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Crude oil consists of a complex arrangement of hydrogen and carbon atoms (hydrocarbon). To be converted into useful products, these atoms must be chemically rearranged.
This was originally accomplished by heating the crude in a vat until it boiled. Vapor rising from the boiling oil was allowed to condense. The product, resulting from this condensation, turned out to be a liquid somewhat like heating oil or kerosene. Oilmen soon discovered they could control the process better if the crude oil was heated at the bottom of a distillation tower. As the vapor rose inside the tower and condensed, different products appeared at different levels in the tower. There, the products were drained off into their own storage tanks.
Gasoline would show up at the top of the distillation tower. But the middle of the tower produced the greatest amount of condensed liquid, mainly heating oil and kerosene. These middle-range products were called distillates, and the oil industry continues to refer to them by that term today.
Early in this century, when automobile usage began to increase, it became clear to petroleum refiners that they were going to have to figure out a way to wring more gasoline out of each barrel of crude. They have managed to accomplish this through development of a series of advanced refining processes: thermal cracking, polymerization, alkylation, catalytic cracking, hydrogenation, reforming, and fluid catalytic cracking.
In each of these processes, refiners continue to use towers in which vapors rising from heated crude oil condense at varying levels. But the processes are far more complex than the original distillation method. In catalytic cracking (cat cracking), the petroleum vapors are passed over catalysts of alumina-silica or other mixtures. This rearranges the hydrocarbon atoms and greatly increases the proportion of gasoline produced, as compared to the original distillation method.
In fluid cat cracking, powdered catalysts are allowed to flow through the crude vapors like a liquid. This process increases the proportion of gasoline even more.
The reforming process, which came into wide usage during World War II, produces important chemicals that enhance the quality of motor fuel. These chemicals are the so-called aromatics: benzene, toluene, and xylene.
See also Refinery.