Shepherd, Benjamin John
(2020)
Development of a liquid inerted microwave pyrolysis process.
PhD thesis, University of Nottingham.
Abstract
The research presented in this thesis evaluates the potential of a new approach to carrying out pyrolysis. A pyrolysis process was demonstrated that combined microwave heating with an external liquid medium at atmospheric pressure. Generally, gases such as nitrogen are used to inert pyrolysis, however in this research liquids were used which had the benefit of acting as a heat-sink to maintain the external temperature at the atmospheric boiling point of the liquid.
An advantage of the presented pyrolysis technology was the ability of the liquid surround to control the external temperature, as this was only possible due to the selective and volumetric heating that occurs with microwaves. The new concept produced a pyrolysis oil that naturally partitioned into a sugar-rich aqueous phase and a phenol-rich organic phase when using solvents that were only miscible with one of the phases, such as n-hexane. The liquid medium overcame many of the challenges encountered in traditional and gas-based microwave pyrolysis processes, acting to eliminate arcing, slow the onset of plasma formation, and prevent carbonaceous residues from forming from the pyrolysis oil produced, phenomena which have so far proved challenging for the scale-up of microwave pyrolysis processes. Energy requirements were as low as 2 kJ/g for 50 % volatilisation, comparable to microwave pyrolysis using inert gases. It was also shown that the liquid inerted concept had the potential to utilise both microwave-transparent and microwave-absorbent solvents effectively.
Two methods were investigated to perform microwave pyrolysis process within a liquid medium. The first process was a feasibility study, which was successful, however due to the scale of the process, oil volumes produced were low and the volatilisation of the sycamore coupled with boiling solvent resulted in challenges in maintaining the surface temperature of the block during pyrolysis.
The second process configuration involved the investigation on controlling the boiling regime and bubble disengagement, along with allowing for sycamore geometries that would allow for larger volumes of pyrolysis oil to be produced. This presented advantages and challenges when compared to the first process. It was found that due to the size of the sycamore blocks used, internal temperature control was reduced in the second process, however there was greater surface cooling potential due to the larger volume of liquid surrounding the block.
It was discovered throughout the work in the thesis that in addition to providing an alternative way of heating the sycamore, microwave pyrolysis offered processing characteristics that were not present in conventionally heated processes. It was known that due to volumetric heating, larger particles could be heated faster than conventional heating. It became apparent, however, that due to the large volume of volatiles being produced within the sycamore block, the pressures achieved would allow for internal temperature control of the sycamore. The volatiles production would potentially result in variations of the dielectric properties of the sycamore that would not be expected when using conventional analysis methods.
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