Spray drying principle : typical stages and process options of spray drying

1. Key stages of spray drying process

How does spray drying work ?

Spray drying is performed along 4 main stages in order to transform a liquid feed into a powder :

• Atomization
• Spray / air contact
• Spray evaporation
• Product separation

For each of these stages many variants of equipment and process sub-units have been developed over time which allows the Process Engineer to chose equipment fitting the most the usage to be done.

The different process options available are summarized in the following diagram.

1.1 STEP 1 : Feed

Prior to spray drying, the slurry must be processed at the right temperature, viscosity and other parameters depending on the application must be reached (for example a Total Solids - Ts - content for milk spraying). Then the feed has to be homogeneized and pumped at high pressure to the spraying nozzles.

1.2 Step 2 : Atomization

The feed is pressurized and go through 1 or several nozzles where it is atomized in very fine droplets. There are 2 big categorizes of atomizers : rotary atomizers or nozzles, the choice depending on the application.

1.3 Step 3 & 4 : Spray-Air contact and evaporation

The droplets, once formed enter in contact with the hot air admitted in the spray dryer, which is causing the evaporation of the solvent (typically water) and the creation of powder.

1.4 Step 5 : Product separation

The resulting product is carried away by the strong air draft used for drying the droplet, thus it is then necessary to separate the desired product from the air. It can be done at the bottom of the spray dryer, using a cyclone or bag filters, or a combination of both.

1.5 Step 6 : Powder

Once separated from the drying air, the powder can be processed further, for example by using an after dryer (fluidized bed) to help dry it further and reach the right product specification.

2. Spray drying stages : summary graph