(235d) Carbon Nanotubes Production by Means of Thermal Treatment of Polymers in Fluidized Bed Reactors

Carbon nanotubes (CNTs) are currently obtained by using processes characterized by high cost and low yield of production. This strongly limits their use in the industrial market specially for low-added value applications [1, 2]. Present methods for production of CNTs utilize graphite, carbon monoxide or hydrocarbons as raw materials, different kinds of metals as catalysts and operating conditions that are generally critical and then very expensive. A number of new procedures to obtain multi-wall carbon nanotubes (MWCNTs) has been recently proposed by several Authors. In particular, the pyrolysis of hydrocarbons (like propylene) in a continuous reactor has been described by Wang et al. [3] as a process that could potentially give mass production of MWCNTs at a reasonable cost, even though there is still the problem of keeping both catalysts and CNT product, fluidized during the whole growing period, within the reactor. Other Authors [4-6] claim the possibility to obtain carbon nanotubes by means different high temperature thermolysis processes, generally carried out under reducing atmosphere, and applied to carbonaceous substrates (as synthesized polymers) that fill metallic templates. More recently Arena and Mastellone [7, 8] proposed a new process that allows a massive, low-cost production of MWCNTs by means of a continuous thermal cracking process of virgin or recycled polyolefins. The technique produces carbon nanotubes having quality similar to MWCNTs available on the market even when the starting materials were polyolefins coming from post-consumers waste collection and recycling. The reactor used is an atmospheric bubbling fluidized bed, operated under different conditions of temperature, fluidizing gas velocity and residence times in the bed and in the freeboard. A very deep investigation on the composition of tar phase of pyrolysis products showed [7, 8] the presence of a large fraction of MWCNTs, with an amount which depends on the type of starting polymer and on operating conditions used (temperature, gas residence time, bed material properties). The paper reports the results obtained under specific pyrolysis conditions, in term of yields and composition of gas, tar and CNT. The composition of the produced gas was evaluated by means of the Agilent Micro GC 3000 that was used for on-line data acquisition (with a frequency of a single measurement of spectrum from hydrogen to methylbenzene each 30min) and of a GC-MS that was used, for off-line measurements, to analyse the content of active carbon phial and/or that of a Tedlar bag used to collect about 5dm3 of gas. The tar was collected in a condenser, kept at -45°C and characterized by means of a Perkin-Elmer/Pfeffer TG-MS by using a flux of 60cm3/min of air or nitrogen, an heating rate of 20°C/min and a temperature range 50-900°C. The MWCNTs, and their yield in the solid phase, were then characterized by termogravimetrical analysis as well as SEM and TEM microscopy.

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