Does Crude Evaporate?
Date: Summer 2010
I Google "Does oil evaporate" and was referred here where
there is some explanation that oil does indeed evaporate. But my
question is, is it really possible that much of the BP oil spill
simply evaporated? And, if it did, will it come down as "rain"
somewhere or is this a case of what goes up doesn't necessarily have
to come back down?
While it is true that oil does have a very small but measurable
vapor pressure (the pressure caused by the liquid particles that
have turned into gaseous particles), the degree to which liquid oil
turns into gaseous oil under ambient conditions is very small that
we could not possibly wait for the oil to evaporate.
We know that liquid oil does turn into gas because we can smell oil.
The liquid has to turn into vapor in order to reach our noses and
give as a scent. However, the amount of oil vapor required to do
that must be very small and it would take a long time for the oil to
Have you ever cooked bacon in the house or gone into someone else's
house after they have cooked bacon (especially if they did not have
the hood on). It could be days and you could still smell it. This
means that when the oil vaporizes, it eventually settles - not
necessarily as rain, but as little, very tiny droplets over any
convenient surface. This is very easy to notice when frying fat
because the high heat allows quite a bit of vaporization, so we're
just speeding up the process (and so we can observe it) that
otherwise takes a long time under normal conditions.
Greg (Roberto Gregorius)
Crude oil is a horrendously numerous and variable mix of chemicals.
Some of it is large molecules that won't evaporate ever,
such as anthracene and carbon-soot of various kinds.
Dissolved in the heavy stuff is some light stuff,
small molecules the oil-companies would
find it easy to make gasoline from,
everything from methane and ethane (natural gas)
to pentane and heptane and benzene (gasoline)
to decane and heavier oils,
Some have functional groups that are nott all C's & H's.
Some might be unsaturated in H, depending on the oil bed.
Anyway, the lighter stuff evaporates,
so the crude remaining behind in the water shrinks smaller
and gets thicker and stickier (i.e., becomes tar)
then later it gets harder (i.e., becomes pitch).
Some of the medium-molecular-weight stuff also seeps out of the tar-blob
and is carried away in water or on nearby solid particles.
Bioactivity around a tar-blob is likely to
deplete the medium-weight chemicals near its surface,
then more diffuses out from inside.
Dust particles get wetted by the tar and stuck in it.
All these processes help the tar become pitch,
or otherwise dry up until it can crumble and
become an indistinguishable part of dirt.
At that point it might be nearly finished degrading.
The evaporated lighter molecules are mostly insoluble in water,
so they may not get caught by the falling raindrops
and washed to the ground.
They can float in the air for years.
Some of them will photo-react in the air and sunlight
and become smog, more-or-less.
This smog probably will not be concentrated enough
to notice like one does in cities.
Anyway, a degradation process that ends mostly in CO2 and H2O
happens to some of the volatiles, but not all, maybe not even most..
Some of it will take a pretty long time to degrade,
and might add to our greenhouse gasses
or float into the upper atmosphere and act as ozone depleters.
Some might adsorb onto surfaces of dust particles
which then can get caught by rain.
Some might get absorbed by the breathing of plants.
Some could exist as micro-droplets in the upper atmosphere
for decades or longer,
reflecting sunlight and causing (I think) global cooling
that partly counteracts global warming.
Probably many things happen, not just one disposal path.
That makes it complicated to predict the results.
The part that evaporates can be something like 50%,
The rest must be dealt with by settling into geologic sediment
(unhealthy because it will be oxygen-starved sediment
with a very different and less desirable ecosystem in it)
or by being digested by micro-organisms.
That's pretty slow.
Crude oil contains many types of hydrocarbon chains.
The lighter hydrocarbons can and do evaporate into the atmosphere as a
The heavier ones float on the surface or fall to the bottom of the sea.
Different types of crude oil have different percentages of the types of
The hydrocarbons that evaporate evaporate as a vapor and disperse in the
There might be particles of hydrocarbons in rain, but there won't be an oil
Unless you are standing directly under a gusher.
Despite what you find in the news, oil is not a single substance, it is a
complex mixture of hydrocarbons. Some have low molecular weight and can
evaporate at ocean water temperatures. Others have high molecular weight
and are non-volatile at ocean temperatures. Oil is "refined" to convert
high molecular hydrocarbons into low molecular weight hydrocarbons. That
conversion is known as "cracking", which you also may wish to use as a
search term. "Crude" oil is what comes out of the ground initially.
Generally "crude" oil is composed of the high molecular fraction and does
not evaporate at ocean temperatures. As an example,
gasoline, charcoal lighter fuel, and similar refined hydrocarbon, are low
molecular weight, and as you know evaporate easily. However, motor oil and
black top are high molecular weight and do not evaporate easily. Both types
are not soluble in water so they are not very soluble in water (that is,
rain). Be assured, if the oil would simply evaporate the problem would be a
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Update: June 2012