Downstream Processing

Additional processing follows crude distillation, "downstream" (or closer to the refinery gate and the consumer) of the distillation process.   Downstream processing is grouped together in this discussion, but encompasses a variety of highly complex units designed for very different upgrading processes.  Some change the molecular structure of the input with chemical reactions, some in the presence of a catalyst, and some with thermal reactions. 

In general, these processes are designed to take heavy, low-valued feedstock -- often itself the output from an earlier process -- and change it into lighter, higher-valued output.  A catalytic cracker, for instance, uses the gasoil (heavy distillate) output from crude distillation as its feedstock and produces additional finished distillates (heating oil and diesel) and gasoline.  Sulfur removal is accomplished in a hydrotreater.  A reforming unit produces higher octane components for gasoline from lower octane feedstock that was recovered in the distillation process.   A coker uses the heaviest output of distillation, the residue or residuum, to produce a lighter feedstock for further processing, as well as petroleum coke. 

U.S. demand is centered on light products, such as gasoline. Refiners in the United States more closely match the mix of products demand by using downstream processing to move from the natural yield of products from simple distillation to the U.S. demand slate.  After simple distillation alone, the output from a crude oil like Arab Light would be about 20 percent of lightest, gasoline-like products, and about 50 percent of the heaviest, the residuum.  After further processing in the most sophisticated refinery, however, the finished product output is about 60 percent gasoline, and 5 percent residuum.  

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