Minimum fluidization velocity — below Umf the gas just percolates through a packed bed; above it, drag on the particles balances their weight and the bed fluidizes. This sim uses the real Wen & Yu correlation, which fits the Ergun pressure-drop equation across the whole laminar-to-turbulent range:
Ar = dp³ρg(ρs−ρg)g / μ² (Archimedes number)
Re_mf = √(33.7² + 0.0408·Ar) − 33.7
U_mf = Re_mf·μ / (dp·ρg)
ρs ≈ 1500 kg/m³ (zeolite/matrix catalyst particle density), ρg and μ are the hydrocarbon-vapor density and viscosity at the chosen bed temperature (ideal-gas density from T, viscosity rising with T). Smaller particles or hotter, less viscous... denser gas lower Umf. Above ~1.2×Umf the bed is bubbling; the visualization scales particle circulation, gas bubble size/rate, and bed expansion with the excess velocity (U−Umf)/Umf — this expansion mapping is a simplified visual model, not a literal solids-hold-up correlation.
Catalytic cracking & deactivation — inside the zeolite's micropores, large hydrocarbon molecules crack to lighter products on acid sites. Conversion follows first-order kinetics with an Arrhenius rate constant and the catalyst's own activity:
k(T) = k0·exp(−Ea / R·T) (Arrhenius)
X = 1 − exp(−k·a·τ) (first-order conversion, τ = vapor residence time)
C_coke = A·t^n (Voorhies coke-laydown correlation)
a = exp(−α·C_coke) (activity falls as coke blocks acid sites)
Coke accumulates on the catalyst as cracking proceeds (the sim runs on an accelerated clock so the decay is visible in seconds rather than minutes), lowering activity a and therefore conversion — exactly why real FCC units continuously withdraw spent catalyst to a regenerator that burns the coke off in air before returning it to the reactor. Regenerate mimics that step, resetting coke and activity to fresh-catalyst values.