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Hydrogen transport in 2.5 m Pd-Agmembranes: experimental and

modeling results

Marco Giacinti Baschetti, Giulio Cesare Sarti

Dipartimento di Ingegneria Chimica, Mineraria e delleTecnologie Ambientali

Alma Mater Studiorum - Università di Bologna

Motivation

† B.C.R. Ewan and R.W.K. Allen, International Journal of Hydrogen Energy, 30 (2005), Pages 809-819.

Conventional Hydrocarbon steam reforming

Membrane Units• Same theoretical efficiency as PSA• more compact and less energy demanding

Alma Mater Studiorun – Università di Bologna

AIM• Study membrane behavior with different feed compositions• Study membrane units• Model and design membrane units

Experimental and Materials

Pd0.8-Ag0.2 membrane. Selective layer ~2.5 m

An extensive study has beenconducted on Pd-Ag membranesproduced by NGK Inc. (Japan)

Membrane featuresDeposition Technique Electroless Deposition

Membrane length 90 mm

Membrane O.D. 10 mm

Support Asymmetric -Al2O3

Layer thickness 2.5 m

Pd-Ag composition 80%w Pd - 20%w Ag


Experimental and Materials

An experimental module has been set up, able to measure permeation of purehydrogen as well as hydrogen mixtures in Pd-Ag membranes at temperatures up to700°C and pressure up to 15 bar.

Operative conditions in the test

Gases H2, N2, CH4, CO,H2O

Temperaturerange 200 - 500 °C

Max pressure 7 bar

Feed flow-rate 1-3 Nl/min


Experimental Results: Pure Hydrogen

723 K

673 K

773 K

The Hydrogen flux:

closely follows the Sieverts’ law;Is not affected by feed-flow rate

Membranes are defect-free

2 2

2 2

u dH H

H H

p p

N

I II

III

I. Dissociativeadsorption

II. Surface-to-Bulktransition

III. Atomic diffusion

Step I and II at equilibrium Sieverts’ Law


Feed flowrate

Feed flowrate

88% H2

50% H2

The Hydrogen flux:

Is lower than for pure H2 at

same driving force;

Is affected by feed-flow rate:Increases by increasing

feed flow-rates

Depends on H2 feed content:Increases with H2 content

in the feed.

Concentration Polarization isimportant in the membrane

module

Experimental Results: H2-N2 Mixtures


Similar behavior in comparisonwith H2-N2 mixtures:

The difference between pureHydrogen data and mixtures one is

not in the chemistry (i.e.competitive adsorption) but in the

transport.

Methane and Nitrogen have to beconsidered as inert gases, andconcentration polarization isconfirmed in the membrane

module

1 NL min-1

2 NL min-1

The mixtures have:

Experimental Results: H2-N2-CH4


2 2 2 2H H H Hint retp p N K

boundarylayer

2

s h e l lHP

2

,s h e l l IHP

2

l u m e nHP

feed permeate

Permeate flux

Bulk flow

nH

Gas phase resistanceHp. high mass transfer rate

Membrane resistance

2

2

2

2 2

2

H ,H ,

H ,

H , H ,

H

1ln

1int

int Gret

ret intG

ML

xk

x

p pk

p p

N

2

shellNP

2

,shell INP

The hydrogen depletion (and theaccumulation of non permeating gases) inthe boundary layer is modeled using the

two resistance in series.

Metal membrane

Modeling inert mixtures behavior


Modeling inert mixtures behavior

Regression results: lines are obtained through model calculations


300°C 250°C

400°C450°C

Water interacts with themetallic membrane and

actively hinders hydrogenpermeation through Pd-

based membranes.

Experimental Results: H2-H2O Mixtures

The inhibition effect iscompletely reversible and

the membrane recoverthe initial permeability

value as soon as the watervapor is removed from

the feed.


H2-CO experiments showed a flux depletion also at 2% of CO. The membrane experienced anon reversible poisoning upon CO exposure; the original permeance of pure H2 could be

restored only by applying a cleaning process

Experimental Results: H2-CO Mixtures


The effects on the hydrogenflux depletion in ternarymixtures experimentscontaining two competitivegases seems to be nonadditive.

Experimental Results: H2-H2O-CO

No substantial changes inhydrogen permeation flux areobserved by substitutingcarbon monoxide to nitrogenin a H2-H2O-N2 mixture.


Kinetic stage Flux equation

(I) Surfaceadsorption/desorption

processes(upstream surface)

(II) Surface to bulktransition

(upstream side)

( III) Solid-state atomicdiffusion

(IV) Bulk to surface(downstream side)

(V) Surfacedesorption/adsorption

processes(downstream surface)

† T. L. Ward and T. Dao, Journal of Membrane Science, 153 (1999), Pages 211-231

H2 transport in Pd membranes can be described by the well known approach proposed by wardand Dao† , that is based on a 5 step process:

2 2

2

0.522

H H , 0 H,1H

2 2,0 H,1

1 12

2 exp

i S

Dd S

p S nkTm

Ek nRT

N

2H 0 H,1 ,1

,1 H,1 0

1 exp 12

1 1 exp2

S BS b s

B SS b s

En n xRT

En n xRT

N

2H 0 ,1 ,2

1 exp /2

diffb s s

En x x

RT

N D

2H ,2 H,2 0

0 H,2 ,2

1 1 exp2

1 exp 12

B SS b s

S BS b s

En n xRT

En n xRT

N

2

2

2

2 2H ,0 H,2

0.522

H ,2 0 H,2H

2exp

1 12

Dd S

S

Ek nRT

p S nkTm

N

Modeling permeability: pure hydrogen


Effects of CO on H2 permeability

Carbon monoxide is known to adsorb on Pd through a rather complex process thatinvolves different type of configurations characterized by different adsorption energies †.

† O. Dulaurent et Al., Journal of Catalysis, Volume 188 (1999), Pages 237-251

Bridge bonded

LinearECO=92-54 kJ/mol

The adsoprtion energy of each species of adsorbedCO has changes with surface coverages.

Relative fraction of the different species adsorbedchanges with temperature and is dependent on thesurface history.

ECO=168-92 kJ/mol


A model has been evelopedconsidering competitiveadsorption of CO and H2.The adsorption energy ECOis linear with CO surfacecoverage and is the onlyfitting parameter.

The model is able todescribe a broad variety ofbehaviors

data from J. Chabot, J. Lecomte, C. Grumet and J. Sannier, Fusion Technology,14 (1988), Pages 614–618

CO COE a b

a = -30 kJ mol-1

b = 140 kJ mol-1


Effects of CO on H2 permeability

data from H. Nguyen, S. Mori, M. Suzuki, Chemical Engineering Journal, 155 (2009), Pages 55-61

Comparison with literature data

When a linear decrease ofECO with CO surface

coverage is consideredexperimental data fromdifferent sources can bedescribed very well usingthis quantity as fittingparameter.At least until the value of

CO does not become toohigh.

CO COE a b

a = -30 kJ mol-1

b = 120 kJ mol-1


In order to model thepermeation of hydrogen inpresence of water vapor bothconcentration polarization andcompetitive adsorption needto be considered.

A local approach has beenconsidered by dividing themembrane in an arbitrarynumber of small segments.

x

Model steps

Transport in the gas phase

Surface adsorption/desorption processes(upstream surface)

Surface to bulk transition(upstream side)

Solid-state atomic diffusion

Bulk to surface(downstream side)

Surface desorption/adsorption processes(downstream surface)

Application to current exp. data


Experimental data ofhydrogen flux in presence ofwater vapor are welldescribed in the rangebetween 450-350°C with asingle fitting parameters thatis:

EH2O = 125 kJ/mol

*J. Catalano et.al., Int J. Hydrogen Energy, 36 (2011) pages 8658-8673

Application to current exp. data


Permeation tests of pure hydrogen and several mixtures were performed on aPd-Ag membranes endowed with a high permeable dense and defect free Pd-Aglayer

Experimental work on several binary mixtures showed decrease in hydrogenflux with respect to pure hydrogen experiments. However the data suggest that:

• Nitrogen can be considered as an inert and H2 flux depletion is due toconcentration polarization phenomena .• Steam and carbon monoxide seems to hinder hydrogen permeation and

possible competitive adsorption on the metal surface has to be considered.A simple model for to account for concentration polarization allowed to

describe the hydrogen flow in the presence of N2

A physically consistent approach was developed to account for the influence onhydrogen flux of non-inert gases.

• Coupling this model allowed to describe also the effect of water vapor onthe hydrogen permeate flux in the membrane

Final Conclusion


Acknowledegements

Dr. K. I. Noda and NGK Insulator Ltd (Japan) for the membranes

Italian Ministry of Research (MIUR) for funding

David Alique, Ryan Worth, Franscesco Fusi, Marco Castellani, Matthew Purcell forthe help in the experimental work


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