Post-combustion Carbon Capture - Toward the Integration of Cooling Crystallization into Ammonia-based Processes

The Chilled Ammonia Process (CAP) enables the separation of CO2 from concentrated and stationary CO2 emission sources. In particular, this process can be used to treat large-scale power plants’ flue gas, thus reducing the carbon dioxide concentration of the stream sent to stack. More recently, the development of an advanced CAP configuration in which a fraction of the absorbed CO2 is precipitated in the form of an inorganic salt via cooling crystallization, allowed for an increase of the CO2 capture efficiency as well as for a reduction of the overall plant’s energy consumption. This novel process, referred as the controlled solid formation-Chilled Ammonia Process (CSF-CAP), produces solids in a closed-loop system described in the following and uses them as a carbon carrier. At first, the ammonium bicarbonate is crystallized from the CO2-loaded solution exiting the absorption unit and partially separated from the mother liquor by means of hydro-cyclones, thus obtaining a concentrated slurry at a reduced mass flow-rate. Then, the slurry is sent to the regeneration section where the solid phase is completely dissolved before to entering the desorption unit where the solvent regeneration cycle is completed. Assessing the feasibility of integrating continuous crystallization into a CO2 capture plant framework necessarily requires the analysis of thermodynamic and kinetics limitations of the system. This thesis seeks to serve as a template for the design and optimization of a continuous solid handling section integrated with the CSF-CAP and by doing so it tackles several engineering challenges related to the process. At first, the use of a bottom-up approach has allowed to achieve a solid understanding of the system thermodynamics as well as of the ammonium bicarbonate solid formation kinetics via a combined experimental and modeling strategy. Then, the knowledge gathered on these fundamental aspects of the system has been used in a rate-based mathematical model that accounts for heat and mass transfer limitations during continuous crystallization. Finally, this thesis shows that the first principle mathematical framework developed, combined with the experimental solid-liquid equilibria data of the CO2-NH3-H2O system, represents a powerful tool for the analysis of the system’s behavior and for the optimization of the overall process performance