A Drying Model of Tobacco Midrib Expanding in Air Flow

Abstract

A drying model was developed to simulate changes in the water content and the temperature of a cylindrical plant material, tobacco midrib, which expands in air flow mixed with or without superheated steam. The model is characterized as follows: (1) mass and heat transfers in the midrib are described by one-dimensional diffusion based on conservation laws; (2) adsorption equilibrium of water is always achieved at a solid-gas interface, so that the internal movement of water is regulated as a rate-limiting step; and (3) transfer phenomena in the expanding diffusion field are simplified by defining the expanded maximum radius as a diffusion length. The curves of water content and temperature calculated by the model were in agreement with each experimental value under various drying conditions: air temperature of 373 to 473 K and flow rate of 10 to 20 m/s. The model also represented drastic rises in temperature caused by condensation heat of water vapor in the initial drying stage. It was therefore judged that the model has validity and can be applied to estimation of the drying curves in expanding diffusion systems for tobacco midribs.