The Order of Selectivity in FischerTropsch Synthesis...2007/08/01 · Product composition in...
Transcript of The Order of Selectivity in FischerTropsch Synthesis...2007/08/01 · Product composition in...
The Order of Selectivity in FischerTropsch Synthesis
Hans Schulz, University of KarlsruheLecture at the 2nd International Freiberg Conference, May 8-12, 2007
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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
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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
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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)
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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
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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)
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%
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
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
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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