Optimal design of an experimental methanol fuel reformer Chih-Chiang Chuang a , Yih-Hang Chen b , Jeffrey D. Ward a , Cheng-Ching Yu a, *, Yen-Chun Liu c , Chiou-Hwang Lee c a Department of Chemical Engineering, National Taiwan University, Taipei 106-07, Taiwan b Energy and Environment Research Laboratories, Industrial Technology Research Institute, Hsinchu 310, Taiwan c Material and Chemical Research Laboratories, Industrial Technology Research Institute, Hsinchu 310, Taiwan article info Article history: Received 4 July 2008 Received in revised form 19 August 2008 Accepted 19 August 2008 Published online - abstract We report on the steady state modeling of an experimental methanol fuel reformer for fuel cell applications. The fuel reformer consists of an AutoThermal Reformer (ATR) followed by an Oxygen Removal (OR) reactor, Steam Reformer (SR) and Water Gas Shift (WGS) reactor. The effluent from the WGS is fed to a series of three Preferential Oxidation (PROX) reactors that reduce the CO concentration to less than 40 ppm. A mathematical model of the reformer is developed and selected parameters of the model are fit to experimental data collected from a fuel reformer that was designed, built and operated by the Material and Chemical Research Laboratories (MCL) of the Industrial Technology Research Institute (ITRI) in Hsinchu, Taiwan. In order to develop a compact and high-performance fuel reformer system, the mathematical model is used to design a reformer that has the minimum possible combined volume of the steam reformer and water gas shift reactor. The result is that the volume of the optimized reactor units can be reduced by 17.2% without a significant change in the overall efficiency. ª 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. 1. Introduction As a method of power generation, fuel cells have the potential to provide higher efficiency and therefore also reduced emissions compared to internal combustion engines [1]. Fuel cells have been designed that can directly oxidize fuels such as methane, methanol, dimethylether, etc. However, the greatest efficiency is achieved when elemental hydrogen is oxidized directly at the anode. However, as a fuel, hydrogen is difficult to store and transport. Therefore, in recent decades, considerable effort has been made in the design of fuel reforming systems that can produce a hydrogen-rich stream from a fuel such as methanol, ethanol, dimethyl ether or hydrocarbon fuels. A detailed review is given by Song [2]. Further background information is available in Refs. [3,4]. There are a number of considerations in the design of fuel reforming systems [1]. Carbon monoxide is a poison for polymer electrolyte membrane (PEM) fuel cells, thus if this kind of fuel cell is used, the carbon monoxide concentration must be reduced to a very low level, typically less than 100 ppm. For transportation applications in particular but for stationary applications as well, it is desirable to make the fuel reforming system as small and lightweight as possible. Because many fuel reforming reactions take place at high temperatures, it is desirable to employ heat integration in order to improve the efficiency of the reforming process. Finally, it is desirable that the fuel reforming system start up * Corresponding author. E-mail address: [email protected](C.-C. Yu). Available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/he ARTICLE IN PRESS 0360-3199/$ – see front matter ª 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2008.08.045 international journal of hydrogen energy xxx (2008) 1–12 Please cite this article in press as: Chuang C-C et al., Optimal design of an experimental methanol fuel reformer, International Journal of Hydrogen Energy (2008), doi:10.1016/j.ijhydene.2008.08.045
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ARTICLE IN PRESSi n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 0 8 ) 1 – 1 2
Avai lab le a t www.sc iencedi rec t .com
j ourna l homepage : www.e lsev ier . com/ loca te /he
Optimal design of an experimental methanol fuel reformer
aDepartment of Chemical Engineering, National Taiwan University, Taipei 106-07, TaiwanbEnergy and Environment Research Laboratories, Industrial Technology Research Institute, Hsinchu 310, TaiwancMaterial and Chemical Research Laboratories, Industrial Technology Research Institute, Hsinchu 310, Taiwan
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important and the endothermic methanol decomposition
reaction, which produces carbon monoxide, becomes more
important. As a result, the concentration of carbon monoxide
in the steam reformer drops quickly initially but then
plateaus. A practical design will include a steam reformer
sized so that the CO concentration is beginning to plateau at
the exit of the reactor. This outcome can be guaranteed by
requiring that the derivative of reactor temperature with
respect to catalyst mass at the exit of the reactor be equal to
zero.
The optimization problem was solved by iterating over the
two optimization variables. For a given value of TSR,in, the
value of THTS,in that minimized the combined volume was
identified. The procedure was repeated for various values of
TSR,in until the global minimum was identified.
Fig. 8 shows certain properties of the fuel reformer system
versus TSR,in with the optimal (volume-minimizing) value of
THTS,in. As the SR inlet temperature increases, the SR volume
decreases (Fig 8(a)). The maximum temperature of the SR also
increases with increasing inlet temperature (Fig 8(d)). The
higher temperature is favorable from a kinetic point of view
and the WGS reaction approaches equilibrium more quickly in
Table 13 – Reaction and kinetics for methanation reactor
Reaction Kinetics Source
CH4þH2O%COþ 3H2rM ¼
kfPCH4 PH2O � krPCOP3H2
P2:5H2ðaÞ2
[21]
a ¼�
1þ KCOPCO þ KH2 PH2
þKCH4 PCH4 þ KH2OPH2O
PH2
�
Please cite this article in press as: Chuang C-C et al., Optimal desJournal of Hydrogen Energy (2008), doi:10.1016/j.ijhydene.2008.08
this case. However, the higher temperature is also disadvan-
tageous from a thermodynamic point of view: the equilibrium
CO concentration is greater at higher temperatures (Fig 8(c)).
As a result, the mole fraction of CO at the SR outlet increases
with increasing temperature, as does the required volume of
the HTS (Fig 8(b)). The higher SR reactor temperature will
cause a higher CO concentration in the SR outlet stream. This
means that a larger HTS reactor volume will be needed to
meet the CO concentration specifications in the HTS outlet
stream. Therefore, there is a tradeoff in the reactor network
design and a minimum in the combined reactor volume.
Fig. 9(a) shows the combined volume of the steam reformer
and HTS reactor versus the inlet temperature of the steam
CH4 (bar ) 6.65� 10 �38.3
CO (bar�1) 8.23� 10�5 �70.7
H2 (bar�1) 6.12� 10�9 �82.9
H2O (bar�1) 1.77� 105 88.7
Table 15 – Modified values of kinetic parameters for themethanation reaction
Pre-exponential factor(rate constant)
Literature Modified
kf mol/(min bar0.5) 7.02� 1016 7.02� 1016
kr mol/(min bar1.5) 5.862� 103 4.66� 104
ign of an experimental methanol fuel reformer, International.045
Fig. 10 – Combined volume of the steam reforming reactor and the methanation reactor (a) and HTS outlet hydrogen flow
rate (b) versus steam reformer inlet temperature.
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reformer. The optimal design is marked with a triangle and
the experimental (base case) design is marked with a circle.
The minimum combined volume occurs at a temperature of
317 �C and a combined volume of 1031 cm3, as compared with
the initial design at a temperature of 365 �C and 1247 cm3, for
a reduction in volume of 17.2%. The steam reformer inlet
temperature in the optimal design is 317 �C.
It is important to verify that the reduction in reformer
size does not come at the expense of a significant reduction
in the hydrogen production rate (i.e. the efficiency of the
reformer). Fig. 9(b) shows the hydrogen flow rate at the exit
of the HTS versus the steam reformer inlet temperature.
Compared with the base-case design, the hydrogen produc-
tion rate of the volume-optimized system is reduced by only
0.69%.
5.2. Optimization of the reaction pathway
Besides optimizing the process as described above, an alter-
native way to reduce the fuel reformer volume is to use
a different process design in which the HTS reactor is replaced
with a methanation reactor. The methanation reactor reduces
the CO concentration by hydrogenating CO to form methane
and water. Table 13 shows the reaction and the kinetic
expression that was used to model the reaction rate [21].
Table 14 shows the literature values of the kinetic parameters
taken from [22]. Table 15 shows the modified kinetic param-
eters that were determined by regressing experimental data
for the methanation reaction from MCL of ITRI.
For the alternative process design with the methanation
reactor, the optimization problem can be stated as:
Minimize : fðXÞ ¼ ðVSR þ VMÞ (9)
Please cite this article in press as: Chuang C-C et al., Optimal desJournal of Hydrogen Energy (2008), doi:10.1016/j.ijhydene.2008.08
X ¼�
TSR;in;TM;in
�(10)
Subject to :
8>><>>:
TSR;out � TM;in � 10 �CdTSR;out
dWcat¼ 0
TM;out < 500 �CYCO;HTS ¼ 1:01%
9>>=>>; (11)
Fig. 10(a) shows the combined volume of the steam
reforming and methanation reactors versus SR inlet temper-
ature for the optimal value of the methanation inlet temper-
ature. In contrast with Fig. 9(a), the combined volume
decreases with increasing temperature over the entire
temperature range 280–370 �C. For temperatures beyond
370 �C it is impossible to satisfy the constraints. The minimum
possible combined volume is just 287 cm3, a reduction of 77%
compared to the base-case system.
However, from Fig. 10(b) this improvement comes at the
expense of a drastic reduction in the hydrogen flow rate. At
the design corresponding to the minimum combined volume,
the hydrogen flow rate is reduced by 19%. The reason for this
is that the methanation reaction consumes three moles of
hydrogen for every mole of CO converted. Although the
methanation reactor is attractive from the point of view of
volume reduction, it results in a prohibitive loss of efficiency
of the reformer. Therefore, this process design alternative is
not considered any further.
6. Conclusions
In this work, we have reported on the modeling and optimi-
zation of an experimental methanol fuel reforming system for
ign of an experimental methanol fuel reformer, International.045
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fuel cell applications developed by MCL of ITRI. Kinetic
parameters for all of the catalysts, including a Pt/CeO2–ZrO2
catalyst developed by MCL of ITRI are identified by fitting
experimental data. The experimental system can produce
a hydrogen stream with a CO concentration less than 40 ppm,
and with an efficiency of 66.1 percent. The method of model
development, in which kinetic parameters are taken from the
literature and the forward reaction rates are adjusted to
match experimental data, has the advantages that it is simple
and can be completed rapidly with minimal experimental
data, thus facilitating rapid product design.
The model was also used to develop an optimized design in
which the volume of the reactor units is minimized. The result
of the optimization suggests that the combined volume of the
SR and HTS reactors can be reduced by 17.2% with a reduction
in the hydrogen flow rate of only 0.69%.
Acknowledgement
This work was supported by the National Science Council of
Taiwan under grant NSC 96-2628-E-002-022-MY3.
Appendix ADetermining capacity parameters from dynamicresponses
Although this contribution deals only with the development
of a steady state model, some of the parameters of the steady
state model were estimated by fitting the dynamic response of
the process to a dynamic process model. This appendix briefly
discusses the development of the dynamic model and the
fitting of these parameters.
Eqs. (1)–(3) in the body of the paper are steady-state equa-
tions. The analogous dynamic equations are partial differen-
tial equations:
3rg
vyi
vt¼ �1
A
v�Fyi
�vz
þ ð1� 3ÞX
j
nijrjrcat (12)
3rgCP;gvTg
vt¼ �1
Av
vz
Xi
FCP;iTg
!þ hS
�TS � Tg
�(13)
�1� 3
rSCP;S
vTS
vt¼ kcond
Av2TS
vz2þ ð1� 3Þ
Xj
��DHrxn;j
�rjrcat
� hS�TS � Tg
�� Qloss (14)
These equations are converted into a series of coupled ordi-
nary differential equations by discretizing the length coordi-
nate into a number of finite positions (lumping):
3rgVndyn;i
dt¼ Fn�1yn�1;i � Fnyn;i þ
Xj
nijrn;jWcat;n (15)
Please cite this article in press as: Chuang C-C et al., Optimal desJournal of Hydrogen Energy (2008), doi:10.1016/j.ijhydene.2008.08
3rgVnCP;gdTg;n
dt¼ Fn�1
Xi
yn�1;iCP;n�1;iTg;n�1 � Fn
Xi
yn;iCP;n;iTg;n
þ hSVn
�Ts;n � Tg;n
�(16)
�1� 3
rSVnCP;S
dTS;n
dt¼ kcond
Dz2ðTS;nþ1 � 2TS;n þ TS;n�1Þ � hSVn
�TS;n
� Tg;n
�þX
j
��DHrxn;j
�rn;jWcat;n � Qloss
(17)
The parameters 3rg, 3rgCp,s, and (1� 3)rsCp,s were adjusted so
that the dynamic trajectories predicted by the model equations
match theexperimentaldata. The resultsareshown inTable12.
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