Oil and gas production handbook An introduction to oil ... - ABB Group

6 Refining
Up to the early 1970s, crude oil prices were kept reasonably stable by major
international oil companies and industrialized nations. Less value was
created in the upstream production operations and relatively more profits
were generated in refining and distribution operations. With the 1973 oil
crisis and rising crude oil prices, more of the value was created upstream.
Now, the success of a modern refinery depends more on economies of scale
and the ability to process a wide range of crudes into the maximum quantity
of high value fuels and feedstock. A refinery that is able to handle multiple
types from heavy to light crude is said to have to have a large swing. Trade
specifications such as ”West Texas Intermediate” (WTI) API 38.3°, “Brent
Blend” API 38.3°, “Heavy Arab Crude” API 27.7° or “Grane” API 18.7° are
examples of such crudes.
Medium light crudes can be used directly in early engines and burners.
Modern consumers, such as gas and diesel engines, aviation turbojet
engines and ship bunkers need fuels manufactured to precise specifications.
This includes removing contaminants and pollutants, such as sulfur.
6.1 Fractional distillation
The basic refinery uses fractional distillation. Incoming crude is heated to its
boiling point. It then enters the distillation column, which separates the
different fractions. The column is of the reflux type, where colder condensed
fluids running down are reheated by rising vapors that in turn condense. This
produces clear thermal zones where the different products can be drained.
NOTE: The schematic on the following page is simplified. Both continuous
and vacuum distillation is used in separate columns to avoid heating the raw
crude to more than 370 °C. Overheating would cause thermal cracking and
excessive coke that may also plug pipes and vessels. Also a sidecut stripper
is used, in addition to the main column, to further improve separation.
Sidecut is another name for the fractions emerging from the side (rather than
top and bottom) of the main column, i.e., naphtha, gasoline, kerosene and
diesel.
The fractions are a mix of alkanes and aromatics and other hydrocarbons, so
there is not a linear and uniformly rising relationship between carbon number
and boiling point and density, although there is a rough fit. Even so, this
means that each fraction contains a distribution of carbon numbers and
hydrocarbons.
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