Unlike Fig. 3, in which downcomer kLa increased slightly when Vgs2 increased, gas hold-up in the downcomer increased as Vgs2 increased significantly. The correlation coefficients for superficial gas velocities shown in Table 3 affirm these findings. WhereVgs1< 0.004 m/s (critical superficial gas velocity), similar gas hold-up values were observed in the downcomer; however, whereVgs1> 0.004 m/s, the gas hold-up in the downcomer increasedsmoothly