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 [email protected]
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
