Ammonia-ammonium nitrate absorption power cycle: Brief thermodynamic properties of binary fluid

• Ammonium nitrate is proposed as a non-volatile absorber for ammonia in absorption power cycle. • The thermophysical properties of ammonium nitrate-ammonia binary system have been experimentally characterised. • Simple correlations have been developed to predict saturation pressure, liquid density, dynamic viscosity and isobaric heat capacity as functions of ammonia mass fraction. This paper presents an experimental study to determine the thermophysical properties of a binary mixture of ammonia and ammonium nitrate. Ammonium nitrate is a non-volatile absorber of ammonia, and its utilisation can have a significant impact on improving the energy efficiency of future electricity generation, particularly for waste heat and geothermal applications. In absorption power cycles, such as the Kalina cycle, ammonium nitrate can be used to mitigate moisture precipitation issues in the turbine path, thereby improving turbine efficiency, lifetime, and size due to increased rotation speed. This work characterised the main thermophysical properties, including the saturation curve, liquid density, liquid dynamic viscosity, and isobaric heat capacity, for a temperature range of 15 to 60°C and an ammonia mass fraction in the mixture between 60% and 100%. The properties have been generalised in the corresponding phenomenological correlations with a feasible accuracy, suitable for use in future thermodynamic analyses and trade-offs between absorption and organic power cycles. In particular, it has been shown that Raoult’s law does not predict well the saturation pressure of the fluid due to the considerable effect of both adhesion and cohesion factors. A newly proposed phenomenological update to Raoult’s law accurately fits the test points within ±3% accuracy. The liquid density and dynamic viscosity increase drastically with the quantity of dissolved ammonium nitrate. For example, the dynamic viscosity of a 60% ammonia-ammonium nitrate solution is more than six times higher than that of pure ammonia. Regarding the thermodynamic cycle aspect, it was briefly shown that the Kalina cycle with a traditional 80% water-ammonia working fluid is almost equivalent to one with a 61% ammonia-ammonium nitrate solution.