The Aromatic Content of Synthetic Aviation Fuels – How Low Can You Go? David Anderson E-Futures DTC [email protected]
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The Aromatic Content of
Synthetic Aviation Fuels – How
Low Can You Go?
David Anderson
E-Futures DTC
Presentation Outline
Background
◦ Aviation Fuels
◦ Elastomers
Current problem and “solution”
Stress Relaxation Testing
◦ Experimental
◦ Results & discussion
Conclusions & Future Work
Aviation Fuels
Petroleum derived jet fuel e.g. Jet A1
◦ Wide range of hydrocarbons
◦ Heteroatoms
◦ Aromatics (12-20% volume)
Synthetic Fischer Tropsch fuels
e.g. GTL (gas to liquid)
◦ Mainly linear alkanes
◦ No aromatics
Synthetic Fuels – the future?
Much better properties
◦ Thermal stability
◦ Particulate emissions
◦ Combustion characteristics
Problems
◦ Lower density
◦ ELASTOMER COMPATIBILITY
Elastomers
What are they? Use in aviation
Amorphous polymer
Long, cross linked polymer
chains
Can be deformed but will
return to original shape
when stress removed
O-rings
Used as a seal in jet engines
and fuel system
3 types: nitrile,
fluorosilicone &
fluorocarbon
Current Problem
Nitrile o-rings sensitive to aromatic
content of the fuel
Solution
Current solution
◦ Any aviation fuel containing a synthetic
component must contain at least 8% volume
aromatics – Defence Standard 91-91
Ideal Solution
◦ Find an additive which promotes swelling of nitrile
o-rings at very low concentrations
Stress Relaxation
Property of all elastomers
Force required to maintain a set
deformation decreases over time.
Both chemical and physical processes
responsible for it
Two test methods:
◦ Tension
◦ Compression
Experimental
Nitrile o-rings
Two different aromatics
2, 4, 6 and 8% volume in GTL
Compare to pure GTL and a Jet A1 type
fuel
Toluene Naphthalene
Experimental
Elastocon Stress Relaxation Tester
Stress Relaxation profile
Ft – counterforce measured at time t
F0 – counterforce measured 30min after initial compression
Stage 3 – gradual decay
Stage 2 –
equilibrium period
Stage 1 – rapid decay
GTL toluene blends
Ft – counterforce measured at time t
F0 – counterforce measured 30min after initial compression
GTL naphthalene blends
Ft – counterforce measured at time t
F0 – counterforce measured 30min after initial compression
% Stress Relaxation
R(t) = [(F0-Ft)/F0] * 100
R(t) - % stress relaxation at time t (hours)
Ft – counterforce measured at time t (hours)
F0 – counterforce measured 30min after initial compression
Fuel Composition Toluene R(120) Naphthalene R(120)
GTL 19.11 19.11
GTL 2% aromatic 16.74 15.88
GTL 4% aromatic 16.54 13.25
GTL 6% aromatic 19.17a 10.88
GTL 8% aromatic 18.48a 10.29a
Jet A1:SPK 80:20 7.61 7.18
a – tests performed as part of different run and therefore potentially exposed to fuel for a
longer period of time before initial compression, effect of the test method
Cause of swelling
Nitrile Polymer
Hydrogen bond
TolueneNaphthalene
Conclusions
Stress relaxation testing is a useful tool
Emphasizes current concerns with nitrile
o-rings and synthetic fuels
Not all aromatics impart same degree of
swell/sealing capability
Toluene no major effect
Naphthalene shows some effect, but still
at ~8% volume.
Future work
Determine whether 8% is true minimum
(test commercial aromatic blends)
Investigate aromatic or other species
which could interact more strongly with
nitrile polymer – polar molecules?
Ensure any additive wouldn’t affect other
properties
Acknowledgements
Prof. Chris Wilson
Michael Liu
Technical team at the Low Carbon
Combustion Centre, Beighton.
Other O-ring Types
Fluorosilicone polymer
Fluorocarbon polymer
Weight Gain Toluene
Weight Gain Naphthalene
Percent Stress Relaxation
R(t) = [(F0-Ft)/F0] * 100
R(t) – percent stress relaxation at time t
Ft – counterforce measured at time t
F0 – counterforce measured 30min after initial compression
Fuel Composition R(120)
GTL 19.11
GTL 2% Naphthalene 15.88
GTL 4% Naphthalene 13.25
GTL 6% Naphthalene 10.88
GTL 8% Naphthalene 10.29
Jet A1:SPK 80:20 7.18
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