Hydroskimming

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Hydroskimming is the simplest type of refinery used in the petroleum industry ^[1] A hydroskimming refinery is defined as a refinery equipped with Atmospheric Distillation, naphtha reforming and necessary treating processes. A Hydroskimming refinery is therefore more complex than a topping refinery (which just separates the crude into its constituent petroleum products by distillation, known as Atmospheric Distillation, and produces naphtha but no gasoline) and it produces gasoline. However a Hydroskimming refinery produces a surplus of fuel with a relatively unattractive price and demand.

Most refineries therefore add vacuum distillation and catalytic cracking, which adds one more level of complexity by reducing fuel oil by conversion to light distillates and middle distillates. A coking refinery adds further complexity to the cracking refinery by high conversion of fuel oil into distillates and petroleum coke.

Catalytic Cracking, Coking and other such conversion units are referred to as secondary processing units. The Nelson Complexity Index, captures the proportion of the secondary conversion unit capacities relative to the primary distillation or topping capacity. The Nelson Complexity Index typically varies from about 2 for Hydroskimming refineries, to about 5 for the Cracking refineries and over 9 for the Coking refineries.^[2]