(70bz) Fluidization Characteristics, Solids Distributions and Gas Mixing of a Carbon Nanotube Agglomerate Fluidized Bed

       The fluidization of nanoparticles in the size range of 1 to 100nm has a variety of industrial applications. Despite the difficulty for fluidizing nanoparticles due to their strong interparticle forces, case studies have indicated that fine particles can be fluidized in the form of agglomerates ^[1-3]. Many efforts have been made to summarize the characteristics of nanoparticle fluidization and to improve the fluidization quality. However, the experimental results on the fluidization in the regimes except for agglomerate particulate and bubbling fluidizations (APF and ABF ^[4]) are quite little, despite the practical significance of a full-regime classification. Furthermore, although the information on solids distribution and gas mixing is obviously significant for design and scale-up, these detailed hydrodynamics behaviors are also scarcely reported.

Carbon nanotubes (CNTs) ^[5] are a novel nano fiber material with excellent physical and chemical properties. Recently, we realized a production of multi-walled CNTs on magnitude of kilograms per hour in a fluidized bed ^[6], which provided an opportunity to investigate the hydrodynamics of CNTs and deepen the knowledge on the fluidization of nanoparticles. We studied the fluidization characteristics, flow regime transitions, and hydrodynamic behaviors, such as solids distributions and gas mixing of CNTs in a fluidized bed. The results were compared with traditional particles to understand some universal characteristics of nanoparticles, which is helpful for the process engineering handling nanoparticles.

CNTs were fluidized in a 280mm inner diameter (ID) fluidized bed. The fluidization phenomena were observed in a wide range of gas velocity from minimum fluidization to 0.3m/s, covering all the fluidization regimes except for pneumatic transport. An electrical conductance probe method was developed to measure the local solids concentration in a CNT fluidized bed. By this method, time-averaged and transient solids fractions can be obtained simultaneously. A steady-state hydrogen tracer technique was used to obtain the axial and radial gas dispersion coefficients.

Via the formation of CNT agglomerates, smooth and highly expanded fluidization was achieved, but strong hysteresis exists in the CNT fluidization, which indicates that the formation of agglomerates changes the interparticle actions significantly. In the defluidization branch of bed expansion experiment, as like as Geldart-A particles, APF, ABF, turbulent and fast fluidizations can be successively observed and distinguished by following criterions: APF: 0.017~0.034m/s, ABF: 0.034~0.1m/s, turbulent fluidization: 0.1~0.205m/s, and fast fluidization: >0.205m/s. However, U_c and U_se of CNTs are relatively lower than that of Geldart-A particles, because of the weaker inter-agglomerate forces. The hysteresis and flow regimes were shown in Fig. 1.

Surprisingly, it was found that the APF was not always uniform as its apparentness, but dependent on gas velocity, as shown in Fig. 2. Furthermore, in the aggregative fluidization, the distribution of time-averaged solids fractions also exhibit stronger radial non-uniformity than that of Geldart-A particles. Empirically, the local solids fraction in the aggregative fluidization can be calculated by:

.

A statistical analysis to the transient density signals showed typical two-phase structures in CNT fluidization, as shown in Fig. 3, and demonstrated that the non-uniformity in radial solids distribution was arisen from the serious aggregation among agglomerates near the wall, in the way of increasing volume fraction of dense phase near the wall. However, on micro-structure scale, the gas-CNT flow is more homogenous than the Geldart-A particle fluidization, which benefit the gas mixing behaviors, leading to high gas dispersion coefficients of about 10¹ m²/s for D_a and 10² cm²/s for D_r.

Figure 1 The bed expansion of a CNT fluidized bed.

(a)                                                                  (b)

Figure 2 Radial solids fraction profiles in APF (a) and ABF (b) of a CNT fluidized bed.

Figure 3 Probability density distributions (PDDs) in CNT fluidization at U_g=0.057m/s, .

Acknowledgements

This work was supported by a grant from the national '863' program (No. 2003AA302630) and National Natural Scientific Foundation of China (NSFC, No. 20236020).

References

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