Lee Min Ji, Kim Sung Won
School of Chemical and Material Engineering, Korea National University of Transportation, Chungju-si, Chungbuk 27469, Korea.
Materials (Basel). 2020 Mar 12;13(6):1289. doi: 10.3390/ma13061289.
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 -cresol known as processing solvents for fabrication of the CNT microbeads. Multiwalled CNT powders were directly mixed with -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/m, 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.
流化床已被用于各种化学和物理应用，包括传热，如气固热交换器。使用具有高导热率的碳纳米管（CNT）作为床层材料，以增强直接气固接触式热交换器中的传热是有利的。然而，由于颗粒之间的强内聚力，CNT的流化性能差是最大的挑战。为了其应用，需要控制CNT粉末的宏观形状。已提出一种制备CNT微珠的方法，该方法适用于流化床应用。该方法的特点是使用被称为加工溶剂的对甲酚来制造CNT微珠。将多壁CNT粉末直接与对甲酚混合，得到浓稠的糊状材料。将糊状物液滴在有和没有表面活性剂的纯水中滚成圆形颗粒。将得到的颗粒在真空烘箱中干燥。所获得的微珠直径范围为300 - 2200μm，表观颗粒密度为350 - 400 kg/m³，在流化分类中对应于 Geldart B组。CNT微珠的显微照片显示表面有堆叠的纳米管阵列，表明原始CNT粉末明显致密化。在含有表面活性剂的水中制备的微珠具有更好的形状因子，如圆形度和紧实度。微珠在块状状态下的热导率约为1.18 W/mK，远高于原始CNT粉末（0.032 W/mK）。多壁CNT（MWCNT）微珠的流动性和流化特性表明其有可能成为适用于流化床热交换器的有前景的床层材料。
