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Chemistry on a very large scale
Another significant portion of refining is converting individual molecules into different sizes and shapes. There are three main conversion processes in the refinery:
- Cracking
- Combining
- Reforming
Cracking takes place in two units: the cokers and the fluid catalytic cracking units (or FCCUs). In the coker, asphalt is heated to 900 to 1000º F in large drums. The asphalt is allowed to cook for 16 to 24 hours. While cooking, the asphalt turns into a synthetic crude oil which is separated into the different fractions. What remains is a black solid, similar in appearance to coal, called petroleum coke. Petroleum coke is used the same way that coal is, primarily to produce electricity in coal fired power plants.
Gasoil, a fraction of crude oil between asphalt and diesel fuel, is cracked in the FCCU. Gasoil is contacted by very hot (> 1300º F), very small (like flour) particles of catalyst, which cracks it into a synthetic crude oil which is again separated into different fractions. In the process, some of the gasoil turns into coke, which attaches to the catalyst particles. This coke must be burned off in the FCCU regenerator, which also reheats the catalyst for another ride around the unit.
Eventually, some of the catalyst wears down and is too small to be captured by the internal capture mechanisms, and escapes from the regenerator. An electrostatic precipitator (like a 14 story electronic dust cleaner on your furnace at home) removes most of these fine pieces of catalyst before they escape into the air.
Combining takes place in three units: the alkylation (or Alky) unit, the dimersol and the polymerization (or Polly) unit. In all three of these units, small LPG molecules are combined into larger, gasoline molecules. FHR uses sulfuric acid, nickel and phosphoric acid to combine different combinations of LPG into gasoline.
Reforming takes place in three units as well: the platformer, the powerformer and the isomerization (or isom) unit. Reforming changes the shapes of molecules so that they have more desirable characteristics. For example, the gasoline which comes directly from the separation of crude oil has too low an octane rating to be used directly in automobiles. By reforming this gasoline with the help of a platinum catalyst, the octane can be raised significantly. FHR uses platinum and other rare metals to rearrange gasoline molecules to improve their octane and other properties.
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