Figure 2a shows that the results of the simulation follows the trends of the experimental data of Goppert and Maurer (1988), which were reported by Darde [
25] in Figure 5-12 and Figure 5-13. The greatest volatility of the CO
2 corresponds to the greater partial pressure (at least 2 orders of magnitude) greater than that exerted by the NH
3 for any of the compositions of the anaerobic digestate investigated in
Figure 2a at the bubble point. The chemistry of this blend also plays a crucial role in determining the volatility of these compounds as partial pressure of the CO
2 started to increase at a greater rate when the moles of this compound in the anaerobic digestate were greater than the moles of NH
3 (
Figure 2a), which was set to 0.6 mol/kg H
2O for the whole test to comply with the experimental data available in Figure 5-12 and Figure 5-13 of Darde [
25]. Typical concentrations of CO
2 and NH
3 in the anaerobic digestate are around 0.12 mol NH
3/kg H
2O and 0.07 mol CO
2/kg H
2O [
20], which are milder concentrations than those tested by Darde et al. [
26] for the development of a carbon capture process. However, given the wide scope of the ELECNRTL property method, the validation could be done at the lower end of the concentration investigated by Darde et al. [
26].
Figure 2b shows the calibration carried out with a concentration of 0.13 mol NH
3/kg H
2O with the experimental data of Pexton & Badger (1938), which is reported in
Figure 2 and
Figure 3 of Darde et al. [
26].
Figure 2c validates the precipitation of the NH
4HCO
3, which is less soluble than the NH
4COONH
2 and it has similar solubility to NH
4Cl and higher solubility than NaHCO
3. Contrary to the explanation of Möller & Müller [
7] on the formation of ammonium carbonate in the anaerobic digestate, Aspen Plus v12 ELECNRTL does not consider the formation of this compound. Modelling the solubilities of NH
4Cl and NaHCO
3 were considered because the HCl and NaOH were tested as titrants of the anaerobic digestate, to assist the flash distillation, tune the ratio inorganic nitrogen to inorganic carbon in the distillate, and promote the precipitation of NH
4HCO
3. Therefore, NH
4Cl and NaHCO
3 might precipitate in the residual stabilized digestate and they are not expected in the distillate despite having similar or lower solubility to NH
4HCO
3. The big difference in the solubility of NH
4HCO
3 modelled above 60 ⁰C with regard to the experimental data available in the literature [
27,
28] is because this compound is not very stable and easily undergoes decomposition [
29].
Figure 2d shows the modelling of the titration of the manure digestate resulting from the process of Rajendran et al. [
17] and considering some of the alkaline elements identified in the comprehensive characterization of various anaerobic digestates, performed by Akhiar et al. [
20]: NaHCO
3, CaCl
2, NaCl, and KHCO
3 (Table S1). The validation of the simulation of the effect of HCl and NaOH on the proposed anaerobic digestate was done by comparing to experimental data of titrations available in the literature. Particularly, the study of Vandré & Clemens [
30] handling raw animal slurry and the previous work of Moure Abelenda et al. [
21] with agrowaste digestate and food waste digestate. The results showed that the amount of CO
2 and carbonates in the anaerobic digestate was responsible of the OH alkalinity at pH > 10. The content of ammonia was found to be the main responsible of the partial (P) alkalinity and the total (M) alkalinity. This means that the volatile fatty acids (i.e. acetic acid and propionic acid) and other components (e.g. hydrogen sulfide; Table S1) previously reported with a role in regulating the pH of the anaerobic digestate [
31], did not have a significant buffer effect in the present model. Simply tuning the ratio CO
2 to NH
3 in the digestate will affect the pH as can be seen in Figure S1, which shows the how pH depends on the composition of the anaerobic digestates and the remaining stabilized digestates (
Figure 1) after the flash separations performed for
Figure 2b.