Tracer ZLC as an Informative Low-cost Technique of Diffusion Measurement

Professor Stefano BrandaniCentre for CO2 Technology Department of Chemical EngineeringUniversity College LondonTorrington Place, London WC1E 7JEs.brandani@ucl.ac.uk

Diffusion in Nanoporous Materials: from Fundamentals to Practical Issues – DECHEMA Kolloquium 15/01/04

Historical Development of Diffusion in Zeolites Measurements

Year1930 1940 1950 1960 1970 1980 1990 2000

Non-equilibriumEquilibrium

DirectVisualObservatTiselius(1934)

TransientUptakeBarrer (1938)

TracerExchangeBarrer(1941)

NMRRelaxationResing, Pfeifer, Michel(1967)

PFG-NMRPfeifer, Kärger(1971)

IR and IR/FRGrenier,Meunier (1998)

Chromatography Haynes,Ruthven(1973)

ZLCEic, Ruthven(1988)

CoherentQENSJobic(1999)

Effectiveness Factor. Haag, Post(1981)

FRYasudaRees (1982)

TAPNijhuis,Baerns,Keipert(1997)

MembranePermeationHayhurst,Weernick(1983)

FTIRKarge(1991)

PEPvan Santen(2000)

IRMicroscopyKarge(1974)

DIF MicroscopyKärger,Schemmert(1999)

IncoherentQENSCohen deLara,Jobic(1983)

Tracer ZLCHufton,Brandani,Ruthven(1994)

Exchange NMRChmelka(1998)

Some recent measurements…

n-Hexane in Silicalite after 1989

1.E-14

1.E-13

1.E-12

1.E-11

1.E-10

1.E-09

1.E-08

1.7 1.9 2.1 2.3 2.5 2.7 2.9 3.1 3.3 3.5

1000/T (K)

D (m

2 /s)

MBRN-PRM TEOscMbal QENS SingCrysMemPiezo FR PFG-NMR ChromatGrav-Uptk (P.Voogd) ZLC Grav-Uptk (SU.Kulkarni) Grav-Uptk (J.Caro et al.)Grav-Uptk (M.Jama & Ruthven)

3 orders of magnitude!

The ZLC apparatus

measuringdeviceFIRC FIRC

purgecylinder

carriercylinder

gas chromatography oven

soap bubble

flowmetersorbate

bath

switchingvalve

ZLC Experimental Set-up

Gaseous Systems

swagelok T fitting

ZLC adsorbent bed

Source: Swagelok Catalogue(http://www.swagelok.com)

capillary to measuring device

The ZLC column

Packing: 0.5 – 2 mg

capillary to measuring

device

swagelok T fitting

The (Tracer) ZLC apparatus

Mass Spec GC Cryo

Soap bubble flow meter

What can be measured (kinetics)?

The transport diffusivity at zero loading

Eic M. and Ruthven D.M., Zeolites, 1988, 8, 40–45.

Liquid phase counter diffusionRuthven D.M. and Stapleton P., Chem. Engng Sci., 1993, 48, 89-98.

The tracer diffusivity – Tracer ZLC

Brandani S., Hufton J.R. and Ruthven D.M., Zeolites, 1995, 15, 624–

631.

The transport diffusivity in mixtures

Brandani S., Jama M. and Ruthven D.M. , Ind. & Eng. Chem. Res.,

2000, 39, 821-828.

What can be measured (equilibrium)?

Henry law constants

Brandani F., Brandani S., Coe C.G. and Ruthven D.M., 2002, Fundamentals of Adsorption 7, 21–28.

Single component isothermsBrandani F., Ruthven D.M. and Coe C., Ind. Eng. Chem. Res., 2003, 42, 1451-1461.

Multicomponent isotherms

Brandani F. and Ruthven D.M., Ind. Eng. Chem. Res., 2003, 42, 1462-1469.

Zero loading heat of adsorption

ZLC parameters - gases.

0K31

KVV

31

S

F ≈≈=γFor gases:

The parameter L controls the ZLC response

Equilibrium

5L

1L

>

<

DKVFRL

S

2

31=

Kinetics

Long time asymptote - gases.

AssumptionsLinear equilibrium

Isothermal

Negligible hold-up in fluid phase: γ < 0.1

Cell is perfectly mixed

( )01Lcot

RtD

1LLL2ln

ccln 112

212

1o

=−+βββ−⎥⎦

⎤⎢⎣

⎡−+β

≈⎟⎟⎠

⎞⎜⎜⎝

⎛

From the slope and intercept of the desorptionplot L and D/R2 can be obtained

Long time asymptote - gases.

C6H14-CaA (T= 150oC )

0.001

0.01

0.1

1

0 50 100 150 200 250 300 350

t [sec]

c/c 0

[ ][ ] [ ]

[ ][ ] [ ] 1

0

0

1

0

0

010.0,21.0min4.38,0.8min8.37,0.17

011.0,16.0min3.64,0.8min4.61,0.17

−

−

==⎭⎬⎫

====

==⎭⎬⎫

====

sSImlFTorrpmlFTorrp

sSImlFTorrpmlFTorrp

Long time asymptote - gases.

0

50

100

150

200

0.01 0.1 1Intercept

L

Intercept2L ≈

Long time asymptote - gases.

0

2

4

6

8

10

0 50 100 150 200L

π2

β2

What can go wrong?

The sorbate is too strongly adsorbed or is too fast diffusing so the regime L > 5 cannot be reached. Only equilibrium measurements.

The sorbate is too weakly adsorbed or is too slow: the desorption curve is almost the same as the system’s blank.

Limits in the assumptions

Isotherm non-linearity.

TheoryBrandani S. Chem. Engng Sci., 1998, 53, 2791-2798.

ExperimentBrandani S., Jama M. and Ruthven D.M., , Chem. EngngSci., 2000, 55, 1205-1212.

Vary gas concentration to verify linearity

Vary gas flow to confirm kinetic control

Run TZLC experiment (always linear)

Isotherm non-linearity.

Non-Isothermal ZLC

Detailed model

Non-LinearNon-Isothermal

Fluid Temperature = To

GS = 0; GF = 0

Non-Linear IsothermalBrandani (1998)

λ = 0

LinearNon-Isothermal

Brandani et al. (1998)

Linear IsothermalEic and Ruthven (1988)λ = 0

α = 0

α = 0

Key Grouping - Isothermal criterion

Brandani S., Cavalcante C.L., Guimaraes A.M. and Ruthven D.M., Adsorption, 1998, 4, 275-285.

so

o2

o haVKFq

T∆H

LeBiL ℜ

⎟⎟⎠

⎞⎜⎜⎝

⎛ℜ

=σδ

=α

1<α For crystals this is generally valid.

2NuforRVK3kFq

T∆H 2

soF

o2

o

=ℜ

⎟⎟⎠

⎞⎜⎜⎝

⎛ℜ

≈α

Surface resistances:fluid film + bed resistance

Change the carrier gas, i.e. He, Ar or N2

Example: Benzene - NaX50 µm crystals 250°C.

Surface resistances: coke deposition

10L

DR 2

Sat

>

≈τ

Surface resistances: Partial loading experiment

Surface resistances: Partial loading experiment

Diffusion control

Either diffusion or surface barrier

Correct time constant

ZLC Experiment - Flexible

Vary system flow rate

Vary charging time – partial loading

Obtain the diffusional time constant from a number of response curves

Tracer ZLC

ZLC measurements are carried out using a tracer, such as a C6D6 for C6H6.

Total concentration constant

ALWAYS LINEAR + ISOTHERMAL

DIRECTLY COMPARABLE TO MICROSCOPIC MEASUREMENTS

Requires a mass spectrometer

Systems reported in literature

ZLC measurements are carried out in several academic and industrial laboratories

More than 70 sorbate-sorbent systems have been studied using the ZLC and reported in the literature

More than 15 systems with commercial pellets, membrane or monolith fragments.

8 liquid sorbate-sorbent systems

10 sorbate-sorbent systems - TZLC

3 Multicomponent systems