Gas - solids fluidized beds are characterized by the presence of gas voids or bubbles causing a mass-transfer resistance. Enhancing the mass transfer fi om the bubbles to the emulsion phase by reducing the bubble size can be advantageous for the chemical performance (like the conversion and selectivity) of the reactor. A model using a 2-D version of the dynamic bubble model of Clift and Grace (1970, 1971) where bubble trajectories are predicted based on an analytic expression for the flow field around a bubble was studied. Bubble coalescence can be reduced by injecting bubbles in certain patterns which leads to a reduction of the average bubble size higher in the bed. A simple proportional feedback control method to force bubbles on a horizontal line made it possible to generate a bubble injection pattern automatically leading to smaller bubbles than without the feedback control method. The principle of the controlled bubble injection could lead to improved gas distributor designs.