Validation of a Model of an Expanding Superheated Steam Flash Dryer for Cut Tobacco Based on Processing Data

Abstract

The role of drying in modern technology is not only to remove moisture but also to control solid properties in a predetermined way. The quality of cut tobaccos can be improved when they are expanded i.e., when their specific volume (filling power) is increased. This effect can be achieved by means of a flash dryer and the process is described in the article. The dryer uses superheated steam as a drying and expanding agent and is composed of a drying pipe, cyclone, circulating fan, and indirect steam superheater. The process atmosphere is virtually oxygen free (O₂ < 4.5%). A distributed parameter model is used to describe solid velocity, temperature, and moisture content as well as steam temperature and velocity along the dryer pipe. Experimentally obtained solid properties (specific heat, characteristic drying curve, particle density, and morphology) are incorporated in the model. Monodisperse particle population representation is used. The above model solutions are iterated in a closed loop lumped parameter model until a steady state solution is reached. Experimental data obtained during test runs of the industrial scale CLED (Closed Loop Expansion Dryer) on lamina cut tobacco throughputs up to 3000 kg/h are used to validate the model. Several test runs were carried out at various processing parameters while both input and output parameters of cut tobacco (in/out moisture and temperature) and steam (in/out temperature, in/out O₂ contents) were measured. The experimental results compare satisfactorily well to model predictions. Optimum processing parameters, leading to improvement of physical properties of the cut tobaccos, have been obtained in the CLED process. This leads to significant mass savings in final products (cigarettes), ca. 10%. Moreover, substantial fuel economies, due to the closed steam cycle, make the superheated steam drying of cut tobacco even more attractive.

Keywords:

• Closed cycle
• Lamina
• Filling power

Acknowledgment

The authors are grateful to ITM B.V. for permission to publish the results.