Morona Murillo, Lorena
(2023)
Hybrid microwave processing of particulate materials using a novel non-conventional fluidised bed.
PhD thesis, University of Nottingham.
Abstract
Microwave technologies have been widely used in various industries for heating applications, offering several benefits when compared to other conventional systems including faster heating rates, selective and volumetric heating, instantaneous control over energy delivery and the potential to be powered by a sustainable energy source. However, challenges arise when it comes to higher temperature industrial processing (exceeding ~100 ºC). Achieving heating homogeneity and uniform material treatment remains a notable issue and the requirement for bespoke designs often leads to expensive equipment and materials of construction. Most of the existing techniques to overcome those challenges rely on the complexity of the use of moving parts. Microwave fluidised-bed reactors have been subject to extensive research for their use in industrial applications, primarily due to their advantageous particle mixing capabilities, accompanied by the rapid and efficient heating provided by microwave energy. Nonetheless, these systems still have limitations, including reactor tube fragility, potential overheating, and limited overall robustness. A novel hybrid microwave toroidal fluidised-bed system, TorWave reactor, was developed collaboratively by the University of Nottingham and Torftech Ltd. to address the mentioned challenges when it comes to scaling up microwave processors.
In this work, a proof of concept one-litre hybrid microwave toroidal fluidised-bed reactor, TorWave 400 (T400) prototype, has been constructed and investigated for its use in microwave processing of particulate materials with particular emphasis in the industrial case study of coffee roasting.
Hydrodynamic characterisation of the T400 prototype has been carried out for the definition of its regimes of operation: packed bed, air channelling/bubbling, slugging, toroidal fluidisation and fast fluidisation or elutriation. Moreover, the developed hybrid microwave system has demonstrated the ability to overcome many of the challenges associated with existing microwave technologies including improved heating uniformity, process reproducibility and excellent temperature control. Moreover, this system has proven to achieve an excellent level of treatment homogeneity with a temperature coefficient of variation across the entire surface of the bed of the order of 2 %, showing an improvement over existing microwave technologies, placing it as a viable candidate for scaling-up. A mathematical model has been created as a possible tool for predicting the temperature profile across given particles for a fixed set of conditions in the T400 prototype. The created mathematical model has been proven to have the ability to predict and reproduce a desired temperature profile for a given material allowing the estimation of the necessary combination of air temperature and microwave power necessary to create any desired temperature profile within a given particle of known properties.
Hybrid microwave roasting of coffee beans in a TorWave reactor was assessed for the first time during this thesis as a potential alternative to conventional coffee roasters. The main hypothesis under this investigation is the increased microwave bloating effect over the coffee grains translating into a superior expansion of the beans during roasting generating enriched flavour and aroma profiles due to better solids extractability. Furthermore, the reduction of roasting times and the possibility of being entirely powered by a sustainable source opens the scope for process intensification of coffee roasting by the use of the TorWave technology. Hybrid microwave coffee roasting tests performed during this study showed superior bloating and porosity of the beans when compared to conventional roasts. Moreover, aroma and flavour profiles obtained proved an enhanced concentration of desired volatile compounds in the coffee brew and stronger overall coffee intensity, thus supporting the main hypothesis presented and placing microwave coffee roasting as an alternative to existing conventional methods.
Future work includes the optimisation of the process conditions for coffee roasting in the TorWave prototype to assess their impact on the final product. Exploratory work has been started for other prospective industrial applications of the TorWave reactor including drying of grains and expansion of snacks. Further research is recommended in those areas as preliminary results showed potential benefits arising from the use of this novel hybrid microwave technology.
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