https://doi.org/10.1016/j.seppur.2020.117789
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The solubility of CO2, in terms of vapor–liquid equilibrium, was measured in solutions of AMP in DMSO using reaction calorimetry. Two concentrations of AMP were studied, 10 and 25 wt%, at temperatures between 25 and 88 °C, corresponding to both absorption and regeneration conditions.
Solubility data collected for CO2 in mixtures of 10 wt% AMP in DMSO at temperatures between 25 and 88 °C are presented in Fig. 1. The results show a decrease in the solubility of CO2 with increasing temperature, which is represented by a higher CO2 partial pressure at lower loadings with increasing temperature. No precipitation was observed in the 10 wt% AMP/DMSO system under the conditions studied. The capacity of the absorption system, i.e., the difference between rich and lean CO2 loading, can be estimated from the solubility data at absorption and regeneration conditions (in terms of temperature and partial pressure of CO2). For the conventional system of 30 wt% aqueous MEA, the rich loading is typically just above 0.5 mol CO2/mol amine (at 40 °C and 10 kPa CO2) [18], [19], [20], [21] and the lean loading around 0.2 mol CO2/mol amine (at 120 °C and 10 kPa CO2) [20], [21], [22]. This results in a capacity of slightly above 0.3 mol CO2/mol amine. If regeneration is performed without stripping gas for 10 wt% AMP/DMSO, this can be represented by the solubility data at 88 °C and 100 kPa, resulting in a lean loading of approximately 0.09 mol CO2/mol AMP. Absorption is usually performed at 40 °C, and thus in order to achieve a capacity of around 0.3 mol CO2/mol amine, the incoming gas must have a CO2 partial pressure of about 50 kPa. If absorption is conducted at 25 °C, a similar capacity is obtained at a CO2 partial pressure of about 20 kPa. However, additional cooling of the incoming gas will probably be needed in such cases. The temperature of the absorber would also have to be kept as low as possible, in order not to reduce its capacity. The CO2 partial pressures needed in the incoming gas (20–50 kPa) indicate that the 10 wt% AMP/DMSO system could be suitable for carbon capture in biogas upgrading and industrial CCS.
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“Fig. 1. Solubility of CO2 in solutions of 10 wt% AMP in DMSO, at temperatures between 25 and 88 °C.”
“The solubility of CO2 in 25 wt% AMP/DMSO obtained in this study at 50–88 °C is presented in Fig. 2, together with the data obtained at 25 and 40 °C in our previous study [17]. A similar trend can be seen to the 10 wt% system, regarding the decreasing solubility with increasing temperature. However, in the 25 wt% AMP/DMSO system, precipitation is observed at the lower temperatures (25–50 °C). This can be seen as a decrease in pressure, with increased loading, in some data points in the data series for those temperatures. Precipitation drives the equilibrium reactions (Reactions 1–4) further to the right, which allows more CO2 to be dissolved in the solution. The lower pressure measured after precipitation indicates that the absorption solution is supersaturated prior to precipitation, likely due to slow crystallization kinetics [23]. The pressure will decrease to values closer to true equilibrium after the initial precipitation takes place. It should be noted that if the solution precipitates at higher amine loadings (past 0.5 mol CO2/mol AMP), and thus a higher partial pressure of CO2, the decrease of pressure in the solubility data is absent. This is the case for one of the experimental runs at 40 °C. The reason for this is believed to be a combination of the driving force for CO2 to absorb in the liquid decreasing after its fully chemically loaded, together with the overall pressure in the reactor being higher at this point, and thus a small decrease in pressure becomes less significant. The precipitation point can however still be seen in the heat of absorption data as discussed below in Section 3.2. If precipitation occurs, a higher rich loading is obtained, and the capacity of the system is increased. At 40 °C, and at a CO2 partial pressure of 10 kPa, the CO2 loading increases from 0.28 to 0.37 mol CO2/mol AMP when there is precipitation in the system. If these conditions are used for the absorption, and the regeneration conditions without a stripping gas are represented by 88 °C and 100 kPa, a difference between rich and lean loading of 0.3 mol CO2/mol AMP can be achieved, based on the solubility data. In the system with 25 wt% AMP in NMP a higher partial pressure of CO2 is needed to reach a similar capacity at 40 °C [13], indicating that the AMP/DMSO system is more suitable for carbon capture at lower CO2 partial pressures. Furthermore, if a stripping gas is used during regeneration, even lower lean loadings can be achieved, i.e., below 0.01 mol CO2/mol AMP at 88 °C and 20 kPa. Also, if it is possible to use a lower absorption temperature of 25 °C, a rich loading of 0.36 mol CO2/ mol AMP can already be achieved at CO2 partial pressures below 1 kPa. However, in order to determine whether this is feasible, the CO2 reaction kinetics needs to be evaluated. The reaction kinetics affects the residence time needed in the absorption step and is thus crucial in order to design the absorption system.”
“Fig. 2. Solubility of CO2 in solutions of 25 wt% AMP in DMSO, at 50, 60, 70, 80 and 88 °C. Data obtained in our previous study, at 25 and 40 °C, are included (indicated by an asterisk in the legend) [17].”
“The solubility of CO2 in the AMP/DMSO systems with (25 wt%) and without (10 wt%) precipitation, at absorption and regeneration temperatures is presented in Fig. 3. The lean loading that can be achieved when regeneration is performed at 100 kPa does not differ between the two systems. However, it can clearly be seen that at a CO2 partial pressure of 10 kPa the rich loading is significantly increased in the 25 wt% AMP/DMSO system, where there is precipitation. This rich loading is almost twice as high as that in the 10 wt% AMP/DMSO system, at 10 kPa. It is thus clear that precipitation is necessary in the AMP/DMSO system in order for it to be competitive with conventional systems, in terms of CO2 loading capacity. The 25 wt% AMP in DMSO is therefore considered the more interesting alternative as a bi-phasic CO2 absorption system.”
“Fig. 3. Comparison of the solubility of CO2 in 10 wt% and 25 wt% AMP in DMSO at absorption (40 °C) and regeneration (88 °C) temperatures. The CO2 partial pressure in the absorption step is assumed to be 10 kPa, and regeneration is carried out at atmospheric pressure. The vertical lines indicate the estimated CO2 loadings that can be achieved for the systems under absorption (green or blue) and regeneration conditions (orange). Data obtained at 40 °C in our previous study are included (indicated by an asterisk in the legend) [17]. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)”