Spray Drying
Spray drying is the method whereby the solids in a solution or slurry are rapidly dried to final particulate form by atomizing the liquid into a heated chamber. If the droplets are small enough and the chamber large enough, they dry before they reach the wall of the chamber. The resulting product is collected as a free-flowing material.
In addition to drying solids from slurries and solutions, spray drying has other more specialized uses. It can be used to encapsulate materials in a matrix, such as oils in starch or any other water-soluble polymer. This provides some protection to the material that is encapsulated. Also, small particles that comprise a dusty powder can be suspended in a liquid and spray dried into larger aggregates that are easier to handle. AVEKA also has experience applying a surface treatment to particles by suspending the particles in a solution containing the surface treatment and then spray drying the particles.
Materials that have been spray dried at AVEKA include ceramics, polymers, surfactants, inorganic salts, pigments, microencapsulated particles, food products such as milk, whey, and corn steep water, inorganic sol gels, and mixtures of the above materials. Almost all of the spray drying is done from aqueous solutions; however AVEKA Inc has the ability to dry materials from solvents in some of its dryers.
AVEKA has experience in spray drying high-value specialty materials that need to be kept free of contaminants and that have rigorous size specifications, down to a d90 less than 10 microns.
Methods
In a typical drying operation, the solution or dispersion is pumped into the chamber through an atomizer or series of atomizers. The simplest type is a single fluid atomizer in which the solution or slurry to be dried is pumped at high pressures (1,000-5,000 psi) through small nozzles. A more complex atomizer is a spinning disk system in which the solution or slurry to be dried is carefully fed onto a disk that is spinning at 10,000-30,000 rpm. The high speed rotation causes the liquid to thin and form droplets at the edge of the disk. A third commonly used nozzle, and the one used the most at AVEKA, is a two-fluid nozzle. In this case, there are two closely spaced nozzles. One nozzle is used to deliver the solution or slurry and the second nozzle directs high pressure air onto the liquid stream causing it to atomize. In all cases, the atomized liquid is then dried by the hot air flowing through the drying chamber.
For rapid drying, a high inlet temperature is desired, although thermal stability of the product or limitations on the air heaters supplying the hot air limits the upper end of inlet temperatures. For most drying, the outlet temperature provides a measure of the final product temperature and an indication of the utilization of the energy in the inlet air for drying. Clearly, for the most economical drying, a low outlet temperature would be desired. However, in many instances, a low outlet temperature leaves the product with a moisture content that is too high. Typical inlet and outlet temperatures are 220-225 C and 70-90 C, respectively.
Finally, in all dryers there is some method for removing the product from the chamber after drying. In tower dryers, like the ones at AVEKA Inc, the chamber is equipped with a cone bottom. Gravity and air flow from the drying air is used to sweep the product from the chamber to a cyclone or baghouse collector. In the box type dryers at AVEKA Manufacturing and Cresco Food Technologies, the product falls to the floor of the chamber and is scraped to a discharge point by a reciprocating unloader.
Most of the spray drying at AVEKA is done from aqueous solutions; however, AVEKA Inc has the cabapility for drying materials out of solvents. The appropriate safety precautions are taken, including running the material at a lowfeed rate that keeps the concentration of the solvent well below its lower explosion limit.
Equipment
AVEKA Inc, the Woodbury Minnesota R&D and small to medium scale production facility, is equipped with a number of spray dryers that have 0.5 to 200 gallons of drying capacity per hour. The batch size for the smallest dryers is typically 1-5 gallons of slurry, scaling up in the other dryers to about 2500 gallons of slurry. These dryers typically produce particles with sizes ranging from 5-50 microns. One of the small dryers is located in the food processing suite and can produce food-grade samples for customers.
AVEKA Manufacturing, the large-scale manufacturing facility in Fredericksburg Iowa, is equipped with a Rogers Box Dryer with 3,200 pounds/hour of water drying capacity. Typical particle sizes range from 75-100 microns.
Cresco Food Technologies also has a Rogers Box Dryer for food processing that dries 4,500 pounds of water/hour. This system is ideal for drying food-grade materials including maltodextrin, yeast, soy milk, and food chemicals.
Characterization
Horiba LA 930 and LB 500, for measuring particle size distributions