1. ASTM D6422 – what’s happening?
Determining water tolerance of gasoline-ethanol-water mixtures

2. Outline Phase equilibria in ethanol-Gasoline blends
Phase stability in Hydrous Ethanol-Gasoline mixtures
Suggestions for improvement

3. Introductory remarks Let’s focus on ethanol
Analysis by simulation of phase equilibria
Ternary mixtures
Gasoline – water - ethanol
Benzene – water – ethanol
Hexane – water - ethanol
Gasoline as single component
All calculations done at 68 °F (20 °C)
Experimental work
Observations at TU Delft
Standard tests at SGS (independent test laboratory)

4. Ternary phase diagram: introduction
Weight Percent Ethanol
20%
40%
60%
80%
ETHANOL
WATER
20%
40%
60%
80%
Weight Percent Water
GASOLINE
20%
40%
60%
80%
Weight Percent Gasoline

5. Phase behavior of gasoline-ethanol-water mixtures
Determining water tolerance involves detecting incipient phase separation. In the region indicated this is very difficult to observe and easily leads to false conclusions.
The location of the binodal curve depends on:
gasoline composition
presence of other oxygenates
temperature
The reasons for this are:
Low free energies of mixing
Meta-stability of phases
Similar properties of phases
Weight Percent Ethanol
20%
40%
60%
80%
ETHANOL
density: how well it separates into lower - and upper layer
surface tension: drop coalescence
refractive index: observability
WATER
20%
40%
60%
80%
Weight Percent Water
binodal curve
GASOLINE
20%
40%
60%
80%
Weight Percent Gasoline

6. Experimental work
It has been experimentally verified that the phase boundary cannot be well observed by visual inspection.
gasoline
water
ethanol
In this example the mixture was well in the two-phase region before haziness persisted and a second liquid layer started to settle. After several hours, the phases were still not clear and the compositions (see graph) were not at equilibrium.

7. When does phase separation occur ?
Free Energy: the arbiter for change
A spontaneous change will only occur if heat can be released to the environment
The Free Energy is the heat that a changing system releases into the environment
A system changes until it obtains minimum Free Energy
However there may be obstacles for spontaneous change to take place

8. Free energy surface of hydrous ethanol-gasoline blends
water
gasoline
water
ethanol
Large free energy gain at low ethanol content
Very low free energy gain at low water concentrations

9. Free energy and phase stability
Mixture is instable:
two liquid phases give lower free energy
Mixture is stable (single liquid)
two liquid phases do not yield lower free energy
If free energy surface is convex (from top)
composition fluctuation lowers free energy
component separation self-sustaining
spinodal decomposition
If free energy surface is concave (from top)
composition fluctuation increases free energy
phase separation not spontaneous
nucleation and growth
meta-stability

10. Phase stability in gasoline-ethanol-water mixtures
Weight Percent Ethanol
20%
40%
60%
80%
ETHANOL
WATER
20%
40%
60%
80%
Weight Percent Water
stability
binodal curve
phases are
homogeneously
mixed
spinodal curve
meta-stability
instability
GASOLINE
20%
40%
60%
80%
Weight Percent Gasoline

11. Spontaneity of phase change
Free energy of phase change (instability)
Free energy curvature of phase change (meta-stability)
Creation of interfacial area
work required due to surface tension > free energy of phase change
phase separation may be delayed or prevented
Other obstacles
diffusion: low concentration gradients, slow mass transport
structured solutions (strong hydrogen bonds at low temperatures) (energy of dissociation > free energy of phase change)
Composition has large effect on binodal & spinodal curve
paraffinic - aromatic content
presence of other oxygenates
ethers, higher alcohols
co-solvency

12. Consequences for the test method
Since:
visual observation of phase separation can be difficult
phase separation may be delayed or even prevented
changes occur gradually with temperature
phase homogeneity plays an important role
the mixtures may be rather hygroscopic
a good test method requires:
advanced detection of phase behavior
good mixing
no cylindrical vessel (bad axial mixing)
controlled stirring
no “dead” zones
minimize water uptake (at low temperatures)
re-define test method’s objective (performance test)

13. Proposed observation of phase behavior
At the phase boundary, the laser light will start to scatter.

14. HE Blends B.V. Process Design Center B.V. Catharinastraat 21f
P.O. Box 7052
NL-4800 GB Breda
The Netherlands
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