1. Introduction to Fischer Tropsch Synthesis
Rui Xu
Department of Chemical Engineering
Auburn University
Jan 29th, 2013
CHEN 4470
Process Design Practice

2. XTL Technology X G L Coal Biomass Natural Gas Gasification Syngas
Processing
Fischer-
Tropsch
Synthesis
Syncrude
Refining &
Upgrading
Fuel
&
Chemicals

3. 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

4. Coal Gasification H/C Ratio Ash Impurities (Sulfur)
2(-CH-) + O2 → 2CO + H2
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

5. Biomass Gasification H/C Ratio Ash Impurities (Sulfur, Nitrogen)
2(-CH-) + O2 → 2CO + H2
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

6. Syngas Processing Purification Water Gas Shift Reaction
CO + H2O ↔ CO2 + H2
Purification
Particulates
Sulfur (<1 ppm) - ZnO Sorbent
Nitrogenates (comparable to Sulfur compounds)
BTX (Below dew point)

7. GTL Technology and Syngas Processing

8. Fischer Tropsch Synthesis
Introduction and History
Reactions and Products
Catalysts and Reactors
Mechanism and ASF plot
Economy

9. Fischer Tropsch Synthesis
Kaiser Wilhelm Institute, Mülheim, Ruhr
1920s
Coal derived gases
Aim to product hydrocarbons
Commercialized in 1930s
Franz Fischer Hans Tropsch

10. 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.

11. Fischer Tropsch Synthesis CO + 2H2 → (CH2) + H2O

12. Fischer Tropsch Synthesis
Introduction and History
Reactions and Products
Catalysts and Reactors
Mechanism and ASF plot
Economy

13. Reactions in FTS

14. Standard LTFT product distribution

15. 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)

16. Fischer Tropsch Synthesis
Introduction and History
Reactions and Products
Catalysts and Reactors
Mechanism and ASF plot
Economy

17. Fischer-Tropsch Catalysts
Fused Iron Catalysts – HTFT
Alkali promotion needed
Products are high olefinic
Cheapest
Reactor: Fluidized bed
Iron oxide
1500 °C
Molten Magnetite (Fe3O4)
Cooled rapidly
Fused Iron
K2O
Crushed in a ball mill
Air
MgO or Al2O3

18. 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
Fe(NO3)3
Na2CO3
K2CO3
pH = 7
Washing
Drying
Calcination
Precipitate Iron Cat.

19. 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
Co(NO3)2
Support
Drying
Calcination
Supported Co Cat.

20. Comparison of Co and Fe LTFTS Catalyst

21. FTS Reactors

22. FTS Reactors

23. LTFT Reactors CO + 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

24. Fischer Tropsch Synthesis
Introduction and History
Reactions and Products
Catalysts and Reactors
Mechanism and ASF plot
Economy

25. FTS Polymerization Process Steps
Reactant adsorption
Chain initiation
Chain growth
Chain termination
Product desorption
Readsorption and further reaction

26. FTS Polymerization process steps
Reactant adsorption
Chain initiation
Chain growth
Chain termination
Product desorption
Readsorption and further reaction

27. FTS Polymerization Process Steps
FTS Mechanisms
Alkyl mechanism
Alkenyl mechanism
CO insertion
Enol mechanism

28. 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

29. 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

30. 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

31. 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

32. 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

33. FTS Mechanisms - ASF Plot
α = Rp / (Rp + Rt)
𝑀 𝑛 = 1−α α (𝑛−1)
𝑊 𝑛 = 𝑀 𝑛 ∗𝑛/( 1 1−α )
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)

34. Standard FTS Product Distribution

35. FTS Kinetics
Iron - based FT catalyst 𝑟= 𝑚 𝑃 𝐻 2 𝑃 𝐶𝑂 𝑃 𝐶𝑂 +𝑎 𝑃 𝐻 2 𝑂
Cobalt - based FT catalyst 𝑟= 𝑚 𝑃 𝐻 2 𝑃 𝐶𝑂 (1+𝑏𝑃 𝐶𝑂 ) 2
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.

36. Fischer Tropsch Synthesis
Introduction and History
Reactions and Products
Catalysts and Reactors
Mechanism and ASF plot
Economy

37. FTS Economics Overall Cost Capital Cost Operating 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

38. 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

39. Syncrude Upgrading Hydrogenation 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

40. Reference www.fischer-tropsch.org Book: Fischer Tropsch Technology
Review Articles:
The Fischer-Tropsch process (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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