Iranian Journal of Chemical Engineering (IJChE)

Abstract Maintaining a constant superficial gas velocity (Usg) is the most common criterion for scaling aerated photobioreactors, yet whether it preserves gas–liquid hydrodynamics is rarely tested directly. Euler–Euler CFD was used to characterize two baffled airlift reactors, a 9.7 L laboratory unit and a 106 L scaled-up unit, over Usg = 0.025–0.10 m·s⁻¹, evaluating gas holdup, gas-phase velocity, and the volumetric mass transfer coefficient (kLa) alongside Reynolds number, Froude number, and power input per unit volume (P/V). Across this range, gas holdup rose 2.4–2.6-fold and kLa rose 1.3–1.6-fold in both reactors, but mean gas velocity increased more in the larger unit (1.4-fold vs. 1.1-fold). Extended-range simulations of the large reactor showed continued but progressively less proportional gains. Froude number was not conserved across scales, and P/V remained consistently higher in the larger reactor. These results show that Usg alone is an insufficient scale-up criterion, gas holdup, kLa, Froude number, and P/V should be evaluated jointly.