Exergy analysis of freeze drying of pharmaceuticals in vials on trays

The expressions for determining the exergy inputs and exergy losses due to heat and mass transfer in the primary and secondary drying stages as well as in the water vapor condenser and vacuum pump of the lyophilization process involving freeze drying of pharmaceuticals in vials on trays, are constructed and presented in this work. The structures of the exergy expressions indicate the importance of the magnitude of the duration times of the primary and secondary drying stages as well as those of the gradient of temperature and of the pressure gradients of water vapor and inerts in the material being dried, on the magnitude of the exergy inputs and exergy losses. This information is then used to indicate policies for the control variables of the lyophilization process that can result in reductions in the exergy losses in the primary and secondary drying stages, as well as in the water vapor condenser and in the vacuum pump system for heat-transfer and mass-transfer controlled freeze drying processes. The distribution of the exergy inputs and exergy losses for a lyophilization system are presented and the results show that most of the exergy input is consumed and most of the exergy losses result by the primary drying stage, the water vapor condenser, and vacuum pump in that order. By using the exergy expressions presented in this work operational control policies can be constructed that minimize the irreversibilities occurring in the operations of a freeze drying system of a given design, thus enhancing the efficiency of energy utilization in lyophilization. (C) 2008 Elsevier Ltd. All rights reserved.