FCC Fundamentals

Chapter from the FCC Operations Guide.

FCC PROCESS FUNDAMENTALS

FCC Process Overview

The FCC (Fluid Catalytic Cracking) unit converts heavy gas oil into lighter, more valuable products (gasoline, LPG, distillate) using a hot, regenerated catalyst.

Process Flow:
1. RISER: Hot regenerated catalyst contacts feed at riser base → instant vaporization + cracking
• Residence time: 2-5 seconds
• Temperature: 950-1,020°F (riser outlet)
• Catalyst/oil ratio: 4-10 wt/wt
• All cracking occurs in the riser — this is where yield is made

2. REACTOR/DISENGAGER: Separates catalyst from cracked vapors
• Cyclones strip entrained catalyst from vapor
• Stripper section: steam strips residual hydrocarbons from catalyst
• Spent catalyst flows to regenerator via standpipe

3. REGENERATOR: Burns coke from catalyst to restore activity
• Full combustion (most modern): burns coke to CO2, T = 1,250-1,400°F
• Partial combustion: burns to CO + CO2, CO boiler downstream
• Regenerated catalyst returns to riser base via standpipe

4. MAIN FRACTIONATOR: Separates cracked products
• Overhead: wet gas + unstabilized gasoline → gas concentration unit
• Side cuts: LCO (light cycle oil), HCO (heavy cycle oil)
• Bottoms: slurry oil (catalyst fines + heavy oil)

Catalyst Circulation System

The FCC catalyst circulation system is the heart of the unit. Catalyst is a fine powder (~70 micron average particle size) that behaves as a fluid when aerated.

Key Components:
• Regenerator standpipe: carries hot regen catalyst from regenerator to riser
• Slide valve (regen side): controls catalyst flow rate to riser
• Riser: catalyst + feed contact zone, vertical pipe
• Reactor/disengager: separates catalyst from vapor
• Stripper: steam strips HC from spent catalyst
• Spent catalyst standpipe: carries spent catalyst to regenerator
• Slide valve (spent side): controls catalyst level in reactor

Pressure Balance:
• Catalyst circulation is driven by pressure differential, NOT by pumps
• Regenerator pressure > Reactor pressure (ΔP = 1-3 psi typical)
• Standpipe provides hydrostatic head (catalyst density × height)
• Slide valve provides flow restriction to control circulation rate
• Aeration: small amounts of steam/air injected into standpipes to maintain fluidity

Source: FCC_Operations_Guide_v1.xlsx · sheet “FCC Fundamentals”

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