Department of Chemical and Environmental Engineering |
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Distillation is an important separations
process in chemical engineering and has come to serve as a symbol
of the profession. This is best illustrated by an oil refinery, which takes
a raw material, namely crude oil, and transforms it into gasoline and hundreds
of other useful products.
A typical large refinery costs billions of dollars to build and millions more to maintain and upgrade. It runs around the clock 365 days a year, employs between 1,000 and 2,000 people and occupies as much land as several hundred football fields. It's so big and sprawling, in fact, that workers ride bicycles from one station to another.
In distillation the crude oil is heated in a still and the resulting liquids and vapors are discharged into towers, the tall, narrow columns that give refineries and other chemical factories their distinctive skylines. Inside the towers, the liquids and vapors separate into components or fractions according to weight and boiling point. The lightest fractions, including gasoline and liquid petroleum gas (LPG), vaporize and rise to the top of the tower, where they condense back to liquids. Medium weight liquids, including kerosene and diesel oil distillates, stay in the middle. Heavier liquids, called gas oils, separate lower down, while the heaviest fractions with the highest boiling points settle at the bottom. These tarlike fractions are literally the "bottom of the barrel."
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