https://doi.org/10.1016/j.petlm.2016.11.002
“Many researchers have measured the equilibrium properties of the CO2-MEA-H2O system at different thermodynamic conditions, observing a relatively high solubility even at low CO2 partial pressures [43], [44], [45], [47], [48], [49]. Microwaves selectively couple with components possessing the highest dielectric loss [38]. The possibility that microwaves specifically target the carbamate to provide the improved CO2 recovery seen in Fig. 3 should be considered. Inspection of the heating profiles in Fig. 2 lends some support to this possibility. We therefore propose that the non-thermal effects are related to one or more of the following: (i) an increase in the free energy (activity, or chemical potential) of the carbamate state relative to the neutral amine, (ii) a reduction in the activation energy for the reverse capture reaction, or (iii) an increase in the Arrhenius pre-exponential factor for the reverse reaction. Investigation into non-thermal effects and targeted microwave heating of the carbamate can be achieved by measuring CO2 solubility for absorption isotherms under both conventional heating and microwave irradiation.
Solution CO2 loadings and outlet gas concentrations were analysed after attainment of steady-state for two different inlet CO2 partial pressures at a constant temperature of 65 °C using microwave irradiation and thermal heating via a water bath. The first inlet CO2 partial pressure was 20 kPa, and after reaching steady-state this was increased to 29 kPa. The case of thermal heating corresponds to an equilibrium absorption measurement. The microwave heating case corresponds to a steady-state process, since heat is generated by the microwaves and dissipated to the surroundings. Nevertheless, due to the constant bubbling, we can expect the temperature distribution within the bubbler to be practically uniform and almost identical for the microwave and thermal cases. Any differences between the two heating methods could illuminate non-thermal contributions to the carbamate activity caused by interaction with the microwave field. The resulting isotherms are depicted in Fig. 4.
Fig. 4. (a) The CO2 outlet concentrations versus time for the absorption isotherm measurements at 65 °C for microwave (MW) and conventional heating (CH). The initial inlet CO2 partial pressure of 20 kPa was increased to 29 kPa after sufficient equilibration time. (b) Equilibrium CO2 loadings and outlet partial pressures from the isotherm measurements.
The two techniques in Fig. 4(a) appear by eye to be very similar. Closer comparison of the CO2 breakthrough curves for the two data sets suggests that the presence of a microwave field actually permits a slightly higher CO2 loading relative to conventional heating. The equilibrium CO2 loadings from the absorption isotherm measurements were confirmed both by comparison to a blank run and by titration of the solution with BaCl2 and are displayed in Fig. 4(b), which agree quite well with literature data for conventional CO2-MEA-H2O solubility measurements [43], [44], [45], [47], [48], [49]. When the inlet partial pressure is increased from 20 kPa to 29 kPa, both techniques follow very similar behaviour with the outlet partial pressures increasing accordingly. If the activity of the carbamate were to increase relative to neutral amine due to interaction with the microwave field, then the reaction equilibrium would shift to favour reactants (compared with true thermal equilibrium at the same temperature), resulting in lower steady-state CO2 loading and faster breakthrough compared to conventional heating. In fact, microwave irradiation gives a slightly longer breakthrough time and marginally higher solubility. Therefore, it appears that we can rule out a relative increase in the carbamate activity in the presence of microwaves as the cause for the very fast recovery of CO2 in Fig. 3. We are then left with the possibility that the microwave field reduces the activation energy barrier, or increases the Arrhenius pre-exponential factor, of the reverse capture reaction.”