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MEMORANDUM
SUBJECT: Water Phase Separation in Oxygenated Gasoline
- Corrected version of Kevin Krause memo
FROM: David Korotney, Chemical Engineer
Fuels Studies and Standards Branch
TO: Susan Willis, Manager
Fuels Studies and Standards Group
On May 26, 1995, Kevin Krause finalized a memorandum
describing the conditions under which water phase separation will
occur in oxygenated gasolines. Recently, several errors were
discovered in that memorandum. I have made the necessary
corrections, and now resubmit the complete text of Kevin's memo
for your review and approval.
Introduction
With the introduction of oxygenated gasoline came the
concern of water phase separation. Water in gasoline can have
different effects on an engine, depending on whether it is in
solution or a separate phase, and depending on the type of engine
being used. While separate water phases in a fuel can be
damaging to an engine, small amounts of water in solution with
gasoline should have no adverse effects on engine components. If
precautions to prevent water from entering the fuel system are
taken, water phase separation will likely not occur.
Discussion
Oxygenated fuels usually contain either ethanol or methyltertiary-
butyl-ether (MTBE). Other possible oxygenates include
ethyl-tertiary-butyl-ether (ETBE), tertiary-amyl-methyl-ether
(TAME), and tertiary-butyl-alcohol (TBA). Chemically, ethanol
and MTBE behave differently. Ethanol, for example, will readily
dissolve water, and is considered infinitely soluble in water.
MTBE, on the other hand, has little affinity for water, and can
only be dissolved in water to a content of 4.3 volume percent (at
room temperature). Therefore, ethanol/gasoline blends can
dissolve much more water than conventional gasoline, whereas
gasoline/MTBE blends act very much like conventional gasoline
when in the presence of water.
-40 -20 0 20 40 60 80
0.2
0.3
0.4
0.5
0.6
Temperature (F)
Water Content (Vol. %)
Water Tolerance of
90% Gasoline/10% Ethanol Blends
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Figure 1
Since ethanol and water readily dissolve in each other, when
ethanol is used as an additive in gasoline, water will actually
dissolve in the blended fuel to a much greater extent than in
conventional gasoline. When the water reaches the maximum amount
that the gasoline blend can dissolve, any additional water will
separate from the gasoline. The amount of water required (in
percent of the total volume) for this phase separation to take
place varies with temperature, as shown in Figure 1. As an
example, at 60 degrees F, water can be absorbed by a blend of 90%
gasoline and 10% ethanol up to a content of 0.5 volume percent
before it will phase separate. This means that approximately 3.8
teaspoons of water can be dissolved per gallon of the fuel before
the water will begin to phase separate.
Since MTBE has much less affinity for water than does
ethanol, however, phase separation for MTBE/gasoline blends
occurs with only a small amount of water, as shown in Figure 2.
A blend of 85% gasoline and 15% MTBE can hold only 0.5 teaspoons
at 60 degrees F per gallon before the water will phase separate.
For comparison, one gallon of 100% gasoline can dissolve only
0.15 teaspoons water at the same temperature. These figures are
far below the 3.8 teaspoons which will cause phase separation in
the 90/10 ethanol blend.
0 20 40 60 80
0
0.02
0.04
0.06
0.08
Temperature (F)
Water Content (Vol. %)
100% Gasoline
15% MTBE
Water Tolerance of
100% Gasoline and 85%/15% MTBE Blends
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Figure 2
Water can enter gasoline engines in two ways: in solution
with the fuel or as a separate phase from the gasoline. Water in
solution operates as no more than an inert diluent in the
combustion process. Since water is a natural product of
combustion, any water in solution is removed with the product
water in the exhaust system. The only effect water in solution
with gasoline can have on an engine is decreased fuel economy.
For example, assuming a high water concentration of 0.5 volume
percent, one would see a 0.5 percent decrease in fuel economy.
This fuel economy decrease is too low for an engine operator to
notice, since many other factors (such as ambient temperature
changes, wind and road conditions, etc.) affect fuel economy to a
much larger extent.
Water as a separate phase, however, can have differing
effects on gasoline engines, depending on whether the engine is
two-stroke (generally, smaller engines) or four-stroke (generally
automobile engines). In the case of conventional and MTBEblended
gasolines, when a water phase forms, it will drop to the
bottom of the fuel tank, and can therefore be drawn into the
engine by the fuel pump. Therefore, large amounts of water will
prevent the engine from running, but no engine damage will
result.
Phase separation in ethanol-blended gasoline, however, can
be more damaging than in MTBE blends and straight gasoline. When
phase separation occurs in an ethanol blended gasoline, the water
will actually begin to remove the ethanol from the gasoline.
Therefore, the second phase which can occur in ethanol blends
contains both ethanol and water, as opposed to just water in MTBE
blends and conventional gasoline. In the case of two-stroke
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engines, this water-ethanol phase will compete with the blended
oil for bonding to the metal engine parts. Therefore, the engine
will not have enough lubrication, and engine damage may result.
In the case of four-stroke engines, the water-ethanol phase may
combust in the engine. This combustion can be damaging to the
engine because the water ethanol phase creates a leaner
combustion mixture (i.e. air to fuel ratio is higher than ideal).
Leaner mixtures tend to combust at higher temperatures, and can
damage engines, particularly those without sensors to calibrate
air to fuel ratios.
Phase separation, however, generally only occurs when liquid
water (as opposed to water vapor) is introduced to the fuel
system. If tank vents are left open, either in the engine being
operated, or at a fuel distribution station, water can enter the
fuel system in the form of rain (or spillage, etc.) or through
the air in the form of moisture. Also, since conventional
gasoline absorbs very little water, there is often a layer of
water present at the bottom of a filling station tank normally
used to store conventional gasoline (water is more dense than
gasoline, and will therefore sink to the bottom). Before an
oxygenated gasoline is added to such a storage tank for the first
time (particularly ethanol-blended fuels), this water must be
purged from the tank to prevent the water from removing any
ethanol from the fuel.
Since the solubility of water in both gasoline and air
decreases with a decrease in temperature, water can enter a fuel
system through condensation when the atmospheric temperature
changes. For example, assume a tank containing conventional
gasoline contains only one gallon of fuel. Assume also that it
is closed while the outside temperature is 100 degrees F with a
relative humidity of 100 percent. If this tank is left sealed
and the temperature drops to 40 degrees F, water will likely
condense on the inside of the tank, and dissolve in the fuel. In
order for enough water to condense from the air to cause
gasoline-water phase separation, however, there must be
approximately 200 gallons of air per gallon of fuel over this
temperature drop (100 to 40 degrees). Since oxygenated fuels can
hold even more water than conventional gasoline, it is even more
unlikely that enough water will condense from the air to cause
gasoline-water phase separation.
Another way water can enter gasoline is through absorption
from the air. Water, in the form of water vapor, can dissolve in
gasoline. The more humid the air, the faster the water vapor
will dissolve in the gasoline. Due to chemical equilibrium,
however, assuming a constant temperature, phase separation will
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never occur if the only source of water is from the air. Only
enough water to saturate the fuel can enter the system, and no
more. Water vapor, however, dissolves in gasoline very slowly,
even at very high humidity. For example, at a constant
temperature of 100 degrees F and relative humidity of 100%, it
would take well over 200 days to saturate one gallon of gasoline
in an open gasoline can (assuming the only source of water is
water vapor from the air). Water absorption from the air is far
slower at lower temperatures and humidities. (At a temperature
of 70 degrees and relative humidity of 70%, it would take over
two years to saturate one gallon of conventional gasoline in the
same gasoline can.) Again, oxygenated gasolines can hold more
water than conventional gasoline, and would therefore take much
longer to saturate with water.
Conclusion
Water phase separation in any gasoline is most likely to
occur when liquid water comes in contact with the fuel. (Water
in the form of moisture in the air will generally not cause phase
separation.) Water which is in solution with gasoline is not a
problem in any engine, but as a separate phase it can prevent an
engine from running or even cause damage. Since oxygenated
gasolines, however, can hold more water than conventional
gasoline, phase separation is less likely to occur with
oxygenates present.
For any gasoline, simple precautions to prevent phase
separation from occuring should be taken. First of all, gasoline
should not be stored for long periods of time, especially during
seasonal changes which usually have large temperature changes
associated with them. (For both oxygenated and conventional
gasolines, gumming can also occur which is detrimental to any
engine.) If it is unavoidable to store gasoline for a long
period of time, one should be sure that the tank if full to
prevent condensation of water from the air, and the addition of a
fuel stabilizer should be considered. Lastly, care should be
taken not to allow water into the fuel sytem while filling fuel
tanks or operating the engine -- in the form of rain or a spash,
for example.
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References
"Alcohols and Ethers: A Technical Assessment of Their
Application as Fuels and Fuel Components." API Publication
#4261, July 1988.
Douthit, W.H., B.C. Davis, E.D. Steinke, and H.M. Doherty.
"Performace Features of 15% MTBE/Gasoline Blends." SAE Technical
Paper Series #881667, October 1988.
"Fuel Ethanol." Technical Bulletin, Archer Daniels Midland
Company, September 1993.
"Storing and Handling Ethanol and Gasoline-Ethanol Blends at
Distribution Terminals and Service Stations." API Recommended
Practice #1626, First Edition, April 1985.
"The Use of Oxygenated Gasoline in Lawn & Garden Power Equipment,
Motorcycles, Boats, & Recreational Equipment." Downstream
Alternatives, Inc. Document #941101, November 1994.
"Use of Oxygenated Gasolines in Non-Automotive Engines." Chevron
Technical Bulletin, December 1992.