Abstract: An important scientific and technological problem is the development of continuous high-performance processes that make it possible to obtain multi-walled carbon nanotubes (MWCNTs), which, due to their unique properties, are one of the key components of rapidly developing nanotechnology. Analysis of various options for the design of such a process allows us to identify the most promising fluidized bed (FB) technology. Despite some progress in understanding the patterns of individual nanotubes catalytic growth and their synthesis in FB under conditions of a significant increase in its volume [1,2], a number of problems related to the design of a continuous technological process remain unresolved. One of the most serious hydrodynamic problems is the creation of a uniform fluidization of the nanoparticle powder. The cause of this problem is the strong influence of the cohesion of nanotubes and their agglomerates on the hydrodynamics of FB. Due to the small size of nanoparticles, they are characterized by very large values of the ratio of surface area to volume. This leads to the significant predominance of cohesion forces between nanoparticles over the forces of gravity and hydrodynamic resistance in the gas flow [3]. The consequence is the formation of agglomerates of nanoparticles. The formation and destruction of agglomerates is a dynamic process. As the nanotube agglomerates grow, the role of the surface interaction of the agglomerates decreases, and their final size and the behavior of the FB consisting of them depend on the conditions under which the hydrodynamic forces destroying the agglomerates balance the cohesion forces [4]. Under unfavorable conditions, a global violation of the homogeneity of FB and formation of stagnant zones and jets can occur, leading to the defluidization and loss of main technological advantages. Here, the relationship between the type and morphology of dispersed powders of catalysts for the synthesis of MWCNTs and cohesive properties of primary nanotube agglomerates growing on a separate catalyst particle are investigated. Three main types of catalysts were studied: 30% Fe-Co/Al2O3; 40% Fe-Co/Al2O3 and 40% Fe-Co/CaO, which provide MWCNTs with varying average diameters (7, 10, and 20 nm, respectively). The morphology of the initial catalysts and the MWCNTs obtained with their use in reactors with fixed and fluidized beds (at T = 660-680 °C, C2H4: Ar = 1: 1) was investigated by microscopy methods. In addition, bulk density of the powders and their specific surface were measured, and the analysis of the particle size distribution and fractality was carried out. The fluidity of MWCNT powders was evaluated using a drum-type powder rheometer. Data were obtained on the fractality of catalysts and MWCNTs produced, and their influence on the size distribution of primary and secondary agglomerates of MWCNTs formed at different stages of the growth process was analyzed. The study of the fluidity of powders by the dynamic method made it possible to establish the dependence of the cumulative energy of avalanches on the time between them, as well as to measure the fractality of the dynamic and stationary layer of powders. These data were used to assess the cohesive properties of catalyst powders and MWCNTs. The cohesive properties obtained on various types of catalysts are compared with the stability of the FB pilot reactor with internal diameter of 12 cm and a capacity of up to 10-12 kg MCNTs per day. The requirements for ensuring this FB-reactor stable operation are determined. The numerical simulation of the hydrodynamic behavior of the MWCNT powder in the drum rheometer and directly in a fluidized bed was also performed. The method of discrete elements coupled with continuous gas flow simulation, CFD-DEM, was used. The established correspondence between the mechanical characteristics of model particles and experimental data on the properties of MWCNT agglomerates made it possible to close the causal chain "catalyst properties - properties of nanotube agglomerates – fluidization regimes of MWCNTs". Together with the experimental results, this allows to formulate requirements for catalysts that provide conditions for uniform fluidization of MWCNTs. References [1] O.S. Rabinovich, V. A. Borodulya, A. N. Blinova, V. L. Kuznetsov, A. I. Delidovich, D. V. Krasnikov. Simulation of transient processes of the catalytic synthesis of carbon nanotubes in a fluidized bed // Theor Found Chem Eng. – 2014. V. 48. – No. 1. – Pp. 1-14. [2] A. Tsytsenka (Blinova), V. Kuznetsov, S. Moseenkov, D. Krasnikov. A model for catalytic synthesis of carbon nanotubes in a fluidized-bed reactor: Effect of reaction heat // Chem. Eng. J., 2017. – V. 329. – P. 305-311. [3] Wang Y., Cheng Y., Jin Y., H.T. Bi . On impacts of solid properties and operating conditions on the performance of gas-solid fluidization systems // Powder Technology. – 2007. – V. 172. – Pp. 167–176. (Department of Chemical Engineering, Tsinghua University, Beijing, China. [4] Valverde Millán, J.M. Fluidization of fine powders: cohesive versus dynamical aggregation // Springer. – 2012. – 134 p. (University of Seville, Spain) Acknowledgments: The work was carried out with the financial support of the Russian Foundation for Basic Research (project No. 20-53-00031) and the Belarusian Republican Foundation for Fundamental Research (project No. T20R-349).

Cite: Kuznetsov V.L. , Moseenkov S.I. , Zavorin A.V. , Golubtsov G.V. , Goidin V.V. , Rabinovich O.S. , Malinovskii A.I. , Lyakh M.Y.
Influence of Catalyst Characteristics on the Formation of MWCNT - Agglomerates During Synthesis in a Fluidized Bed Reactor
XXIV International Conference on Chemical Reactors. CHEMREACTOR-24 12-17 Sep 2021