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Atomization Techniques

Proper atomization is essential for satisfactory drying and for producing a powder of prime quality. To meet varying parameters, APV offers a selection of atomizing systems: centrifugal, pneumatic nozzles, or pressure nozzles.

Centrifugal Atomization. With centrifugal or spinning-disk atomization (Fig. 18), liquid feed is accelerated to a velocity in excess of 800 ft/s to produce fine droplets that mix with the drying air. Particle size is controlled mainly be liquid properties and wheel speed. There are no vibrations, little noise, and small risk of clogging. Furthermore, the system allows maximum flexibility in feed rate, provides capacities in excess of 200 tons/h and operates with low power consumption.

Steam Injection. To produce a product with significantly increased bulk density and fewer fines, the APV steam injection technique (Fig. 19) has been refined to a point that allows its use with centrifugal atomizers in large drying operations.

During centrifugal atomization, air around the rotating wheel may be entrapped within the atomized droplets.

When heat is transferred from the drying air into the feed droplet, water evaporates and diffuses out through the surface, at the same time creating a hard shell around the particle and some hollow spaces inside. If the droplet already contains some air bubbles, this incorporated air will expand to fill out the created vacuoles and the particle will not shrink much during the drying process, resulting in porous particles as shown in Figure 20 microphoto of spray-dried autolyzed yeast.

In the steam injection process the air atmosphere around the spinning disk is replaced with a steam atmosphere, thus reducing the amount of air incorporated in the droplets. This means that there is no air that will expand within the particle and the created vacuoles will collapse, thus shrinking the particle and resulting in the type of dense, void-free particles illustrated in the Figure 21 microphoto. Control of the amount of steam injected permits a precise adjustment in powder bulk density. Furthermore, reduction of air-exposed surfaces often reduces product oxidation and prolongs powder shelf life.

Figure 19. Steam is added around and into the atomizing disk to minimize air in the atomized liquid droplet.

Figure 18. Flat-bottomed drying chamber incorporates rotating powder discharger and can be equipped with an air broom.

Pressure Nozzles. With the pressure nozzle system (Fig. 22), liquid feed is atomized when forced under high pressure through a narrow orifice. This approach offers great versatility in the selection of the spray angle, direction of the spray, and positioning of the atomizer within the chamber. It also allows cocurrent, mixed-current, or countercur-rent drying, with the production of powders having par-

Figure 20. Spray-dried autolyzed yeast (2000 X magnification). Particle is hollow and filled with crevices.

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Figure 18. Flat-bottomed drying chamber incorporates rotating powder discharger and can be equipped with an air broom.

Figure 19. Steam is added around and into the atomizing disk to minimize air in the atomized liquid droplet.

Figure 21. Spray-dried autolyzed yeast with steam injection (2000 X magnification). Particle is solid and essentially void-free.

ticularly narrow particle size distribution and/or coarse characteristics. Since the particle size is dependent on the feed rate, dryers with pressure nozzles are somewhat limited relative to changing product characteristics and operating rates.

Pneumatic (Two-Fluid) Nozzles. This type of nozzle uses compressed air to accomplish the atomization of the feed product. The advantage with this type of atomizer is that it allows a greater flexibility in feed capacity than what can be obtained with pressure nozzles. However, particle size distribution is not as good as what can be obtained with the centrifugal or high-pressure nozzle-type atomizers, and because of the large quantities of compressed air required, operating costs tend to be quite high.

tide. During this operation, the temperature of the particles will not increase much owing to the cooling effect resulting from water evaporation.

The rate of drying will be very high at first but then declines as the moisture content of the particles decreases. When the powder reaches the bottom of the drying chamber, it has attained its final moisture and normally is picked up in a pneumatic cooling system to be cooled down to a suitable bagging or storage temperature. Powder that is carried with the air is separated in cyclone collectors. As the powder is cooled in the pneumatic system, it also is subjected to attrition. This results in a powder that is fine, dusting, and has relatively poor redissolving properties.

Single-stage dryers are made in different configurations. The basic ones are described below.

Single-Stage Drying

Defined as a process in which the product is dried to its final moisture content within the spray-drying chamber alone, the single-stage dryer design is well known throughout the industry, although some difference in airflow patterns and chamber design exist.

As illustrated, the drying air is drawn through filters, heated to the drying temperature, typically by means of a direct-combustion natural-gas heater, and enters the spray drying chamber through an air distributor located at the top of the chamber. The feed liquid enters the chamber through the atomizer, which disperses the liquid into a well-defined mist of very fine droplets. The drying air and droplets are very intimately mixed, causing a rapid evaporation of water. As this happens, the temperature of the air drops, as the heat is transferred to the droplets and used to supply the necessary heat of evaporation of the liquid. Each droplet thus is transformed into a powder par-

Box Dryers. A very common type of dryer in the past, the drying air enters from the side of a boxlike drying chamber. Atomization is from a large number of high-pressure nozzles also mounted in the side of the chamber. Dried powder will collect on the flat floor of the unit, from which it is removed by moving scrapers. The exhaust air is filtered through filter bags that also are mounted in the chamber. In some instances, cyclones will be substituted for the filter bags.

Tall-Form Dryers. These are very tall towers in which the airflow is parallel to the chamber walls. The atomization is by high-pressure nozzles. This type of dryer is used mainly for the production of straightforward, commodity-type dried powders.

Wide-Body Dryers. While the height requirement is less than for the tall-form type, the drying chamber is consid erably wider. The atomizing device can be either centrifugal or nozzle type. The product is sprayed as an umbrella-shaped cloud, and the airflow follows a spiral path inside the chamber. As shown in Figs. 18 and 22, this type of dryer is designed either with a flat or conical bottom.

Flat-Bottom Dryer. The flat-bottom dryer is not a new development, but the design has gained renewed interest in recent years.

The chamber has a nearly flat bottom from which the powder is removed continuously by means of a rotating pneumatic powder discharger. The dryer has the obvious advantage of considerably reduced installed height requirements compared to cone-bottom dryers, and it is also easy for operators to enter the chamber for cleaning and inspection. Another advantage is that the rotating powder discharger provides a positive powder removal from the chamber and a well-defined powder residence time. This has been shown to be important in the processing of heat-sensitive products such as enzymes and flavor compounds. Depending on the product involved, a pneumatic powder cooling system also may be installed.

Typically, this type of dryer is used for egg products, blood albumin, tanning agents, ice cream powder and toppings. The flat- and bottom chamber, incidentally, may be provided with an air broom, which is indicated by the colored section in Figure 18. By blowing tempered air onto the chamber walls while rotating, this device blows away loose powder deposits and cools the chamber walls to keep the temperature below the sticking point of certain products. Some items with which the air bottom technique has been successful are for fruit and vegetable pulp and juices, meat extracts, and blood.

Cone-Bottom Dryer. Figure 22, meanwhile, shows a conical-bottom chamber arrangement with a side air outlet, high-pressure nozzle atomization, and pneumatic product transport beneath the chamber. This single-stage dryer is very well suited to making relatively large particles of dairy products or proteins for cattle feed. If the product will not withstand pneumatic transport, it may be taken out unharmed directly from the chamber bottom.

moisture within the particles that can move by capillary action are extracted and (2) the falling-rate period when diffusion of water to the particle surface becomes the determining factor. Since the rate of diffusion decreases with the moisture content, the time required to remove the last few percent of moisture in the case of single-stage drying takes up the major part of the residence time within the dryer. The residence time of the powder thus is essentially the same as that of the air and is limited to between 15 and 30 s. As the rate of water removal is decreasing toward the end of the drying process, the outlet air temperature has to be kept fairly high in order to provide enough driving force to finish the drying process within the available air residence time in the chamber.

In multiple-stage drying, the residence time is increased by separating the powder from the main drying air and subjecting it to further drying under conditions where the powder residence time can be varied independently of the airflow. Technically, this is done either by suspension in a fluidized bed or by retention on a moving belt. Since a longer residence time can be allowed during the fallingrate period of the drying, it is possible and desirable to reduce the drying air outlet temperature. Enough time to complete the drying process can be made available under more lenient operating conditions.

The introduction of this concept has led to higher thermal efficiencies. Fifteen years ago, the typical inlet and outlet temperatures of a milk spray dryer were 360-205°F. Today, the inlet temperature is often above 430°F, with outlet temperatures down to about 185°F.

Two-Stage Drying. In a typical two stage drying process as shown in Figure 23, powder at approximately 7% moisture is discharged from the primary drying chamber to an APV fluid bed for final drying and cooling. The fact that the powder leaves the spray dryer zone at a relatively high moisture content means that either the outlet air tem

Multiple-Stage Dryers

The best way to reduce energy usage in spray drying is, of course, to try and reduce the specific energy consumption of the process. With advances in atomizer and air distributor designs it has been possible with many products to operate with higher dryer inlet temperature and lower outlet temperature. While this procedure substantially cuts energy needs and does not harm most heat-sensitive products, care must be taken and proper balance struck. The nature of the product usually defines the upper limit, ie, the denaturation of milk protein or discoloring of other products. A higher inlet temperature requires close control of the airflow in the spray-drying chamber, and particularly around the atomizer. Furthermore, it must be noted that a lower outlet temperature will increase the humidity of the powder.

Generally speaking, the drying process can be divided into two phases: (1) the constant-rate drying period when drying proceeds quickly and when surface moisture and Figure 23. Two-stage drying with external fluid bed.

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Integrated fluid bed

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Air out perature can be lowered or the inlet air temperature increased. Compared to single-stage drying, this will result in better thermal efficiency and higher capacity from the same size drying chamber. As the product is protected by its surrounding moisture in the spray-drying phase, there normally are no adverse effects on product quality resulting from the higher inlet air temperature.

The outlet air from the chamber leaves through a side air outlet duct, and the powder is discharged at the bottom of the chamber into the fluid bed. This prolongs the drying time from about 22 s in a typical single-stage dryer to more than 10 min, thus allowing for low-temperature use in the fluid bed.

In the development of this type of drying system, an initial difficulty was to provide a means to reliably fluidize the semimoist powders. This stems from the fact that milk powder products, and especially whey-based ones, show thermoplastic behavior. This makes them difficult to fluidize when warm and having high moisture content, a problem that was largely overcome by the development of a new type of vibrating fluid bed.

The vibrating fluid bed has a well-defined powder flow and typically is equipped with different air supply sections, each allowing a different temperature level for a optimum temperature profile. The last section normally is where the product is cooled to bagging and storage temperature.

While the specific energy consumption in the fluid-bed process may be relatively high, the evaporation is minor compared with the spray-drying process and the total energy use therefore is 15-20% lower.

Figure 24 shows the specific energy costs as a function of the water evaporated in the fluid bed or the residual moisture in the powder from the spray drying to the fluid-bed process. The curves are only shown qualitatively because the absolute values depend on energy prices, inlet temperatures, required residual moisture content, and other such parameters. The total cost curve shows a minimum that defines the quantity of water to be evaporated in the fluid bed to minimize the energy costs.

The introduction of two-stage drying also created the potential for a general improvement in powder quality, especially as far as dissolving properties are concerned. The slower and more gentle the drying process produces more solid particles of improved density and solubility and opens up the possibility of producing agglomerated powders in a straight-through process. If desired, the fluid bed can be designed for rewet instantizing or powder agglomeration and for the addition of a surface-active agent such as lecithin.

Three-Stage Drying. This advanced drying concept basically is an extension of two-stage drying in which the second drying stage is integrated into the spray-drying chamber with final drying conducted in an external third stage (Fig. 25). The design allows a higher moisture content from the spray drying zone than is possible in a two-stage unit and results in an even lower outlet air temperature. An added advantage is that it improves the drying conditions for several difficult products. This is accomplished by spray drying the powder to high moisture content but at the same time avoiding any contact with metal

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