Saturday, January 22, 2011

Getting at the gasoline, part three

Crude oil contains many components, all of them useful. Gasoline is one of those things. These wonderful things are separated from each other by heating the crude oil to the various boiling temperatures of the components, then recovering them by cooling the vapors back to liquid state and collecting those new liquids.

Some of the components are very liquid, some are thicker, some are almost solid, or actually solid. The crud ones that are solid/almost solid require the highest boiling temperature to boil them.

Below is a list of the components of crude oil and show their boiling temperatures. Somewhere in this discussion I should work in an explanation of carbon chains, but I'm not going to do that right now. Here is a list (heavy on the bottom and the light stuff on the top.) Click to enlarge the chart:

You have most likely passed by an oil refinery and noticed the tall distilling towers. Basically, the crude oil supply is piped into the bottom of those towers up to a certain height and then heated (usually by steam). The bottom is hottest and so the lighter parts "boil" off and the vapors go upwards, of course. As the vapors get higher and higher in the tower, they get cooler and cooler since they are farther from the heat source at the bottom. When the various vapors get high enough (cool enough) they turn back into liquids. The various liquids are collected into trays at the various heights and drawn off out of the tower and piped to separate containers/holding tanks. One for gasoline. One for Diesel. One for kerosene. One for motor oil. And so forth. The "trays" at the various heights all have perforations ("bubble caps" like a soda pop bottle with a loose cap?) in them so the lighter components which are still vapor can keep going up higher into the tower until they too cool enough to liquify. Each catch tray is located at the height in the tower that is at the right temperature. See?

I will put up another picture to show this better. But the main thing to remember is that the entire crude oil is being boiled at the bottom of the tower and that means everything but the very heaviest components boil off and rise up the tower. When they cool enough to pass back through their boiling temperature, they liquify again, at different heights in that distilling tower. So thats how crude oil gets separated into the various parts we use for many different things.

Oil refinery distillation towers (columns) and holding tanks

You can go here to see an animation of the distillation process. (Go to the link and then scroll down toward the bottom of the page to see the flash animation.) Please do go look, because it will give you a much better understanding of the process than I can describe in words here. This is located at a very interesting site called "How Stuff Works". If you have an interest in more details of how crude oil is refined, stay on that site and read for hours! I can't tell you how much I like that website or how highly I commend it to you.

Do you know what octane is? Do you know what "Cat Cracked" means? If you visit this space tomorrow, you will know these things and even more!



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