Microwave Pyrolysis of Plastic for Production of Fuel and Other Chemicals

Abstract

Microwave heating has been used in the chemical industry for many years for diverse applications due to advantages, such as volumetric heating, high power density, and fast and easier temperature control. One of the major challenges in the application of microwave heating is the design of an efficient and uniform-heating reactor to pyrolyse a highly microwave transparent material, such as plastic, on a scale larger-than-laboratory. The aim of this study was to improve the understanding of the pyrolysis of plastic considering the use of both thermal and microwave energy as a heat source. A goal was to use the available knowledge to design and build a rotating microwave reactor, to attain homogeneous temperature distribution in anticipation of producing pyrolysis products with uniform molecular hydrocarbon distribution. Finally, it was sought to investigate the effect of process parameters on products’ yield and composition, and examine their suitability as fuel and useful chemicals. In this study, thermal kinetic of pyrolysis of plastic was studied quantitatively under isothermal and non-isothermal conditions using a gas chromatography flame ionisation detector (GC/FID) and a thermal gravimetric analysis apparatus, respectively. Microwave heating was evaluated qualitatively taking multiphysiscs into consideration (the chemical reaction, electromagnetic field and heat transfer). This was done by solving conservation equations using MATLAB code. An understanding of the microwave field distribution, and intensity and absorbed microwave power inside the microwave system was obtained using Microwave Studio® software. Determination of the dimensions of the microwave system was assisted by an electromagnetic simulation study aiming to maximise the effective heating in the reactor. An innovative rotating microwave reactor was designed and fabricated using a coaxial transmission structure without the limitations of the commonly-used enclosed glass quartz reactor, which made the design appealing for any future industrial-sized microwave reactor. To the author’s knowledge, no one else has designed and built a similar reactor. The pyrolysed products (oil/waxes) were quantitatively analysed using GC/FID. The optimum pyrolysis operating condition (low nitrogen flow rate and controlled microwave power) led to the production of a suitable product for a fuel application. However these products cannot be used directly as phase change materials due to their low latent heat and very broad melting range. Fractionation of the products may also be used as phase change materials, capable of storing/releasing heat at suitable application temperature. The microwave heating provided a more uniform heating distribution, although it did not alter product composition in comparison to the conventional pyrolysis of plastic. In the small reactor used in this project, microwave benefit was not evident with regards to its effect in creating uniform temperature distribution. The benefit may be clearer in an industrial-sized reactor in which heat penetration in conventional thermal reactor can be a serious problem. The results of microwave pyrolysis showed 73% oil/wax yield, with silicon carbide used as a microwave absorbent.