Energetic and exergetic assessment of two- and three-stage spray drying units for milk processing industry

In order to avoid perishable characteristics and secure long shelf life of liquid milk, it is mostly converted into more stable derivatives such as butter, cheese, yogurt and milk powder. Amongst them, milk powder contains highest shelf life at ambient conditions and accounts for 76% of global annual trade of milk and milk derivatives. The milk powder production and its qualitative features are often characterized by huge amount of high- and low-grade energy requirements depending upon the number of stages employed for drying, e.g. single, two or three. In the present work, comparative thermodynamic analysis of two- and three-stage milk drying units has been executed. Both the units have been extensively explored on the grounds of parameters such as exergy efficiency (72.01%/43.67%), drying efficiency (67.50%/65.71%), drying chamber efficiency (57.77%/53.51%), exhaust air temperature (346.15 K/353.15 K), overall specific exergy destruction (3081.64 kJ/kg/5006.28 kJ/kg), overall specific exergy consumption (0.74 kWh/kg/1.34 kWh/kg) and sustainability indices (3.57/1.78) so as to establish the fact that the thermodynamic behaviour of two-stage spray dryer was far better than its three-stage counterpart, at the cost of slightly higher specific steam consumption (3.53 kg/kg/2.77 kg/kg).