Exergy analysis for a freeze-drying process

Abstract

A mathematical model for exergy loss analysis of a freeze-drying process was established to evaluate the exergy losses in the individual operations and the distribution of exergy losses in a freeze-dryer. The exergy losses of five operations, namely, freezing, primary drying, secondary drying and vapor condensation as well as vacuum pumping were included in the model. The unique feature of the model is the incorporation of dynamics into the expressions of the exergy loss analyses for freezing, primary drying and secondary drying stages. The distribution of exergy losses at various operating parameters of freeze-drying was investigated using this model. Take freeze dying beef for an example. The effects of various operation conditions on the exergy losses in the three stages were investigated. The results show that the exergy consumption in the primary drying reaches 35.69% of the total exergy input, while exergy consumption in vapor condensing is 31.76% of the total exergy input. In the vacuum pumping 23.29% of the total exergy input is consumed. In contrast the exergy consumption in the freezing and the secondary drying is only 3.56% and 5.71% of the total exergy input, respectively. The exergy analyses based on various operating parameters show that the exergy losses of the drying process can be remarkably reduced by increasing the temperature of the cooling source in the vapor condenser. In this study, when the temperature of the cooling source in the vapor condenser increases from −70 °C to −25 °C, it leads to the total exergy losses reducing from 1409 kJ/kg(moist basis) to 604 kJ/kg(moist basis). It indicates that increasing the temperature of vapor condensation is an effective way to reduce the total exergy losses of the system, as long as the conditions of drying dynamics are satisfied during drying. Moreover, there is an optimal surface temperature corresponding to the minimum total exergy losses during the primary and secondary drying stages. If the surface temperature is higher than the optimal temperature, an increase in the surface temperature leads to a dramatic increase in the total exergy losses. This knowledge will provide insights into the optimization of a freeze-dryer and the operating parameters that reduce energy consumption and losses in freeze-drying.