Particle mixing and residence time when drying sawdust in a continuous spouted bed

When unscreened sawdust is dried in a spouted bed dryer using superheated steam with a temperature of 240 °C, there is no active discharge of the dried material. Instead the particles lose density as they dry, while being pneumatically transported out of the dryer. The residence time distribution was measured by injecting LiCl tracer added sawdust into the dryer. The measured residence time distributions show that particle mixing in the spouted bed dryer is very similar to the complete mixing occurring in an ideal continuous stirred tank reactor. A comparison between the residence time distribution and the results of stop-and-empty trials indicates that the experiments were made with a small accumulation of inert material in the dryer. Under identical conditions regarding in- and outgoing steam temperatures, steam flow, material flow and moisture contents, the spouted bed dryer can operate under different sets of operating conditions as regards the mass of the material in the dryer and the residence time.