Formation conditions of NH4HCO3(s) in CAP

https://doi.org/10.1039/C9RA00164F

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Fig. 11(a) shows the NH₄HCO₃(s) mole fraction in the solution at temperatures between 2 and 40 °C and for m(NH₃) = 3.1 mol kg⁻¹ H₂O. The corresponding overall heat of CO₂ absorption is shown in Fig. 11(b). As low temperature favors the formation of solid phase NH₄HCO₃(s),⁴⁶ there is little solid formed (less than 8%) for temperatures over 20 °C. CO₂ loading above 0.7 mol CO₂/mol NH₃ and temperatures less than 20 °C promotes NH₄HCO₃(s) precipitation, which can dramatically increase the heat of CO₂ absorption. For instance, NH₄HCO₃(s) begins to form when CO₂ loading is greater than 0.7 mol CO₂/mol NH₃ at T = 2 °C, and almost 50% of CO₂ is converted to NH₄HCO₃(s) at CO₂ loading = 1 mol CO₂/mol NH₃. The overall heat of absorption changes from −43.43 to −76.09 kJ mol⁻¹ CO₂ caused by NH₄HCO₃(s) formation at T = 2 °C (see Fig. 11(b)).

As shown in Fig. 11(b), the model results show a good agreement with the experimental data²⁰ at T = 40 °C. The predicted average heat of absorption is about −74.4 kJ mol⁻¹ CO₂ at low CO₂ loadings (0.2 mol CO₂/mol NH₃ < CO₂ loading < 0.5 mol CO₂/mol NH₃). This is consistent with Liu et al.‘s results (−74.8 kJ mol⁻¹ CO₂).²⁰Fig. 11(b) also shows that temperature has almost no effect on the heat of CO₂ absorption at low CO₂ loadings (less than 0.5 mol CO₂/mol NH₃), which is consistent with the results from the model of Que and Chen.⁵⁵ However, at high CO₂ loadings (above 0.7 mol CO₂/mol NH₃), the decrease in temperature shows a negative effect on the overall heat of CO₂ absorption. The overall heat of CO₂ absorption at a CO₂ loading of 0.9 mol CO₂/mol NH₃ are −77.1, −75.7, −73.3, −45.3 and −36.6 kJ mol⁻¹ CO₂ for temperatures of 2, 5, 10, 15 and 20 °C, respectively. This is likely the more amounts of NH₄HCO₃(s) at low temperature (see Fig. 11(a)) the more heat is released through NH₄HCO₃(s) formation reaction (R6). The formation of solid at low temperature greatly increases the overall heat of CO₂ absorption. CO₂ loading with the lowest absorption heat, 0.67, 0.75, 0.8, 0.83 and 0.92 mol CO₂/mol NH₃ at the corresponding temperature of 2, 5, 10, 15 and 20 °C are recommended in this study to avoid solid formation, which can, not only minimize the overall heat of CO₂ absorption, but also mitigate fouling and blocking problems in stripper and tubes.

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