JAEA-Review-2010-065.pdf:15.99MB - 日本原子力研究開発機構

4-09
Gas Permeation Characteristics of Silicon Carbide
Membrane Prepared by Radiation-curing of
Polycarbosilane Film
A. Takeyama, M. Sugimoto, A. Idesaki and M. Yoshikawa
Environment and Industrial Materials Research Division, QuBS, JAEA
Hydrogen has attracted much attention because it could
produce energy without exhausting a popular greenhouse
gas, carbon dioxide (CO 2). Hydrogen is produced
industrially by use of steam reforming of methane at
temperature range of 1,073 to 1,273 K, with consuming lots
of energy to maintain the reactant gases at such high
temperature. When the inorganic membrane is used to
separate hydrogen from the product gases, it is expected the
reaction equilibrium would be shifted to the product side,
consequently the reaction temperature are lowered and the
amount of energy required for the process is decreased.
Silica (SiO 2) membranes have been expected as a candidate
for such hydrogen separation because of their high H 2
permeance and selectivity, however, there is a serious issue
that they are unstable in steam at high temperature.
Silicon Carbide (SiC) membrane has a potential
advantage of hydrothermal stability at high temperature due
to its high chemical inertness. So far, SiC membranes with
lower H 2 permeance and selectivity compared than SiO 2
membranes were prepared and much effort to improve its
gas permeation character have been made 2) . In this study,
SiC membranes with high H 2 permeance and selectivity
were prepared by the modified coating method using
precursor (polycarbosilane, PCS) solution and curing of PCS
film by electron beam irradiation.
Alpha alumina tubes coated with thin gamma alumina
layer were used as supports. Appropriate amount of
polycarbosilane (PCS) were dissolved into cyclohexane to
prepare 10 mass% solution. Porous supports whose both
ends were plugged were dipped in PCS solution for 60 s and
drawn up by 1.5 mm/s. After drying in air, supports coated
with PCS film were immersed in the same PCS solution and
drawn up again. Subsequently, curing and cross-linking of
PCS film was carried out by an electron beam irradiation in
helium atmosphere or thermal oxidation in air at 493 K
followed by the pyrolysis at 973 K for 30 minutes in argon
atmosphere. Single gas permeance of hydrogen (H 2) and
nitrogen (N 2) of the membrane were measured using
home-made apparatus and the selectivity (H 2/N 2) was
calculated.
Figure 1 shows H 2 permeance and selectivity of SiC
membrane prepared from PCS film cured by electron beam
irradiation or thermal oxidation. For SiC membrane
prepared from thermally cured PCS film, H 2 permeance
(closed circle) was almost inversely proportional to the
temperature. This temperature dependence indicates H 2
molecule diffuse through pore of the membrane colliding
with the pore wall by Knudsen diffusion mechanism 3) .
JAEA-Review 2010-065
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Knudsen diffusion occurs when the pore diameter is smaller
than about one tenth of the mean free path of H 2 molecule,
which increase proportionally to the temperature.
Therefore, the H 2 permeance and selectivity were declined
as the temperature and mean free path were increased.
In contrast, H 2 permeance of SiC membrane prepared
from radiation-cured PCS film (opened circle) was
exponentially increased. This is typical of thermally
activated diffusion of H 2 molecule by molecular sieving
mechanism, indicating pore diameter of SiC membrane was
smaller than that of membrane prepared from thermally
cured PCS film. Considering the gas evolution behavior of
SiC fiber during the pylolysis 4) , for radiation-cured PCS
film, smaller amount of gas evolved during the pyrolysis and
this caused the formation of small pore in SiC membrane.
Assuming molecular sieving mechanism, H 2 permeance at
773 K in which steam reforming of methane is performed, is
calculated from plots of H 2 permeance. Extrapolated H 2
permeance is 7.6 × 10-7 mol/m 2 /s/Pa and the selectivity is
over 116, which shows SiC membrane prepared by modified
coating method and radiation-curing have both high H 2
permeance and selectivity at the temperature preferable to
H 2 production.
H2 permeance/ mol/m 2 /s/Pa
10 -6
10 -7
10 -8
10 -9
EB
300 400 500
Temperature/ K
References
1) T. Tsuru et al., J. Membr. Sci. 316 (2008) 53.
2) T. Nagano et al., J. Ceram. Soc. Jpn. 114 (2006) 533.
3) J. Dong et al., J. Appl. Phys. 104 (2008) 121301.
4) M. Sugimoto et al., J. Am. Ceram. Soc. 78 (1995) 1013.
80
60
40
20
0
Selectivity, H2/N2
Fig. 1 H 2 permeance and selectivity of SiC
membranes. Closed circles and squares are for
the membrane prepared by thermally cured PCS.