Study on the enhancement of mass transfer in convective drying process of cut tobacco
The cylinder drier is widely used in the tobacco primary process. It combines the convective heat from the hot air inside the cylinder with the conductive heat from the cylinder wall to dry cut tobacco. Most of the former researches have focused on the hot air temperature. In this study, the enhancement of mass transfer in convective drying process of cut tobacco (temperature and velocity of hot air and temperature of cylinder wall) was investigated. The cylinder drier was simulated by a fixed bed drier in the laboratory, in which the convective heat from hot air and the conduction heat from thin wall were provided. Three kinds of cut tobacco (A: Burley from China; B and C: flue-cured tobacco from China and Brazil respectively) were chosen as experimental materials. The ranges of hot air temperature, hot air velocity and thin wall temperature were chosen as 60~100°C, 0.37~0.93 m/s and 50~90°C respectively. The results shows that: 1) A mathematical model which characterizes the enhancement of mass transfer in cut tobacco drying process was set up based on Fick's Second Law. The effects of hot air temperature, velocity and thin wall temperature on mass transfer of cut tobacco drying process can be expressed by the calculated effective diffusion coefficient (De) using this mathematical model. 2) The Arrhenius Equation can be used to describe the effect of hot air temperature on De. According to the Arrhenius Equation, the apparent activation energies (Ea) are 8.31, 12.11 and 21.13 kJ/mol for samples A, B and C respectively. 3) The effects of thin wall temperature and hot air velocity on De can be expressed by using the dimensionless analysis method based on Fick's Second Law. The calculated enhancement factors of thin wall temperature are 10.08, 12.21, 10.82 for samples A, B and C respectively. The calculated enhancement factors of hot air velocity are 0.65, 0.42, 0.81 for samples A, B and C respectively.