Page 42 - Fluid Catalytic Cracking Handbook

Process Description 25

The hydrocarbon vapors flow to the wet gas compressor. This gas
stream contains not only ethane and lighter gases, but about 95% of
the C 3 and C 4 and about 10% of the naphtha. The phrase "wet gas"
refers to condensable components of the gas stream.
The hydrocarbon liquid is split. Some is pumped back to the main
column as reflux and some is pumped forward to the gas plant,
Condensed water is also split. Some is pumped back as wash to the
overhead condensers and some is pumped away to treating. Some
might be used as wash to the wet gas compressor discharge coolers,

GAS PLANT

The FCC gas plant (Figure 1-14) separates the unstabilized gasoline
and light gases into the following:

• Fuel gas
• C 3 and C 4 compounds
• Gasoline

C 3's and C 4's include propane, propylene, normal butane, isobutane,
and butylene. Propylene and butylene are used to make ethers and
alkylate, which are blended to produce high-octane gasoline. Most gas
plants also include treating facilities to remove sulfur from these products.
The gas plant starts at the wet gas compressor. A two-stage centrifugal
compressor is typically employed. This type of compressor generally
incorporates an electric motor or a multistage turbine that is driven
typically by high-pressure steam. The steam is exhausted to a surface
condenser operating under vacuum. It should be noted that there are
FCC units in which single-stage wet gas compressors are used.
In a two-stage system, the vapors from the compressor's first stage
discharge are partially condensed and flashed in an interstage drum.
The liquid hydrocarbon is pumped forward to the gas plant, either to
the high pressure separator (HPS) or directly to the stripper.
The vapor from the interstage drum flows to the second-stage
compressor. The second-stage compressor discharges through a cooler
to the high pressure separator. Gases and light streams from other
refinery units are often included for recovery of LPG. Recycle streams
from the stripper and the primary absorber also go to the high pressure
separator. Wash water is injected to dilute contaminants, such as