The knowledge of the speciation and of the supersaturation of aqueous solutions of CO2 and NH3 is pivotal for the design and optimization of unit operations, e.g. absorption or crystallization, in the framework of ammonia-based CO2 capture systems.

In this work, we estimate the macroscopic crystal growth and dissolution rates of ammonium bicarbonate in aqueous ammonia solutions as a function of the solution supersaturation in a range of process conditions of industrial relevance. The electrolytic and reactive nature of the mixtures requires the computation of the driving force for crystallization using an activity-based speciation model. The model parameters have been estimated based on experimental speciation data obtained by applying tailored multivariate data analyses on the measured ATR-FTIR spectra of the solutions. Moreover, the volatility of CO2(aq) and NH3(aq) requires operating the system under its vapor pressure in order to avoid depletion of the solute in the liquid phase. For this reason, a sealed crystallizer equipped with custom-made connections for the online monitoring tools, such as FBRM and ATR-FTIR probes, has been used during the experiments. In addition to estimating the crystallization kinetics, it has been found that the speciation in solution and the supersaturation can affect the relative growth of the ammonium bicarbonate crystal facets, thus leading to different crystal habits. Finally, this work provides a sound thermodynamic framework in which the kinetics models for crystal growth and dissolution of ammonium bicarbonate can be applied to the design and optimization of ammonia-based CO2 capture absorption processes that exploit solid formation.

A systematic method is presented to design multistage continuous crystallization processes with heat integration. A general method has been applied to a specific case, namely the integrated absorption-based process for postcombustion CO2 capture in which solid formation is exploited. At first, a set of viable process flow schemes based on different equipment configurations for crystallization has been selected. Then, a rate-based model capable of describing the phenomena occurring during crystallization and dissolution of particles coupled with heat transfer has been developed for design and optimization purposes. Because of the broad design space and the interplay between unit operations, the mathematical model stands out as the key tool for the assessment of the process feasibility. Furthermore, a multiobjective optimization of the solid handling section has been performed, the aim of which is twofold: identifying optimal operating conditions and gathering information on the effect of and the synergy among the different design variables on the process performance. The results are evaluated on the basis of key performance indicators such as productivity and specific energy penalty, as well as of the particle size distribution of the crystals obtained during crystallization.

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In this study, we estimated the growth kinetics of ammonium bicarbonate crystallized from aqueous ammonia solutions containing an impurity in the form of ammonium sulfate. The evolution of the ammonium bicarbonate concentration of seeded desupersaturation experiments, estimated by attenuated total reflection−Fourier transform infrared measurements, has therefore been fitted to a population balance model that accounts for the real speciation of the system in the computation of the solution supersaturation. Furthermore, nonidealities of the liquid phase have been considered through the extended-UNIQUAC model. In this work, it has been found that the presence of sulfate ions in solution leads to a progressive decrease in the ammonium bicarbonate crystal growth rates as the concentration of the sulfate ions increases. A competitive adsorption model based on a Langmuir-type isotherm in which the sulfate ions represent the impurity in the system has been found to describe the experimental data rather well. Moreover, the analysis of the ammonium bicarbonate crystal purity revealed that sulfate ions have been sparingly included in the crystals at the experimental conditions reported, thus indicating that the foreign species is only temporary adsorbed on the crystal surface.