Page 310 - Introduction to Petroleum Engineering

THE DOWNSTREAM SECTOR: REFINERIES 299

chapter. Notice that the fraction of feed moles in gas G is greater at 600°F than
it was in Example 15.3 which had the same feed at 330°F.

Component Feed (z ) k‐Value x y
i i i
n‐C H 12 0.20 42.2 0.006 0.237
5
n‐C H 18 0.20 10.3 0.023 0.234
8
n‐C H 26 0.20 2.25 0.097 0.219
12
n‐C H 34 0.20 0.636 0.288 0.183
16
n‐C H 0.20 0.216 0.586 0.127
20 42
Additional distillation of the bottom stream, or residue, from a crude distillation unit
as shown in Figure 15.5 is achieved by recognizing that the boiling point of a
hydrocarbon component decreases as pressure is lowered. Feeding the residue stream
into a vacuum distillation tower that operates at a much lower pressure allows
separation of components with lower boiling points.

15.2.2 Conversion
The separation process separates the crude oil into product streams that need to be
changed into mixtures that are suitable for the consumer. The process of converting
low‐value, high molecular weight hydrocarbon mixtures into high‐value, lower
molecular weight hydrocarbon products is known as the conversion process.
Conversion is achieved by breaking the hydrocarbon chains of higher molecular
weight molecules to produce hydrocarbon molecules with lower molecular weights.
High molecular weight hydrocarbon chains are broken in conversion units like
the fluidized catalytic cracker (FCC), the hydrocracker, and the delayed coker.
The FCC uses a catalyst to convert the high molecular weight product stream into
LPG, gasoline, and diesel. The hydrocracker also uses a catalyst at high temperature,
but the reactions are performed in the presence of high concentrations of hydrogen.
The input product stream flows over a fixed position catalyst in a hydrogen
environment. Product streams from the hydrocracker have low levels of sulfur.
The heaviest product from the distillation tower is Vacuum Tower Bottoms (VTB),
which is also known as “resid” or residue. Resid would become a solid if allowed to
cool to ambient temperature. It is sometimes used as a blend component for the pav-
ing asphalt market. It is possible to convert VTB to more commercially valuable
products by breaking the high molecular weight hydrocarbon chains. This cannot be
achieved in the FCC because resid is too heavy and typically contains too many con-
taminants. The delayed coker is a conversion unit that uses high temperature to break
long carbon chains and convert resid to more valuable products.
Another function of the conversion process is to change the way carbon chains are
put together. For example, butane molecules are by‐products of some conversion
units. They can be combined to form larger, more commercially valuable, hydro-
carbon molecules in alkylation units.
The last stage of the conversion process is catalytic reforming. The reformer gen-
erates hydrogen and increases the octane number of components used in a gasoline