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Sequential simulation of dense oxygen permeationmembrane reactor for hydrogen production from oxidativesteam reforming of ethanol with ASPEN PLUS

Yun Jin a, Zebao Rui a, Ye Tian a, Yuesheng Lin b, Yongdan Li a,*aTianjin Key Laboratory of Catalysis Science and Technology, School of Chemical Engineering, Tianjin University, Tianjin 300072, ChinabDepartment of Chemical Engineering, Arizona State University, Tempe, AZ 85287-6006, USA

a r t i c l e i n f o

Article history:

Received 25 January 2010

Received in revised form

8 April 2010

Accepted 10 April 2010

Available online 18 May 2010

Keywords:

Dense tubular membrane reactor

Sequential simulation

Oxidative steam reforming of ethanol

Oxygen permeation

* Corresponding author. Tel.: þ86 22 2740561E-mail address: ydli@tju.edu.cn (Y. Li).

0360-3199/$ e see front matter ª 2010 Profedoi:10.1016/j.ijhydene.2010.04.042

a b s t r a c t

Hydrogen production via oxidative steam reforming of ethanol in a dense tubular

membrane reactor (DMR) is sequentially simulated with ASPEN PLUS. The DMR is divided

into multi-sub-reactors, and the Gibbs free energy minimization sub-model in ASPEN PLUS

is employed to simulate the oxidative steam reforming of ethanol process in the sub-

reactors. A FORTRAN sub-routine is integrated into ASPEN PLUS to simulate the oxygen

permeation through membranes in the sub-separators. The simulation result indicates

that there is an optimal length of the tubular membrane reactor at the operating

temperature and steam-to-ethanol (H2O/EtOH) ratio, under which hydrogen and carbon

monoxide formation reach their maxima.

ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.

1. Introduction Hydrogen may be generated from ethanol by different

The potential benefits of a hydrogen economy coming from

renewable energy sources are creating a large consensus [1,2].

Hydrogen production from fossil fuels has been investigated

for many years. Among various liquid fuels, either for direct

feeding or indirect via conversion to hydrogen rich mixtures,

methanolhasbeen thoroughly studied [3e6], sincemethanol is

available as an abundant feedstock already largely distributed.

However, the main drawback of methanol, besides its rela-

tivelyhigh toxicity, is that its production is essentiallybasedon

reforming of non-renewable fossil fuels (mostly natural gas).

Ethanol which can be produced in large quantities from

renewable bio-resources appearsas anattractive alternative to

methanol since it is much less toxic, almost CO2 neutral and

nowmore and more available as industrial feedstock.

3; fax: þ86 22 27405243.

ssor T. Nejat Veziroglu. P

technologies, including steam reforming (SR) (Eq. (1)), partial

oxidation (POX) (Eq. (2)) and oxidative steam reforming (OSR)

(Eq. (3)) [7].

CH3CH2OH ðlÞ þ 3H2O ðlÞ42CO2 ðgÞ þ 6H2 ðgÞ; DH298

¼ 347:4 kJ=mol (1)

CH3CH2OH ðlÞ þ 1:5O2 ðgÞ42CO2 ðgÞ þ 3H2 ðgÞ; DH298

¼ �554:0 kJ=mol (2)

CH3CH2OHðlÞþ1:8H2OðlÞþ0:6O2ðgÞ42CO2ðgÞþ4:8H2;DH298

¼2:4kJ=mol (3)

SR generates a high H2/CO ratio product but has the disad-

vantage of high endothermicity and catalyst deactivation due

ublished by Elsevier Ltd. All rights reserved.

Nomenclature

aik numbers of the kth atom in molecule i

Cep permeation capacity of membrane surface area/

thickness, cm

Ci density of oxygen ions, mol cm�3

Da ambipolar diffusion coefficient (the diffusion

coefficient of oxygen ion-electron hole pairs),

cm2 s�1

d1 outer diameter of the membrane tube, cm

d2 inner diameter of the membrane tube, cmbf i fugacity of species i in the gas mixture, Pa

f0i fugacity of species i at its standard state, Pa

F inlet flow rate, mol s�1

G molar Gibbs free energy, Jmol�1

Gi partial molar Gibbs free energy of species i in the

gas mixture, Jmol�1

G0i Gibbs free energy of species i at its standard state,

Jmol�1

nG total Gibbs free energy of the system, J

DG0fi standard Gibbs free energy of formation of

compound i, Jmol�1

DH298 heat of the reaction at 298 K, kJmol�1

JO2 oxygen permeation flux through membrane,

mol s�1m�2

L length of a tubular dense membrane reactor, cm

m number of sub-separators used in the model

ni yield of compound i, mol

P total pressure in the tube membrane reactor, Pa

P1 oxygen partial pressure at oxygen-lean side, Pa

P2 oxygen partial pressure at oxygen-rich side, Pa

S effective oxygen permeation area of the

membrane tube, cm2

T operating temperature, K

yi molar fraction of compound I in the gas mixture

Greek letters

lk Lagrange multiplierb4i Fugacity coefficient of species i in the gas mixture

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 5 ( 2 0 1 0 ) 6 6 9 1e6 6 9 86692

to coke deposition. On the other hand, POX is exothermic but

it generates a lowH2/CO ratio than SR. OSR is a combination of

POX and SR and can produce suitable H2/CO ratio without

external energy consumption. OSR seems to be a reasonably

good alternative whose salient features are reduced rate of

carbon deposition and more favorable thermal equilibrium

that can be varied as a function of the oxygen feed [8].

Within the category of dense membranes, ionic oxygen

conducting membranes (IOCM) offer the unique advantage to

provide activated oxygen at its surface while preventing

hydrocarbon losses to the opposite side [9]. Intensive studies

on the IOCM process in the membrane reactors have been

performed taking into account: the catalytic properties of

perovskite-type [10,11], tubular membrane [12e15], packed

catalyst in the tubular membrane reactor [15], the effects of

oxygen flux, temperature and feed composition [16] and the

comparison of the membrane reactor with a co-feed reactor

[17,18].

Modeling of membrane reactor presents interesting chal-

lenges because of the coupling of selective diffusion through

the permeable membrane with chemical reactions and mass

transfer on the both sides of reactor. Rui et al. [19,20] consid-

ered the differences of partial oxidation of methane in

a conventional fixed-bed reactor and a dense oxygen perme-

ationmembrane reactor (DMR) caused by the different oxygen

feed method, and set up a more reasonable one-dimensional

DMR model considering the oxidation of the products, i.e., H2

and CO. Akin [21] and Rui et al. [22] recently simulated the

oxygen permeation through oxygen ionic or mixed-conduct-

ing ceramic membranes under reaction conditions with

a model taking into account of different electrical transport

mechanisms (p-type and n-type transports), and found that

the effect of downstream conditions, especially the rate of

chemical reactions involving oxygen, have strong effects on

the oxygen permeation.

However, except for Sarvar-Amini et al. [23] and Ye et al.

[24], all other models were solved by FORTRAN, MATLAB or

other computer programs, which are not easily accessible to

design engineers in industry. Various process simulators,

such as ASPEN PLUS and HYSYS, are employed widely for

industrial process simulations. ASPEN PLUS includes standard

and ideal unit operations, such as Gibbs reactionmodels. This

course of action is useful when the kineticmodels of reactions

are unknown or are high in number due to many components

participating in the reactions. A FORTRAN sub-routine was

integrated into ASPEN PLUS to simulate oxygen permeation

through membrane in the sub-separator. A DMR model based

on such popular process simulators can facilitate the appli-

cation of such models, and help chemical engineers to design

such reactors and to simulate alternative hydrogen produc-

tion processes.

There is fewpaper reportedwork of sequential simulation of

DMR for hydrogen production from oxidative steam reforming

of ethanol with ASPEN PLUS. This work performs such a simu-

lationandprovidessomeusefulguidanceonthedesignofaDMR

configuration for the reaction. The DMR used in this work is

composed of a perovskite-type oxide Ba0.5Sr0.5Co0.8Fe0.2O3�d

(BSCF5582) tubular membrane.

2. Modeling and simulation

2.1. Simulation with ASPEN PLUS

ASPEN PLUS 11.1 (Aspen Technology, Inc., MA, USA), one of

the most important process simulators in the chemical

industry and oil refining processes, is utilized. As there is no

separation model based on the oxygen permeation flux in

ASPEN PLUS and the DMR operation does not exist in the

simulator. To simulate the DMR process with ASPEN PLUS

11.1, a sequential modular approach was implemented and

a FORTRAN sub-routine was built and integrated into ASPEN

PLUS to simulate the permeation of oxygen through the

membrane.

CH3CH2OH+H2O

Air

Air

O2

O2

l=0 l=L

dFO2

dl

Fig. 1 e Schematic of a dense membrane reactor.

i 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 en e r g y 3 5 ( 2 0 1 0 ) 6 6 9 1e6 6 9 8 6693

In the simulation, the DMR ismodeled using the sequential

modular approach by dividing the reactor into multi-sub-

reactors. At each sub-reactor, the Gibbs reactor model in

ASPEN PLUS is employed to simulate the production process

with an assumption that the OSR reaction reach thermody-

namic equilibrium locally. In the Gibbs reactor model, Gibbs

free energy minimization is performed to determine the

product composition. The equilibrium computation brings in

temperature dependence, without requiring detailed infor-

mation regarding the specific reaction or catalyst perfor-

mance. The total Gibbs function for the system is given by

nG ¼XNi¼1

niGi (4)

If Gi is substituted by

Gi ¼ G0i þ RTln

bf if 0i

(5)

then

nG ¼X�

niG0i

�þ RTX

nilnbf if 0i

!(6)

Moreover, in the gas phase bf i can be written as bf i ¼ yiPb4i.

With the restraints of the material balances, the minimum

Gibbs free energy of the system then is expressed as

DG0fi þ RTln Pþ RTln yi þ RTln b4i þ

Xk

ðlkaikÞ ¼ 0 (7)

where yi is replaced by ni/Sni.

In this simulation, the product species of CH3CH2OH, H2O,

O2, H2, CO, CO2 CH4, C2H4 and CH3CHO are considered to exist

in the equilibrium system.

2.2. Oxygen permeation equations

For the oxygen permeation through tubular BSCF5582

membrane, the oxygen flux equation (8), whichwas developed

by Kim et al. [25], can be used to predict the permeation flux

when bulk diffusion is the controlling step for oxygen

permeation [26]. Lu et al. [27] deduced the ambipolar diffusion

coefficients Da for the BSCF5582 material based on the bulk

oxygen ionic diffusion current model. Table 1 presents the

ambipolar diffusion coefficients Da of the oxygen permeation

fluxes of the BSCF5582 membrane at 973e1123 K.

JO2¼ pLCiDa

2Slnðd1=d2Þln�P1

P2

�(8)

dFO2¼ pLCiDa

2lnðd1=d2Þln�P1

P2

�dl (9)

There is a distribution of oxygen in the tube side along the

dense membrane, the oxygen flow rate built up from zero.

Table 1 e The ambipolar diffusion coefficients (Da) at different

T(K) 973 1023

Da (cm2 s�1) 1.62� 10�6 2.24� 10�6

Therefore, based on Eq. (8), the local oxygen flow rate in

a differential element across the membrane is given as Eq. (9).

As there is no separation model based on Eq. (9) in ASPEN

PLUS, a FORTRAN sub-routine was built and integrated into

ASPEN PLUS to simulate the permeated oxygen flow rate

through the membrane as described by Eq. (9). After the user

model (sub-routine) was built, it was compiled using the

“aspcomp” procedure in ASPEN PLUS.

2.3. The membrane reactor model

A one-dimensional dense membrane reactor assumed in DMR

for pure hydrogen production by OSR of ethanol is shown

schematically in Fig. 1. The tubular BSCF82 membrane has an

outer diameter of 8 mm, an inner diameter of 5 mm, and length

up to 30 cm. According to the cross-flow pattern, the feed of

steam and ethanol is a plug flow in the tube side while perme-

ated oxygen are perfectlymixed in the tube side,where theOSR

takes place, closely approaching thermodynamic equilibrium.

To illuminate the simulation of OSR of ethanol, the domain

sketched in Fig. 1 is considered for model development. To

represent the characteristics of dense oxygen permeation

tubular membrane reactor, following assumptions have been

adopted:

1. The separator is under steady-state isothermal operation.

2. The radial diffusion of gases in DMR is negligible.

3. Ideal gas law can be applied to describe the gas behavior of

a single component or gas mixture.

4. The gas-phase mass-transfer resistances are negligible.

Thus, the oxygen partial pressures on both membrane

surfaces are the same as that in the shell side or tube side,

respectively.

5. The total pressure in both sides is kept at constant.

To develop a sequential modular the dense tubular

membrane reactor is axially divided intomulti-sub-reactor, as

illustrated in Fig. 2, where sub-reactor consists of an oxygen

permeation tubular membrane sub-separator and a Gibbs

sub-reactor. At each section, the flow of gas is considered as

a plug flow with axial dispersion on both the tube and shell

sides. Overall, the OSR process is represented by m sub-

separators and m Gibbs sub-reactors, where in each sub-

temperatures (973e1123 K) [26].

1073 1098 1123

2.98� 10�6 3.47� 10�6 4.12� 10�6

Fig. 2 e Sequential modular simulation diagram of DMR

with ASPEN PLUS.

0 10 20 30 40 50 602.25

2.30

2.35

2.40

2.45

2.50

2.55

2.60

2.65

Mol

es o

f H

2 per

mol

e of

eth

anol

(m

oles

/mol

e)

m = number of sub-reactors

Fig. 3 e Influence of the number of sub-reactors on the

predicted yield of H2 per mole of ethanol, P[ 0.1 MPa, T

[ 973 K, H2O/EtOH[ 0.5, Ln(P2/P1)[ 2, L[ 30 cm.

1.5

2.0

2.5

3.0

3.5

es o

f H

2 pe

r m

ole

of e

than

ol (

mol

es/m

ole)

Temperature 973 K 1023 K 1073 K

a

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 5 ( 2 0 1 0 ) 6 6 9 1e6 6 9 86694

separator it has a membrane permeation capacity of Cep/m.

The separator oxygen from the mth sub-separator is fed to

the mth Gibbs sub-reactor. For example, at m¼ 1, the model

represents a case with one sub-separator and a regular

Gibbs sub-reactor, so that the oxygen separation process is

ahead of OSR process. m¼ 2 represents a case with two

sub-separators and two Gibbs sub-reactors, where in each

sub-separator has a membrane permeation capacity of Cep/

2. The greater the number of the sub-separators employed

in the model, the more closely the sequential modular

approach method should represent the real reaction

process. In practice, m should be high enough to ensure

that the predicted hydrogen production rate from the

model is virtually independent of m.

b

0.5

1.0

Mol 1098 K

1123 K

0 5 10 15 20 25 30

0.0

0.3

0.6

0.9

1.2

1.5

Mol

es o

f C

O2 p

er m

ole

of e

than

ol

(mol

es/m

ole)

Axial length of reactor (cm)

Temperature 973 K 1023 K 1073 K 1098 K 1123 K

Fig. 4 e Influence of operating temperature at different

length of membrane reactor on the H2 and CO2 yields, P

[ 0.1 MPa, Ln(P2/P1)[ 2, H2O/EtOH[ 0.5.

3. Results and discussion

3.1. Number of sub-reactors

Fig. 3 shows the influence of m on the predicted hydrogen

production rate for a typical case: reactor pressure P¼ 0.1 MPa,

operating temperature T¼ 973 K, steam-to-ethanol ratio (H2O/

EtOH) is 0.5, the oxygen partial pressure gradient term Ln(P1/

P2)¼ 2, and the axial length of membrane reactor is 30 cm. It

can be seen that for m> 55, the influence of m on hydrogen

yield is negligible. So m can be taken as 55 for the simulation.

In the following simulations for various length values

(<30 cm) of the reactor, the m value is set as 55, and it is

considered in each case that its influence on the hydrogen

production rate is negligible.

3.2. Influence of temperature

Figs. 4 and 5 show the four major products at different

temperature along DMR with different length for a restraint

that H2O/EtOH ratio is 0.5. The conversion of ethanol is almost

100% over the whole temperature range (973e1123 K). As

described in Fig. 4(a) that hydrogen yield increases with the

length of the reactor at 973 K, while at the other temperatures,

hydrogen yield increases with the length of the reactor and

0 5 10 15 20 25 30

0.0

0.1

0.2

0.3

0.4

0.5

0.6

Mol

es o

f C

H4

per

mol

e of

eth

anol

(m

oles

/mol

e)

Axial length of reactor (cm)

Temperature 973 K 1023 K 1073 K 1098 K 1123 K

0.3

0.6

0.9

1.2

1.5

1.8

Mol

es o

f C

O p

er m

ole

of e

than

ol (

mol

es/m

ole)

Temperature 973 K 1023 K 1073 K 1098 K 1123 K

a

b

Fig. 5 e Influence of operating temperature at different

length of membrane reactor on the CH4 and CO yields, P

[ 0.1 MPa, Ln(P2/P1)[ 2, H2O/EtOH[ 0.5.

i 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 en e r g y 3 5 ( 2 0 1 0 ) 6 6 9 1e6 6 9 8 6695

reaches maximum and then decreases with the increase of

the length of the reactor. For example, the maximal yield of

hydrogen is 2.942 with 10 cm at 1023 K, 3.184 with 6 cm at

1073 K, 3.248 with 4 cm at 1098 K, and 3.296 with 4 cm at

1123 K.

The OSR process in a DMR consists of SR reaction (1) and

POX reaction (2). Both of these two reactions consume ethanol

and produce hydrogen. Since the supply of oxygen in DMR is

distributed along the axial length of the membrane tube and

a fixed oxygen partial pressure ratio between the two sides is

assumed during the calculation, the oxygen feeding amount

increases with the increase of the length of the membrane

tube. When the membrane length is short, the permeated

oxygen will react with the excess ethanol by reaction (2) along

with reaction (1) to producemorehydrogen.As a consequence,

the hydrogen yield increases with an increase of the length of

themembrane tube for this case.When the permeated oxygen

reaches the stoichiometric amount of reaction (2) with the

excess ethanol, the hydrogen yield reachesmaximum. Beyond

this point, the permeated oxygen may react with the product

hydrogen by the following reaction [19,20,28].

2H2 ðgÞ þO2 ðgÞ42H2O ðgÞ; DH298 ¼ �483:63 kJ=mol (10)

As a result, the hydrogen yield decreases with the further

increase of the length of the membrane after reaching the

maximum point for the temperatures 1023 K, 1073 K, 1098 K

and 1123 K, respectively, as shown in Fig. 4(a).

The change of temperature has two main effects. Firstly,

the oxygen permeation flux, as expressed by Eq. (8), increases

with the increase of temperature when the oxygen partial

pressure ratio between the two sides is fixed. Therefore, the

effect of the permeated oxygen is more significant at a higher

temperature for the same length of the membrane tube. As

indicated in Fig. 4(a), the length for the maximum hydrogen

yield decreases with the increase of temperature. For the

temperature 973 K, the hydrogen yield cannot yet reach the

maximumpoint at the length of 30 cm. On the other hand, the

change of temperature affects the equilibrium states of reac-

tions (1), (2) and (10). Since reaction (1) is endothermic and

reactions (2) and (10) are exothermic, increasing temperature

favors reaction (1), while shifts reactions (2) and (10) to the left

side. The final hydrogen yield depends on the relative domi-

nances of these two effects. For example, when the length of

the membrane tube is shorter than 4 cm (the maximum

hydrogen yield point at 1123 K), the hydrogen yield increases

with the increase of temperature due to both the favorable

effect of improved oxygen permeation amount and the equi-

librium shift of reaction (1), as shown in Fig. 4(a).

Fig. 4(b) reveals the trend of carbon dioxide yield which

increases with the increase of the operating temperature and

the length of the reactor. When themembrane length is short,

the permeated oxygen reacts with the excess ethanol by

reaction (2) along with reaction (1) to produce more carbon

dioxide. Therefore, the carbon dioxide yield increaseswith the

length of the membrane tube for this case. When the

membrane length is long, the permeated oxygen moles in

DMR is more than the stoichiometric amount of reaction (2)

for the excess ethanol, the excess permeated oxygen leads to

the combustion of other products to produce more carbon

dioxide [29e31,36], for instance, CO, CH4, C2H4 and CH3CHO,

which are considered in the equilibrium system. Therefore, in

the excess presence of oxygen, the following reactions (11)e

(14) can also occur.

2CO ðgÞ þO2 ðgÞ42CO2 ðgÞ; DH298 ¼ �565:93 kJ=mol (11)

CH4 ðgÞ þ 2O2 ðgÞ4CO2 ðgÞ þ 2H2O ðlÞ; DH298

¼ �890:29 kJ=mol (12)

C2H4 ðgÞ þ 3O2 ðgÞ42CO2 ðgÞ þ 2H2O ðlÞ; DH298

¼ �1411:14 kJ=mol (13)

2CH3CHO ðgÞ þ 5O2 ðgÞ44CO2 ðgÞ þ 4H2O ðlÞ; DH298

¼ �2384:63 kJ=mol (14)

The carbon dioxide yield increases with the increase of the

operating temperature due to two positive behaviors of OSR in

DMR. The first, the oxygen permeation flux increases with the

increaseof temperatureasexplained in thepreviousparagraph,

which leads to the production of more carbon dioxide. Mean-

while, the increase of temperature affects the equilibrium state

of reactions (1)e(2) and (11)e(14). Since reaction (1) is endo-

thermic, and reactions (2) and (11)e(14) are exothermic,

increasing temperature favors reaction (1), while shifts reac-

tions (2) and (11)e(14) to the left side. However, the final carbon

dioxide yield depends on the dominant reaction (1) at high

0

1

2

3

4

5

H2O/EtOH 0 0.5 1 2 5

Mol

es o

f H

2 pe

r m

ole

of e

than

ol

(

mol

es/m

ole)

0 5 10 15 20 25 30

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Mol

es o

f C

O2

per

mol

e of

eth

anol

(mol

es/m

ole)

Axial length of reactor (cm)

H2O/EtOH 0 0.5 1 2 5

a

b

Fig. 6 e Influence of steam-to-ethanol ratio at different

length of membrane reactor on the H2 and CO2 yields, T

[ 1073 K, P[ 0.1 MPa, Ln(P2/P1)[ 2.

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 5 ( 2 0 1 0 ) 6 6 9 1e6 6 9 86696

temperatures. As a consequence, the increase of the operating

temperature favorsmorecarbondioxideas indicated inFig. 4(b).

Fig. 5(a) shows the carbon monoxide yield increases with

the length of the DMR at 973 K, while it increases with the

length of the DMR and reaches maxima and then decreases

with the increase of the length at the other temperatures. For

example, the maximal yield of carbon monoxide is 1.669 with

10 cm at 1023 K, 1.818 with 6 cm at 1073 K, 1.867 with 4 cm at

1098 K, and 1.894 with 4 cm at 1123 K, respectively.

The SR reaction (1) can bewell represented by the following

reactions (15) and (16) [32,33].

C2H5OH ðlÞ þH2O ðlÞ44H2 ðgÞ þ 2CO ðgÞ; DH298

¼ 341:73 kJ=mol (15)

CO ðgÞ þH2O ðgÞ4H2 ðgÞ þ CO2 ðgÞ; DH298 ¼ �41:2 kJ=mol

(16)

The permeated oxygen, which is distributed along the axial

length of DMR, oxidizes excess ethanol along with reaction

(15) to produce more carbon monoxide by the following

reaction (17) when the length of membrane tube is short.

2CH3CH2OH ðlÞ þO2 ðgÞ44CO ðgÞ þ 6H2 ðgÞ; DH298

¼ 111:80 kJ=mol (17)

As a result, the carbon monoxide yield increases with the

increase of the length of the DMR. When the permeated

oxygen reaches the stoichiometric amount of reaction (17) for

the excess ethanol, the carbon monoxide yield reaches

maximum. Beyond this point, the excess permeated oxygen

reacts with carbon monoxide according to the reaction (11),

therefore, the carbon monoxide yield decreases with the

further increase of the length of the reactor after reaching the

maximum point for the temperatures 1023 K, 1073 K, 1098 K

and 1123 K, respectively, as shown in Fig. 5(a).

As described in the previous paragraph, an increase of

temperature leads tomore oxygen permeated into DMR, so the

effect of the permeated oxygen is more significant at a higher

temperature for the same length of the membrane tube. As

indicated in Fig. 5(a), the length for the maximum carbon

monoxide yield decreaseswith the increase of the temperature.

For the temperature 973 K, the carbon monoxide yield cannot

reach the maximum point at the length of 30 cm. On the other

hand, the change of temperature affects the equilibrium states

of reactions (11) and (15)e(17). Since reactions (15) and (17) are

endothermic, and reactions (11) and (16) are exothermic,

increasing temperature favors reaction (15) and (17), while

shifts reactions (11) and (16) to the left side. The final carbon

monoxide yield depends on the relative dominances of these

two effects. For example, when the length of the membrane

tube is shorter than 10 cm (the maximum carbon monoxide

yield point at 1023 K), the carbonmonoxide yield increaseswith

the increase of the temperature due to both the favorable effect

of improved oxygen permeation amount and the equilibrium

shift of reactions (15) and (17), as shown in Fig. 5(a).

Fig. 5(b) gives the trend ofmethane yield with the change of

operating temperature and the length of the DMR. The

production of methane at high temperature was suggested to

followtheethanoldecompositionreactionaccording to (18) [34].

C2H5OH ðlÞ4H2 ðgÞ þ CH4 ðgÞ þ CO ðgÞ; DH298 ¼ 91:57 kJ=mol

(18)

Since the permeated oxygen reacts with the formed

methane by reaction (12), the equilibrium methane yield

decreases with the increase of the length of the membrane

tube as shown in Fig. 5(b). Increase the temperature cannot

only improve the oxygen permeation through the membrane

and also favor the endothermic methane steam reforming

reaction (19) [35], which in turn decrease the methane yield.

CH4 ðgÞ þH2O ðgÞ4CO ðgÞ þ 3H2 ðgÞ; DH298 ¼ 206:16 kJ=mol

(19)

3.3. Influence of steam-to-ethanol ratio

Figs. 6 and 7 plot the relationship between the four major

product yields and the H2O/EtOH ratio and the length of the

DMR at 1023 K. It can be seen from Fig. 6(a) that the hydrogen

yield is significantly influenced by the H2O/EtOH ratio. For the

H2O/EtOH ratio of 0, 0.5 and 1, hydrogen yield shows the

volcano shape and reaches themaximawith different lengths

of reactor, respectively. For example, the maximal yield of

hydrogen is 2.723 with 10 cm at H2O/EtOH¼ 0, 3.183 with 6 cm

at H2O/EtOH¼ 0.5 and 3.645 with 2 cm at H2O/EtOH¼ 1,

respectively. However, for H2O/EtOH ratio of 2 and 5, hydrogen

yield decreases with the length of the DMR monotonically.

Additionally, for the fixed length of DMR, moles of hydrogen

are higher at a higher H2O/EtOH ratio than at a lower H2O/

EtOH ratio. This is because the amount of the H2O affects the

0 5 10 15 20 25 30

0.0

0.2

0.4

0.6

0.8

1.0

Mol

es o

f C

H4

per

mol

e et

hano

l(m

oles

/mol

e)

Axial length of reactor (cm)

H2O/EtOH 0 0.5 1 2 5

0.0

0.5

1.0

1.5

2.0

Mol

es o

f C

O p

er m

ole

of e

than

ol (

mol

es/m

ole)

H2O/EtOH 0 0.5 1 2 5

a

b

Fig. 7 e Influence of steam-to-ethanol ratio at different

length of membrane reactor on the CH4 and CO yields, T

[ 1073 K, P[ 0.1 MPa, Ln(P2/P1)[ 2.

i 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 en e r g y 3 5 ( 2 0 1 0 ) 6 6 9 1e6 6 9 8 6697

moles of the excess ethanol, which can react with the

permeated oxygen by POX reaction (2) and affects the change

of hydrogen yield with themembrane tube length. In addition,

for the fixed length of DMR, the hydrogen yield increases with

the increase of H2O/EtOH ratio due to its favorable effect on

the SR reaction (1), as depicted in Fig. 6(a).

It can be found from Fig. 6(b) that carbon dioxide yield

increases both monotonically with the increase of the H2O/

EtOH ratio and the length of the reactor. According to the SR

reaction (1), the increase of H2O/EtOH ratio favors the carbon

dioxide yield by shifts reaction (1) to the right. Furthermore,

the oxygen feeding amount, which increases with the length

of the membrane tube, favors reaction (2) to produce more

carbon dioxide. Reactions (11)e(14) can also occur in the

excess presence of oxygen, which produces more carbon

dioxide. As a result, carbon dioxide yield increases with the

increase of the H2O/EtOH ratio and the length of the DMR, as

shown in Fig. 6(b).

Fig. 7(a) provides the trend of carbon monoxide yield

changes with the H2O/EtOH ratio and the length of membrane

tube. It can be seen that carbonmonoxide yield at a lowerH2O/

EtOH ratio is higher than that at a higher H2O/EtOH ratio for

the fixed length of the DMR. Moreover, carbonmonoxide yield

shows the volcano shaped curves and reaches themaxima for

the H2O/EtOH ratios of 0, 0.5 and 1, respectively. For example,

the maximal yield of carbon monoxide is 1.824 with 10 cm at

H2O/EtOH¼ 0, 1.821 with 6 cm at H2O/EtOH¼ 0.5 and 1.815

with 2 cm at H2O/EtOH¼ 1, respectively. However, for the

H2O/EtOH ratio of 2 and 5, the carbon monoxide yield

decreases monotonically with the length of the DMR.

The H2O amount in the reactant affects the equilibrium

state of the water gas shift reaction (16) and the H2 yield, as

depicted in Fig. 7(a). For the effect of the membrane length,

when it is short, the permeated oxygen reacts with the excess

ethanol to produce more carbon monoxide by reaction (17).

Therefore, the carbon monoxide yield increases with the

increase of the length of themembrane tube for the H2O/EtOH

ratio of 0, 0.5 and 1. When the permeated oxygen reaches the

stoichiometric amount of reaction (17), carbon monoxide

yield reaches maximum. Beyond this point, the permeated

oxygen reacts with the producced carbon monoxide by reac-

tion (10) [29], as a result, the carbon monoxide yield decreases

with the further increase of the length of the membrane, as

shown in Fig. 7(a). However, for the H2O/EtOH ratios of 2 and

5, there is no excess ethanol to consume the permeated

oxygen, which will react directly with the product carbon

monoxide by reaction (10). The carbon monoxide yield

decreases with the length of the DMR, as shown in Fig. 7(a).

Fig. 7(b) shows the trend ofmethane yield changeswith the

H2O/EtOH ratio and the length of membrane tube. The

methane yield decreases with the increase of H2O/EtOH ratio

and the length of DMR both monotonically and approaches

zero along with the tube length with different slope for the

different H2O/EtOH ratio. This is because both H2O and O2 can

affect themethane yield by reacting with CH4 according to the

steam reforming reaction (19) [36] and reaction (12).

4. Conclusion

Hydrogen production via oxidative steam reforming of

ethanol with a dense tubular membrane reactor (DMR) has

been sequentially simulated with ASPEN PLUS. Simulation

result on DMR performance shows that there exists an

optimal length of the tubular membrane reactor at an oper-

ating temperature and a fixed steam-to-ethanol ratio. Under

this condition, the carbon monoxide and hydrogen formation

reaches its maximum. For example, at the fixed ratio H2O/

EtOH¼ 0.5, the maximum hydrogen yield occurred at

a temperature of 1123 K with the length of tubular DMR of

4 cm. For the temperature of 1073 K, the maximum hydrogen

yield appears at H2O/EtOH¼ 5 with the length of tubular DMR

of 0 cm. Using the approach proposed in this work, the

prediction of the DMR’s behavior under different operating

conditions becomes meaningful.

Acknowledgements

This work has been supported by the Natural Science Foun-

dation of China under contract number 20425619 and

20736007. Thework has been also supported by the Programof

Introducing Talents to the University Disciplines under file

number B06006, and the Program for Changjiang Scholars and

Innovative Research Teams in Universities under file number

IRT 0641.

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 5 ( 2 0 1 0 ) 6 6 9 1e6 6 9 86698

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