Water Inlet

Figure 30. Thermoplate for air-to-liquid plate recuperator.

erally is recycled to preheat the drying air but can be used for other purposes such as heating water, clean-in-place (CIP) liquids, or buildings.

Other Heat-Recovery Methods. While many spray-drying plants are equipped with bag filters to minimize emissions to the environment and recover valuable powder, the filtration system can be coupled with a finned-tube recuperator for heat-recovery purposes. This type of recuperator is particularly effective when the air is not dust-loaded. It is compact, very flexible and normally, very inexpensive.

Some degree of heat recovery can also be accomplished by using available low-temperature waste streams to preheat the drying air through a finned-tube heat exchanger. Examples of such sources of waste heat include evaporator condensate and scrubber liquids.


Most powderlike products produced by spray drying or grinding are dusting, exhibit poor flow characteristics, and are difficult to rehydrate.

It is well known, however, that agglomeration in most instances will improve the redispersion characteristics of a powder. Added benefits of the agglomerated powder is that it exhibits improved flowability and is nondusting. All of these are characteristics for which the demand has increased in recent years.

Depending on the application or the area of industry where the process is being used, the process sometimes is also referred to as granulation or instantizing.

Instant Powders

Powders with particle size less than about 100 ¡urn typically tend to form lumps when mixed with water and require strong mechanical stirring in order to become homogeneously dispersed or dissolved in the liquid. What is happening is that as water aided by capillary forces penetrates into the narrow spaces between the particles, the powder will start to dissolve. As it does, it will form a thick, gellike mass that resists further penetration of water. Thus, lumps will be formed that contain dry powder in the middle and, if enough air is locked into them, will float on the surface of the liquid and resist further dispersion.

In order to produce a more readily dispersible product, the specific surface of the powder has to be reduced and the liquid needs to penetrate more evenly around the particles. In an agglomerated powder with its open structure, the large passages between the individual powder particles will assist in quickly displacing the air and allow liquid to penetrate before an impenetrable gel layer is formed. The powder thus can disperse into the bulk of the liquid, after which the final dissolution can take place.

Although there always is some degree of overlap between them, the reconstitution of an agglomerated product can be considered as consisting of the following steps:

1. Granular particles are wetted as they touch the water surface.

2. Water penetrates into the pores of the granule structure.

3. The wetted particles sink into the water.

4. The granules disintegrate into their original smallest particles, which disperse in the water.

5. The small dispersed particles dissolve in the water.

It is important to realize that it is the total time required for all these steps that should be the criterion in evaluating a product's instant properties. It is not unusual to see products characterized only on their wettability. This neglects the importance of the dispersion and disso

Figure 31. Straight-through agglomeration.

lution steps, the time for which may vary considerably with different agglomeration methods.

For powders that are produced by spray drying, there are a number of ways in which the agglomeration can be accomplished in the spray dryer itself. This often is referred to as the "straight-through" process and is illustrated in Figure 31. Note that fines powder from the cyclone is conveyed up to the atomizer, where it is introduced into the wet zone surrounding the spray cloud. Cluster formation will occur between the semimoist freshly produced particles and the recycled fines. The agglomerated product then is removed from the bottom of the drying chamber, cooled, and packaged. This method produces a degree of agglomeration that is sufficient for many applications.

An alternative approach to agglomeration is referred to as the "rewet method." This is characterized by processing an already existing fine dry powder into an agglomerate using fluidized-bed technology.

The Agglomeration Mechanism

Two particles can be made to agglomerate if they are brought into contact and at least one of them has a sticky surface. This condition can be obtained by one or a combination of the following means:

1. Droplet humidification whereby the surface of the particles is uniformly wetted by the application of a finely dispersed liquid.

2. Steam humidification whereby saturated steam injected into the powder causes condensation on the particles.

3. Heating—for the thermoplastic materials.

4. Addition of binder media, ie, a solution that can serve as an adhesive between the particles.

The steam condensation method usually cannot provide enough wetting without adversely heating the material and is used less frequently on newer systems.

After having been brought into a sticky state, the particles are contacted under such conditions that a suitable, stable agglomerate structure can be formed. The success of this formation will depend on such physical properties as product solubility and surface tension as well as on the conditions that can be generated in the process equipment.

For most products, possible combinations of moisture and temperature can be established as shown in Figure 32. Usually, the window for operation is further narrowed down by the specifications for product characteristics. Once the agglomerate structure is created, the added mois-

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