Closed-cycle drying of heat-sensitive powders
Increasing public awareness of potential chemical manufacturing hazards has caused the chemical process industry to take steps toward minimizing plant effluent. Drying equipment has historically been a major contributor to air pollution. Although evaporated water, air, and a maximum of 0.02 grains per cubic foot of powder can be exhausted to the atmosphere, the long-term environmental impact of atmospheric discharge for many newly developed products is unknown.
An alternative approach is to refrain from discharging any air. This can be accomplished by condensing the evaporated solvent vapors and recirculating the air or gas stream back to the dryer. The condensed solvent can then be returned to process or to storage. Though this closed-cycle drying technique is not new, it has been used almost exclusively with materials that oxidize, explosive materials, and products other than water-based solids. The current trend in industry, however, is to "close the loop," even with water-based products whose potential long-term hazardous effects are unknown.