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					International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 – 6480(Print),
 INTERNATIONAL JOURNAL OF ADVANCED RESEARCH
ISSN 0976 – 6499(Online) Volume 5, Issue 5, May (2014), pp. 40-50 © IAEME
    IN ENGINEERING AND TECHNOLOGY (IJARET)
ISSN 0976 - 6480 (Print)
ISSN 0976 - 6499 (Online)
                                                                              IJARET
Volume 5, Issue 5, May (2014), pp. 40-50
© IAEME: www.iaeme.com/ijaret.asp                                             ©IAEME
Journal Impact Factor (2014): 7.8273 (Calculated by GISI)
www.jifactor.com




       GAS PERMEATION PROPERTIES OF HYDROGEN PERMEABLE
       MACROPOROUS ALUMINA CERAMIC MEMBRANES AT HIGH
                         TEMPERATURE

                                Abubakar Alkali1*,       Edward Gobina1
                              Robert Gordon University, Riverside East,
                        Garthdee Road, Aberdeen, AB10 7GJ, United Kingdom




ABSTRACT

        The main purpose of this work is to investigate the hydrogen permeation behavior and
selectivity of a commercial ceramic alumina membrane and compare same with that of a γ-alumina
membrane graded with AlOOH sol using the dip coating method. The permeance of hydrogen and 5
other single gases (He, N2, CH4, CO2 and Ar) were investigated at high temperature and results show
that the permeance of H2 increased with increasing temperature for the graded γ- Al2O3 membrane
while it decreased for the α-Al2O3 support. For the α-alumina membrane, a hydrogen permeance of
up to 1.30 × 10-6 mol m-2 s-1 Pa-1 was observed which increased by about four-fold to 4.61 × 10-6
mol m-2 s-1 Pa-1 when the membrane was graded with Boehmite sol. However, the graded membrane
was permeable to only hydrogen at fifth coating. The selectivity of the membranes for hydrogen with
respect to the other 5 single gases decreased with increasing temperature and the order of gas
permeation didn’t exactly follow the order of gas kinetic diameter and molecular weight.

Key words: Hydrogen, Porous Alumina Membranes, Dip Coating, Gas Permeance, Selectivity.

INTRODUCTION

        Membrane technology for hydrogen separation, purification and production processes is
becoming an important and enabling technology in the current global decarbonisation efforts aimed
at combating climate change and ensuring energy security1. Ultra pure hydrogen (99.99%) is
required for use in chemical industries, domestic power and clean energy applications2. More
recently, the dawn of a new energy epoch in polymer electrolyte fuel cells (PEMFC) has motivated
continued research in membrane technology for hydrogen processes2. It is still ‘work in progress’
before hydrogen is fully adopted as the global energy carrier to replace fossil fuels. During this

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International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 – 6480(Print),
ISSN 0976 – 6499(Online) Volume 5, Issue 5, May (2014), pp. 40-50 © IAEME

transition period from fossil fuels to H2, research interest into several H2 production, separation and
purification processes is expected to rise.
        Recently, inorganic membranes have assumed wider applications in hydrogen separation
processes as a result of their ability to withstand harsh operating conditions and they can also be
permeable to specific molecules3. Inorganic membranes can be divided into 2: Dense (metallic)
membranes and porous ceramic membranes4. Palladium membranes are mostly used in hydrogen
processes due to their infinite selectivity to hydrogen when defect free5. However, due to the high
cost of palladium membranes, there is great motivation for research into comparably cheaper
membranes for hydrogen processes. Porous alumina ceramic membranes with different pore sizes
can be considered for several gas separation applications including hydrogen by reducing the pore
sizes or by improving specific surface properties6. Interest is growing in porous ceramic alumina
membranes for hydrogen processes due to their ability to withstand harsh operating conditions,
thermal and mechanical stability as well as their resistance to corrosion4,7. The gas transport
mechanisms through inorganic membranes generally involves Knudsen diffusion, Hagen –
Poiseulles’s flow, surface diffusion and viscous flow but molecular sieving also occurs in porous
membranes with small pore sizes less than 1 nm4.8.
        In Knudsen diffusion gas molecules collide with the pore wall and diffuse into the pores of
the membrane6. It occurs when the pore radius is less than the mean free path of the gas molecules
and is based on the ratio of inverse square root of the molecular weights of the 2 gases A and B as
follows1:

                                         1/ 2
                              M     
                        αAB =  B                                                  (1)
                              M A
                                    
                                     

        The Knudsen diffusion is usually accompanied by high permeance but with a low selectivity
while surface diffusion involves adsorption of the gas molecules that takes place on the membrane
surface and it favors gases with high adsorption capacity while limiting the diffusion of less
adsorbing gases6. Molecular sieving occurs when the membrane pore diameter is close to that of the
diffusing species. However, the permeance is low in this mechanism but an infinite separation factor
can be achieved6. Porous ceramic membranes have several layers with different pore sizes hence the
mechanisms can occur simultaneously. Thus, gas transport mechanisms in a porous ceramic
membrane can be evaluated by taking into account the separate transport mechanisms based on the
properties of the membrane and the diffusing gases8.
        The hydrogen permeation and transport behavior in porous alumina membranes has been
reported by several authors. Li et al4 (2012) investigated the permeance of hydrogen across porous
alumina ceramic support which was graded with a top γ – Al2O3 layer of pore diameter 4 nm using
the sol-gel technique. H2 permeation tests were conducted at temperature from 250 C to 5000 C and
the group reported a drastic decrease in H2 permeance with increasing temperature from 250 C to
2500 C but for temperatures from 3500 C to 5000 C, the hydrogen permeance was more stable. The
H2/N2 selectivity reported by the group was between 2.9 – 3.4 which was lower than the theoretical
Knudsen value of 3.74. The lower selectivity value below the theoretical Knudsen value indicates
that there was a contribution of viscous flow since hydrogen and nitrogen cannot transport through
the membrane by surface diffusion4. Y.S Cheng et al7 (2002) also investigated the hydrogen
permeance of a mesoporous commercial alumina support with a nominal pore size of 5 nm. Although
a hydrogen permeance of over 700 cm2 cm-3 min-1 bar-1 was achieved, the alumina support could not
separate hydrogen from town gas mixture. For H2/He selectivity, the transport mechanism was
Knudsen and a separation factor of 1.5 was achieved which was above the theoretical Knudsen value
of 1.41.

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International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 – 6480(Print),
ISSN 0976 – 6499(Online) Volume 5, Issue 5, May (2014), pp. 40-50 © IAEME

       One of the ways of enhancing hydrogen permeation in alumina supports is through surface
modification with AlOOH Boehmite sol to convert the topmost α- alumina layer to the more stable
γ- alumina layer. In this work, the hydrogen permeation behavior of a commercial alumina support
was investigated and the effect of temperature, pressure on gas permeance through the membrane
and the selectivity were investigated. The hydrogen permeation behavior and selectivity of the
modified γ-alumina membrane was also investigated and compared to that of the unmodified
commercial alumina support.

EXPERIMENTAL

       The ceramic alumina support is a macroporous membrane supplied by CTI (Ceramiques
Techniques Industrielles SA) France with 6000 nm average pore size, I.D = 20.07 mm, O.D = 26
mm and effective length = 0.32 m. A second alumina support with same specifications was graded
with AlOOH sol using the sequential dip coating method. Both tubes were dried in an oven at 650 C
for 2 hours to remove any water vapor prior to permeation test for the commercial alumina support
and also before the support was modified. In the dip coating method for preparation of γ-alumina
membrane, the Boehmite sol was prepared into which the support was inserted for 30 minutes for
each dipping under continuous stirring. To prepare the Boehmite sol, 46.1 g of the AlOOH powder
was weighed and diluted in 1000 mL of distilled water under constant stirring.
       A fresh Boehmite sol was used for each dipping in order to obtain a uniformly coated
membrane. Prior to dipping, both ends of the support were sealed with plastic seals to avoid inner
deposition. 5 sequential dippings were conducted and after each dipping, the modified support was
dried for 10 hours at 650 C and calcined at 873 K for 24 hours and a permeation test for 6 single
gases carried out. These gases are: H2, He, CH4, CO2, N2 and Ar. The experimental set up for gas
permeation consists of a shell and tube membrane reactor module, a gas flow system comprising of 3
connected parts: The feed, permeate and retentate. The third part of the experimental set up consists
of the mass flow meter which monitors the gas flow rate. Gas permeation tests were conducted for
both the modified and unmodified membranes at 298, 323, 373, 473 and 573 K. Membrane
characterization was conducted using a scanning electron microscopy (SEM) and the elemental
composition of the membrane was analyzed using energy dispersive x-ray analysis (EDXA).




                      Figure 1: A Picture of the commercial alumina membrane




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International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 – 6480(Print),
ISSN 0976 – 6499(Online) Volume 5, Issue 5, May (2014), pp. 40-50 © IAEME




                          (a)                                                     (b)

     Figure 2: SEM Micrographs of the α-alumina membrane: (a) cross section (b) outer section

RESULTS AND DISCUSSION

The Dip Coating Method
        After each dipping, the weight of the membrane was calculated using the weight gain
technique in order to monitor and make an accurate estimate of the thickness of the γ-alumina layer.
The amount of AlOOH deposited was recorded after each dipping. The amount of deposited AlOOH
W was measured by subtracting the weight of the alumina support before dipping from the weight
after dipping. The average layer thickness L was calculated using the equation9.

                     W
                L=                                                                (2)
                      ρ

ρ is the density. The estimated thickness of the deposited layer after each dipping in this work using
equation (2) is shown in Table 1.

                           Table 1: Estimated layer thickness after each dip
                                 Dip            Estimated Thickness (µm)
                                    1                         14.86
                                    2                         27.10
                                    3                         39.34
                                    4                         48.96
                                    5                         55.95

       It is necessary to mention that the thickness was not factored into the calculation for gas
permeance. Unit thickness is used to standardize permeability measurements but not for permeance
measurements. Nonetheless, layer thickness affects gas permeation across the membrane and the
high the thickness the high the membrane resistance to gas permeation.


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International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 – 6480(Print),
ISSN 0976 – 6499(Online) Volume 5, Issue 5, May (2014), pp. 40-50 © IAEME

Gas Permeance and Selectivity
        The permeation behavior of a gas through a membrane is denoted as permeance or
permeability4. Permeance in mol m-2 s-1 Pa-1 is defined as the quantity of gas crossing a unit area in
unit time or flux per unit pressure difference between the higher pressure and the lower pressure
sides of the membrane10. To obtain the permeability of the membrane, the unit thickness of the
membrane is used to normalize the permeance10.
Permeance is represented in the equation4:

                                  Q / 22.4
                             J=                                                     (3)
                                   A× P

Where J is the gas permeance (mol/m2. s. Pa), Q is the permeate gas flow rate (L/min) in permeate
side, A is the membrane area for permeation (m2) and P is the pressure difference across the
membrane (Pa).
        Selectivity denotes the relationship between gas permeance and the permeance of another
gas4. If JH2 is the permeance of Hydrogen and JN2 is the permeance of Nitrogen through the alumina
membrane, then the selectivity of hydrogen relative to nitrogen can be represented as follows4:

                                       JH2
                            αH2/N2 =                                               (4)
                                       JN2




 Figure 3: SEM micrograph for inner section of             Figure 4: EDXA of inner section of the
            the α-alumina membrane                                α- alumina membrane


SEM Micrographs and EDXA Results
        The SEM micrographs for the cross section of the α-alumina membrane shown in Fig. 2 (a)
shows the morphology of the cross section with the pores becoming open and flowery but more
closely knitted and finer at the surface. The outer section in Fig. 2 (b) shows a more homogeneous
pore structure with a smaller grain size compared to both the inner and cross sections. Smaller grain
sizes usually lead to better gas permeation through the membrane due the relatively smaller
boundaries associated with small grains 11.

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International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 – 6480(Print),
ISSN 0976 – 6499(Online) Volume 5, Issue 5, May (2014), pp. 40-50 © IAEME

                                                                                        Table 2: EDXA Elemental composition of the α-alumina membrane
                                                                                                 Element         Weight%             Atomic %

                                                                                                               O                                    49.67                                                              72.96
                                                                                                               Al                                    6.06                                                              5.28
                                                                                                               P                                     0.13                                                              0.10
                                                                                                               Ti                                   44.14                                                              21.66



                                      1.40E-06                                                                                             (a)                                                        2.50E-06                                               (c)




                                                                                                                                                             Permeance (mol m-2 s-1 Pa-1)
   Permeance (mol m-2 s-1 Pa-1)




                                      1.30E-06
                                                                                                                                                                                                      2.00E-06
                                      1.20E-06                                                                                                                                                                                                          1st Coating
                                                                                                                                            298 K
                                      1.10E-06                                                                                                                                                        1.50E-06                                          2nd Coating
                                                                                                                                            323 K
                                                                                                                                                                                                                                                        3rd Coating
                                      1.00E-06                                                                                              373 K
                                                                                                                                                                                                      1.00E-06                                          4th Coating
                                                                                                                                            473 K
                                      9.00E-07
                                                                                                                                                                                                                                                        5th Coating
                                                                                                                                            573 K
                                      8.00E-07                                                                                                                                                        5.00E-07
                                      7.00E-07
                                                                                                                                                                                                 0.00E+00
                                      6.00E-07
                                                                                                                                                                                                                 1   1.05   1.1   1.15    1.2    1.25
                                                                                    1   1.05     1.1         1.15          1.2    1.25
                                                                                        Average Pressure (Bar)                                                                                                       Average Pressure (Bar)


                                                                         5.00E-06
                                                                                                                                            (b)                                                                                                              (d)
                                  P e r m e a n c e (mol m-2 s-1 Pa-1)




                                                                                                                                                                                                      9.00E-07
                                                                         4.50E-06
                                                                                                                                                                       Permeance (mol m-2 s-1 Pa-1)




                                                                                                                                                                                                      8.00E-07
                                                                         4.00E-06
                                                                                                                                                                                                      7.00E-07
                                                                         3.50E-06
                                                                         3.00E-06                                                                                                                     6.00E-07                                               298 K
                                                                                                                                                298 K                                                                                                        323 K
                                                                         2.50E-06                                                                                                                     5.00E-07
                                                                                                                                                323 K                                                                                                        373 K
                                                                         2.00E-06                                                                                                                     4.00E-07
                                                                                                                                                373 K                                                                                                        473 K
                                                                         1.50E-06                                                                                                                     3.00E-07                                               573 K
                                                                                                                                                473 K
                                                                         1.00E-06                                                                                                                     2.00E-07
                                                                                                                                                573 K
                                                                         5.00E-07
                                                                                                                                                                                                      1.00E-07
                                                                         0.00E+00
                                                                                                                                                                                                      0.00E+00
                                                                                    1     1.05         1.1          1.15         1.2     1.25
                                                                                                                                                                                                                 1   1.05   1.1    1.15    1.2    1.25
                                                                                            Average Pressure (Bar)                                                                                                    Average Pressure (Bar)


     Figure 5: Permeance of hydrogen through (a) commercial alumina at different temperature
  (b) modified γ-alumina at different temperature for 1st coating, (c) modified γ-alumina at different
          coatings, (d) N2 permeance for the commercial alumina at different temperature

       Fig.5 (a) shows hydrogen permeance of up to 1.30 × 10-6 mol m-2 s-1 Pa-1 at 298 K which
decreased with increasing temperature for the commercial unmodified alumina membrane.
Impliedly, there is a reverse temperature dependence on gas permeation. The same trend was also
observed for the 5 other single gases i.e. He, CO2, CH4, Ar and N2. Table 3 shows the gas permeance
and permselectivity for both the α– alumina and γ-alumina membranes including the theoretical
Knudsen selectivity.



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International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 – 6480(Print),
ISSN 0976 – 6499(Online) Volume 5, Issue 5, May (2014), pp. 40-50 © IAEME

   Table 3: Gas Permeance and selectivity for the α-alumina membrane and the graded γ-alumina
                                  membrane after first dipping


             Gas Permeance (mol m-2 s-1 Pa-1)                            Selectivity


       Gas         α-alumina         γ-alumina                α-alumina     γ-alumina   Knudsen S.F

                    (298 K)           (573 K)                  (573 K)       (298 K)

        H2        1.30 × 10-6        4.61 × 10-6    H2/N2       11.47          2.92         3.73
       He         1.15 × 10-6        3.79 × 10-6    H2/He        2.78          1.86         1.41
       CO2        1.05 × 10-6        3.13 × 10-6    H2/CO2       2.70          2.25         4.67
       CH4        9.97 × 10-7        2.72 × 10-6    H2/CH4       7.03          2.54         2.82
       Ar         8.61 × 10-7        2.01 × 10-6    H2/Ar        6.24          2.53         4.45
        N2        7.96 × 10-7        1.92 × 10-7


        For the modified alumina membrane, a permeance of up to 4.61E × 10-6 mol m-2 s-1 Pa-1 was
observed at 573 K after the first dipping. The results indicate that there was about 4-fold increase in
the hydrogen permeance of up to 4.61 × 10-6 mol m-2 s-1 Pa-1 after the alumina support was modified
with the Boehmite sol. Hence the modification with AlOOH sol reduced the hydrogen permeation
resistance of the membrane. The modification enhanced surface diffusion of hydrogen molecules
through the membrane pores leading to increased hydrogen permeance. However, the permeance was
negated by increase in temperature. Surface adsorption is generally inversely proportional to
temperature hence the low temperature enhances the surface diffusion of H2 in α-alumina
membrane4. Molecular diffusion has been known to enhance the permeance of gases in nanoporous
inorganic membranes hence those gas molecules with higher diffusivity permeate faster than those
with lower diffusivity 12. Hydrogen is more diffusive than CO2 hence hydrogen molecules enjoy
more mobility and can retain in the permeate stream longer than CO2 and by implication exhibit a
higher permeance compared to CO212.
        The decrease in permeance at high temperature also indicates low mobility of the gas
molecules in the alumina membrane pores. Li and Liang4 (2012) reported similar results in their
investigation of Hydrogen permeance across porous alumina ceramic membrane with an average
pore size of 0.1 µm at temperature 298 K to 773 K and transmembrane pressure difference of 0.0005
– 0.050 Mpa. In this work, for the γ-alumina membrane, 5 successive coatings were carried out and a
permeation test conducted after each dip. However, all the 6 gases permeated after the first 3
dippings but only hydrogen and CO2 permeated after the 4th dipping although the membrane was not
permeable to CO2 at room temperature at the 4th dip. CO2 permeated only at 323, 373 473 and 573 K
but hydrogen permeated at all the temperatures. After the fifth dipping, the membrane was permeable
to only hydrogen. As shown in Fig. 5 (b) the hydrogen permeance increased with temperature up to
4.61 × 10-6 mol m-2 s-1 Pa-1 at 573 K but reduced with the number of successive coatings as more
layers of the AlOOH sol were deposit leading to increase in layer thickness of the membrane which
increased the membrane resistance to permeation. As shown in Table 1, the layer thickness of the
composite membrane increased with the number of dipping.
        Gu and Oyama13 also reported an increase in hydrogen permeance of up to
       -7
5 × 10 mol m-2 s-1 Pa-1 in their work on a macroporous alumina support modified with AlOOH sol.
The gas permeation behavior observed in this work for the modified γ- alumina membrane was in
contrast to that of the unmodified commercial alumina. This is because in the commercial alumina,

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International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 – 6480(Print),
ISSN 0976 – 6499(Online) Volume 5, Issue 5, May (2014), pp. 40-50 © IAEME

gas permeance decreased with temperature while for the modified membrane, it increased with
temperature. The high increase in hydrogen permeance after modification suggests that there are
cracks and defects in the commercial alumina membrane which were repaired by the deposition of
the AlOOH layer. This led to a more uniform and finer surface which enhanced hydrogen permeation
through the membrane13. The apparent opposite trend in hydrogen permeation dependence on
temperature for the modified membrane indicates that modifying the membrane with AlOOH sol has
enhanced surface diffusion and mobility of the molecules.


                         14                                                            (a)                                                   4                                                   (c)
                         12




                                                                                                               H 2 /C O 2 S e le c tiv ity
                                                                                                                                             3                                                  298 K
                         10
         H2/N2 selectivity




                                                                                       298 K
                                                                                                                                                                                                323 K
                             8
                                                                                       323 K                                                 2                                                  373 K
                             6                                                         373 K                                                                                                    473 K
                                                                                       473 K                                                 1                                                  573 K
                             4
                                                                                       573 K
                             2
                                                                                                                                             0
                             0                                                                                                                   0          0.2          0.4            0.6
                                   0   0.1       0.2    0.3        0.4     0.5
                                                                                                                                                           Feed Pressure (Bar)
                                             Feed Pressure (Bar)


                              4
                                                                                       (b)                                       2.5                                                            (d)
                             3.5
                                                                                                     H2/CO2 se le ctiv ity




                                                                                                                                             2
                              3
   H2/N2 selectivity




                                                                                                                                                                                                 298 K
                             2.5
                                                                                                                                 1.5                                                             323 K
                                                                                        298 K
                              2                                                                                                                                                                  373 K
                                                                                        323 K
                             1.5                                                                                                             1                                                   473 K
                                                                                        373 K
                                                                                                                                                                                                 573 K
                              1                                                         473 K
                                                                                        573 K                                    0.5
                             0.5

                              0                                                                                                              0
                                   0    0.1       0.2      0.3       0.4         0.5                                                             0   0.1          0.2   0.3       0.4     0.5
                                              Feed Pressure (Bar)                                                                                           Feed Pressure (Bar)


                    Figure 6: (a) H2/N2 selectivity commercial α-alumina support
  (b) H2/N2 selectivity γ-alumina membrane (c) H2/CO2 selectivity commercial α- alumina support
                             (d) H2/CO2 selectivity γ-alumina membrane

        The selectivity of the membrane for hydrogen in relation to the other gases was also
investigated. In Fig. 6 (a) and Table 3, it can be observed that the highest H2/N2 selectivity for the α-
alumina support is 11.47 at 573 K which was well above the theoretical Knudsen value of 3.74.
However, for H2/CO2, the selectivity was 2.70 which is below the theoretical Knudsen selectivity of
4.67. For the α-alumina support as shown in Fig. 6 (a) and (c) for H2/N2 and H2/CO2 selectivity
respectively, the selectivity increased with increase in temperature. But a different trend was
observed for the H2/N2 and H2/CO2 selectivity in the modified γ-alumina membrane as shown in Fig.

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International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 – 6480(Print),
ISSN 0976 – 6499(Online) Volume 5, Issue 5, May (2014), pp. 40-50 © IAEME

6 (b) and (d) where there is a clustered selectivity. Fig. 6 (b) and (d) show the H2/N2 and H2/CO2
selectivity appearing closer to decreasing with temperature but it is generally clustered.
        It can be observed that hydrogen permeance for the α-alumina support decreased with
temperature while the H2/N2 and H2/CO2 selectivity increased with temperature. For the modified
γ-alumina membrane, the hydrogen permeance increased with temperature while a clustered H2/N2
and H2/CO2 selectivity that appears closer to decreasing with temperature was observed.
Modification of the membrane decreased the hydrogen selectivity although the selectivity is
generally low for both membranes.
        The lower selectivity below Knudsen denotes the contribution of another gas transport
mechanism such as viscous flow.

Gas Permeance and Kinetic Diameter

                          1.00E-02                 H2
     (mol m-2 s-1 Pa-1)




                                                                                                                          1.40E-06
                                           He                                                                                            H2



                                                                                        P erm eanc e (mol m-2 s-1 Pa-1)
                          8.00E-03                                                                                        1.30E-06
                                                         CO2
                                                                                                                          1.20E-06
                                                                                                                                          He
                          6.00E-03
                                                               Ar        CH4                                              1.10E-06                                     CO2
                                                                    N2
                          4.00E-03                                                                                        1.00E-06
    Perm eance




                                                                                                                                                   CH4
                                                                                                                          9.00E-07
                          2.00E-03                                                                                                                          N2
                                                                                                                          8.00E-07                                    Ar

                                                                                                                          7.00E-07
                          0.00E+00
                                                                                                                                     0        10     20      30       40     50
                                     2.5           3           3.5             4
                                                                                                                                                   Molecular Weight
                                                Kinetic Diameter (A)

                                                    (a)                                            (b)
                                            Figure 7: Gas Permeance for α-alumina membrane as a function of
                                                        (a) Kinetic diameter, (b) molecular weight

The order of permeance of the single gases at different temperatures is as follows:

Order of permeance: H2>He>CO2>CH4>Ar>N2

Order of Kinetic Diameter: CH4 (3.8 Ǻ)>N2 (3.64 Ǻ)>Ar (3.4 Ǻ)>CO2 (3.3 Ǻ)>H2 (2.89 Ǻ)>He
(2.65 Ǻ)

Order of molecular weight: CO2 (44.01)>Ar (39.948)> N2 (28.0134)>CO (28.011)>CH4
(16.044)>He (4.02)> H2 (2.016).

       Fig. 7 (a) shows the relationship between gas permeance and kinetic diameter for the
α-alumina membrane. The permeance of the gases didn’t exactly follow the order of their kinetic
diameter. Although there is a trend such that the smaller molecules seem to permeate faster than the
bigger molecules. He and H2 have the lowest kinetic diameter but the highest permeance although H2
(2.89 Ǻ) with a larger kinetic diameter permeated faster than He (2.65 Ǻ) which could be explained
in terms of the higher sorptivity of H2 compared to that of He.

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International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 – 6480(Print),
ISSN 0976 – 6499(Online) Volume 5, Issue 5, May (2014), pp. 40-50 © IAEME

        Moreover, CH4 with a larger kinetic diameter (3.8 Ǻ) has a higher permeance compared to
both N2 (3.64 Ǻ) and Ar (3.4 Ǻ). This could also be explained in the same vein as for the H2 and He
permeance difference.
        As shown in Fig. 7 (b), the order of gas permeance didn’t exactly follow the order of gas
molecular weight for the α- alumina membrane. H2, He, CH4, N2 and Ar belong to the group of
nonadsorbed gases for alumina but CO2 is a strongly adsorbing gas for alumina membranes4. It can
be observed from Fig. 7 (b) that the permeance of hydrogen was higher than that of all the other
gases which was expected based on the fact that hydrogen has the lowest molecular weight.
However, hydrogen is a nonadsorbed gas for alumina hence its higher permeance is explained in
terms of surface diffusion. The same could be said of CO2 which has the largest molecular weight;
CO2 is a strongly adsorbing gas for alumina which explains why CO2 with a higher molecular weight
has a higher permeance than N2, Ar and CH4.

CONCLUSION

        The hydrogen permeation properties of a commercial α-alumina membrane including those of
5 other single gases: He, CH4, CO2, Ar, N2 were investigated and compared with the permeation
properties of γ-alumina membrane modified with Boehmite sol through the dip coating method.
Results indicated that the modification of the membrane with Boehmite sol increased the hydrogen
permeance by 4 fold. The gas permeance of the α-alumina membrane decreased with temperature
and the highest hydrogen permeance of 1.30 × 10-6 mol m-2 s-1 Pa-1 was observed at 298 K. For the
modified γ-alumina membrane, the gas permeance increased with temperature and the highest
hydrogen permeance of 4.61 × 10-6 mol m-2 s-1 Pa-1 was achieved at 573 K. Conversely, the
membrane selectivity for hydrogen in relation to the other 5 single gases increased with temperature
for the α-alumina membrane while it decreased with temperature for the modified γ-alumina
membrane. The modified γ-alumina membrane was permeable to all the gases at the first, second and
third dips. However, after the 4th dip, the membrane was permeable to only H2 and CO2 and after the
fifth dip, the membrane was permeable to only H2. Both the commercial α-alumina membrane and
the modified γ-alumina membrane show excellent thermal stability at higher temperature.

REFERENCES

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International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 – 6480(Print),
ISSN 0976 – 6499(Online) Volume 5, Issue 5, May (2014), pp. 40-50 © IAEME

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