Reactivity enhancement in a catalytic zeolite grain through molecular traffic control (MTC) rests on the basic notion that the reactant and product molecules prefer to diffuse along different channels inside the grain and therefore do not mutually hinder their transport in and out of the grain. We investigate the conditions of reactivity enhancement in the presence of MTC for a realistic channel topology that describes the pore structure of a TNU-9 zeolite. We compare the output current of an MTC system with a reference system, which does not show any channel selectivity. For a wide range of reaction rates and for different grain sizes, we find that there is a very significant enhancement of reactivity for the MTC system. This effect remains strong as the grain size increases. The mechanism behind reactivity enhancement is argued to be generic rather than being confined to the particular structure of TNU-9.