https://doi.org/10.1016/j.petlm.2016.11.002
“Heat of absorption has an influence in the energy of regeneration because it is believed to be equal to the heat of desorption [156]. Therefore, low heat of absorption promotes reduction in energy of regeneration because less heat will be required to break the amine–CO2 species produced during CO2 absorption. Reactive amine solvents usually have high heat of absorption and it is the reason why carbamate forming amines have higher heat of absorption than bicarbonate forming amines [157]. This is also the reason why heat of absorption decreases from primary to tertiary monoamines [158]. MEA and MDEA both possess the highest (80–86 kJ/mol-CO2) and lowest (54.6–60.9 kJ/mol-CO2) heat of absorption respectively [69], [79], [159], [160].
From experimental analysis, integral heat of absorption of amines can be determined using the differential reaction calorimeter (DRC) [70], [161], [162]. This method gives an accurate absorption heat of amine solutions because it takes into account the effect of heats due to physical dissolution of CO2 into the amine solvent and the amine–CO2 chemical reactions [53]. In the absence of a DRC, the Gibbs–Helmholtz correlation given in Eq. (20) has often been used to predict the differential heat of absorption [53], [63], [68], [88], [160]. According to Kim and Svendsen the differential absorption heat is believed to have a huge error (±20–30%) when there is a slight error (±2–3%) in the CO2 solubility results [53]. Another disadvantage of the differential absorption heat is that it does not take into account the effect of temperature because the plots of ln P vs 1/T are considered linear [53].
where; PCO2 is the CO2 partial pressure (kPa), T is the temperature (K), R is the universal gas constant (8.314 J/mol.K), αCO2 is the CO2 loading (mol CO2/mol amine) and ΔHabs is the absorption heat (kJ/mol CO2).
Apart from using DRC or the Gibbs–Helmholtz correlation, other researchers have deployed alternative methods of determining the absorption heat of amine solutions [100], [163], [164], [165], [166]. Abdulkadir and Abu-Zahra analysed the heat of absorption of single solvents (i.e. 5 kmol/m3 MEA, 2 kmol/m3 PZ, 1 kmol/m3 AMP, and 0.9 kmol/m3 MDEA) and bi–solvent blends (i.e. 1 kmol/m3 AMP–2 kmol/m3 PZ and 0.9 kmol/m3 MDEA–2 kmol/m3 PZ) [167]. The bi–solvent blend of 0.9 kmol/m3 MDEA–2 kmol/m3 PZ was found to have a lower absorption heat that those of 5 kmol/m3 MEA and 2 kmol/m3 PZ, respectively. The absorption heat of 1 kmol/m3 AMP–2 kmol/m3 PZ however, was higher than 5 kmol/m3 MEA but significantly lower than 2 kmol/m3 PZ. It was also seen that the heat of absorption of the bi–solvent blends were between those of their parent solvents [167], [168]. In addition, the bi–solvent blend, particularly the one containing tertiary amine (MDEA) had a lower heat of absorption than that of the one with sterically hindered (AMP). From a preliminary in–house experimental study using Gibbs–Helmholtz correlation, it was also discovered that the absorption heat of the AMP–MDEA–DETA tri–solvent blends decreased with decreasing AMP/MDEA molar concentration ratio.
From these studies, it can be inferred that the flexibility of blended amine solutions (i.e. bi–solvent, tri–solvent) will enable careful selection of the individual solvents in order to reduce the absorption heat. Further studies need to be conducted to confirm and validate this.”