Page 58 - Fluid Catalytic Cracking Handbook

Process Description 37

removal of mercaptan. There are two options for treating mercaptans.
In each option, the mercaptans are first oxidized to disulphides. One
option, extraction, dissolves the disulfides in caustic and removes them.
The other option, sweetening, leaves the converted disulfides in the
product. Extraction removes sulfur, while sweetening just removes the
mercaptan odor. Extraction is used for light products (up to light naphtha)
and sweetening for heavy products (gasoline through diesel).
Sweetening of the FCC gasoline is usually sufficient to meet its
sulfur specifications. However, in areas where "reformulated" gasoline
is marketed, sulfur specifications in the gasoline may require more
treatment. The mercaptans in the LPG need to be extracted to protect
the downstream processes, such as alkylation. Sulfur increases acid
consumption and produces undesirable by-products.
Both sweetening and extraction processes (Figure 1-16) commonly
use caustic and catalyst. If the LPG and the gasoline contain high
levels of H 2S, a caustic prewash is needed to protect the catalyst.
The sweetening process utilizes a caustic solution, catalyst, and air,
Mercaptans are converted to disulfides in a mixing vessel or fiber film
contactor. The reactions take place according to the following equations:

H 2O + catalyst -> RSSR + 2NaOH (1-14)

The mixture of caustic and disulfides is transferred to a settler. From
the settler, the treated gasoline flows to a coalescer, sand filter, or wash
water tower, before going to storage. The caustic solution is recir-
culated to the mixing vessel/fiber film contactor.
In the extraction process, the LPG from the prewash tower enters
the bottom of an extractor column. The extractor is a liquid/liquid
contactor in which the LPG is counter-currently contacted by a caustic
solution. Another option is the use of a fiber film contacting device.
The mercaptans dissolve in the caustic (Equation 1-14). The treated LPG
leaves the top of the extractor and goes on to a settler, where entrained
caustic is separated.
From the bottom of the extractor, the caustic solution, containing
sodium mercaptide, enters the regenerator. Plant air supplies oxygen
to react with the sodium mercaptide to form disulfide oil (Equation
1-11), which is insoluble in caustic. The oxidizer overhead stream