This research uses the newly-developed Computational Fluid Dynamics (CFD) model which employs free surface mass transfer and bubble mass transfer using the small eddy model (SEM), and investigates the effects of multiple factors, including SEM coefficient, airflow rate, and bubble diameter. The study reveals that (1) two-domain simulations supports experimental data better than one-domain simulations, (2) free surface mass transfer occupies a small portion of overall mass transfer, irrespective of the SEM coefficient and (3) flow rate and bubble size is not a critical factor on free surface mass transfer in low airflow rate condition. Thus, to enhance free surface mass transfer, high flow rate condition first of all should be applied. This research demonstrates a new approach for evaluating mass transfer using the CFD model, making it helpful for deriving optimal bubble reactor design and operation. (C) 2018 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.