nvestigators estimate the storage tank was nearly empty and contained only 14 barrels of crude oil. One barrel contains 42 gallons of crude oil. Crude oil is a complex mixture of hydrocarbons, which are molecular compounds made of carbon and hydrogen atoms. The precise composition of crude oil varies, depending on where it is from. Crude oil itself is neither explosive nor very flammable. It is not easy to get crude
oil to burn because it is so thick, unless its temperature is
raised to 140 °F or more.
From crude to refined
In order for crude oil to be useful, it has to be refined. The main step in refining crude oil is fractional distillation.
In this process, the raw petroleum is heated in a boiler with high-pressure steam to temperatures of about 1,100 °F. At this temperature, most substances in petroleum go into the vapor phase. The vapor then enters the bottom of a long column, called a fractional distillation column. As the temperature rises, the lightest and most volatile compounds in the petroleum mixture begin to vaporize.
These compounds are separated according to the temperature at which they go from liquid to gas, also called their boiling point. The refining process does not separate each compound in crude oil, but produces mixtures of compounds, called fractions. Hence the name of the technique — “fractional distillation.”
The smaller, lighter molecules that have the lowest boiling points are drawn off the top of the tower as gases. Fractions with larger molecules and higher boiling points condense to liquid lower down the tower. At the bottom, the heaviest and least volatile molecules remain.
As the crude oil sat in the Mississippi storage tank, something similar happened. The lighter and more volatile parts of the mixture vaporized, filling the upper part of the storage tank with a dangerous mixture of hydrocarbon gases. Given the right conditions, this vapor could react explosively.
The three ingredients necessary for most fires are fuel, heat, and oxygen. In this case, the volatile hydrocarbons provided the fuel, but what about the heat and oxygen?
Investigators were not able to determine the source of the heat that set off the explosion. It may have come from a discarded match or a lighter, if the students were smoking or playing with fireworks. Another possibility is a spark from, say, a tool hitting the metal tank or railing. Any of these heat sources might have set off the explosion.
An explosive atmosphere
In order for an explosion to occur, the hydrocarbon-to-air ratio has to be 1%–6% (for example, 1% fuel and 99% air). This ratio varies depending on the specific hydrocarbons that are present.
Gasoline is one component of crude oil. It, too, is a mixture of hydrocarbons, ranging from 4 to 12 carbon atoms per molecule, but we can represent it as octane (C8H18) because it is in the median range of hydrocarbons making up gasoline.
If there is too little oxygen, the fuel will not burn or will not burn completely. Too much oxygen, and there may not be enough fuel to sustain combustion. Somehow, enough air got into the tank to make the fuel burn explosively. For instance, holes or leaky seams may have allowed in air, or a hatch may have been left ajar.
It’s one thing for a fuel to burn, but another for it to explode. What are the characteristics of an explosion? Explosions result from a rapid increase in volume and a large release of energy. Gasoline packs a lot of energy; it contains 10 times the energy of trinitrotoluene, a common explosive. But what about the increase in volume?