Validation of thermodynamic model by heat of absorption

https://doi.org/10.1039/C9RA00164F

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Fig. 6 shows the heat of CO₂ absorption predicted by the model and the experimental data of Liu et al.²⁰ and Qin et al.²¹ at different temperatures. In addition, another model from Que et al.⁵⁵ is also cited in Fig. 6 for comparison. As we can see that all the model values and experimental data decrease with CO₂ loading except Qin et al. Qin et al. found that the absorption heat of CO₂ with NH₃ at 40 °C and 60 °C decreases at first with increasing loading, but between 0.2 and 0.6 mol CO₂/mol NH₃ in loading it rapidly increases. When the loading is around 0.6 mol CO₂/mol NH₃, the absorption heat of CO₂ with NH₃ reaches a maximum (∼100 kJ mol⁻¹ CO₂ at 60 °C). The absorption heat then starts to decrease again. This trend is more pronounced at high temperature (60 °C). No theoretical justification for this strange trend is presented in their paper. However, according to all prior researchers’ results, there is no reaction between CO₂ and NH₃ that should release a heat of absorption higher than 100 kJ mol⁻¹ CO₂.^20,36,55 The estimated absorption heat of CO₂ with NH₃ using the speciation data of Mani et al.,⁶¹ measured by NMR, also gives a value of around 80 kJ mol⁻¹ CO₂. In addition, as CO₂ is gradually absorbed, the concentration of ammonia in the solution decreases attenuating the reaction. The amount of heat released during the absorption process should be gradually reduced. So, the validity of the data obtained by Qin et al. needs further discussion. In general, the agreement between the current model values and Liu’s experimental data, as well as agreement with the model values of Que et al. clearly support the model validity and accuracy. The subtle difference between the model values and experimental data may be caused by the activity change of species conjectured by Kim.⁶⁴ The contribution to the heat of absorption from the liquid-phase nonideality is neglected in this study. It should be better to consider the heat from the liquid-phase nonideality in the model to examine Kim’s guess in our future works. In addition, the modeling deviation may also be from the chemical equilibrium constants chosen from literature. As shown in Fig. 2(a), the chemical equilibrium constants chosen from different literature have some differences with each other and may cause a difference in the calculation of enthalpy change using eqn (8) (see Fig. 2(b)). The heat of CO₂ absorption predicted by the model decreases from −81 to −37 kJ mol⁻¹ with the CO₂ loading increasing from 0.1 to 1 mol CO₂/mol NH₃. In addition, the current model results indicate that the overall heat of CO₂ absorption does not change significantly with NH₃ concentration. This implies that the reaction between NH₃ and CO₂ at different NH₃ concentration has almost the same reaction products distribution.

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