Inorganic Microporous Membranes for Gas Separation in Fossil Fuel ...

2 Theoretical background
pO2′′
RT
j = − ∫ t σ d ln pO
(2)
O2 2 e ion
16F
L pO2′
2
Thus, the oxygen flux (jO2) through dense ceramic membranes is dependent on the
temperature (T), thickness (L), the electronic conductivity (σe; te= σe/[σe+σion]) and ionic
conductivity (σion) of the membrane material and the oxygen partial pressure gradient
(∆pO2) over the membrane. 16
pO’2
e
A
O 2-
A B
pO’’2
10
pO’2
O 2-
pO’’2
Figure 2 Dense membrane concepts. A): oxygen pump. b): Mixed ionic and electronic conductor 16 .
The most promising materials for oxygen separation and methane reforming membranes
are the acceptor-doped perovskite-type oxides with the general formula La1-xAxByO3-δ
(A = Sr, Ba and B = Fe, Cu, Ni, Cr, Co). These compounds show high electronic and
ionic conductivity. The highest oxygen flux has been obtained by Vente et al. 17,18 for a
strontium barium iron doped cobaltate at 900˚C, see Table 2. The highest oxygen flux
found in literature has been obtained with an oxygen partial pressure at the feed side of
1 bar and a membrane thickness of only 0.2 mm, whereas the other results are based on
membranes thicker than 1 mm. If the oxygen flux is normalised by the thickness than
Mertins et al. 19 measured the highest oxygen flux with atmospheric air at the feed side.
The flux of the membrane reported by Mertins et al. 19 is obtained by using methane as a
sweep gas in combination with a Ni-catalyst for methane reforming application.
Decreasing the thickness would improve the bulk diffusion. The oxygen transport will be
rate determined by the oxygen surface exchange 20 for membranes with a thickness less
than 500 µm. Thin La1-xSrxCo1-yFeyO3-δ films supported on tailored ceramic substrates
can be prepared 21 . Fluxes between 1 and 30 ml/cm 2 ·min can be expected. 7
2e -