Transcript MIECs Membrane reactors

Mixed ionic electronic conductors (MIECs).
Ceramic membranes for gas separation and chemical reactors
MIECs
Membrane reactors
Dense ceramic membranes made of MIECs have attracted interest for the realization of
membrane reactors.
>Mass transport by ionic diffusion through the lattice + electronic conductivity for charge
compensation
>Superior chemical and thermal stability in comparison to polymeric membranes;
>High selectivity for oxygen and hydrogen separation;
>Production of pure oygen, enriched air, hydrogen;
membrane
>Selective oxidation of hydrocarbons (uniform and well-controlled O2 flux); reactors
>Splitting of oxygen containing molecules (H2O, N2O, NOx)
>High fluxes;
>Scalability of reactors;
>Cost reduction and energy saving;

MIEC
membrane
MIECs
d
Transport process
Mass flux in a chemical potential gradient
J O2
Ji  
'
RT
1 pO 2
driving force
 amb ln ''


2
16 F
d pO 2
thickness
 amb 
 e  ion
 e   ion
If e>> ion
amb = ion
 ion 
Di ci di
RT dx
Wagner equation
2
 

O
4F V
DV
RT Vm
DV  A exp Ea / RT 
If the diffusion rate is very high (d < dc), surface oxygen
exchange reactions become rate controlling.
dc 
ion: ionic conductivity;
e : electronic conductivity;
Dv: VO diffusion coefficient;
Vm: molar volume
DO
ks
ks = surface exchange
reaction constant
MIECs
Materials
Most used MIECs are ABO3 perovskites with general composition BaCoxFeyZrzO3– (x+y+z=1)
or La1-xSrxCoxFe1-xO3-.
1250K
1000K
830K
P
P
B
P
Bi2O3-based (B)
P
P
C
Ceria-based (C)
B
B
B
B
Perovskites (P)
MIECs
Materials
900°C, 100 m
BaCoxFeyZrzO3–
MIECs
Materials
Chemical expansion of perovskites
Formation of additional oxygen vacancies at high temperature produces an increase in thermal
expansion. Compatibility problems. Same or similar material for the support porous tube and
active dense membrane.
A0.68Sr0.3Co0.2Fe0.8O3−δ
Slope change
MIECs
Tubular membranes
Activation layer
Dense layer
Porous support layer
Ba0.5Sr0.5Co0.8Fe0.2O3−δ
MIECs
Membrane module design
Planar design
Tubular design
MIECs
Applications: oxygen separation
(1) Using a sweep gas (steam)
(2) Two-step process
5 bar
50% O2
Oxygen production: 10 mL cm-2 min-1 at 900°C
MIECs
Applications: dehydrogenation of light alkanes
(1) Conventional catalytic thermal dehydrogenation of light alkanes suffers from low alkane
conversion due to thermodynamic limitation.
alkane alkene H 2
G0  0
(2) Oxidative dehydrogenation of alkanes improve conversion efficiency but leads to
byproducts. Steam suppresses coke formation.
1
H 2  O2  H 2O
2
G 0  220 kJ / mol
Use of MIEC membranes allows for the controlled supply of oxygen (no co-feeding) leading to
high selectivity and can exploit the advantages of both methods by realizing a sequence of
thermal dehydrogenation/oxydative dehydrogenation steps.
O2 permeable membrane
Passivated membrane
DH: dehydrogenation
HC: hydrogen combustion
MIECs
Applications: dehydrogenation of light alkanes
Literature: conventional
catalytic reactor
ethene
propene
MIECs
Applications: hydrogen production from water splitting
Water splitting coupled with syngas or alkene production
Water splitting
Syngas production
Alkene production
1
H 2  O2  H 2O
2
G 0  220 kJ / mol
The reaction is shifted to the right even at high
temperature. To increase H2 production rate, p(O2)
must be very low at the opposite side of the
membrane by means of an oxidation reaction.
H2 production 3.1 mL cm-2 min-1 at 950°C