Plastics & Microwave Pyrolysis by Dr. Elham Khagami. (This article first published in 2016)
Energy is fundamental to economy and survival in today’s social activity. Clearly the search of alternative energy sources would have a high priority. Therefore, minimizing the quantity of waste being generated from the source energy such as recycling could be a potential remedy in waste management activities. Disposal of used consumer plastic has increased with industrial development. It is in everyone’s interest to reduce waste plastic, reuse goods containing high calorific values as high as crude oil calorific value, and recycle materials as landfills are no longer cheap and appropriate way to dispose the wastes.
Pyrolysis of plastic wastes is a well-known method to minimise the environmental effect. Pyrolysis of plastic wastes can be described as a chemical process and thermal decomposition of organic components in an oxygen-free atmosphere to yield char, oil and gas. Pyrolysis of plastic is an endothermic process, so that it requires a supply of heat. Two types of the heat sources can be addressed here i.e. conventional heat source and microwave. The convectional plastic thermal decomposition has been established to operate in industrial scale around the world. Using microwave as a heat source would open a new horizon in this topic.
Microwave Energy in Pyrolysis
Microwave energy can be delivered directly to the reacting or processing species by using their dielectric properties or by adding absorbers to materials which allows more volumetric heating of materials. The high heating rate can be several orders of magnitude greater than with conventional heating. Microwave generators can respond quickly to changes in process parameters with a feedback loop of an automated process.
Microwave heating results from induced currents so the heating tends to be volumetric however the penetration of microwaves is influenced by the properties of the material. The field penetrates it losses power and therefore the field intensity will decrease suggesting that heating may not be uniform.
Microwaves interact in three ways with different materials. It is reflected by conductors, transmitted by perfect insulators, or absorbed and decayed on the way inside materials depending on their dielectric properties. The heat is generated in dielectric materials due to agitation of molecules by the alternating electromagnetic field.
Advantages of microwave technology may facilitate moving forward to produce clean, fast and high quality product. Further study should be performed to get a clear picture of microwave pyrolysis of plastic process at high temperature. Challenges such as controlling electromagnetic field and uniformity, temperature measurements may require more sophisticated approaches to be tackled. Albeit in the light of present technique and instrument of temperature measurement, it is not easy to acquire a precise result of the temperature distribution from the interior of the medium at high frequency and high temperature. The only question remains to be answered is whether it is possible to achieve microwave pyrolysis of plastic with uniform heating in much less time comparing to conventional heating on a reasonable scale and economical.
Microwave technology can be very useful in chemical processing because products can be heated volumetrically instead of surface heating via convection and conduction. It has the unique selective heating feature that cannot be provided by conventional methods.
Biomass, scrap tyre and municipal solid waste are among the most common types of materials that have been subjected to microwave energy. Microwave technology has established in different field areas such as processing, heating and drying food, medical, waste management, pyrolysis of different materials (tyres, plastics, timber, and biomass), sintering, cooking, pasteurizing, and synthesis of chemical compounds, ceramics and many other processes.
Plastics cannot absorb microwave energy, as it has a very low dielectric loss factor. Therefore, an absorbent must be mixed with the plastic to aid in heating the plastic in pyrolysis. Materials with high dielectric loss factor are good candidates as an absorbents for plastic pyrolysis e.g. tyre shredded, silicon carbide and carbon. The mechanism of plastic microwave pyrolysis is based on absorbing the microwave energy via absorbent and subsequently transferring thermal heat to the plastic via conduction. The physical properties and the volume ratio of the absorbent affects the uniformity of heating distribution.
Microwaves are widely used in the chemical industry to accelerate chemical reactions. The biggest challenges in this area of study are the nonlinear response prediction of the reaction system to the microwave and the design of highly efficient and homogeneous-heating reactors. Not many investigations have quantitatively evaluated microwave heating taking multiphysiscs into consideration (chemical reaction, electromagnetic field and heat transfer). Modelling of the microwave heating is vital to achieve a good understanding of the volumetric heating process. Microwave heating is complicated and is not easily modelled because the rate of energy absorption and temperature distribution within the product is governed by the physical, thermal and electrical properties of the products, which change with temperature during radiation and field distribution. The main aim in microwave reactor design based on modelling results is to efficiently couple the microwave energy with the plastic.
There are some limitations in the design of the microwave reactor, related to the thermal runaway, voltage breakdown and arcing. In the design of microwave unit the following tasks must be taken into account: 1. The field intensity should be limited to prevent arcing. On the other hand, the field intensity must be high enough to allow rapid treatment without significant heat loss. 2. Indifferent contacts in the inner surfaces of the unit (flanges, fittings, etc.) create high electrical resistance, leading to localised overheating, which causes arcing, so all gaps must be eliminated to prevent the sapping of microwave energy. 3. Dust must not be allowed to enter the waveguide system, as it will cause overheating and arcing. 4. Sharp corners and edges must be avoided. 5. Correct and proper tuning procedures must be conducted to prevent coupling the arc with reflected power.
The Bottom Line
The volumetric heating, higher power densities, selective energy absorption, instant on and off control without pre or post preparation, improved yields and enhanced product performance make microwave heating a better option to conventional heating in some applications although creating uniform energy distribution in large scale can be challenging.
A few studies have outlined the scale up challenges in microwave process. There is currently no industrial scale application of microwave pyrolysis of plastic. The reason may be explained by the difficulties of combining the chemical and electrical engineering technologies to meet the requirements for a high temperature microwave processing of plastic degradation. Due to the complex nature of pyrolysis of plastic a very particular and detailed design with the help of a robust electromagnetic simulation model is needed in order to achieve this goal. The available software in the market like QuickWave-3d, COMSOL and Microwave Studio can provide some insight into microwave behavior with regards to energy dissipation and electromagnetic wave distribution.
In other words the key to overcome this hurdle is a precise step by step approach to design a reactor for a specific application. This includes conducting a numerical modelling considering multiphysiscs. In addition, study the kinetic parameters of the plastic under microwave radiation. In which cannot be done unless the temperature monitoring techniques within the sample at 2.45 GHz frequency and high operating temperature well established in advance.
Khaghani, E. and Farid, M.M., Mathematical Modelling of Microwave pyrolysis, International Journal of Chemical Reactor Engineering, 2013, Volume 11, Issue 1, Pages 543–559, DOI: 10.1515/ijcre-2012-0060
Khaghani E, Farid MM, Holdem J and Williamson A, Microwave Pyrolysis of Plastic, Journal of Chemical Engineering & Process Technology, 2013, Volume 4, Issue 3, DOI:10.4172/2157-7048.1000150
About the Author
Dr. Khaghani is a chemical process engineer, who conducted research and developed the thermo-chemical processes for converting plastic waste into fuels using thermal and microwave energy in University of Auckland during my PhD. The study was involved kinetic studies, mathematical modelling, reactor design, interpretation of product composition. She has worked as a process engineer in oil and gas sector and carried out engineering investigations, feasibility studies, preliminary designs, cost estimations, detailed designs, Front End Engineering Design (FEED), process simulation and modelling and prepared associated reports (Basis of Design, Technical Notes and Queries, Design Reviews, HAZOPs).