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Powder moisture (%)

Figure 32. Typical combinations of conditions for agglomeration.

Exhaust air

Bag collector

system

To powder storage

Exhaust air

Bag collector system

To powder storage ture is dried off and the powder cooled below its thermoplastic point.

Agglomeration Equipment

While slightly different in equipment design and operation, most commercially available agglomeration processes fundamentally are the same. Each relies on the formation of agglomerates by the mechanism already described. This is followed by final drying, cooling, and size classification to eliminate the particle agglomerates that are either too small or too large. Generally, designs involve a re-wet chamber followed by a belt or a fluid bed for moisture removal. Such a system is shown in Figure 33.

It is obvious that this system is quite sensitive to even very minor variations in powder or liquid rates. A very brief reduction in powder feed rate will result in overwetting of the material with consequent deposit formation in the chamber. Conversely, a temporary reduction in liquid rate will result in sufficiently wetted powder and, therefore, weak agglomerates. Many designs rely on the product impacting the walls of the agglomeration chamber to build up agglomerate strength. Other designs include equipment for breaking large lumps into suitably sized agglomerates before the final drying. Obviously, deposit formation always will be a concern in agglomeration equipment as the process depends on the creation of conditions where the material becomes sticky.

Typically, equipment designs are very 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 that involves the use of a fluidized bed for the rewetting and particle contact phase. This approach provides the following advantages:

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, rewet 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 always is constant, controlled by an overflow weir. Thus, the rewetting section will not be emptied of powder. Even during a complete interruption of powder flow, the fluidized material will remain in the rewet 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. Startup, shutdown, and operation of the fluid bed agglomerator are greatly simplified owing the stabilizing effect of the powder volume in the rewet zone.

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

Features of Fluid-Bed Agglomeration

Figure 34 shows a typical agglomerator system where the process is implemented through the use of a vibrated, continuous fluid bed.

The powder is fed into the agglomerator by a volumetric screw feeder. As a result of the previously mentioned stabilizing effect of the material already in the fluid bed, the

Steam

Exhaust,

Exhaust,

Steam

Figure 33. Typical agglomeration system.

Agglomerated product

Figure 33. Typical agglomeration system.

ft Exhaust air

Figure 34. Fluid bed rewet agglomeration system.

Figure 34. Fluid bed rewet agglomeration system.

Compressed air

liquid

Fines and oversize return

Compressed air

liquid

Fines and oversize return

Table 5. Frequently Used Binders

For food products

For chemical products

Malto dextrines

Gum arabic

Starch

Gelatin

Molasses

Sugar

Lignosulfonates

Poly(vinyl alcohol) (PVA)

Any of the food product binders reproducibility of a volumetric feeder is satisfactory and there is no need for a complicated feed system such as a loss-in-weight or similar type.

The fluid bed unit is constructed with several processing zones, each with a separate air supply system. The first section is the rewet and agglomeration section where agglomerates are formed. Here, the powder is fluidized with heated air in order to utilize its thermoplastic characteristics.

The binder liquid almost always is water or a water-based solution, whereas steam, as already explained, rarely is used. The binder liquid is sprayed over the fluidized layer using two fluid nozzles driven by compressed air. For large systems, numerous nozzles are used. Powder deposits are minimized by accurate selection of spray nozzle angles and nozzle position patterns. Powder movement is enhanced by the vibration of the fluid-bed unit itself and by the use of a special perforated air distribution plate with directional air slots. A proper detailed design is vital in order to have a trouble-free operation.

From the agglomeration zone, the powder automatically will flow into the drying area where the added moisture is removed by fluidization with heated air. In some instances, more than one drying section is required, and in such cases, these sections are operated at successively lower drying temperatures in order to reduce thermal exposure of the more heat-sensitive dry powder.

The final zone is for cooling where either ambient or cooled air is used to cool the agglomerates to a suitable packaging temperature.

During processing, air velocities are adjusted so that fine, unagglomerated powder is blown off the fluidized layer. The exhaust air is passed through a cyclone separator for removal and return of entrained powder to the inlet of the agglomerator. When there are high demands for a narrow particle size distribution, the agglomerated powder is passed through a sifter, where the desired fraction is removed while over- and undersize material is recycled into the process.

As with all rewet agglomeration equipment, the operation must be performed within certain operating parameters. Overwetting will lead to poor product quality, while underwetted powder will produce fragile agglomerates and an excessive amount of fines. However, fluid-bed agglomeration does offer a great degree of flexibility in controlling the final result of the process. The characteristics of the formed agglomerate can be influenced by operating conditions such as binder liquid rate, fluidizing velocity, and temperature. Typically, the fluid-bed rewet method will produce agglomerated products with superior redispersion characteristics.

As indicated by this partial list, this method has been used successfully with a number of products.

Dairy products Infant formula Calf milk replacer Flavor compounds Corn syrup solids Sweeteners Detergents Enzymes Fruit extracts

Malto dextrines

Herbicides

Egg albumin

In most cases, the agglomeration can be accomplished using only water as a rewet medium. This applies to most dairy products and to malto dextrine-based flavor formulations. For some products, increased agglomerate size and strength has been obtained by using a solution of the material itself as the binder liquid. In the case of relatively water-insoluble materials, a separate binder material has been used, but it must be one that does not compromise the integrity of the final product. The addition of the binder material may have a beneficial effect on the end product at times. This is seen, for example, in flavor compounds when a pure solution of malto dextrine or gum arabic may further encapsulate the volatile flavor essences and create better shelf life. In other instances, the added binder can become part of the final formulation as is the case with some detergents.

For some materials, the addition of a binder compounds is an unavoidable inconvenience. At such times, the selected binder must be as neutral as possible and must be added in small quantities so that the main product is not unnecessarily diluted. An example of this is herbicide formulations, which often have a very well defined level of active ingredients.

For products containing fat, the normal process often is combined with a step by which the agglomerates are coated with a thin layer of surface-active material, usually lecithin. This is done by mounting an extra set of spray nozzles near the end of the drying section where the surfactant is applied.

Variations of the fluid-bed rewet technology have been developed whereby the system serves as a mixer for several dry and wet products. An example of this may be seen in the APV fluid mix process for the continuous production of detergent formulations from metered inputs of the dry and wet ingredients. This provides an agglomerated end product that is produced with minimum energy input when compared to traditional approaches.

Spray-Bed Dryer Agglomeration

While the fluid-bed rewet agglomeration method produces an excellent product that is, in most respects, superior to that made directly by the straight-through process, a new generation of spray dryers has evolved that combines fluid-bed agglomeration with spray drying. These are referred to as "spray-bed" dryers. The concept was developed from spray dryers having a fluid bed integrated into the spray chamber itself and is depicted in Figure 35. What distinguishes the spray-bed dryer is that it has the drying air both entering and leaving at the top of the chamber. At-omization can be with nozzles or by a centrifugal atomizer.

During operation, the chamber fluid bed is vigorously agitated by a high fluidization velocity, and as the particles from the spray-drying zone, they enter the fluid bed with a very high moisture content and agglomerate with the powder in the bed. Fines carried upward in the dryer by the high fluidizing velocity have to pass through the spray cloud, thus forming agglomerates at this point as well. Material from the integrated fluid bed is taken to an external fluid bed for final drying and cooling.

The spray-bed dryer is a highly specialized unit that can produce only agglomerated powder. It is best suited for small to medium-sized plants since the efficiency of the agglomeration process unfortunately decreases as the plant increases in size. This is because the spray zone becomes too far removed from the fluid-bed zone as the size of the dryer increases.

Agglomerates from the spray-bed dryer exhibit excellent characteristics. They are very compact and show high agglomerate strength and good fiowability.

Considerations and Conclusions

While the agglomeration process improves the redispersion, fiowability, and nondustiness of most fine powders, it invariably decreases the bulk density. The comparison in Table 6 clearly shows that agglomeration improves the powder wettability and dispersibility. Individual powder particles with a mean diameter of less than 100 /im are converted into agglomerates ranging in size from 250 to 400 jum, with the rewet method being able to produce the coarser agglomerate. The powder bulk density will decrease from about 43 lb/fit3 to approximately 28 lb/ft3. Use of the rewet method will expose the product to one additional processing step that, in this case, will somewhat affect the proteins and result in a slightly poorer solubility.

Since fluid bed agglomeration can be operated as an independent process, it can be used with already existing power-producing equipment. It offers great flexibility and ease of operation, and provides a convenient way to add functionality, nondustiness, and value to a number of products.

SPIN FLASH DRYERS Background Information

While mechanical dewatering of a feed slurry is significantly less expensive than thermal drying, this process results in a paste or filter cake that cannot be spray dried and can be difficult to handle in other types of dryers. The spin flash dryer is one option available for continuous powder production from pastes and filter cakes without the need for grinding.

Powders generally are produced by some form of drying operation. There are several generic types of dryers, but all must involve the evaporation of water, which can take anywhere from 1000 to 2500 Btu/lb depending on dryer type. The most common of these dryers probably is the spray dryer because of its ability to produce a uniform powder at relatively low temperatures. However, by its definition, a spray dryer requires a fluid feed material to allow its atomization device to be employed. Generally, there is a maximum viscosity limitation in the range of 10001500 SSU (Saybolt Seconds Universal viscosity unit) (see Fig. 36).

Figure 37 illustrates the amount of water that must be evaporated to produce 1 lb of bone dry powder from a range

Feed

Figure 35. Spray bed-type agglomerating spray dryer.

Table 6. Reconstitutability and Physical Structure of Different Types of Skim Milk Powder

Ordinary spray dried Integrated fluid-bed Rewet agglomerated powder agglomeration powder

Wettability, s >1000 <20 <10

Dispersibility: % 60-80 92-98 92-98

Insolubility index <0.10 <0.10 <0.20

Average particle size, fim <100 >250 >400

Density, lb/ft3 40-43 28-34 28-31

of different feed solids. It can clearly be seen that even a 5% increase in total solids will reduce the water evaporation and, hence, the dryer operating costs by about 20%. If this water removal can be done mechanically by, for example, filtration or centrifuging, the cost will be infinitely lower than that required to heat and evaporate the same water. The direct energy cost can be calculated as equivalent to 3-8 Btu/lb, compared to an average 1500 Btu/lb for evaporation. This increase in solids, however, inevitably will result in an increase in viscosity that may exceed the limitations of a spray dryer.

Options available for drying these higher-viscosity feed materials are listed in Table 7. The spin flash dryer is among the newest of the dryer options and has the capability of drying most materials ranging from a dilatent fluid to a cohesive paste.

Description

The spin flash dryer was developed and introduced in 1970 in response to a demand by the chemical industry to produce a uniform powder on a continuous basis from high-viscosity fluids, cohesive pastes, and sludges.

The spin flash dryer can be described as an agitated fluid bed. As shown in Figure 38, the unit consists primarily of a drying chamber (9), which is a vertical cylinder with an inverted conical bottom, an annular air inlet (7), and an axially mounted rotor (8). The drying air enters the air heater (4), is typically heated by a direct-fired gas burner (5), and enters the hot-air inlet plenum (6) tangentially. This tangential inlet, together with the action of the rotor, causes a turbulent whirling gas flow in the drying chamber.

The wet feed material, typically filter cake, is dropped into the feed vat (1) where the low-speed agitator (2) breaks up the cake to a uniform consistency and gently presses it down into the feed screw (3). Both agitator and feed screw are provided with variable speed drives.

In the case of a dilatent fluid feed, the agitated vat and screw would be replaced with a progressive cavity pump and several liquid injection ports at the same elevation as the feed screw.

As the feed material is extruded off the end of the screw into the drying chamber, it becomes coated in dried powder. The powder-coated lumps then fall into the fluid bed and are kept in motion by the rotor. As they dry, the friable

Feed

Figure 35. Spray bed-type agglomerating spray dryer.

Agglomeration nozzles

Anhydro Spin Flash Drying

Final product

Bag collector

Exhaust air

Agglomeration nozzles

Bag collector

Final product

Anhydro Spin Flash Drying

Figure 36. Typical APV Anhydro spin flash dryer with general dryer characteristics. Dryer characteristics: drying method— direct gas contact; flow—cocurrent; food material—dilatent fluids, cohesive paste, filter cake, moist granules; drying medium—air, inert gas, low-humidity waste gas; inlet temperature—up to 1800°F; capacity—up to 10 tons per hour of final product; product residence time—5-500 s.

Figure 36. Typical APV Anhydro spin flash dryer with general dryer characteristics. Dryer characteristics: drying method— direct gas contact; flow—cocurrent; food material—dilatent fluids, cohesive paste, filter cake, moist granules; drying medium—air, inert gas, low-humidity waste gas; inlet temperature—up to 1800°F; capacity—up to 10 tons per hour of final product; product residence time—5-500 s.

surface material is abraded by a combination of attrition in the bed and the mechanical action of the rotor. Thus, a balanced fluidized bed is formed that contains all intermediate phases between raw material and finished product.

The dryer and lighter particles become airborne in the drying airstream and rise up the walls of the drying chamber, passing the end of the feed screw and providing, in effect, a continuous backmixing action within the heart of the dryer. At the top of the chamber, they must pass through the classification orifice, which can be sized to prevent the larger particles from passing on to the bag collec-

Table 7. Dryer Options for High-Viscosity Materials

Direct suspension dryers

Pneumatic or flash dryers Spin flash dryers Fluid-bed dryers

Direct nonsuspension dryers

Tray dryers Tunnel dryers Belt dryers Rotary dryers

Indirect dryers

Screw conveyor dryers Vacuum pan dryers Steam tube rotary dryers tor. These larger lumps tend to fall back into the fluid bed to continue drying.

Air exiting from the bag collector (10) passes through the exhaust fan (12) and is clean enough for use in a heat-recovery system. Dried powder is discharged continuously from the bottom of the bag collector through the discharge valve (11).

Two important features make the spin flash dryer suitable for products that tend to be heat-sensitive: (1) the dry powder is carried away as soon as it becomes light enough and therefore is not reintroduced into the hot-air zone, and (2) the fluid bed consists mainly of moist powder, which constantly sweeps the bottom and lower walls of the drying chamber and keeps them at a temperature lower than the dryer air outlet temperature. In addition to this self-cooling capacity, the lower edge of the drying chamber directly above the hot-air inlet can be provided with an auxiliary cooling ring.

Figure 39 illustrates the very rapid reduction in air temperature that occurs as a result of the high heat-transfer rate obtained in the fluid bed.

Spin Flash Dryer Apv
Figure 38. Fluid bed provides rapid air temperature reduction.
Spin Flash Dryer
Figure 39. Standard spin flash dryer configuration.

Operating Parameters

Inlet temperature of the drying air introduced into the chamber is dependent on the particular characteristics of the product being dried but generally would be similar to that used on a spray dryer for the same product.

Outlet temperature is selected by test work to provide the desired powder moisture and is controlled by the speed of the feed screw. Since the spin flash dryer produces a finer particle size than does a spray dryer, it has been found that a slightly lower outlet temperature may be used to obtain

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