Spray Dryer - How it Works?

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 many heat-sensitive pharmaceuticals. A consistent particle size distribution can be achieved using spray drying of some industrial products such as catalysts.

Principle of Spray Dryer

Spray drying is the continuous transformation of feed from a fluid state into dried particulate form by spraying the feed into a hot drying medium. The feed is either solution, slurry, emulsion, gel, or paste which is provided through a pump in atomized form.

Construction of Spray Dryer

Many different types of spray dryers exist, each with different features for meeting various spray drying needs. 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 and process control systems. It consists of a large cylindrical drying chamber with a short conical bottom, made-up of glass (lab scale) or stainless steel (large scale). The diameter of the chamber is 2.5- 9 meters and height 25 meters or more. An inlet for hot air is placed in the roof of the chamber and another inlet carrying spray disk atomizer is set in the roof. The spray disk atomizer is about 300 mm in diameter and rotates at a speed of 3000 to 50,000 r.p.m. The bottom of the dryer is connected to a cyclone separator.

 Fig.1: Spray Dryer and Its Stepwise Working Sequence

Working of Spray Dryer

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. The three stages that occur in a spray dryer before drying is accomplished include atomization, spray-air mixing, moisture evaporation, and dry product separation from the exit air. The spray drying process begins with atomization. During atomization, a nozzle or rotary atomizer turns the liquid feedstock into small liquid droplets. This is followed by the separation of the solute or suspension as a solid and the solvent into a vapor. It is during this stage that many of the desired product qualities such as particle size and viscosity are developed. When droplets exit the nozzles or atomizers, they are dried to form a powder that is easily packed and transported. Solids form as moisture quickly leaves the droplets. The solid is usually collected in a drum or cyclone. The nature of the final product depends on the design and operation of the spray dryer and the physicochemical properties of the feed. Drying of the powder is commonly completed using hot air. The final moisture content in the powder is controlled by adjusting the hot air temperature. The recovery process is the last step that takes a few seconds to recover the powder from the exhaust gas within the cyclone.

The main advantage of spray drying is its versatility of the technology. Spray drying offers multiple opportunities that no other single drying technology provides. The flexibility and reproducibility of spray dryers make spray drying the process of choice for many industrial drying operations. It has the following other advantages :

1. This technique can operate in aseptic pharmaceutical processing.
2. Feed rates in spray drying can range from a few pounds per hour to over 100 tons per hour and thus can be designed to any capacity required.
3. The spray drying process is very rapid, with the major portion of evaporation taking place in less than a few seconds.
4. It is adaptable to a fully automated control system that allows continuous monitoring and recording of a very large number of process variables simultaneously.
5. Wide ranges of equipment designs are available to meet various product specifications.
6. It has few moving parts and careful selection of various components can result in a system having no corrosion and wear and tear problems.
7. It can be used with both heat-resistant and heat-sensitive products.
8. It can handle feed for drying in solution, slurry, paste, gel, suspension, or melt form.
9. It can have control over properties such as particle size, bulk density, degree of crystallinity, impurities, and residual solvents.
10. It can produce nearly spherical particles with uniform size and thus reduce the bulk density of the product. Powder quality remains constant during the entire run of the dryer.

1. The equipment is very bulky and the ancillary equipment components are expensive to install.
2. The overall thermal efficiency is low, thus large volumes of heated air are wasted as pass through the chamber without contacting a particle.
3. It is difficult to maintain and clean after use.
4. It needs material in liquid form and thus solid materials cannot be dried using spray dryers.
5. Product degradation or fire hazard may result from product deposit in the drying chamber.

Applications of Spray Dryer

Spray drying technology is widely applied in pharmaceutical fields as well as non-pharmaceutical fields.

1. Many pharmaceutical and biochemical products are spray dried, including antibiotics, enzymes, vitamins, yeasts, vaccines, and plasma.
2. It can be used for drying algae, antibiotics and molds, bacitracin, penicillin, streptomycin, sulphathiazole, tetracycline, dextran, enzymes, hormones, lysine (amino acids), pharmaceutical gums, sera, spores, tableting constituents, vaccines, vitamins, yeast products, tannin products, etc.
3. Spray drying stands out as a unique method in making granules and tablets. The composite particles with good compatibility and excellent micrometric properties as filler for direct tableting of controlled release matrix tablets. It has been used for granulating, for slow-release granulations of magnesium carbonate, theophylline, and acetaminophen. The spherical composite particles consisting of amorphous lactose and sodium alginate are prepared by spray drying.
4. It can be used in preparing dry powder aerosol formulations. For example, salbutamol sulfate particles are prepared by spray drying, using a mini spray dryer, and liposomal ciprofloxacin powder for inhaled aerosol drug delivery.
5. It can be used in preparing microparticles for the preparation of dried liposomes, amorphous drugs, mucoadhesive microspheres, microcapsules, gastro-resistant microspheres, and controlled-release systems.
6. Spray drying has proved extremely useful in the coating and encapsulation of both solids and liquids. Spray-dried microparticles of theophylline were prepared with a coating polymer in an aqueous system.
7. Dry emulsions can be prepared by spray-drying various liquids. For example, o/w emulsions containing fractionated coconut oil dispersed in aqueous solutions of HPMC (solid carrier).
8. It can also be used to prepare dry elixirs. For example, Flurbiprofen Dry Elixir.
9. Non-pharmaceutical applications of spray drying include drying of various materials in the food, chemical, and ceramic industry.

For example, detergents, soaps, and surface-active agents, pesticides, herbicides, fungicides and insecticides, dyestuffs, pigments, fertilizers, mineral floatation concentrates, inorganic chemicals, organic chemicals, spray concentration (purification), milk products, egg products, food and plant products, fruits, vegetables, carbohydrates, and similar products, slaughterhouse products, fish products, and many others.

Make sure you also check our other amazing article on : Drum Dryer