The Order of Selectivity in FischerTropsch Synthesis

Hans Schulz, University of KarlsruheLecture at the 2nd International Freiberg Conference, May 8-12, 2007

2

Overview

• Industrial FT-activity, High-Diesel option, Olefin production

• FT-self-organization, Catalyst re-assembling

• FT-selectivity and elementary reactions:

Chain growth, Branching, Methanation, Olefin reactions

• Essentials with iron and cobalt as catalysts

4

C3 C6 C9

C12 C15 C18C21

Hydro-genated

Original

n-H

EXAN

E

minutes 10 35

70

5

Original

C6 - F r a c t i o n

Hydrogenated

n-Hexanen-Hexane

2-Methyl-pentane

3-Methyl-pentane

3-Methyl-pentene-1+ 4-Methyl-pentene-1

n-Propanol2-Methyl-pentane

Butanal3-Methyl-pentane

2-Methyl-pentene-1n-Hexene-1

trans-Hexene-3trans-Hexene-2

3-Methyl-pentene-2cis-Hexene-2

7

Fischer-Tropsch Fluid-Bed Olefinsx) (Sasol at Secunda)(Fused alkalized iron catalyst, ca. 350 oC)

Product Fraction 1 - O l e f i n Content, C-% 1-Olefin Production

Carbon-No. Amount, C-% in Fraction in Product t/a .

_______________________________________________________________________________

(methane) (7 )

C2 7 30 2 .1 170.000

C3 1 3 80 1 0 . 4 830.000

C4 1 3 75 9 .8 780.000

C5 1 2 65 7 .8 620.000

C6 1 0 55 5 .5 440.000

C7 8 50 4 .0 320.000

C8 6 45 2 .7 220.000

_______________________________________________________________________________

69 C-% 42 C-% 3.4 x 106 t/a

Foot notes:

- Estimated values from various sources

- Total fluid bed hydrocarbon production is reported as ca. 8 x 106 t/a

- In August, 2004, Sasol announced to build a 3rd

octene-1 plant of 100.000 t/a capacity.

(Increase of the octene-1 capacity to 196.000 t/a in 2006)

9

Ideal FT-Product Distribution

Ideal: pg= constant (independent of chain length NC), (no branching, only one sort of products, no secondary reactions)

CO, H2P3

PN

↑→

desorption pdN

Sp1pgN

↑

Sp2

↑→ Sp3

↑→ → →

growthSpN

↑ Sp Surface speciesP ProductP Probability of reactiong Growthd DesorptionN Carbon number

Pg + pd = 1

Σ PN = 1 (= Sp1)

PN = Sp1 • pgN-1 • pd

PN = 100 • pd/pg • pgN

PN = const •pgN

Log PN = const + N •Log pg

Log

Mol

es

Carbon Number,N

pg = Δ Log Moles/Δ N Carbon Number, N

pg0,85

The ideal product distribution is defined by only one number, the value of growth probability pg

10

Product composition in dependence of chain growth probability pgassuming ideal polymerization kinetics

Maximum Fischer-Tropsch Diesel selectivity is only ~ 40 C% at pg ~0.87

With ideal hydrocracking of FT-waxes Diesel selectivity is enhanced to ~ 60 C %

Adapted from M. Dry (1990)

11

C h a i n g r o w t h

IRON COBALT

Pro

babi

lity

of

cha

in g

row

th, p

g

Pro

babi

lity

of c

hain

gro

wth

, pg

C a r b o n N u m b e r , NC C a r b o n N u m b e r , NC

Riedel, Schaub, Schulz (1999) Thesis Zh. Nie (1996)

Iron (Fe-Al-Cu-K); 250°C, 10 bar, H2/CO = 2.3:1Cobalt (Co-Zr-Pt-Aerosil); 190°C, 5 bar, H2/CO = 2:1

M e t h a n a t i o n, Fe-/Co-C o m p a r i s o n

Thermodynamic Trend: Methanation much favored against FT-Synthesis

With Iron (K-promoted) With Cobalt Lowest CH4-Selectivity ca. 5 C% ca . 5C% Increasing Temperature little change strong increase Decreasing pCO little change strong increase Increasing pH2O decrease Initial time-on-stream little change decrease Methanation sites static/merely present dynamic

On methanation sites: CO-Dissociation

C CH CH2 CH3 CH4+ H+ H + H + H

On growth-sites

CH3

+ H

+ CH2

CH4 20% CH3 − CH2 80%

14H. Pichler 1952

Self-assembling/Re-structuringFe

, A

t o

m -

% Episodes of Synthesis

IRONCOBALT

T i m e, texp, min

α-Fe

Fe3O4

Fe5C2

Thermodynamic view of cobalt surfacesegregation in FT-synthesis

On-Plane-Sites disproportionationfor low and high Coordination:• On-Peak-Sites• In-Pit-Sites

Ent

halp

y

Big particles+CO

+CO

+CO

Smallparticlessegregated

Small particles

Iron phase compositionby Moessbauer spectroscopy

Catalyst: 100Fe-13Al-11Cu-9KFT-Synthesis: 250°C,10 bar, H2/CO2 = 3:1

Ref.: Riedel, Schaub, Schulz, Jun, Hwang, Lee

Ref.: Schulz

C h a i n B r a n c h i n g

IRON COBALT

P

roba

bilit

y o

f bra

nchi

ng

Pr

obab

ility

of b

ranc

hing0.1

0 2 4 6 8 10 12

0.1

0

C a r b o n N u m b e r , NCC a r b o n N u m b e r , NC

Riedel, Schaub, Schulz (1999) Thesis Zh. Nie (1996)

Iron (Fe-Al-Cu-K); 250°C, 10 bar, H2/CO2 = 3:1Cobalt (Co-Zr-Pt-Aerosil); 190°C, 5 bar, H2/CO = 2:1

17,000 min

2,270 min

Olefin Selectivity

IRON COBALT

Ole

fin c

onte

nt, C

-%

Ole

fin c

onte

nt, C

-%

C a r b o n n u m b e r , NC C a r b o n n u m b e r , NC

Ref. : Riedel, Schaub, Schulz Ref. : Thesis Zh. Nie

Iron (Fe-Al-Cu-K); 250°C, 10 bar, H2/CO = 2.3:1Cobalt (Co-Zr-Aerosil); 190°C, 5 bar, H2/CO = 2:1

18

D o u b l e B o n d S h i f t

C a r b o n n u m b e r

Ole

fins-

1in

lin.

ole

fins,

mol

-%

R–CH2–CH=CH2

R–CH=CH2 70 - 80%

R–CH2–CH2-R–CH2–CH2 20 - 30%

-H

+HPrimary olefin formation on FT-sites

R–CH2–CH2–CH2 R–CH2–CH2–CH3

R–CH=CH–CH3 R–CH2–CH–CH3

+H+H

+H +H

-HSecondary olefin-reactionsOn “non-FT-sites”

Kinetic Schemes of Olefin Reactions

Activity in dependence of time (texp)

Y I

E L

D ,

C-%

YFT

I II III

T i m e , texp, min T i m e , texp, min

Riedel, Schaub, Schulz (1996) Thesis Claeys (1997)

Yields YFT: CO + 2H2 = ( CH2) + H2 + H2O YRWGS: CO2 + H2 = CO + H2O Ycarb: CO = Ccarb + (O)

Iron (Fe-Al-Cu-K); 250°C, 10 bar, H2/CO2 = 3:1Cobalt (Co-Zr-Pt-Aerosil); 190°C, 5 bar, H2/CO = 2:1

IRON COBALT

1hr 10hr 4days

Y I

E L

D

YFT

21

Real FT-Synthesis: The Order of SelectivityKinetic Scheme and Alternative Reactions

SpN

Sp*N

POlefin-1

POlefin-2PParafin

SpN-1 SpN+1

SpN+1,br

PParafin

dolefindparaffin

glinear

gbranched

P ProductSp Species on surfaced Desorptiong Growth br BranchedN Carbon number

SpN Species on On-Peak-SiteSp*N Species on On-Plane-Site

Rate constants depend on carbon number and are ruled by frustrations

CH2-from-CO formation on In-Pit-Sites

22

Understanding Fischer-Tropsch synthesis fundamentally

will be usefull for further process development