Modeling of membrane contactors as absorber units requires a rigorous absorption model where the liquid phase is discretized in the radial direction. The current paper describes such a model for absorption of CO2 into aqueous alkanolamines. The model is validated with a large volume of experimental data for absorption of CO2 in membrane contactor units. A simplified version of the model is developed to enable fast design and scale up assessment of membrane contactor systems. Simulations are used to show the importance of introducing mixing points in order to avoid mass transfer limitations on the liquid side. This can be achieved by a novel design of the membrane fibers or by operating the contactors with liquid flow on the shell side and gas on the tube side.