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Hydrogen permeability of PdeAg membrane modules withporous stainless steel substrates

Donglai Xie a,*, Jinfeng Yu a, Fang Wang a, Ning Zhang a, Weixing Wang a, Hao Yu a,Feng Peng a, Ah-Hyung A. Park b

aMOE Key Laboratory of Enhanced Heat Transfer & Energy Conservation, South China University of Technology, Guangzhou 510640, ChinabDepartment of Earth and Environmental Engineering, Columbia University, 9500 W. 120th Street, NY 10027, USA

a r t i c l e i n f o

Article history:

Received 5 May 2010

Received in revised form

10 October 2010

Accepted 11 October 2010

Available online 10 November 2010

Keywords:

Hydrogen

Palladium membrane

Porous stainless steel

* Corresponding author. Tel./fax: þ86 20 2223E-mail address: [email protected] (D. Xie

0360-3199/$ e see front matter ª 2010 Profedoi:10.1016/j.ijhydene.2010.10.030

a b s t r a c t

Palladium-based membranes are attractive for their nearly perfect permselectivity to

hydrogen. Membrane modules, consisting of a membrane foil, porous stainless steel

substrate, test frame and flange were assembled and tested in an electrically heated vessel.

Instantaneous hydrogen permeation flux was measured. Influences of operation condi-

tions on the membrane performance were examined. Microstructure and morphology of

the membrane surface and the cross-sectional surface of the substrate and membrane foil

were characterized by scanning electron microscopy. It was observed that for an operation

temperature higher than 755 K, the hydrogen permeation flux through the membrane

module with 0.2 mm grade porous 316L stainless steel substrate decayed continuously due

to the inter-metallic diffusion between the membrane and the substrate. For a temperature

of around 869 Ke943 K, a stable hydrogen permeation flux through the membrane module

with 0.5 mm grade stainless steel substrate was observed. Pretreatment of the 0.5 mm grade

substrate with polishing and etching helped to smooth the membrane foil surface.

However, it changed the surface structure of the material and led to a decrease in hydrogen

permeability. Under the conditions investigated, the permeation factor of the module

increased by raising the hydrogen pressure in the vessel side and decreasing the

membrane module temperature. By decreasing the hydrogen exit partial pressure by

sweep gas, the membrane module permeation flux increased, while the permeation factor

decreased.

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

1. Introduction dwindling fossil fuel supply and an approach to a future lasting

The increased demand for pure hydrogen gas in recent years in

many sectors, ranging from petroleum processing, materials

treatment to renewable energy related applications, has led to

a revival of interest in economical hydrogen production tech-

nologies. Hydrogen energy is looked upon as a savior in

combating the deterioration of the global environment, as

a means of securing energy that is independent of the

6985.).ssor T. Nejat Veziroglu. P

supply of an energy resource [1e3]. Most of the world’s

hydrogen is generated by steam reforming or partial oxidation

of natural gas in parallel fixed bed reactors within huge top-

fired or side-fired furnaces, coupled with Pressure Swing

Adsorption (PSA) for hydrogen purification [4]. Hydrogen

separation accounts for a large fraction of energy expenditure

and capital investment in the hydrogen production process.

The most widely used technology for hydrogen purification is

ublished by Elsevier Ltd. All rights reserved.

mailto:[email protected]

http://www.sciencedirect.com

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http://dx.doi.org/10.1016/j.ijhydene.2010.10.030



Fig. 1 e Structure of the PdeAg membrane module

assembly.

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 6 ( 2 0 1 1 ) 1 0 1 4e1 0 2 6 1015

PSA. Palladium and its alloy membranes have attracted

growing interests for their capability to separate ultra-pure

hydrogen from gaseous mixtures [5e7]. They can also be inte-

grated with chemical reactors where chemical reaction and

hydrogen separation occur simultaneously to simplify the

hydrogen production process. Various membrane reactors

havebeenproposedand tested for hydrogenproduction [8e13].

The driving force for hydrogen transportation through

a membrane is the hydrogen partial pressure difference

between the two surfaces of the palladium membrane [14].

Thin palladium membrane itself cannot stand the pressure

difference imposed on it. Hence, membrane modules should

be constructed with thin palladium or palladium alloy

membranes supported on porous substrates, such as ceramics,

porous glass and porous stainless steel [6,15]. Of all of these

substrates, porous stainless steel has shown advantages for its

close thermal expansion coefficient to palladium [16,17].

The fabrication and performance of palladiummembranes

have been investigated by many researchers. Hydrogen

permeates through palladium or palladium alloy membranes

via the “solution e diffusion” mechanism. It can be described

by the Sieverts’ Law [18,19] as:

Ms ¼ KS1

t1e�

EpRT�PnH � Pn

L

�(1)

where MS is the hydrogen permeation rate, K is the pre-

exponential factor, S1 is the effective area of membrane

surface for hydrogen permeation, t1 is the thickness of palla-

diumor palladium alloymembrane, Ep is the activation energy

for permeation, R is the gas constant, T is the temperature, PHis the hydrogen partial pressure in vessel side, PL is the

average hydrogen partial pressures in the membrane

permeate side, and n is the parameter whose value depends

on the limiting transport mechanism of hydrogen permeation

through palladium or its alloy membrane. The hydrogen flux

follows the Sieverts’ Law when the hydrogen pressure expo-

nent n is equal to 0.5, which is usually valid for thick Pd films

[15]. Deviations from the Sieverts’ Law (n > 0.5) were reported

for very thin membranes [20,21]. Based on a hydrogen

permeation model, Ward and Dao [22] showed that for

temperatures above 673 K, n was equal to 0.5 for membranes

thicker than 1 mm. Usually to use Sieverts’ Law correctly with

an exponent of 0.5, the thickness of membrane should be

higher than 10 mm [15].

When stainless steel substrates are applied to form

membrane modules, it can affect the membrane permeability

by adding a flow resistance to the hydrogen transportation

process. It can also decrease the membrane foil permeability

by inter-diffusion between the stainless steel substrate and

the membrane metal under high temperature [23]. Other

factors, such as the existence of gas species other than

hydrogen, can also affect the membrane permeability [24].

Some researchers used an efficiency factor h to denote the

difference between the actual permeability (Ma) and those

predicted from Sieverts’ Law (Ms) [8,25e28]:

Ma ¼ hMs (2)

h is reported in literatures to be from 0.39 to nearly 1.0

[8,25e27].

The permeability of membrane modules is critical to the

design and sizing of membrane reactors and separators.

Experiments were carried out to study the permeability

performance of membrane modules with porous stainless

steel substrates. The work can help to understand the influ-

ence of stainless steel substrate on the permeability of the

membrane module and find measures to improve the

membrane module performances.

2. The test membrane module andexperimental setup

2.1. The test membrane module

As shown in Fig. 1, the membrane module consists of the

following parts: frame, substrate, membrane foil, graphite

gasket and flange. These parts were tightened together with

bolts and nuts through the holes on the edge of the flange and

frame. Channels were machined inside the frame for

permeate side hydrogen flow. The geometries of the frame

and flange are shown in Fig. 2. Three types of membrane

module sets, denoted as type I, II and III, were fabricated, with

their dimensions listed in Table 1.

PdeAg membrane foils of 75% (wt) palladium and 25% (wt)

silver with thicknesses of 10 mm, 25 mm and 50 mmwere tested

in the experiments. The membrane foils were supplied by

Good-Fellow (10 mm) and Alfa-Aesar (25 mm and 50 mm). The

following performance data of suchmembrane foils was used:

activation energy 9.18 kJ/mol, pressure order 0.5, pre-expo-

nential factor 2.07 � 10�3(mol m)/(m2 min bar0.5) [29]. Two

types of porous stainless steel material were employed as

a substrate: one with a thickness of 1.2 mm and media grade

of 0.5 mm, while the other with a thickness of 1.0 mm and

media grade of 0.2 mm. The media grade is defined by the

supplier of the material (Mott Corporation) as over 95% of

particles or the other fluid with the size of the grade (in mm)

cannot pass through the substrate during filtering.



Fig. 2 e Geometry of the test frame (top) and flange

(bottom).


2.2. Experimental setup

After the membrane module was assembled, it was installed

inside an electrically heated pressure vessel. The vessel was

designed and fabricated for pressure up to 2.0 MPa and

temperature of 973 K. As illustrated in Fig. 3, hydrogen and

Table 1 e Dimensions in Fig. 2 (unit: mm).

Module type A B C D E F G H I J

I 3 5 5 28 32 1.2 8 24 28 6

II 3 5 5 28 32 1.2 8 22 20 6

III 3 5 5 24 26 1.0 8 20 22 6

argon were directed to the vessel from gas cylinders through

Mass Flow Controllers (MFC). The vessel pressure was

controlled by a back pressure regulator in the off gas stream. A

cylindrical electrical heater of 10 kWwas installed around the

vessel for controlling the vessel temperature. The vessel and

electrical heater were sufficiently insulated. Sweep gas

nitrogen was delivered to the permeate side of the membrane

module. Pure hydrogen, or a mixture of the sweep gas and

hydrogen from the permeate side of the membrane was

metered by a bubble gas meter.

Seven sets of experiments have been carried out. The

configurations and test conditions of these tests are listed in

Table 2.

2.3. Experimental procedure

2.3.1. Porous stainless steel substrate pretreatmentsFor tests 1 to 6, the porous stainless steel substrates were

treated with ultra-sonic cleaning only. It was suspected that

the rough surface of the substrate could lead tomembrane foil

failure. Hence for the test 7, the substrate surface was pre-

treated by a process similar to that described by Li et al. [23]:

1. Polishing: the surface of the substrate was polished using

sandpaper with increasing grits step by step. The substrate

was finally polished with 1200 grit sandpaper.

2. Etching: the substrate was etched at an ambient tempera-

ture with a mixed solution of nitric acid and hydrochloric

acid (volumetric ratio 1HNO3: 3HCl) for several minutes.

After etching, the substrate was immediately washed with

clean water in an ultra-sonic bath to remove acid solution

remaining in the pores. Fig. 4 shows the substrate surface

as received, after polishing by sandpapers and after etching

with acid solution under a Hitachi S-3700N Scanning Elec-

tronic Microscopy (SEM).

2.3.2. Substrate pressure drop measurementsFor all tests, the flow resistance of these substrates under

ambient temperature was measured before they were

assembled to the module. Bottled air was employed to

measure the pressure drop across the substrate at certain air

flow fluxes. For test 7, the pressure drops across the substrate

after it was polished and etched were also measured. The

pressure drop can also be calculated by the equation provided

by the supplier of these materials:

DP ¼ KG � fS2

� y� t2 (3)

where KG is a constant given by the supplier of the porous

substrate, f is the gas flow rate, S2 is the area of substrate, y is

the gas viscosity, and t2 is the thickness of substrate.

Fig. 5 shows the pressure drops across the 0.5 mm grade

substrates for tests 1, 2, 6 and 7 and the 0.2 mm grade

substrates for tests 3 to 5, respectively. It can be seen that for

the 0.5 mm grade substrate, polishing with sandpaper added

a strong flow resistance to the material. As can be seen from

Fig. 4, the substrate pores near the surface were blocked by

sandpaper polishing, which contributed to the flow resistance

increase. The etching process helped to open these pores and



Fig. 3 e Schematic diagram of the experimental setup.


lower the flow resistance. The pressure drops across the un-

treated substrates were very close to that calculated from

Equation (3).

The supplier of the porous sintered metal suggested that

the maximum application temperature for the 316L stainless

steel porous material under reducing atmosphere is 755 K.

Usually, palladium alloy membrane modules are operated

under a temperature range of 773e973 K [8,11,28]. Too high

a temperature will damage the membrane, while too low

a temperature will cause a low chemical reaction conversion

in the membrane reactor and membrane permeability. This

temperature range is beyond the recommended operating

temperature of the substrate. The high temperature may

destroy the porosity of the substrate, hence block the gas

transportation passage and lead to low membrane perme-

ability. To study this possibility, a 0.5 mm grade test substrate

module and a 0.2 mm grade test substrate module were

assembled. The 0.5 mm grade test substrate module was

identical to the Type II membrane module, without assem-

bling the membrane foil. The 0.2 mm grade test substrate

modulewas identical to the Type IIImembranemodule, again,

without assembling the membrane foil. Both modules were

installed in the pressure vessel as shown in Fig. 3, and heated

at temperature of 923 K under hydrogen environment for 8 h

(the heating period from ambient temperature to 923 K was

not included in the 8 h). The pressure drop across these

modules under certain hydrogen flux was measured before,

Table 2 e Test configurations and conditions.

Test t2mm

Substrate grademm

t1mm

Module typeP

1 1.2 0.5 25 I

2 1.2 0.5 25 I

3 1.0 0.2 10 III

4 1.0 0.2 25 III

5 1.0 0.2 25 III

6 1.2 0.5 50 II

7 1.2 0.5 25 I

after, and during the heating process as shown in Fig. 6. It can

be seen that during the heating process, the flow resistances

across the 0.2 mmgrade substrate increased slightly with time,

while the increase of pressure drop with time across the

0.5 mmgrade substrate was not noticeable. The pressure drops

across the substrates after the heating process were slightly

higher than those before the heating process for both

substrates. From the stability point of view, 0.5 mm grade

sintered metal was more suitable to be used as membrane

substrate than the 0.2 mmgrade one. Considering the pressure

potential required for hydrogen to permeate through the

membrane layer in a membrane module was much higher

than the pressure drop for hydrogen to flow across the

substrate, this slow increase in pressure drop across the

0.2 mmsubstrate itself should not cause any quick decay in the

module permeability.

2.3.3. Membrane module permeability test procedureAfter the membrane module was assembled, it was installed

in the pressure vessel. The permeability of the membrane

module was studied by the following procedure:

1. Displacement of the air in the pressure vessel. Pure argon

gas was directed to the vessel until its pressure reached

0.2 MPa, and then released through the back pressure

regulator. This procedure was repeated 8 times. The vessel

pressure was then kept at 0.2 MPa with argon inside.

Substrateretreatment

TemperatureK

PressureMPa

Vesselenvironment

No 869e917 0.2 H2

No 923 0.2 H2

No 923 0.2 H2

No 913 0.4 H2 þ Ar

No 723 0.3 H2

No 943 0.3 H2

Yes 920 0.2 H2



Fig. 4 e Surface of the 0.5 mm grade substrate under SEM as

received (top), after polishing with sandpaper (middle) and

after etching with acid solution (bottom).

0 50 100 150 200 250 300 350 400 4500.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0.18

)aPM(

porderusser

P

Air flow flux (Nm3 h-1m-2)

Equation (3) Test 7 after polishing Test 7 without pretreatment Test 7 after etching Test 1 Test 2 Test 6

50 100 150 200 250 300 350 400 4500.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

)aPM(

porderusserP

Air flow flux (Nm3 h-1m-2)

Equation (3) Test 3 Test 4 Test 5

Fig. 5 e Pressure drops across the 0.5 mm grade substrates

under ambient temperature for tests 1, 2, 6 and 7 (top) and

0.2 mm grade substrates for tests 3e5 (bottom).


2. Displacement of the air in the permeate side of the

membrane module. When the vessel was pressurized,

nitrogen was directed to the permeate side of the

membrane module to purge the air out. A small flow of

nitrogen (about 1 ml/min) was maintained during the test,

until hydrogen was confirmed to have been permeated

from the vessel to the permeate side of the membrane.

3. Heating in argon environment: the vesselwas heated by the

electrical heater to 523 K. The vessel pressure was main-

tained at 0.2 MPa during the heating process.

4. Displacement of argon with hydrogen: when the vessel

temperature reached 523 K, the argon gas inside the pres-

sure vessel was released. For test 4, pure hydrogen was

forced into the vessel until the vessel pressure reached

0.4 MPa for four times. Then both hydrogen and argon with

molar flow rates controlled at 1:1 were charged to the vessel

and the vessel pressure was maintained at 0.4 MPa by the

back pressure regulator. For other tests, pure hydrogen gas

was directed to the vessel until its pressure reached

0.2 MPa, and then released through the back pressure

regulator. This procedure was again repeated 8 times. For

tests 5 and 6, the final vessel pressure was controlled to be

0.3 MPa, while for tests 1, 2, 3 and 7, the final pressure was

0.2 MPa. As soon as the displacement of argon was

completed, the hydrogen permeation flow rate through the

membrane module was measured by the bubble gas meter

at a time interval of approximately 30 min. For each

measurement, three readings were performed and an

average value was taken.

5. Heating in hydrogen (and for test 4 hydrogen/argon) envi-

ronment until the desired temperature (590 Ke913 K) was

reached. The membrane permeability data was continu-

ously recorded during this period.



200 400 600 800 1000 12000.00

0.02

0.04

0.06

0.08

0.10

0.5 m before heating0.5 m after heating Pr

essu

re d

rop

(MPa

)

Hydrogen flow flux (Nm3 h-1m-2)

0.2 m before heating 0.2 m after heating

200 400 600 800 1000 12000.00

0.05

0.10

0.15

0.20

0.25

0.30

0.5 m, 0 h after being heated @ 923K0.5 m, 4 h after being heated @ 923K0.5 m, 8 h after being heated @ 923KPr

essu

re d

rop

(MPa

)

Hydrogen flow flux (Nm3 h-1m-2)

0.2 m, 0 h after being heated @ 923K 0.2 m, 4 h after being heated @ 923K 0.2 m, 8 h after being heated @ 923K

Fig. 6 e Pressure drops across the 0.2 and 0.5 mm grade

substrates before, after (top) and during (bottom) being

heated under hydrogen environment at temperature of

923 K for 8 h.

Fig. 7 e Variation of membrane module permeation flux

(top) and factor (bottom) with time for tests 1e7.


6. Permeability tests under stable vessel temperatures: the

vessel inner temperature was maintained at �2 K around

the desired temperature by the temperature controller for

several days. For test 1, the vessel temperature was main-

tained at 869 K for the first 47 h, and then 917 K for the next

47e75 h. The membrane permeability data was continu-

ously recorded.

7. For the tests 1, 2, 5 and 6, the hydrogen permeability of the

membrane module became almost stable after hours test

under stable vessel temperatures. Hence the vessel pres-

sure was changed by adjusting the back pressure regulator,

inner temperature was varied by adjusting the set point of

the electrical heater controller, and the module perme-

ability under various conditions was measured.

8. After all tests were performed, the vessel was again

charged with pure argon,maintained at pressure of 0.2 MPa

and temperature around 873 K. No flowwas observed in the

permeate side of the membrane module. Hence the

membrane integration was confirmed.

3. Experimental results and discussion

3.1. Instant hydrogen permeation flux and permeationfactor

As the effective membrane surface area varies slightly

between Type I and Type II, III membrane modules, molar

hydrogen permeation flux (Q) is used to denote module

hydrogen permeation performance, and it is defined as

Q ¼ Ma

S1(4)

Fig. 7 shows the variation of the measured permeation flux

and permeation factor with time from these membrane

modules. Since the membrane module permeability was

measured as soon as hydrogen was charged into the vessel at

the point that the vessel temperature reached 523 K, the

module permeation fluxes increased at the heating period for

all tests. At the period when the vessel temperature was

maintained stable, the membrane module permeability in

different tests behaved differently. It can be observed that:




1. For test 1 (0.5 mm grade substrate and 25 mm thick

membrane foil), when the vessel temperature was around

869 K and pressurewas 0.2MPa, themembranemodule had

an initial quick decay in permeation flux from 0.25 to

0.23 mol/(m2 s). The corresponding membrane permeation

factor decreased from 0.89 to 0.78, and then maintained at

around 0.86. When the vessel temperature was adjusted to

917 K, the membrane module permeation flux changed to

0.27 mol/(m2 s), and the membrane permeation factor

changed to around 0.88. The initial decrease in the

membrane permeability could be caused by the inter-

metallic diffusion between the PdeAg membrane and the

sintered stainless steel substrate. Although the inter-

metallic diffusion could be a very slow process, it could be

speeded up in the hydrogen environment. Evidence of the

diffusion bonding is that the membrane foil was totally

bonded with the metal substrate, and it could not be

detached from the support after the experiments. Some

researchers actually used the diffusion bonding to fabricate

membranemodules [5,30]. After the test, the substrate with

the attached membrane foil was cut into halves and the

cross-sectional surface was characterized by SEM (Hitachi

S-3700N) as shown in Fig. 8(a). The distribution of Pd, Ag

and Fe elements along the line (from point A to B) in the

SEM image was characterized using a line scan (Hitachi S-

3700N), as shown in Fig. 8(b). It can be seen that there was

approximately a 0.06 mm thick layer that contains Pd, Ag

and Fe elements. This indicates that some diffusion

occurred between the surface of the PdeAg membrane foil

and the sintered metal, which may have caused the initial

decay in membrane permeability.

2. For test 2, the same substrate and membrane foil as test 1

were applied. The vessel temperature was maintained at

around 923 K and pressure 0.2 MPa. Under such conditions,

the membrane permeation flux was around 0.26 mol/(m2 s)

0

20

40

60

80

100

0

tisnetnIy

020406080

100

0

tisnetnIy

a

c

A BA

C D A

Fig. 8 e (a) SEM micrograph showing the cross-sectional micros

(b) line scan of the cross-sectional elemental distributions of Pd, A

(c) SEM micrograph showing the cross-sectional microstructure

scan of the cross-sectional elemental distributions of Pd, Ag and

to 0.28 mol/(m2 s) and the permeation factor was around

0.83 to 0.87. These were very close to the results from test 1

under the conditions of temperature of 917 K and pressure

of 0.2 MPa. It suggests that the experiments had very good

repeatability.

3. For test 3, 1.0 mm thick 0.2 mm grade substrate and 10 mm

thick membrane foil were employed in the membrane

module. The vessel temperature was maintained at 923 K

and pressure 0.2 MPa. The membrane permeation flux was

observed decreasing from 0.50 to 0.20 mol/(m2 s) during the

stable temperature period, and the corresponding perme-

ation factor decreased from 0.70 to 0.25. The differences of

the membrane module configurations between test 3 and

tests 1 and 2 were the substrate grade and membrane foil

thickness. Hence the continuous decay of hydrogen

permeation performance with time in test 3 should be

caused by the thinner PdeAg membrane foil thickness, the

substrate grade, and/or the inter-action between the

membrane and the substrate under current operation

conditions.



module. The vessel temperature was again maintained at

913 K. Here both hydrogen and argon with their molar flow

rate controlled to be 1:1 were directed to the vessel. The

hydrogen flow rate was controlled at 10 LPM, which was

muchhigher than thehydrogenpermeation rate through the

membranemodule (maximum0.1LPM).Hence thehydrogen

concentrationwasmaintained almost uniform in the vessel.

The vessel pressure was maintained at 0.4 MPa by the back

pressure regulator. Hence the hydrogen partial pressure

inside the vessel was 0.2 MPa. The membrane permeation

flux was observed decreasing from 0.13 to 0.05 mol/(m2 s)

during the period of experiment, and the corresponding

permeation factor decreased from 0.65 to 0.16.

0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

Distance from C to D (µm)

0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2Distance from A to B (µm)

b

d

Pd

g Fe

Pd

gFe

tructure of the 0.5 mm substrate and membrane after test 1;

g and Fe of the 0.5 mmsubstrate andmembrane after test 1;

of the 0.2 mm substrate and membrane after test 4; (d) line

Fe of the 0.2 mm substrate and membrane after test 4.




The configuration differences between test 4 and tests 1e2

were the substrate material grade and vessel environment.

The vessel was filled with hydrogen and argon in test 4, while

it was filled with pure hydrogen in tests 1e2. The original

purpose of introducing argon to the vessel was to see if it can

help smoothing the membrane surface. Gallucci et al. [31]

have studied the effect of mixture gas on hydrogen perme-

ation through a palladium membrane and found that N2/H2,

Ar/H2 and CO2/H2 feed mixtures had no remarkable surface

effects on hydrogen permeation through membrane.

Unemoto et al. [32] also concluded in their studies that the

interference effect of the co-existing gas is negligible at

temperatures higher than 873 K for the membranes thicker

than 10 mm. Hence the influence of argon on the membrane

permeability can be neglected, as long as the hydrogen partial

pressure is used for predicting the permeability with Sieverts’

Law. As for the substrate, test 4 used 0.2 mm grade substrate

while tests 1 and 2 used 0.5 mm grade one. It was then sus-

pected that the substrate had caused the continuous decrease

in permeation flux.

After the test, the substrate with the attached membrane

foil was again cut into halves and the cross-sectional surface

was characterized by SEM as shown in Fig. 8(c). The distribu-

tion of Pd, Ag and Fe elements along the line (frompoint C toD)

in the SEM imagewasmeasured using a line scan, as shown in

Fig. 8(d). It can be seen that there was approximately a 0.16 mm

thick layer that contains Pd, Ag and Fe elements. Comparing to

Fig. 8(b) it can be concluded that themetal element diffusion in

this case was much stronger that that in test 1. Due to the

strong metallic diffusion, the performance of the membrane

foil changed. This should be the reason for the continuous

decay of membrane permeability in tests 3 and 4. Both the

0.2 mm and 0.5 mm substrate plates were supplied by the same

supplier (Mott Corporation). Somehow the 0.2 mm substrate

was more active on molecule diffusion than the 0.5 mm

substrate, possibly due to its low porosity and high effective

contact area with the membrane.




which was below 755 K. The membrane permeation flux

was observed at 0.30 mol/(m2 s) during the stable temper-

ature period under vessel pressure of 0.3 MPa, and the

corresponding permeation factor was around 0.99e1.01. No

initial decrease of the permeation flux was observed. After

the test, it was observed that the membrane foil was not

attached to the substrate, and the foil could be detached

Table 3 e Membrane module permeation flux and factor.

Test Substrate grademm

Membrane thicknessmm

TemperaK

1 0.5 25 869

1 0.5 25 917

2 0.5 25 923

5 0.2 25 723

6 0.5 50 943

from the substrate easily. Comparing to the permeation

flux of tests 4 and 5, it can be confirmed that the permeation

flux decay in tests 3 and 4 were caused by the temperature

impact on the inter-metallic diffusion between the

membrane foil and the 0.2 mm substrate. The metallic

diffusion was a strong factor of temperature. Under the

temperature of 723 K, the inter-diffusion of Pd/Ag and the

substrate metal was not initiated. The permeation factor

was around 1, which means under such operation condi-

tions, the influence of substrate on membrane module

permeability was negligible and the Sieverts’ Law held.




and pressure of 0.3 MPa. When the vessel temperature was

stable, the membrane module permeation flux was main-

tained at w0.20 mol/(m2 s). The corresponding permeation

factor was w0.92. The initial decrease in permeation flux

was not noticeable. It could be because that the membrane

foil was so thick that themetallic diffusion near the surface

to the substrate had a limited influence on its total

permeability.

7. For test 7, the 0.5 mmsubstrate was pre-treated as described

previously. It can be observed from Fig. 7 that the perme-

ation flux kept decreasing, similar to what happened with

tests 3 and 4. The pore structure near the surface of the

0.5 mm grade substrate was destroyed by the pretreatment,

as shown in Fig. 4. Hence the actual grade of the substrate

surface was much less than 0.5 mm after the pretreatment

and it behaved like the0.2mmgradesubstrateunder thehigh

temperature operation conditions.

For tests 1, 2, 5 and 6, the permeation flux reached

a constant during the tests under experimental conditions.

The permeation fluxes and factors of these tests are listed in

Table 3 for the reader’s convenience.

3.2. Influences of operation conditions on membranepermeation factor

For tests 1, 2 and 6, the membrane module permeation flux

became stable after more than 40 h of experiments at

temperature ranging from 869 K to 943 K. These threemodules

were used to investigate the influences of the operation

conditions (vessel side temperature, pressure and permeate

side hydrogen partial pressure) on membrane permeation

factor. The vessel pressure was changed by adjusting the back

pressure regulator, inner temperature was varied by adjusting

ture PressureMPa

Permeation fluxmol/(m2 s)

Permeation factor

0.2 w0.22 w0.82

0.2 w0.27 w0.87

0.2 w0.27 w0.85

0.3 w0.30 w1.02

0.3 w0.30 w0.92



Hydrogen + Sweep gas

Permeate flow channel

H2 partial pressure in permeate flow channel

H2 partial pressure in permeate side surface of membrane foil

H2 pressure in vessel


the set point of the electrical heater controller. A wide range of

temperature points were tried and those permeation factors

measured at temperatures within maximum 6 K differences

were grouped together, as shown in Fig. 9. It can be seen that

the permeation factor increasedwith increasing the hydrogen

0.20 0.25 0.30 0.35 0.40 0.45 0.50 0.55 0.600.92

0.94

0.96

0.98

1.00

1.02

1.04

1.06

1.08

1.10

1.12

1.14

1.16

1.18

1.20

659-665K 758-764K 911-917K

PH

0.5-PL

0.5 (MPa0.5)

)-(rotcaf

noitaemreP

0.1 0.2 0.3 0.4 0.5 0.60.84

0.86

0.88

0.90

0.92

0.94

0.96

0.98

1.00

1.02

1.04

1.06

721-727K 820-825K 921-924K

)-(rotcaf

noitaemreP

PH

0.5-PL

0.5 (MPa0.5)

0.20 0.25 0.30 0.35 0.40 0.45 0.50 0.550.92

0.94

0.96

0.98

1.00

1.02

1.04

1.06

1.08

1.10

1.12

1.14

672-676K 718-724K 912-918K

)-(rotcaf

noitaemreP

PH

0.5-PL

0.5 (MPa0.5)

Fig. 9 e Influence of vessel temperature and hydrogen

pressure on permeation factor for test 1 (top), test 2

(middle) and test 6 (bottom) (permeate side pressure:

0.1 MPa).

0 PEPL1 PL2 PH

Hydrogen partial pressure

Sweep gas

Porous substrate

Membrane

Fig. 10 e Profile of hydrogen partial pressure in the

membrane module.

pressure in the vessel side and decreasing the vessel inner

temperature, i.e., the membrane temperature. Permeation

factors higher than 1 were observed for some conditions. The

highest permeation factor observed was approximately 1.18.

One possible contribution for this phenomenon could be the

error on estimating the membrane foil thickness. The

suppliers of the membrane foils (Alfa-Aesar for 25 and 50 mm

foils, and Good-Fellow for 10 mm foils) claimed an error of

�15% on these membrane foil thicknesses. For an extreme

case, if the membrane foil was 15% thinner than the claimed

thickness, the calculated hydrogen permeation flux from

Sieverts’ Law would be 15% higher than actual one. Another

possible reason could be the contact between the membrane

foil and the metal substrate. The vessel was under pressure

(0.2e0.4 MPa) during the tests. Some substance of the

0.16 0.18 0.20 0.22 0.24 0.26 0.28 0.30 0.32

0.65

0.70

0.75

0.80

0.85

0.90

0.95

1.00

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

)-(rotcaf

noitaemreP

PL

0.5 ( MPa0.5)

672-678K 720-724K 820-825K

mlom(

xulfnoitae

mreP2-

s1-)

Fig. 11 e Influence of permeate side hydrogen pressure at

the module exit and vessel temperature on permeation

flux and factor for test 2 (Vessel pressure: 0.2 MPa solid

symbols: permeation flux; open symbols: permeation

factor).



Fig. 12 e SEM images of the PdeAg membrane foils after the test (Test conditions were listed in Table 2).


membrane material was squeezed into the pores of the

substrate, leading to a thinner effective foil thickness than the

original foil thickness.

To study the influence of permeate side hydrogen partial

pressure on the permeation factor, the permeate side of the

membrane module was swept by nitrogen. The sweep gas

entered the membranemodule from the bottom and left from

the top. The hydrogen partial pressure at the module exit (PE)

can be calculated from:

PE ¼ qt � qN

qtPP (5)

where qt is the total permeate flow rate of gas at the exit of

membranemodule, qN is the sweep gas flow rate, and PP is the

permeate side total pressure.

The actual hydrogen partial pressure in the permeate side

of the membranemodule is then between 0 at the entrance of

the sweep gas and PE at the exit. When calculating the

hydrogen permeation rate from Equation (1), PE was used to

represent the hydrogen partial pressure at the low pressure

side. The permeation factor can be calculated from Equation

(2). As a substrate layer was placed between the membrane

foil and the permeate flow channel, the sweep gas could not

fully flush the permeated hydrogen out from the membrane

surface, and the hydrogen partial pressure at the permeate

side of the membrane surface should be higher than that in

the permeate flow channel. A possible hydrogen partial

pressure profile in the membrane module is illustrated in

Fig. 10. Using PE to represent the hydrogen partial pressure at

the permeate side of the membrane could introduce



Table 4 eMajor features of the SEM images of membranefoil after test.

Test Major feature of membrane surface

1 Grain-like bump on surface

2 Smooth, tiny and discrete cracks observable

3 Bumps on and cracks in surface

4 Frost-like bumps on surface

6 Continuous cracks

7 Smooth, cracks hardly observable


a calculation error on estimating the permeation flux.

However, it could reveal the correct trend of the influence of

the permeate side hydrogen partial pressure on the perme-

ation factor. Fig. 11 shows the influence of the permeate side

hydrogen pressure at the module exit and vessel temperature

on the permeation flux and factor for test 2. It can be seen that

with decreasing the hydrogen exit partial pressure, hydrogen

permeation flux was increased, while the permeation factor

was decreased. Similar results were obtained from test 6.

Hence, it was concluded that if PE is used as the permeate side

hydrogen partial pressure, it can actually over-estimate the

membrane module permeation flux under the conditions

studied in these experiments.

3.3. Membrane foil surface smoothness

After each test, themembranemodulewas taken out from the

pressure vessel. SEM was used to characterize microstructure

and morphology of the membrane surfaces. Various types of

membrane surface morphologies were observed as shown in

Fig. 12. Major features of these SEM photos are summarized in

Table 4. It can be seen that for test 7, a very smooth surface

was achieved via the pretreatment of the porous substrate.

However, the pretreatment caused a decrease in membrane

permeability. The surface cracks in the 10 mm thickness

membrane used in test 3 were very serious. These differences

betweenmembrane surface features, however, are difficult to

explain. For example, the membrane modules in test 1 and

test 2 were made on the same type of membrane supports

under similar operating temperatures and vessel environ-

ment, but the surfaces shown in Fig. 12 were extremely

different. It is difficult to draw any conclusions on the effects

of the operation temperature, atmosphere, and the substrate

grade on the morphological structures of membranes. More

studies are desired to understand the mechanisms that

caused such morphologies.

4. Conclusions and recommendations

Membrane modules, consisting of PdeAg membrane foil with

thickness of 10 mm, 25 mm and 50 mm, porous stainless steel

substrate of 0.5 mm and 0.2 mm grade, test frame and flange

were assembled and tested in an electrically heated vessel. It

can be concluded from the experimental observations that:

1. For operation temperatures higher than 755 K, hydrogen

permeation flux through the membrane module with

0.2 mm grade porous 316L stainless steel substrate contin-

uously decayed due to the inter-metallic diffusion between

the membrane and the substrate. Hence the 0.2 mm grade

porous 316L stainless steel material is not suitable as

a membrane module substrate.

2. Under the conditions studied (temperatures around

869 Ke943 K), stable hydrogen permeation flux through the

membrane module with 0.5 mm grade stainless steel

substrate was observed. Although the supplier of the mate-

rial does not recommend the application of such material

above 755 K in reducing environment, the flow resistances

across the 0.5 mm grade substrate did not significantly

increase during the 8 h test period that was performed in

hydrogen environment at temperature of 923 K.

3. Pretreatment of the 0.5 mm grade substrate helped to

smooth the membrane foil surface. However, it changed

the surface structure of thematerial and led to a decrease in

the permeability of the membrane module.

4. For temperatures below 755 K, the influence of porous

stainless steel substrate on the membrane module perme-

ability was negligible and the Sieverts’ Law held.

5. Under the operation conditions investigated, the perme-

ation factor of the module increased by increasing the

hydrogen pressure in the vessel side and decreasing the

membrane temperature. By decreasing the hydrogen exit

partial pressure using sweep gas, the membrane module

permeation flux increased, while the permeation factor

decreased.

6. Various membrane surface morphologies were observed

via SEM. Small crackswere observed inmost of SEM images,

which could lead to failure of these membrane modules in

future. Efforts need to be made to smooth the substrate

surface while avoiding the reduction in membrane module

permeability.

Acknowledgment

Financial support fromtheNationalHighTechnologyResearch

and Development Program of China (2009AA05Z102) and the

Fundamental Research Funds for the Central Universities

(project # 2009ZZ0013) are gratefully acknowledged.

Nomenclature

A-G dimensions in Fig. 2, mm

Ep activation energy for permeation, J mol�1

f gas flow rate, m3 s�1

H-J dimensions in Fig. 3, mm

K pre-exponential factor, mol m�1 s�1 MPa�n

KG constant given by the supplier of the porous

substrate, m�2

Ma actual hydrogen permeation rate, mol s�1

MS hydrogen permeation rate calculated from Sieverts’

Law, mol s�1

n parameter whose value depends on the limiting

transport mechanism of hydrogen permeation

through palladium or its alloy membrane




PE permeate side hydrogen partial pressure at the exit

of membrane module, MPa

PH hydrogen partial pressure in vessel side, MPa

PL average hydrogen partial pressures in themembrane

permeate side, MPa

PL1 hydrogen partial pressure at the bottom of the

surface between membrane foil and substrate, MPa

PL2 hydrogen partial pressure at the top of the surface

between membrane foil and substrate, MPa

PP permeate side total pressure, MPa

Q hydrogen permeation flux, mol m�2 s�1

qN sweep gas flow rate, mol s�1

qt total permeate flow of gas at the exit of membrane

module, mol s�1

R gas constant, J mol�l K�1

S1 effective area of membrane surface for hydrogen

permeation, m2

S2 area of substrate, m2

T temperature, K

t1 thickness of palladium or palladium alloy

membrane, m

t2 thickness of substrate, m

Greek letter

y gas viscosity, MPa s

h permeation factor

DP pressure drop of substrate, MPa

r e f e r e n c e s

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