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Transcript of Introduction to Fischer Tropsch Synthesis Rui Xu Department of Chemical Engineering Auburn...
Introduction to Fischer Tropsch Synthesis
Rui XuDepartment of Chemical Engineering
Auburn UniversityJan 29th, 2013
CHEN 4470Process Design Practice
CHEN 4470Process Design Practice
Coal
Biomass
Natural Gas
Fuel&
ChemicalsGasification Syngas
Processing
Fischer-TropschSynthesis
SyncrudeRefining &Upgrading
X LG
XTL Technology
CHEN 4470Process Design Practice
Natural Gas Gasification
Steam Reforming• CH4 + H2O → CO + 3H2 (Ni Catalyst)
• H2/CO = 3• Endothermic• Favored for small scale operations
Partial Oxidation• CH4 + ½O2 → CO + 2H2
• H2/CO ≈ 1.70
• Exothermic• Favored for large scale applications
Autothermal Reforming• A combination of Steam Reforming and Partial Oxidation
CHEN 4470Process Design Practice
Coal Gasification
H/C Ratio• Produces Leaner Syngas (Lower H2:CO Ratio)
Ash• Non-flammable material in coal complicates Gasifier design
Impurities (Sulfur)• Necessitates greater syngas cleanup
2(-CH-) + O2 → 2CO + H2
CHEN 4470Process Design Practice
Biomass Gasification
H/C Ratio• Similar issues to coal
Ash• Biomass aggressively forms ash
Impurities (Sulfur, Nitrogen)• Necessitates greater syngas cleanup
Moisture• High moisture levels lower energy efficiency
Size Reduction• The fibrous nature of biomass makes size reduction difficult
2(-CH-) + O2 → 2CO + H2
CHEN 4470Process Design Practice
Syngas Processing
Water Gas Shift Reaction
• CO + H2O ↔ CO2 + H2
Purification• Particulates
• Sulfur (<1 ppm) - ZnO Sorbent
• Nitrogenates (comparable to Sulfur compounds)
• BTX (Below dew point)
CHEN 4470Process Design Practice
GTL Technology and Syngas Processing
CHEN 4470Process Design Practice
Fischer Tropsch Synthesis
Introduction and History
Reactions and Products
Catalysts and Reactors
Mechanism and ASF plot
Economy
CHEN 4470Process Design Practice
Fischer Tropsch Synthesis• Kaiser Wilhelm Institute,
Mülheim, Ruhr• 1920s• Coal derived gases• Aim to product
hydrocarbons• Commercialized in
1930s Franz Fischer Hans Tropsch
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FTS Industrial History
Germany• 1923, Franz Fischer and Hans Tropsch• 1934, first commercial FT plant• 1938, 8,000 barrels per day (BPD)
U.S.A• 1950, Brownsville, 5,000 BPD
South Africa• 1955, Sasol One, 3,000 BPD• 1980, 1982, Sasol Two and Sasol Three, 25,000 BPD
Malaysia and Qatar• 1993, Shell, Bintulu, 12,500 BPD• 2007, Sasol, Oryx GTL, 35,000 BPD
China, Nigeria etc.
CHEN 4470Process Design Practice
Fischer Tropsch SynthesisCO + 2H2 → (CH2) + H2O
CHEN 4470Process Design Practice
Fischer Tropsch Synthesis
Introduction and History
Reactions and Products
Catalysts and Reactors
Mechanism and ASF plot
Economy
CHEN 4470Process Design Practice
Reactions in FTS
CHEN 4470Process Design Practice
Standard LTFT product distribution
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Fischer-Tropsch Products Hydrocarbons Types
Olefins• High chemical value• Can be oligomerized to heavier fuels
Paraffins• High cetane index• Crack cleanly
Oxgenates Branched compound (primarily mono-methyl
branching) Aromatics (HTFT)
CHEN 4470Process Design Practice
Fischer Tropsch Synthesis
Introduction and History
Reactions and Products
Catalysts and Reactors
Mechanism and ASF plot
Economy
CHEN 4470Process Design Practice
Crushed in a ball mill
Fischer-Tropsch Catalysts
Fused Iron Catalysts – HTFT• Alkali promotion needed• Products are high olefinic• Cheapest• Reactor: Fluidized bed
Iron oxide
K2O
MgO or Al2O3
1500 °C
AirMolten Magnetite
(Fe3O4)Cooled rapidly
Fused Iron
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Fischer-Tropsch Catalysts
Precipitated iron catalysts - LTFT• Co-precipitation method• Alkali promotion is also important• Cost more than fused iron catalyst• Reactor: slurry phase or fixed bed
pH = 7
Na2CO3
Fe(NO3)3
Washing Drying Calcination Precipitate Iron Cat.
K2CO3
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Fischer-Tropsch Catalysts
Supported cobalt catalysts - LTFT• Incipient wetness impregnation method• Oxide support: silica, alumina, titania or zinc oxide• Products: predominantly paraffins • Low resistance towards contaminants
Support Drying Calcination Supported Co Cat.
Co(NO3)2
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Comparison of Co and Fe LTFTS Catalyst
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FTS Reactors
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FTS Reactors
CHEN 4470Process Design Practice
LTFT ReactorsCO + H2 → (CH2) + H2O + 145 kJ/mol
1800 oC Adiabatic Temperature Rise
• Fixed Bed (Gas Phase Reaction Media) – Shell SMDS– 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– Thermal uniformity– Excellent product extraction– Excellent economies of scale– Requires separation of wax (media) from catalyst– High development cost
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Fischer Tropsch Synthesis
Introduction and History
Reactions and Products
Catalysts and Reactors
Mechanism and ASF plot
Economy
CHEN 4470Process Design Practice
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• Alkyl mechanism
• Alkenyl mechanism
• CO insertion
• Enol mechanism
FTS Polymerization Process Steps
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FTS Mechanisms
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
CHEN 4470Process Design Practice
FTS Mechanisms
– 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
CHEN 4470Process Design Practice
FTS Mechanisms
– 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
3i). CH2CH3R terminates to alkane by α-hydrogenation
3ii). CH2CH3R terminates to alkene by β-dehydrogenation
3iii). CHR(OH) terminates to aldehyde by dehydrogenation
3iv). CHR(OH) terminates to alcohol by hydrogenation
CHEN 4470Process Design Practice
FTS Mechanisms
– The Enol Mechanism 1i). CO chemisorbs non-dissociatively
1ii). CO hydrogenates to CH(OH) and CH2(OH)
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)
3i). chain termination to aldehyde is by desorption
3ii). Chain termination to alkane, alkene, and alcohol, is by hydrogenation
CHEN 4470Process Design Practice
FTS Mechanisms - ASF Plot
• Propagation is exclusively by the addition of one monomer
• αi + bi = 1 (by definition)
• Propagation probability is independent of carbon number
CHEN 4470Process Design Practice
FTS Mechanisms - ASF Plot
α = Rp / (Rp + Rt)
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)
CHEN 4470Process Design Practice
Standard FTS Product Distribution
CHEN 4470Process Design Practice
FTS KineticsIron - based FT catalyst
Cobalt - based FT catalyst
•Iron catalyst: at low conversion (P H2O ≈0 ), the reaction rate is only a
function of hydrogen partial pressure.
•The kinetic equations imply that water inhibits iron but not cobalt.
•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 .
•Both denominators involve partial pressure of CO, indicating CO’s general
status being a (reversible) catalyst poison.
•Both kinetic equations indicate hydrogenation as the rate-limiting step.
CHEN 4470Process Design Practice
Fischer Tropsch Synthesis
Introduction and History
Reactions and Products
Catalysts and Reactors
Mechanism and ASF plot
Economy
CHEN 4470Process Design Practice
FTS Economics
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
CHEN 4470Process Design Practice
XTL technology Economy
• Cost Distribution• NTL case 1: 25% for the gas, 25% for the operations and 50% for the capital
• 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)
• 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
CHEN 4470Process Design Practice
Syncrude Upgrading
Extraction and Purification• Terminal Olefins, Oxygenates, and FT Wax have high value
Hydrocracking• Converts wax into liquid fuels
Oligomerization• Converts light olefins to liquid fuels
Other Reactions • Alkylation, Isomerization, Aromatization, etc.
Polymerization• HTFT ethylene and propylene can be made into polymers
Hydrogenation• Promoted fuel stability
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Reference
www.fischer-tropsch.org Book: Fischer Tropsch Technology Review Articles:
• 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)
CHEN 4470Process Design Practice