Spray Drying

Spray drying

Spray drying is a commonly used method of drying a liquid feed through a hot gas. hot gas is air but sensitive materials such as pharmaceuticals, and solvents like ethanol require oxygen-free drying and Nitrogen gas is used instead. The liquid feed varies depending on the material being dried and is not limited to food or phamaceutical products.

The liquid feed is pumped through an atomiser device that produces fine droplets into the main drying chamber. Atomisers vary with rotary, single fluid, two-fluid, and ultra-sonic designs. These different styles have different advantages and disadvantages depending on the application of the spray drying required.

The hot drying gas can be passed as a co-current or counter-current flow to the atomiser direction. The co-current flow enables the particles to have a lower residence time within the system and the particle separator (typically a cyclone device) operates more efficiently. The counter-current flow method enables a greater residence time of the particles in the chamber and usually is paired with a fluidised bed system.

Process

The raw material fluid is sprayed into hot air and transformed into droplets and then dried particles. The feeding product remains in the spray dryer from 5 to 100 seconds, which is vital for heat sensitive products. The particle size of the dry product is very small (10 to 500 m m) in comparison with other drying methods. The open cycle and the close loop are the most common types of spray dryers.

Open cycle, co-current spray drying layout. (Barbosa-Canovas and Vega-Mercado,

1996).

The open cycle heats the incoming air and cleanses it by using cyclones and scrubbers. After drying the food product, the hot air goes back to the environment and its heat is wasted. The close loop system is more efficient since the hot air is recycled. According to Barbosa-Canovas and Vega-Mercado (1996), since the atomization produces moisture evaporation, modifying the flow and temperature of the air and the product can control the drying degree of food particles.

Principles

Every spray dryer consists of:

Ø Feed pump

Ø Atomizer

Ø Air heater

Ø Air disperser

Ø Drying chamber

Ø 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 thedrying chamber design.

Atomization

The formation of sprays having the required droplet size distribution is vital to any

successful spray dryer. 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.

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
• Counter- current
• Mixed flow

Co- current

Drying air and particles move through the drying chamber in the same direction.

Product temperatures on discharge form 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.

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

temperatures.

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.

Spray Dryer Chamber Design

As drying characteristics and product specifications vary from product to product, there

is no one spray drying chamber design suitable for all applications. By offering a full

range of designs, impartially selects the most suitable type of plant.

Spray Drying Systems

The essential elements of the spray dryer: atomizer, air disperser, drying chamber,

inlet and exhaust air handling are combined into a system that meets individual

operational safety, environmental protection, and powder handling requirements. All

systems can be provided with post- treatment equipment: fluid bed dryer/cooler,

agglomerator, de-duster and conveyor.

Open

Featuring once- through airflow with exhaust to atmosphere. The majority of industrial

spray dryers handles aqueous feed stocks and uses this system. Both direct and

indirect air heating are applicable. Exhaust air cleaning in cyclones, bag filters,

electrostatic precipitators, and scrubbers.

Closed cycle

Featuring drying in an inert gas atmosphere where nitrogen recycles within the dryer.

This system must be used for the spray drying of feedstocks containing organic

solvents or where the product must not contact oxygen during drying. Closed cycle

plants are gas and powder tight, and are designed to the strictest safety standards.

The inflammable solvent vapours are fully recovered in liquid form.

Semi-closed cycle

Featuring either the partial recycle mode (recycle of up to 60% of the exhaust air as

inlet air to the dryer, for effective waste heat utilization) or the self- inertizing mode,

where direct air heating and a minimal air bleed create the low oxygen atmosphere

necessary for drying aqueous feedstocks that form explosive powder-air-mixtures. If

odour is generated during drying, the small volumes of air vented from the system can

be effectively and economically incinerated.

Aseptic

Featuring sterile feed atomization and air filtering systems. These dryers are used

where any form of powder contamination must be avoided. They are fabricated to

special standards of finish and operate under a slight pressure. Fully automatic

cleaning and sterilization systems are available. Plant layout is integrated with the

laminar flow packing room.

Advantages

Spray dryers can be used from small to very high productions depending on their design. Cost of the equipment is expensive,

The cost of maintenance is low due to the small number of moving parts and the use of resistant materials

The purity of the product will be maintained since the food particles do not have any contact with the surface of the equipment until they are dried

Minimizing problems in sticking and corrosion.

The simple operating system and the cleaning conditions for spray dryers contributes to the low labor cost.

Another advantage of using the spray drying method is that a low bulk density of the product can be obtained