Fluidised Bed Dryers Dr J Badshah University Professor

Fluidised Bed Dryers Dr. J. Badshah University Professor – cum - Chief Scientist Dairy Engineering Department Sanjay Gandhi Institute of Dairy Science & Technology, Jagdeopath, Patna (Bihar Animal Sciences University, Patna)

Plug Flow Fluid - Bed Dryer • 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. • 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.

Contact Fluidizers • This is a rectangular fluid bed dryer incorporating back-mix and mix 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 the gas through the system. This is particularly important for heat sensitive products. • Subsequent plug flow sections are used for post drying and cooling, if required.

Conventional Two stage Dryer and Compact Dryer • Conventional Two-Stage Dryer • Powder discharged from the drying chamber can be further dried and cooled in a vibrating fluid bed, Vibro-Fluidizer. • This two-stage drying concept achieves better overall heat economy and is suitable for many food and dairy products. • When non-agglomerated powders of non-fat products are dried, a pneumatic transport system can replace the fluid bed. • Compact Dryer • Atomization is created by either a rotary atomizer or spray nozzle atomizer. • The location of the fluid bed within the drying chamber permits drying to be achieved at lower temperature levels resulting in higher thermal efficiencies and cooler conditions for powder handling. • The plant can be equipped with pneumatic transport system for many powders or with an external vibrating fluid bed for agglomerated powders.

Vibrating Fluid Bed Dryer (VFBD) • It is specially applied for drying and cooling the products that fluidize poorly. It is due to a broad particle size distribution, highly irregular particle shape or require relatively low fluidization velocities to prevent attrition and high stickiness. • It is worth considering a batch or continuous vibrated bed dryer, which is basically of plug flow type. • An application of nearly vertical sinusoidal mechanical vibration (half-amplitude 3 -5 mm; frequency 10 -50 Hz) allows ‘pseudofluidization’ of the bed with rather low airflow rates. • 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

Fluidized Spray Dryer – FSD/Multi Stage Dryer • • • Agglomerated powders are obtained in systems based upon the integrated fluid bed or belt and a multi-stage concept where moist powder, produced during the first drying stage, forms agglomerates, which are after-dried and cooled in the following stages. The Fluidized Spray Dryer and the Multi Stage Dryer combine spray drying and fluid bed drying technologies and offer excellent product flexibility, thermal efficiency. Pressure nozzles or a rotary atomizer spray the feed down towards the fluid bed where agglomeration incorporating finer, recycled material takes place. Exhaust air outlet is let through the roof causing further agglomeration in the spray zone. Sticky products can be dried successfully, and the concept is ideal for drying heat sensitive products, and improved aroma retention is accomplished. It operates with low outlet-temperatures, achieving high thermal efficiency. Dry ingredients for additional flavor or nutrient fortification can be added within the system to further promote capabilities and improve formulation efficiencies. This design concept is successful for drying high fats, hygroscopic, and sticky products that are difficult to handle in more conventional designs.

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 pre drying and the lower tier (plug flow) for the post drying. • 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. • 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.

Mechanically Agitated Fluidized Bed Dryer • • • For drying of pastes, one variant uses a cylindrical vessel with a fast spinning agitator. The feed drops by gravity on agitator bottom for dispersion into an upward spiral of hot drying gas. Other versions use a high rpm chopper that disperse the feed in to hot air. More commonly, slowly rotating agitators (or rakes) are used to facilitate fluidization in the feed zone, where highly wet feed is fed into a continuous plug-flow dryer. The drying of micrometer-sized, cohesive particles belonging to the group C type materials according to Geldart's classification is always challenging. An agitated fluid bed dryer (AFBD) of pilot-scale capacity was designed to study the effect of the type of agitator and its speed, gas velocity, and inlet temperature and feed loading on the hydrodynamic performance of the AFBD. Key hydrodynamic parameters such as pressure drop have a profound influence on determining the fluidization characteristics. The pressure drop across the AFBD system was expressed in terms of flow coefficient, ξ, which can be conveniently used as a design parameter. The choice of agitator has an even greater influence on the drying kinetics and the results have been summarized, paving the way for a more efficient spiral agitator of the helical ribbon type.

Centrifugal Fluidized Bed Dryers • To intensify heat and mass transfer rates for rapid drying of surfacewet particles, a centrifugal-type device may be used so that the drag force due to the fluidizing gas can be balanced with an ‘artificial gravity’ generated by rotating the bed on a vertical axis. • The rotating fluidized bed equipment is complex and the decrease in drying times for most material is normally not high enough or essential enough to justify the cost and complexity. .

Centrifugal Fluidized Bed Dryers* • Drying characteristics of the fluidized food materials such as rice, potato and carrot have been investigated in a centrifugal fluidized bed whose effective length and diameter have been 0. 2 m and 0. 1 m, respectively. The effects of air velocity and initial moisture content on the hydrodynamics and drying rate have been determined. • The pressure drop has exhibited a maximum in the relatively thicker bed, while it has showed a plateau in the relatively thinner bed, with an increase in the air velocity. • The drying rate has been increased with increases in the initial moisture content of the materials and air velocity. • The variation of drying rate with respect to the moisture content has been almost linear at the falling rate period, and it has been well correlated by means of the rotational power acting on the bed. *Yong Kang et. al. , 1995. Drying of fluidized materials in a centrifugal fluidized bed. In Korean Journal Of Chemical Engineering. Volume 12, pages: 325 -330.

Tall Form and Multi-stage Dryer • Tall Form Dryer (TFD) • The atomized droplets dry while gently falling down the tower. • Further drying and cooling are carried out in a vibrating fluid bed located under the tower. • The tall form dryer is suited for both non-fat and fat-containing products, producing non-agglomerated and agglomerated free-flowing powders.

FILTERMAT Dryer • The FILTERMAT Spray Dryer is frequently used in food and dairy applications. • It operates at a low outlet temperature, achieving high thermal efficiency. • It is the recommended system for drying high fat, sugar-based, hydrolyzed, and fermented products.

Schematic Diagram of FBD

Three stage Spray Dryer

FBD Image