Heat of absorption of CO2 in mixtures of AMP and DMSO

https://doi.org/10.1016/j.seppur.2020.117789

“The heat of absorption (−ΔH_abs) is another important factor when evaluating absorption systems, as it is an essential part in determining the amount of heat needed to regenerate the system. Heat of absorption includes the heat of dissolution of CO₂ into the liquid system and the heat of reaction between CO₂ and the amine (Reactions 1–3). These are usually exothermic reactions, resulting in heat being released during the absorption process, causing an increase in temperature. The differential heat of absorption obtained from the absorption of CO₂ in 10 wt% AMP in DMSO is presented in Fig. 4. In general, it can be seen that the heat of absorption decreases with increasing CO₂ loading. At low loadings, where chemical absorption dominates the reaction mechanism, the heat released upon the absorption of CO₂ is much higher than for CO₂ absorption in pure DMSO (i.e. around 10–14 kJ/mol CO₂ in DMSO [17], [26], [27] where only physical absorption occurs. As the loading approaches its maximum theoretical value of 0.5 mol CO₂/mol AMP, based on the reaction mechanism (Reactions 1–3), the absorption mechanism will shift towards physical absorption as more amine has reacted, thus causing a decrease in the heat released. Additionally, the heat of absorption decreases with increasing temperature, and the heat released is more than twice as high at 25 °C than at 88 °C for CO₂ absorption in 10 wt% AMP/DMSO.”

“Fig. 4. Differential heat of absorption at different CO₂ loadings for the absorption of CO₂ in the system consisting of 10 wt% AMP in DMSO.”

“The same trend, of decreasing heat of absorption with increasing loading and temperature, was seen for the absorption of CO₂ in 25 wt% AMP in DMSO, as shown in Fig. 5. The points at which precipitation occurs can be clearly seen in the heat of absorption data as the points with much higher values, ranging between 140 and 240 kJ/mol CO₂. Precipitation of the AMP carbamate is an exothermic reaction and, as the absorption solution is supersaturated, more carbamate will precipitate at the same time than in the equilibrium case. Thus, the heat released as precipitation occurs is higher than in the equilibrium case, which gives rise to the high values observed. After the initial precipitation points, the heat of absorption starts to decrease again, and above a loading of 0.5 mol CO₂/mol AMP, the values approach values that are typical of physical absorption, as in the case of 10 wt% AMP/DMSO.”

“Fig. 5. Differential heat of absorption at different CO₂ loadings for the absorption of CO₂ in the system consisting of 25 wt% AMP in DMSO. Data obtained at 25 and 40 °C in our previous study are included (indicated by an asterisk in the legend) [17].”

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Decreasing heat of absorption with increasing temperature was also observed in our previous study of 15 and 25 wt% AMP in NMP, in the temperature interval 25–88 °C [13]. In the present study it was verified that this trend is true also at lower amine concentrations (2.5 wt% AMP in NMP) and the data can be found in the Supplementary Material. A similar decreasing trend in heat of absorption has also been reported by Mobley et al. [28] in the non-aqueous system of 2-fluorophenethylamine in 2,2′,3,3′,4,4′,5,5′-octafluoropantanol at 40–120 °C. They speculated that changes in hydrogen bonding around the carbamate, and changing interactions in the aromatic groups, could explain this behavior. In our previous study, we suggested that this decreasing trend could be the result of a larger fraction of the dissolved species being physically absorbed, rather than chemically absorbed, at higher temperatures [13]. Overall, less CO₂ will be absorbed in the system at higher temperatures, but we suggest that the fraction of physically absorbed CO₂ will increase at higher temperatures. This hypothesis is supported by the results obtained in the NMR study for the AMP/DMSO and AMP/NMP systems, discussed below in Section 3.3, showing how the fraction of physically dissolved species increases as the temperature is raised.

The opposite trend has been observed for aqueous MEA, where the heat of absorption increased at higher temperatures [29], [30]. In aqueous MEA, CO₂ reacts to form the MEA carbamate, but further reaction to bicarbonate also occurs in aqueous solutions. These reaction products are more stable in solution than the sterically hindered AMP carbamate formed in AMP/DMSO, which affects the temperature required to reverse the reactions for regeneration. The difference in the observed trends, regarding the change in heat of absorption with increasing temperature, between aqueous MEA and non-aqueous AMP could be due to differences in the amount of physically dissolved and chemically reacted CO₂. The solubility of CO₂ in DMSO is much higher than in water, and since the heat of absorption includes both the heat of dissolution and the heat of reaction, it is likely that the heat of dissolution has a greater effect on the heat of absorption in AMP/DMSO than in aqueous MEA. Even if the heat of reaction resulting from Reactions 1–2 were to increase with temperature for CO₂ in AMP/DMSO, the fraction of physically absorbed CO₂ would still lead to a higher contribution to the measured heat of absorption at higher temperatures.

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