Liquid drying is an important unit operation in chemical production, and the common method of drying liquid is spray dryer. However, the aim is not necessarily evaporation in water or a solvent, but the formation of a powder.
Like all drying, spray drying is a mass transfer phenomenon. The concentration gradient at the gas-liquid interface and the surface area of the gas-liquid interface are critical. The former uses dry air to maximize the concentration gradient. Dispersing a liquid into fine droplets makes the latter larger.
Spray drying is usually carried out in cyclone-shaped vessels. The liquid enters the cyclone separator through a nozzle called an atomizer. This rotary or pressure device atomizes it into very small droplets.
Dry air is forced into contact with fine droplets, which have a high surface area to volume ratio. The extra surface area enhances the evaporation process and the powder settles quickly.
Depending on the application, air and liquid flow configurations are arranged in many different ways. Patterns are often concurrent or countercurrent, but even then there are variations on each theme. Depending on the type of application, the exact configuration needs to be carefully selected.
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