Preparation of MWCNT Microbeads for the Application of Bed Materials in a Fluidized Bed Heat Exchanger.

Min Ji Lee,Sung Won Kim

doi:10.3390/ma13061289

Abstract

Fluidized beds have been utilized for various chemical and physical applications including heat transfer such as the gas–solid heat exchanger. It is advantageous to use carbon nanotubes (CNTs) with high thermal conductivity as bed materials for heat transfer enhancement in a direct gas–solid contacting heat exchanger. However, the poor fluidization of CNTs is the biggest challenge due to the strong cohesive force between the particles. A control over the macroscopic shapes of CNT powders is required for their application. A preparation method of CNT microbeads has been proposed to be suitable for fluidized bed applications. The method is characterized by using m-cresol known as processing solvents for fabrication of the CNT microbeads. Multiwalled CNT powders were directly mixed with m-cresol to yield a thick paste-like material. The paste droplets were rolled into round particles with in pure water with and without surfactant. The obtained particles were dried in a vacuum oven. The obtained microbeads have diameters ranging 300–2200 μm and apparent particle density of 350–400 kg/m3, which corresponds to Geldart group B in the fluidization classification. The micrograph of the CNT microbeads exhibited stacked nanotubes array on the surface, indicating obvious densification of the raw CNT powders. The microbeads prepared in water containing surfactant have better shape factor such as circularity and solidity. The thermal conductivity of the microbeads is about 1.18 W/mK in a bulk state, which is much higher than raw CNT powder (0.032 W/mK). The flowability and fluidization characteristics of the multiwalled CNT (MWCNT) microbeads showed a possibility as promising bed material suitable for the fluidized bed heat exchanger.

Highlights

Fluidization is a physical phenomenon that makes solid particles into a fluid-like state using a fluid medium such as a gas or liquid [1]
The Carbon nanotubes (CNTs) used in this study are multiwalled CNT (MWCNT) produced in a fluidized bed reactor, which is based on the method of catalytic chemical vapor deposition (CCVD)
The shift of the maximum oxidation temperature of the microbeads is because the regularly stacked array structure inside the microbead obstructs the oxidation process of the CNT [26]. These results suggest that the CNT microbeads can be used stably up to about 500 ◦ C in the air–solid fluidized bed such as a heat exchanger operated at low and medium temperature [27], considering the possibility of initial oxidation of disordered carbon

Summary

Introduction

Fluidization is a physical phenomenon that makes solid particles into a fluid-like state using a fluid medium such as a gas or liquid [1]. The fluidized bed reactors have been utilized for fluid catalytic cracking, combustion, heat or mass transfer such as gas–solid heat exchange and solid coating [2,3]. Due to the mentioned reasons, there have been growing interests in an effective application of the nanoparticle in the fluidized bed in the last decade [4]. Carbon nanotubes (CNTs) among various nanoparticles have been mentioned as attractive materials for application in the fluidized bed because of their excellent thermal and electrical conductivities and mechanical strength [5]. Since the fluidized beds have high rates of heat and mass transfer, it could be advantageous to use the CNTs with high thermal conductivity as bed Materials 2020, 13, 1289; doi:10.3390/ma13061289 www.mdpi.com/journal/materials

Methods

Results

Conclusion