Crude oil, as it comes from underground, is a mixture of hydrocarbon –compounds.
The basic purpose of a refinery is to separate and transform these various hydrocarbon groups so that they may be used,
combined or further treated to create the thousand of products made from petroleum

One of the fundamental processing units is the fractionating tower (shown at right in the drawing)
because hydrocarbon vapors and liquid goes into the tower; the liquid residual oil is drawn off at the bottom to be used as asphalt or heavy fuel. As the vapors rise they cool, condense and are drawn off at various levels in the tower. The most volatile are drawn off as gases at the top; these "Streams" are piped to other areas of the refinery such as the catalytic cracker and reformer.