The ability of computational fluid dynamics (CFD) to model the complex two-phase hydrodynamics of sieve trays is examined. The key to a proper description of the flow is the estimation of the momentum exchange, or drag, coefficient between the gas and liquid phases. In the absence of sound theoretical models, empirical correlations for the average gas fraction on the tray, such as those of Bennet, Agrawal and Cook [AIChEJ, 29 (1983): 434], can be used to estimate the drag coefficient. Transient simulations of sieve trays of 0.3 and 0.9 m in diameter, operating in the bubbly flow regime, reveal the chaotic, three-dimensional character of the flow and the existence of circulation patterns in all three dimensions. The CFD simulations underline the limitations of simpler approaches wherein the flow is assumed to be two-dimensional or where the interaction of the liquid phase with the gas phase is either ignored completely or simplified greatly. The major advantage of the CFD approach is that geometry and scale effects are properly encapsulated and do not require further inputs. It is concluded that CFD can be a powerful investigative and design tool for sieve tray columns.