Energetic optimization of lyophilisation

Abstract

Lyophilisation is a special method of dehydration carried out through a process of sublimation. Particularly in the food sector, this operation is increasingly used because it always ensures the quality of food. This technique is widely used in fruit, high protein foods and juices. In contrast, the disadvantage of this method is its high cost. This leads to a major constraint: economically speaking, freeze-dried foods are not available to everyone. Furthermore, to carry out the lyophilisation process it is necessary to operate under very low temperature conditions, which is an additional reason why, from the energy standpoint, it is a very expensive process. For all these reasons, the aim of this research is the energy optimization of lyophilisation and the study of the possible integration of the refrigeration cycle in the lyophilisation process. This study consists in the following stages: • Bibliographical study about lyophilisation parameters. • Selection of refrigerant fluids to be studied. • Simulation with ASPEN to select the best refrigerant and the best conditions.The bibliographical study has permitted to establish that the lyophilisation parameters generally use a range between -5°C for the sublimation chamber, assuring to be below the triple point, and 20°C to remove the final adsorbed water. For the condensation chamber, the temperature fluctuates between -10°C and -50°C, assuring enough driving force for vapour transfer. The refrigerants selected through the study have been: R32, ammonia, R152A, R1270A and R134A, which are the most generally used satisfying environmental concerns by avoiding chlorinated compounds. After ASPEN simulations and having compared variables such as power, energy consumption and operation time, depending on temperature difference, it can be concluded that although these parameters vary depending on the refrigerant used, the greater the temperature difference, the more power consumption. The greater temperature difference implies the less drying time, whilethe energy consumption, which is power multiplied by time, could present a minimum. However, according to the temperature differences studied, only refrigerant R152A and ammonia present a minimum. Taking into account energy considerations and operating safety, the R32 refrigerant has been chosen as the best one for carrying out the lyophilisation process, since this refrigerant operates at sublimation and condensation temperatures of -2°C (7,6 bars) and -40°C (1,8 bars), respectively, which are reasonable temperatures for carrying out the lyophilisation process. Consumption of energy is not too high, and pressures implied are appropriate from the operation safety point of view.