Spray drying is the most widely used industrial process involving particle formation and drying. It is highly suited for the continuous production of dry solids in either powder, granulate or agglomerate form from liquid feedstocks as solutions, emulsions and pumpable suspensions. Spray drying is an ideal process where the end-product must comply with precise quality standards regarding particle size distribution, residual moisture content, bulk density and particle shape.
Spray drying involves atomizing a liquid feedstock into a spray of droplets and contacting the droplets with hot air in a drying chamber. The sprays are produced by either rotary (wheel) or nozzle atomizers. Evaporation of moisture from the droplets and the formation of dry particles proceed under controlled temperature and airflow conditions. Powder is discharged continuously from the drying chamber. Operating conditions and dryer design are selected according to the drying characteristics of the product and powder specification.
Every spray dryer consists of a feed pump, atomizer, air heater, air disperser, drying chamber and systems for exhaust air cleaning and powder recovery. The drying characteristics and quality requirements determine the selection of the atomizer, the most suitable airflow pattern and the drying chamber design.
With co-current systems, the drying air and particles move through the drying chamber in the same direction. Product temperatures on discharge from the dryer are lower than the exhaust air temperature, making this a suitable mode for drying heat-sensitive products. When operating with rotary atomizer, the air disperser creates a high degree of air rotation, giving uniform temperatures throughout the drying chamber.
With counter-current systems, the drying air and particles move through the drying chamber in opposite directions. This mode is suitable for products that require a degree of heat treatment during drying. The temperature of the powder leaving the dryer is usually higher than the exhaust air temperature.
Finally, with mixed flow systems, the particle movement through the drying chamber experiences both co-current and counter-current phases. This mode is suitable for heat-stable products where coarse powder requirements necessitate the use of nozzle atomizers, spraying up into an incoming airflow, or for heat-sensitive products where the atomizer sprays droplets down toward an integrated fluid bed.
System Design. Open systems have once-through airflow with exhaust to atmosphere. Most industrial spray drying systems handle aqueous feedstocks and use this system. Both direct and indirect air heating are applicable. Exhaust air cleaning is accomplished with cyclones, bag filters, electrostatic precipitators, and scrubbers.
Closed systems dry in an inert gas atmosphere where nitrogen recycles within the dryer. This system must be used for spray drying of feedstocks containing organic solvents or where the product must not contact oxygen during drying. Closed cycle plants are gas- and powder-tight and are designed to the strictest safety standards. The inflammable solvent vapors are fully recovered in liquid form.
Semi-closed systems have either a partial-recycle mode (recycling up to 60 percent of the exhaust air as inlet air to the dryer for effective waste heat utilization) or a self-inertizing mode, where direct air heating and a minimal air bleed create the low oxygen atmosphere necessary for drying aqueous feedstocks that form explosive powder-air mixtures.
Fore more information on spray drying systems from Niro Inc., Columbia, Md., call (410) 997-8700 or visit www.niroinc.com.