Composite Mixed Ion-Electron Conducting (MIEC) Membranes for Hydrogen Generation and Separation Haibing Wang, Srikanth Gopalan, and Uday B. Pal Division of Materials Science and Engineering & Department of Mechanical Engineering Boston University Boston University
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• Effect of the surface catalyst on hydrogen generation process.
• Conclusions.
• Suggested future work.
Motivations and Research Objectives
Motivations:• Currently, more than 600 billion cubic meters of hydrogen consumed each year.
• Simultaneous generation of pure hydrogen and syn-gas production at commercially
attractive rates.
Research Objective:
• Develop and analyze a new approach for hydrogen generation and separation from steam • Develop and analyze a new approach for hydrogen generation and separation from steam
employing a MIEC based membrane architecture.
Conventional Membranes for Hydrogen Separation
Feed side (High PH2)
Permeate side (Low PH2)
Mixed proton-
electron
conducting
membrane
''
H
'
H 22PP >
'e+H
'
H2P ''
H2P
Feed side (High PH2)
Permeate side (Low PH2)
Pd alloy
''
H
'
H 22PP >
'
H2P ''
H2P
H
Approach 1: (dense ceramics)Hydrogen separation using mixed proton-electron conducting membrane
Permeation mechanism: Coupled diffusion of protons and electrons
Drawback:Low hydrogen production rate
membrane
X=0 X=L X=0 X=L
Approach 2: (dense metals and alloys)Hydrogen separation using Pd and its alloys
Material Selection: Gd0.2Ce0.8O1.9(GDC)/Gd0.08Sr0.88Ti0.95Al0.05O3±±±±δ(GSTA)
• GDC: Ionic conducting phase
• GSTA: n-type electronic conducting phase
⋅⋅×
++ → OO
'
Ce
CeO2
32 V3O2GdOGd 2
(g)O2
16OTi22GdTiO2OGd 2O
'
TiSr
2SrTiO
2323
+++ →+×⋅
'
O2O 2eVO2
1O ++→
⋅⋅×
'
Ti
'
Ti TieTi =+×
Low PO2
MIEC dense membrane
Critical Thickness of the Membrane
Surface exchange is the rate controlling step
OH-H 22
22
2 121
)/1/1(2
PD
Lk
KD
L
K
PPk
k
Js
feed
exv
permeate
ex
feed
O
permeate
O
f
r
Hτ
+++
−
=
Flux JCritical thickness
permeate
exK
1 Resistance of surface exchange
reaction on permeate side
Regime I:Bulk diffusion controlled
Regime II:Surface exchange reaction controlled
)/1/1(222
feed
O
permeate
O
f
rvH PP
Lk
kDJ −=
Bulk diffusion is the rate controlling step 1/L
vD
L2Bulk diffusion resistance
feed
exK
1 Resistance of surface exchange
reaction on feed side
L: membrane thickness
Lc
S. J. Xu and W. J. Thomson, Chem. Eng. Sci., 54, 3839 (1999).
OH-H 22PD
Lk sτ Resistance of gas diffusion
through porous support
Derivation of the Permeation Equation
(a) Five steps involved in the oxygen permeation
through mixed ion-electron conducting membrane:
(b) Two Steps not considered:
Step 1: gas phase mass transfer of gaseous species on feed side
Step 5: gas phase mass transfer of gaseous species on permeate side
(c) Three important steps are considered:
Step 2: surface exchange reaction on feed sideMixed
conducting
membrane'P''P
Step 1 Step 5
Step 2 Step 4
Step 3
Interface I Interface II
r
'
V
5.0'
OfO k-CPkJ =
O 2eVO2
1 x
O
'
O2 ++•• '
O2
x
O 2e VO2
1 O +++
••
rk
fk→←
fk←→
rk
Step 3: oxygen bulk diffusion process
Step 4: surface exchange reaction on permeate side
S. J. Xu and W. J. Thomson, Chem. Eng. Sci., 54, 3839 (1999).
2O 2Omembrane
2e'
••
OVFeed side (High PO2)
Permeate side (Low PO2)
X=0 X=L
L is the membrane thickness
'
O2P
''
O2P
''
O
'
O 22PP >
)C-(CL2
DJ
2
1J '
V
''
VVOV
2==
rVOfO k-CPkJ22
=
''
V
5.0''
OfrO CPk-kJ22
=
'
V
''
V C ,C
Oxygen vacancy concentration at the low and high oxygen partial pressure sides of the membrane
k and k rf
The forward and reverse reaction rate constants for oxygen incorporation reaction. Functions of gas compositions, microstructure and properties of the membrane surface
VD Oxygen vacancy diffusivity
Oxygen permeation flux equation:
)PP(DPP2Lk
)PP(kDJ
5.0''
O
5.0'
OV
5.0''
O
5.0'
Of
5.0''
O
5.0'
OrV
O
2222
22
2++
−
=
Continued:
Derivation of the Permeation Flux Equation
Equation (4) can be further simplified as:
Pk
1
Pk
1
D
2L
)PP(k
k
J
5.0'
Of
5.0''
OfV
5.0''
O
5.0'
O
f
r
O
22
22
2
++
−
=
−−
The area specific hydrogen generation rate can be expressed as:
feed
ex
permeate
exV
5.0''
O
5.0'
O
f
r
5.0'
Of
5.0''
OfV
5.0''
O
5.0'
O
f
r
OH
K
1
K
1
D
2L
)PP(k
2k
Pk
1
Pk
1
D
2L
)PP(k
2k
J2J22
22
22
22
++
−
=
++
−
==
−−−−
(both surface exchange reactions and bulk diffusion process are included)
Continued:
)PP( 5.0''
O
5.0'
O 22
−−
− Driving force for the permeation process
Derivation of the Permeation Flux Equation
permeate
ex
5.0''
Of K
1
Pk
1
2
= Resistance on the permeate side
feed
ex
5.0'
Of K
1
Pk
1
2
= Resistance on the feed side
VD
2LBulk diffusion resistance
For the bulk diffusion controlled permeation process with