site

an

rive,

004; a

(OT

lar d

se p

model. This model describes oxygen flux as a function of temperature, pressure, and oxygen recovery across composite OTMs. The

istry [3,4,5] resulted in an analytical solution for the ing the oxygen transport membrane (OTM) in partnership

with the U.S. Department of Energys National Energy

Technology Laboratory (NETL). The aim of this project is

iency of integrated

Solid State Ionics 174 (2004model predicts an optimum thickness for porous mixed-conducting layers to improve oxygen surface exchange. Layer thickness

depends on a number of parameters, such as pore size distribution, porosity, and tortuosity in each layer, and basic material

parameters, such as surface exchange rate and ambipolar conductivity. The transfer model shows the importance of optimizing these

parameters and a reactor design that enables a high mass transfer coefficient on the airside of the OTM element for optimum

performance.

D 2004 Elsevier B.V. All rights reserved.

PACS: 66.30; 81.05; 81.20; 82.20; 82.65

Keywords: Air separation; Oxygen permeation; Mixed ionic electron conductor; Surface exchange; Ambipolar conductivity; Diffusion; Ionic conduction;

Porous materials; Computational modeling; Membrane processes; Mass transfer

1. Introduction

The energy-related materials research group, led by

Masayuki Dokiya, conducted basic research on thermody-

namic and transport properties of complex oxides at the

former National Chemical Laboratory for Industry in

Tsukuba, Japan. The first oxide they evaluated,

La1yCayCrO3d, was considered a candidate materialfor interconnects in a solid oxide fuel cell. Oxygen

transport through that mixed conductor was undesirable

because it could result in oxidizing part of the fuel

without generating electrical power. The application of

irreversible thermodynamics [1,2] and point defect chem-

oxygen flux and oxygen chemical potential gradient [6,7].

This analytical solution was used to evaluate experimental

results [8].

Teraoka et al. [9,10] combined one of the basic

functions of interconnects (separating air from fuel) with

materials that were engineered for high ambipolar con-

ductivity. Their work started a new field of research:

mixed-conducting oxide membranes for air separation

[11]. This paper describes oxygen transport processes

through a mixed-conducting oxide membrane to achieve

air separation that produces oxygen as a product. The

model has been used in Praxair to determine some

preferred membrane architectures [12]. Praxair is develop-Oxygen transfer across compo

Bart A. v

Praxair, Inc., 175 East Park D

Received 30 March 2

Abstract

The transfer of oxygen across an oxygen transport membrane

across a boundary layer on the airside, surface exchange, ambipo

viscous flow of oxygen through the porous support. Each of the0167-2738/$ - see front matter D 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.ssi.2004.07.034

* Tel.: +1 716 897 2389; fax: +1 716 879 7931.

E-mail address: bart_vanhassel@praxair.com.oxygen transport membranes

Hassel*

Tonawanda, NY 14150, USA

ccepted 3 July 2004

M) is limited by a number of processes, such as mass transfer

iffusion through the mixed-conducting gas separation layer, and

rocesses was incorporated into a comprehensive oxygen transfer

) 253260

www.elsevier.com/locate/ssito reduce costs and improve the efficoxygen-fired coal gasification in combined cycle power

plants [1316].

2. Theory

2.1. Model outline

Fig. 1a and b shows a cross-section photograph and a

schematic drawing of an OTM that was used for model

development. The membrane consists of a dense gas

separation layer of a mixed-conducting oxide on a porous

support. Porous mixed-conducting oxide layers can be

applied on either the airside, oxygen product side, or both,

to improve the rate of oxygen transfer. Each porous layer

has its own thickness, pore size, porosity, and tortuosity.

B.A. van Hassel / Solid State Io254Fig. 1. (a) A cross-section of an OTM using scanning electron microscopy.

(b) A schematic diagram of the OTM used in model development.Compressed air is supplied to the side of the membrane with

the thin gas separation layer, while a low-pressure, high-

purity oxygen product is collected from the porous support

side.

2.2. Oxygen transport

The simultaneous transport of oxygen molecules inside

the pores and oxygen vacancies in the mixed-conducting

oxide phase of the membrane is described using an effective

medium approximation. Kenjo et al. [17], Murygin [18], and

Maggio et al. [19] first related this common approximation

to fuel cells, and Thorogood et al. [20] and Deng et al.

[21,22] applied it to the modeling of OTMs. The difference

between this study and previous works is shown in the

expressions used to describe oxygen transfer, and how the

boundary conditions are applied.

When modeling oxygen transfer across membranes with

a commercially viable oxygen flux, it is important to

account for the mass transfer resistance in the boundary

layer on the airside of the membrane. The following

expression was used to compute the oxygen flux across

the airside boundary layer around the OTM tube in axial

flow:

JO2 ShDO2;N2

IDShroud ODOTM Pair

RgTln

1 xO2;e1 xO2;s

1

where JO2=oxygen flux [mol/m2/s]; T=absolute temperature

[K]; Sh=Sherwood number []; DO2N2=binary diffusion co-

efficient for oxygen and nitrogen [m2/s]; ODOTM=outside

diameter of OTM tube [m]; IDShroud=inside diameter of

shroud around OTM tube [m]; Pair=airside absolute pressure

[Pa]; Rg=molar gas constant [J/(mol K)]; xO2,s=O2 mole

fraction outside the boundary layer []; xO2,e=O2 mole

fraction at the OTM wall [].

The Sherwood number was computed using well-known

correlations [23]. A positive oxygen flux indicates the

transport of oxygen from the airside of the membrane to the

oxygen product side.

The dusty gas model [24] was used to explain the

diffusion of oxygen gas against stagnant nitrogen on the

airside of the OTM, which resulted in the following

expression for the oxygen flux:

JO2 es

1

1

DKn;O2 1 xO2

Dmol;O2;N2

Pair

RgT

1

tpa

dxO2dx

2

where xO2=oxygen mole fraction []; Dmol,O2,N2=molecular

diffusion coefficient of oxygen in an O2/N2 mixture [m2/s];

DKn,O2=Knudsen diffusion coefficient of oxygen [m2/s];

Pair=airside absolute pressure [Pa]; e=porosity []; s=

nics 174 (2004) 253260tortuosity []; x=dimensionless coordinate (0bxb1) [];tpa=thickness of porous layer on the airside [m].

ate IoThe molecular diffusion coefficient was calculated from

[25]:

Dmol;O2;N2 0:00266 105T 32

Pair 103 MwO2MwN2

MwO2 MwN2

12

rO2;N2XD

3

where MwN2=molar weight of nitrogen [g/mol]; MwO2=

molar weight of oxygen [g/mol]; rO2,N2=characteristiclength [2]; XD=diffusion collision integral [].

The Knudsen diffusion coefficient is given by:

DKn;O2 2

3r

8RgT

pMO2

s4

where DKn,O2=Knudsen diffusion coefficient of oxygen [m2/

s]; Rg=molar gas constant [J/mol K]; T=absolute temper-

ature [K]; P=pressure [Pa]; r=pore radius [m]; MO2=

molecular weight of oxygen [kg/mol].

The ambipolar diffusion of oxygen ions through the

dense mixed-conducting oxide film results in the following

relation between oxygen flux and driving force:

JO2 ramb42F2

Dltg

5

where ramb=ambipolar conductivity [V1m1]; F=Faraday

constant [C/mol]; Dl=oxygen chemical potential dropacross dense gas separation layer [J/mol]; tg=thickness of

the dense gas separation layer [m].ramb stands for theaverage ambipolar conductivity (teltionrtot, with tel=elec-tronic transport number, tion=ionic transport number, and

rtot=total conductivity) in the oxygen chemical potentialgradient. The ambipolar conductivity was assumed to have

an Arrhenius dependence on temperature:

ramb r0ambeEambR

1T 1

1273:15 6

with ramb0 as the preexponential factor and Eamb as the

activation energy for the ambipolar conductivity.

The viscous flow of oxygen on the oxygen product

side of the OTM results in the following relationship

between the oxygen flux and the oxygen pressure

gradient:

JO2 esDKn;O2

1

RgT

1

tpp

dP

dx

e

sr2

8RgTgP

1

tpp

dP

dx

7

where Pl=pressure at the low-pressure side of the inert

support [Pa]; Ph=pressure at the high-pressure side of the

inert support [Pa]; tpp=thickness of the active porous layer

on oxygen side [m]; g=gas viscosity [N s/m2].The same equation is used to describe the oxygen flow

B.A. van Hassel / Solid Stthrough the inert porous support layers, but with the

appropriate thickness, porosity, and tortuosity.The exchange between oxygen in the gas phase, and the

oxygen ions in the mixed-conducting oxide is described

by:

JO2 kPnO2DlRgT

8

where k=rate constant for surface oxygen exchange

[mol O2/(m2 s (Pa/105)n)]; PO2=oxygen partial pressure

[Pa]; Dl=oxygen chemical potential drop across inter-face [J/mol]; n=exponent for oxygen partial pressure

dependence [].

The surface exchange rate constant was assumed to have

an Arrhenius dependence on temperature:

k k0eEkR 1T 11273:15 9

with k0 as the preexponential factor, and Ek as the activation

energy for surface exchange.

In the porous layers, an exchange occurs between the

oxygen gas molecules and the oxide ions, when there is a

difference in the oxygen chemical potential. Such an

exchange is accounted for by the following system of

differential equations:

Airside:

1

tpa

dJO2dx

kPnO2lg ls

RT

2er

10

1

tpa

dJO2

dx 2kPnO2

lg ls

RT

2er

11

O2 product side:

1

tpp

dJO2dx

kPnO2lg ls

RT

2er

12

1

tpp

dJO2

dx 2kPnO2

lg ls

RT

2er

13

where lgls=oxygen chemical potential differencebetween the gas phase and the mixed conducting oxide

phase [J/mol].

This results in two second-order differential equations for

each porous active layer. The oxygen flux in the gas phase is

indicated by JO2, and the flux of oxygen ions in the mixed-

conducting oxide is indicated by JO2.The following boundary conditions were used to solve

the system of differential equations and to obtain the oxygen

flux through the membrane.

On the airside, the oxygen partial pressure at the entrance

of the pore is equal to the pressure inside the boundary layer

with PO2,feed=Pair xO2,e:

nics 174 (2004) 253260 255lg RT ln PO2;feed 14

ate IoThere is also some exchange of oxygen in the gas

atmosphere and the oxygen ions in the mixed-conducting

oxide:

1 ess

ramb42F2

1

tpa

dlsdx

1 e kPnO2

lg ls

RT15

where ss=tortuosity of the solid phase in the porous mixedconducting oxide [].

This boundary condition that is based on a limited rate of

surface exchange is different from previous studies [1722],

in which it was assumed that the chemical potential of

oxygen in the gas phase was equal to the chemical potential

of oxygen in the solid at this interface. Tanner et al. [26]

applied a similar boundary condition in their two-dimen-

sional model of a porous composite electrode structure of a

solid oxide fuel cell.

At the location where the porous mixed-conducting

oxide layer comes into contact with the dense gas separation

layer, it is assumed that both oxygen chemical potentials in

the solid material are equal:

ls ls;dense 16Some oxygen diffuses all the way through the gas phase

in the pore into the dense gas separation layer interface.

Here oxygen can directly exchange with the oxygen ions in

the dense gas separation layer:

esg

1

1

DKn;O2 1 XO2

Dmol;O2;He

1

RgT

1

tpa

dPO2dx

ekPnO2

lg ls

RT

17where sg=tortuosity of the gas phase in the porous mixedconducting oxide [].

Similar boundary conditions are applied on the airside

and on the oxygen product side. The chemical potential of

oxygen in the dense gas separation layer on the oxygen

product side is equal to the chemical potential inside the

dense phase of the mixed-conducting oxide layer on the

oxygen product side:

ls ls;dense 18

Some oxide ions from the dense gas separation layer will

exchange with oxygen gas molecules, and these oxygen

molecules will diffuse and flow out of the porous mixed-

conducting oxide layer on the product side of the

membrane:

ekPnO2lg ls

RT e

sg

DKn;O2tpp

1

RT

1

tpp

dPO2;g

dx

esg

1

tpp

r2p

8RTgPO2;g

1

tpp

dPO2;g

dx

B.A. van Hassel / Solid St25619On the way out of this layer, there is a continuous

exchange between oxide ions and oxygen molecules, with a

net transfer of oxide ions as oxygen molecules into the gas

phase of the pore:

1 ess

ramb42F2

1

tpp

dlsdx

1 e kPnO2

lg ls

RT20

This boundary condition that is based on a limited rate of

surface exchange is also different from previous studies

[1722], in which it was assumed that the chemical potential

of oxygen in the gas phase was equal to the chemical

potential of oxygen in the solid at this interface.

As it exits the pore, the oxygen pressure is equal at the

interface between the mixed-conducting oxide layer and the

inert support:

lg RT ln PO2;inter 21

The oxygen flux through the dense mixed-conducting

layer must match the sum of the oxygen transported through

the dense phase of the porous mixed-conducting layer and

the oxygen directly exchanged with the dense gas separation

layer:

1 ess

ramb42F2

1

tpa

dlsdx

ekPnO2lg ls

RT

rion42F2

ls;h ls;ltd

22

1 ess

ramb42F2

1

tpp

dlsdx

ekPnO2lg ls

RT

rion42F2

ls;h ls;ltd

23

A pressure drop across each inert porous support layers

must be accounted for:

ramb42F2

ls;h ls;ltd

esup1

sg;sup1

DKn;O2tsup1

1

RT

dPO2;g

dx

esup1sg;sup1

1

tsup1

r 2sup1

8RTgPO2;g

dPO2;g

dx

24

ramb42F2

ls;h ls;ltd

esup2

sg;sup2

DKn;O2tsup2

1

RT

dPO2;g

dx

esup2sg;sup2

1

tsup2

r 2sup2

8RTgPO2;g

dPO2;g

nics 174 (2004) 253260dx

25

ramb42F2

ls;h ls;ltd

esup3

sg;sup3

DKn;O2tsup3

1

RT

dPO2;g

dx

surface exchange enhancement layer. Those layers are

considered inactive in oxygen transfer, since they are

Fig. 3 shows the oxygen flux versus the layer

thickness of the porous mixed-conducting oxide on the

airside of the membrane for various pore radii. Little

improvement in oxygen flux is achieved by mixed-

conducting oxide layers with large pore radii. Significant

improvements in oxygen flux are achieved by mixed-

Table 1

Operating conditions and oxygen viscosity

Temperature 1073.15 K

Airside total pressure 106 Pa

Oxygen mole fraction at airside 0.209

Oxygen product pressure 105 Pa

Oxygen viscosity at 1073.15 K 56.837106 Pa s

Table 3

High-performance oxygen ion transport membrane architecture

Porous mixed-conducting oxide layer on airside

Thickness 0.075 Am1.26 mmPorosity 0.32

Tortuosity gas phase 2.2

Tortuosity dense phase 2.2

Pore radius 0.00520 Am

Dense mixed-conducting gas separation layer

Thickness 10 Am

Porous mixed-conducting oxide layer on oxygen product side

Thickness 8 AmPorosity 0.32

Tortuosity gas phase 2.2

Tortuosity dense phase 2.2

Pore radius 0.05 Am

Porosity 0.32

Tortuosity gas phase 2.2

Pore radius 3 AmLayer 3

Thickness 1 mm

Porosity 0.32

Tortuosity gas phase 2.2

Pore radius 15 Am

B.A. van Hassel / Solid State Ionics 174 (2004) 253260 257located too far away from the dense gas separation layer

to have any influence on oxygen flux.

The system of differential equations with boundary

conditions at two points was solved numerically by using

a variable order, variable step-size finite difference method

with deferred corrections, which was implemented in the

bBVPFDQ solver from International Mathematics andStatistics (IMSL). Tables 13 provide a list of model

parameters that were used to determine preferred membrane

architectures.

3. Results and discussion

The oxygen partial pressure profiles in Fig. 2 were

computed with the model parameters found in Tables 13.

These profiles show an oxygen chemical potential differ-

ence between the gas phase and the mixed-conducting oxide

phase of the porous layer that serves as the driving force for

oxygen transfer. The thickness of the oxygen transfer zone

extended over about 8 Am in this example.

Table 2 esup3sg;sup3

1

tsup3

r2sup3

8RTgPO2;g

dPO2;gdx

26

where ei=porosity of support layer i []; si=tortuosity ofsupport layer i []; ti=thickness of support layer i [m].

Eqs. (24) Eqs. (25) Eqs. (26) show that the model allows

for three inert porous support layers below the porousModel parameters of a simulated OTM material

Ambipolar conductivity

at 1273.15K

214/V/m

Activation energy for ionic

conductivity

48 kJ/mol

Surface exchange coefficient

at 1273.15K

0.26 mol O2/

(m2 s (Pa/105)n)

Activation energy for surface

exchange coefficient

134 kJ/mol

Power dependence on oxygen

partial pressure

n=0.5Inert porous support layers

Layer 1

Thickness 6 AmPorosity 0.32

Tortuosity gas phase 2.2

Pore radius 0.3 AmLayer 2

Thickness 60 AmFig. 2. Oxygen partial pressure profiles across the mixed-conducting oxide

parts of the oxygen ion transport membrane. The difference in oxygen

partial pressure between the gas phase and the solid phase provides a

driving force for oxygen surface exchange, in which oxygen molecules

exiting the gas phase are incorporated in oxygen vacancies of the mixed-

conducting oxide layers.

Fig. 3. Oxygen flux versus layer thickness of the porous mixed-conducting oxide on the airside of the membrane. All other model parameters were kept

constant. The pore radius in the porous layer is indicated in micrometers. The oxygen flux is indicated in standard (273.15 K; 101,325 Pa) cubic centimeters per

minute per square centimeter (cm3/cm2/min).

B.A. van Hassel / Solid State Ionics 174 (2004) 253260258conducting oxide layers with small pore radii and Fig. 3

shows that there is an optimum layer thickness. Beyond

that thickness, the oxygen flux declines due to a gas

phase diffusion limitation. This optimum thickness shows

a complex relationship with the pore radius. The smaller

the pore radius, the thinner the porous layer must be to

achieve optimum oxygen flux.

Fig. 4 compares the optimum thickness/pore radius ratio

to the pore radius on the airside of the mixed-conducting

oxide layer. Fig. 4 demonstrates that the thickness appa-

rently has a one over square root dependence on the pore

radius. Fig. 4 also shows that a 10-fold change in the surface

exchange rate constant only results in about a factor 3.2

change in the optimum thickness/pore radius ratio, with a

smaller ratio for the material with the faster surface

exchange rate constant. A sensitivity analysis shows that

the following expression holds for the optimum thickness/

pore radius ratio for given values of the pore radius,porosity, tortuosity, materials parameters (ion conductivity

Fig. 4. The solid symbols show the optimum ratio of porous mixed-conducting

corresponding oxygen flux. The oxygen flux is indicated in standard (273.15 K;

min). The linear relation between the logarithm of that ratio and the logarithm ofand surface exchange rate), and operating conditions

(temperature and oxygen partial pressure):

t

r

optimum

1F

R

p

22

p Tp e EsigmaEk 2R 1T 11273:15 !

r0ambk0PnO2

1 ee

1

s1

rp

s27

When the activation energy for the ionic conductivity is

lower than the activation energy for the surface exchange,

the optimum thickness/pore radius ratio will decrease with

an increase in temperature. The optimum thickness/pore

radius ratio will increase with a decrease in oxygen partial

pressure (assuming nN0), which may occur when asignificant fraction of the oxygen is recovered from the air

stream. A material with a smaller ion conductivity/surface

exchange rate ratio will need a smaller thickness/pore radiusratio to achieve optimum flux. If the surface exchange rate

oxide layer thickness and pore radius, and the open symbols show the

101,325 Pa) cubic centimeters per minute per square centimeter (cm3/cm2/

the pore radius is consistent with Eq. (27).

layer thickness is adjusted to its optimum value.

research group, especially Yukiko and Masayuki Dokiya,

for their hospitality and for letting me perform basic

this copyrighted paper.

ate IoReferences

[1] C. Wagner, Prog. Solid State Chem. 10 (1975) 3.

[2] H. Schmalzried, Solid State Reactions, Monographs in Modern

Chemistry, vol. 12, Hans F. Ebel (series editor), completely revised

2nd ed., Verlag Chemie, Weinheim, 1981.

[3] J. Mizusaki, S. Yamauchi, K. Fueki, A. Ishikawa, Nonstoichiometryresearch while supported by a fellowship from the Science

and Technology Agency (STA) of Japan. Dokiya-Sensei let

me combine theoretical work with practical experimentation

in order to verify the modeling results. I would like to

acknowledge the financial support for OTM development at

Praxair by the Department of Energy under contract no.

DE-FC26-99FT40437. I also would like to acknowledge the

entire Praxair team for their relentless support for OTM

development. This paper was written with support of the

U.S. Department of Energy under Contract No. DE-FC26-

99FT40437. The Government reserves for itself and others

acting on its behalf a royalty-free, nonexclusive, irrevoca-

ble, worldwide license for Governmental purposes to

publish, distribute, translate, duplicate, exhibit and perform4. Conclusions

An oxygen transport model of composite OTMs must

incorporate mass transfer across a boundary layer on the

airside of the membrane, surface exchange, ambipolar

diffusion through the mixed-conducting gas separation

layer, and viscous flow of oxygen through the porous

support. There is an optimum ratio for layer thickness, over

average pore radius for porous mixed-conducting surface

modification layers for OTMs, which depend on the pore

radius and various materials properties, such as surface

exchange rate and ambipolar conductivity. This relation-

ship can be used to engineer membranes with a superior

oxygen flux.

Acknowledgement

I would like to thank the energy-related materialswould be infinite, then no porous mixed-conducting oxide

layers would be required.

Fig. 4 shows both the optimum thickness/pore radius

ratios for each pore radius value, and the oxygen flux for

each value. This shows that the highest oxygen flux is

achieved at the smallest pore radius, but only if the porous

B.A. van Hassel / Solid Stof the perovskite-type oxide La1xSrxCrO3d, Solid State Ionics 12(1984) 119124.[4] G.F. Carini, H.U. Anderson, D.M. Sparlin, M.M. Nasrallah, Electrical

conductivity, Seebeck coefficient and defect chemistry of Ca-doped

YCrO3, Solid State Ionics 49 (1991) 233.

[5] I. Yasuda, T. Hikita, in: F. Grosz, P. Zegers, S.C. Singhal and, O.

Yamamoto (Eds.), Proceedings of the Second International Symposium

on Solid Oxide Fuel Cells (Commission of the European Community

Report EUR13546 EN), Athens, Greece, July 25, 1991, pp. 645652.

[6] B.A. van Hassel, T. Kawada, N. Sakai, H. Yokokawa, M. Dokiya,

Oxygen permeation modelling of La1yCayCrO3d, Solid State Ionics66 (1993) 4147.

[7] B.A. van Hassel, T. Kawada, N. Sakai, H. Yokokawa, M. Dokiya,

H.J.M. Bouwmeester, Oxygen permeation modelling of perovskites,

Solid State Ionics 66 (1993) 295305.

[8] N. Sakai, T. Horita, H. Yokokawa, M. Dokiya, Oxygen permeation

measurement of La1xCaxCrO3d by using an electrochemicalmethod, Solid State Ionics 8688 (1996) 12731278.

[9] Y. Teraoka, T. Fukuda, N. Miura, N. Yamazoe, Development of

oxygen semipermeable membrane using mixed conductive perovskite-

type oxides (Part 1), J. Ceram. Soc. Jpn., Int. Ed. 97 (1989) 459462.

[10] Y. Teraoka, T. Fukuda, N. Miura, N. Yamazoe, Development of

oxygen semipermeable membrane using mixed conductive perovskite-

type oxides (Part 2), J. Ceram. Soc. Jpn., Int. Ed. 97 (1989) 523529.

[11] H.J.M. Bouwmeester, A.J. Burggraaf, Dense ceramic membranes for

oxygen separation, in: P.J. Gellings, H.J.M. Bouwmeester (Eds.), The

CRC Handbook of Solid State Electrochemistry, CRC Press, Boca

Raton, FL, 1997, pp. 481553.

[12] A.V. Virkar, C.F. Gottzmann, R. Prasad, B.A. van Hassel, Method of

separating oxygen with the use of composite ceramic membranes,

U.S. Patent 6,368,383, issued April 9, 2002.

[13] R. Prasad, J. Chen, B.A. van Hassel, J. Sirman, J. White, Advances in

oxygen transport membrane technology for integrated oxygen

production in IGCC, Paper presented at the Eighteenth Annual

International Pittsburgh Coal Conference, Newcastle Town Hall,

NSW, Australia, December 37, 2001.

[14] R. Prasad, J. Chen, B.A. van Hassel, J. Sirman, J. White, P. Apte, T.

Aaron, E. Shreiber, Advances in OTM technology for IGCC, Paper

presented at the Nineteenth Annual International Pittsburgh Coal

Conference, Pittsburgh, PA, USA, September 2327, 2002.

[15] R. Prasad, J. Chen, B.A. van Hassel, J. Sirman, J. White, E. Shreiber,

J. Corpus, J. Harnanto, OTMan advanced oxygen technology for

IGCC, Paper presented at the Gasification Technologies 2002

Conference, San Francisco, CA, USA, October 2830, 2002.

[16] R. Prasad, J. Chen, H. Chen, J. Lane, J. White, J. Corpus, E. Shreiber,

J. Spero, B.A. van Hassel, Oxygen transport membranes for future

IGCC power plants: Session 19. Gasificationadvanced technolo-

gies1, Proceedings of the Twentieth Annual International Pittsburgh

Coal Conference, Pittsburgh, PA, USA, September 1519, 2003.

[17] T. Kenjo, S. Osawa, K. Fujikawa, High temperature air cathodes

containing ion conductive oxides, J. Electrochem. Soc. 138 (2) (1991)

349356.

[18] I.V. Murygin, Steady polarization of distributed gas electrodes in cells

with solid electrolyte. The method of effective coefficients, Elek-

trokhimiya 23 (6) (1987) 740747.

[19] G. Maggio, I. Ielo, V. Antonucci, N. Giordano, Morphological

optimization of a SOFC anode based on theoretical considerations: a

preliminary approach, in: F. Grosz, P. Zegers, S.C. Signhal, O.

Yamamoto (Eds.), Proceedings of the Second International Symposium

on Solid Oxide Fuel Cells, Commission of the European Community

Report EUR13546 EN, Athens, Greece, July 25, 1991, pp. 611619.

[20] R.M. Thorogood, R. Srinivasan, T.F. Yee, and M.P. Drake, Composite

mixed conductor membranes for producing oxygen, U.S. Patent US

5,240,480, issued August 31, 1993.

[21] H. Deng, M. Zhou, B. Abeles, Diffusion-reaction in mixed ionic

electronic solid oxide membranes with porous electrodes, Solid State

Ionics 74 (1994) 7585.

nics 174 (2004) 253260 259[22] H. Deng, M. Zhou, B. Abeles, Transport in solid oxide porous elec-

trodes: effect of gas diffusion, Solid State Ionics 80 (1995) 213222.

[23] D.K. Edwards, V.E. Denny, A.F. Mills, Transfer Processes, An

Introduction to Diffusion, Convection, and Radiation, 2nd ed.,

McGraw-Hill, New York, 1979.

[24] E.A. Mason, A.P. Malinauskas, Gas transport in porous media: the

dusty-gas model, in: Chem. Eng. Monogr., vol. 17, Elsevier,

Amsterdam, 1983.

[25] R.C. Reid, J.M. Prausnitz, B.E. Poling, The Properties of Gases and

Liquids, 4th ed., McGraw-Hill, New York, 1987.

[26] Cameron W. Tanner, Kuan-Zong Fung, Anil V. Virkar, The effect of

porous composite electrode structure on solid oxide fuel cell

performance, J. Electrochem. Soc. 144 (1) (1997) 2131.

B.A. van Hassel / Solid State Ionics 174 (2004) 253260260

Oxygen transfer across composite oxygen transport membranesIntroductionTheoryModel outlineOxygen transport

Results and discussionConclusionsAcknowledgementReferences