Production of CO2 Hydrates in Aqueous Mixtures Having (NH4)2SO4 at Different Concentrations; Definition of Consequences on the Process Evolution, Quantification of CO2 Captured and Validation of Hydrates Production as Technique for Ammonium Removal from Waste Water

Carbon dioxide hydrates were formed in fresh water and in aqueous mixtures containing ammonium sulfate, at concentrations equal to 1.9, 6.3, and 9.5 wt%. The moles of hydrates formed were compared, to define the inhibiting strength of the electrolyte solution and the dependence of inhibition from concentration. The addition of salt strongly inhibited the process and the number of hydrates produced passed from 0.204–0.256 moles, obtained in fresh water, to 0.108–0.198 moles, obtained at the lowest concentration tested. The further addition of salt still lowered the production of the hydrates; at the highest concentration tested, only 0.092–0.177 moles were obtained. The pressure-temperature evolutions of the hydrates were then discussed and compared with the ideal process and with the experimental results obtained in demineralised water. Finally, further samples of CO2 hydrates, produced in the presence of 9.5 wt% salt in the aqueous phase (corresponding to 1.5 wt% NH4+), were recovered and dissociated in a separated environment. The liquid phase, resulting from their dissociation, was subjected to spectrophotometric analyses. Its NH4+ content was measured and compared with the initial concentration in water. Therefore, it was possible to quantify the capability of the system to remove the (NH4)2SO4 from the water (involved in hydrate formation) and to concentrate it in the remaining liquid phase. Considering the portion of water involved in hydrates formation, the concentration of ammonium passed from 1.5 wt% to 0.38–0.449 wt%.