The choice of material for a crucible is essential in hybrid microwave heating and graphite is one of the strong microwave absorbers which can sustain high temperatures. Experiments were carried out to investigate the microwave heating behavior of graphite combined with other materials to enhance efficiency. Mixtures comprising of graphite and magnetite or graphite and silicon carbide were found to have better microwave heating properties than pure components. The enhancement was likely due to microplasma generated by graphite in a microwave environment. Microwave heating of graphite-silicon carbide mixtures provided the best results in terms of high heating rates without arcing. The optimal combination of graphite-silicon carbide was identified and can be used as a starting material for making efficient crucibles and susceptors. © 2013 Elsevier Ltd.

Tanmay Basak

Department Of Chemical Engineering

Srinivasan Ramanathan

Sekaran Chandrasekaran

HIGH HEATING RATES

High temperature

MICRO-PLASMAS

Microwave absorbers

OPTIMAL COMBINATION

POWDER MIXTURES

Pure components

SUSCEPTORS

magnetite

metamaterials

Microwave heating

Mixtures

Plasma devices

Silicon carbide

Graphite

IIT Madras is a public technical and research university located in Chennai, Tamil Nadu. Founded in 1959, it is recognised as an Institute of National Importance.

IIT Madras has been ranked as the top engineering institute in India for four years in a row by the National Institutional Ranking Framework of the MHRD

It currently offers undergraduate, postgraduate and research degrees across 16 disciplines in Engineering, Sciences, Humanities and Management. About 596 faculty belonging to science and engineering departments and centres of the Institute are engaged in teaching, research and industrial consultancy.

IIT Madras

International Communications in Heat and Mass Transfer

Microwave pyrolysis is an efficient technique to valorize the abundantly available Prosopis juliflora (PJF) biomass into fuel intermediates. In this study, the effects of microwave power, susceptor, PJF particle size, PJF to susceptor mass ratio, and initial mass of PJF on bio-oil, gas, and char yields, composition of bio-oil, and energy recovery in bio-oil and char were evaluated. Five different susceptors, namely, graphite, char, aluminum, silicon carbide, and fly ash, an industry waste, were utilized. A high bio-oil yield of 40 wt % with a heating value of 26 MJ kg-1 was achieved with fly ash at a microwave power of 560 W, PJF particle size of 2-4 mm, and PJF (50 g)/fly ash composition of 100:1 (wt/wt). The bio-oil contained a mixture of phenolic compounds, aromatic hydrocarbons, cyclopentanones, carboxylic acids, ketones, and furan derivatives. Nearly 51% deoxygenation of PJF was achieved with an atomic O/C ratio of 0.24 in bio-oil. This work demonstrates that the yield and quality of bio-oil are dependent on key parameters such as microwave power, biomass particle size/composition, and type of susceptor. © 2015 American Chemical Society.

Energy and Fuels

Bio-oil production from Prosopis juliflora via microwave pyrolysis

Abstract In this study, microwave assisted pyrolysis of polypropylene (PP) was conducted, and the effects of microwave power, susceptor type, and PP to susceptor ratio on (i) temperature and heating rate profiles, (ii) overall oil, gas and char yields, (iii) composition of oil and gas, and (iv) heating value of oil, were evaluated. Pyrolysis experiments were carried out in a multimode microwave reactor equipped with accurate temperature monitoring system. The individual constituents in oil and gaseous fractions were analyzed using gas chromatograph equipped with mass spectrometer and flame ionization detectors. Six different microwave susceptors such as graphite, aluminum, silicon carbide, activated carbon, lignin and fly ash were utilized. At optimal conditions of 450 W microwave power and 100:1 wt./wt. of PP (5 g):graphite, nearly 48 wt.% of oil with heating value of 44.45 MJ kg -1 was obtained along with 50 wt.% of C3-C6 gaseous hydrocarbons. The scalability of the process was evaluated by pyrolyzing 50 g of PP under the above conditions, which resulted in 83% energy recovery in the oil. While alkenes and cycloalkanes constituted a major fraction in the oil, their relative yields varied significantly with different susceptors suggesting that the susceptors also acted as catalysts and interacted with the polymer besides transferring the incident microwave energy. The applicability of the technique was also demonstrated for pyrolysis of commercial polyethylenes and polyisoprene. © 2015 Elsevier B.V.

Journal of Analytical and Applied Pyrolysis

Resource recovery from synthetic polymers via microwave pyrolysis using different susceptors

Microwave heating is caused by the ability of the materials to absorb microwave energy and convert it to heat. This article represents a review on fundamentals of microwave heating and their interaction with materials for various applications in a comprehensive manner. Experimental studies of single, multimode, and variable frequency microwave processing were reviewed along with their applications. Modeling of microwave heating based on Lambert's law and Maxwell's electromagnetic field equations have also been reviewed along with their applications. Modeling approaches were used to predict the effect of resonances on microwave power absorption, the role of supports for microwave heating, and to determine the nonuniformity on heating rates. Various industrial applications on thermal processing have been reviewed. There is tremendous scope for theoretical and experimental studies on the athermal effects of microwaves. Some of the unresolved problems are identified and directions for further research are also suggested. © 2011 American Institute of Chemical Engineers (AIChE).

AIChE Journal

Microwave material processing-a review

A theoretical analysis has been carried out to analyze efficient microwave assisted heating processes of food cylinders with various shapes of metallic annuli in presence of lateral and radial irradiation. Lateral irradiation represents the sample incident at one direction with source at infinity whereas the radial irradiation represents the situation where the sample is incident with microwave radiations from co-axial cylindrical cavity at infinity. A generic finite element method has been used to obtain computational mesh for all shapes of metallic annulus and Galerkin finite element method has been used to solve electric field and energy balance equations. Influence of metallic annulus has been illustrated for three representative sample lengths or regimes (I, II and III) as a function of wave number (Nw). Sample radius with regime I is smallest and largest sample radius corresponds to regime III. In general, power absorption due to lateral irradiation is larger for regime I whereas identical power with lateral or radial irradiation or larger power absorption with radial irradiation corresponds to regime II in absence of metallic annulus. The metallic annuli are found to enhance power absorption or heating rate for specific aspect ratios of both bread and beef samples. The aspect ratios are defined as the ratio of the dimension of metallic annulus and outer radius of the food sample. Lateral irradiation is found to give larger heating rates especially for regimes II and III for bread samples (low dielectric loss), but radial irradiation gives smaller degree of thermal runaway. On the other hand, radial irradiation gives significantly large heating rates with minimal thermal runaway for beef samples (high dielectric loss) with all regimes. It is interesting to observe that the metallic annuli enhance two to three times microwave power for beef samples involving regimes II and III. Various shapes of metallic annuli are also found to play dominant role either to focus or to optimize heating effects. © 2010 Elsevier Ltd. All rights reserved.

International Journal of Heat and Mass Transfer

Theoretical analysis on the role of annular metallic shapes for microwave processing of food dielectric cylinders with various irradiations

Experiments were conducted for microwave heating and melting of lead, tin, aluminium and copper with the aid of susceptors and the detailed results were presented for various microwave power levels and sample loading. Aluminium and copper samples were heated in presence of inert gas to minimize oxidation. Compared to conventional melting, microwave melting was twice as fast and more energy efficient. Lumped parameter model of the heating process showed that the conversion of microwave to thermal energy was enhanced at higher temperatures, justifying this a favourable process for metal melting applications. © 2010 Elsevier B.V. All rights reserved.

Journal of Materials Processing Technology

Experimental and theoretical investigation on microwave melting of metals

The heating of porous dielectrics, beef-air (b/a) and beef-oil (b/o), attached with ceramic, metallic or ceramic-metallic composite supports, has been studied analytically using microwaves. Three test porosities were considered, 0.3, 0.45 and 0.6. A preliminary understanding of enhanced heating and power absorption within the material was obtained from the average power vs slab depth plots. The maxima in power, also termed as 'resonances', are observed for specific sample thicknesses and the two consecutive resonances are termed as R1 and R2 modes. It is observed that, for both b/a and b/o samples, higher intensity of resonances for power absorption occur in the presence of metallic and alumina-metallic composite supports. The detailed spatial distribution of power illustrates that the sample attached with the metallic support attains lower power distribution for both b/a and b/o samples. The low power absorption near the unexposed face has been enhanced by ceramic-metallic composite support. The overall heating efficiency is also enhanced with ceramic-metallic composite supports. The optimal heating strategy has been derived based on large heating rate with small thermal runaway. Metallic support may be recommended as optimal heating strategy for higher porosities (φ{symbol} ≥ 0.45) whereas alumina-metallic composite support may be suitable for smaller porosities (φ{symbol} ≤ 0.45) for b/a samples. Metallic or alumina-metallic composite supports give optimal heating effects for b/o samples. For both b/a and b/o samples, the thermal runaway is larger at higher porosities. © 2006 Elsevier Ltd. All rights reserved.

Role of metallic and composite (ceramic-metallic) supports on microwave heating of porous dielectrics

A theoretical analysis has been carried out to study efficient microwave heating of porous dielectrics. The heating effects are analyzed for two types of porous material: beef-air (b/a) and beef-oil (b/o) with and without ceramic supports (Al2O3 and SiC). Three test cases for porosities (φ{symbol}) 0.3, 0.45 and 0.6 are considered. The maxima in average power corresponding to resonances occur at various sample thicknesses for all porous materials with and without supports and two dominant resonance modes R1 and R2 are considered where the average power at R1 is larger than that at R2. It is interesting to observe that average power absorption is enhanced for samples (b/a and b/o) in presence of Al2O3 support whereas the average power is smaller with SiC support. From the analysis on spatial distribution of electric field, power and temperature, it is seen that runaway heating is observed at the face which is not attached with support for b/a samples, and the intensity of thermal runaway increases with porosity whereas lower thermal runaway is observed for b/o samples at all porosity values. An efficient heating strategy has been investigated for various distributions of microwave incidences. It is observed that one side incidence may correspond to the largest heating rates whereas distributed sources may correspond to smaller thermal runaway for both beef-air (b/a) and beef-oil (b/o) samples. © 2006 Elsevier Ltd. All rights reserved.

Journal	International Communications in Heat and Mass Transfer
ISSN	07351933
Open Access	No