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Co-current and counter-current configurations for ethanolsteam reforming in a dense Pd–Ag membrane reactor

F. Galluccia, M. De Falcob, S. Tostic, L. Marrellib, A. Basiled,*aFundamentals of Chemical Reaction Engineering Group, Faculty of Science and Technology, University of Twente, Enschede,

The NetherlandsbUniversita di Roma La Sapienza, Dip. Ing. Chimica e Materiali, Via Eudossiana 18, 00184 Roma, ItalycENEA,Dipartimento Fusione, Tecnologie e Presidio Nucleare, C.R. ENEA Frascati, Via E. Fermi 45, 00044 Frascati (RM), ItalydInstitute on Membrane technology, ITM-CNR, c/o University of Calabria, Via Pietro Bucci, Cubo 17C, 87030 Rende (CS), Italy

a r t i c l e i n f o

Article history:

Received 8 April 2008

Received in revised form

3 July 2008

Accepted 3 July 2008

Available online 25 September 2008

Keywords:

Ethanol steam reforming

Membrane reactor

Pd membranes

Hydrogen production

Modelling

* Corresponding author. Tel.: þ39 0984 49201E-mail address: a.basile@itm.cnr.it (A. Ba

0360-3199/$ – see front matter ª 2008 Interndoi:10.1016/j.ijhydene.2008.07.026

a b s t r a c t

The ethanol steam-reforming reaction to produce pure hydrogen has been studied theo-

retically. A mathematical model has been formulated for a traditional system and

a palladium membrane reactor packed with a Co-based catalyst and the simulation results

related to the membrane reactor for both co-current and counter-current modes are pre-

sented in terms of ethanol conversion and molar fraction versus temperature, pressure,

the molar feed flow rate ratio and axial co-ordinate. Although the counter-current mode

does not always give an ethanol conversion higher than the one obtained in membrane

reactor operated in co-current mode, in the first case it is always possible to extract more

hydrogen from the reaction zone. With this theoretical analysis, different values of the

operating parameters that allow to have a CO-free hydrogen stream and a complete

recovery of the hydrogen from the lumen side of the reactor are investigated.

ª 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights

reserved.

1. Introduction on Rh, Ru, Pd, Pt, Ni, Co and Cu are used on supports of Al2O3,

Polymeric fuel cell systems are electrochemical devices able to

generate electrical power by the electrochemical oxidation of

hydrogen with atmospheric oxygen. Many studies about

hydrogen production for fuel cells deal with the use of two

types of carbon compounds: the first is an oxygen-containing

compound, such as methanol or ethanol, while the second is

hydrocarbons, such as natural gas, propane gas, gasoline, etc.

[1–8]. Production of hydrogen from ethanol is very attractive

since bio-ethanol is a renewable material mainly produced

from biomass fermentation [9,10].

Many authors studied the ethanol steam reforming [11–17]

being this an endothermic catalysed reaction whose conver-

sion increases with the temperature. Usually, catalysts based

3; fax: þ39 0984 402103.sile).ational Association for H

SiO2, MgO and La2O3. The reaction conversion and selectivity

of the products (mainly H2, CO, CO2 and in minor part, CH4,

CH3CHO and C2H4) depend on the catalyst used as well as on

the operating temperature. The ethanol steam reforming was

studied in innovative reactors such as membrane reactors and

microreactors [18].

The use of membrane reactors for carrying out the steam

reforming is proposed in order to increase the ethanol

conversion at lower temperature. In fact, a membrane reactor

is a device in which a reaction and a selective separation takes

place simultaneously: in this way, the continuous removal of

one of the product permits obtaining reaction conversion

beyond the thermodynamic equilibrium that is an upper limit

to be considered in a traditional reactor (shift effect) [19–21].

ydrogen Energy. Published by Elsevier Ltd. All rights reserved.

FeedRetentate OUT

Permeate OUT

Co-current

Sweep Gas

Sweep Gas Permeate OUT

a

b

Nomenclature

D reactor diameter, m

Ji membrane flux of component i, mol/m2 s

MR membrane reactor

Ni molar flow rate component i, mol/s

p pressure, bar

rj reaction rate of reaction j, mol/m2 s

T temperature, K

TR traditional reactor

z reactor co-ordinate, m

DH enthalpy of reaction, kJ/mol

ni,j coefficient of component i in reaction j

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 e n e r g y 3 3 ( 2 0 0 8 ) 6 1 6 5 – 6 1 7 16166

The palladium and its alloys are permeable to the hydrogen

and are used for manufacturing permeators and membrane

reactors aimed at separating and producing hydrogen: several

studies report the use of these membranes [22–24]. Especially,

the use of dense Pd–Ag membranes allows maximizing the

shift effect and producing directly ultra pure hydrogen

without using any other purification unit.

The application of dense Pd–Ag membrane reactors was

also suggested for carrying out methanol and ethanol steam

reforming reactions [25–32].

In a previous work [33], the authors studied the ethanol

steam reforming reaction in a TR and in an MR confirming that

the MR is able to give better results in terms of ethanol

conversion than the TR. Moreover, by using an MR a COx-free

stream can directly be produced and used in a fuel cell system.

When using an MR, an important parameter to be taken into

account is the relative direction of both the retentate and

permeate streams. In particular, there are differences in terms

of both ethanol conversion and hydrogen recovery when the

co-current mode or the counter-current one is used. The use of

the counter-current mode was already studied from a theo-

retical point of view in reaction systems such as methane

steam reforming [34], methanol steam reforming [29,31] and

water gas-shift reaction [30], carried out in Pd–Ag membrane

reactors. These studies found that the counter-current mode is

able to increase the hydrogen recovery with respect to the co-

current one for a wide range of operative conditions. Differ-

ences between the performances of MR operated in co-current

and counter-current modes have also been observed for other

reaction systems [31,35,36]. In the present work, a theoretical

model has been used for simulating, the first time for the

ethanol steam reforming reaction, the behaviour of a dense Pd–

Ag membrane reactor operated in both co-current and

counter-current modes. The performances in terms of ethanol

conversion and hydrogen recovery of both the configurations

have been compared taking into account the effect of the

various operating conditions (temperature, pressure, sweep

gas mode and the water/ethanol feed molar ratio).

FeedRetentate OUT

Counter-current

Fig. 1 – Sketch of a membrane reactor in co-current mode

(a) and counter-current mode (b).

2. Theoretical model of ethanol steamreforming

The model has been developed using all the thermodynamic

values for the reaction species and the kinetic expressions;

hydrogen removal has been calculated using experimental

permeabilities [30].The equations considered in the ethanol

reaction system carried out on a Co-based catalyst are as

follows:

C2H5OHþ 3H2O52CO2 þ 6H2 DH298k ¼ 157:09 kJ=mol (1)

COþH2O5CO2 þH2 DH298k ¼ �41:19 kJ=mol (2)

C2H5OH5COþ CH4 þH2 DH298k ¼ 33:18 kJ=mol (3)

In particular, reactions (1) and (3) are both reversible and

endothermic reactions and proceed under increased volume,

suggesting that the highest ethanol conversions are obtained

at high temperature and low pressures. The reaction (2) is the

(exothermic) water gas-shift reaction, which proceeds simul-

taneously with the ethanol steam reforming, and with no

volume change.

A mass balance for a differential reactor volume along the

z-axis (Fig. 1) of MR for both co-current and counter-current

mode has been written for the reaction zone (lumen) and for

the permeating zone (the last mass balance changes for the

two modes).The following common hypotheses have been

considered for the model formulation:

� plug flow,

� kinetic control of the reaction system,

� isothermicity,

� constant catalyst bed density through the reactor.

The model consists of the following equations:

Mass balances:

Reaction zonedNi

dz¼ pD

X3

j¼1

vijrj � JipD (4)

nversio

n, %

60

80

100

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 e n e r g y 3 3 ( 2 0 0 8 ) 6 1 6 5 – 6 1 7 1 6167

Permeation zonedNi

dz¼ �JipD ½þ for co-current mode;

� for counter-current mode� (5)

where the reactions rates ri are calculated according to Sahoo

et al. [37].

Boundary conditions (z¼ 0).

Reaction zone (lumen side):

N10 CH3CH2OH molar flow rate at the inlet side of the lumen

N20¼mN1

0 H2O molar flow rate at the inlet side of the lumen

N30¼ 10�7 N1

0 H2 molar flow rate at the inlet side of the

lumen1

N40¼ 10�7 N1

0 CO2 molar flow rate at the inlet side of the

lumen

N50¼ 10�7 N1

0 CO molar flow rate at the inlet side of the

lumen

N60¼ 10�7 N1

0 CH4 molar flow rate at the inlet side of the

lumenwhere m is the H2O/CH3CH2OH molar feed flow ratio.

Permeation zone (shell side):

N70¼NSweep gas

0 Sweep gas (H2O or N2) flow rate at the inlet

side of the shell

N80¼N8

0 H2 flow rate at the inlet side of the shell (co-current)

or outlet side of the shell (counter-current).For the co-current

mode, N80 is zero, whereas for the counter-current mode N8

0 is

found by a shooting procedure, being N8jz¼L ¼ 0.

The set of differential equations has been solved using a IV

order Runge–Kutta method with variable step in order to over-

come the instabilities in the first part of the reactor. In addition,

the counter-current mode has been solved using the shooting

method. In fact, for the counter-current mode, the initial value

N80 is unknown, whereas the final value N8 is known to be zero.

The solution for the counter-current mode is possible by

guessing a value for N80, by solving the equation system after-

wards with the Runge–Kutta method and finally controlling the

final solution for N80. When this solution for N8 is zero, the whole

solution can be considered as the right solution, otherwise

a new guess value for N80 is necessary. Of course, during the

shooting procedure N8 can reach zero before z¼ L and in this

case a new guess value is automatically chosen. The code was

written by using the Salford Plato3 FORTRAN95 platform.In

order to compare the results of the two reactors, the ethanol

conversion and the hydrogen recovery are defined as follows:

CH3CH2OH conversion; % ¼ CH3CH2OHIN � CH3CH2OHOUT

CH3CH2OHIN100

(6)

H2 recovery; % ¼ H2;out-shell

H2;out-lumen þH2;out-shell100 (7)

W/F, kg cat/(mol/s)

0 2 4 6 8 10 12 14 16 18

Eth

an

ol C

o

0

20

40 673 K773 K873 K773 K model 873 K model673 K model

3. Results and discussion

3.1. Validation of the model

The model was validated by using the experimental results

reportedbySahooetal. [37] for theTR.Unfortunately, thereareno

1 N03 ¼ 00ri/N:

data in literature regarding the use of membrane reactors for the

ethanol steam reforming on Co-based catalyst. For this reason,

a direct comparison of the modelling results and membrane

reactor experimental results at the moment is not possible. The

results of the model validation are reported in Fig. 2 where a very

good agreement can be seen between the experimental results

reported by Sahoo et al. [37] and the theoretical model presented

in the whole range of parameters investigated.

3.2. The effect of the sweep gas mode (co-current andcounter-current modes)

The main difference between the co-current mode and the

counter-current one is the distribution of the gas partial

pressures along the reactor in the two modes. Figs. 3 and 4

sketch the hydrogen partial pressure profile in the shell side

and lumen side for the co-current and counter-current modes,

respectively. The operating conditions investigated are the

same for both the figures: water/ethanol¼ 3, sweep gas/

ethanol¼ 2, T¼ 673 K, plumen¼ 2 bar, pshell¼ 1 bar.

As it can be seen in Fig. 3, the hydrogen partial pressure in

both the lumen and shell sides increases along the reactor

axes. Moreover, the driving force for the hydrogen perme-

ation, related to the difference between the hydrogen partial

pressure in the lumen side and in the shell side reported in

Fig. 3, decreases by increasing the reactor length because the

hydrogen partial pressure in the shell side tends to reach

the hydrogen partial pressure in the lumen side. When the

hydrogen partial pressures in the two sides became equal,

the hydrogen permeation through the membrane equals zero

and the membrane reactor starts to work as a traditional one.

From the example reported in Fig. 3, it is clear that, by using

the co-current mode it is not simple to recovery, as COx-free

stream, all the hydrogen produced in the reactor. The only

way to do it is to keep the hydrogen partial pressure in the

shell side equal to zero, that is to say to use a very large sweep

gas flow rate, with all the relative problems in terms of pres-

sure drop in the shell side and the extremely high dilution of

the hydrogen containing stream and so on.

On the other hand, Fig. 4 shows that the hydrogen partial

pressure profile in the counter-current mode is completely

Fig. 2 – Comparison between experimental results and

theoretical ones for TR; p [ 1 bar.

Reactor length, cm

0 10 15 20

Hyd

ro

gen

p

artial p

ressu

re, b

ar

0.0

0.2

0.4

0.6

0.8

Lumen side

Shell side

5

Fig. 3 – Hydrogen partial pressure profiles along the reactor

length in both the shell side and the lumen side. Co-

current mode, water/ethanol [ 3, sweep gas/ethanol [ 2,

T [ 673 K, plumen [ 2 bar, pshell l[ 1 bar.

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 e n e r g y 3 3 ( 2 0 0 8 ) 6 1 6 5 – 6 1 7 16168

different. In fact, the hydrogen partial pressure in the lumen

side increases in the first part of the reactor (reactor

length< 2 cm), mainly due to the reaction, and afterwards it

diminishes mainly due to the permeation through the

membrane. It is evident that, in the example reported in this

figure, the hydrogen partial pressure at the exit of the lumen

side (i.e., reactor length¼ 20 cm) is almost zero, but still there

is a driving force for the permeation higher than zero because

this is the inlet zone for the sweep gas and so the hydrogen

partial pressure in the shell side is zero. It is clear that, in

almost the whole reactor there is a positive driving force for

the hydrogen permeation and consequently, by just using the

counter-current mode instead of the co-current one, it is

possible to recover 100% of the hydrogen produced in the

reactor. There is a reactor zone (i.e., reactor length< 1 cm)

where the driving force is negative and so a counter diffusion

of hydrogen through the membrane occurs from the shell side

to the lumen side (see the zoom in Fig. 4).

Reactor length, cm

0 5 10 15 20

Hyd

ro

gen

p

artial p

ressu

re, b

ar

0.0

0.2

0.4

0.6

0.8

1.0

Lumen side

Shell side

Fig. 4 – Hydrogen partial pressure profiles along the reactor

length in both the shell side and the lumen side. Counter-

current mode, water/ethanol [ 3, sweep gas/ethanol [ 2,

T [ 673 K, plumen [ 2 bar, pshell [ 1 bar.

Finally, it should be noted that the differences in terms of

ethanol conversion and hydrogen recovery between the co-

current mode and the counter-current one became negligible

if a very large sweep gas flow rate is used. In fact, as shown in

Fig. 5, which is the same for co-current and counter-current

mode, when a very large sweep gas flow rate is considered (in

the case reported in this figure, the sweep gas flow rate is

12,500 times the ethanol feed flow rate), the hydrogen partial

pressure in the shell side is practically zero along the whole

reactor. Moreover, the hydrogen recovery is almost 100% since

the hydrogen partial pressure in the lumen side is two orders

of magnitude lower that the one reported in both Figs. 3 and 4.

The theoretical results show that, by increasing the sweep gas

flow rate, both the hydrogen recovery and the ethanol

conversion in co-current and counter-current mode converge.

Eventhough the results of both modes are the same for

a sweep gas flow rate> 12,500, at the sweep gas flow rate> 90,

the results are quite similar and the differences could be

considered negligible.

3.3. The combined effect of temperature and pressure

Fig. 6 shows a three dimensional graph in which the ethanol

conversion versus the temperature and the reaction pressure

is reported for both co-current and counter-current modes.

For both modes, the conversion increases with increasing the

temperature and the pressure. There is a part of the domain

where the ethanol conversion in co-current mode is higher

than the ethanol conversion in counter-current mode.

Generally, it happens at low pressures for the whole range of

temperature considered as well as at low temperatures for the

whole range of pressures investigated. Vice versa, at high

temperatures and high pressures, the ethanol conversion in

the counter-current mode is always higher than the conver-

sion in the co-current mode. In order to maximise the ethanol

conversion, from this graph it is evident that the choice

between the two configurations depends on the values of the

parameters investigated.

Reactor length, cm

0 10 15 20

Hyd

ro

gen

p

artial p

ressu

re, b

ar

0

1e-3

2e-3

3e-3

4e-3

5e-3

6e-3

Lumen side

Shell side

5

Fig. 5 – Hydrogen partial pressure profiles along the reactor

length in both the shell side and the lumen side. Counter-

current mode, water/ethanol [ 3, sweep gas/

ethanol [ 25000, T [ 673 K, plumen [ 2 bar, pshell [ 1 bar.

0

20

40

60

80

100

573598

623648

673698 723 748 773

1.01.5

2.02.5

3.03.5

4.04.5

5.05.5

6.06.5

Eth

an

ol co

nversio

n, %

Temperature, °C

Pressure, b

ar

Co-current

Counter-current

Fig. 6 – Ethanol conversion versus pressures and

temperatures. Co-current and counter-current modes,

water/ethanol [ 3, sweep gas/ethanol [ 2, pshell [ 1 bar.

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 e n e r g y 3 3 ( 2 0 0 8 ) 6 1 6 5 – 6 1 7 1 6169

For what concerns the effect of the two modes on the

hydrogen recovery, Fig. 7 illustrates well the situation: the

counter-current mode is able to give always a higher hydrogen

recovery with respect to the co-current one. This aspect can be

explained considering again the comments reported for Figs. 3

and 4 or, in other words, the difference in terms of hydrogen

partial pressure profile in the two modes.

An interesting result is that, by using the counter-current

mode for pressures higher than 2.5 bar the maximum

hydrogen recovery (100%) is obtained for all the temperatures

considered. Vice versa, in the case of the co-current mode, the

30

40

50

60

70

80

90

100

598623

648673

698723

748773

1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0

Hyd

ro

gen

reco

very, %

Tem

perature,°

C

Pressure, bar

Co-current

Counter-current

Fig. 7 – Hydrogen recovery versus pressures and

temperatures. Co-current and counter-current modes,

water/ethanol [ 3, sweep gas/ethanol [ 2, pshell [ 1 bar.

best result (94%) is obtained only at high pressure and high

temperature.

3.4. The effect of the sweep gas flow rate

The effect of the sweep gas flow rate on both the ethanol

conversion and the hydrogen recovery is well illustrated on

Fig. 8. By increasing the sweep gas molar ratio, calculated as

the ratio between the sweep gas flow rate and the ethanol feed

flow rate, the ethanol conversion in both co-current and

counter-current mode increases. In particular, for sweep ratio

higher than 2 the counter-current mode gives higher conver-

sion than the co-current mode. This fact indicates that the

domain where the co-current mode gives better results than

the counter-current one depends on the pressure and the

temperature (as already indicated in Fig. 6), but also from the

sweep gas flow rate. Moreover, for sweep ratio higher than 10,

the counter-current mode is able to give complete ethanol

conversion while this value is never reached in the co-current

mode in the whole range of sweep ratio investigated. Con-

cerning the hydrogen recovery, the same Fig. 8 clearly indi-

cates that the counter-current mode gives complete recovery

and reaction conversion for sweep ratios higher than 5, while

the highest hydrogen recovery obtained in the co-current

mode is reached at sweep ratio¼ 10 and it is 95%. In particular,

this value is a plateau for the co-current mode operated at the

conditions illustrated in Fig. 8.

3.5. The combined effect of pressure and feed molar ratio

The three dimensional Figs. 9 and 10 report for both co-current

and counter-current modes the effect of pressure and water/

ethanol feed molar ratio on the ethanol conversion and

hydrogen recovery, respectively. In particular, Fig. 9 shows that

for both co-current and counter-current modes the ethanol

conversion sharply increases for feed molar ratio between 2

and 3, for each value of pressure investigated, while it slightly

increases for feed molar ratios higher than 3. Moreover, for

pressures lower than 3 bar and feed molar ratio higher than 3,

Sweep ratio, -

0 5 10 15 20 25 30

Eth

an

ol co

nversio

n, %

75

80

85

90

95

100 Counter-current

Co-current

Sweep ratio, -

0 5 10 15 20 25

Hyd

ro

gen

reco

very, %

75

80

85

90

95

100

Co-current

Counter-current

Fig. 8 – Ethanol conversion and hydrogen recovery versus

sweep gas/ethanol ratio. Co-current and counter-current

modes, water/ethanol [ 3, T [ 673 K, plumen [ 3 bar,

pshell [ 1 bar.

50

60

70

80

90

100

2 3 4 5 6 7 8 9 10 11

23

45

67

Eth

an

ol co

nverstio

n, %

Water/Ethanol feed ratio, -

Pressure, b

ar

Counter-current

Co-current

Fig. 9 – Ethanol conversion versus pressures and water/

ethanol feed ratio. Co-current and counter-current modes,

sweep gas/ethanol [ 2, T [ 673 K, pshell l[ 1 bar.

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 e n e r g y 3 3 ( 2 0 0 8 ) 6 1 6 5 – 6 1 7 16170

the co-current mode is able to give better results in terms of

ethanol conversion than the counter-current one does.

Considering the effect of the molar feed ratio on the

hydrogen recovery, it is evident in Fig. 10 that the hydrogen

recovery in counter-current mode is not influenced by the

feed molar ratio while the co-current mode is strongly influ-

enced by this parameter. In fact, in co-current mode, for the

whole range of pressure investigated, the hydrogen recovery

decreases by increasing the feed molar ratio, even though this

decrease is lower at higher pressures.

On the one hand, by increasing the feed molar ratio part of

the excess of water dilutes the hydrogen produced in the

50

60

70

80

90

100

23

45

67

89

1011

23

45

67

Hyd

ro

gen

reco

very, %

Counter-current

Co-current

Wate

r/Eth

anol feed ra

tio, -

Pressure, b

ar

Fig. 10 – Hydrogen recovery versus pressures and water/

ethanol feed ratio. Co-current and counter-current modes,

sweep gas/ethanol [ 2, T [ 673 K, pshell [ 1 bar.

reaction zone resulting in a lower hydrogen partial pressure

difference between the lumen side and the shell side of the

reactor and so in a lower hydrogen recovery. On the other

hand, from Fig. 10 it seems that the counter-current mode

does not suffer by this aspect and this fact could be related to

the different partial pressure profiles between the two modes.

Therefore, it is also worth noting that, at low pressures the

counter-current mode is able to give an increase at least of

30% on the hydrogen recovery with respect to the co-current

mode for each feed molar ratio considered, while at high

pressure the difference between the co-current mode and the

counter-current one decreases up to 5% at 8 bar and water/

ethanol¼ 2.

4. Conclusions

The ethanol steam-reforming reaction to produce pure

hydrogen has been studied theoretically in a Pd-based

membrane reactor for both co-current and counter-current

modes. The results indicate that the counter-current mode is

able to give higher hydrogen recovery than the co-current

mode in the whole range of operative conditions investigated.

For what concerns the ethanol conversion, this theoretical

study also indicates that attentions should be paid in order to

identify the set of parameters able to maximise it in the

counter-current mode. In fact, low pressures and low

temperatures give higher ethanol conversion in the co-current

mode while high pressures and high temperatures give higher

ethanol conversion in the counter-current mode.

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