Effects of downcomer-to-riser cross sectional area ratio on operation behaviour of external-loop airlift bioreactors

Experiments performed in two external-loop airlift bioreactors of laboratory and pilot scale, (1.880-1.189). 10(-3) m(3) and (0.170-0.157) m(3), respectively, are reported. The A,IA, ratio was varied between 0.111-1.000 and 0.040-0.1225 in the laboratory and pilot contractor respectively. Water and solutions of different coalescence (2-propanol 2% vol, 1 M Na (glucose 50% wt/vol) and rheological behaviour (non-Newtonian starch solutions with consistency index K = 0.061-3.518 Pas(n) and flow behaviour index n = 0.86-0.39), respectively, were used as liquid phase. Compressed air at superficial velocities nu(SGR) = 0.016-0.178 m s(-1) in the laboratory contactor and nu(SGR) = 0.010-0.120 m s(-1) in the pilot contactor, respectively was used as gaseous phase. The A(D)/A(R) ratio affect gas-holdup behaviour as a result of the influence of A(D)/A(R) on liquid circulation velocity. Experimental results show that A(D)/A(R) ratio affect circulation liquid velocity by modifying he resistence to flow and by varying the fraction of the total volume contained in down-comer and riser. A(D)/A(R) ratio has proven to be the main factor which determines the friction in the reactor. Mixing time increases with increasing of the reactor size and decreases with A(D)/A(R) decreasing. The volumetric gas-liquid mass transfer coefficient increases with A(D)/A(R) ratio decreasing, as a result of variations of the liquid velocity with A(D)/A(R), which affect interfacial areas. Correlations applicable to the investigated contactors have been presented, together with the fit of some experimental data to existing correlation in literature.