Carbon nanotube reactor: Ferrocene decomposition, iron particle growth, nanotube aggregation and scale-up
Carbon nanotube reactor: Ferrocene decomposition, iron particle growth, nanotube aggregation and scale-up
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DOI：
10.1016/j.ces.2010.01.019
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发表时间：
2010-05-15
影响因子：
4.7
通讯作者：
Davidson, John
中科院分区：
文献类型：
--
作者：
Conroy, Devin;Moisala, Anna;Davidson, John
我们描述了一个数学模型的铁粒子的形成和生长的化学反应器中，我们用来研究碳纳米管（CNT）的形成。该模型用于理解反应器条件（例如温度和流速）对CNT直径的影响，已知CNT直径与铁颗粒动力学耦合，因为CNT尺寸对最终产品的材料性质具有主要影响。碳纳米管是在1144-1265摄氏度下在直径7.0厘米x高1米的垂直管式反应器中制备的，其中加入了由氢气带入顶部的乙醇、二茂铁和噻吩。二茂铁分解形成铁颗粒，其充当催化剂以从分解的乙醇形成CNT。然后CNT聚集，得到与反应器同心但直径较小的“袜”;袜在反应器底部附近会聚，作为缠绕在主轴上的垂直线出现。本文给出了分析预测：（一）速度和温度分布的气体;（二）铁粒子数，平衡化学速率的二茂铁分解对扩散和对流;（三）铁粒子的体积分数，由二茂铁分解，凝聚，和扩散。结果表明，在反应器的顶部，从直径为1 cm的注射管的流动产生的氢气的环形涡流;然而，低于约10 cm的顶部，氢气流是很好地描述了一个抛物线速度分布（Poiffille）。气体温度在距顶部约40 cm以下是均匀的（1265 ℃）。反应器中的铁颗粒尺寸由帽（c）上的无量纲基团（k）和帽上的无量纲基团（β）控制，其将二茂铁解离和凝聚与通过反应器的轴向气体对流相关联。对于这些基团的增加值，铁颗粒尺寸在喷射器下方的给定距离处较小。然后，我们将该模型与纤维内CNT直径的实验结果进行比较，该结果与催化剂尺寸的结果定性一致。两种假设可以解释袜子的形成：（1）该模型预测，铁颗粒在壁面附近长大，因为它们的停留时间较长。这些大颗粒不适合CNT生长，而且可能因焦化而中毒。在反应器中心线附近，没有足够的剪切力来促进CNT团聚。这两种效应意味着最佳的团聚半径，因此袜子的形成。(2)几位研究人员已经观察到，在Poiffille流中，球形颗粒在从壁测量的约0.185 x管半径处积聚。如果这适用于碳纳米管，它可能解释袜子的形成。此外，在cap上使用基团（beta）和在cap（c）上使用基团（k）意味着对于放大，反应器中半径与平均垂直速度的比率保持恒定，因此反应器输出与反应器半径的立方成比例。因此，工业规模的反应器应该是可行的。(C)2010爱思唯尔有限公司版权所有。
We describe a mathematical model for iron particle formation and growth within a chemical reactor, which we use to study carbon nanotube (CNT) formation. This model was used to understand the effect of reactor conditions (e.g. temperature and flow rate) on the CNT diameters, which are known to be coupled to the iron particle dynamics, since the CNT size has a major effect on material properties of the final product. The CNTs were made at 1144-1265 degrees C in a vertical tubular reactor, 7.0 cm diameter x 1 m high, fed with ethanol, ferrocene and thiophene carried into the top by hydrogen gas. The ferrocene decomposed to form iron particles, which acted as a catalyst to form CNTs from the decomposed ethanol. The CNTs then agglomerated, giving a 'sock' concentric with the reactor, but of smaller diameter; the sock converged near the bottom of the reactor, emerging as a vertical thread wound onto a spindle. This paper gives analyses to predict: (i) velocity and temperature profiles for the gas; (ii) iron particle number, balancing chemical rate of ferrocene decomposition against diffusion and convection; (iii) iron particle volume fraction, governed by ferrocene decomposition, coagulation, and diffusion. The results show that at the top of the reactor, flow from the 1 cm diameter injection tube generates an annular vortex of hydrogen; however below about 10 cm from the top, the hydrogen flow is well described by a parabolic velocity profile (Poiseuille). The gas temperature is uniform (1265 C) below about 40 cm from the top. The iron particle size in the reactor is controlled by the non-dimensional groups (k) over cap (c) and (beta) over cap, which relate ferrocene dissociation and coagulation to axial gas convection through the reactor. For increasing values of these groups the iron particle size is smaller at a given distance below the injector. We then compare this model to experimental results for CNT diameter within the fibre, which show good qualitative agreement with the results for catalyst size. Two hypotheses may account for sock formation: (1) The model predicts that the iron particles grow bigger near the wall, because of their longer residence times. These big particles are unsuitable for CNT growth and moreover likely to be poisoned by coking. Near the reactor centre-line, there is insufficient shear to promote CNT agglomeration. These two effects imply an optimum radius for agglomeration, and hence sock formation. (2) Several investigators have observed that, in Poiseuille flow, spherical particles accumulate at about 0.185 x tube radius measured from the wall. If this applies to CNTs, it might explain sock formation. Furthermore, use of the groups (beta) over cap and (k) over cap (c), implies that for scale-up, the ratio of the radius to the average vertical velocity in the reactor remains constant, so the reactor output is proportional to the cube of the reactor radius. Thus, an industrial-scale reactor should be practicable. (C) 2010 Elsevier Ltd. All rights reserved.