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

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

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)

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

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

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.

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

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

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

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

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

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)

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

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