Spray Drying Technology

Process

Spray drying is the most widely used industrial process involving particle formation and drying. It is highly suited for the continuous production of dry solids in either powder, granulate or agglomerate form from liquid feedstocks as solutions, emulsions and pumpable suspensions. Therefore, spray drying is an ideal process where the end-product must comply to precise quality standards regarding particle size distribution, residual moisture content, bulk density, and particle shape.

Spray drying involves the atomization of a liquid feedstock into a spray of droplets and contacting the droplets with hot air in a drying chamber. The sprays are produced by either rotary (wheel) or nozzle atomizers. Evaporation of moisture from the droplets and formation of dry particles proceed under controlled temperature and airflow conditions. Powder is discharged continuously from the drying chamber. Operating conditions and dryer design are selected according to the drying characteristics of the product and powder specification.

Principles

Every spray dryer consists of feed pump, atomizer, air heater, air disperser, drying chamber, and systems for exhaust air cleaning and powder recovery.

Widely varying drying characteristics and quality requirements of the thousands of products spray dried determine the selection of the atomizer, the most suitable airflow pattern, and the drying chamber design.

Atomization

The formation of sprays having the required droplet size distribution is vital to any successful spray dryer operation so that powder specifications can be met. Atomization is a high technology area where Niro has played a central role in the development and use of nozzles and rotary atomizers in spray drying.

Spray Drying Technology: rotary atomizer in a spray dryer

A video of rotary atomization in a spray dryer: Length: 0:00:23

Spray Drying Technology: nozzle atomizer in a spray dryer

A video of nozzle atomization in a spray dryer: Length: 0:00:12

Airflow

The initial contact between spray droplets and drying air controls evaporation rates and product temperatures in the dryer. There are three modes of contact:

• Co-current

  Drying air and particles move through the drying chamber in the same direction. Product temperatures on discharge from the dryer are lower than the exhaust air temperature, and hence this is an ideal mode for drying heat sensitive products. When operating with rotary atomizer, the air disperser creates a high degree of air rotation, giving uniform temperatures throughout the drying chamber. However, an alternative non-rotating airflow is often used in tower or FILTERMAT®-type spray dryers using nozzle atomizers with equal success.

• Counter-current

  Drying air and particles move through the drying chamber in opposite directions. This mode is suitable for products which require a degree of heat treatment during drying. The temperature of the powder leaving the dryer is usually higher than the exhaust air temperature.

• Mixed flow

  Particle movement through the drying chamber experiences both co-current and counter-current phases. This mode is suitable for heat stable products where coarse powder requirements necessitate the use of nozzle atomizers, spraying upwards into an incoming airflow, or for heat sensitive products where the atomizer sprays droplets downwards towards an integrated fluid bed and the air inlet and outlet are located at the top of the drying chamber.

Niro has a vast experience in spray drying a wide range of food and dairy products. Please access this page to see some typical Niro spray drying applications.

Spary drying and fluid bed agglomeration systems in a large dairy facility.

Powders and agglomerates from a Niro spray drying plant.

Inquiries outside North America and more information on Spray Drying