It is a continuous direct contact dryer employed for Drying solutions, slurries, and pastes. Spray dryer, a liquid solution or slurry is introduced in the form of very fine droplets into a steam of hot gas inside a large drying chamber, thereby a large by a large contact area becomes available for perfect drying. 

Construction

Fig. 12.14 shows a typical spray dryer. The essential components of the dryer are a drying chamber (a vertical cylindrical chamber with short conical bottom), where the feed material is contacted with a hot gas (air) a heater for heating the fresh air sucked by a fan or blower, which are assembled for dust separation and collection, a pneumatic conveying duct and blowers, which are assembled as shown in fig. 12.14. 

{tocify} $title={Table of Contents}

The materials are usually spread in the form of a mist of fine droplets by Spray nozzles or high-speed rotating spray discs into a hot gas stream inside the chamber as shown in fig. 12.14.

Spray Dryer Working Principle 

The feed is pumped to the top of the dryer where it is disintegrated into small droplets by an atomizer. The large quantity of fresh air is taken in by a fan, it is heated in a heater and finally fed below the atomizer in the drying chamber. As the surface area of drops is very large, the liquid portion of these drops rapidly evaporates and before they touch the bottom of the drying chamber they are completely dried.

This dried product is taken out and conveyed to a cyclone dust collector -2 by the steam of air. The major portion of the air is taken out through the air outlet duct which mostly contains dust and sent to a cyclone-1. The solids collected by the cyclone - 1 are fed to a pneumatic conveying duct. The air leaving cyclone - 2 may contain some dust and therefore it is sent to cyclone - 1, for further separation, by the fan. The air from cyclone -1 is thrown out into the atmosphere by a blower. The dried product from cyclone -2 is collected in a dry product collector. 

An atomizer is a device that causes the liquid to be disintegrated into fine drops. The atomizers commonly employed are. 

1. Pressure nozzles make use of pressure energy for atomization. 

2. Two fluid nozzles wherein air or steam at a certain pressure is used to tear a liquid into droplets, I. e. they make use of gas energy and

3. Rotating discs which make use of centrifugal energy for atomization. Spray nozzles (type -1 and type -2) are relatively inflexible in operation and also subject to erosion and tear. The rotating disc rotates at a speed of about 3000 to 12000 r. p. m. The feed introduced at the center of the disc is centrifugally accelerated to the periphery and ultimately thrown in an umbrella-shaped spray. 

The rotating disc is very flexible in its operating characteristics and can handle thick slurries without the danger of clogging. In this dryer, care must be taken to ensure that the droplets or wet particles of solids do not strike the solid surface before complete drying. So as to avoid this, a large drying chamber is used. 

Spray dryers are versatile pieces of equipment used in various industries for the conversion of liquid or slurry materials into dry powder or granular form. Their primary purpose is to remove moisture from a liquid feed by spraying it into a hot drying chamber, where the liquid quickly evaporates, leaving behind solid particles. Here are some common uses and applications of spray dryers:

Spray Dryer Uses 

1. Food Industry

Spray dryers are extensively used to produce powdered dairy products like milk powder, whey protein powder, and coffee creamer.

They are used to make fruit and vegetable powders used in instant drink mixes, baby food, and flavorings.

Spray drying is employed to create powdered food flavorings, such as vanilla extract and essential oil extracts.

2. Pharmaceutical Industry

Spray dryers are used to produce pharmaceutical powders, including antibiotics, inhalable drugs, and enzyme formulations.

They are used to create powdered vaccines for easier storage and transportation.

Spray drying is used to produce excipients, which are inactive substances added to medications for various purposes.

3. Chemical Industry

Spray dryers help create powdered detergents, cleaning agents, and detergent additives.

Catalysts used in chemical reactions are often produced using spray drying techniques.

Spray dryers are used to produce powdered pigments and dyes for the textile, paint, and ink industries.

4. Ceramics and Advanced Materials

Spray drying is used to create ceramic powders for the production of ceramic products like tiles, insulators, and electronic components.

It is used in the production of advanced materials like battery materials and fuel cell components.

5. Environmental and Waste Management

 Spray drying is used to solidify liquid waste, making it easier to handle and dispose of safely.

In power plants, it's used to remove sulfur dioxide from flue gases by converting it into dry powder for disposal.

6. Chemical and Petrochemical Industries

Spray drying is used to produce various chemical powders used in industrial processes.

It can be used for drying petrochemical products like resins and polymers.

7. Biotechnology

Spray drying is employed to create powdered biological products such as probiotics, enzymes, and bacterial cultures.

8. Cosmetics and Personal Care

It's used to create cosmetic powders, including talcum powder and powdered foundation.

9. Nanoparticle Production

Spray drying is used in nanoparticle production, where precise control over particle size and characteristics is required.

Spray dryers are valued for their efficiency, consistency, and ability to produce fine, free-flowing powders with controlled properties. 

Spray Dryer Advantages and Disadvantage

Advantages

Very short drying times (2 - 20 s). 

Handle heat-sensitive products. 

Control of a product particle size. 

Rapid dehydration. 

Relatively low operating costs, particularly in large capacity units. 

Disadvantages

Low solids content. 

Relatively large units. 

Maintenance of atomiser. 

Inefficient in (its) energy use. 

Product built-up on interior.

Take these Notes is, Orginal Sources: Unit Operations-II, KA Gavhane