Comparison between microwave and conventional regeneration

“An aqueous 30 wt% MEA solution was first loaded with CO2 via an initial absorption step with an inlet gas stream of 20% CO2 balanced by N2 (this CO2 composition is broadly representative of a wide range of large-scale flue gas compositions, from fossil fuelled power plants through to industrial cement works). The CO2-rich solution was then regenerated by microwave heating in the presence of a purge flow of 100 mL/min N2 to produce a CO2-lean solution. The CO2 desorption profiles and solution temperatures are outlined in Fig. 3. In order to provide a benchmark against conventional heating, the microwave regeneration is compared with that using only a thermal heat source, with the aim to closely reproduce the temperature profile within the sample generated by the microwave experiments. This was achieved by heating in a water bath at 75 °C, followed by removal of the heat bath and maintaining a purge flow of 100 mL/min N2, the results of which are also shown in Fig. 3. The two temperature profiles in Fig. 3 are quite similar overall, however the microwave heating rate is somewhat faster than the conventional heating rate. This demonstrates the remote and volumetric nature of microwave heating compared to conventional heating, the latter being restricted by conductive heat transfer. Both temperatures reduce quickly under nitrogen purge when heating is stopped, although cooling following microwave heating is somewhat faster. This is again illustrative of the direct interaction of microwaves with the solution without appreciably heating the reactor vessel.

Fig. 3. Comparison of microwave (MW) and conventional heating (CH) to regenerate the CO2-loaded MEA solution after an initial absorption step of 20 min with 20% CO2.

However, despite similar temperature profiles, the CO2 desorption profiles in Fig. 3 display strikingly different characteristics. During microwave heating CO2 is rapidly released, peaking at early times (around 4 min) and steadily decreasing as CO2 is removed until microwave irradiation stops. The thermal heating CO2 desorption profile on the other hand emerges slowly and with a much smaller peak towards the end of the heating period. Overall, microwave heating releases more than double the amount of CO2 compared to conventional heating during this single-cycle experiment. The observed differences between conventional and microwave regeneration do not appear to reflect the differences between the temperature profiles in Fig. 3, since these are quite close. In particular, the temperature profiles are not very different after two minutes, yet the microwave CO2 recovery profile peaks at around 4 min, while the thermal CO2 recovery profile peaks only when thermal heating stops. Clearly, there is a significant microwave effect at play, i.e. the microwave field enhances regeneration beyond that caused by a change in the temperature alone. It therefore appears that a portion of the microwave energy is channelled into reversing the capture reaction without heating the solution.

For microwave regeneration of solid adsorbents, authors frequently refer to ‘inverted thermal temperature gradients’ that arise as a consequence of heat being generated directly inside the adsorbent bed rather than from a stripping medium or the column surface [24][29][37]. The heat transfer direction therefore flows from the inside to the outside of the irradiated material, potentially promoting mass transport and diffusion of the desorbing molecules. Even if this hypothesis were shown to be correct for microwave desorption from a solid adsorbent fixed bed, the continuous mixing of the solution by the bubbling gas in this work will provide a much more isotropic heating profile. Others suggest that changes to the structure and dynamics of the solution due to the microwave field may promote diffusion [42]. However, we consider this unlikely in this case given that CO2 itself is quadrupolar and therefore largely unaffected by microwaves. The nature of this apparent non-thermal effect might be due to the oscillation of polar functional groups under microwave irradiation to facilitate breaking the bond between CO2 and amine moieties. This will be investigated next.”

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