Spray Dryers

Fundamentally, the spray-drying process is a simple one. However, the design of an efficient spray-drying plant requires considerable expertise along with access to large-scale test facilities, particularly where particle size and bulk density requirements in the dried product are critical. The sizing of spray dryers on a purely thermal basis is a comparatively simple matter since the evaporation is entirely a function of the At across the dryer. Tests on pilot-scale equipment are not sufficient in the face of such imponderables as possible wall buildup, bulk density, and particle size predictions. Atomization of the feed is of prime importance to efficient drying and three basic feed devices are used extensively:

1. Single-fluid nozzle or pressure type

2. Two-fluid nozzle or pneumatic type

3. Centrifugal (spinning disk)

The single-fluid nozzle produces a narrow spray of fine particles. While a multiplicity of nozzles of this type are used in tonnage plants to obtain the desired feedrate, because of the high pressures employed (up to 7000 psig), excessive wear can result, particularly with abrasive products, as an alternative, the two-fluid nozzle with external mixing is used for a variety of abrasive materials. This system generally is limited to small-capacity installations. Normally, the feed is pumped at about 25 psig merely to induce mobility while the secondary fluid is introduced at 50-100 psig, thus producing the required atomization.

Centrifugal atomization achieves dispersion by centrifugal force; the feed liquor is pumped to a spinning disk. This system is suitable for and generally used on large productions. When stacked or multiple disks are employed, feed rates of 40,000-60,000 lb/h are not uncommon.

Many spray dryer configurations are in current use along with a variety of airflow patterns. The nature of the chamber geometry selected is strictly related to the system

roller feed

Discharge

Figure 5. Feeding arrangements for drum dryers.

Discharge

Discharge

Flaking and chilling machine with deep feed

Dryer with top roller feed

Flaking and chilling machine with deep feed

Double drum dryer with center feed

Discharge Side trough feed

Discharge

Discharge Side trough feed

Discharge

Dryer plain dip feed

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