Page 54 - Fluid Catalytic Cracking Handbook

34 Fluid Catalytic Cracking Handbook

(text continued from page 31)

Amine and caustic solutions are used to remove these impurities,
The amine solvents known as alkanolomines remove both H 2S and
CO 2. Hydrogen sulfide is poisonous and toxic. For refinery furnaces
and boilers, the maximum H 2S concentration is normally about 160 ppm.
Amines remove the bulk of the H 2S; primary amines also remove
the CO 2. Amine treating is not effective for removal of mercaptan. In
addition, it cannot remove enough H 2S to meet the copper strip
corrosion test. For this reason, caustic treating is the final polishing
step downstream of the amine units. Table 1-3 illustrates the chemistry
of some of the important caustic reactions.

Sour Gas Absorber

An amine absorber (Figure 1-15) removes the bulk of H 2S from the
sour gas. The sour gas leaving the sponge oil absorber usually flows
into a separator that removes and liquefies hydrocarbon from vapors,
The gas from the separator flows to the bottom of the H 2S contactor
where it contacts a countercurrent flow of the cooled lean amine from
the regenerator. The treated fuel gas leaves the top of the H 2S absorber,
goes to a settler drum for the removal of entrained solvent, and then
flows to the fuel system.
Rich amine from the bottom of the H 2S contactor goes to a flash
separator to remove dissolved hydrocarbons from the amine solution.
The rich amine is pumped from the separator to the amine regenerator.

Table 1-3
Acid/Base Reactions Encountered Most Frequently
by Oil Industry Caustic Treaters

Carbon Dioxide
CO 2 + 2 NaOH — » Na 2CO 3 + H 2O
Hydrogen Sulfide
H 2S + 2 NaOH — > Na 2S + 2 H 2O
Mercaptan Sulfur
RSH + NaOH — » RSNa + H 2O
Naphthenic Acid
RCOOH + NaOH — > RCOONa + H 2O