In other to obtain further information on the behavior of the external loop airlift reactor operating with air-water mixtures, the liquid circulating velocity, gas holdup, oxygen mass transfer coefficient, mixing time and bubble properties were studied. The effects of gas velocity (0.02-0.18 m/s) and the length of horizontal connection pipe (0.1-0.5m) with respect to the previous parameters have been investigated. The local holdup in each section of the airlift reactor was measured by manometric technique and the overall holdup by volume expansion method. The effects of operating variables on the bubble properties were measured by using an electro-resistivity probe. The oxygen mass transfer coefficient between air bubble and water was determined by dynamic absorption method. The mixing time and the liquid circulating velocity were measured by the salt (KCl)-base impulse method. The experimental results in the present investigations can be summarized as follows;
1). The length of the horizontal connection pipe was a key design parameter for the external circulation loop airlift reactor.
2). The overall oxygen transfer coefficient in the external loop airlift reactor with a short connection length was higher than that with a long one due to higher gas holdup.
3). The mixing performance of the external loop airlift reactor with a long connection length was more efficient than that with a short one.
4). Both Chisti et al.'s model and Hatch's model used in predicting the liquid circulating velocity were satisfactory.
5). Some correlations for the estimation of the mixing time, liquid circulating velocity and oxygen transfer coefficient were proposed.
·$V_{Ld} = 1.04 U_G^{0.17} (L_h/L_o)^{0.18}$
·$t_m = 13.1 U_G^{-0.16} (L_h/L_o)^{-0.04}$
·$K_La_L = 0.24 U_G^{0.84} (L_h/L_o)^{-0.26}$
·$K_La_L = 0.45 E_t^{1.14} (L_h/L_o)^{-0.04}$
