Natural Gas to Liquid Fuels Using Ion-Transport Membrane Technology

1
Natural Gas to Liquid Fuels Using Ion-Transport
Membrane Technology

• Process Design Profitability Analysis of

Doug Muth, Eve Rodriguez, Christopher
Sales Faculty Advisor Dr. Stuart
Churchill Senior Design Project 2005
2
Gas-to-Liquids (GTL)

• What is Gas-to-Liquids?
• Conversion of natural gas to liquid fuels (e.g.,
  diesel).

Natural Gas
SYNGAS PRODUCTION
FISCHER-TROPSCH SYNTHESIS
HYDROCRACKING
Diesel
3
Motivations for GTL

• Transportation
• Natural gas reserves are often too far from gas
  pipelines.
• Transporting liquid or electricity is more
  feasible.
• Environmental
• US EPA regulations require reduction of sulfur
  content.
• GTL naturally produces extra low-sulfur diesel.
• High Fuel Quality
• High cetane numbers for GTL diesel.
• Low aromatic and olefin content.

4
Design Basis

• GTL plant to be located in Ohio.
• Minimum DCFRR of 12 required.
• Evaluate ion-transport membrane technology for
  syngas production.

5
GTL Conversion Strategies

• Direct Conversion of CH4 to Methanol
• Too difficult to control.
• High activation energy required.
• No suitable catalyst.
• Indirect Conversion Via Synthesis Gas
• Syngas converted to long-chain hydrocarbons by
  the Fischer-Tropsch (FT) reaction.
• Syngas production methods
• Steam reforming of methane.
• Dry reforming of methane.
• Partial oxidation of methane.

6
Production of Synthesis Gas

• Steam reforming of methane.
• CH4 H2O ? CO 3H2 (endothermic)
• Dry reforming of methane.
• CH4 CO2 ? 2CO 2H2 (endothermic)
• Partial oxidation of methane.
• CH4 ½ O2 ? CO 2H2 (exothermic)

Optimal H2/CO ratio (21) for FT synthesis.
7
Partial Oxidation of Methane

• Nearly pure O2 required for partial oxidation.
• O2 Purification Methods
• Cryogenic distillation of air.
• Hundreds of equilibrium stages required.
• High energy costs of refrigeration.
• Substantial capital cost (insulation,
  compressors, etc).
• New Alternative Oxygen ion-transport membrane.
• Potentially cheaper than cryogenic air separation
  plant.
• Membrane not yet commercialized, cost not known.

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Ion-Transport Membrane
Courtesy of Air Products, Inc.

• Membrane provides O2 for syngas production.

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Ion-Transport Membrane

• Membrane material non-porous mixed conducting
  metallic oxides.
• Perovskites
• LaxA1-xCoFe1-yO3-z
• (La Lanthanides, 0ltxlt1, A Sr, Ba, or Ca,
  0ltylt1, z number which renders the compound
  charge neutral)
• Conducts O2- through membrane vacancies.
• O2 partial pressure gradient creates
  electrochemical driving force.

Courtesy of Air Products, Inc.
10
GTL Process Overview

• GTL process divided into four main parts
• Convert CH4 to CO/H2 with membrane reactor.
• CH4 1/2O2 ? CO 2H2
• Convert CO/H2 to synthetic hydrocarbons
  (Fischer-Tropsch).
• nCO 2nH2 ? (-CH2-)n H2O
• Hydrocrack synthetic hydrocarbons to fuels
  (mainly diesel).
• Separate hydrocracked product into standard oil
  fractions.

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GTL Process Overview
Natural Gas
HP Steam
1.
Air
ITM Reactor (Syngas Production)
Turbine and Generator
Syngas
Excess Electricity
2.
Fischer-Tropsch Synthesis
H2
Synthetic wax
3.
Hydrocracking
Fuel Gas
Hydrocracked wax
Heavy Gas Oil
4.
Fractionation Tower
Naphtha
Kerosene
Diesel
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Key Equipment Details
Fischer-Tropsch Reactor
Hydrocracker
Distillation
ITM Reactor
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ITM Reactor Vessel

• Function
• Convert methane into synthesis gas.
• Design Details
• T 1650oF, P 300 psig
• Horizontal shell/tube reactor.
• 45,000 ft2 membrane area required.
• 2900 membrane tubes (OD 2 in, L 30 ft).
• Inconel Alloy for shell and tubes.
• Shell packed with 39,300 lbs nickel/alumina
  catalyst.

CH4 ½O2 ? CO 2H2 CH4 H2O ? CO
3H2
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Praxair ITM Reactor Model (2002)
Steam Reforming Catalyst Bed
ITM Membrane
Gottzmann et al. (US Patent)
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Praxair ITM Reactor Model (2003)
Methane Steam
Syngas
Steam Reforming Catalyst Bed
ITM Membrane
Air
Halvorson et al. (US Patent)
O2-depleted Air
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Entire ITM Section
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Key Equipment Details
Fischer-Tropsch Reactor
Hydrocracker
Distillation
ITM Reactor
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Fischer-Tropsch Reactor

• Function
• Produce long-chain hydrocarbons from synthesis
  gas.
• Design Details
• T 400oF, P 400 psig
• Slurry volume of 3400 ft3.
• 276,000 lbs cobalt/ruthenium catalyst required.
• Synthesis gas bubbled through bottom.
• 13 ft diameter ensures proper gas superficial
  velocity.
• Dynamic settler separates molten wax from
  catalyst particles.
• Stainless steel construction.

nCO 2nH2 ? (-CH2-)n H2O
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Fischer-Tropsch Products
Anderson-Schulz-Flory Distribution

• Mn (1 - ?) ?n-1
• Wn n (1 - ?)2 ?n-1
• For Co/Ru, ? 0.94

Mole Fraction Mn
Weight Fraction Wn
Diesel
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Entire Fischer-Tropsch Section
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Key Equipment Details
Fischer-Tropsch Reactor
Hydrocracker
Distillation
ITM Reactor
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Hydrocracker Reactor

• Function
• Cracks and isomerizes long-chains to shorter
  chains.
• Design Details
• Molten wax trickles down from top.
• Hydrogen-rich stream fed through bottom.
• T 725oF, P 675 psig
• Height 27 ft, ID 9 ft
• 30,000 lbs catalyst bed (0.6 Pt on alumina).
• H2/Wax 0.105 kgH2 / kgwax
• WHSV 2 kgwax / hour - kgcat.
• Stainless steel construction.

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Modeling the Hydrocracker

• From Hydrocracking Kinetic Model
• (developed by Pellegrini et al).
• Lumped hydrocarbon groups
• Hydrocracking reaction pathways
• Cracking and isomerization occur.

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Modeling the Hydrocracker

• From Hydrocracking Kinetic Model
• (developed by Pellegrini et al).
• MW drops along reactor length.
• Longer chains crack more quickly.
• Isomerization improves cold properties.

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Entire Hydrocracker Section
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Key Equipment Details
Fischer-Tropsch Reactor
Hydrocracker
Distillation
ITM Reactor
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Fractionation Tower

• Function
• Separates HC effluent into standard oil
  fractions.
• Design Details
• P 10 psig, Tcondenser 100oF, Tbottoms 725oF
• Feed preheated to 725oF and fed on bottom stage.
• Steam injected on bottom stage (0.5 lb / bbl
  bottoms)
• 16 Koch Flexitrays.
• 6 ft tray diameter, 2 ft spacing ? 40 ft height
• Diesel drawn off at tray 12.
• Kerosene drawn off at tray 8.
• Naphtha Fuel Gas in overhead.
• Heavy Gas Oil recycled to hydrocracker
• Sidedraws eliminate need for multiple towers.

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Entire Fractionation Tower Section
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Auxiliary Units
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Process Summary
Raw Materials Raw Materials Products Products
Natural Gas 1.95 MM SCFH Diesel 2,700 bbl/day
Kerosene 1,800 bbl/day
Naphtha 200 bbl/day
HGO 90 bbl/day
Electricity 10,500 kW
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US Proven Natural Gas Reserves (2003)

• For a 15 year plant life, required puddle size
  for plant is 0.263 trillion standard cubic feet
  (TSCF).
• 1.126 trillion SCF available in Ohio.
• U.S. Total 189 TSCF

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Profitability Analysis

• Membrane price unknown.
• Determine maximum membrane cost that still allows
  profitability.
• Criterion minimum IRR of 12

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Variable Cost Summary

• Natural gas price trumps other variable costs of
  operation.

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Assumed Product Prices

• Diesel Fuel 1.82/gal
• Kerosene 1.24/gal
• Naphtha 1.00/gal
• HGO 70/gal
• Electricity 6/kW-hr
• Assumed 15 higher than typical diesel due to
  low-sulfur and high cetane number.

Source US DOE
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Membrane Cost Tolerability

• Membrane costs that give IRR 12
• NG cost cannot exceed 6/MSCF.

PROBLEM Analysis assume no price
correlation between liquid fuels and natural gas!
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Energy Price Correlation

• Hydrocarbon prices historically show correlation.

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Energy Price Correlation

• Regression Analysis
• R 0.85
• Diesel changes 16/gal for every 1/MSCF change
  in NG.

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New Membrane Cost Tolerability

• Shallower slope ? less dependence on NG price.
• Maximum tolerable NG cost increases to
  12.5/MSCF.

Without price correlation
With price correlation
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Sample Cash Flow

• Assume NG costs 5/MSCF
• Assume 5 million membrane investment.
• NPV _at_ 12 is 59 million.

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Why not just burn it?

• Alternative Burn the natural gas across a gas
  turbine to create electricity.
• Using data from 2001 senior design project
  Combined-Cycle Power Generation by Beaver,
  Matamoros, and Prokopec.
• 290 MW possibility _at_ 57 total efficiency.
• Total BM cost of plant 150 million (22.6
  million membrane)
• Annual sales 145 million (68 million)
• Total annual costs 120 million (38 million)
• NPV _at_ 12 is -118 million (59 million)
• IRR is only 2.7

Not a competitive alternative!
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Conclusions

• Profitability is weakly dependent upon natural
  gas cost.
• ITM/GTL profitable at any NG price below
  12.5/MSCF.
• Current NG prices well-below this limit
  (6/MSCF).
• Membrane price unlikely to be a prohibitive
  factor.
• NG price of 5/MSCF ? 26 million max membrane
  cost allowed (572/ft2).
• BM Cost of plant without membrane is approx.
  22.6 million.
• Power plant unlikely to be competitive
  alternative.

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Recommendations

• Confirm ITM O2 flux rate
• Flux rate of 10 cm3/cm2-min assumed for design.
• Current research report values from 0.1 to 20
  cm3/cm2-min.
• Required membrane area strongly dependent on O2
  flux rate.
• Determine ITM stability and durability.
• Investigate low-sulfur, high cetane diesel price.

Recommend ITM/GTL plant construction once
membrane is commercialized.
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Acknowledgements

• Professor Leonard Fabiano
• Dr. Stuart Churchill
• Industrial Consultants
• Gary Sawyer
• Peter Schmeidler
• Adam Brostow
• William Retallick
• David Kolesar
• Henry Sandler
• John Wismer