Spray drying and its applications

Spray drying is a method of turning a liquid or slurry into a dry powder by rapidly drying with hot gas.This is the method of choice for drying many heat-sensitive materials,such as food and pharmaceuticals or where extremely consistent fine-grained materials may be required.Air is the heated drying medium; however, nitrogen is used if the liquid is a flammable solvent such as ethanol or the product is oxygen sensitive.All spray dryers use some type of atomizer or nozzle to disperse a liquid or slurry into a spray of controlled droplet sizes.The most common of these are rotary disc and single-fluid high-pressure swirl nozzles.Atomizer wheels are known to provide a wider particle size distribution but both methods allow for a consistent particle size distribution.Alternatively, for some applications, a two-fluid or ultrasonic nozzle is used.Depending on process requirements, droplet sizes from 10 to 500 μm can be achieved with proper selection.The most common applications are in the 100 to 200 μm diameter range.Dry powders are usually free-flowing.

The most common type of spray dryer is called a single effect.There is a single source of dry air at the top of the chamber.In most cases, the air is in the same direction (cocurrent) as the sprayed liquid Produces a fine powder, but has poor fluidity and produces a lot of dust.In order to overcome the problems of excessive dust and poor fluidity, a new generation of spray dryer, called multi-effect spray dryer,has been produced. Instead of drying the liquid in one stage, drying is done in two steps:the first step at the top (according to single effect), and the second step at the bottom of the chamber using an integrated static bed.The bed provides a moist environment, which leads to agglomeration of smaller particles, resulting in a more uniform particle size, typically in the range of 100 to 300 μm.Due to the large particle size, these powders are free flowing.The fine powder from the first stage of drying can be recovered in continuous flow at the top of the chamber (around the spray liquid) or in the integrated fluidized bed at the bottom.Drying of the powder can be done on an external vibrating fluidized bed.The hot drying gas can enter cocurrently in the same direction as the spray liquid atomizer, or countercurrently,with hot air exiting the atomizer countercurrently.With co-current, particles spend less time in the system and in the particle separator (usually a cyclone device).With countercurrent flow, the particles stay in the system longer and are often used with fluidized bed systems.Parallel flow often allows the system to operate more efficiently.

Alternatives to spray dryers are: 

1.Freeze Dryer: A more expensive batch process for products that degrade in spray drying.Dry product is not free flowing.

2.Drum Dryer: A lower-cost continuous process for low-value products; produces flakes rather than a free-flowing powder.

3.Pulse Combustion Dryer: A less costly continuous process that can handle higher viscosities and solids loadings than spray dryers, sometimes producing free-flowing freeze-dried quality powders.

Spray dryer

Spray dryers take a liquid stream and separate solutes or suspensions into solids and solvents into vapors.Solids are usually collected in drums or cyclones.The liquid input stream is sprayed through the nozzle into the hot steam stream and evaporated.Solids form when moisture leaves a droplet quickly.Nozzles are typically used to keep the droplets as small as possible,thereby maximizing heat transfer and water evaporation rates.Droplet size ranges from 20 to 180 μm, depending on the nozzle.There are two main types of nozzles: high-pressure single-fluid nozzles (50 to 300 bar) and dual-fluid nozzles: one fluid is the liquid to be dried and the second is a compressed gas (usually air from 1 to 7 bar) .Spray dryers dry products very quickly compared to other drying methods.They also turn solutions (or slurries) into dry powders in one step, simplifying the process and increasing margins.In pharmaceutical manufacturing, spray drying is used to create amorphous solid dispersions by uniformly dispersing active pharmaceutical ingredients into polymer matrices.This state will place the active compound (drug) in a higher energy state, thereby facilitating the diffusion of the drug flavor in the patient.

Spray drying applications

Food: milk powder, coffee, tea, eggs, grains, spices, flavoring agents, blood,starch and starch derivatives, vitamins, enzymes, stevia, health products, pigments, animal feed, etc.Pharmaceuticals: antibiotics, pharmaceutical ingredients,additives Industry: paint pigments, ceramic materials, catalyst supports,microalgae.