Page 57 - Fluid Catalytic Cracking Handbook

36 Fluid Catalytic Cracking Handbook

In the amine regenerator, the rich amine solution is heated to reverse
the acid-base reaction that takes place in the contactor. The heat is
supplied by a steam reboiler. The hot, lean amine is pumped from the
bottom of the regenerator and exchanges heat with the rich amine in
the lean-rich exchanger and a cooler before returning to the contactor.
A portion of the rich amine flows through a particle filter and a
carbon bed filter. The particle filters remove dirt, rust, and iron sulfide.
The carbon filter, located downstream of the particle filters, removes
residual hydrocarbons from the amine solution.
The sour gas, containing small amounts of amine, leaves the top of
the regenerator and flows through a condenser to the accumulator. The
sour gas is sent to the sulfur unit, while the condensed liquid is
refluxed to the regenerator.
For many years, nearly all the amine units were using monoethanola-
niine (MEA) or diethanolamine (DEA). However, in recent years the use
of tertiary amines such as methyl diethanolamine (MDEA) has increased.
These solvents are generally less corrosive and require less energy to
regenerate. They can be formulated for specific gas recovery requirements.

LPG Treating

The LPG stream containing a mixture of C 3's and C 4's must be
treated to remove hydrogen sulfide and mercaptan. This produces a
noncorrosive, less odorous, and less hazardous product. The C 3's and
C 4's from the debutanizer accumulator flow to the bottom of the H 2S
contactor. The operation of this contactor is similar to that of the fuel
gas absorber, except that this is a liquid-liquid contactor.
In the LPG contactor the amine is normally the continuous phase
with the amine-hydrocarbon interface at the top of the contactor. This
interface level controls the amine flow out of the contactor. (Some
liquid/liquid contactors are operated with the hydrocarbon as the
continuous phase. In this case, the interface is controlled at the bottom
of the contactor.) The treated C 3/C 4 stream leaves the top of the contactor.
A final coalescer is often installed to recover the carry-over amine.

Caustic Treating

Mercaptans are organic sulfur compounds having the general formula
of R-S-H. As stated earlier, amine treating is not effective for the