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Determination of Absolute Gas Adsorption Isotherms by Combined Calorimetric and Dielectric Measurements

J. U. Keller, W. Zimmermann, R. Staudt*Inst. Fluid- and Thermodynamics, University of Siegen, D-57068 Siegene-mail: keller@ift.maschinenbau.uni-siegen.de

*Inst. Nichtklassische Chemie (INC), Universität Leipzig, D-04318 Leipzig

1. Gas Adsorption Systems, Basic Concepts

2. Masses of Adsorbed Phases (Adsorbates)Definition, Measurements Methods (Volumetry, Gravimetry)

3. Calorimetric Measurements of Pure Gas AdsorptionEquilibria

4. Dielectric Measurements of Pure Gas Adsorption Equilibria

5. Calculation of the Absolute Mass of the Adsorbate

6. Example

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Adsorptive 1

Adsorptive 2Adsorption(exothermic) Desorption

(endothermic)

Adsorbent

Adsorbate

Gas Adsorption Systems, Basic Concepts

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x

f

d

s a f

ms m*, V*pT

Sorbent material(s)

Absolute mass adsorbed ma

Mass of sorptive gas mf

Gibbs excess mass mGEa

Molecule adsorbedMolecule in gas phase

Vf

Vs

Va

V*=V +V +Vf=V +Vfas as

Plane Surface Sorption System: Absolute/Excess Mass

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Zeolite Molecular Sieve UOP/Linde MS13XCapillaries/Macropores, Mesopores,Micropores

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Experimental Setup for Volumetric- ChromatographicMeasurements of Gas Adsorption Equilibria

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Installation for Densimetric-Gravimetric Measurements (DGMs) of Gas Adsorption Equilibria * *

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Measurement Methods of Masses of Adsorbed Phases

1.Volumetry / Manometry 2.Gravimetry

Result of measurements: Gas Adsorption System

Approximations:

1.Gibbs excess mass 2.Absolute mass adsorbed

a f asm V * f asV V V

,as sHeV V 0aV

a f sGE Hem V

0( / )as s a LHeV V m

01 ( / )

aa aGE

GEf L

mm m

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~~I(t)U(t)

m*T*p*

ms

ma

VC

D Ep

T*=constQSV

.

Thermopiles Ti(t) QAV

.

mf

SV AV

Impedance Calorimeter*Experimental setup for calorimetric - dielectric measurements of gasadsorption equilibria.

*)P DE 100 191 22 A1

StorageVessel

AdsorptionVessel

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Schematic diagram of a sensor gas calorimeter (SGC)

Sensor gas calorimeter (SGC) forsimultaneous measurements of adsorption isotherms and enthalpies. © IFT, University of Siegen, 2003.

Air Thermostat

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Calibration experiments in the SGC 0.5J to 5JSensor gas N2 (1.6bar), T=298K, =10s

Ohm’s heat release (red lines) Pressure signal (blue lines)

0

20

40

60

80

100

120

0 100 200 300 400 500 600 700 800Time [s]

Diff

eren

ce P

ress

ure

[Pa]

0

50

100

150

200

250

300

350

400

450

500

Ele

ctri

cal

Pow

er [

mW

]

Calibration SGC

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Differential and integral heat of adsorption for activated carbon AC BAX 1500 / n-butane (C4H10) at 298K.

0 1 2 3 4 5 60

10

20

30

40

50

60

70

80

90

100

Heat of Condensation for n-butane (20,95 kJ/mol)

Mesured differential heat of adsorption Differenciated from integral heat of adsorption

Diff

eren

tial h

eat o

f ads

orpt

ion

[kJ/

mol

e]

n-butane ads. [mmole/g]

0

50

100

150

200

250

300

350

400

Measurend integral heat of adsorption Interpolated integral heat of adsorption

Inte

gral

hea

t of a

dsor

ptio

n [J

/g]

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a a s aDE DE (p,T,m ,m )

aa a aDE s

mp,T, mm

Calorimetric - Dielectric Measurements (1)1.Dielectric polarizability of the adsorbed phase (a)

Asm/(V/m) DEOS

2.Enthalpy of adsorbed phase (a)

( , , , )

( , , ).a

s

f a

a s

amm

H H HH H p T m m

H h p T m

J, kJ CEOS

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Calorimetric - Dielectric Measurements (2)

Inversion of (DEOS)

aa

s

mp p ,T,m

Insert in CEOS

a aa a

s s

m mH h p ,T, ,T, mm m

/a a a

DE m

Extensivity relation

Dielectric - caloric equation of state (DCEOS)

a aDE

a a s

H mh ,T,m m m

Experimental data: , , ,a s

DEH T m Calculated quantity: am

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13

38

95

116 118

900

660 648

333

137

000E -4

020E -4

040E -4

060E -4

080E -4

100E -4

120E -4

0 1000 2000 3000 4000 50000

200

400

600

800

1000

1200

p/[hPa]

100 499 1009 1961 4003

s

s

GE GE

2

112 2

mWessalite, DAY zeoliteCO , T 298K

GE g/mg

Reduced dielectric polarizability () and Gibbs excess specificpolarizability (=/mGE) of CO2 on DAY-zeolite at T = 298K.

^

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fa

fa

GEGE

2

sorbens

sorbate

H J/ g

Hh J/mgm

Wessalite, DAY zeoliteCO , T 298K

-2,6

-8,25

-12,8

-5,3-0,584

-0,43-0,345

-0,809

-1,83

-20

-15

-10

-5

00 500 1000 1500 2000 2500 3000 3500 4000 4500

p/[hPa]

Hfa/[J/g]

-2

-1,5

-1

-0,5

0

hGE/[J/mg]

100

499 1009 1961 4003

Sorbate!Sorbens!

Enhalpy (Hfa) and Gibbs excess specific enthalpy (hGE=Hfa/mGE)of CO2 adsorbed on DAY-zeolite at T=298K.

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-0,809

-0,584-0,430

-1,830

-0,345

-3

-2,5

-2

-1,5

-1

-0,5

00 200 400 600 800 1000

137 33 455 660 900

Henryregion

h h( )

GE g/mg

Liquidstate

GEh J/mg

Gibbs excess specific enthalpy (hGE=Hfa/mGE) - reduced Gibbs excess specificpolarizability (GE=/mGE) - diagram of CO2 on DAY-zeolite at T=298K.Curve: h=h(), =P/ma, h=Hfa/ma.^

^^

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14.12

29.74

1.41 6.60

86.45

14.66

34.83

1.445.76

20.87

89.67

1.44

5.77

45.54

0

10

20

30

40

50

60

70

80

90

100

0 1000 2000 3000 4000 5000

mg/

g

p [hPa]

100 499 1009 1961 4003

a

GEV

GEG

m ... Dielectric Caloric Meas.(RS, ESch, JUK)

m ... Volumetry, He Vol. (ESch)

m ... Gravimetry, He Vol. (RS)

Absolute and Excess Masses of CO2 Adsorbates onWessalite (DAY -Zeolite) at T=298K

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Conclusions (1)

0. Gas adsorption phenomena on porous solids (zeolites,activatedcarbons) provide the basis for a still growing number of separation /purification processes for air and other technical gases.

1. Traditional measurement methods for the mass of gas adsorbed on

a porous solid like volumetry and gravimetry need auxiliary model

assumptions on the sorbent/sorbate system and hence deliver only

approximate values of that mass.

Deviations between apparent and true values of masses are expected

to increase with increasing preasure and decreasing temperature.

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Conclusions (2)2. Combined calorimetric-dielectric measurements of gas adsorption

systems allow on principle to calculate the mass adsorbed from the

dielectric - caloric equation of state (DCEOS) of the adsorbate.

3. Measurements are presently laborious and cumbersome.

Hence development of a (fully automated) instrument,probably based

on a gas sensor calorimeter (SGC) would be desirable and useful.

References:(1) Keller,J.U.,Staudt R.

Gas Adsorption Equilibria, Experimental Methods and Adsorption Isotherms, Springer, New York, 2004.

(2) Keller J.U.,Zimmermann W., Schein E.Determination of absolute gas adsorption isotherms by combinedcalorimetric and dielectric measurements, Adsorption 9(2003),p.177-188.