 7 mm at 1050 K
have more freedom for chemical engineers to tailor
the desirable transfer and reaction performances by
adjusting monolith channel shape, channel size, gas
velocity and so on.
4
CONCLUSIONS
A computational fluid dynamic model is applied
to simulate the SCR of N2O with CO as reductant in
the monolith reactor with different channel shapes, i.e.
circle, regular triangle, rectangle, square and hexagon.
It is found that under the same operating condition,
pressure drop in monolith catalysts is much lower than
that in traditional pellet packed-bed reactor by three
orders of magnitude, and for five kinds of monolith
channel shapes it follows the order of regular triangle >
rectangular > square > hexagon > circle.
For monolith catalysts, Nu and Sh are in the order
of regular triangle > rectangular ≈ square > hexagon >
circle. N2O conversion depends on the joint contributions of reaction and transfer while mainly controlled
by chemical reaction, and finally is in the order of
regular triangle > rectangular ≈ square ≈ circle > hexagon.
Among five kinds of monolith catalysts with different
channel shapes, circular channel possesses the lowest
pressure drop with a slightly lower N2O conversion.
The channel size is another important structural
parameter influencing N2O conversion. The smaller
size of monolith reactors leads to higher N2O conversion. Monolith catalysts may be more suitable for the
processes where low pressure drop is required, although
traditional packed-bed reactor is superior in N2O conversion. N2O conversion in monolith catalysts can be
improved by decreasing the channel size. In summary,
monolith catalysts can be referred to as “designer
catalysts”.
NOMENCLATURE
a
c
cv
permeability, m2
molar concentration, kmol·m−3
constant volume heat capacity of gas, J·kg−1·K−1
b
w
Di,carrier
de
dp
ΔHr
k N 2O
M
Nu
p
Δp
Re
RN 2O
S
Sh
T
T
u
V
Vi
wi
ε
λg
μ
ρ
Ω
volume average concentration of N2O in the gas bulk phase,
kmol·m−3
interface concentration of N2O between bulk phase and catalyst layer, kmol·m−3
binary diffusion coefficient of different carrier gases, m2·s−1
equivalent diameter of the channel, mm
diameter of particles, mm
reaction heat, J·mol−1
reaction rate constant of N2O, (kmol·m−3)−0.2·s−1
molecular mass, kg·mol−1
Nusselt number ( Nu = α d p / λ )
pressure, Pa
pressure drop, Pa
Reynolds number ( Re = d e u ρ / μ )
reaction rate of N2O, kmol·m−3·s−1
momentum source term
Sherwood number ( Sh = kd p / D )
temperature, K
bulk average temperature, K
gas velocity, m·s−1
volume, m3
molecular diffusion volume of species, m3·mol−1
mass fraction of species
porosity
thermal conductivity, W·m−1·K−1
molecular viscosity, kg·m−1·s−1
gas density, kg·m−3
bulk phase domain
Subscripts
b
e
g
i
w
bulk phase
equivalent
gas phase
species (N2O, CO, N2 or He)
wall
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