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Methanol steam reforming reaction in a Pd–Ag membranereactor for CO-free hydrogen production

Adolfo Iulianelli, Tiziana Longo, Angelo Basile*

CNR-ITM, Institute on Membrane Technology, c/o University of Calabria, Via Pietro Bucci, Cubo 17/C, 87030 Rende (CS), Italy

a r t i c l e i n f o

Article history:

Received 1 July 2008

Accepted 15 July 2008

Available online 18 September 2008

Keywords:

Methanol steam reforming

CO-free hydrogen production

Membrane reactor

Pd–Ag membrane

* Corresponding author. Tel.: þ39 0984492013E-mail address: a.basile@itm.cnr.it (A. Ba

0360-3199/$ – see front matter ª 2008 Interndoi:10.1016/j.ijhydene.2008.07.044

a b s t r a c t

A dense tubular Pd–Ag membrane reactor was used to carry out the methanol steam

reforming reaction for producing a CO-free hydrogen stream. A Cu/Zn/Mg-based catalyst

was packed in the lumen side of the membrane reactor and the experimental tests were

performed at a reaction temperature of 300 �C and at a H2O/methanol feed molar ratio of

3/1. The effects of the different flow configurations, as well as the sweep factor and the

reaction pressure were analysed. Experimental results in terms of CO-free hydrogen

recovery, hydrogen yield, CO-free hydrogen yield and hydrogen selectivity are presented.

Moreover, a comparison between the performances of the membrane reactor and a tradi-

tional reactor working at the same operative conditions is proposed and discussed.

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

reserved.

1. Introduction a consequent drastic loss in performance. Therefore,

Conventionally, there are three processes to produce

hydrogen by using methanol: decomposition, partial oxida-

tion and steam reforming reactions. In particular, the meth-

anol steam reforming (MSR) reaction is carried out in

traditional reactors (TRs) using mainly Cu-based catalysts due

to their high reaction activity (at a relatively low temperature

range: 200–300 �C) and their high selectivity towards the

hydrogen and carbon dioxide production [1–4]:

CH3OH þ H2O ¼ CO2 þ 3H2 DH�298 K ¼ 49:7 kJ=mol (1)

In order to find a good balance between the catalytic activity

and the product selectivity, other catalysts are also studied.

For example, the hydrogen selectivity of metals such as Pd, Pt,

Ru and Au is lower than Cu-based catalysts, however their

stability is much higher [5].

The MSR reaction produces CO as by-product. Unfortu-

nately, a concentration higher than 10 ppm in the reformed

stream is able to poison the Pt electrode of a PEMFC with

; fax: þ39 0984402103.sile).ational Association for H

hydrogen purifying devices are used to remove the CO, but the

addition of further steps affects negatively the overall process

in terms of costs and efficiency [6–8]. Hence, a process able to

produce a CO-free hydrogen stream in only one system should

be developed to utilize hydrogen for feeding a PEMFC. With-

in the purview of the process intensification strategy,

a membrane reactor (MR) is a system able to both carry out

steam reforming reactions and remove pure hydrogen in the

same device [9,10]. In fact, using steam as a sweep gas, the

removed hydrogen from the shell side of the MR can be

directly suitable for feeding a PEMFC.

In our previous works [11–16], we carried out the MSR

reaction in membrane reactors in order to study the influence

of the temperature, the reaction pressure, the sweep gas flow

rate, the water/methanol feed molar ratio and the oxygen/

methanol feed ratio on the following parameters: methanol

conversion, hydrogen production and hydrogen selectivity. In

particular, in Ref. [16] we studied the MSR reaction by packing

a new and non-commercial Cu/Zn/Mg-based catalyst into

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

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 ) 5 5 8 3 – 5 5 8 85584

both a Pd–Ag MR and a TR in order to compare their perfor-

mances by varying the temperature and the water/methanol

feed molar ratio at a low reaction pressure. As a result, no

more than 10% of CO-free hydrogen recovery was achieved in

the MR due to the low reaction pressure employed.

Therefore, the aim of this experimental work is to carry out

the MSR reaction in a dense Pd–Ag MR packed with the same

catalyst used in Ref. [16] in order to investigate the influence of

the different flow configurations (co-current and counter-

current), the sweep factor (SF) and, in particular, the reaction

pressure on parameters such as the hydrogen yield, CO-free

hydrogen yield, hydrogen selectivity and CO-free hydrogen

recovery. Moreover, a comparison between the experimental

results of the MR and a TR working at the same operative

conditions is proposed and discussed.

2. Experimental

2.1. Description of the Pd–Ag MR

The MR consists of a tubular stainless steel module (length

280 mm, i.d. 20 mm) containing a tubular pine-hole free Pd–Ag

membrane permeable only to hydrogen (thickness 50 mm, o.d.

10 mm, length 145 mm). Such membrane differs from the one

used in our previous works [12,13]. The dense Pd–Ag

membrane, produced by cold-rolling and diffusion welding

a

b

Pd-Ag membr

Sweep Gas

Cu/Zn/Mg-based catalyst

Cu/Zn/Mg-based cata

Glass

Permeate

Glass

Pd-Ag membrane

Fig. 1 – Scheme of the Pd–Ag MR: co-current mode flow co

technique [17], has the upper temperature limit around 450 �C.

This membrane is joined to two stainless steel tube ends

useful for the membrane housing. As shown in Fig. 1, catalyst

pellets are packed in the lumen of the MR and glass spheres

(2 mm diameter) are placed at both the extremities of the

membrane. Moreover, in Fig. 1 both the MR flow configura-

tions, co-current and counter-current are shown.

The hydrogen permeating flux through the Pd–Ag

membrane used in the present work follows Richardson

equation:

JH2¼

Pe0exp��Ea

RT

�� ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffipH2 ;retentatep � ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

pH2 ;permeatep �

d(2)

where d is the membrane thickness (50 mm), R the universal

gas constant, T is the absolute temperature, pH2 ;retentate and

pH2 ;permeate are the hydrogen partial pressures in the retentate

and permeate side, respectively.

2.2. Experimental details

The Pd–Ag MR is placed in a temperature-controlled P.I.D.

(Proportionalþ IntegralþDerivative Control) furnace. The

reaction temperature is kept constant at 300 �C, while the

absolute reaction pressure ranges between 1.5–3.5 bar, regu-

lated by means of a back pressure controller. The sweep gas

(N2) is fed by means of a mass-flow controller (Brooks

Instruments 5850S) driven by a computer software furnished

Retentate

ane

Sweep Gas

Retentate

Feed

Feed

Glasslyst

Glass

Permeate

Retentate

Retentate

nfiguration (a), counter-current flow configuration (b).

R2= 0.999

pH2-retentate0.5 - pH2-permeate

0.5 [bar0.5]

0 5x10-3 10x10-3 15x10-3 20x10-3 25x10-3

J H2[

mol

/m2 ·s

]

0,0

0,1

0,2

0,3

0,4

0,5

0,6

T = 300 °C

Fig. 2 – Sieverts’ plot at 300 8C.

[m

ol·m

/s·m

2 ·bar

0,5 ]

140

160

180

200

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 ) 5 5 8 3 – 5 5 8 8 5585

by Lira (Italy) and used for all the experiments. Liquid water

and methanol are fed by means of a HPLC pump furnished by

Dionex. The methanol feed molar rate is 1.26$10�3 mol/min,

while the H2O/methanol feed molar ratio is 3/1.

The absolute permeate pressure of the MR is always kept

constant at 1.0 bar, while the sweep gas flow rate depends on

the sweep factor (SF), which is defined as the ratio between

the sweep gas molar rate and the methanol feed molar rate.

The SF ranges between 1.1 and 9.7. The liquid reactants are

mixed and vaporised and, then, fed into the reactor. Moreover,

a constant nitrogen molar rate (1.33$10�3 mol/min) is used as

internal standard gas and fed with the reactants into the

reaction side. The retentate stream is released over a cold-trap

in order to condensate unreacted methanol and water. Both

the permeate and retentate stream compositions are analysed

using a temperature programmed HP 6890 GC with two

thermal conductivity detectors at 250 �C and Ar as carrier gas.

The GC is equipped by three packed columns: Porapack R 50/

80 (8 ft� 1/8 in) and Carboxen� 1000 (15 ft� 1/8 in) connected

in series, and a Molecular Sieve 5 A (6 ft� 1/8 in). Each exper-

imental point reported in the present work represents the

average value of the 5 experimental values recorded during

65 min at steady state conditions. The MR was packed with 4 g

of a CuOAl2O3ZnOMgO based catalyst. Before reaction, the

catalytic bed was pre-heated using nitrogen up to 300 �C under

atmospheric pressure and, afterwards, reduced by using

hydrogen (15 ml/min) at the same temperature for 2 h. After

each experimental cycle (65 min), the catalyst was regen-

erated using hydrogen (15 ml/min) for 2 h. The experimental

tests for the TR were performed using the MR with the inlet

and outlet permeate sides completely closed. A flat tempera-

ture profile along the reactor was confirmed during the reac-

tion by means of a three-point thermocouple inserted into

both reactors.

The following definitions are used to describe the TR and

MR performances:

X selectivity; ðSx; %Þ ¼ XOUT

H2;OUT þ COOUT þ CO2;OUT100 (3)

where X is H2, CO2, and CO, respectively.

H2 recovery; ð%Þ ¼ H2;permeate

H2;permeate þH2;retentate100 ðonly for the MRÞ

(4)

H2 yieldð%Þ ¼ H2;OUT

3CH3OHIN100 (5)

H2;CO�free yieldð%Þ ¼ H2;permeate

3CH3OHIN100 (6)

As regards MR, the subscript ‘‘OUT’’ indicates the total

(retentate and permeate sides) outlet flow rate of each species,

while ‘‘IN’’ refers to the feed stream. Furthermore, the

hydrogen recovery is used only for the MR.

1000/T [K-1]

1,5 1,6 1,7 1,8 1,9 2,0

-ln

Pe

100

120

pH2-retentate0.5 - pH2-permeate

0.5 = 26.7·10-2 bar0.5

Fig. 3 – Arrhenius plot for the membrane reactor.

3. Results and discussion

The experimental tests on the dense Pd–Ag membrane are

summarised in Figs. 2 and 3. They show that H2/other gas

selectivity is infinite and that both Sieverts’ law and Arrhenius

equation are followed. The apparent activation energy (Ea)

is 17.64 kJ/mol and the pre-exponential factor (Pe0)

6.6$10�5 mol m/(m2 s bar0.5). Thus, since the temperature

dependence of the hydrogen permeability can be expressed by

means of an Arrhenius-like equation: Pe¼ Pe0 exp(�Ea/RT ),

the hydrogen flux permeating through the Pd–Ag membrane

can be expressed by means of Richardson equation, which

combines Sieverts’ law and Arrhenius equation. Moreover,

before and after each reaction test, the hydrogen flux was

evaluated to confirm both that the Richardson equation is still

followed and that no changes on the permeation properties

have been observed.

Fig. 4 illustrates that, at SF¼ 1.1, the hydrogen selectivity of

the MR in counter-current mode slightly increases by

increasing the reaction pressure, while it is almost constant

for SF� 5.1. Moreover, at a reaction pressure of 1.5 bar, the

hydrogen selectivity is 68.6% at SF¼ 1.1 and 74.8% at SF¼ 9.7,

while at 3.5 bar it is 71.1% at SF¼ 1.1 and 74.9% at SF¼ 9.7.

These results can be explained by taking into account that, for

a Pd–Ag MR, both a high reaction pressure and a high SF

maximize the hydrogen partial pressure square root differ-

ence between the retentate and the permeate side, inducing

Reaction pressure [bar]1 2 3 4

0

20

40

60

80

100

MR - SF = 1.1MR - SF = 5.1MR - SF = 9.7TR

Hyd

roge

n se

lect

ivit

y [

]

Fig. 4 – Hydrogen selectivity vs reaction pressure for the MR

in counter-current flow configuration and the TR at

T [ 300 8C, H2O/CH3OH [ 3/1 and different sweep factor (SF).

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 ) 5 5 8 3 – 5 5 8 85586

an increase in the hydrogen permeation driving force.

According to Richardson equation, such an effect results in

a higher hydrogen flux permeating through the membrane,

which affects (following Le Chatelier principle) the MSR

reaction equilibrium, and shifts the reaction towards further

products formation. This allows a higher hydrogen production

and, thus, higher hydrogen selectivity. As regards MR in co-

current flow configuration, Fig. 5 shows that, by increasing the

reaction pressure, hydrogen selectivity increases in the whole

range of SF. In particular, at SF¼ 1.1, the hydrogen selectivity

is 64.6% at 1.5 bar and 68.3% at 3.5 bar, while at SF¼ 9.7, it is

67.8% at 1.5 bar and 73.9% at 3.5 bar. At a fixed reaction pres-

sure, a SF higher than 1.1 yields higher hydrogen selectivity,

but for SF� 5.1 no relevant benefits are achieved. However, at

the maximum reaction pressure and SF used in the present

work, no difference in terms of hydrogen selectivity is

observed within the co-current and the counter-current flow

configuration, while in the other cases the counter-current

mode shows higher hydrogen selectivity than the co-current

Reaction pressure [bar]1 2 3 4

0

20

40

60

80

100

MR - SF = 1.1MR - SF = 5.1MR - SF = 9.7TR

Hyd

roge

n se

lect

ivit

y [

]

Fig. 5 – Hydrogen selectivity vs reaction pressure for the MR

in co-current flow configuration and the TR at T [ 300 8C,

H2O/CH3OH [ 3/1 and different sweep factor (SF).

one. By varying the reaction pressure, the TR hydrogen

selectivity (reported in both Figs. 4 and 5) shows a constant

value of around 64.0%. It is evident that, at the same operative

conditions and in both flow configurations, the MR gives

better hydrogen selectivity than the TR.

Regarding CO formation, CO selectivity for the MR is

always lower than 1.0%, while for the TR it ranges between 1.0

and 2.0%.

With the aim to produce and maximize a CO-free hydrogen

stream, the key parameter of the present work is the CO-free

hydrogen recovery (HR). It is defined as the molar ratio

between the CO-free hydrogen stream in the permeate side

and the total amount of hydrogen produced. Fig. 6 shows that,

in both co-current and counter-current modes, the HR

increases by increasing both the reaction pressure and the SF.

In fact, a higher pressure results in a higher retentate

hydrogen partial pressure and a higher SF results in a lower

permeate hydrogen partial pressure; therefore, both effects

improve the hydrogen permeation driving force by inducing

a higher CO-free hydrogen stream recovered in the permeate

side. In details, at 1.5 bar and counter-current flow configu-

ration, the HR is 31% at SF¼ 1.1 and 73.7% at SF¼ 9.7, while at

3.5 bar the HR is 70.6% at SF¼ 1.1 and 94.4% at SF¼ 9.7. Only at

1.5 bar and SF¼ 1.1, the two flow configurations give almost

the same HR, while, in other cases, the counter-current mode

can be considered a better choice than the co-current one. In

particular, in the whole range of reaction pressure and at

SF¼ 5.1 the counter-current mode gives better HRs than the

co-current ones at SF¼ 9.7.

Another important parameter studied in the present work

is the hydrogen yield, defined as the molar ratio between the

total hydrogen produced and 3 times the methanol fed to the

reactor (representing the maximum hydrogen theoretically

producible from the stoichiometric of the MSR reaction). Table

1 shows the hydrogen yield of both the MR and the TR by

varying the flow configuration as well as the reaction pressure

and the SF. The MR in counter-current mode is able to give

a better hydrogen yield than the co-current one. For both the

configurations, at 1.5 bar the hydrogen yield increases by

Reaction pressure [bar]

1,0 2,0 3,0 4,0

CO

-fre

e hy

drog

en r

ecov

ery

[]

0

20

40

60

80

100

Counter-current - SF = 1.1Counter-current - SF = 5.1Counter-current - SF = 9.7Co-current - SF = 1.1 Co-current - SF = 5.1Co-current - SF = 9.7

Fig. 6 – CO-free hydrogen recovery for the MR in co-current

and counter-current flow configuration vs reaction

pressure at T [ 300 8C, H2O/CH3OH [ 3/1 and different

sweep factor (SF).

Table 1 – Hydrogen yield for the MR (counter-current and co-current flow configuration) and the TR at T [ 300 8C, differentreaction pressure and sweep factor (SF)

H2 yield [%]

p¼ 1.5 bar p¼ 2.5 bar p¼ 3.5 bar

SF¼ 1.1 SF¼ 5.1 SF¼ 9.7 SF¼ 1.1 SF¼ 5.1 SF¼ 9.7 SF¼ 1.1 SF¼ 5.1 SF¼ 9.7

MR co-current 60.6 61.8 63.5 61.6 63.2 63.1 63.3 63.6 63.1

MR counter-current 62.5 63.5 65.4 64.7 65.3 64.8 64.6 65.3 65.4

TR 60.1 58.2 57.3

Table 2 – CO-free hydrogen yield in co-current and counter-current flow configuration, at T [ 300 8C, different reactionpressure and sweep factor (SF)

H2,CO-free yield [%]

p¼ 1.5 bar p¼ 2.5 bar p¼ 3.5 bar

SF¼ 1.1 SF¼ 5.1 SF¼ 9.7 SF¼ 1.1 SF¼ 5.1 SF¼ 9.7 SF¼ 1.1 SF¼ 5.1 SF¼ 9.7

MR co-current 19.7 35.2 41.7 30.3 45.1 48.6 37.4 51.3 52.1

MR counter-current 19.4 40.7 46.0 35.7 55.5 58.2 44.5 57.4 58.5

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 e n e r g y 3 3 ( 2 0 0 8 ) 5 5 8 3 – 5 5 8 8 5587

increasing the SF, while a constant value of around 63% for the

co-current and 65% for the counter-current flow configuration

is observed at a reaction pressure� 2.5 bar. Moreover, the

hydrogen yield of the MR is always higher than the TR one,

and it is more evident at a higher reaction pressure and

a higher SF. The hydrogen yield in the TR ranges between

60.1% at 1.5 bar and 57.3% at 3.5 bar. Such decreasing trend

occurs since the increase in reaction pressure does not favour

the thermodynamic of the MSR reaction that proceeds with an

increase of the moles number and, then, a lower hydrogen

yield is achieved. On the other hand, for the MR the negative

effect of a higher pressure on the thermodynamic of the MSR

reaction is compensated by the positive effect, induced on the

hydrogen permeation driving force, which allows an

enhancement of the hydrogen yield.

As regards MR, Table 2 shows the CO-free hydrogen yield at

different flow configurations, reaction pressure and SF. As

reported in Eq. (6), such yield is defined as the molar ratio

between the hydrogen stream recovered in the permeate side

and 3 times the methanol fed to the reactor (representing the

maximum hydrogen theoretically producible from the stoi-

chiometric of the MSR reaction). Apart from the result at

1.5 bar and SF¼ 1.1, the CO-free hydrogen yield is always

higher in counter-current mode and increases by increasing

the pressure and the SF in both flow configurations. The best

result is obtained at 3.5 bar, SF¼ 9.7 and counter-current

mode, where almost 60.0% of CO-free hydrogen yield is

achieved.

4. Conclusions

The scope of the present experimental work was to carry out

the MSR reaction in a Pd–Ag MR in order to study the perfor-

mances in terms of hydrogen selectivity, hydrogen yield and,

in particular, CO-free hydrogen recovery by varying the flow

configuration mode, the reaction pressure and the sweep

factor. Moreover, the comparison between the experimental

results of the Pd–Ag MR and a TR working in the same oper-

ative conditions has been proposed. As a result of the exper-

imental investigation carried out in the present work, the MR

in counter-current flow configuration represents a better

choice than the co-current one, even though at low reaction

pressure (1.5 bar) and SF (1.1) no difference in terms of CO-free

hydrogen recovery has been observed. However, at the best

operative conditions (3.5 bar and SF¼ 9.7) and counter-current

flow configuration, the MR is able to achieve 94% of CO-free

hydrogen recovery, almost 75% of hydrogen selectivity, 65% of

hydrogen yield and almost 60% of CO-free hydrogen yield. At

any rate, the MR shows better performance than the TR,

asserting itself as the best solution to carry out the MSR

reaction in a MR for a CO-free hydrogen production.

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