Process Variables

Chapter from the FCC Operations Guide.

FCC PROCESS VARIABLES & YIELD EFFECTS

Key Process Variables

Variable	Increase Effect on Conversion	Effect on Gasoline	Effect on Coke	Effect on LPG/Gas	How to Adjust	Typical Range
Riser Outlet Temperature	Increases	Increases then decreases (overcracking)	Increases	Increases (more dry gas)	Feed preheat, cat/oil ratio	950-1,020°F
Cat/Oil Ratio	Increases	Increases to a point	Increases	Increases	Regen slide valve, feed rate	4-10 wt/wt
Feed Preheat Temperature	Slight decrease (less cat/oil)	Slight decrease	Decreases	Slight decrease	Feed preheat exchanger/heater	400-700°F
Feed Quality (CCR)	Less conversion per lb catalyst	Lower gasoline selectivity	More coke (contamination)	More dry gas	Feed blending, pretreating	0.1-1.0 wt% CCR
Feed Quality (metals Ni/V)	Dehydrogenation activity	Lowers gasoline	Increases	More H2 and dry gas	Passivators (Sb, Bi)	<5 ppm Ni+V desired
Catalyst Activity (MAT)	Higher activity = more conversion	More gasoline per ton	More coke per ton	More LPG	Fresh catalyst addition rate	60-75 MAT typical
Catalyst Circulation Rate	Increases	Increases	Increases	Increases	Regen slide valve	Per unit design
Regenerator Temperature	Indirect (hotter cat = more cracking)	Indirect	Lower coke on regen cat	Indirect	Catalyst cooler, feed quality	1,250-1,400°F

Yield Structure Overview

Typical FCC Yield Structure (VGO feed, 70-80% conversion):

Product Yield (vol%) Value
Fuel Gas (C2-) 3-5 wt% Low (fuel value)
LPG (C3/C4) 12-20 vol% High (propylene, butylene)
FCC Gasoline 45-55 vol% High (blending component)
LCO (Diesel) 10-20 vol% Moderate (needs hydrotreating)
Slurry Oil (CLO) 5-10 vol% Low (fuel oil or carbon black)
Coke 4-6 wt% Burned in regenerator (heat)

Key Relationships:
• Conversion vs. gasoline: bell-shaped curve — max gasoline at ~70-75% conversion
• Beyond max gasoline: overcracking produces more LPG and gas
• Coke yield increases monotonically with conversion
• LCO yield peaks at low conversion then declines
• Slurry yield decreases with conversion

Source: FCC_Operations_Guide_v1.xlsx · sheet “Process Variables”

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