Spray drying pesticides

Pesticides formulations fall into three categories: fungicides, herbicides, and insecticides. For the manufacture of these products in powdered form, spray drying is widely used. Due to their toxicity, great attention is given to the dryer design and component layout to prevent any form of environmental pollution from the spray dryer. The optimum layout is decided on the basis of the required powder form (fine, dustless, agglomerated), residual moisture content, and toxicity properties.

The spray drying process

Spray drying produces powder directly from liquids. The pesticide formulation is sprayed by an atomizer (rotary atomizer or nozzle) into hot drying air. Moisture in the spray droplets evaporates rapidly leaving dried particles, which are discharged from the spray dryer as a continuous flow of powder.

Tall form spray dryer

Fungicides

• Colloidal Sulphur
• Copper compounds
• Dithicoarbamates
• Thiuram disuphides

Herbicides

• Benzoic acid derivates
• Chlorates
• Halogenated acid derivates
• Phenoxy derivates
• Phenyl acetic acid derivates

Insecticides

• Arsenates
• Chloride containing compounds
• Fluoride containing compounds
• Thiophosphates

Fluidized spray dryer

Spray dryer with single point product discharge

System selection

The plant layout is selected according to the desired powder characteristics.

For standard powders compromising fine particles, a co-current spray drying chamber, type SD, is suitable. The drying air enters through a roof air disperser that houses a rotary atomizer.

Spray dryer with rotary atomizer (Type SD) mean size: 80 micron

For free-flowing, dustless powders consisting of individual particles, a co-current tower drying chamber, type TFD, is used. Low pressure nozzle atomization creates a coarse spray that dries while falling gently down the tower under lenient temperature conditions. This design, however, is being superseded by a new mixed-flow spray dryer, type FSD, having a fluid bed integrated within the drying chamber base. These fluidized spray dryers have the advantage of producing dustless, free-flowing agglomerates of excellent dispersibility in a more energy efficient operation. Drying is accomplished at lower product temperatures giving much better activity retention. The operation of the integrated fluid bed gives greater product flexibility and enables the handling of products that previously could not be spray dried due to heat sensitivity or stickiness.

Tall form spray dryer with nozzle atomizer (Type TFD) mean size: 200 micron

Stationary or vibrating fluid beds as separate attachments are used with SD and TFD plant for post-drying/cooling in connection with low dust powder production.

Fluidized spray dryer (Type FSD) mean size: 300 micron

Powder recovery and air pollution prevention systems

The method of powder discharge from the drying chamber and selection of exhaust system depends upon pesticide toxicity and emission control standards.

Recycle system with air bleed incineration

The basic exhaust system features air cleaning of entrained powder air cleaning of entrained powder in a cyclone or bag filter. A wet scrubber follows the cyclone, and often also the bag filler. Where toxic particulate emissions must be fully prevented, the exhaust air is recycled within the spray drying plant. Any vented air is first deactivated by thermal incineration. A heat recuperation system is utilized to recover the incineration heat, thus maintaining a high overall thermal efficiency of the spray drying operation.