Analysis of Residence Time Characteristics According to Carrier Gas Composition in a Reactor for Carbon Nanotube Synthesis

This study employed computational fluid dynamics (CFD) simulations to quantitatively evaluate how variations in carrier gas composition affect the residence time of a primary species in a carbon nanotube (CNT) synthesis reactor based on the floating catalyst chemical vapor deposition (FC-CVD) process. Once the flow field within the reactor stabilized, a tracer CO species— identical in physical properties but labeled distinctly—was introduced to calculate residence time from its time-dependent concentration distribution. The carrier gas composition was controlled by adjusting the mixing ratio of hydrogen (H₂) and argon (Ar), where a higher Rx value indicates a greater proportion of Ar relative to H₂ in the total carrier gas flow. As Rx increased, the average residence time within the reactor consistently increased. Specifically, the average residence time under the Rx = 0.57 condition was approximately 19% longer than that under Rx = 0.10, indicating that carrier gas composition strongly influences flow behavior, including velocity reduction and expansion of stagnation zones. The increase in residence time observed in this study is consistent with earlier studies reporting lower CNT quality at higher Rx values, suggesting a possible link between gas residence characteristics and synthesis performance. This method establishes a structured baseline for analyzing how gases behave inside CNT reactors and can be extended to future studies that include chemical reaction pathways and precursor decomposition.