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Chemical reactions and kinetics of a low-temperature watergas shift reaction heated by microwaves
Wei-Hsin Chen a,*, Tsung-Chieh Cheng b, Chen-I Hung b, Bo-Jhih Lin a
aDepartment of Greenergy, National University of Tainan, Tainan 700, Taiwan, ROCbDepartment of Mechanical Engineering, National Cheng Kung University, Tainan 701, Taiwan, ROC
a r t i c l e i n f o
Article history:
Received 20 July 2011
Received in revised form
30 August 2011
Accepted 14 September 2011
Available online 20 October 2011
Keywords:
Water gas shift reaction (WGSR)
Microwave irradiation
CueZn-based catalyst
Chemical kinetics
Electromagnetic fields
Exothermic reaction
* Corresponding author. Tel.: þ886 6 2605031E-mail address: weihsinchen@gmail.com
0360-3199/$ e see front matter Copyright ªdoi:10.1016/j.ijhydene.2011.09.089
a b s t r a c t
Chemical reaction characteristics of a water gas shift reaction (WGSR) heated by micro-
waves are investigated experimentally where a CueZn-based catalyst is employed. The
experiments indicate that the performance of the low-temperature shift reaction (LTSR)
increases with increasing temperature and steam/CO molar ratio. The effect of increasing
temperature on CO conversion with microwave heating is contrary to that with conven-
tional heating where the thermodynamic equilibrium dominates the LTSR in the latter. It
follows that the reactions of the LTSR with microwave heating are governed by chemical
kinetics. To further figure out the reaction phenomena inside the catalyst bed with
microwave irradiation, a new chemical kinetic model accounting for the behavior of the
LTSR are developed and the reaction phenomena are simulated numerically. In the
numerical method, the continuity, momentum, energy and species equations as well as
the electromagnetic fields are simultaneously solved. It is of interest that the temperature
distribution in the catalyst bed is nearly uniform due to the exothermic reaction featured.
When the thermal behavior of the LTSR is examined, heat generation stemming from
microwave irradiation is always larger than that from the chemical reaction.
Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
reserved.
1. Introduction atmosphere [10,11]. In this aspect, since WGSR can enrich the
Water gas shift reaction (WGSR) is an important reaction in
industry in that it can occur in the reactions of steam
reforming (SR) [1,2], partial oxidation (POX) [3,4], autothermal
reforming [5,6], gasification [7,8] and ironmaking process in
a blast furnace [9]. For the prospective development of
hydrogen economy, WGSR will also play a vital role for
hydrogen production because it will convert CO into CO2 and
thereby produces H2 from steam. In particular, on account of
the global concern of greenhouse effect, carbon (or carbon
dioxide) capture and storage (CCS) has been considered
a potential route to reduce anthropogenic CO2 emissions into
; fax: þ886 6 2602205.(W.-H. Chen).2011, Hydrogen Energy P
concentration of CO2 in the product gas of gasification and this
is conducive to the subsequent CCS, it has thus been thought
of as a powerful tool for achieving the mitigation of global
warming.
Conceptually, when CO and H2O co-exist in a system,
WGSR may occur and it is expressed as
COþH2O4CO2þH2 DHWGSR¼�41.2 kJmol�1 (1)
The reaction is a reversible and moderately exothermic
reaction in nature [12]. However, by virtue of energy barrier
ublications, LLC. Published by Elsevier Ltd. All rights reserved.
Nomenclature
A Pre-exponential factor, m3mol�1K�as�1
ci Molar concentration of species i, molm�3
Cp Gas mixture specific heat, J kg�1 K�1
D Diffusion coefficient, m2 s�1
E Electric field intensity, Vm�1
Ea Activation energy, Jmol�1
f Frequency, Hz
F S/C ratio function, dimensionless
h Convective heat transfer coefficient, Wm�2 K�1
hi0 Standard-state enthalpy of species i, J mol�1
H Magnetic field intensity, Am�1
k Thermal conductivity, Wm�1 K�1
keff Effective thermal conductivity, Wm�1 K�1
kf Fluid phase thermal conductivity, Wm�1 K�1
ks Solid medium thermal conductivity, Wm�1 K�1
kWGSR Reaction rate constant of water gas shift reaction,
m3mol�1 s�1
K Catalyst layer permeability, m�2
Keq Equilibrium constant, dimensionless
Mi Molar mass of species i, kgmol�1
N Number of species
p Pressure, Pa
patm Atmospheric pressure (¼1.013� 105 Pa)
Q Energy equation source term, Jm�3 s�1
Qmw Energy generation due to microwave heating,
Jm�3 s�1
Qreaction Energy consumption due to chemical reactions,
Jm�3 s�1
R Universal gas constant (¼8.314 m3 Pa K�1mol�1 or
8.314 J K�1mol�1)
Ri Reaction rate of species i, molm�3 s�1
RWGSR Reaction rate of water gas shift reaction,
molm�3 s�1
si0 Standard-state entropy of species i, J mol�1 K�1
T Temperature, K
Ta Ambient air temperature in the oven (¼298 �C)Tw Cavity wall temperature (¼298 �C)
V Velocity, m s�1
V Volume of catalyst bed (dimensionless)
w Velocity, m s�1
Xi Molar fraction of species i, dimensionless
Greek letter
DHWGSR Heat of water gas shift reaction (¼�41,200 Jmol�1)
a Temperature exponent, dimensionless
g Porosity, dimensionless
d S/C ratio, dimensionless
ε0 Free space permittivity (¼8.854� 10�12 Faradm�1)
εr Complex relative permittivity, dimensionless
εrad Emissivity, dimensionless
ε
00r Relative dielectric loss factor, dimensionless
f Pre-exponential function, m3mol�1 s�1
ni Stoichiometric coefficient of species i,
dimensionless
m Viscosity, Pa s
m0 Free space permeability (¼4p� 10�7 TmA�1)
r Gas mixture density, kgm�3
s StefaneBoltzmann constant
(¼5.67� 10�8 Wm2K4)
u Angular frequency, Rad s�1
Subscript
a Air of the cavity
CO Carbon monoxide
CO2 Carbon dioxide
eq Equilibrium state
f Fluid
H2 Hydrogen
H2O Water
i Species i
in Inlet
mw Microwave
n Normal direction
reaction Reaction
t Tangent direction
w Wall
WGSR Water gas shift reaction
i n t e r n a t i o n a l j o u rn a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 2 7 6e2 8 9 277
encountered in the reaction, catalysts are required all the time
to trigger the formation of hydrogen. Based on adopted cata-
lysts or reaction temperature, WGSR is generally classified
into two different reactions; one is the high-temperature shift
reaction (HTSR) and the other the low-temperature shift
reaction (LTSR). HTSR is approximately operated at the
temperatures between 350 and 500 �C and the typical adopted
catalysts are FeeCr-based catalysts. In contrast, the temper-
ature of LTSR is usually in the range of 150e250 �C [13] and
CueZn-based catalysts are themost commonly used ones. On
the one hand, from Arrhenius law it is known that increasing
reaction temperature will facilitate the reaction rate of CO; on
the other hand, according to thermodynamics or Le Chate-
lier’s principle a lower reaction temperature is conducive to
CO conversion or hydrogen yield, as a consequence of the
exothermic reaction involved. In general, HTSR is governed by
chemical kinetics, whereas LTSR is dominated by thermody-
namic equilibrium.
In reviewing the literature concerning WGSR, a number of
methods have been conducted to induce hydrogen formation.
Conventional heating via external burner [14] or electric
heater [15,16] is a typical mean to provide a reaction envi-
ronment with controlled temperature. Chao et al. [17] acti-
vated the partial oxidation of methane in association with
WGSR in a plasma-assisted heating environment. Byrd et al.
[18] and Voll et al. [19] investigated the behavior of WGSR
using a supercritical water method. Chen et al. [20] designed
a Swiss-roll reactor to elicit WGSR by means of recovering
waste heat from the partial oxidation of methane; in another
study [21], the reaction characteristics for WGSR in a rotating
packed bed (RPB) was highlighted to account for the CO
conversion in a high gravity (Higee) environment.
Aside from the aforementioned methods, microwave
irradiation is another route which can be employed to trigger
WGSR [12]. In fact, microwaves have been widely used in
industrial processes and household appliances [22e24]
i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 2 7 6e2 8 9278
because of its advantages of minimizing heating time, saving
space and high energy efficiency. Unlike conventional heating
through conduction and convection, microwaves can pene-
trate into a catalyst bed by radiation; hence the heating
mechanism with microwave irradiation is different from that
of conventional heating. In particular, when microwaves are
applied for activating a catalytic reaction such as methanol
steam reforming (MSR), it has been addressed [2] that the
reaction is characterized by the microwave double absorption
in that both the reactants and catalyst pellets can absorb
microwaves and convert them into thermal energy, thereby
achieving a rapidly heating process.
In the past, a few studies concerning endothermic reaction
of MSR in an environment with microwave irradiation has
been studied [2,25,26]. However, very little research has been
performed on the exothermic reaction of WGSR along with
microwave heating, especially for the numerical study. To the
authors’ knowledge, LTSR with microwave-assisted heating
has not been investigated yet. For this reason, the interest of
the present study is focused on the aforementioned topic. To
provide a comprehensive insight into the reaction behavior of
LTSR, the aims of this work are to: (1) explore the reaction
behavior of LTSR experimentally; (2) develop the chemical
kinetics of the LTSR based on the experimental measure-
ments; and (3) simulate the reaction phenomena of the LTSR
and observe detailed reaction characteristics. Moreover, the
difference of reaction behavior between the endothermic
reaction (e.g. MSR) and exothermic reaction (i.e. WGSR) will be
addressed.
Fig. 1 e A schematic of the conducted reaction system and expe
controller readout; E: gasmixer; F: water; G: pump; H: microwave
catalyst layer; M: power controller; N: condenser; O: drier; P: ga
2. Methodology
2.1. Microwave reaction system
The schematic of the experimental setup is demonstrated in
Fig. 1. The experimental microwave reaction system mainly
comprised four components, consisting of a magnetron,
a waveguide, a cavity and a cylindrical quartz reaction tube.
Microwaveswith the frequency of 2.45 GHzwere emitted from
the magnetron and guided into the cavity by the waveguide.
The reaction tube was placed at the center of the cavity and it
included two zones; the upper zone was non-porous zone and
the lower zone was porous zone in which CueZn-based
catalyst pellets were packed. In the upper zone, the reagents
were preheated without chemical reactions. When the reac-
tants entered the porous zone, they were heated and WGSR
was driven. Detailed physical sizes of the cavity and reaction
tube have been illustrated elsewhere [25].
2.2. Governing equations
The mathematical modeling of the microwave reaction
system and the geometries of the cavity and the reaction tube
were constructed based on the experimental setup. To
simplify the physical problem, the following assumptions are
adopted. (1) The physical phenomena are symmetric along the
vertical center plane of the reaction tube and the cavity; (2) the
porous zone is homogeneous and thermal equilibrium
rimental procedure (A: CO; B: N2; C: mass flow controller; D:
reactor; I: reaction tube; J: thermocouple; K: mixing layer; L:
s chromatography; Q: gas analyzer; R: recorder).
i n t e r n a t i o n a l j o u rn a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 2 7 6e2 8 9 279
prevails at the catalyst surface; (3) the thermal conductivity,
specific heat, porosity and bulk density of the catalyst are
temperature independent; (4) WGSR occurs in the catalyst bed
alone; (5) the body force of the reagents is ignored and the gas
mixture inside the reaction tube abides by the ideal gas law;
and (6) the values of complex relative permittivity in the non-
porous zone and the porous zone are set as 10þ 0.05i and
10þ 1i, respectively [26].
The governing equations include the continuity,
momentum, energy and species equations and they are dis-
played in Table 1. The electric field equation derived from
Maxwell’s equations [25] is employed as well and it is
expressed as
V� �m�10 V� E
. �� u2ε0εr E
. ¼ 0 (2)
where εr is the complex relative permittivity. Accordingly, the
volumetric power absorbed by a dielectric material (Qmw) can
be calculated by the following
Qmw ¼ 12uε0ε
}r jE
. j2 ¼ pfε0ε}r jE
. j2 (3)
where f is the excitation frequency.
2.3. Boundary conditions
The boundary conditions can be partitioned into two parts;
one is in the cavity and waveguide and the other in the reac-
tion tube. In the cavity and waveguide, the boundary condi-
tions of the electromagnetic field are described as follows.
(1) The inner wall of the rectangular cavity is thought of as
a perfect electric conducting wall
Et ¼ 0 and Hn ¼ 0 (4)
(2) The symmetric plane is treated as a perfect magnetic
conducting wall
Ht ¼ 0 and En ¼ 0 (5)
In the reaction tube, the boundary conditions are divided
into the upstream inflow, the downstream outflow, the
Table 1 e A list of governing equations, equation of state and
Governing equations Non-porou
Continuity V$ðrV. Þ ¼ 0
Momentum rV.
$VV. ¼ �Vpþ V$½mðVV.
Energy rCpV.
$VT ¼ V$ðkVTÞ þ Q
k ¼ kfQ ¼ Qmw
Species V$ðV. ciÞ ¼ V$ðDVciÞEquation of state
p ¼ rRTPNi
1XiMi
Electric field V� ðm�10 V� E
. Þ � u2ε0εr E
.
Source terms in energy equation
Microwaves Qmw ¼ 12uε0ε
}r jE. j2 ¼ pfε0
Chemical reaction Qreaction ¼ RWGSRDHWGSR
symmetric plane and the tube wall. They are described as
the following.
(1) The upstream inflow
V/
¼ win k.
; T ¼ Tin and ci ¼ ci;in (6)
(2) The downstream outflow
VV. ¼ VT ¼ Vci ¼ 0 and p ¼ patm (7)
(3) The symmetric plane
V$V. ¼ VT ¼ Vci ¼ 0 (8)
(4) The tube wall
V. ¼ Vci ¼ 0 (9)
�kVT ¼ hðT� TaÞ þ εrads�T4 � T4
w
�(10)
2.4. Modeling of LTSR
To aid in predicting the reaction behavior of LTSR with
microwave heating, a new chemical kinetics is developed in
this study. The reaction rate of WGSR is expressed as
RWGSR ¼ kWGSR
�cCOcH2O � K�1
eq cCO2cH2
�(11)
where kWGSR and Keq are the reaction rate constant and the
equilibrium constant of the WGSR, respectively. In general,
the reaction rate constant kWGSR obeys Arrhenius law and it is
written as
kWGSR ¼ ATaexp
��Ea
RT
�(12)
The pre-exponential factor A and the temperature exponent
a are determined by experiments. The past studies [27e29]
indicated that the activation energy of LTSR ranged from 35
source terms in non-porous zone and porous zone.
s Porous
þ ðVV. ÞTÞ� m
KV. ¼ �Vpþ V$½m
gðVV. þ ðVV. ÞTÞ�
k ¼ keff ¼ g kf þ ð1� gÞ ksQ ¼ Qmw � Qreaction
V$ðV. ciÞ ¼ V$ðgDVciÞ þ Ri
¼ 0
ε}r jE. j2
Table 2 e Values of standard-state enthalpy h0 andstandard-state entropy s0 [32].
Species h0 (J mol�1) S0 (J mol�1 K�1)
CO �110,530 197.556
CO2 �393,510 213.677
H2O �241,814 188.724
H2 �1.881377 130.571
Table 3 e Properties of fluid at the inlet of the reactor aswell as properties of the adopted catalyst [27].
Properties of fluid
Mass diffusion coefficient (m2 s�1) 2.88� 10�5
Thermal conductivity (Wm�1 K�1) 4.54� 10�2
Viscosity (Pa s) 1.72� 10�5
Properties of the catalyst
Porosity 0.3
Density (kgm�3) 6877.2
Specific (J kg�1 K�1) 475.32
Thermal conductivity (Wm�1 K�1) 183.25
Table 4eA list of operating temperature, supplied power,volumetric flow rate of feed gas and CO mole fraction inthe feed gas at various S/C ratios.
Temperature(�C)
Power(W)
Volumetric flowrate of feed
gas (mLmin�1)
Steam/CO ratio
2 4 6
200 237 934 21.84 10.92 7.28
250 320 806 25.31 12.66 8.44
300 409 700 29.14 14.57 9.71
Fig. 2 e Three-dimensional profile of CO conversion with
respect to reaction temperature and S/C ratio from
experimental measurements.
i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 2 7 6e2 8 9280
to 90 kJmol�1. In the present study, the activation energy of
35 kJmol�1 is adopted in that the predicted results are in good
agreement with the experimental data. On the other hand,
Keiski et al. [30] reported that the reaction of WGSR was not
a simple order reaction if the S/C ratio was high. Therefore,
a function of S/C ratio is taken into account in the reaction rate
constant. The reaction rate constant is expressed as the
following
kWGSR ¼ fðd;TÞexp��Ea
RT
�¼ ATaFðdÞexp
��Ea
RT
�(13)
The parameter d denotes the S/C ratio; f(d,T ) and F(d) stand for
the pre-exponential function and the S/C ratio function,
respectively. The pre-exponential function will be determined
Fig. 3 e (a) Distributions of pre-exponential function and (b)
comparisons of CO conversion between experimental
measurements and numerical predictions.
i n t e r n a t i o n a l j o u rn a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 2 7 6e2 8 9 281
in accordance with the experimental measurements. The
equilibrium constant Keq shown in Eq. (11) is given below
Keq ¼ cCOcH2O
cCO2cH2
(14)
According to thermodynamics [31], the equilibrium
constant of a reaction is expressed as
Keq ¼ exp
"Xi¼1
ðn00i � n0iÞs0iR�Xi¼1
ðn00i � n0iÞh0i
RT
#�patm
RT
�Pi¼1
ðn00i�n0
iÞ
(15)
In the preceding equation, n0i and n00i are the stoichiometric
coefficients for the reactant and product i in the reaction; si0,
Fig. 4 e Isothermal contours in the non-porous zone at (a) 200, (b
(e) 250 and (f) 300 �C (S/C[ 2).
hi0, R, patm and T are the standard-state entropy (Jmol�1 K�1),
standard-state enthalpy (Jmol�1), universal gas constant
(¼8.314 J K�1mol�1), atmospheric pressure (¼1.013� 105 Pa)
and temperature (K), respectively. The values of si0 and hi
0 are
given in Table 2.
2.5. Numerical method
The commercial software COMSOL Multiphysics 4.0a
through the utilization of finite element method was
employed to solve the governing equations along with the
boundary conditions. To seek an appropriate grid system,
) 250 and (c) 300 �C as well as in the porous zone at (d) 200,
i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 2 7 6e2 8 9282
four different unstructured grid systems with triangle
element of (i.e. cavity� reaction tube� catalyst bed) -
¼ (20,561� 5312� 1107), (40,567� 10,373� 2040), (60,567
� 15,686� 2922) and (81,070� 20,413� 3889) were tested and
compared with each other. It was found that the grid system
of (60,567� 15,686� 2922) satisfied the requirement of grid
independence. Therefore, the aforementioned grid system
was utilized for simulations. Moreover, the SPOOLES and
GMRES (generalized minimum residual) solvers were used to
solve the momentum, energy and mass equations and the
electromagnetic fields. Details of the numerical method and
software setting have been illustrated elsewhere [26].
In the following discussion, the mean temperature of the
catalyst bed from numerical simulations is defined based on
volumetric integration and expressed as
Mean temperature ¼
Z�V
TV d�VZ�V
V d�V(16)
3. Results and discussion
Seeing that attention is paid to LTSR under the impact of
microwave irradiation, WGSR triggered in a catalyst bed
packed by CueZn-based catalyst pellets serves as the basis of
the present study. In the reaction tube, the length of the
catalyst bed was 4 cm. Detailed properties of the adopted
catalyst are provided in Table 3. Regarding the operating
conditions, the volumetric flow rate of water was fixed at
0.3 mlmin�1 (25 �C) and the gas hourly space velocity (GHSV)
of the reactant streamwas 28,000 h�1. Three different reaction
temperatures of 200, 250 and 300 �C were taken into account
and the steam/CO molar ratio, namely, the S/C ratio, was in
the range of 2e6. Corresponding to the reaction temperatures
of 200, 250 and 300 �C, the supplied microwave powers were
237, 320 and 409 W, respectively. The volumetric flow rates of
feed gas (COþN2) and themolar fractions of CO in the feed gas
at various reaction temperatures and S/C ratios are shown in
Table 4. In Section 3.1, experimental results are illustrated,
whereas numerical predictions are described in other
sections.
Fig. 5 e Contours of CO conversion in the catalyst bed at the
mean temperatures of (a) 200, (b) 250 and (c) 300 �C (S/C
[ 2.0).
3.1. Experimental measurement of LTSR
Three-dimensional profile of CO conversion of LTSR with
respect to reaction temperature and S/C ratio from experi-
mental measurements is demonstrated in Fig. 2. Because only
two species, namely, CO and N2, are contained in the feed gas,
the CO conversion can be defined as
CO conversionð%Þ ¼ cCO2
cCO þ cCO2
� 100% (17)
It was reported that the reaction characteristics of LTSR with
conventional heating were governed by thermodynamic
equilibrium [15] in that the CO conversion decayed mono-
tonically with increasing reaction temperature. However,
Fig. 2 depicts that the CO conversion under microwave-
assisted heating increases as the reaction temperature is lif-
ted. Specifically, with the conditions of 200 and 300 �C at S/
C¼ 2, the values of CO conversion are 42 and 62%, respec-
tively. For the case of 300 �C and S/C¼ 6, the CO conversion is
further promoted to 95%. The foregoing behavior is contrary to
the results with conventional heating. It is thus recognized
i n t e r n a t i o n a l j o u rn a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 2 7 6e2 8 9 283
that the LTSR under microwave irradiation is dominated by
chemical kinetic or Arrhenius law. Moreover, with microwave
irradiation, the reaction temperature of 300 �C in association
with S/C¼ 6 is a feasible operating condition for achieving H2
production from the LTSR.
3.2. Modeling of LTSR
As noted above, the chemical kinetics of LTSRwithmicrowave
irradiation is different from that with conventional heating.
Furthermore, an accurate and precise chemical kinetic model
plays an important role in predicting H2 production and
figuring out the detailed reaction behavior in the catalyst bed.
Fig. 3a displays the distributions of the pre-exponential
function f(d,T ) versus mean temperature of the catalyst bed
Fig. 6 e Concentration contours of CO at (a) 200, (b) 250 and (c)
catalyst bed (S/C[ 2).
at three S/C ratios (viz. S/C¼ 2, 4 and 6) where the values of
f(d,T ) are determined based on the experimental data. From
the values shown in Fig. 3a, the reaction rate constant of the
LTSR can be correlated as the following
kWGSR ¼ 2:67� 1013T�4�� d2 þ 11:288d� 10:541
�exp
��35;000
RT
�(18)
where the S/C ratio d ranges from 2 to 6. According to the
foregoing developed model, the predicted values of CO
conversion at various S/C ratios and temperatures are
comparedwith the experimental data. As shown in Fig. 3b, the
numerical predictions agree well with the experimental
results, revealing that the developed model can predict the
LTSR accurately.
300 �C as well as H2 at (d) 200, (e) 250 and (f) 300 �C in the
i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 2 7 6e2 8 9284
3.3. Reaction phenomena of LTSR
The isothermal contours along the symmetrical plane in
the non-porous and porous zones at three reaction
temperatures of 200, 250 and 300 �C are presented in Fig. 4
where the S/C ratio is 2. Fig. 4aec clearly displays that
Fig. 7 e Contours of CO conversion in the catalyst bed with
S/C ratios of (a) 2, (b) 4 and (c) 6 (mean temperature
[ 300 �C).
Fig. 8 e Distributions of CO along the centerline of the
catalyst bed at various mean temperatures with S/C ratios
of (a) 2, (b) 4 and (c) 6.
Fig. 9 e Distributions of H2 concentration along the
centerline of the catalyst bed at various mean
temperatures with S/C ratios of (a) 2, (b) 4 and (c) 6.
i n t e r n a t i o n a l j o u rn a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 2 7 6e2 8 9 285
a hotspot is exhibited approximately at the center of the
non-porous zone. This is attributed to that the emitted
microwaves are absorbed the reactants followed by con-
verting the electromagnetic waves into heat. The charac-
teristic of skewness in isothermal contour is owing to the
reactants in the tube being not spatially uniformly heated
by microwaves. Alternatively, it is of interest that the
isothermal contours in the porous zone are almost uniform,
and this feature is obviously different from the behavior of
endothermic reaction, say, methanol steam reforming [26].
For example, the maximum temperature differences in Figs.
4d, e and f are 4.4, 5.6 and 6.7 �C, respectively. In the
catalyst bed, heat liberated is contributed by two factors,
with one the microwave irradiation and the other the
exothermic reaction. This implies, in turn, that the thermal
behavior in the catalyst bed is dominated by the afore-
mentioned two factors and the influence of heat loss along
the tube surface is relatively slight. Furthermore, from
the viewpoint of practical operation, the uniform distribu-
tion of temperature in the catalyst bed is conducive to the
durability of catalyst pellets because of no hotspot
exhibited.
In examining the contours of CO conversion in the cata-
lyst bed at various temperatures (S/C¼ 2), the influence of
increasing temperature on the enhancement of CO conver-
sion can be clearly observed in Fig. 5 in that the contours at
the bottom of the catalyst bed turns bright blue (Fig. 5a) to
yellow (Fig. 5c). Nevertheless, the CO conversion at the exit
of the catalyst bed is not high because the maximum CO
conversion at conditions of S/C¼ 2 and 300 �C is around 62%.
The concentration contours of CO and H2 at the three
temperatures are shown in Fig. 6. With the reactants
marching from the entrance to the exit of the catalyst bed, it
can be seen that CO is consumed progressively, whereas the
concentration of H2 undergoes ascent gradually. The more
drastic variation in CO and H2 concentrations at a higher
temperature is partially due to more preheating of the
reactants prior to entering the catalyst bed (Fig. 4aec). Upon
inspection of the effect of varying S/C ratio on CO conversion
at 300 �C, Fig. 7 indicates that the CO conversion can be
improved markedly once the S/C ratio is as high as 4 in that
the red zone extends greatly (Fig. 7b) in comparison with
that at S/C¼ 2 (Fig. 7a). In view of the same power supplied
(i.e. 409 W) in Fig. 7aec, it reflects that increasing S/C ratio
from 2 to 4 or 6 is an effective route to improve the perfor-
mance of the LTSR.
Spatial distributions of CO concentration along the
centerline of the catalyst bed at five mean temperatures are
plotted in Fig. 8. Because the GHSV of the experiments is
fixed at 28,000 h�1, a higher mean temperature leads to
a higher CO concentration in the feed gas (Table 1).
However, the higher the mean temperature, the more
pronounced the abatement of CO concentration. Fig. 8a
depicts that the decaying extent in CO concentration is
relatively not notable, regardless of what the temperature is.
With the conditions of S/C¼ 4 and 6, Fig. 8b and c reveals
that the concentration of CO can be reduced to a small
value, except for the case of 200 �C. It is thus concluded that
the condition of S/C¼ 2 is not recommended for the oper-
ation of LTSR and the ratio should be controlled at 4 at
i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 2 7 6e2 8 9286
least. With further examination of H2 concentration shown
in Fig. 9, it suggests that the higher the S/C ratio, the more
pronounced the growth of H2 in the vicinity of the entrance
of the catalyst bed. Despite the enlargement of CO conver-
sion with increasing S/C ratio (Fig. 8), it should be pointed
out that the operation of increasing S/C ratio is accompa-
nied by the disadvantage of lower H2 concentration in the
product gas (Fig. 9), resulting from fixed flow rate of water
and less CO being sent into the reactor so that less H2 is
produced.
Fig. 10 e Contours of electric field at (a) 200, (b) 250 and (c) 300 �Ccatalyst bed (S/C[ 2).
3.4. Electromagnetic field, chemical reaction and heatgeneration
The contours of electric field andmagnetic field in the catalyst
bed at three different temperatures are plotted in Fig. 10. The
profiles indicate that the largest intensities electric and
magnetic fields always develop at the entrance of the catalyst
bed. This is the reason that the largest intensity of CO
consumption occurs adjacent to the entrance, as shown in
Fig. 8. However, in each horizontal cross-sectional area it can
as well as magnetic field at (d) 200, (e) 250 and (f) 300 �C in the
i n t e r n a t i o n a l j o u rn a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 2 7 6e2 8 9 287
be seen that the location of the maximum electric field is
accompanied by the minimum magnetic field all the time,
revealing the nature of the arrangement of the electric and
magnetic fields in a crisscross pattern [33].
Fig. 11 e Distributions of the forward and backward
reaction rates along the centerline of the catalyst bed at
various reaction temperatures with S/C ratios of (a) 2, (b) 4
and (c) 6.
Fig. 12 e Distributions of volumetric heat generation of
microwaves and WGSR along the centerline of the catalyst
bed with S/C ratios of (a) 2, (b) 4 and (c) 6.
i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 2 7 6e2 8 9288
As far as the chemical kinetics of the LTSR is considered,
Eq. (11) indicates that the chemical reaction rate comprises
the forward reaction rate (i.e. kWGSRcCOcH2O) and the backward
one (i.e. kWGSRK�1eq cCO2cH2 ). To recognize the reaction mecha-
nisms in more detail, the two reaction rates along the
centerline of the catalyst bed are displayed in Fig. 11. It is
evident that the distributions of the forward reaction rate
decay downward. On the contrary, the backward reaction rate
increases with the reagents marching. Moreover, the higher
the reaction temperature, the faster the forward reaction rate
decays and the backward reaction rate rises.
Considering the thermal behavior of LTSR, Fig. 12 shows
the values of volumetric heat generation originating from
microwave heating (MW) and WGSR at three S/C ratios. It is
noteworthy that heat generated frommicrowave irradiation is
larger than that from chemical reaction to a great extent. In
the current study, because the flow rate of water is fixed,
a higher S/C ratio represents a lower CO concentration in the
feed gas. Consequently, heat produced from WGSR decreases
as the S/C ratio goes up.
4. Conclusions
Water gas shift reaction triggered in a CueZn-based catalyst
bed, namely, the LTSR, with microwave-assisted heating has
been studied experimentally and numerically. Unlike LTSR
with conventional heating, the microwave heating leads to
the growth of the CO conversion with increasing reaction
temperature. This reveals that the phenomena of LTSR under
microwave irradiation are dominated by chemical kinetics
rather than thermodynamic equilibrium. The experimental
results suggest that the reaction temperature of 300 �C along
with S/C ratio of 6 is an appropriate operating condition for H2
production in that the CO conversion can reach around 95%. A
chemical kinetics accounting for H2 production from the LTSR
has also been successfullymodeled. In themodel, not only the
reaction temperature is taken into account, a polynomial of S/
C ratio is also embedded in the kinetics, yielding a pre-
exponential function. In view of chemical kinetics domi-
nating the LTSR, when the CO concentration along the
centerline of the catalyst bed is examined, it is found that the
decaying rate of CO concentration is faster at a higher reaction
temperature. The numerical simulations reveal that the
temperature distribution in the catalyst bed is uniform which
is significantly different from that of an endothermic reaction,
such as methanol steam reforming. This arises from the fact
that heat released in the catalyst bed comes from microwave
irradiation and exothermic reaction of the LTSR, and the
former is substantially larger than the latter. From the
perspective of practical operation, a uniform temperature
distribution in the catalyst bed is conducive to the durability of
catalyst.
Acknowledgments
The authors acknowledge the financial support of the
National Science Council, Taiwan, ROC, in this research.
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