Page 215 - Fluid Catalytic Cracking Handbook

Products and Economics 183

• LCO
« HCO
• Decanted Oil
• Coke

Dry Gas

The gas (C 2 and lighter) leaving the sponge oil absorber is com-
monly referred to as dry gas. Its main components are hydrogen,
methane, ethane, ethylene, and hydrogen sulfide (H 2S). Once the gas
is amine-treated for removal of H 2S and other acid gases, it is blended
into the refinery fuel gas system. Depending on the volume percent
of hydrogen in the dry gas, some refiners recover hydrogen using
processes such as cryogenics, pressure-swing absorption, or membrane
separation. The recovered hydrogen is often used in hydrotreating.
Dry gas is an undesirable by-product of the FCC unit; excessive
yields load up the wet gas compressor (WGC) and are often a con-
straint. The dry gas yield is primarily due to thermal cracking, metals
in the feed, and nonselective catalytic cracking. The main factors that
contribute to the increase of dry gas are:

• Increase in the concentration of metals (nickel, vanadium, etc.)
on the catalyst
• Increase in reactor or regenerator temperatures
• Increase in the residence time of hydrocarbon vapors in the reactor
• Decrease in the performance of the feed nozzles
8
Increase in the aromaticity of the feed

The overhead stream from the debutanizer or stabilizer is a mix of
C 3's and C 4's, usually referred to as LPG (liquefied petroleum gas).
It is rich in olefins, propylene, and butylene. These light olefins play
an important role in the manufacture of reformulated gasoline (RFC),
Depending on the refinery's configuration, the cat cracker's LPG is
used in the following areas:
* Chemical sale, where the LPG is separated into C 3's and C 4's.
The C 3's are sold as refinery or chemical grade propylene. The
C 4 olefins are polymerized or alkylated.