Spray drying is the most widely used industrial process involving particle formation coupled with drying. It’s well suited for continuous large-scale production of dry solids in powder, granulate, or agglomerate form from solutions, emulsions, and pumpable dispersions
By Dr. Joseph Iannicelli
Drying, the unit process in which a liquid is removed from a solid by vaporization, is an important, often vital, process in the chemical, food, pharmaceutical, mineral, and ceramic industries.
More than a score of major drying systems are used, and each system is available in numerous configurations. A partial list of the more important dryer types include tunnel, rotary, spray, fluid bed, flash, drum, and hearth. More specialized equipment includes infrared, microwave, and freeze dryers.
Dryers are classified as direct in which the product is directly contacted by heated gases (air, combustion gases, inert gases) or indirect in which the product makes contact with a heated surface. Both configurations are often available as continuous or less efficient batch dryers.
The choice of dryer type depends on the physical state of the feed material as well as the form of the dried product. Feed types include solutions, emulsions, dispersions, pastes, powders, crystals, granules, fabricated shapes, and sheets.
Often, several dryer types are used on the same feed material, but no single dryer type is practical for all of the feed types listed.
Besides feed form, considerations when choosing a dryer include the following:
1. Drying characteristics
2. Conveyance of material to and from dryer
3. Product properties (stability, form)
4. Product collection (main product, dust)
5. Infrastructure requirements (space, fuel, power, utilities, environmental)
6. Economics (capital, operating cost, maintenance, labor, scale)
Pilot and preferably scaled-up tests by experienced personnel at well equipped facilities are an absolute necessity prior to selection of equipment. Such tests should also allow sampling to prospective customers to confirm that product meets their requirements and specifications.
The majority of industrial dryers are continuous units operating on direct fired combustion gases. Theoretical operating relations between inlet temperatures, air flow (ACFM), and BTU required per pound of water evaporated are given in the nomograph (Figure 1).
Figure 1: Dryer Shortcut Chart for Temperatures to 1,500°F
(Click image for larger version.)
For example, an inlet of 250°F and outlet of 200°F at 7.2 ACFM per lb of water requires 2,150 BTU per lb of water evaporated. On the other hand, at an inlet of 1,000°F, an outlet of 200°F, and an air flow of 2.1 ACFM, only 1,440 BTU per lb of water is required.
However, the efficiency of a dryer also depends on the dryer configuration, feed form, and product form. The Georgia kaolin industry has used and evaluated a wide variety of dryer types on different products in their plants. Kaolin is a naturally occurring aluminosilicate clay produced by a sophisticated flow sheet involving up to a dozen steps including electro-kinetic dispersion, classification, delamination, magnetic separation, selective flocculation, filtration, and drying. The U.S. industry is concentrated in Georgia (90 percent of value produced) and is highly competitive intrastate as well as from alternative pigments and imports.
As a result, there has been relentless R&D continuing to improve and optimize process performance and economics. A good part of this effort is spent on drying, which is the most energy-intensive step. An example of the results of such investigation is summarized in Table 1.
| Table 1: Comparison of Evaporative Efficiency of Dryer Types | ||
| Dryer Type | Feed Solids | BTU/lb H2O Evaporated |
| Rotary | 82% | 2,720 |
| Scott | 82% | 2,400 |
| Impact Mill | 82% | 2,387 |
| Drum | 60% | 2,288 |
| Flash | 80% | 1,962 |
| Fluid Bed | 70% | 1,518 |
| Spray | 60% | 1,470 |
Over the years, the industry has adopted spray drying as its principal drying method. As a result, Georgia now has more than 40 large spray dryers (30 to 46 ft diameter) which is the highest concentration of large spray dryers in the world.
Table 1 shows the competitive efficiency, which other studies have confirmed, for spray dryers. Spray drying is the most widely used industrial process involving particle formation coupled with drying. It is well suited for continuous large-scale production of dry solids in either powder, granulate, or agglomerate form from solutions, emulsions, and pumpable dispersions. The spray drying process is relatively easily controlled for products meeting precise quality control with respect to particle size, particle size distribution, bulk density, particle shape, and moisture content. Thermal exposure is usually limited to 15-30 seconds at the outlet temperature.
Sensitivity of the spray drying process allows drying of complex organic compounds, botanicals, and pharmaceuticals. A recent report states that spray drying of mycobacterium tuberculosis yields a powder that shows promise as a TB vaccine — one that can be inhaled instead of injected!
Where applicable, spray drying can take advantage of inlet temperatures up to 1,400°F and the resulting high evaporative efficiency. Dryers are available in large sizes having evaporative capacities of over 20 tons of water per hour. Operation lends itself to total automation and one person can operate one or more dryers from a single control room.
Although spray dryers require use of pumpable slurries, numerous commercial processes actually use thick filter cakes, pastes, and sludges that are then re-slurried with dilution water so that these can be dried on spray dryers to take advantage of the ease of processing and the resulting product form.
Within the kaolin industry, some companies have been able to compensate for the large increases in the price of natural gas by coupling the exhaust of their calciners (1,000°F) to the inlet of their spray dryers. In addition, many companies have installed wet scrubbers to capture the latent heat of vaporization of water in their exhaust. Recovered heat is then used to operate evaporators to increase solids of their feed slurry to dryers.
Dr. Joseph Iannicelli is the president and technical director of Aquafine/Aero-Instant in Brunswick, GA. He has degrees in organic chemistry from M.I.T. and has worked for DuPont and JM Huber, where he accumulated about a hundred U.S. and foreign patents. In 1971, he founded Aquafine Corp. to commercialize his development of high-gradient magnetic separators to the kaolin industry. For 25 years, he also represented Niro Atomizer and sold nearly all the spray dryers acquired by the Georgia kaolin industry during that time. In 1987, Iannicelli started Aero-Instant Spray Drying Services, where he operates a dozen Niro dryers for contract drying including a closed system unit for drying solvents under nitrogen. More information is available by calling 912-265-2000.