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thermodynamics of N-(2-Hydroxyethyl) Piperazine/H2O/CO2

https://doi.org/10.3389/fenrg.2021.785039

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TABLE 4. Parameters regressed for HEPZ/H2O/CO2 and their deviations (α= 0.2α= 0.2).”

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FIGURE 6(A) CO2 solubility into 5 wt% HEPZ/H2O (lines, model results; points, this study: ◆, 313.15 K; ■, 343.15 K; ▲, 373.15 K; and ●, 393.15 K); (B) CO2 solubility into 15 wt% HEPZ/H2O (lines, model results; points, this study: ◆, 313.15 K; ■, 343.15 K; ▲, 373.15 K; and ●, 393.15 K); (C) CO2 solubility into 30 wt% HEPZ/H2O (lines, model results; points, this study: ◆, 313.15 K; ■, 343.15 K; ▲, 373.15 K; and ●, 393.15 K).”

Cyclic Capacity

Circulating capacity is an important property to characterize the properties of amines, and there are two ways to calculate cyclic capacity. When considering a CO2 removal rate of 90% in the absorber, one way (way 1) is defining lean loading as the CO2 loading when the partial pressure of CO2 is 1 kPa at a temperature of 313.15 K, and the rich loading of CO2 partial pressure is 10 kPa. Meanwhile, cyclic capacity represents the difference between the rich and lean loadings with the unit of gg of CO2/kg of the solvent.

However, in the actual operation of the absorption tower, the absorption tower is not at a constant temperature, and at the bottom of the tower, the rich loading is determined by the equilibrium partial pressure of CO2 in the flue gas as well as the temperature of the liquid. Also, the lean loading is defined by the desorption tower and not by the equilibrium partial pressure of CO2 in the top gas of the absorption tower. The other way (way 2) to calculate cyclic capacity is defining lean loading as the CO2CO2 loading when the partial pressure of CO2CO2 is 15 kPakPaat 393.15 KK and rich loading as 15  kPa kPa of CO2CO2 partial pressure at 313.15KK. All results are shown in Table 5.

TABLE 5. Cyclic capacity for 5 wt%, 15 wt%, and 30 wt% HEPZ solutions by way 1 and way 2.

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Speciation

Figure 7 shows the speciation data for 5 wt%, 15 wt%, and 30 wt% HEPZ solutions at a temperature of 313.15 K forecast by the model. For 5 wt% and 15 wt% HEPZ solutions, when the loading is 0–0.5, most of the CO2 absorbed is converted into CO23CO32− and HEPZCOOHEPZCOO−, and the other is converted into HCO3HCO3−; most HEPZ is converted into HEPZH+HEPZH+. At the loading of 0.5–1, the main reactants are CO2, HEPZ, and HEPZH+HEPZH+, and some HEPZCOOHEPZCOO− and CO23CO32− are also consumed to generate HCO3HCO3− and HHEPZCOOHHEPZCOO. For the 30 wt% HEPZ solutions, when the loading is lower than 0.3, HEPZ is consumed and converted to HEPZH+HEPZH+HEPZCOOHEPZCOO−CO23CO32−, and HHEPZCOOHHEPZCOO, and the important products are HEPZH+HEPZH+ and HEPZCOOHEPZCOO−. At a loading of 0.3–0.7, HEPZCOOHEPZCOO− becomes a reactant which is converted to HHEPZCOOHHEPZCOO. At a greater loading, the proportion of CO23CO32− and HEPZH+HEPZH+ continues to increase, and the proportion of HEPZHEPZ and HEPZCOOHEPZCOO− decreases, showing the CO2 absorbed mainly converted to CO23CO32−. It is because the solution is more alkaline in this loading range, which is consistent with the theoretical analysis. As the concentration of HEPZ increases, the HCO3HCO3− produced by the reaction gradually decreases.”

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FIGURE 7. Speciation for CO2 absorbed into 5 wt% (A), 15 wt% (B), and 30 wt% (C) HEPZ/H2O solutions at 313.15 K.”

“Reaction Equilibrium Constant and Heat of Reaction”

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TABLE 6. Parameters of equations for reaction equilibrium constants.”

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