Spray drying is one of the oldest forms of drying -- and one of the few technologies available for converting a liquid, slurry or low viscosity paste to a dry solid (free-flowing powder) in one unit operation. Spray dryers are found in almost every industry, including pharmaceuticals and detergents, paint and pigments, food and dairy, and mining and minerals. They can dry at rates from several pounds to several tons per hour but are expensive to operate at the higher rates.
Spray dryers, or towers, as they are sometimes called, atomize or spray the feed material into the drying chamber in fine droplets. They are continuous processing machines that come in a range of configurations. Traditional vertical configurations are broadly grouped into two categories depending on the method of introducing the feed. The first of these, tall-form dryers, use nozzles to atomize the feed. They are so termed due to the relatively narrow spray angle of the nozzle and the relative velocity of the droplets, requiring a tall drying chamber with a proportionally small diameter to provide sufficient residence time to achieve drying.
The second group, which are shorter and fatter, use rotary disk atomizers (also referred to as centrifugal atomizers) to generate the spray. The angle of the rotary atomizer spray is flat with a wide spray pattern, requiring a large diameter tower that is relatively short. Other spray dryer configurations include box spray dryers, which rely on the same operating principles but extract the product from the bottom of the dryer housing by means of a conveyor, and pulse-combustion spray dryers.
Of all drying systems, spray drying is perhaps the simplest in principle but remains one of the most difficult to achieve the desired product characteristics. The theoretical design of the system is simple compared to many other dryers, but the mechanical designs are art.
Principle of OperationIn the pulse-combustion dryer, the burner is used to both atomize the product and introduce the carrier into the system.
The feed liquid is pumped into the atomization system that, by virtue of its technology, either will immediately atomize the feed to a spray of droplets (pressure nozzles) or feed the rotary disk that will atomize the product (rotary atomizer). Pressure nozzles rely on the pump supply pressure (or fluid such as compressed air in a two-fluid nozzle) to effect the atomization. Therefore, in tall-form dryers, high-pressure pumps or high pressure fluids usually are required for operation. A low-pressure pump is required for rotary atomizers to overcome the head associated with the change in elevation required to supply the feed from the ground to the top of the tower. The disk of the atomizer is driven at a high rate of revolution by an electric motor. The feed is metered onto the disk that creates a spray of material into the drying chamber by centrifugal forces.
The technology behind atomization, be it from pressure nozzles or rotary atomizers, is a science in itself. Without going into great detail, I can tell you that by adjusting parameters and components of the atomizer in conjunction with dryer configuration, you can effectively produce a final product that will meet specific product characteristics such as rehydration, moisture content, particle size and bulk density. Atomizer selection is based on the requirements of the final product, and each type of atomizer offers unique advantages outside of a common broad regime.
All atomizers (pressure or rotary) create a fine spray of feed into the tower’s drying chamber. In so doing, the surface area of the feed has been increased dramatically to allow for intimate contact between the carrier and product.
Spray dryers can be co-current, counter-current or fountain flow, depending on the material’s sensitivity to temperature, desired dry product characteristics and selection of the system. They may be direct or indirect and can use a variety of heat sources. In co-current systems, the hot gas (carrier) is introduced with the feed at the top of the tower and extracted at the discharge cone through an extraction duct. With counter-current systems, the carrier is introduced by means of a bustle above the dryer cone and exhausted from the top of the tower. With fountain flow, the product is sprayed vertically upward and changes direction within the drying chamber.
Pulse drying first was introduced as a commercially available method of drying in the early ‘80s. This method of spray drying uses a pulse combustion burner to both atomize the product and introduce the carrier into the system. This design obviates the requirement for both insulated hot gas ducting and high-pressure pumps and nozzles or rotary disk atomizers. The principle of atomization relies on the burner producing a high frequency combustion/detonation within the burner. The so-formed gases are channeled into a resonating chamber, where the frequency of the wave so formed is amplified. This wave atomizes the feed, which is metered in at low pressure, and entrains the particles in the hot gases, which are diluted by a secondary process air makeup within the burner/nozzle assembly to the required process temperature. The remainder of the dryer is identical to a tall-form cocurrent spray dryer.
Evaporation of moisture from the droplets and formation of dried particles proceed under controlled temperature and airflow conditions within the tower, and the dried product is discharged from the tower cone. The dryers have relatively short residence times. Varying the gas velocity within the dryer can alter the residence time inside the dryer.
Spray dryers typically employ induced-draft fans to extract the moisture-laden air from the system. In some instances, special fans are installed to achieve dryer-specific operations such as air sweeps.
Due to the fine product that is produced on spray dryers, they inherently require dedicated dust-collection systems such as cyclones, bag houses, scrubbers and electrostatic precipitators.
Spray dryers are controlled by programmable logic controllers (PLCs) or solid-state controllers. In spray drying systems, the exhaust air temperature or humidity provides an input signal that, by way of a setpoint, will modulate the energy supplied to the process. Mechanically, these dryers are relatively low maintenance units. They can be fabricated from materials ranging from basic carbon steel to sophisticated duplex stainless steels. These dryers must be fully insulated to allow energy-efficient operation. Tall-from dryers have a pump and exhaust fan that require differing amounts of maintenance depending on the service, environment and abrasion characteristics of the product. Likewise, nozzles may wear -- specifically, the orifice plates -- and may require frequent replacement due to the wear adversely affecting the spray pattern. Dryers using rotary atomizers can become somewhat of a maintenance challenge, having to lift relatively large motors and gearboxes to the top of the tower for replacement. Facilities to assist in the maintenance and replacement of rotary atomizers can be designed into the system.
Spray dryers do have limitations. They are extremely energy intensive and have a correspondingly high operating cost. This is due to the fact that considerably more moisture is being thermally evaporated from the feed than in most other types of dryers. It is more expensive to thermally evaporate moisture than mechanically dewatering. Many spray dryers have problems relating to product buildup on the dryer walls. In some instances, this buildup is so significant it adds additional load to the tower, stressing the structure.
Spray dryers have a unique position in the arena of thermal drying. There is no other high volume method of producing a free-flowing powder from a liquid in one step. They offer unmatched versatility in the production of powders and can control the powder characteristics to a specified requirement.