Spray dryers are used for drying only the liquid materials like aqueous solutions, organic solutions, emulsions, slurries and thin pastes. The basic concept of spray drying is the production of highly dispersed powders from fluid feed by evaporating the solvent. Spray drying is used for wide range of products like:
- Soaps and detergents
- Milk and egg products
- Colourings, flavours and plant extracts
- Bone and tooth amalgams
- Plastics, resins, polymers
Principle: in this method, the liquid is dispersed or atomized (sprayed) as fine droplets into a moving stream of hot air where they are evaporated rapidly before reaching the wall of chamber. The temperature of the droplets is immediately increased and fine droplets get dried in the form of spherical particles. This process completes in a few seconds before the droplets reach the wall of the dryer. The product dries into fine powder which is collected into a collection system.
All the spray dryers consists of the following components
- Feed delivery system
- Heated air supply
- Drying chamber
- Solid-gas separator
- Product collection system
All other processes or dryers for drying i.e Tray dryer, freeze dryer, rotary evaporators produce lumps and mass of material which needs further processing- grinding and produce irregular size of particles.
Design and control: The main priority in making and processing of spray dryer is:
- Improve powder quality
- Increase production
- Reduce cost
It consists of large cylindrical drying chamber with conical bottom, usually made up of stainless steel. Diameter of drying chamber ranges between 2.5 to 9.0 meters and height is 25.0 metres or more. Hot air inlet is placed in the roof of the chamber. Second inlet for spray disc atomizer is set in the roof. Spray disc atomizer rotates at a speed of 3000-50000 rpm. Cyclone separator is connected at the bottom of dryer.
Feed preparation: It is homogenous, pumpable and free from impurities solution, suspension or paste.
- Atomization: it is dispersion of feed solution in small droplets. which can be achieved by:
- Pressure nozzle atomization: spray is produced by forcing the fluid through an orifice either by gravity or by using suitable pump to form fine droplets.
- Two-fluid nozzle atomization: spray is produced by mixing the feed with compressed gas. It is used for making fine particles.
- Centrifugal or spinning disc atomization: spray is produced by passing the feed through a rotating disc.
For Example: 80-100ml of solution are sprayed, resulting in approximately 8×10⁸=80,000,0000 drops representing 12m² of surface area. It shows that the solvent is vaporized very rapidly.
A constant rate phase ensures moisture evaporates rapidly from the surface of the particles.
The manner in which spray contacts the drying air is an important factor spray dryer design
- Co-current flow: The feed solution is sprayed in the same direction as the flow of hot air. The droplets come into contact with hot air when they are moist.
- Counter-current flow: The feed solution is sprayed in the opposite direction of the flow of hot air. The hot air flows upwards and the spray feed falls through increasingly hot air into the collection system. The residual moisture dries up and eliminated and product becomes very hot. This method is useful for thermo stabile products.
Combined: the advantages of both spraying methods are combined. The product is sprayed upwards and only remains in the hot zone for less time to remove residual moisture. Gravity then pulls the product into the cooler zone. Due to the fact that product is only in the hot zone for a short time; the product is treated with care.
- Disk atomizer (Rotary wheel): the material to be sprayed flows into rapidly rotating atomizing disk and converted into fine mist. The Drying air flows in the same direction. The product is handled with care.
Drying: Fine droplets are dried in the drying chamber by supplying hot air through the inlet. The surface of the liquid drop is dried rapidly to form a tough shell. The liquid inside must escape by diffusing through the shell at a particular rate. At the same time, heat transfer from outside to inside takes place at a rate greater than liquid diffusion rate. As a result, heat inside increases which allow the liquid to evaporate at a faster rate. This tendency of a liquid leads to rise in the internal pressure, which causes the droplets to swell.
Recovery of the dried product: centrifugal force of atomizer drives/forces the droplets to follow helical path.
All these processes are completed in a few seconds. Particle size depends on solid content in the feed, feed rate, liquid viscosity, disc speed. Particle size of product ranges from 2-500mm
|Application||Use||Practical use/ Application|
|Spray drying||Drying of organic/inorganic products||Dried milk, Pigments, Starch|
|Micro-encapsulation||Liquid is embedded in solid matrix||Peach oil, Perfumes|
|Micronization||Particle size reduction of product||Dyes, Salt|
|Englobing||A solid product is embedded in another solid or mixture of solids||Carotenoids in gelatin|
Products that are dried using spray dryer are: vaccines, vitamins, penicillin, milk, fruit juices, adrenaline, barium sulphate, blood, chloramphenicol succinate, dextran, lactose, plasma, serum, starch, acacia, bacitracin, citric acid, hormones, gelatin, borax, methyl cellulose, pancreatin, yeast, pepsin, sulphur, coffee extract, calcium sulphate etc.
- It is suitable for drying of sterile products.
- It is a continuous process and drying is very rapid which completes within 3-30 seconds.
- Reconstituted product seems similar to fresh material.
- Fine droplets provide large surface area for heat and drying and further show good solubility.
- Through atomizer, uniform size of product can be obtained.
- Globules of emulsion can be dried which again on reconstitution makes good emulsion.
- Thin pastes, emulsions, suspensions, organic or inorganic solutions can be dried in one step.
- it can be operated in oxygen free environment.
- it is useful for encapsulation of solid and liquid particles.
- it can be constructed in such a way that the flow of liquid and gas may be co-current, counter current or a combination of both in the same unit.