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Chain Growth in Fischer-Tropsch:Cobalt versus Iron
Irving Wender, Yulong Zhang, Li Hou, and John Tierney
Department of Chemical and Petroleum EngineeringUniversity of Pittsburgh
CFFS Annual Meeting, Roanoke, WVAugust 1-4, 2004
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Outline
Driving forces Fischer-Tropsch plants
Fischer-Tropsch vs ethylene
polymerization
Comparison of FT on cobalt and iron
Conclusions
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Driving Forces
Monetizing stranded gas reserves
Technology improvements
EPA regulations for low S, clean fuels
High oil prices Seeking alternative fuel route for energy
security
Co-production of electricity and fuels inFutureGen
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Current GTL Situation
*Coal feedstock
Existing Plants
Owner/Developer Location Capacity (b/d) Process Start Date
BP Alaska 300 BP/Davy 2002PetroSA South Africa 30,000 Sasol 1992
SASOL I -III* South Africa 175,000 Sasol 1956-1982Shell Bintulu Bintulu 12,500 Shell SMDS 1993
Syntroleum Oklahoma 2 Syntroleum 1990
Commercial Scale Plants Under Development
SasolChevron Nigeria 34,000 Sasol 2005-2006
Sasol/Qatar Petr. Qatar 34,000 Sasol 2005Pilot Scale Under Construction or Development
Conoco Oklahoma 400 Conoco 2002JNOC Japan 7 N/A 2002Mossgas/Statoil South Africa 1,000 Statoil 2002Syntroleum/Marathon Oklahoma 70 Syntroleum 2003
Fe
ExxonMobil has signed a $7 billion deal to develop a 154,000 barrels per dayGTL project in Qatar, scheduled production in 2011
Shell signed a $5 billion, 140,000 barrels per day GTL project in Qatar,
scheduled production in 2009
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China: Coal to Liquids Planned
China signed a letter of intent with SASOLto build two coal-to-liquid fuel plants
Cost $6 billion, $3 billion each.
440 million barrels/year, 8 times as large
as Sasols current production, covers 60%of China current oil imports
Financial Times, June 28, 2004
David Dapice, YaleGlobal, July 15 2004
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COdissociatesat roomtemp.
CO dissociates at200-300 oC
Non-dissociativeadsorption of CO
Ethylenepolymerization
Catalysts
FT catalysts, metal carbides FT catalysts, metal
Activity of Metallic Elements inPeriodic Table
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Fischer-Tropsch vs Ethylene polymerization
Fischer-Tropsch and ethylene
polymerization both follow stepwise chaingrowth mechanism (Schultz-FloryDistribution) to form high molecular weighthydrocarbons with the same structure(polymethylene and HDPE)
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Molecular Weight Distribution of Polymethylene asCompared with Polyethylene (HDPE).
Pichler, H., and Bellstedt, F., Erdoelund Kohle, Erdgas, Petrochemie
vereinigt mit Brennstoff-Chemie26,560 (1973).Schulz, H., Chemierohst. Kohle, 334(1977).
Ru catalyst, 1000atm, 100-120oC
CH3-CH2-(CH2)n-CH2-CH3
Polymethylene and polyethylene have the same structure
Both follow Schultz-Flory distribution
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IR Spectra of polymethylene and Zieglers polyethylene
Polymethylene
Polyethylene (HDPE)
Schulz, H., Chemierohst. Kohle, 334 (1977).
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Chain Growth vs. Polymerization
Schultz-FloryAnderson-Schultz-FloryProductdistribution
Straight chain hydrocarbonsStraight chain hydrocarbonsProducts
Ethylene monomer addedMethylene (-CH2-) produced insitu by CO hydrogenation
Monomer
High molecular weight
polyethylene
Gasoline, diesel and wax,polymethylene
Productrange
Polymerization by stepwise
addition of ethylenemonomer
Chain growth (polymerization)
by stepwise addition ofmethylene monomer
Reaction
PolyethyleneFischer-Tropsch
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Cobalt vs Iron Although many metals have Fischer-Tropsch activity,
only cobalt and iron catalysts are in industrial use. Cobalt and iron share similarities in FT Very active
Produces broad range of straight chain hydrocarbons
Product distribution follows ASF equation Cobalt and iron are also quite different in many aspects
Cobalt is active as Co metal
Iron is active as Fe carbides
Products differ Response to probes differs
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Iron in the presence of alkali is very active
for the WGS Allows use of CO2 in the FT synthesis
CO2+3H2CO2 + H2 = CO + H2O rwgs
CO + 2H2 = -CH2-+ H2O FTCO2+3H2 = -CH2- +2H2O
Iron as WGS Catalyst
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Cobalt vs IronDifferences in Catalysts
Iron carbidesCobalt metalActive catalyst
SiO2, Al
2O
3as structural
stabilizers
Iron loading: >85% weight
Supports are necessary
SiO2, Al2O3, TiO2, ZrO2Cobalt loading: ~20% weight
Supports
Difficult to regenerateRegenerate by oxidation andreduction
Regeneration
CuNoble metals (Ru, Rh, Re, Pt)Promoters
Essential for activity and
chain growth
Decreases activityAlkali (K)
Iron ($252/t)Cobalt ($57,300/t)
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Cobalt vs IronReaction Conditions
2/32/1H2/CO
10-40 atm.Pressure is necessary foractivity and chain growth.
1-30 atm.High pressure tends to formCo2(CO)8.
Pressure
180-250 C in LTFT
300-350 C in HTFT200-250 CTemperature
IronCobalt
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Cobalt vs IronProducts
HighLowOxygenates
Moderate to highHighWaxes
CO2 and H2OH2OOxygen sink
More olefinsFewer olefinsOlefins
Less sensitive to reaction conditions.Alkali promoter greatly decreases CH4selectivity
Very sensitive to hightemperature andH2/CO ratio
CH4
IronCobaltSelectivity
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Cobalt vs IronResponse to Probes
Acetylene inhibits adsorption of CO on FT catalyst1
surfaces and Initiates reaction at low temperature (120oC)
IronCobalt
Yield oxygenates with ASFdistribution
Yield oxygenates onlywith one carbon morethan the probe
Acetylenes
Difficult to incorporateIncorporates easilyEthylene(Olefins)
Incorporates 100 times fasterthan ethylene.
CH3CH2OHCH3CHO + H2
CH3CHO -CH2- + CO + H2
Not incorporatedEthanol
(Alcohols)
R C CH +CO+H2 RCH2CH2CH2OH
1Jackson, S. D., Hussain, N., and Munro, S., J Chem Soc Faraday T94, 955 (1998).
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Evidence for Initiation of FT by addition of acetylenes
5.00 10.00 15.00 20.00 25.00 30.00 35.00 40.00 45.00 50.00 55.00 60.000
500000
1000000
1500000
2000000
2500000
3000000
3500000
4000000
4500000
5000000
5500000
6000000
6500000
7000000
7500000
8000000
8500000
9000000
Time-->
AbundanceTIC: CO05103V.D
C6 C7 C8C9 C11 C12 C14 C16 C17
C18C10 C13 C15 C19
Ph-C3
Ph-C4
Ph-C5
Ph-C6
Ph-C7
Ph-C8
Ph-C9
Ph-C10
Ph-C11
Ph-C12
Ph-C13
Ph-C8-Ph
Ph-C5OH
C5
Straight chain substituted benzene
Co10Al90, T=220
o
C, P=100psi, H2/CO=2, 4-phenyl-1-butyne were added inpentane (10% in volume) with a flow rate of 2ml/h
CH2 CH2 C CH
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Initiation of Chain Growth at Lower Temperature
10.00 20.00 30.00 40.00 50.00 60.00
0
500000
1000000
1500000
2000000
2500000
3000000
3500000
4000000
4500000
5000000
5500000
6000000
6500000
7000000
7500000
8000000
8500000
9000000
9500000
1e+07
1.05e+07
1.1e+07
1.15e+07
1.2e+07
1.25e+07
1.3e+07
Time-->
Abundance
TIC: CO05202N.D
pentane
Ph-C3
Ph-C2Ph-C1
Ph-C4
Ph-C5
Ph-C6
Ph-C7
Ph-C5OH
Ph-C8
Ph-C9
Ph-C10
Co10Al90, T=180
o
C, P=100psi, H2/CO=2, 4-phenyl-1-butyne were added inpentane (10% in volume) with a flow rate of 2ml/h
Ph-C8-Ph
Ph-C11
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Oxygenated products without/with acetylene addition
on Co and Fe catalysts
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
C2OH C3 C4 C5 C6 C7 C8 C9 C10
n
Yields(mg/h)
without acetylene
with acetylene
100Fe/4.4Si/1.25K, T=180oC,
P=300psi, H2/CO=1, 1% acetylene
0
0.5
1
1.5
2
2.5
3
C2OH C3 C4 C5 C6 C7 C8 C9 C10
n
Yields(mg/h) without acetylene
with acetylene
Co10Al90, T=180oC, P=300psi,
H2/CO=1, 1% acetylene
Iron catalyst
Cobalt catalyst
Addition of acetylene to ironincreases oxygenates viaSchultz-Flory chain growth.
Addition of acetylene to cobaltyields C3 oxygenates only byhydroformylation
C5 oxygenates are formed by
hydroformylation of the dimerof acetylene on cobalt
C3H5OH
C3H5OH
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ConclusionsCobalt and iron differ in many ways:
Co will be used with natural gas: GTL
Iron will be used with coal: CTL
Iron is a water gas shift catalyst so that H2 + CO2can yield FT products.
Cobalt metal is a strong hydrogenation catalyst
producing saturated chains; iron carbides, in thepresence of alkali, produces more olefins
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Conclusions Cobalt, although much more expensive than iron, is
used in smaller amounts and can be used forseveral years by regeneration (oxidation/reduction).
Iron, because of formation of carbides (contraction)and carbon deposition (expansion), tends to
disintegrate during operation.
Schultz-Flory distribution is not peculiar to FT. Itdescribes stepwise addition of a monomer to a
growing chain. Similarity to ethylene polymerizationadds evidence for chain growth by stepwiseincorporation of methylene (-CH2-) in FT
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Conclusions Acetylene initiates FT at more than 100C below
normal FT temperatures for both iron and cobalt
Addition of acetylene to iron yields oxygenatesvia ASF chain growth; addition of acetylene tocobalt yields only a single oxygenate via
hydroformylation. The oxidative tendency of iron is shown by the
easy incorporation of ethanol and other alcohols
Ethylene (olefins) incorporates easily into cobalt,but not into iron catalyzed FT products.
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AcknowledgementWe thank the Department of Energy for
financial support.
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Characteristics of FT Catalysts
Strong and dissociative adsorption of CO
Products follow ASF distribution; stepwise chaingrowth by addition of C1 monomers (-CH2-)
FT catalysts form full-blown metal carbonyls.
Optimum conditions of pressure and temperature forFT synthesis are close to conditions at which formationof metal carbonyl can be detected
H. Pichler in Advances in Catalysis (1952) IV, 272-341
Sensitive to sulfur compounds
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CO Adsorption Properties on
Transition Metals Fe, Mo and W can dissociate CO at room
temperature and readily form active carbides. Co, Ru, Ni and Rh adsorb CO associatively at
room temperature; dissociatively at elevated
temperatures, active for FT synthesis in themetal form.
Nondissociative adsorption of CO, methanol
synthesis catalysts (Cu, Pd)
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Schultz-Flory Distribution
Assumptions Stepwise chain growth
(polymerization) byaddition of monomer
Chain growth andtermination probability aresame for all intermediates
)1(2)1( = nn
nW