Spray Drying Parameters

Spray drying is one of the most widely used technologies in the chemical process industries (CPI) for producing dry powder products, wherein a liquid or slurry is atomized into hot gas. In spray drying, a broad range of pumpable solutions, suspensions and emulsions can be used as feed. Spray drying enables the production of powders with precisely defined characteristics. By controlling process variables including liquid feed, atomization method, and the properties of the dryer and other system equipment, the shape, flow properties and porosity of the solid particles produced can be controlled. Below are fundamental considerations relating to spray drying.

Atomization

Various methods exist for atomizing the liquid feed into a spray drying system (Figure 1). In many spray equipment, liquid is separated from the spray head as a thin liquid film. The film separated from the spray head breaks into droplets, which immediately assume droplet form due to the surface tension of the liquid. For this reason, droplet formation depends predominantly on the rheological properties of the liquid and its interaction with the heated drying air located immediately outside the spray device.

Rotary Centrifugal (rotary) atomization is the most common method used in spray drying; a rotating disk

or wheel converts the liquid flow into droplets. A liquid mist is produced laterally from the spray disk. Centrifugal atomizers rotate in the range of 5,000 to 25,000 rpm. The sizes of the resulting droplets are approximately inversely proportional to the peripheral speed of wheels or disks having diameters in the range of 5 to 50 cm. Through the use of variable-speed drives, droplet size can be controlled directly. The smallest rotary atomizers enable processing of liquid feed with capacity of 1-10 kg/hour in the laboratory, while large commercial units operated with 1,000 kW motors can achieve well over 200 tons/hour. Nozzle In atomization performed with a pressure nozzle, pressure is applied to the liquid using a pump and the liquid is forced toward an orifice or nozzle. A typical orifice size ranges from 0.5 to 3.0 mm, and the applied pressure, viscosity and solid content limit the nozzle capacity to 750-1,000 kg/hour of liquid feed. Larger pressure drops across the orifice produce smaller droplets, and to reduce particle size for a given feed rate while maintaining the same mass flow, a smaller orifice and higher pump pressure must be applied. Despite its simplicity, maintenance of the pressure nozzle is quite difficult, particularly in multi-nozzle systems. The most important factor creating this difficulty is erosion and clogging of the nozzle inlet, which can alter the nozzle characteristics. Two-Fluid Pneumatic In this method, atomization is achieved by the feed interacting with a second fluid (usually compressed air) through a two-fluid nozzle. Particle size is controlled by adjusting the ratio of compressed air to the feed. Two-fluid pneumatic spraying is commonly used in smaller drying systems. Sonic Atomization Can be used in applications requiring fine droplets at low flow rates. In this technique, the feed liquid is passed through a surface vibrating at ultrasonic frequencies. It is used in small-capacity dryers primarily where a narrow particle size distribution is required.

Dryer Configuration

To enable liquid evaporation, the flow profiles of the droplets and the gas passing through the dryer must remain in contact for sufficient time. For this reason, the size and geometry of the spray drying chamber and gas distributor become important variables. In many spray drying systems, the atomizer is positioned at the top of a drying chamber with large diameter and heated gas is introduced around the atomizer through an air or gas distributor mounted at the top. In this manner, co-current flow of gas, droplets and particles is achieved. The chamber height must be designed to allow particles sufficient time to dry. Larger particle sizes require drying chambers with greater diameters. Residence time should be selected based on the known drying characteristics of the product and the desired particle size. In this way, the drying chamber volume can be calculated directly. In another configuration, a pressure nozzle is positioned at the bottom of the drying chamber, thus performing the spray operation from bottom to top. This configuration can be used when the desired product is coarse powder and production rate is lower.

Collection of Dried Solids

Coarse powders are collected easily from the bottom of the drying chamber. For collection of fine powders, cyclones or bag filters are used. Particles must be separated from the drying air, which is cooler (due to evaporation) and more humid than before drying. Gas Flow Heating of the drying gas used in spray dryers can be accomplished by direct combustion of natural gas, indirectly using shell-and-tube type heat exchangers or electric heaters (in small spray dryers). Many gas distributors are configured with the aid of computational fluid dynamics (CFD) analysis to define air flow profiles and heat distribution in the drying chamber. In many applications, gas distributors feature adjustable guide valves that permit fine-tuning. Industrial radial fans are used to move gas throughout the system. System components can be sized according to gas flow.

Evaporation Rate

Evaporation rate is directly proportional to the product of the mass flow and the temperature difference between inlet and outlet to the spray dryer. Exit temperature values are determined by experimental methods since they depend on the isothermal equilibrium of the material and true equilibrium is never achieved. Inlet temperature values are likewise determined by experimental methods and should be at the highest level possible without risking product degradation. Safety In spray drying operations, safety procedures associated with dust explosions, including pressure rise rate (Kst), maximum dust explosion pressure (Pmax), minimum ignition energy (MIE), minimum ignition temperature (MIT) and minimum auto-ignition temperature (MAIT), must be carefully addressed. İlker Damar / Sales Manager Industrial Applications - GEA Jens Thousig Møller / Process Design Manager - GEA Søren Fredsted / Senior Process Technology Specialist - GEA