Spray drying is a method of producing a dry powder from a liquid or slurry by rapidly drying with a hot gas. This is the preferred method of drying of many thermally-sensitive materials such as foods and pharmaceuticals. A consistent particle size distribution is a reason for spray drying some industrial products such as catalysts. Air is the heated drying medium; however, if the liquid is a flammable solvent such as ethanol or the product is oxygen-sensitive then nitrogen is used.

A spray dryer takes a liquid stream and separates the solute or suspension as a solid and the solvent into a vapor. The solid is usually collected in a drum or cyclone. The liquid input stream is sprayed through a nozzle into a hot vapor stream and vaporized. Solids form as moisture quickly leaves the droplets. A nozzle is usually used to make the droplets as small as possible, maximizing heat transfer and the rate of water vaporization. Droplet sizes can range from 20 to 180 μm depending on the nozzle.There are two main types of nozzles: high pressure single fluid nozzle (50 to 300 bars) and two-fluid nozzles: one fluid is the liquid to dry and the second is compressed gas.

Spray dryers can dry a product very quickly compared to other methods of drying. They also turn a solution (or slurry) into a dried powder in a single step, which simplifies the process and improves profit margins.

In pharmaceutical manufacturing, spray drying is employed to manufacture Amorphous Solid Dispensation, by uniformly dispersing Active Pharmaceutical Ingredients into a polymer matrix. This state will put the active compounds (drug) in a higher state of energy which in turn facilitates diffusion of drug spices in patient body.

The spray drying process begins with atomization.  This is where a nozzle or rotary atomizer turns the liquid feed stock into small liquid droplets. Here many of the desired qualities for the final product such as particle size and viscosity are developed.

The drying process is next.  After the droplets exit the nozzles or atomizer, they need to be dried to form a powder that can be easily packed and transported.  Drying of the powder is commonly accomplished using air that has been heated with natural gas.  Final moisture of the powder is controlled by fine-tuning the temperature of that hot air.

The recovery process is last and takes just a few seconds to recover the powder from the exhaust gas, by way of a bag filter or cyclone.

• 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.
• Feedstock 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.

• 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.

We encounter spray dried ingredients and food products all the time. Whenever a liquid has been converted to a shelf-stable powder, there’s a good chance that spray drying was used. The most commonly spray dried foods include:

• milk powders
• dried eggs
• instant coffees
• instant teas
• dried fruit juices
• honey powders
• molasses powders
• powdered flavors

While there are several drying processes available for food, the spray drying process is distinguished by its unique equipment that allows for rapid drying with minimal heat exposure. In spray drying, a liquid is sprayed through an atomizer into a chamber that contains streams of hot air. The moisture quickly evaporates, leaving behind solid powder particles that fall to the bottom of the chamber.

Spray drying is ideal for heat-sensitive materials and whenever a free-flowing, uniform powder is required. Whereas other drying techniques typically produce flakes that then must be ground to size, spray dryers create a free-flowing powder with a narrow size distribution, making a subsequent grinding step unnecessary. Furthermore, spray drying is considered the method of choice for commercial-scale encapsulation applications and is used to encapsulate flavors, carotenoids, and lipids.