Powell, Christopher
(2023)
High sulphur activated carbon regeneration.
EngD thesis, University of Nottingham.
Abstract
Companies such as CPL Ltd, who specialise in the production and regeneration of activated carbon, are looking at ways in which they can reduce their impact on the climate by switching to greener methods for their operations. At current, their aim is to move towards producing more regenerated activated carbon rather than procuring virgin carbonaceous material such as coal. However, their current regeneration method, a conventional thermal method, entails long processing times which require vast amounts of fossil fuels and consume large quantities of energy. Additionally, the regeneration of spent sulphurous activated carbon via thermal regeneration results in the production of sulphuric and sulphurous acids, which cause structural damage to their equipment and present a health & safety issue with their workforce. The company have been looking into possible replacement methodologies including the use of microwave technology, with the hope that the volumetric and selective heating involved can reduce their carbon footprint and reduce/eliminate the production of unwanted by-products.
The aim of this research was to study the effects if microwave heating on spent sulphurous activated carbons and whether a process could be developed that would provide a suitable replacement for their current method. This would go on to become a matter of process development as the study was carried out.
The effects of exposing the spent sulphurous activated carbon samples directly to microwaves via a high-density electromagnetic field were studied and found to cause localised overheating. The microwave energy absorbed by the adsorbates was converted into heat energy, which resulted in damage to the surrounding carbon structure. However, the butane activity of this initial method was found to return adsorptivity to a relatively high degree, being 33.43 % (+/- 0.87 std) compared to the thermal methods 40.88 % (+/- 0.92 std). This was achieved over the course of 360s as opposed to upwards of 6 hours needed for thermal regeneration. There were no signs of sulphuric or sulphurous acid having been produced which was a result of being able to remove the steam activation stage needed for thermal regeneration methods.
The next stage of process development involved exposing the spent sulphurous activated carbon samples directly to microwaves via a low-density electromagnetic field to reduce the localised overheating. With this, the idea being that the more chaotic way low-density fields interact with a target would allow for a slower rate of heating, thus reducing the damage caused by the heat energy emitted from the desorbing sulphurous adsorbates. However, although to a lesser extent than seen in the initial study, the technique still led to damage to the carbon structure, resulting in a loss of potential returned adsorptivity. Additionally, the use of a multimode waveguide cavity also resulted in a different heating profile. The butane activity was also found to be lesser than that of high-density field exposure (32.78 % (+/- 0.03 std) at best – at 800 W over 360s), due to reduced number of incidents between microwaves and the samples. Extended irradiation times did show that certain sulphurous adsorbates desorb before others, and that damage to the carbon structure increased as irradiation time increased. This suggested that the rate at which the desorbing material was desorbing and exiting the activated carbon was determining how much damage was being caused to the carbon structure – exiting at a rate at which emitted heat energy could cause more damage.
A third iteration of microwave processing was established which utilised an inert gas flow directly over the samples during irradiation to control the rate of mass transfer. This design resulted in butane activities surpassing that seen in thermal regeneration, reaching 48.34 % (+/- 1.20 std), and greatly reduced the damage being caused to the overall activated carbon structure. This was achieved by increasing the rate at which material desorbed during the initial stages of irradiation, and by increasing the rate at which desorbed materials were removed from the activated carbon pores. Additionally, the heating rate could be controlled via controlling the microwave power, which gave rise to higher internal pressure and vapor velocity, promoting more efficient removal of adsorbates from the carbon surface. Moreover, the increasing vapor velocity and internal pressure served to perpetuate removal of adsorbates with stronger chemical interactions.
The final method was found to be more efficient than the currently used thermal methods: reducing the processing time from several hours of a matter of minutes; eliminating the production of unwanted acidic by-products; and, increasing the achievable adsorptivity level.
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