safe storage of ethyl alcohol

Kragen Javier Sitaker kragen at canonical.org
Mon Jul 26 03:37:01 EDT 2010


I keep 96% pure ethyl alcohol on hand in the house for use as a fuel
and for cleaning.  Here are the results of an experiment about its
safe storage.

Applications of alcohol
-----------------------

Ethyl alcohol is useful for several purposes:

- a cooking fuel
- an engine fuel
- a mild solvent for removing greases and adhesives
- a thinning agent to spread oils more evenly (e.g. in Pam)
- a drying agent to reduce the surface tension of water so that it
  will form a thin film that dries up faster
- an antidote to ethylene glycol poisoning
- in a spray bottle to shoot fireballs across the room at
  mosquitoes. (Do not try this if you live in combustible housing.)
- as a mild disinfectant
- to remove water from fuel tanks, although I think for that you may
  need purer alcohol than what I have.
- setting your hands on fire without injuring yourself, as a party
  trick. (This is less dangerous if you mix some water into the
  ethanol.)

Some people drink that shit for fun, too, but that’s just crazy. It’s
much less toxic than many alternative fuels (methanol, most gasolines,
isopropyl alcohol) but it’s still toxic. It only takes a few ounces of
it to kill even a full-grown adult.

Background
----------

Any liquid with a combustible vapor will burn if it’s warm enough that
it emits enough vapor to sustain a flame. The temperature at which a
liquid emits that much vapor is called its “flashpoint”. Liquids that
can burn at some temperature are called “combustible”; liquids whose
flashpoints are below room temperature are called “inflammable” or, in
modern pidgin English, “flammable”; you can generally ignite them with
a single spark. OSHA’s definition of “flammable” uses 100°F as the
definition of “room temperature”.

Ethyl alcohol is inflammable. Its flashpoint is 13°C, only a little
higher than isooctane’s 4.5°C. This is a very useful property when
you’re trying to get your alcohol stove lit in the winter, but it also
makes it dangerous.

Ethyl alcohol is miscible with water in all proportions. This turns
out to be very useful when you have a spill and you want to keep it
from accidentally catching on fire before you can clean it up. The
more water in the mixture, the higher the flashpoint. So, if you add
some water to the spill, although it makes the spill bigger, it can
also keep it from being inflammable.

I tried to calculate once how much water you could mix with ethyl
alcohol and have the resulting mixture remain combustible at
all. Water, being an ash, isn’t combustible, and when you try to burn
a mixture of ethyl alcohol and water, you lose some energy to
evaporating the water. If there’s enough water in the mixture, you’ll
lose more from the evaporation than you gain from the combustion. I
tried to calculate this based on the assumption that the mixture had
to evaporate as a whole, and I got a ratio of about 14 parts water to
one part alcohol.

But actually, when you burn a mixture of alcohol and water, the
alcohol evaporates faster than the water, and the alcohol has a lower
latent heat of evaporation than the water, so you might need even more
water than that to keep the mixture from being combustible at all. And
I don’t know how to calculate that.

I searched the web for information on flashpoints of ethanol-water
mixtures to no avail.

Experimental procedure
----------------------

So I tried the experiment myself. I used a small ceramic vessel to mix
the liquids and a bottlecap to measure them. In between experiments, I
cooled it back down to the temperature of the tap water. I used a
cigarette lighter to try to light the mixture. Eventually I started
stirring the mixture, because I was concerned it might not mix
instantly without any help, reducing the reproducibility of my
experiment. At some point I also started trying to burn the mixture
soaked into a string as well as in the ceramic container, on the
theory that maybe I could get that to light more easily.

I didn’t measure the temperature that all this was happening at, which
is obviously kind of a crucial omission.

The alcohol has mostly been in sealed bottles, but it may have
absorbed some water from the air when the bottles are open.

Results
-------

1 bottlecap water to 1 bottlecap alcohol: burned easily at room
temperature.

2 parts water to 1 part alcohol: burned at room temperature.

4 parts water to 1 part alcohol: would not burn at room temperature.
When I put it into a tin can and heated it over the stove burner,
after heating it to boiling, I could light it and it sustained a
flame.

3 parts water to 1 part alcohol: would not burn at room temperature

2 parts water to 1 part alcohol: would not burn at room temperature,
contradicting my earlier result. Must have been sloppy.

1 part water to 1 part alcohol: burned easily at room
temperature. Also burned on the string, not as easily as the purer
alcohol. However, the string didn’t burn after the alcohol burned off,
as it had with the pure alcohol. This is probably often a desirable
feature.

3 parts water to 2 parts alcohol: would not burn at room temperature,
further contradicting my earlier result with the 2:1 ratio.

Conclusion
----------

My tentative conclusion is that, to lower the flashpoint to (roughly
defined) room temperature, you need somewhere between 40% and 50%
alcohol. So if you mix two or more parts of water into spilled
alcohol, it will generally stop being flammable.

Improvements to procedure
-------------------------

A better experiment would gradually heat up the mixture with a
continuous ignition source (one of those electrical sparking things?)
until it caught, and then extinguish it and measure the
temperature. That would give me a little more information about the
different runs of the experiment and give me more confidence that any
particular result wasn’t a one-off error.

Application
-----------

I have a clever scheme for storing fuel alcohol in an inherently safe
fashion: float the alcohol bottles in a larger tank of drinking water,
which is itself a crucial emergency supply. Most in-storage accidents
that would result in an alcohol spill would mix it with the water,
which would keep it from creating an inflammable hazard. (For example,
or the bottle falling and breaking, or something heavy falling on the
bottle and puncturing or crushing it, or if your emergency consists of
a military invasion of your city, a stray bullet puncturing the
bottle.)

Based on this crude experiment, I think that I could probably float
each liter of alcohol in three or four liters of drinking water and be
inherently safe. Two thirds of an ounce of alcohol (20mℓ) is enough to
cook a dish with Mark Jurey’s “penny stove”, so a liter is 50 dishes —
if you cook five dishes a day, one liter is ten days’ worth of cooking
fuel.

I did this experiment after having my second alcohol spill in a
week. The first time, I knocked over the bottle in the bathroom, and
it wasn’t properly capped. The second time, I dropped a small squirt
bottle of it on the floor because I’d forgotten that I hadn’t yet
screwed the lid on. Both times, I dumped water on the spill to make it
safe.

[I wrote this some time ago and haven’t had any alcohol spills since.]


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