Fresh Options in Drying

Continuous gas-suspension dryers offer a variety of choices in removing moisture from particles and loose aggregates

Selection of the right drying system requires a thorough knowledge of the properties of the feed material as well as the desired characteristics of the final product. The feed material can vary from a dilute solution of solids in water to heavy cakes of wet solids containing only a small amount of moisture. In between are liquid slurries of suspended solids, colloidal suspensions, gels and pastes.

Often, drying systems are required to preserve product quality while controlling the moisture content and avoiding thermal or mechanical degradation. The dried product may also have to be produced at a specified particle size distribution.

Widely employed in the chemical process industries (CPI) for removing moisture from particulate solids and various aggregate materials, gas-sus-pension drying makes up a broad class of technologies. These include spray, flash and fluid-bed drying as well as hybrid systems. Drying takes place as the dispersed feed is kept suspended in a stream of hot gas, usually air.

Getting through the option maze

Many factors go into the selection of a gas-suspension drying system. The selection process is best illustrated by a decision tree (Figure 1).

Figure 1 decision tree for gas-suspension dryer selection

Generally, a solution or slurry is best handled by spray drying. Otherwise it is necessary to backmix the dry product with the feed to produce a cake-like material that can be fed to a flash or fiuid-bed drying system. Filter cakes and centrifuge cakes, if suitably dry and friable, can be fed directly to flash or fluid-bed dryers. Various means are used in these systems to disperse or distribute the wet materials into the gas stream.

In order to assure good fluidization in a fluid-bed drying system, the material being dried must have sufficiently large (> 50 μm) mean particle size. Additionally, the particle size distribution must be rather uniform. Also, irregularly shaped particles, such as needles and platelets, present difficulties in fluidization during drying.

It is possible, however, to alter the feed characteristics to force another selection. Examples of such feed conditioning are the addition of dispersants, wet milling, and backmixing of dried solids. Each of these feed-conditioning processes can sufficiently modify the characteristics of the feed so as to alter the equipment choice.

Moreover, the non-Newtonian behavior of pseudoplastic and thixotropic materials often influences the selection of a system. These materials reduce in viscosity with the application of shear force. As a result, they may discharge from a vacuum filter as a dry, crumbly cake, but during handling, revert to a liquid form.

These non-Newtonian fluids can present problems in feeding to flash or fluid-bed dryers. However, if enough shear can be applied to make the cake pumpable and thus transport it to an atomization system, the cake can usually be spray-dried.

Nevertheless, one must keep in mind that some materials are inherently not suitable for gas-suspension drying. These include stringy, fibrous or particularly sticky materials, and large (> 10 mm) particles, such as chopped vegetables and briquettes. Tray, rotary or conveyor dryers are usually recommended for such materials.

Powder characteristics

Flash and fluid-bed drying systems have little or no influence on the dried powder characteristics. On the other hand, spray drying presents a number of possibilities in creating custom-designed products.

The mean particle size of the dried powder can be controlled and often the bulk density can be influenced by adjustment of the dryer operating parameters. Because of spray drying's ability to produce specific powder properties, it is often desirable to find a method by which an otherwise unsuitable material can be conditioned into a spray-dryer feed.

The control of particle size can be important for several reasons. For example, in the manufacture of catalysts used in gas-phase reactions, the particle size of the catalyst is critical to its distribution in the gas stream. Other properties, such as pore volume and surface area, can be affected by the drying process, which in turn affects the catalyst's activity.

Very fine particles are required for inhalation therapeutics [1] and slightly larger particles are desired for plasma-spray coatings in the ceramics and metals industries [2]. Very large particles are required for dyestuffs and pesti-cides to eliminate dustiness and provide good flowability and redispersion. Flowability is critical to ceramics manufacturers in products ranging from hi-tech electronics to wall tile [3].

Basics of spray drying »