The Atomizer Effect
A typical laboratory spray drying system starts with a heated air source that is introduced into a drying chamber along with a liquid feed. An atomizer is used to break the liquid feed into a spray or mist so that the hot air can evaporate the liquid, leaving the feed in a dry particulate form (see Figure 1). The dried particles and moisture-laden air pass from the drying chamber into the powder separator, which is usually a cyclone, baghouse or a combination of the two. The dried particles are discharged from the bottom of the cyclone, while the air is pulled out of the top and is released to the atmosphere or sent to a baghouse separator for further cleaning.
These basic systems vary depending on whether rotary, mixed flow nozzle or co-current nozzle atomization is used. The atomizer type determines the drying chamber proportions and ultimately affects the resulting powder properties.
Rotary AtomizersA rotary atomizer centrifugally accelerates the liquid feed to a high velocity (see Figure 2). The feed extends over the rotating wheel surface as a thin film and is sheared away horizontally into discrete droplets.
The system uses a short chamber with a large diameter, which allows the particles to dry in a horizontal direction before hitting the walls. When different particle sizes are needed, the diameter of the chamber becomes the limiting factor. However, the particle size can be varied by changing the speed of the atomizer as related to the peripheral velocity of the wheel. A larger-diameter wheel running at a fixed speed will produce finer particles, while a smaller-diameter wheel running at the same fixed speed will produce larger particles.
Another way to change the particle size in a rotary atomizer is to change the wheel design. The most common wheel types are high-vane, low-vane, abrasion-resistant and flat disc. While high- and low-vane wheels are similar in design, the high-vane wheel has a greater surface area for the liquid feed to spread out on, so it forms a thinner film and a finer particle size. In a lab-size spray dryer, a low-vane wheel will produce powders in the range of 20-40 microns, while a high-vane wheel in the same dryer, running under the same conditions, will likely produce powders in the range of 10-30 microns.
The abrasion-resistant wheel was designed to have replaceable wear parts and contains fewer openings for the liquid feed to leave the wheel. This results in a greater amount of feed being forced out of each hole, thus creating a larger particle (approximately 30-50 microns).
The flat disc wheel produces the largest particle size and widest distribution of the four designs because of its low peripheral velocity. As the feed is dropped onto the wheel, it has a tendency to slip on the wheel's surface, which limits the amount of particle reduction that can be achieved. The amount of slippage, and therefore the resulting particle size, varies depending on the material being dried.
Yet another way to change particle size, which applies to all types of atomization, is to change the solids content of the liquid feed. For example, a liquid feed with a high solids content will put more material into the dryer at one time, resulting in greater feed interaction and larger particles. However, this is probably the least desirable method because it requires a change in the feed formula.
Generally, laboratory or pilot spray dryers with rotary atomization produce fine, uniform, discrete, spherical particles in the 10- to 40-micron range.
Mixed-Flow AtomizersWhen a larger or coarser particle is desired, mixed-flow two-fluid (pneumatic) nozzle atomization is typically used (see Figure 3). A two-fluid nozzle uses air to break up the liquid feed into droplets. The air is rotated within the nozzle at a high velocity and comes in contact with the feed either inside the nozzle, which is known as internal mix, or outside at the nozzle tip, known as external mix, to atomize the liquid. The degree of atomization is affected by the properties of the liquid, such as surface tension, density and viscosity.
A mixed-flow nozzle is placed in the bottom of the spray dryer and sprays upward into the hot inlet air. Liquid evaporation takes place shortly after the feed leaves the nozzle tip. As the particles travel upward into the hotter section of the dryer, any latent moisture is driven off. The particles then turn with the airflow and are carried to the powder separator. This type of atomization usually requires a drying chamber that is slightly taller than its diameter.
With a two-fluid nozzle, the particle size can be varied by the feed-to-airflow ratio at the nozzle head. A change in this ratio also affects the spray angle. The optimum spray angle is typically 70 to 80 degrees. Decreasing the feed rate at a constant air pressure or increasing the air pressure at a constant feed rate will decrease both the spray angle and the resulting particle size. Conversely, decreasing the air pressure will increase both the spray angle and particle size.
Generally, a mixed-flow two-fluid nozzle produces coarse, spherical particles that might include some satellites because of the interaction within the drying chamber. A laboratory or pilot mixed-flow spray dryer will typically produce particles in the 60- to 110-micron range.
Co-Current AtomizersA third type of atomization commonly found in small spray dryers is the co-current two-fluid nozzle (see Figure 4). This atomizer functions in the same way as a mixed-flow nozzle, but because the former is placed in the top of the drying chamber and sprays downward with the air flow, it is capable of producing fine particles like a rotary atomizer.
The co-current atomizer is often used in place of a rotary atomizer because of either the chamber size or equipment cost. Generally, a co-current two-fluid nozzle costs less then a rotary atomizer, and the chamber for a nozzle arrangement can be smaller in diameter, although it is typically taller. However, the powder produced by any two-fluid nozzle will have a larger particle size distribution compared to the powder produced by a rotary atomizer. Additionally, the operation of a two-fluid nozzle is harder to duplicate from one test run to the next because there are more control variables, such as the feed rate compared to the atomizing air rate.
The maximum particle size that can be achieved with a co-current nozzle is limited to the length of the drying chamber. A short drying chamber (e.g., 0.60 to 0.75 meter) will not allow enough time to dry a large droplet (e.g., 50-80 microns). By increasing the straight side length of the drying chamber, the large droplet will have a greater distance to travel and a greater amount of time to dry before it comes into contact with the chamber cone, which allows the possibility of making a larger particle. At a given point, however, increasing the straight side length will cause the spray to contact the chamber walls instead of the chamber cone, and the chamber diameter will also have to be increased proportionally to achieve the desired results.
With all spray dryers, the final particle size affects not only the powder's flow characteristics but also its bulk density, and this must be kept in mind when selecting spray drying equipment. For example, a fine particle that is made by a rotary atomizer or co-current nozzle will usually have a higher bulk density than a large particle made by a mixed flow two-fluid nozzle because the latter has a tendency to make particles that are somewhat hollow. However, hollow particles can be desirable when combined with different types of binders in a pressing operation.
Interdependent VariablesSpray drying is a process with many variables, and these variables often work against each other. Powder properties such as particle size and bulk density depend on the atomization form used in the spray dryer, but they are also affected by the chamber size, drying temperatures and liquid feed formulation. For example, a fine particle of 10 to 20 microns is easy to produce in a laboratory size spray dryer; however, the results cannot be duplicated in a production size dryer simply because the larger size of the dryer will increase the average particle size. On the other hand, a production dryer can use single fluid-pressure nozzle atomization to produce particles up to 300 microns in size. To achieve reasonable results from a pressure nozzle, the dryer must have a diameter and cylinder height of at least 2 meters (~6.5 ft). Lab and pilot plant dryers are simply too small to accommodate these dimensions. As with the atomizer type, careful consideration should be given to the needs of the facility when choosing between a lab or production spray dryer. Additionally, when trying to change powder properties, it is best to change only one variable at a time so that a direct cause and effect can be observed.
With the right equipment and a basic understanding of spray dryer operation, today's ceramic labs and pilot plants can optimize their results and speed their research and development efforts to commercialization.
For more information about spray drying, contact Niro Inc. - Chemical Division, 9165 Rumsey Rd., Columbia, MD 21045; (410) 997-8700; fax (410) 997-5021; e-mail firstname.lastname@example.org; or visit http://www.niroinc.com.