We use a computational fluid dynamics model coupled with a particle dynamics model to simulate how catalyst nanoparticles nucleate, grow and evaporate over the length of a floating catalyst reactor. We focus on the influence of the flowrate in the reactor and the ferrocene mass fraction on the production of the catalyst nanoparticles. In the downstream region of the reactor, where the majority of CNT growth occurs, we find that, as either the flowrate or the ferrocene mass fraction increases, the particle mass fraction profile changes, with the mass fraction peak shifting away from the centreline. This displacement away from the centreline of the mass fraction peak may explain why the CNTs form a hollow, sock-like, aerogel at the downstream end of the reactor.