Spray Drying Process Stages

Atomization is the process by which a liquid is disintegrated into many fine droplets. The formation of a spray with high surface/mass ratio is highly critical for optimum liquid evaporation conditions and, consequently, the desired properties of the resulting product. Although ideally the sizes of all droplets should be the same, in practical terms formation of droplets with a narrow size distribution would be satisfactory. 

Formation of the atomized spray requires application of a force. The commercially available systems employ one of the following in order to create an atomized spray centrifugal energy, pressure energy, kinetic energy or sonic energy, and vibrations. 

Rotary atomizers normally operate in the range of 5000-25,000 rpm with wheel diameter of 5 50 cm. The mean size of the droplet produced is inversely proportional 

Flow Dryer I Flow Dryer I Flow Dryer I 

I I Conical Flat I Flat I I Conical Flat I Flat 

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Dry Product Recovery Product recovery is the last stage of the spray drying process. Two distinct systems are used ... 

FIG. 21-174 Typical stages of a spray-drying process atomization, spray-air contact/evaporation, and product collection. [Master, Spray Drying Handbook, 5th ed., Longman Scientipc Technical, 1991. With permission.)... 

FIGURE 23.23 A two-stage spray drying process for milk (1) air filter (2) heater (3) cooler (4) spray dryer (5) cyclone (6) exhaust fan (7) fluidized bed dryer (8) return line of fine powder. 

FIGURE 32.5 The main stages involved in the spray-drying process 1, nehulization of feed liqnid into small droplets (spray formation) 2, spray-air contact (mixing and flow), which can he made hy several modes cocurrent (the liquid is nebulized in the same direction as the airflow, as illustrated), countercurrent (liqnid droplets and hot air flowing in opposite directions), or mixed flow (the liqnid is sprayed upward and only remains in the hot zone for a short time) 3, droplet evaporation and 4, separation of dried product from the air. 

FIGURE 39,7 Spray-drying process scheme (one stage). Hot air drying/drops cocurrent = ao high T for dry product. Residence time 10 s, continuous process). (Adapted from Pisecky, J., Handbook of Milk Powder Manufacture, Niro A/S/, Copenhagen, Denmark, 1997, 261p.)... 

A variation of the spray drying process, known as spray pyrolysis, uses a higher temperature and a reactive (often an oxidizing) environment in the chamber. This allows the salts to be dried and decomposed directly. Figure 20.4 shows the stages in the spray pyrolysis process. In addition to producing powders this technique has been used to produce thin films and fibers. 

Each stage is carried out according to the dryer design and operation and, together with the physical and chemical properties of the feed, determines the characteristics of the final product. A typical example of a spray drying process with the most important ancillary equipment included is shown in Figure 9.1. 

In the food industry, because the spray dryer is commonly available, economical, fast, and produces good-quality material [16], it becomes the most common means of encapsulation. The encapsulation process is simple and similar to the one-stage spray drying process. The coated material is called the active or core material, and the coating material is called the shell, wall material, carrier, or encapsulant [43]. The active material to be encapsulated, such as an oil or fia-vor in an oil base, is dispersed in a hydrocolloid carrier, e.g., gelatin, modified starch, dextrin or maltodextrin, or gum arabic. After the emulsifier is added, the mixture must be... 

The spray-drying process is carried out in three fundamental stages as shown schematically in Figure 3. The first stage is atomization of a liquid feed into fine... 

Figure 3 Schematic of spray-drying process shown in stages stage I atomization stage II spray-air contact and evaporation stage III product separation. (From Ref. 5.)...

Manufacturing Processing and Uses. In commercial production, aqueous urea solution is mixed with acetaldehyde in 1 1 molar ratios. An acid catalyst is introduced into the reaction mixture which is staged at various process temperatures. After neutralization with a base, the CDU is separated from the mother hquor by filtration or spray drying. 

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