Effect of phase change kinetics on energy efficiency of secondary refrigeration with hydrate slurries

Secondary refrigeration is one of the currently available means for reducing refrigerant leaks in large cooling units. Compared to the well-known ice slurry, hydrate slurries present new features, especially when they involve a gaseous phase, the pressure of which influences the fusion temperature. The clathrate-hydrate system considered herein is the mixed hydrate of CO2 plus tetra-n-butyl-phosphonium-bromide (TBPB) melting around 12°C. The concern of energy efficiency focuses attention on the total energy consumption of the process (herein air-conditioning), and therefore includes the primary cooling unit into the analysis. The present issue is: is very slow phase-change kinetics (compared to the breakthrough time of the slurry in the heat exchangers) problematic with respect to the energy consumption of the whole process? The equations of phase change in the heat exchangers are modified for including a first-order kinetics law characterized by a time constant and using the temperature shift from equilibrium as the driving force. The mixed hydrate is compared to the ice slurry. The results are unexpected but fully explained by a thermodynamic analysis.