In this research, the residence time distribution (RTD) approach was used for the down comer of the special-shaped triangular external loop airlift reactor (EL-ALR) by applying a modified downcomer to describe the flow pattern and dispersion behavior. Moreover, the effects of the three parameters of the superficial gas velocities of the downcomer and riser (V-gs1 and V-gs2, respectively), and the angle of the hypotenuse (alpha) were studied. Response surface methodology (RSM) was applied to analyze the results statistically and also to optimize the mixing conditions. In addition, the Bodenstein number (Bo), liquid velocity (U-L), and axial dispersion coefficient (D-ax) were determined. A mathematical relationship with a high accuracy was developed for the prediction of Bo number in the reactor downcomer. Also, it was shown that the reactor configuration was the most significant factor among all the studied parameters that affected the reactor performance. As a result of the RTD analysis, the flow pattern had been tended to be perfectly mixed. In this regard, the obtained values of Bo number were within the range 7.18-11.21 that confirmed this finding. The predicted optimum conditions (alpha = 60 degrees, V-gs1 = 0.005 m/s, and V-gs2 = 0.019 m/s) were validated using the experimental control test, indicating a small deviation of +/- 0.39. (C) 2020 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.