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


Coal

Biomass

Natural Gas

Fuel&

ChemicalsGasification Syngas

Processing

Fischer-TropschSynthesis

SyncrudeRefining &Upgrading

X LG

XTL Technology


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• 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


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


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)


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 SynthesisCO + 2H2 → (CH2) + H2O







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)







Economy


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



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



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


Comparison of Co and Fe LTFTS Catalyst


FTS Reactors


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







Economy


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 Polymerization Process Steps


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)

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 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.







Economy


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


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

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)

Introduction to Fischer Tropsch Synthesis Rui Xu Department of Chemical Engineering Auburn...

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