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Ind. Eng. Chem. Res. 2008, 47, 7686–7692
Process Parameter Interaction Effects during Carbon Nanotube Synthesis in Fluidized Beds Chee Howe See,* Oscar M. Dunens, Kieran J. MacKenzie, and Andrew T. Harris Laboratory for Sustainable Technology, School of Chemical and Biomolecular Engineering, UniVersity of Sydney, Australia, NSW 2006
The interaction effects between temperature, catalyst properties, fluidization conditions, and deposition time during carbon nanotube (CNT) synthesis by chemical vapor deposition in a fluidized bed were investigated. While numerous investigations have attempted to correlate process parameters with CNT characteristics, selectivity and yield, the interaction between process parameters is often ignored. Parametric interactions in this process have been investigated using a factorial design methodology. Besides the main effects of synthesis temperature, deposition time, and catalyst type, the interaction parameters temperature-time and temperature-catalyst were found to significantly influence the resultant carbon and CNT yields. These results lay the foundation for a detailed parametric analysis toward the optimization of CNT synthesis in fluidized beds, which takes into account these interaction effects. 1. Introduction Carbon nanotubes (CNTs) have been investigated for a multitude of applications including energy storage,1 field emission devices,2 and composite material strengthening,3 because of their unique mechanical, optical, and electrical properties.4 Both multiwalled (MWCNTs) and single-walled (SWCNTs) carbon nanotubes can be used, depending on the value-added functionality required in the product. Nevertheless, the use of CNTs in both research and end-use applications is currently inhibited by production throughput.5 Of the three dominant techniques used for CNT synthesis, that is, laser ablation, arc discharge, and chemical vapor deposition (CVD), the latter is recognized as having the most potential for large-scale, economically viable CNT production.6,7 In particular, we concur with De Jong and Geus8 that the most likely low cost scenario for CNT production is to conduct the CVD reaction in a fluidized bed reactor. The most prominent advantages of fluidized bed CVD (FBCVD) compared to traditional fixed-bed CVD are the enhanced heat and mass transfer, continuous operation, and scalability; characteristics which lead to lower costs of production.9 Despite its potential, we highlighted in a recent review, the dearth of research investigating the (FBCVD) technique for large-scale CNT capability.10 When the results of several studies are analyzed collectively,7,9-20 there appears to be no clear correlation between the synthesis parameters and CNT characteristics, yield, and selectivity. Taken to an extreme, this suggests that the synthesis of CNTs in a fluidized bed is a noncontrollable process. This is almost certainly incorrect, runs counterintuitive to the behavior of similar fluidized bed processes, and at the very least, requires verification. Hence, in this work, we have undertaken an experimental study to elucidate the influential parameters; employing a statistical experimental design methodology to investigate the influence of (i) synthesis temperature, (ii) deposition time, (iii) catalyst type (Fe, Fe-Co), (iv) catalyst loading (2.5, 5 wt %), and (v) total gas flow rate on carbon yield in a 0.5 kg/h FBCVD process. * To whom correspondence should be addressed. E-mail: c.see@ usyd.edu.au. Tel.: +61-2-9036-6244. Fax: +61-2-9351-2854.
Although fractional factorial design (FFD) has been employed in CNT research previously,21-23 we report for the first time, the use of this methodology for CNT synthesis via FBCVD. The advantage of using FFD is that statistically meaningful insights can be obtained using a reduced experimental set. Furthermore, besides the main effects, interaction effects (between different process parameters) can be analyzed simultaneously. In addition, the FFD design tool provides a simple validity test of the “optimal parametric envelope” during reactor scale-up, again using minimal experiments. We note that, to our knowledge, this is the first time that process interaction effects have been investigated for CNT synthesis via FBCVD, although this information may well exist in a commercial environment. The results from this study verify the degree of influence of process parameters and their interactions on the desired output criteria, laying the foundation for future detailed parametric analyses toward the optimization of CNT synthesis via FBCVD. 2. Experimental Details A typical apparatus setup is shown in Figure 1. The 52 mm internal diameter, 1000 mm long cylindrical fluidized bed, constructed entirely of Inconel 601 and enclosed within a high temperature furnace, was operated as a batch reactor in this study. It has an approximate capacity of 0.5 kg CNT/h. While this diameter is less than the required minimum for many classical fluidized bed scale-up “rules-of-thumb”, rig commissioning fluidization tests show that well-developed fluidization occurs, and the diameter is sufficiently large to negate the substantial wall effects of very small reactors (e.g., 6.4 mm)16 without requiring impracticably large catalyst volumes. An expansion unit, 100 mm in diameter and 500 mm long, was affixed to the top of the reactor to minimize particle entrainment. Particle scrubbers were incorporated to treat effluent gases prior to release. Gas flow to the reactor was controlled via a series of Alicat Scientific, Series 16 mass flow controllers. Catalysts were prepared using impregnation.24 In brief, a weighed amount of sieved calcined alumina (-90 µm, +106 µm) substrate was added to an ethanolic solution of Fe or Fe-Co salts in the appropriate proportions to result in either Fe or Fe-Co (1:1) catalyst, with a total metal loading of either
10.1021/ie701786p CCC: $40.75 2008 American Chemical Society Published on Web 09/20/2008
Ind. Eng. Chem. Res., Vol. 47, No. 20, 2008 7687
Figure 1. Sketch of the fluidized-bed reactor setup. A cylindrical reactor is affixed within a high-temperature furnace with temperature, pressure, and gas flow controls, connected to a data logging system, as described in ref 10.
2.5 or 5 wt %. These metal loadings are representative of ratios reported in the literature.15,25 The mixture was air-dried overnight at 40 °C prior to calcination in air for 12 h at 900 °C. Approximately 80 ( 5 g of calcined catalyst was used in each experiment, corresponding to an initial bed height of 38 ( 2 mm. This was reduced in situ at 700 °C in 1.5 SLPM, 30% H2/N2 for 1 h before ethylene (the carbon source) was introduced. The synthesis of CNTs was carried out according to the conditions depicted in the full factorial design given in Table 1, investigating the effect of (i) synthesis temperature (600 or 800 °C), (ii) deposition time (20 or 60 min), (iii) catalyst type (Fe or Fe-Co), (iv) catalyst loading (2.5 or 5 wt %), and (v) total gas flow rate, reported as the ratio of the gas velocity through the bed to the minimum fluidization velocity at 800 °C under N2 (U/Umf ) 3 or U/Umf ) 6). The output variables were the total carbon yield, defined as TGA weight loss occurring in the temperature range 250-800 °C, and carbon nanotube yield as defined in section 3. A total of 32 runs were conducted in randomized order. The as-synthesized products were analyzed using thermogravimetric analysis (TGA; TA Instruments SDT Q600), transmission electron microscopy (TEM; Philips CM120, 120 kV) and Raman spectroscopy. For TGA analysis, a sample weight of ≈60 mg was used. The large sample size was chosen to negate the effects of instrument error across the board, because in some instances a weight loss of F
F value
p-value prob > F
model A, temperature B, U/Umf C, deposition time D, catalyst AB AC AD BC BD CD R2
6.3 41.3 3.1 28.8 1.1 0.2 23.6 3.1 2.9 0.2 0.261 0.898
