A 3D volume of fluid (VOF) model was used and a convection-diffusion equation was implemented to simulate the reactive absorption process of CO2 into NaOH aqueous solution. The simulations were performed based on a representative element unit (REU) of Mellapak 500Y structured packing using direct numerical simulation (DNS) and turbulent simulation. We proposed a method to locate the gas-liquid interface, based on which the effective area of mass-transfer was determined, using the information provided by the VOF model. Using the simulation results and material balance around the liquid phase in the REU, the liquid mass transfer coefficient was calculated. The obtained mass transfer coefficient was shown to agree well with existing models. We also showed a two-equation turbulent model which consumes much less computation efforts than the DNS, but it can also be applied for the same purpose. The procedure proposed in this paper is expected to be a general approach to determining systematically the mass-transfer coefficient in structured packing without relying on empirical correlations or experiments. (C) 2017 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.