GEOL 115: The Earth’s Energy Resources

Now we know what compounds are present in natural gas and crude oil.  Crude oil is a complex solution that can exhibit considerable variability in its composition.  It would be useful if there were a quick way of evaluating the composition of crude oil.  There is a parameter that can do this; it is called the API gravity.

API gravity is a measure of the specific gravity of the oil and it is defined as

API Gravity = (141.5/specific gravity of oil at 60^oF)-131.5

The gravity of an oil is quoted in “degrees.”  An oil that had the same denisty as water would have an API gravity of 10^o. Higher numbers designate oils that are less dense than water; whereas oils with API gravities less than 10^o are denser than water.

API Gravity is an important parameter because it is related to the amount of gasoline that we can recover from a barrel of oil. High API gravity oils (light oils) contain more gasoline range hydrocarbons than do heavy oils. We’re interested in the amount of gasoline that we can get out of a barrel of oil because that is among the most valuable products.

Making gasoline from a barrel of oil is the job of a petroleum refinery. As the number of carbon atoms in a molecule increases, the boiling point of that molecule will also increase. We are familiar with this in everyday life. It takes a modest amount of pressure to keep butane as a liquid in a butane lighter, whereas gasoline (mainly pentane through decanes) is a liquid at room temperature. This relationship is used to distill the different products in crude oil.

The crude is heated in a furnace to 600 degrees C and largely vaporized. The crude oil vapor then passes up through a distillation tower. As the vapor rises different compounds begin to condense (higher boiling point, heavier compounds condensing first) and the condensed products are drawn off the tower. Larger molecules may then be sent to a reformer where the compounds are broken and reformed as smaller hydrocarbons.

Gasoline is a solution in which every compound has a slightly different temperature of preferred combustion. If proper care is not taken when making the gasoline the mixture will not combust evenly and will cause the engine to knock. The addition of cyclic and branched hydrocarbons to gasoline will increase its octane number and decrease the probability of engine knock. Ethanol will also increase octane number.

Early efforts to combat engine knock discovered that the addition of a small amount of tetraethyl lead eliminated engine knock. This process was adopted widely had some severe public health consequences. You can read a rather lurid history of lead use written in 1985 here. A long but interesting history of lead in gasoline can be found here (if you are accessing from a university computer or have a UNL ID). A study that relates lead and crime is here (that link will also require UNL ID or access from a university computer).