https://doi.org/10.1039/C9RA00164F
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The heat of each individual reaction ((R1)–(R7)) is expressed in terms of enthalpy change, ΔHk, which can be calculated from the van’t Hoff’s equation47 with corresponding equilibrium constant written as in eqn (8). The results are summarized in Table 1 (the values of C2 to C4 will be discussed later).
(8)
| Reaction no. | Enthalpy change (ΔHk, kJ mol−1) | ||||||
|---|---|---|---|---|---|---|---|
| 2 °C | 5 °C | 10 °C | 15 °C | 20 °C | 30 °C | 40 °C | |
| (R1) | 60.37 | 59.81 | 58.88 | 57.94 | 57.01 | 55.13 | 53.27 |
| (R2) | 16.39 | 15.48 | 13.95 | 12.42 | 10.89 | 7.83 | 4.77 |
| (R3) | 22.19 | 21.30 | 19.83 | 18.35 | 16.88 | 13.93 | 10.98 |
| (R4) | 7.83 | 7.36 | 6.56 | 5.74 | 4.91 | 3.19 | 1.42 |
| (R5) | −24.11 | −24.11 | −24.11 | −24.11 | −24.11 | −24.11 | −24.11 |
| (R6) | −23.72 | −22.39 | −20.21 | −18.04 | −15.91 | −11.70 | −7.63 |
| (R7) | −21.51 | −21.11 | −20.46 | −19.79 | −19.13 | −18.56 | −16.43 |
The overall heat of CO2 absorption in the NH3–CO2–H2O system depends on the endothermic or exothermic properties, as well as the extent and direction, of each individual reaction (R1)–(R7) at different CO2 loadings. The extent and direction of (R1) to (R7) are determined by the key species change in the solution with changing CO2 loading. By increasing the CO2 loading gradually, all of these reactions will move in one direction or the other. Some may move forward and the others backward, depending on the variation of key species, Δni, as shown in the following equations:
(9)
(10)
(11)
(12)
(13)
(14)
The change in the total number of moles of CO2, ΔnCO2,tot is determined by
(15)where superscripts F and I stand for final and initial states, respectively.
The extent and direction of each individual reaction absorbing per unit CO2 can be quantified by Ek:
(16)where Δni is the increment of key species in mole, Ek is the specific extent for each reaction ((R1)–(R7)), i.e. per mole of CO2 absorbed. Ek value can be positive or negative depending on the direction of the reaction.
The overall heat of CO2 absorption can be calculated by the summation of the heat of absorption of all the reactions:
(17)where ΔHabs is the overall heat of CO2 absorption.
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