Introduction to
Fischer Tropsch Synthesis
Rui Xu
Department of Chemical Engineering
Auburn University
Jan 29th, 2013
CHEN 4470
Process Design Practice
XTL Technology
CHEN 4470
Process Design Practice
Coal
Biomass
Natural Gas
L
G
X
Gasification
Syngas
Processing
FischerTropsch
Synthesis
Syncrude
Refining &
Upgrading
Fuel
&
Chemicals
Natural Gas Gasification
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 Steam Reforming
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CH4 + H2O → CO + 3H2 (Ni Catalyst)
H2/CO = 3
Endothermic
Favored for small scale operations
 Partial Oxidation
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CH4 + ½O2 → CO + 2H2
H2/CO ≈ 1.70
Exothermic
Favored for large scale applications
 Autothermal Reforming
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A combination of Steam Reforming and Partial Oxidation
Coal Gasification
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2(-CH-) + O2 → 2CO + H2
 H/C Ratio
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Produces Leaner Syngas (Lower H2:CO Ratio)
 Ash
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Non-flammable material in coal complicates Gasifier design
 Impurities (Sulfur)
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Necessitates greater syngas cleanup
Biomass Gasification
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2(-CH-) + O2 → 2CO + H2
 H/C Ratio
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Similar issues to coal
 Ash
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Biomass aggressively forms ash
 Impurities (Sulfur, Nitrogen)
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Necessitates greater syngas cleanup
 Moisture
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High moisture levels lower energy efficiency
 Size Reduction
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The fibrous nature of biomass makes size reduction difficult
Syngas Processing
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 Water Gas Shift Reaction
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CO + H2O ↔ CO2 + H2
 Purification
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Particulates
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Sulfur (<1 ppm) - ZnO Sorbent
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Nitrogenates (comparable to Sulfur compounds)
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BTX (Below dew point)
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GTL Technology and Syngas
Processing
Fischer Tropsch Synthesis
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Process Design Practice
 Introduction and History
 Reactions and Products
 Catalysts and Reactors
 Mechanism and ASF plot
 Economy
Fischer Tropsch Synthesis
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Franz Fischer
Hans Tropsch
• Kaiser Wilhelm Institute,
Mülheim, Ruhr
• 1920s
• Coal derived gases
• Aim to product
hydrocarbons
• Commercialized in
1930s
FTS Industrial History
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Process Design Practice
Germany
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•
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U.S.A
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1923, Franz Fischer and Hans Tropsch
1934, first commercial FT plant
1938, 8,000 barrels per day (BPD)
1950, Brownsville, 5,000 BPD
South Africa
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1955, Sasol One, 3,000 BPD
1980, 1982, Sasol Two and Sasol Three, 25,000 BPD
Malaysia and Qatar
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1993, Shell, Bintulu, 12,500 BPD
2007, Sasol, Oryx GTL, 35,000 BPD
China, Nigeria etc.
Fischer Tropsch Synthesis
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Process Design Practice
CO + 2H2 → (CH2) + H2O
Fischer Tropsch Synthesis
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Process Design Practice
 Introduction and History
 Reactions and Products
 Catalysts and Reactors
 Mechanism and ASF plot
 Economy
Reactions in FTS
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Standard LTFT product
distribution
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Fischer-Tropsch Products
Hydrocarbons Types
 Olefins
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High chemical value
Can be oligomerized to heavier fuels
 Paraffins
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High cetane index
Crack cleanly
 Oxgenates
 Branched compound (primarily mono-methyl
branching)
 Aromatics (HTFT)
Fischer Tropsch Synthesis
CHEN 4470
Process Design Practice
 Introduction and History
 Reactions and Products
 Catalysts and Reactors
 Mechanism and ASF plot
 Economy
Fischer-Tropsch Catalysts
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 Fused Iron Catalysts – HTFT
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Alkali promotion needed
Products are high olefinic
Cheapest
Reactor: Fluidized bed
Iron oxide
1500 °C
K2O
MgO or
Al2O3
Air
Molten Magnetite
(Fe3O4)
Cooled rapidly
Crushed in a ball mill
Fused Iron
Fischer-Tropsch Catalysts
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Process Design Practice
 Precipitated iron catalysts - LTFT
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•
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Na2CO3
Co-precipitation method
Alkali promotion is also important
Cost more than fused iron catalyst
Reactor: slurry phase or fixed bed
Fe(NO3)3
K2CO3
pH = 7
Washing
Drying
Calcination
Precipitate
Iron Cat.
Fischer-Tropsch Catalysts
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Process Design Practice
 Supported cobalt catalysts - LTFT
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Incipient wetness impregnation method
Oxide support: silica, alumina, titania or zinc oxide
Products: predominantly paraffins
Low resistance towards contaminants
Co(NO3)2
Support
Drying
Calcination
Supported
Co Cat.
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Comparison of Co and Fe LTFTS
Catalyst
FTS Reactors
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FTS Reactors
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LTFT Reactors
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CO + H2 → (CH2) + H2O + 145 kJ/mol
1800 oC Adiabatic Temperature Rise
• Fixed Bed (Gas Phase Reaction Media) – Shell SMDS
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–
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Excellent reactant transport
Simple design
Poor product extraction, heat dissipation
Limited scale-up
Potential for thermal runaway
• Slurry Bed (Liquid Phase Reaction Media) – Sasol SPR
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Thermal uniformity
Excellent product extraction
Excellent economies of scale
Requires separation of wax (media) from catalyst
High development cost
Fischer Tropsch Synthesis
CHEN 4470
Process Design Practice
 Introduction and History
 Reactions and Products
 Catalysts and Reactors
 Mechanism and ASF plot
 Economy
FTS Polymerization Process
Steps
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
Reactant adsorption

Chain initiation

Chain growth

Chain termination

Product desorption

Readsorption and further reaction
FTS Polymerization process
steps
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• Reactant adsorption
• Chain initiation
• Chain growth
• Chain termination
• Product desorption
• Readsorption and further reaction
FTS Polymerization Process
Steps
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 FTS Mechanisms
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Alkyl mechanism
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Alkenyl mechanism
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CO insertion
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Enol mechanism
FTS Mechanisms
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The Alkyl mechanism

1i). CO chemisorbs dissociatively

1ii). C hydrogenates to CH, CH2, and CH3

2). The chain initiator is CH3 and the chain propagator is CH2

3i). Chain termination to alkane is by α-hydrogenation

3ii). Chain termination to alkene is by β-dehydrogenation
FTS Mechanisms
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Process Design Practice
– The Alkenyl Mechanism

1i). CO chemisorbs dissociatively

1ii). C hydrogenates to CH, CH2

1iii). CH and CH2 react to form CHCH2

2i). Chain initiator is CHCH2 and chain propagator is CH2

2ii). The olefin in the intermediate shifts from the 2 position to the
1 position

3). Chain terminates to alkene is by α-hydrogenation
FTS Mechanisms
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– The CO Insertion Mechanism

1i). CO chemisorbs non-dissociatively

1ii). CO hydrogenates to CH2(OH)

1iii). CH2(OH) hydrogenates and eliminates water, forming CH3

2i). Chain initiator is CH3, and propagator is CO

2ii). Chain propagation produces RC=O

2iii). RC=O hydrogenates to CHR(OH)

2iv). CHR(OH) hydrogenates and eliminates water, forming CH2R
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3i). CH2CH3R terminates to alkane by α-hydrogenation
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3ii). CH2CH3R terminates to alkene by β-dehydrogenation
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3iii). CHR(OH) terminates to aldehyde by dehydrogenation

3iv). CHR(OH) terminates to alcohol by hydrogenation
FTS Mechanisms
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Process Design Practice
– The Enol Mechanism

1i). CO chemisorbs non-dissociatively
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1ii). CO hydrogenates to CH(OH) and CH2(OH)
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2i). Chain initiator is CH(OH) and chain propagator is CH(OH) and CH2(OH)

2ii). Chain propagation is by dehydration and hydrogenation to CR(OH)
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3i). chain termination to aldehyde is by desorption
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3ii). Chain termination to alkane, alkene, and alcohol, is by hydrogenation
FTS Mechanisms - ASF Plot
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Process Design Practice
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Propagation is exclusively by the addition of one monomer
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αi + bi = 1 (by definition)
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Propagation probability is independent of carbon number
FTS Mechanisms - ASF Plot
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Process Design Practice

α = Rp / (Rp + Rt)

𝑀𝑛 = 1 − α α(𝑛−1)

𝑊𝑛 = 𝑀𝑛 ∗ 𝑛/(1−α)
1

The weight fraction of a chain of length n, Wn, can be measured as a function
of the chain growth probability.
 Wn = nαn-1(1- α)

The logarithmic relation is as follows:
 ln (Wn / n) = nln α + ln((1- α)/ α)
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Standard FTS Product
Distribution
FTS Kinetics
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𝑚 𝑃𝐻2 𝑃𝐶𝑂

Iron - based FT catalyst
𝑟=

Cobalt - based FT catalyst
𝑟=
•
Iron catalyst: at low conversion (P H2O ≈0 ), the reaction rate is only a
𝑃𝐶𝑂 +𝑎𝑃𝐻2 𝑂
𝑚 𝑃𝐻2 𝑃𝐶𝑂
(1+𝑏𝑃𝐶𝑂 )2
function of hydrogen partial pressure.
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The kinetic equations imply that water inhibits iron but not cobalt.
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For cobalt catalyst, when the CO partial pressure is very high, (1+bPCO) 2→
(bPCO) 2, the reaction rate is proportional to the ratio of P H2 ⁄PCO .
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Both denominators involve partial pressure of CO, indicating CO’s general
status being a (reversible) catalyst poison.
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Both kinetic equations indicate hydrogenation as the rate-limiting step.
Fischer Tropsch Synthesis
CHEN 4470
Process Design Practice
 Introduction and History
 Reactions and Products
 Catalysts and Reactors
 Mechanism and ASF plot
 Economy
FTS Economics
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Process Design Practice
Overall Cost

Capital Cost
• 50% to 65% of total production cost is due to capital cost
• $10 per BBL for Natural Gas feedstock, $20 per BBL for Coal or Biomass feedstock

Operating Cost
• 20% to 25% of total production cost is due to operating costs
• $5 per BBL for Natural Gas, $10 per BBL for Coal or Biomass

Raw Material Cost
• Waste or stranded resources are preferred
• At market value ($4.50 / MMBTU), natural gas costs $45 / BBL
• At market value ($70 / ton), coal costs $35 / BBL
• At market value ($30 / ton), biomass costs $30 / BBL
XTL technology Economy
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• Cost Distribution
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NTL case 1: 25% for the gas, 25% for the operations and 50% for the capital
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NTL case 2: 15% for the gas, 21% for the operations and 64% for the capital (28% reforming,
24% FTS system, 23% oxygen plant, 13% product enhancement and 12% power recovery)
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BTL capital (21% for biomass treatment, 18% for gasifier, 18% for syngas cleaning, 15% for
oxygen plant, 1% for water-gas-shift (WGS, CO + H2O → CO2 + H2) reaction, 6% for FTS
system, 7% for gas turbine, 11% for heat recovery / steam generation, 4% for other)
• Recycle, power and heat integration
• CO2 transport and storage
Syncrude Upgrading
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
Extraction and Purification
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
Hydrocracking
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
Converts light olefins to liquid fuels
Other Reactions
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
Converts wax into liquid fuels
Oligomerization
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
Terminal Olefins, Oxygenates, and FT Wax have high value
Alkylation, Isomerization, Aromatization, etc.
Polymerization
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HTFT ethylene and propylene can be made into polymers
 Hydrogenation
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Promoted fuel stability
Reference
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 www.fischer-tropsch.org
 Book: Fischer Tropsch Technology
 Review Articles:
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The Fischer-Tropsch process 1950-2000 (Dry, 2002)
High quality diesel via the Fischer–Tropsch process – a review (Dry,
2001)
Kinetics and Selectivity of the Fischer–Tropsch Synthesis: A Literature
Review (Gerard, 1999)
Design, synthesis, and use of cobalt-based Fischer-Tropsch synthesis
catalysts (Iglesia, 1997)
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