- Longer-chain hydrocarbons generally have higher boiling points. During fractional distillation, the oil is heated and drawn into a distillation column, which is basically like a still used to distill alcohol. The vapor cools as it ascends the column, and different fractions, or chains of hydrocarbons, condense at different temperatures. Trays in the distillation column collect the condensate so it can be recovered and processed further.
- Butane and smaller hydrocarbons ascend farther than any of the other fractions -- all the way to the top of the column. Light straight naphtha is the next most volatile fraction, condensing between 90 and 200 degrees Fahrenheit. Naphtha condenses further down the column, and below that kerosene is recovered. Home heating oil and diesel fuel condense below kerosene in the 460 to 650 degree range, while heavy gas oil condenses further on down, from 650 to 1000 degrees.
- The heaviest fraction is the residual fuel oil, which does not ascend the column at all. This residual oil is used for paraffin waxes and tar. It's also broken down through a process called cracking to yield shorter-chain hydrocarbons for gasoline. The naphtha and light straight naphtha are used for gasoline, and the kerosene and home heating oil find use as fuel. The butane and other very light components are sent for gas processing; they too can find use as fuel.
- Crude oil comes in both sweet and sour flavors. The sweet stuff has less than 0.5 percent sulfur content, while the sour stuff has more than 0.5 percent sulfur. Refineries prefer the former, since it has less sulfur to remove and a greater fraction of shorter-chain hydrocarbons that make gasoline. It's important to note that crude contains a mere 25 to 35 percent gasoline on average, so distillation alone is not enough; refineries have to increase the amount of gasoline by breaking down larger hydrocarbons through cracking.
Boiling
Fractions
Next Steps
Considerations
SHARE
