from diffusion research to industrial process

8/3/2019 From Diffusion Research To Industrial Process

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From Diffusion Research to

Industrial ProcessesDechema, Frankfurt,Oct26th2006

Douglas M.Ruthven, University of Maine,

Orono, ME04469, U.S.A.


2/24

From Diffusion Research to

Industrial ProcessesKinetic vs Equilibrium Separations:

Most adsorption Separations Selectivitydepends on equilibrium differences.

A few important separations depend on

differences in kinetics.

Examples: Linear/Branched paraffins

Air Separation for N2Olefin/Paraffin separation

N2/CH4Separation (Natural Gas)


3/24

Olefin/Paraffin SeparationsHigh demand for light olefins (for

polyethylene/polypropylene production).Recovery of olefins from cat-cracker off-gas is

preferred route.Requires C2H4/C2H6 and C3H6/C3H8 separation.

C3H6separation is especially important.


4/24

Processes for Olefin/Paraffin

SeparationCryogenic Distillation: Relative volatility is

small so process is energy intensive.Extractive Distillation.

Adsorption offers promising alternative.

Cationic zeolites show equilibrium selectivity forolefins (~12 on 5A).

Olex process (UOP) uses simulated counter-current flow to achieve a pure product withlimited selectivity.


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Traditional 8-Ring Zeolites for

Olefin/Paraffin Separation (5A)Equilibrium and Kinetic Data at 323K

K Kratio D(cm2s-1) DratioC2H4 5100 ~10

-6

} 15 } 1.0

C2H6 340 ~10-6

C3H6 8.3x104

1.4x10-8

} 12 } 2

C3H8 6800 7x10-9


6/24

Traditional 8-Ring Zeolites for

Olefin/Paraffin Separation (4A)Equilibrium and Kinetic Data at 323K

K Kratio D DratioC2H4 4600 1.5x10

-11

} 15 } 3

C2H6 300 5.5x10-12

C3H6/C3H8 Kinetics too slow on 4A


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Olefin/Paraffin Separation by Adsorption

Olefins are preferentially adsorbed (stronger

equilibrium and faster kinetics).Recovery of preferentially adsorbed component

at high purity is difficult requires very high

equilibrium selectivity or diffusivity ratio.Traditional adsorbents (4A, 5A or 13X) do not

give required product purity.Look for adsorbents with high enough kineticselectivity to give molecular sieve separation.


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Structure of DDR38-ring silica framework: 2-dimensional channels

(4.4x3.6A)


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Chabazite Structure (CHA)

Cages (free volume ~3803) interconnected through tetrahedrallyoriented 8-ring windows free aperture 3.7 4.1

SiCHA, SAPO-34: cation free versions


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CHA VariantsBond Lengths (Angstroms)

AlO =1.75;

(1/2)(AlO + P-O) =1.64Si-O= 1.61

Reduction in unit cell volume and windowdimensions with Si/Al Ratio:

CHA > SAPO34 > AlPO34 > SiCHA


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Modified CHA Adsorbents8-Ring Zeolites (Angstroms)

4A 3.8x4.25A 4.2x4.2

CHA 3.9x4.1

SAPO34 3.8x4.3

AlPO34 3.7x4.5

SiCHA 3.65x4.3

DDR3 3.6x4.4 (not CHA)


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Olefin/Paraffin SeparationDiffusion in Type A and CHA Zeolites

Diffusion in CHA Zeolites

1.00E-15

1.00E-14

1.00E-13

1.00E-12

1.00E-11

1.00E-10

1.00E-09

1.00E-08

2.3 2.5 2.7 2.9 3.1 3.31000/T

Do(cm

2/sec)

C3H6 - SAPO 34

C3H6 - ALPO 34

C3H6 - SiCHA

C3H8 - ALPO 34

C3H8 - Si CHA

D and E are sensitive to subtle differences in T Odistance

Diffusion in Zeolite A

1.00E-12

1.00E-11

1.00E-10

1.00E-09

1.00E-08

1.00E-07

1.00E-06

2.6 2.7 2.8 2.9 3 3.1 3.2 3.3 3.4

1000/T

Do

(cm

2/sec)

C2H6 -5A

C3H8 - 5A

C2H4 - 4A

C2H6 - 4A


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C o rre la t io n o f D if fus iv ity with Windo w D im e ns io n

1 . 0 0 E - 1 2

1 . 0 0 E - 1 1

1 . 0 0 E - 1 0

1 . 0 0 E - 0 9

1 . 0 0 E - 0 8

1 . 0 0 E - 0 7

3 . 5 3 . 6 3 . 7 3 . 8 3 . 9 4 4 . 1 4 . 2 4 . 3

Mi ni m um D ia m e t e r ( A ng s t r o m )

C 3 H6 a t 3 2 3 K


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Olefin/Paraffin SeparationVariation of D and Kinetic Selectivity with Unit Cell Size

From Reyes et al. U.S. Patent 6,730,142 B2 May 4, 2004

Diffusion of Propane and Propylene in CHA Zeolites

1.00E-11

1.00E-10

1.00E-09

1.00E-08

1.00E-07

1.00E-06

770 780 790 800 810 820 830

Unit Cell Volume (A3)

Diffusivity(cm

2/sec)an

d

DiffusivityRatio

DC3H6/DC3H8

DC3H6

T = 323K

10,000

1,000

100

1.00E-11

1.00E-10

1.00E-09


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Comparison of Activation Energies

0

2

4

6

8

10

12

14

16

18

CaA SAPO34 ALPO34 SiCHA

Zeolite Type

.

E

(kc

al/mole)

Propane

Propene


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Preferred Adsorbents for C3H6 (323K)

K Kratio D(cm2s-1) DratioSiCHA:

C3H6 700 8x10-11

} 0.8 } 11,500

C3H8 900 7x10-15

DD3RC3H6 1000 5X10

-12

} 1 ?? } 10,000

C3H8 No data 6x10-16


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Olefin/Paraffin SeparationComparative Uptake Rates for C3H6 and C3H8

in SiCHA at 80oC

From Olson et al. Microporous and Mesoporous Mats. 67, 27-33(2004)


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N2/CH4 SeparationPipeline specifications for natural gas limit N2

content to < 3%.Many gas reservoirs contain higher N2 %.

On non-polar adsorbents CH4

is adsorbedmore strongly.

On polar adsorbents N2 and CH4 are adsorbed

at similar rates and with similar equilibria.For an efficient separation adsorbent should

adsorb N2 preferentially


19/24

Titanosilicates ETS-4A Tuneable Adsorbent

Dimensions of unit cell

(and 8-ring windows)depend on dehydration

temperature

From Kuznicki et al.Nature, 412, 720 (2001)


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ETS-4 (270o

C dehydration)

Sr-ETS-4; High kinetic selectivity N2/CH4 (Farooq)


21/24

N2 CH4 SeparationLittle difference in either kinetics or equilibrium

on most adsorbents.SrETS4 offers good kinetic selectivity with

N2 (minor component) as the preferredspecies.

PSA Process with periodic thermalregeneration to remove higher hydrocarbons.


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ConclusionsOlefin/Paraffin Separation

Small differences in T O distances lead to

changes in free aperture of 8-rings. This has a large impact on the diffusional activation

energy (and hence on D) for critically sized

molecules. SiCHA andDDR3 have high kinetic selectivity for

C3H6/C3H8 (Dratio> 104).

Under properly selected operating conditions (PSAor TSA) propylene can be recovered at high purityand high yield.


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Conclusions (contd.)Purification of N2containing Natural Gas

CH4 and N2 are adsorbed at similar rates andsimilar strength on most polar adsorbents.

SrETS-4 dehydrated at 270DegC shows high kinetic

selectivity for N2/CH4, so N2 is preferentiallyadsorbed, yielding a pure CH4 product.

Traces of higher hydrocarbons are slowly adsorbed

necessitating periodic regeneration at elevatedtemperature.

Further Details


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Further Details

D.M.Ruthven and S.C.Reyes

Adsorptive Separation of Light Olefins from

Paraffins Microporous and Mesoporous Materialsin press.

D.H.Olson

U.S.Patent 6,488,741 (Dec3, 2002)D.H.Olson et al.

Micro and Mesoporous Mats. 67,27 (2004)

S.C.Reyes et al. Ibid. in press (2006)S.C.Reyes et al.

U.S.Patent 6,730,142 (May 4, 2004)

from diffusion research to industrial process

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