Drying kinetics of single porous particles in superheated steam under pressure

Abstract

The kinetics of single porous particles of ceramic and lignite in pressurised superheated steam was investigated experimentally. Particles of diameters in the range 10–14 mm were used. The experiments were performed over a range of pressure, from 1.7 to 8.4 bar, and a range of superheated steam temperature, from 155 to 197 °C. The steam velocity was also varied, from 1.1 to 4.3 m/s. The results from these experiments show that the rate of drying is limited by the rate of external convective heat transfer. The operating pressure appears to have no influence on the drying rate. The experimental results, however, show that the equilibrium moisture content is higher in a higher pressure steam and that this equilibrium is attained earlier when drying is carried at higher pressure. A mathematical model for the drying of single particles in pressurised superheated steam was also developed and tested against experimental results of ceramic drying. This model is based on the receding core assumption and is solved using the Crank–Nicholson method. Prediction of drying rate, drying time and moisture content in the case of porous ceramic particles is encouraging. In the case of lignite particles the model always under-predicts the drying rate and this is thought to be due to cracking of the lignite particles which increases the area for heat transfer.