Spray drying is a one-step continuous unit operation that employs liquid atomization to produce droplets that are dried to individual particles when moved in a hot gaseous drying medium. A spray dryer consists of a feed pump, atomizer, air heater, air dispenser, drying chamber and systems for exhaust air cleaning and powder recovery/separator. The three stages that occurs in a spray dryer before drying is accomplished includes.

1. Atomization
2. Spray-air mixing and moisture evaporation.
3. Dry product separation from the exit air.

The nature of the final product obtained after drying in a spray dryer depends on;

• The design and operation of the spray dryer.
• The physicochemical properties of the feed.

Spray dryer process layout

The different process layouts of spray dryer include;

• Open cycle layout.
• Closed cycle layout.
• Semi-closed (standard) layout.
• Semi-closed (self-inertizing) layout.
• Two stages layout.
• Combination layout

See also: rotary dryer

Pharmaceutical uses of Spray dryer

• Spray dryer is used in drying pharmaceuticals like penicillin, blood products, enzymes, vaccines etc.
• It is used in the production of excipient and co-processed excipient with increased flowability, compatibility and tablet disintegration.
• To improve drug compressibility and reduce capping tendencies in crystals.
• It is equally used in the preparation of matrix microcapsule containing drugs substances and a biodegradable polymer in order to obtain controlled drug release formulation.
• It is employed in enhancing solubility and dissolution rates of poorly soluble drugs by formation of pharmaceutical complexes or via the development of solid dispersion thus increasing bioavailability.
• It is used in the production of dry powder formulation/dry powder aerosol and thermoabile materials.

Apart from its applications in the pharmaceutical industries, spray dryers also find use in;

• Chemical industries e.g. phenol-formaldehyde resin, catalysts, PVC emulsion type, amino acids etc.
• Ceramic industries e.g. aluminium oxide, carbides, iron oxides, kaoline etc.
• Dye stuffs and pigments e.g. chrome yellow, food colour, titanium dioxide, paint pigments etc.
• Fertilizer production e.g. nitrates, ammonium salts, phosphates etc.
• Detergent and surface active agents e.g. detergent enzymes, bleach powders, emulsifying agents etc.
• Food industries e.g. milk, whey, egg, soya protein etc.
• Fruits and vegetables e.g. banana, tomatoes, coconut milk etc.
• Beverage e.g. coffee, tea etc.
• Biochemical industries e.g. algea, fodder antibiotics, yeast extracts, enzymes etc.
• Environmental pollution control e.g. flue gas desulfurization, black liquor from paper-making etc.

Advantages of Spray Dryers

• Product quality and properties can be effectively controlled and maintained through the entire drying operation.
• Thermolabile products/pharmaceutical can be dried at atmospheric pressure and low temperature.
• Spray dryer permits high- tonnage production in continuous operation adaptable to conventional PLC control (Programmable Logic Controller) and it is relatively simple to operate.
• Feed stock in solution, slurry, emulsion, paste and melt form can be dried if pumpable.
• Corrosion problem is minimal and the selection of materials of construction of spray dryer is simplified since the dried material comes in contact with the equipment surfaces in an anhydrous condition.
• Spray dryer produces dry powder particle of controllable particle size, shape, form, moisture content and other specific properties irrespective of dryer capacity and heat sensitivity.
• Spray dryer handles a wide range of production rate and provides extensive flexibility in its design that is product specification are readily met through the selection of appropriate spray dryer design and its operation from a wide range of available design.
• It is an energy intensive equipment because;
  1. Specific heat of evaporation can be supplied in a short time.
  2. The temperature difference across the drying chamber is relatively small and
  3. An appreciable amount of heat is lost with exhaust air.

Disadvantages of spray dryers

• Spray dryer is bulky and also expensive to install.
• It is difficult to clean after use.
• It has a low thermal efficiency that is a lot of heat is wasted during operation.
• Solid materials cannot be dried using spray dryers.
• Product degradation or fire hazard may result from product deposit on the drying chamber.

REFERENCES
Arun S. Mujumdar (2015). Handbook of Industrial Drying. CRC Press, Taylor and Francis Group, New York.
http://m.authorstream.com/presentation/keerthi09-443902-spray-drying-pharmaceutical-industry/
http://www.niro.com/niro/cmsdoc.nsf/webdoc/ndkw5y7gne
http://www.niroinc.com/technologies/spray_drying_tech.asp