Dairy Process Engineering

Module 3. Fluidization

Lesson 19
BATCH FLUIDIZATION, FLUIDIZED BED DRYER

19.1 Introduction

Typically, equipment designs are complicated, probably reflecting the fact that agglomeration actually is a complicated process. Despite the complexity of the process, however, it is possible to carry out agglomeration by means of comparatively simple equipment, which involves the use of a fluidized bed for the re-wetting and particle contact phase.

Advantages of fluidized bed system for agglomeration

1. There is sufficient agitation in the bed to obtain a satisfactory distribution of the binder liquid on the particle surfaces and to prevent lump formation.
2. Agglomerate characteristics can be influenced by varying operating parameters such as the fluidizing velocity, re-wet binder rate and temperature levels.
3. The system can accept some degree of variation of the feed rate of powder and liquid as the product level in the fluid bed is always constant, controlled by an overflow weir. Thus, the re-wetting section will not be emptied of powder. Even during a complete interruption of powder flow, the fluidized material will remain in the re-wet section as a stabilizing factor in the process.
4. By using fluid bed drying and cooling of the formed agglomerates, it is possible to combine the entire agglomeration process into one continuously operating unit.
5. Start-up, shut-down and operation of the fluid bed agglomerator are greatly simplified due to the stabilizing effect of the powder volume in the re-wet zone.

 

Proper implementation of a fluid bed agglomeration system requires detailed knowledge of the fluidization technology. Fluidization velocities, bed heights, air flow patterns, residence time distribution and the mechanical design of vibrating equipment must be known.

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

(i) The back-mix flow design for feeds that requires a degree of drying before fluidization is established.

(ii) The plug flow design for feeds those are directly fluidizable on entering the fluid bed.

19.2 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 mixing (back-mix flow) within its confines. The product temperature and moisture are uniform through out the fluidized layer.

19.3 Plug Flow Fluid Bed

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.

19.4 Vibrating Fluid Bed Dryer (VFBD)

For beds of particles, which are difficult, to fluidize due to strong poly dispersity, particle size, or particle-to-particle adhesive forces (stickiness) it is worth considering a batch or continuous vibrated bed dryer. An application of nearly vertical sinusoidal mechanical vibration (half-amplitude 3-5 mm; frequency 10-50 Hz) allows ‘pseudo-fluidization’ of the bed with rather low airflow rates. In this case, the requirements of hydrodynamics and heat/mass transfer are effectively coupled. Vibrated bed dryers can also be used to reduce attrition by gentle processing. Most vibrated fluidized bed dryers are continuous units.

This design is known as vibro-fluidizer, which is basically of plug flow type. It is specially applied for drying and cooling the 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. Fig. 19.1 shows Schematic diagram of vibro fluidizer, while (Fig. 19.2) shows vibro fluidizer animation and (Fig. 19.3) shows animated multi purpose rewet agglomeration two stage spray drying plant.

Fig. 19.1 Schematic diagram of vibro fluidizer

During agglomeration process fine particles are returned from the cyclone separator, which is animated in the (Fig. 19.4) and (Fig. 19.5)

19.5 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.

19.6 Batch Fluidized Bed Dryer

Batch fluidized bed dryers are used for low through out, multi product applications. Drying air is heated directly or indirectly usually to a fixed temperature. The drying air flow rate is also usually fixed. However, it is possible to start drying at a higher inlet gas temperature and flow rate and lower it since the product moisture content falls below the critical value. Mechanical agitators or vibration may be needed if the material is difficult to fluidize.

19.7 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. 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.

19.8 Continuous Fluidized Bed Dryer

In this type of dryer, the bed temperature is uniform and is equal to the product and exhaust gas temperatures. However, due to inherent product residence time distribution, product moisture content will sway the range from inlet moisture content to lower value. One advantage of the perfect mixing dryer is that the feed falls in to a bed of relatively dry material and so is easy to fluidize.

19.9 Mechanically Agitated Fluidized Bed Dryer

Several designs of such dryers are in use today. For drying of pastes or sludges one variant uses a cylindrical vessel with a fast spinning agitator the bottom on to which the feed drops by gravity 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.

19.10 Centrifugal Fluidized Bed Dryers

To intensify heat and mass transfer rates for rapid drying of surface-wet 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.