Basic principle of operation

When a stream of air is passed upwards through a bed of material at a certain velocity the bed will first expand, then become suspended and agitated by the air stream to form a fluidised bed. This has the appearance of a boiling liquid due to the formation of many small bubbles the so called "bubbling fluidisation"

At higher air velocities, large bubbles and plugs of material are formed resulting in a more violent type of fluidisation called slugging or sprouting. Although smooth bubbling is preferred, some materials, especially those uniform in size, may be more amenable to sprouting fluidisation. The optimum operating air velocity for bubbling fluidisation lies above the minimum fluidising velocity but below the velocity of entrainment of the material

If a bed of the wet material is fluidised by a heated air stream, as in the laboratory batch dryer, the conditions are ideal for drying. The very efficient contact between air and solid particales due to the turbulence of the bed results in high heat transfer rates causing rapid evaporation of moisture which is carried away by the exit air. This process has a high thermal efficiency because most of the heat input is used in vaporising the moisture and the exit air only rises in temperature as drying is nears completion

Because of the very good mixing of particles in the fluidised state, the conditions of temperature and moisture content are uniform throughout the bed, therefore a uniform product is obtained. Also the product is not affected because fluidisation causes very little abrasion and the temperature can be adjusted to ensure no loss of properties for heat sensitive materials

The same principles apply for industrial fluidised bed dryers, both batch and continuous types, therefore this JMC laboratory fluid bed dryer can be used to assess the feasibility of different materials for large scale fluidised drying