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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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