Optimization and scale-up of CNF production based on intrinsic kinetic data obtained from TEOM

Abstract

Optimizing the operating parameters for growing carbon nanofibers (CNFs) is an important step toward the large-scale production of CNFs with a high degree of structure control. The present work demonstrates that a tapered-element oscillating microbalance (TEOM) is an excellent tool for the kinetic study of the highly dynamic process of CNF growth. Initial CNF growth rates, deactivation rates, and yields can be studied simultaneously as a function of temperature, pressure, hydrogen partial pressure, or residence time. The kinetic data from TEOM studies are directly implemented to scale up the production of CNF. For a hydrotalcite (HT)-derived Ni catalyst (77 wt% Ni), the highest CNF growth rate was obtained at a temperature of around 580 °C and a hydrogen partial pressure of around 0.1 bar. High temperatures and low partial pressures of hydrogen resulted in a high initial growth rate and deactivation rate, whereas low temperatures and high partial pressures of hydrogen resulted in low initial growth rate and deactivation rate. A carbon capacity (maximum carbon yield) of 50 g/g cat could be achieved at optimized conditions. Growth rates and CNF yields were found to increase with increasing total pressure (0.3–3.8 bar). The mechanisms of CNF growth and deactivation were explored based on kinetic data and DFT calculation studies from the literature. The results obtained from the TEOM studies were verified in a PFR fixed-bed reactor and a horizontally placed fixed-bed reactor similar to a CSTR reactor. The residence time of methane was identified as the most important parameter for scaling up the process. The data on the effect of hydrogen from the TEOM were reproduced, and a successful scale-up of 10,000 times was achieved.