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Name: Mark
Status: other
Grade: other
Location: MD

What happens to crude oil, or petroleum when it freezes at extreme temperatures? Does the viscosity or usability, of the oil change if it crystallizes?

The viscosity of petroleum definitely changes with temperature. As crude oil warms up, its viscosity decreases (gets thinner). When cooled, its viscosity increases (gets thicker). It's not a linear relationship though -- the changes in the oil that occur are quite complicated because petroleum is a complex mixture of many components.

Each component can be a liquid, a solid, or a gas depending on the temperature and pressure. The "lighter" components (meaning lower molecular weight) will become gases a lower temperatures compared with the "heavier" components. For example, propane is one fraction of crude oil that is a gas at room temperature and pressure. Propane can be liquefied at high pressure or very low temperature. A heavier fraction of petroleum like asphalt is very thick at room temperature, but asphalt flows much more easily when heated.

When the asphalt cools, however, it does not 'crystallize' the way water might -- first, it is a complex mixture, and forms a complex structure that is a little bit solid and a little bit liquid (it is called a 'colloid' -- but that is probably outside the scope of this question). Unlike ice, which has a well-defined structure, asphalt's structure is more disordered, not a 'crystal'.

Also, if you increase the temperature of the oil, volatile components will evaporate off; this is the principle that refineries use to separate the components. Components may also break down chemically at high temperature. Refineries use heat and pressure to chemically change components of petroleum into more useful materials.

In general, refineries can be designed to separate all kinds of petroleum into its useful components. If petroleum were 'frozen' -- if it were cooled to a point where it would not flow -- it could still be warmed and separated.

Hope this helps,
Burr Zimmerman

Hi Mark,

It all depends on what petroleum derivative you are cooling. Crude oil becomes very viscous as you cool it. The cooler it gets, the "thicker" it gets. Much below freezing, crude oil becomes as thick and sluggish as tar.

If you cool diesel oil much below 0°F, the waxes dissolved in it start precipitating out, and the result is a whitish mass that will not flow at all. Some grades of motor oil at the same temperature become as thick as molasses. Gasoline on the other hand, being a lighter "fraction" of crude oil, contains no waxes and can be cooled to very low temperature with only moderate changes in viscosity.

No petroleum product actually "crystallizes" in cold temperature, but they do become very much more viscous, and in some cases, like diesel fuel mentioned above, the wax content separates out. The much greater viscosity of oils can seriously affect their function in an engine, and the separation of wax and thickening of diesel fuel in the cold causes serious clogging of fuel filters and injectors.

Bob Wilson

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