Important and unique metal powders are made industrially by a variety of vapor condensation processes in tube reactors. Often, however, the fundamental mechanisms for particle formation and growth are still not well understood. In this article, a computational fluid dynamics (CFD) model was developed to examine a tube reactor's internal flow characteristics. The model identified a massive zone of fluid recirculation in the top half of the reactor. In-situ sampling from an experimental reactor under the same conditions revealed a large increase of aerosol particle size corresponding to the region of recirculation. A first principles mass balance model based on chemical kinetics and aerosol physics was developed for this system which showed that the average particle size grew monotonically with time in the reactor. On the basis of this firmly established link between residence time and particle size, a new reactor geometry was proposed to produce a "plug-flow" velocity profile with a narrower particle size distribution. A CFD model was used to prototype the new configuration, and then this new reactor design was tested experimentally to confirm that the design objective was achieved. This work shows the potential synergies between first principles models for process understanding and CID models for process prototyping and optimization. (c) 2007 American Institute of Chemical Engineers