Drying of granulated materials. Part I. Drying of a single granule

✍ Scribed by Van Krevelen, D. W. ;Hoftijzer, P. J.

• Publisher: Wiley (John Wiley & Sons)
• Year: 1949
• Weight: 770 KB
• Volume: 68
• Category: Article
• ISSN: 0368-4075
• DOI: 10.1002/jctb.5000680207

✦ Synopsis

Abstract

Granules of wet marl, suspended from the beam of a torsion balance, were dried in a current of dry air. Two rate periods were observed; during the first the rate of drying remains constant, during the second it decreases. With the aid of the conception of Ceaglske and Hougen on moisture distribution in granular materials equations for the rate of drying during the second stage could be derived which arc proved to be in agreement with the experiments. Besides these theoretical equations dimensionless empirical expressions were derived for the two drying stages which enable the rate of drying of single granules under arbitrary conditions to be calculated.

Granules of nitrochalk fertilizer, when dried in the same apparatus, showed quite a different behaviour. At first the rate of drying decreased rapidly and soon reached a constant value. This constant rate was over a thousand times smaller than the rate of drying in the case where mass transfer through the gas film is rate determining. The rapid decrease of the drying me at the start could be ascribed to n filling of the pores by solid ammonium nitrate. As soon as the rate became constant drying took place exclusively by diffusion of water vapour through a thin crystal film which had an average thickness of 10^‐3^ to 10^‐5^ cm. In this period the rate of drying was only influenced by temperature and by size and structure of the granule.