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Fluid bed evaporation

Figure 24. Specific energy cost as a function of the water evaporated in fluid bed.

surfaces by handling the powder directly on a fluidized powder layer in the integrated fluid bed. Moist powder particles thus are surrounded by already dry particles, and any tendency to stick to the chamber walls will be reduced.

In this drying concept, it is not necessary to vibrate the chamber fluid bed, although often the powders entering from the spray drying zone are difficult to fluidize because of their high moisture content and thermoplastic and hygroscopic characteristics. The fluidization characteristics are improved as the wet powder from the spray-drying zone is mixed and coated with dryer powder in the integrated fluid bed. The fluidization is further assisted by the use of a special type of perforated plate with directional airflow. The stationary fluid bed operates with high fluid-izing velocities and high bed depths, both optimized to the product being processed.

The air outlet of the integrated fluid-bed system is located in the middle of the fluid bed at the bottom of the chamber. This forms an annulus around the air outlet duct and creates an aerodynamically clean design, completely eliminating the mechanical obstructions found in older two-point discharge designs. The dryer is equipped with tangential air inlets, so called Wall Sweeps, for conditioned air. These are important to the operation of the dryer in two ways: they will cool the wall and remove powder that may have a tendency to accumulate on the wall, and they also serve to stabilize the airflow within the chamber. The dryer thus operates with a very steady and well-defined airflow pattern that minimizes the amount of wall buildup.

Figure 25. (a,b) Three-stage dryer combines conical drying chamber, integrated stationary fluid bed with directional air flow and external vibrating fluid bed. Provides optimum conditions for producing nondusty, hygroscopic, and high-fat-content products.

Figure 25. (a,b) Three-stage dryer combines conical drying chamber, integrated stationary fluid bed with directional air flow and external vibrating fluid bed. Provides optimum conditions for producing nondusty, hygroscopic, and high-fat-content products.

Atomization can be either by pressure nozzles or by a centrifugal-type atomizer.

The three-stage system is exceptionally suitable for the production on nondusty, hygroscopic, and high-fat-content products. The dryer can produce either high-bulk-density powder by returning the fines to the external fluid bed, or powder with improved wettability by straight-through agglomeration with the fines reintroduced into an atomizing zone. The concept also allows the addition of liquid into the internal fluid bed, thus opening the path to the production of very sophisticated agglomerated and instantized products, ie, excellent straight-through lecithinated powders. Whey powder from such a system shows improved quality because of the higher moisture level present when the powder enters the integrated fluid bed, providing good conditions for lactose after crystallization within the powder.

A three-stage dryer also offers a high production capacity in a small equipment volume. The specific energy requirement is about 10% less than for a two-stage dryer.

Spray-Bed Dryer

While the concept of a three-stage drying system with an integrated fluid evolved from traditional dryer technology, it served to spur the development of a modified technique referred to by APV as the "Spray Bed Dryer." This machine is characterized by use of an integrated fluid bed at the bottom of the drying zone but with drying air both entering and exiting at the top of the chamber. Atomization can be with either nozzles or a centrifugal atomizer.

The chamber fluid bed is vigorously agitated by a high fluidization velocity. Particles from the spray-drying zone enter the fluid bed with a moisture content as high as 1015% depending on the type of product and are dried in the bed to about 5%. Final drying and cooling take place in an external fluid bed.

The structure of the powder produced in the Spray Bed differs considerably from the conventional. It is coarse, consists of large agglomerates, and consequently has low bulk density but exhibits excellent flowability. The dryer is very suited to the processing of complicated products having high contents of fat, sugar, and protein. Two of the many possible variations of this type drying are shown in Figures 26 and 27.

Spray-drying systems divided into two or more stages undoubtedly will be a characteristic of almost all future dryer installations. The advantages resulting from this technology will provide dairy and food processors with the necessary flexibility and energy efficiency required to meet today's uncertain market and whatever changes will be called for in the future.

Pollution-Control Devices

Virtually, all plants have a system to ensure clean exhaust air while collecting powder at an efficiency of about 99.5%, and may be supplied with secondary pollution-control collection equipment if necessary. In most new spray dryers this is accomplished using bag filter collectors after the primary collection device. In the case of such products as acid whey where the use of bag collectors is not practical, high-efficiency scrubbers are used. Some multipurpose plants have both plus an elaborate system of ducts and dampers to switch between the devices as different products are dried.

An added benefit from the use of sanitary bag collectors is an improvement in the yield of sale product. Although this increase typically is not more than 0.5%, it still can be of substantial value over a year of operation.

Heat-Recovery Equipment

Although it is possible to reduce the direct heating energy consumption by the use of multiple-stage drying, optimum thermal conditions generally require that heat recuperators be used as well. A few approaches are shown in Figure 28.

Despite the many recuperators available for recovering heat from drying air, only a few are suited for the spray-

Figure 27. Spray bed dryer with added external fluid bed for final powder drying and cooling.

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