Low Reynolds Number Gaseous Mixing with Applications to Carbon Nanotube Production

Compared to traditional materials, carbon nanotubes (CNTs) have been shown to exhibit superior properties in mechanical, electrical, and thermal domains, leading researchers to propose and study a number of different new composite materials to harness this potential. One key problem preventing greater development of these materials is the cost and availability of high-purity CNTs, which can be reliably produced using the High Pressure Carbon Monoxide (HiPCO) process. Low Reynolds number gaseous mixing is a key step in this process, and it must be precisely executed to minimize loss of catalyst and ensure uniformity of CNT diameter. Above 250°C, iron pentacarbonyl particles decompose into Fe and five CO molecules, but CNT production does not begin until the mixture exceeds 500°C. Because free iron is highly reactive, the mixture must be heated from 250°C through 500°C, referred to herein as the transition temperature regime, rapidly to minimize loss of reagents. Several techniques, culminating with Computational Fluid Dynamics (CFD), are used to study the mixing of particle-laden gaseous flows in the low Reynolds number (Re < 300) regime present in this reactor. The effects of geometry, inlet temperature, flow rate, and reactor orientation are quantified, and the results indicate the importance, from a mixing standpoint, of smaller diameter chambers and higher temperatures. The results of this study can be used to guide design of more efficient injectors for the next generation of HiPCO reactors, including laboratory scale units and development of larger scale reactors. Larger reactors are required to lower costs of CNTs and enable widespread use of CNT-based nano-composite