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

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

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

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

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

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)

GTL Technology and Syngas Processing

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

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

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.

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

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

Reactions in FTS

Standard LTFT product distribution

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)

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

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

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.

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.

Comparison of Co and Fe LTFTS Catalyst

FTS Reactors

FTS Reactors

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

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

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

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

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

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

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

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

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

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

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)

Standard FTS Product Distribution

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.

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

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

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

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

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)

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