Heat Balance

Chapter from the FCC Operations Guide.

FCC HEAT BALANCE FUNDAMENTALS

The Basic FCC Heat Balance

The FCC is a heat-balanced process: the coke burned in the regenerator provides ALL the heat needed for:
1. Vaporizing the feed
2. Heating feed to reaction temperature
3. Supplying heat of cracking (endothermic)
4. Heating combustion air in the regenerator
5. Heat losses

Fundamental Heat Balance Equation:
Heat from coke burning = Heat to vaporize feed + Heat of cracking + Sensible heat changes + Losses

Q_coke = m_cat × Cp_cat × (T_regen - T_reactor) = m_feed × [ΔH_vap + ΔH_cracking + Cp × ΔT]

This means:
• More coke → more heat → higher regenerator temperature → hotter catalyst
• Higher catalyst temperature → more heat transfer to feed → more conversion
• MORE COKE → MORE CONVERSION (the FCC is self-reinforcing)

But there are limits:
• Regenerator metallurgy limits temperature (typically 1,350-1,400°F max)
• Excessive coke production requires a catalyst cooler
• Air blower capacity limits total coke burning rate
• The heat balance CONSTRAINS the unit — you cannot independently set all variables

Heat Balance Levers

Lever	Effect on Heat Balance	When to Use	Impact on Yields	Constraint
Feed preheat (increase)	Reduces heat demand from catalyst	Heat surplus (regen T too high)	Slight decrease in conversion	Feed heater/exchanger capacity
Feed preheat (decrease)	Increases heat demand → more cat circulation	Heat deficit, need more conversion	More conversion and coke	Minimum preheat for vaporization
Catalyst cooler (engage)	Removes excess heat from regenerator	Regen T at limit, need more throughput	Allows higher coke make	Cooler capacity, fouling
Fresh catalyst addition	Higher activity, more conversion per cat/oil	Declining yields, low activity	More conversion and coke	Catalyst cost, E-cat equilibrium
Feed quality change	Heavier feed = more coke, lighter = less	Balancing heat, optimizing yields	Changes entire yield slate	Feedstock availability, specs
Recycle (HCO/slurry)	Increases conversion, increases coke	Need more coke for heat balance	More gasoline from recycle	Riser capacity, regenerator air

Source: FCC_Operations_Guide_v1.xlsx · sheet “Heat Balance”

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