south africa fischer-tropsch synthesis - uni · pdf filecentre for catalysis research...

Centre for Catalysis Research

Department of Chemical EngineeringUniversity of Cape Town • Rondebosch 7701

South Africa

Fischer-Tropsch synthesis

Reaction pathwayReactors

Michael Claeys Eric van Steen

1. Mechanistic ideas2. Fischer-Tropsch synthesis – a

polymerization reaction3. Re-adsorption of reactive product

compoundsa. reaction kinetic modelb. diffusion model

4. Reactors and processes


Fischer-Tropsch synthesis

F.Fischer, H. Tropsch, Brennstoff-Chem. 7 (1926), 97

Catalyst:Fe, Co (Ru, Ni)

Synthesis gas

C1 C2 C3 C4 C5

C9

C7 C8C6

C10 C11 C12 C13 C14 C15 C16 C17 C18


Products of Fischer-Tropsch synthesis

n-Olefins - mainly α-olefins

- olefins with internal double bonds

n-Paraffins

Oxygenates- alcohols, aldehydes

- ketones

branched compounds- olefins/paraffins/oxygenates

1-pe

nten

e

tran

s 2-

pent

ene

cis

2-pe

nten

e

n-pe

ntan

eet

hano

l2-

met

hyl b

utan

e


Product formation in Fischer-Tropsch synthesis

60

70

80

90

0 1 2 3 4

Space time, g.s/ml

Ole

fin c

onte

nt in

C3,

mol

-%

cobalt (220oC)

iron (240oC)

80

85

90

95

100

0 1 2 3 4

Space time, g.s/ml

1-O

lefin

con

tent

in

n-C

5-ol

efin

s, m

ol-%

cobalt (220oC)iron (240oC)

0

10

20

30

40

50

0 0.5 1 1.5

Space time, g.s/ml

Oxy

gena

te c

onte

nt in

C

2-pr

oduc

ts, m

ol-%

promoted iron (240oC)

Primary formation of olefins and paraffins Most olefins are primarily α-olefins

0

5

10

15

20

0 0.5 1 1.5

Space time, g.s/ml

Ald

ehyd

e co

nten

t in

C2-

oxyg

enat

es, m

ol-%

promoted iron (240oC)

Primary formation of alcohols/aldehydes Most oxygenates are primarily alcohols


Mechanistic ideas:“Modern surface carbide theory”

Initiation:

CO + 2 H- H2O

C + H CH + H CH2+ H CH3

Chain initiatorMonomerPropagation:

R CH2 CH2

R

Termination/desorption:

CH2

CH2R

+ H

- H

CH3 CH2R n-paraffin

CH2 CHR α-olefin

+ OH CH2OH CH2R n-alcohol


Mechanistic ideas:“Alkylidene mechanism

Initiation:

CO + 2 H- H2O

C + H CH + H CH2

Chain initiator

Monomer

Propagation:

CH2 CH

CH2

CH

CH2 CH2

CH

CH

CHRCHR

CH

CH

CH2R


+ H CH2 CHR α-olefinCH

CH

R


Mechanistic ideas:“CO-insertion mechanism”

Initiation:

+ 3 H

monomer

Propagation:


CH2

OH

CO + 2 H

+ 2 H

Chain initiator

CH3- H2O

C

R O

CH

R OH+ 2 H CH2

R

- H2O

CH2

CH2R

+ H

- H

CH3 CH2R n-paraffin

CH2 CHR α-olefin

CH

R OH

+ H

- H

RCH2OH n-alcoholRCHO aldehyde

CO

R


Mechanistic ideas:“Enol mechanism”

Initiation:

C + 2 H

Chain initiator& main monomer

Propagation:

O

C

H OH

C

R OH

C

H OH- H2O C

R

C

OH+ 2 H C

H2C OH

R

CH

R OH

C

H OH

- H2O+ 2 H C

H2C OH

R

+ H

Alternativemonomer

CH

H OH

C(CH3)

H OH

or


+ 2H RCH2CH2OH

RCH2CHOC

H2C OH

R

C

H2C OH

R

C

H OH

+ R=CH2


Fischer-Tropsch synthesis:polymerization reaction

Sp1

Pr1

d gSp2

Pr2

d gSp3

Pr3

d g. . .

gSpn

Prn

d gCOH2

1)(nggn p)p(1x −⋅−=ASF-kineticsChain growth probability

dg

gg rr

rp

+=

-6

-4

-2

0

2

0 10 20 30Carbon Number, n

lg(x

N)

0.5

0.6

0.7

0.8

0.9

1.0

0 10 20 30Carbon Number, N

pg


Constraint due to ASF-distribution

0

20

40

60

80

100

0.0 0.2 0.4 0.6 0.8 1.0

Chain growth probability, pg

wt.%

C1

C2-4

C5-9

C10-20

C21+

(after M. Dry, 1990)


Ideal chain growthdesorption as olefin and paraffin

Sp1

P1

d gSp2

Ethane

dP

Sp3 . . . Spn

gCOH2

dOl

Ethene

g dP gdOl g dP dOl

0

20

40

60

80

100

0 10 20 30Carbon Number, N

Ol/(

Ol+

P),

mol

%Desorption of alkyl surface species:

R - CH = CH2

R - CH2 - CH2

-H

+H R - CH2 - CH3


Real product distributiondesorption as olefin and paraffin

-6

-4

-2

0

2

0 10 20 30 40 50 60

Carbon Number, n

lg(x

N)

Gas Phase

Liquid Phase

0.5

0.6

0.7

0.8

0.9

1.0

0 5 10 15Carbon Number, n

p g(0.28)

0

20

40

60

80

100

0 5 10 15

Carbon Number, n

Ol i

n H

Cs,

mol

-%

primary selectivity

(Reaction conditions: CoZrRu-SiO2, T=190°C, pH2=5.1 bar, pCO=2.2 bar, pH2O=1.1 bar)

Deviations from ideal kinetics: “non ASF-distributions”- Relatively high methane selectivity- Anomaly in C2- Curvature in distribution at low n, chain length dependent pg- Chain length dependent olefin content


Secondary olefin reactions

R CH CH CH2

R CH CH CH3

+H

-H

+HR CH CH CH2 2 2

R CH CH CH2 3

R CH CH CH2 2 3

+H

+H

+CH2

+CH2

Growth

Growth

(increase of pg)

sterically hindered

2

(Schulz et al., 1977 and 1996)

Known reactions• Hydrogenation• Incorporation• Double bond shift• (Hydroformylation)• (Hydrogenolysis)

Extent and selectivity of secondary olefin reactions determine product distribution


Impact of secondary olefin reactions on chain growth

(Iglesia et al., 1991)

Bed residence time effects on carbon number distributions and α-olefin to n-paraffin ratio(reaction conditions: 1.2% Ru/TiO2, T=203°C, H2/COin=2.1, ptot=6 bar, 5-60% conversion,

Fixed bed reactor)


Incorporating secondary olefin reactions in Fischer-Tropsch model

Fischer-Tropsch products are both vapour and liquid products under reaction conditions

• Desorption as linear α-olefin Ol-(1) and non-reactive end product EP• Readsorption as species Sp or Sp’; Sp’ doesn’t participate in chain growth• All rate constants are assumed carbon number independent (except ka,2)• Thermodynamic equilibrium between gas and liquid phase (carbon number dependent)• No effects of diffusional limitations or concentration gradients• Steady state conversion, convective product removal via gas and liquid phase

Sp SpSp

EP Ol-(1)

dg

N-1 N N+1

Ol

N,l N,l

gdP

a

Gas Phase

Liquid Phase

Vl

Vg

KN K N

Sp'Nd'EP

a'd'Ol

CatalystSurface

EPN,g Ol-(1)N,g

Schulz & Claeys, 1999


Distribution over liquid and gas phase

0

1000

2000

3000

4000

5000

0 2 4 6 8 10 12

Carbon Number, N

Mea

n re

acto

r res

iden

ce

time

of h

ydro

carb

ons,

τN,

min

REACTOR

gV

lVKN

lNg

lNg

N

NN VKV

VKVnn

⋅+

⋅+==τ

Partition coefficient KN:

2)(N2

Ng,

Nl,N bK

cc

K −⋅==

(CoZrRu-SiO2, T=190°C, pH2=5.1 bar, pCO=2.2 bar, pH2O=1.0 bar

Vg=210 ml, Vl=270 ml, Vg=5.38 ml/min, Vl=6.28.10-4 ml/min) T=190ºC: K2=1.91; b=1.53

Rate of convective removal of products depends on removal of gas and liquid phase and and is therefore carbon number dependent

..


Comparison of extended model with real product distributions

0

20

40

60

80

100

0 5 10 15 20

Carbon Number, N

Ol-(

1) in

HC

s, m

ol-%

pH2O=8.5 bar

pH2O=0.9 bar

0.5

0.6

0.7

0.8

0.9

1.0

0 5 10 15 20

Carbon Number, N

pg,N

pH2O=8.5 bar

pH2O=0.9 bar

(0.42)-3

-1

2

0 5 10 15 20 25

Carbon Number, N

lg(M

N /M

N )

pH2O=0.9 bar

pH2O=8.5 bar

*

Primary Sel

(CoZrRu-SiO2, T=190°C, pH2=5.8 bar, pCO=2.9 bar, pH2O=varied)


Diffusion enhanced olefin re-adsorption

Assumptions:

• Product desorption as paraffin or α-olefin only• Product transport limitations in liquid filled pores of catalyst

⇒ increase in local olefin concentration / residence time inside liquid filled pores⇒ diffusion enhanced olefin readsorption: Dn∝e-0.3n

• All rate constant assumed carbon number independent (except C2: kR,2=10kR,n)• rs negligible at CO conversion >5% and ptotal>5 bar (inhibition by CO and H2O)

Cn

rSα-Olefinn-Paraffin

rO rRrH

rP rP

Diffusion / Reaction

Φ2 = (kR/Dn) . (L2 . ε . (site density) / RP)

Property of Catalyst (χ)Catalyst pore

(Iglesia et al., 1991)


Diffusion enhanced olefin re-adsorption

• Complete description of Fischer-Tropsch reaction in fixed bed reactors for Cobalt and Ruthenium catalysts, H2/CO~2

• Non-ASF distributions and olefin/paraffin ratios caused by different product diffusivities• C5+ selectivity dependent on catalyst properties (χ)

(Reaction conditions: Cobalt catalysts, T=200°C, ptotal=20 bar, H2/CO=2.1, XCO~60%)(Iglesia et al., 1991, 1993)


Reactors for Fischer-Tropsch synthesis

Fischer-Tropsch synthesis is highly exothermic (ca. 160 kJ/mol of CO converted)

Heat removal essential

ObjectiveDiesel production maximize wax formation

hydrocracking of wax to diesel Cobalt (200-220oC) or iron-based (240-260oC) catalyst

Chemicals (olefins)/petrol productionFused iron catalyst (300-350oC)



Liquid products

Synthesis gas

Water

Catalyst bed

Steam

Gaseous productsLiquid products

Synthesis gas

Water

Catalyst bed

Steam

Gaseous products

Product stream

Synthesis gas

Gas distributor

Wax

Water Steam

Suspension

Product stream

Synthesis gas

Gas distributor

Wax

Water Steam

Suspension



Steam

Water

Fresh catalyst

Synthesis gas

Product stream

Stand pipe

FT-Reactor

Steam

Water

Fresh catalyst

Synthesis gas

Product stream

Stand pipe

FT-Reactor

Synthesis gas

Water Steam

Fluidised bed

Cyclone orFilter

Product stream

Synthesis gas

Water Steam

Fluidised bed

Cyclone orFilter

Product stream


Concluding remarks

Main primary products of Fischer-Tropsch synthesis:linear α-olefinsparaffins(alcohols)

Mechanism: more than one single mechanism in operationdebate on the importance of the various mechanisms still ongoing

Fischer-Tropsch synthesis = polymerization reactionconstraints in the product formationmaximum liquid productivity by re-adsorption and re-

incorporation of reactive product compounds

Reactors for Fischer-Tropsch synthesisHeat removal primary concern

south africa fischer-tropsch synthesis - uni · pdf filecentre for catalysis research...

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