Understanding Petroleum

In oil refineries crude oil is distilled into fractions that provide our major fuels and other petroleum products. Photo: BP. Click on image on enlarge.

This week I'll take a detour from practical energy-conserving solutions to take a look at oil (petroleum)--the fluid that has powered our automobile-based society. Be prepared for some new terminology and a little bit of chemistry!

The name petroleum comes from the Greek "petra" (rock) and the Latin "oleum" (oil). Most scientists believe petroleum is derived from biomass (algae and zooplankton) that was present in bodies of water millions of years ago. This biomass settled to the bottom of lakes and oceans and, over geologic time in oxygen-deprived conditions, become compressed by sediments that were constantly laid down in ancient lakes and seas. High pressure and heat transformed this biomass into the mix of hydrocarbons that we refer to as crude oil and natural gas. Along with coal and natural gas, petroleum is referred to as a "fossil fuel," owing to this biological origin. There is another theory that petroleum is not derived from living organisms, but rather from some other, "abiogenic" process. This theory was advanced in the 1950s by several Soviet scientists, and it has had a few proponents in the U.S. and Europe. This school believes that petroleum is inorganic and exists within the Earth's core. Evidence supporting the other theory (biological origin of petroleum) comes, in part, from certain markers found in petroleum that are also found in algae.

Petroleum (crude oil--which is sometimes defined to include natural gas) is a complex mix of hydrocarbons (carbon- and hydrogen-containing compounds), including--most prominently--alkanes like pentane, hexane, and octane. These are linear molecules with hydrogen atoms attached to those carbon atoms, the name indicating the number of carbon atoms (five, six, and eight, respectively, in the examples listed).  

Along with these fairly straight-forward alkanes are more complex substances like benzene, a six-carbon compound in which the carbon atoms are arranged into a ring. Compounds containing these benzene rings are referred to as "aromatic," and because they are biologically reactive, many such chemicals are carcinogenic or otherwise hazardous.

Just a few years ago, gasoline hitting $3.00 per gallon was front-page news. Photo: Alex Wilson. Click on image on enlarge.

The more carbon atoms found in the hydrocarbons that comprise crude oil, the heavier the oil. The crude oil "distillation" process separates these many hydrocarbons into collections of similar-sized compounds that have different properties--these groups are called "distillate fractions." The distillation process is what petroleum refineries do, and the principle is fairly simple. The crude oil is heated and first the lightest molecules boil (become gaseous) and are collected. Then heavier and heavier fractions boil and are collected.

The distillate fraction from pentane to octane (hydrocarbons with five to eight carbon atoms) is used to produce gasoline. The distillate fraction from nonane to hexadecane (9 to 16 carbons) is a heavier mix that is made into kerosene, jet fuel, diesel, and heating oil. Paraffin wax and asphalt are heavier mixes still, with paraffin made of molecules containing about 25 carbon atoms and asphalt made from hydrocarbons with 35 or more carbons.

Hydrocarbons that are lighter than pentane are usually gasses at atmospheric pressure. Propane is a three-carbon-chain hydrocarbon that has to be kept under pressure to maintain its liquid state, and when a valve is opened--on our propane grill, for example--the liquid immediately vaporizes and can be burned to grill those burgers.

Natural gas is lighter still. It's mostly methane (a single carbon atom with four hydrogen atoms), plus some ethane (two carbon atoms with six hydrogen atoms), and its boiling point is so low that it has to be kept super-cold to maintain a liquid state. That's why "liquefied natural gas" (LNG) terminals are controversial; if a LNG tanker were to rupture, the natural gas would quickly vaporize and be highly explosive.

The separation of crude oil into its constituent parts is a lot more complex than this, but that should give you a general sense of what's going on when you drive down the New Jersey Turnpike or live in the Gulf Coast and see all those refineries with tall stacks rising from them.

Over the next few weeks, we'll look at the history of oil and some issues relating to its use and supply.

In addition to this Energy Solutions blog, Alex contributes to the weekly blog BuildingGreen's Product of the Week, which profiles an interesting new green building product each week. You can sign up to receive notices of these blogs by e-mail--enter your e-mail address in the upper right corner of any blog page.

Alex is founder of BuildingGreen, Inc. and executive editor of Environmental Building News. To keep up with his latest articles and musings, you can sign up for his Twitter feed.