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 6 ( 2 0 1 1 ) 1 0 1 4 e1 0 2 6
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Hydrogen permeability of PdeAg membrane modules with
porous 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
a
b
MOE Key Laboratory of Enhanced Heat Transfer & Energy Conservation, South China University of Technology, Guangzhou 510640, China
Department of Earth and Environmental Engineering, Columbia University, 9500 W. 120th Street, NY 10027, USA
article info
abstract
Article history:
Palladium-based membranes are attractive for their nearly perfect permselectivity to
Received 5 May 2010
hydrogen. Membrane modules, consisting of a membrane foil, porous stainless steel
Received in revised form
substrate, test frame and flange were assembled and tested in an electrically heated vessel.
10 October 2010
Instantaneous hydrogen permeation flux was measured. Influences of operation condi-
Accepted 11 October 2010
tions on the membrane performance were examined. Microstructure and morphology of
Available online 10 November 2010
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
Keywords:
temperature higher than 755 K, the hydrogen permeation flux through the membrane
Hydrogen
module with 0.2 mm grade porous 316L stainless steel substrate decayed continuously due
Palladium membrane
to the inter-metallic diffusion between the membrane and the substrate. For a temperature
Porous stainless steel
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
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 technologies. 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
dwindling fossil fuel supply and an approach to a future lasting
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 topfired 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
* Corresponding author. Tel./fax: þ86 20 22236985.
E-mail address: [email protected] (D. Xie).
0360-3199/$ e see front matter ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.
doi:10.1016/j.ijhydene.2010.10.030
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 6 ( 2 0 1 1 ) 1 0 1 4 e1 0 2 6
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 integrated with chemical reactors where chemical reaction and
hydrogen separation occur simultaneously to simplify the
hydrogen production process. Various membrane reactors
have been proposed and tested for hydrogen production [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 palladium membranes
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 K
S1 Ep n
e RT PH PnL
t1
(1)
where MS is the hydrogen permeation rate, K is the preexponential factor, S1 is the effective area of membrane
surface for hydrogen permeation, t1 is the thickness of palladium or palladium alloy membrane, Ep is the activation energy
for permeation, R is the gas constant, T is the temperature, PH
is 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 exponent 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
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 influence 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 and
experimental 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 mm were 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 such membrane foils was used:
activation energy 9.18 kJ/mol, pressure order 0.5, pre-exponential factor 2.07 103(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.
(2)
h is reported in literatures to be from 0.39 to nearly 1.0
[8,25e27].
1015
Fig. 1 e Structure of the PdeAg membrane module
assembly.
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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 kW was 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 pretreatments
For 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 to membrane foil
failure. Hence for the test 7, the substrate surface was pretreated 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 temperature 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 Electronic Microscopy (SEM).
2.3.2.
Fig. 2 e Geometry of the test frame (top) and flange
(bottom).
2.2.
Substrate pressure drop measurements
For 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:
Experimental setup
DP ¼ KG 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
II
III
3
3
3
5
5
5
5
5
5
28
28
24
32
32
26
1.2
1.2
1.0
8
8
8
24
22
20
28
20
22
6
6
6
f
y t2
S2
(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
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Fig. 3 e Schematic diagram of the experimental setup.
lower the flow resistance. The pressure drops across the untreated 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 permeability. 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 assembling the membrane foil. The 0.2 mm grade test substrate
module was identical to the Type III membrane module, 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,
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 mm grade substrate increased slightly with time,
while the increase of pressure drop with time across the
0.5 mm grade 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 mm grade 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 mm substrate itself should not cause any quick decay in the
module permeability.
2.3.3.
Membrane module permeability test procedure
After 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.
Table 2 e Test configurations and conditions.
Test
1
2
3
4
5
6
7
t2
mm
Substrate grade
mm
t1
mm
Module type
Substrate
Pretreatment
Temperature
K
Pressure
MPa
Vessel
environment
1.2
1.2
1.0
1.0
1.0
1.2
1.2
0.5
0.5
0.2
0.2
0.2
0.5
0.5
25
25
10
25
25
50
25
I
I
III
III
III
II
I
No
No
No
No
No
No
Yes
869e917
923
923
913
723
943
920
0.2
0.2
0.2
0.4
0.3
0.3
0.2
H2
H2
H2
H2 þ Ar
H2
H2
H2
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0.18
Equation (3)
Test 7 after polishing
Test 7 without pretreatment
Test 7 after etching
Test 1
Test 2
Test 6
0.16
Pressure drop (MPa)
0.14
0.12
0.10
0.08
0.06
0.04
0.02
0.00
0
50
100
150
200
250
3
300
-1
-2
-1
-2
350
400
450
400
450
Air flow flux (Nm h m )
0.16
Equation (3)
Test 3
Test 4
Test 5
0.14
Pressure drop (MPa)
0.12
0.10
0.08
0.06
0.04
0.02
0.00
50
100
150
200
250
3
300
350
Air flow flux (Nm h m )
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).
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).
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 vessel was heated by the
electrical heater to 523 K. The vessel pressure was maintained 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 pressure 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) environment until the desired temperature (590 Ke913 K) was
reached. The membrane permeability data was continuously recorded during this period.
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 6 ( 2 0 1 1 ) 1 0 1 4 e1 0 2 6
0.10
0.2 m before heating
0.2 m after heating
Pressure drop (MPa)
0.08
0.5 m before heating
0.5 m after heating
0.02
Experimental results and discussion
3.1.
factor
Instant hydrogen permeation flux and permeation
Q¼
0.00
200
400
600
800
1000
3
-1
1200
-2
Hydrogen flow flux (Nm h m )
0.30
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
0.25
Pressure drop (MPa)
3.
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
0.06
0.04
1019
Ma
S1
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:
0.20
0.15
0.5 m, 0 h after being heated @ 923K
0.5 m, 4 h after being heated @ 923K
0.5 m, 8 h after being heated @ 923K
0.10
0.05
0.00
200
400
600
800
3
-1
1000
1200
-2
Hydrogen flow flux (Nm h m )
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.
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 maintained at 869 K for the first 47 h, and then 917 K for the next
47e75 h. The membrane permeability data was continuously 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 pressure 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 permeability 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 flow was observed in the
permeate side of the membrane module. Hence the
membrane integration was confirmed.
(4)
Fig. 7 e Variation of membrane module permeation flux
(top) and factor (bottom) with time for tests 1e7.
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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 permeation 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.
4. For test 4, 1.0 mm thick 0.2 mm grade substrate and 25 mm
thick membrane foil were employed in the membrane
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
much higher than the hydrogen permeation rate through the
membrane module (maximum 0.1 LPM). Hence the hydrogen
concentration was maintained 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.
A
B
Intensity
1. For test 1 (0.5 mm grade substrate and 25 mm thick
membrane foil), when the vessel temperature was around
869 K and pressure was 0.2 MPa, the membrane module 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 intermetallic diffusion between the PdeAg membrane and the
sintered stainless steel substrate. Although the intermetallic 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
membrane modules [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 S3700N), 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)
100
80
60
40
20
0
Pd
Ag
0
0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8
Distance from A to B (µm)
a
D
c
2
b
100
80
60
40
20
0
Pd
Intensity
C
Fe
Ag
0
Fe
0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8
Distance from C to D (µm)
2
d
Fig. 8 e (a) SEM micrograph showing the cross-sectional microstructure of the 0.5 mm substrate and membrane after test 1;
(b) line scan of the cross-sectional elemental distributions of Pd, Ag and Fe of the 0.5 mm substrate and membrane after test 1;
(c) SEM micrograph showing the cross-sectional microstructure of the 0.2 mm substrate and membrane after test 4; (d) line
scan of the cross-sectional elemental distributions of Pd, Ag and Fe of the 0.2 mm substrate and membrane after test 4.
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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 permeation 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 suspected 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 distribution of Pd, Ag and Fe elements along the line (from point C to D)
in the SEM image was measured 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 the metal 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.
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 conditions, the influence of substrate on membrane module
permeability was negligible and the Sieverts’ Law held.
6. For test 6, 1.2 mm thick 0.5 mm grade substrate and 50 mm
thick membrane foil were employed in the membrane
module. The vessel temperature was maintained at 943 K
and pressure of 0.3 MPa. When the vessel temperature was
stable, the membrane module permeation flux was maintained 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 the metallic diffusion near the surface
to the substrate had a limited influence on its total
permeability.
7. For test 7, the 0.5 mm substrate was pre-treated as described
previously. It can be observed from Fig. 7 that the permeation 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 the 0.2 mm grade substrate under the high
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 membrane
permeation factor
5. For test 5, 1.0 mm thick 0.2 mm grade substrate and 25 mm
thick membrane foil were employed in the membrane
module. The vessel temperature was maintained at 723 K
which was below 755 K. The membrane permeation flux
was observed at 0.30 mol/(m2 s) during the stable temperature 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
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 three modules
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
Table 3 e Membrane module permeation flux and factor.
Test
1
1
2
5
6
Substrate grade
mm
Membrane thickness
mm
Temperature
K
Pressure
MPa
Permeation flux
mol/(m2 s)
Permeation factor
0.5
0.5
0.5
0.2
0.5
25
25
25
25
50
869
917
923
723
943
0.2
0.2
0.2
0.3
0.3
w0.22
w0.27
w0.27
w0.30
w0.30
w0.82
w0.87
w0.85
w1.02
w0.92
1022
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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 increased with increasing the hydrogen
H2 partial pressure in
permeate flow channel
H2 pressure in vessel
H2 partial pressure in permeate
side surface of membrane foil
Hydrogen + Sweep gas
Permeate flow channel
Porous substrate
1.20
Membrane
1.18
1.16
Permeation factor (-)
1.14
1.12
1.10
1.08
1.06
Sweep gas
1.04
1.02
PL1
0
1.00
0.98
659-665K
758-764K
911-917K
0.96
0.94
0.92
0.20
PL2
PH
0.25
0.30
0.35
0.40
0.45
0.5
0.50
0.5
0.55
Fig. 10 e Profile of hydrogen partial pressure in the
membrane module.
0.60
0.5
PH -PL (MPa )
1.06
1.04
1.02
Permeation factor (-)
PE
Hydrogen partial pressure
1.00
0.98
0.96
0.94
0.92
0.90
721-727K
820-825K
921-924K
0.88
0.86
0.84
0.1
0.2
0.3
0.4
0.5
0.5
0.5
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
0.6
0.5
PH -PL (MPa )
1.14
1.12
1.00
0.35
0.95
0.30
0.90
0.25
1.06
1.04
1.02
1.00
0.85
0.20
0.80
0.15
0.75
0.10
0.70
0.98
0.96
672-676K
718-724K
912-918K
0.94
0.92
0.20
0.25
0.30
0.35
0.5
0.40
0.5
0.45
0.50
-1
-2
Permeation factor (-)
Permeation factor (-)
1.08
Permeation flux (mol m s )
1.10
672-678K
720-724K
820-825K
0.65
0.05
0.00
0.16
0.55
0.18
0.20
0.22
0.24
0.5
0.26
0.28
0.30
0.32
0.5
PL ( MPa )
0.5
PH -PL (MPa )
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).
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).
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1023
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 membrane module 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
PP
qt
(5)
where qt is the total permeate flow rate of gas at the exit of
membrane module, 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 membrane module 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
1024
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 6 ( 2 0 1 1 ) 1 0 1 4 e1 0 2 6
Table 4 e Major features of the SEM images of membrane
foil after test.
Test
1
2
3
4
6
7
Major feature of membrane surface
Grain-like bump on surface
Smooth, tiny and discrete cracks observable
Bumps on and cracks in surface
Frost-like bumps on surface
Continuous cracks
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 permeation 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.
3.
4.
5.
Membrane foil surface smoothness
After each test, the membrane module was 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
between membrane 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 environment, 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.
2.
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:
6.
Acknowledgment
Financial support from the National High Technology Research
and Development Program of China (2009AA05Z102) and the
Fundamental Research Funds for the Central Universities
(project # 2009ZZ0013) are gratefully acknowledged.
Nomenclature
A-G
Ep
f
H-J
K
KG
Ma
MS
n
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 continuously 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.
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 material 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.
Pretreatment of the 0.5 mm grade substrate helped to
smooth the membrane foil surface. However, it changed
the surface structure of the material and led to a decrease in
the permeability of the membrane module.
For temperatures below 755 K, the influence of porous
stainless steel substrate on the membrane module permeability was negligible and the Sieverts’ Law held.
Under the operation conditions investigated, the permeation 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.
Various membrane surface morphologies were observed
via SEM. Small cracks were observed in most 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.
dimensions in Fig. 2, mm
activation energy for permeation, J mol1
gas flow rate, m3 s1
dimensions in Fig. 3, mm
pre-exponential factor, mol m1 s1 MPan
constant given by the supplier of the porous
substrate, m2
actual hydrogen permeation rate, mol s1
hydrogen permeation rate calculated from Sieverts’
Law, mol s1
parameter whose value depends on the limiting
transport mechanism of hydrogen permeation
through palladium or its alloy membrane
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 6 ( 2 0 1 1 ) 1 0 1 4 e1 0 2 6
PE
PH
PL
PL1
PL2
PP
Q
qN
qt
R
S1
S2
T
t1
t2
permeate side hydrogen partial pressure at the exit
of membrane module, MPa
hydrogen partial pressure in vessel side, MPa
average hydrogen partial pressures in the membrane
permeate side, MPa
hydrogen partial pressure at the bottom of the
surface between membrane foil and substrate, MPa
hydrogen partial pressure at the top of the surface
between membrane foil and substrate, MPa
permeate side total pressure, MPa
hydrogen permeation flux, mol m2 s1
sweep gas flow rate, mol s1
total permeate flow of gas at the exit of membrane
module, mol s1
gas constant, J moll K1
effective area of membrane surface for hydrogen
permeation, m2
area of substrate, m2
temperature, K
thickness of palladium or palladium alloy
membrane, m
thickness of substrate, m
Greek letter
y
gas viscosity, MPa s
h
permeation factor
DP
pressure drop of substrate, MPa
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