Process Calculations

Chapter from the FCC Operations Guide.

FCC PROCESS CALCULATIONS

Key FCC Process Calculations

Calculation	Formula	Inputs	Units	Typical Value	Notes
Conversion	(Feed - (LCO + Slurry)) / Feed × 100	Feed, LCO, slurry volumes	vol%	65-85%	Based on 430°F or 650°F cut point
Cat/Oil Ratio	Catalyst circ rate / Feed rate	Circulation rate (tph), feed rate (tph)	wt/wt	4-10	Key lever for conversion
Coke Yield	Coke burned / Feed rate × 100	Air rate, flue gas analysis, feed rate	wt% on feed	4-6%	From air balance or carbon balance
Delta Coke	Coke on spent - Coke on regen	Lab analysis of catalyst samples	wt%	0.5-1.2%	Key heat balance parameter
Coke on Regen Cat	Lab measurement or calculation	Regen catalyst sample	wt%	0.01-0.10%	Lower = better regeneration
Heat of Combustion (coke)	~14,000 BTU/lb (pure C) to ~17,000 (with H)	Coke composition (C, H, S)	BTU/lb coke	14,000-17,000	H/C ratio affects heat release
Excess O2	O2 in flue gas / (21 - O2) × 100	Flue gas O2 analyzer	% excess air	1-3% O2 (full comb)	Low O2 = afterburn risk
Stripping Efficiency	(Entrained HC removed) / (Total HC on catalyst) × 100	Spent cat HC, stripped cat HC	wt%	>95%	Measured by H/C ratio in coke
Catalyst Activity (MAT)	Microactivity test per ASTM D5154	E-cat sample to lab	MAT number	60-75	Higher = more active
Catalyst Metals (Ni+V)	XRF or ICP analysis of E-cat	E-cat sample	ppm	<5,000 ppm total	Metals deactivate and dehydrogenate

Carbon Balance Approach for Coke Calculation

The most rigorous method to determine coke yield is the carbon balance (or air balance):

Coke Rate (lb/hr) = Air Rate × (CO2% + CO%) / 100 × (12/44) × correction factors

More precisely:
1. Measure dry flue gas composition: CO2, CO, O2, N2 (by difference)
2. Calculate carbon burned per mole of air from flue gas analysis
3. Coke = carbon burned + hydrogen burned + sulfur burned
4. H/C ratio estimated from delta coke or lab analysis (typically 0.05-0.08 for good stripping)

Air Balance:
• Total air (measured by blower) = O2 consumed in combustion + excess O2 + N2
• From flue gas analysis: back-calculate O2 consumed → moles of C, H, S burned
• Coke rate = Σ(elements burned) in lb/hr
• Coke yield = coke rate / feed rate × 100 (wt%)

Source: FCC_Operations_Guide_v1.xlsx · sheet “Process Calculations”

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