In petroleum refining, there are many processes that are import to achieving the overall fuels necessary for a refinery to sell. One of the main processes crude must undergo is distillation, where the crude is heated to a desired temperature in order to separate the crude into desired feedstock to undergo different treatments to get out different products. There are two types of distillation, the first is atmospheric distillation which is done at atmospheric pressure. The second is vacuum distillation.
The reason that vacuum distillation is so vital to the process of refining is due to the fact that if the temperature used in atmospheric distillation were to be raised any higher, it would cause thermal cracking which is not ideal. The reason thermal cracking is not desired is because it would compromise the ending product or fuel in many cases, and lead to coking. In order to avoid thermal cracking, and changing the chemical composition of the feedstock, vacuum distillation is used in succession to atmospheric distillation. The atmospheric residual oil is further distilled in the vacuum distillation column. This distillation must be performed at very low pressures, as low as 10-40 mmHg, in order to further distil the oil without the presence of thermal cracking and eventual coking. Vacuum distillation towers are very large, as tall as 164 feet with a 46 foot diameter. The reason they are so large is due to the expansion of the oil under such low pressures.
In order to complete vacuum distillation, you must know what temperature to perform the distillation at. This is done with the use of the Watson Characterization Factor. The factor is the ratio of mean average boiling point and specific gravity of the oil. This factor is used to further determine the chemical makeup of an oil. Higher characterization factors such as 12.5 or greater indicate a compound is mostly paraffinic, while a lower factor tells us it is naphthenic or aromatic. Once this has been determined, you can better select the temperature at which to Vacuum distil.
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