Simulation of the Effect of Air Humidity Ratio in the Drying Room on the Rate of Material Evaporation in Vacuum Freeze Drying

Abstract

In the drying process there are several parameters that influence the evaporation rate. Apart from pressure and temperature, air humidity in the drying room has been shown to influence the evaporation rate. To find out how much influence air humidity has on the evaporation rate in a Vacuum Freeze Dryer (VFD) system, calculations and simulations need to be carried out. Evaporation in a VFD occurs by sublimation at evaporation conditions below the triple point of water. Ice weighing 2x10-2 kg was used as dry material to characterize the VFD process at a temperature of -10oC. The ice temperature and operating pressure are selected at conditions below the triple point so that sublimation evaporation occurs. The temperature of the drying room for heating was varied, namely (20, 25, 30)(oC) and pressure (0.5, 0.4, 0.3)(kPa). Air humidity ratio, varies at (300, 200, 100, 10, 1, 0.1, 0.01, 0.001) (kg H2O/kg dry air). The simulation is carried out in a drying room, the ice receives heat due to the temperature difference with room temperature, heat transfer occurs by radiation and diffusion. Heat is used to evaporate the ice through sublimation. The temperature of the ice upon sublimation is close to the wet bulb temperature and remains constant until the ice has completely evaporated. From the simulation it is known that changing the air humidity ratio in the drying room from 300 (kg H2O/kg dry air) to 0.001 (kg H2O/kg dry air) will increase the evaporation rate between 6.2% - 9.5%. The evaporation rate increased by 6.2% was obtained at P∞=0.3kPa, T∞=30oC. This increase in evaporation rate shortened the drying time by 0.42 hours. The evaporation rate increased by 9.5% at a pressure of P∞=0.5kPA, T∞=20oC thereby shortening the drying time by 1.03 hours. Simulations show that increasing the evaporation rate and shortening the drying time has the potential to increase the energy efficiency of the VFD. The increase in evaporation rate is influenced by operating temperature and pressure. Simulations confirmed that the evaporation rate increased when the drying chamber temperature was increased and the air pressure was decreased.