Study On Heat And Mass Transfer Of Filamentous Materials In Vacuum Freeze Drying

China is currently the world’s largest producer and consumer of tobacco.Drying tobacco leaves is a crucial step in cigarette processing.However,the widely used method for drying tobacco leaves in Chinese cigarette companies is hot air high-temperature drying.This drying method leads to significant loss of aroma in the tobacco leaves,thereby affecting the sensory quality.On the other hand,low-temperature vacuum drying can provide a more stable and controllable drying process for tobacco leaves.It minimizes the loss of aromatic substances and enhances the sensory quality of the product,thus better preserving the refreshing and sweet characteristics of the tobacco leaves.To determine the optimal freeze-drying process parameters,this study investigates the heat and mass transfer of tobacco leaves during the freeze-drying process.The main aspects covered are as follows:(1)The theoretical analysis of tobacco leaf sublimation drying process was conducted.Experimental measurements and empirical calculations were performed to obtain the basic thermal properties of tobacco leaves.This data serves as a basis for subsequent freezedrying experiments and simulations.Parameters include the density of dried tobacco leaves with moisture content below 5% measured using an electronic balance,the porosity of the drying layer determined through immersion medium method,and the effective thermal conductivity of the drying layer obtained using the quasi-steady-state plate method.Similarly,the effective thermal conductivity of the freezing layer was calculated using empirical equations.(2)A mathematical model for calculating the temperature field in the freeze-drying chamber was established.Finite element software was utilized to simulate and analyze the temperature distribution inside the freeze-drying chamber.Simulation analysis of the radiative heat transfer between two parallel plates provided insights into the temperature distribution and temperature gradient on the surface of the receiver.To ensure uniform drying of tobacco leaves and avoid excessive drying due to localized high temperatures,a reasonable stacking shape and position for the tobacco leaves were determined.(3)A heat and mass transfer model for the sublimation drying process of tobacco leaves was constructed.The COMSOL software was used to simulate the sublimation drying stage of tobacco leaves,obtaining the temperature field,velocity field,pressure field,and changes in moisture content inside the tobacco leaf pile during the sublimation drying stage.Vacuum freeze-drying experiments were conducted on the tobacco leaves,and the internal temperature and weight of the tobacco leaf pile were measured.The experimental results demonstrated a close agreement between the measured and simulated values,with a small error,thus confirming the accuracy of the model.Finally,the impact of different parameters such as shelf temperature,vacuum level in the freeze-drying chamber,freeze-drying chamber temperature,and tobacco leaf pile thickness on the sublimation drying time of tobacco leaves was simulated,and reasonable tobacco freezedrying process parameters were provided.