Individual reaction contribution to the overall heat of CO2 absorption

https://doi.org/10.1039/C9RA00164F

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Fig. 7 shows the predicted solution speciation and heat of CO₂ absorption in the NH₃–CO₂–H₂O system, respectively, all at m(NH₃) = 3 mol kg⁻¹ H₂O and T = 2 °C. Because the formation of carbamate (NH₂COO⁻) and NH₄HCO₃(s) significantly impact the heat of CO₂ absorption, the whole absorption process is divided into three stages according to carbamate and NH₄HCO₃(s) formation, as shown in Fig. 7, i.e. Stage I: CO₂ loading < 0.5 mol CO₂/mol NH₃; Stage II: 0.5 < CO₂ loading < 0.7 mol CO₂/mol NH₃; and Stage III: CO₂ loading > 0.7 mol CO₂/mol NH₃. They are discussed in detail in the following paragraphs.

At low CO₂ loading (Stage I), there is an excess of free NH₃, and carbamate is the main product in the solution via the forward reaction of carbamate formation (R5). For example, 0.333 mol CO₂/mol NH₃, 72% of CO₂ converts to carbamate and only 12.5% and 15.4% converts to bicarbonate and carbonate, respectively. Fig. 7(b) shows that the overall heat of CO₂ absorption first decreases and then increases rapidly with increasing CO₂ loading. As explained above, (R5) moves forward to form carbamate with increasing CO₂ loading in Stage I. In this stage, (R5) is an exothermic process (−ΔH of (R5) has a positive value) and thus releases heat.

As the absorption proceeds to Stage II, carbamate is decomposed via the backward reaction of carbamate formation (R5) to form bicarbonate, with 56.9% of CO₂ turns into bicarbonate, 13.6% into carbonate, and 29.5% into carbamate at CO₂ loading of 0.667 mol CO₂/mol NH₃. In this stage, (R5) moves backward with increasing CO₂ loading. As shown in Fig. 7(b), (R5) is still the dominant reaction, but becomes an endothermic, thus reducing the overall heat of CO₂ absorption (the overall process remaining exothermic).

Fig. 7(a) shows that for CO₂ loading greater than 0.7 mol CO₂/mol NH₃ (Stage III), NH₄HCO₃(s) is gradually formed via the forward reaction of NH₄HCO₃(s) formation (R6) at 2 °C. The amount of bicarbonate produces by carbamate decomposition is equal to that consumes by solid formation, so the concentration of bicarbonate remains constant. The corresponding overall heat of CO₂ absorption increases due to the heat release from the solid formation, which can be seen in Fig. 7(b). The overall heat of CO₂ absorption is about −78 kJ mol⁻¹ CO₂ at CO₂ loading of 1 mol CO₂/mol NH₃, which is similar to the initial stage of absorption. Now, NH₄HCO₃(s) formation (R6) contributes most to the overall heat of CO₂ absorption. Water as a main reactant is continuously consumed by CO₂ dissociation (R2), CO₃²⁻ formation (R3) and NH₃ protonation (R4), causing water ionization (R1) to move backward and to release heat in the entire absorption process. It is worth pointing out that the heat of CO₂ physical absorption (R7) remains −21 kJ mol⁻¹ CO₂ or so in Fig. 7(b). This is because the Henry’s law constant of CO₂ physical absorption (R7) depends on temperature, and the physical absorption amount of CO₂ increases linearly with increasing CO₂ loading.²⁸

Fig. 8 shows the contribution of each reaction to the overall heat of CO₂ absorption at m(NH₃) = 3 mol kg⁻¹ H₂O and T = 2 °C. The share of CO₃²⁻ formation (R3) is very small due to the small amount of CO₃²⁻ in the solution. The water dissociation (R1), CO₂ dissociation (R2), carbamate formation (R5), and CO₂ physical absorption (R7) are the main contributors to the overall heat of CO₂ absorption at the initial phase (CO₂ loading = 0.25 mol CO₂/mol NH₃). This is quite different from amine-based system. Kim et al.²⁷ reported that the main contributors to the overall heat of CO₂ absorption in MEA solution were carbamate and MEAH⁺ formation reactions. When CO₂ loading is 0.5 mol CO₂/mol NH₃, the contribution of carbamate formation (R5) becomes minimum. This is because carbamate formation (R5) is at a tipping point from forward to backward reaction, when the extent of carbamate formation reaction (R5) is very weak. After the solids appear at CO₂ loadings greater than 0.7 mol CO₂/mol NH₃, the NH₄HCO₃(s) formation (R6), water dissociation (R1), and CO₂ physical absorption (R7) become the main contributors to the overall heat. The contribution of NH₄HCO₃(s) formation (R6) is 32% at a CO₂ loading = 1 mol CO₂/mol NH₃.

Fig. 9 and 10 show the prediction of solution speciation change and heat of CO₂ absorption in the NH₃–CO₂–H₂O system at T = 15 °C and 40 °C, respectively. At T = 15 °C (Fig. 9), three stages, similar to the process at T = 2 °C (Fig. 7), are observed, but with a higher turning point of CO₂ loading (moving from 0.7 at T = 2 °C to 0.85 mol CO₂/mol NH₃ at T = 15 °C). Additionally, speciation data reported by Jilvero et al.³¹ at m(NH₃) = 3.5 mol kg⁻¹ H₂O and room temperature is also include in Fig. 9. The trend of the model results agree well with those of experimental data. However, the model values of NH₂COO⁻ are distinctly lower than the experimental data. This is because the NH₃ concentration in Jilvero et al. (m(NH₃) = 3.5 mol kg⁻¹ H₂O) is higher than that in this study (m(NH₃) = 3 mol kg⁻¹ H₂O). According to (R5), Higher NH₃ concentration promotes the formation of NH₂COO⁻, so the NH₂COO⁻ concentration in Jilvero et al. is higher than our model results. When the absorption temperature increases further to 40 °C, only two stages can be seen in Fig. 10. The third stage caused mainly by the formation of NH₄HCO₃(s) disappears at higher temperature, as shown in Fig. 10.

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