Fluid Bed Types

Fluid bed processors may be configured for either continuous or batch operation.

There are two types of basic fluid bed designs according to the solids flow pattern in the dryer.

• The continuous back-mix flow design for feeds that require a degree of drying before fluidization is established.
• The plug flow design for feeds that are directly fluidizable on entering the fluid bed.

Transport of the solids through the fluid bed may be achieved either by the fluidization alone or a combination of fluidization and vibration.

The flow of gas relative to the solids is characterized either as cross flow in a single tier fluid bed or as cross/counter-current in a multi-tier fluid bed.

Back - mix flow fluid bed

Back-mix flow fluid beds

These are applied for feeds that are non-fluidizable in their original state, but become fluidizable after a short time in the dryer, e.g. after removal of surface volatiles from the particles.

The condition of the fluidizing material is kept well below this fluidization point. Proper fluidization is obtained by distributing the feed over the bed surface and designing the fluid bed to allow total solids mixing (back-mix flow) within its confines. The product temperature and moisture are uniform throughout the fluidized layer. Heating surfaces immersed in the fluidized layer improve the thermal efficiency and perfomance of this system. Back-mix fluid beds of both rectangular and circular designs are available.

Plug flow fluid beds

Plug Flow Fluid Beds

These are applied for feeds that are directly fluidizable. Plug flow of solids is obtained by designing the fluid bed with baffles to limit solids mixing in the horizontal direction. Thereby the residence time distribution of the solids becomes narrow. Plug flow fluid beds of either rectangular or circular designs are especially used for removal of bound volatiles or for heating and cooling. The volatile content and temperature vary uniformly as solids pass through the bed, and the plug flow enables the solids to come close to equilibrium with the incoming gas.

Plug flow may be achieved in different ways depending upon the shape and size of the bed.

• In rectangular beds, baffles are often arranged to create an alternating flow of solids from side to side.
• In circular beds, baffles are spiral.
• In relatively small circular beds with high powder layers, baffles are radial.

Vibrating fluid beds

Vibro - Fluidizers

This design, marketed under the name Vibro-Fluidizer®, is basically of the plug flow type. It is especially applied for drying and cooling products that fluidize poorly due to a broad particle size distribution, highly irregular particle shape, or require relatively low fluidization velocities to prevent attrition. The Vibro-Fluidizer® operates with a shallow powder layer of less than 200 mm. This gives a much lower product residence time per unit bed area than non-vibrating beds which can have powder layers up to 1500 mm.

Vibro-Fluidizers incorporate pressure shock resistance and sanitary features if clean operation is required.

Contact Fluidizers

Contact Fluidizer™

This is a rectangular fluid bed dryer incorporating back-mix and plug flow sections. A rotary distributor disperses the wet feed evenly over the back-mix section equipped with contact heating surfaces immersed in the fluidized layer.

The heating surfaces provide a significant portion of the required energy, and therefore, it is possible to reduce both the temperature and the flow of gas through the system. This is particularly important for heat sensitive products.

Subsequent plug flow sections are used for postdrying and cooling, if required.

Advantages of the Contact Fluidizer™ - compared to fluid beds without heating surfaces, two-stage flash/fluid bed dryers, or rotary dryers - include its compact design, high thermal efficiency, and low gas throughput.

Multi-tier fluid beds

These fluid beds consist of two or more stacked fluid beds. The upper tier (back-mix or plug flow) is for predrying and the lower tier (plug flow) for postdrying. The drying gas travels counter-current to the solids. The gas leaving the lower tier contains sensible heat which is transferred to the upper tier. Furthermore, each fluid bed may be provided with immersed heating surfaces. These designs result in a low gas throughput and high thermal efficiency which are of great importance in closed cycle drying systems.

Multi- tier Fluid Beds