Regeneration units for thermolytic salts applications in water & power production: State of the art, experimental and modelling assessment

Abstract

Thermolytic solutions are often proposed as high salinity or “draw” stream to generate a chemical potential driving force in Salinity Gradient Power (SGP) and Forward Osmosis (FO) technologies. Depleted “draw” solutions exiting the process can be regenerated by a thermal process powered at very-low grade heat, which is able to decompose the salt into gaseous ammonia and carbon dioxide, which can be stripped and then reabsorbed in the draw solution, restoring its initial concentration. In this work, two different experimental prototypes for the regeneration of ammonium bicarbonate aqueous solution were designed, built and tested. The effect of several operating parameters on the regeneration efficiency was experimentally investigated also identifying technological limitations and relevant solutions. A process simulation tool has been developed, and for the first time in the literature, successfully validated against original experimental results. Results from modelling analysis suggest that among the investigated processes, only the vapour stripping is viable for such applications. Models were used to evaluate the performance of ideal forward osmosis desalination and ideal SGP heat engines, finding, in the case of forward osmosis desalination, specific thermal consumptions between 180 and 250 kWh/m3 and, in the case of SGP heat engines, exergy efficiency up to almost 5%.