Page 22 - Fluid Catalytic Cracking Handbook

6 Fluid Catalytic Cracking Handbook

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which cannot be distilled in the atmospheric tower, is heated and sent
to the vacuum tower where it is split into gas oil and tar. The tar from
the vacuum tower is sent to be further processed in a delayed coker,
deasphalting unit, or visbreaker, or is sold as fuel oil or road asphalt.
The gas oil feed for the conventional cat cracker comes primarily
from the atmospheric column, the vacuum tower, and the delayed
coker. In addition, a number of refiners blend some atmospheric or
vacuum resid into the feedstocks to be processed in the FCC unit.
The FCC process is very complex. For clarity, the process descrip-
tion has been broken down into six separate sections:

• Feed Preheat
• Riser—Reactor—Stripper
« Regenerator—Heat/Catalyst Recovery
• Main Fractionator
• Gas Plant
« Treating Facilities

FEED PREHEAT

Most refineries produce sufficient gas oil to meet the cat crackers'
demand. However, in those refineries in which the gas oil produced
does not meet the cat cracker capacity, it may be economical to
supplement feed by purchasing FCC feedstocks or blending some
residue. The refinery-produced gas oil and any supplemental FCC
feedstocks are generally combined and sent to a surge dram, which
provides a steady flow of feed to the charge pumps. This drum can
also separate any water or vapor that may be in the feedstocks.
From the surge drum, the feed is normally heated to a temperature
of 500°F to 700°F (260°C to 370°C). The main fractionator bottoms
pumparound and/or fired heaters are the usual sources of heat. The
feed is first routed through heat exchangers using hot streams from
the main fractionator. The main fractionator top pumparound, light
cycle oil product, and bottoms pumparound are commonly used (Fig-
ure 1-5). Removing heat from the main fractionator is at least as
important as preheating the feed.
Most FCC units use fired heaters for FCC feed final preheat. The
feed preheater provides control over the catalyst-to-oil ratio, a key
variable in the process. In units where the air blower is constrained.