Powder Spray Drying

What is a Spray Dryer?

A spray dryer is an industrial drying unit that can convert a liquid feed into a powder in a single step, continuously. This type of drying is especially important when it's necessary to have consistency in the particle size, composition and quality of the finished powder; and to get the powder dried rapidly.

Spray dryers are generally used in food and beverages, pharmaceuticals, chemicals, ceramics, detergents, and other advanced materials.

What is Spray Drying?

Spray drying is a thermal process to convert a liquid feed to dry powder through the use of an atomizer, which converts the liquid feed into very small droplets. These droplets then enter a stream of heated gas and, because of their large surface area, the moisture inside these droplets will rapidly evaporate, resulting in the formation of a powder.

Since drying occurs extremely rapidly (in seconds), spray drying is well-suited to heat sensitive materials and to products that require precision in terms of moisture content and particle characteristics.

Steps Used In A Spray Dryer

Although there are variations based upon the intended application and capacity of the spray dryer, the majority of spray dryers work in the same basic steps:

Preparation Of The Liquid Feed

Preparation of the liquid feed typically begins with preparation of the liquid feed, which could be a solution, suspension or slurry. The feed is often filtered, blended or homogenized to ensure a uniform composition and flow characteristics.

Atomization

The liquid feed is pumped into the spray dryer and atomized into small droplets using one of the following methods:

- Rotary atomizers (e.g., spinning disk or wheel)

- Pressure nozzles

- Atomization is critical since the size of the droplets produced will determine how long the droplets take to dry; the size of the final powder particles; and the final properties of the product.

Drying Chamber And Hot Gas Contact

The atomized droplets are introduced into a drying chamber where they come into contact with a heated air or inert gas. The typical inlet temperature of the hot gas ranges from approximately 150°C to above 300°C, depending upon the product.

In addition to the heat provided to the droplets by the hot gas, as the droplets pass through the drying chamber, the moisture within the droplets evaporates rapidly. The material forming the powder will also solidify at this point. Although the inlet temperature of the hot gas is quite high, the product itself is relatively cool due to the evaporative cooling effect during drying.

Particle Formations

As drying continues, the particles form into the desired shape and structure. Manufacturer controls include:

- Particle size distribution

- Bulk density

- Flowability

By changing the operating conditions (i.e., temperature, airflow rate, and droplet size) of the spray dryer, manufacturers can achieve the desired properties of the final powder.

Powder Collection

Powdered products are separated from the exhausted hot gas using cyclones, bag filters, or wet scrubbers. Once the powdered product has been separated from the exhaust gas, it is ready for packaging or further processing.