Spray drying is a method of producing a dry powder of liquid or slurry quickly drying with hot gas. This is the preferred method of drying of many heat-sensitive ingredients such as food and medicine. A consistent particle size distribution is the reason for spray drying of some industrial products such as catalysts. Air is a heated drying medium; however, if the liquid is a flammable solvent such as ethanol or the product is sensitive to oxygen, nitrogen is used.
All spray dryers use several types of sprayers or nozzle spray to disperse liquids or slurries in controlled drip spray spray. The most common are rotary discs and high velocity single fluid swirl nozzles. The atomizing wheels are known to provide a wider particle size distribution, but both methods allow for a consistent distribution of particle sizes. Alternatively, for some applications, two liquid or ultrasonic nozzles are used. Depending on the needs of the process, a drop size of 10 to 500 Âμm can be achieved with the right choice. The most common applications are in the diameter range of 100 to 200 Âμm. Dry powder often flows freely.
The most common type of spray dryer is called a single effect. There is an air drying source at the top of the chamber (see n  ° 4 on the diagram). In many cases the air is blown in the same direction as the co-currents. Finely produced powder, but can have a bad flow and produce a lot of dust. To cope with dust and poor powder flow, a new generation of spray driers called multiple effects spray dryers has been produced. Instead of draining the liquid in one stage, the drying is done through two steps: the first at the top (as a single effect) and the second with the integrated static bed at the bottom of the chamber. The bed provides a humid environment that causes smaller particles to clot, resulting in a more uniform particle size, typically in the range of 100 to 300 Âμm. These powders are free flowing because of the larger particle size.
The fine powder produced by the first-stage drying can be recycled in a continuous stream either at the top of the chamber (around the sprayed liquid) or at the bottom, in an integrated fluidized bed. Powder drying can be finalized in a vibrating external vibrating bed.
The hot-drying gas can be passed as a direct current direction, just like a sprayed liquid sprayer, or a reverse current, in which hot air flows against the flow from the atomizer. With a shared current flow, the particles spend less time in the system and particle separators (usually cyclone devices). With reverse flow, the particles spend more time in the system and are usually paired with a fluidized bed system. The shared current flow generally allows the system to operate more efficiently.
Alternatives to spray dryers are:
- Freeze dryer: a more expensive batch process for degradable products in spray drying. The dry product does not flow freely.
- Drum dryer: a cheaper sustainable process for low value products; creating flakes instead of free-flowing powders.
- Pulse burner: A cheaper continuous process that can handle higher viscosity and solid loading than a spray dryer, and sometimes provide free flowing freeze-dried powder.
Video Spray drying
Spray dryer
The spray drier takes a liquid stream and separates the solute or suspension as solids and solvents into steam. Solids are usually collected in drums or cyclones. The liquid input stream is sprayed through the nozzle into the hot steam stream and evaporated. The solids are formed because the moisture quickly leaves the droplets. A nozzle is usually used to make the smallest possible droplets, maximizing heat transfer and the rate of water evaporation. The droplet size can range from 20 to 180 m depending on the nozzle. There are two main types of nozzles: high-pressure fluid nozzle (50 to 300 bar) and two-fluid nozzle: one liquid is dry liquid and the second is compressed gas (generally air at 1 to 7 bar).
Spray dryers can dry the product very quickly compared to other drying methods. They also change the solution, or pulp into a dry powder in one step, which can be advantageous because it simplifies the process and increases the profit margin.
Maps Spray drying
Micro-encapsulation
Spray drying is often used as an encapsulation technique by the food industry and others. A substance to be encapsulated (load) and an amphipathic carrier (usually a modified starch) is homogenized as a suspension in water (slurry). The slurry is then fed into the spray dryer, usually a tower heated to a temperature well above the boiling point of water.
As the porridge enters the tower, it is atomized. Partly because of the high water surface tension and partly due to the hydrophobic/hydrophilic interaction between the amphipathic carrier, the water, and the load, the atomized pulp forms the micelles. The small size of the drops (an average of 100 micrometers in diameter) results in a relatively large surface area that dries quickly. As the water dries, the carrier forms a hardened shell around the load.
Load loss is usually a function of molecular weight. That is, lighter molecules tend to boil in larger quantities at the processing temperature. Losses are minimized by industry spraying to higher towers. Larger air volumes have lower average humidity during the process. With the principle of osmosis, water will be driven by differences in fugacities in the vapor and liquid phases to leave micelles and enter the air. Therefore, the same percentage of water can be dried from particles at lower temperatures if larger towers are used. Alternatively, the slurry may be sprayed into a partial vacuum. Since the solvent boiling point is the temperature at which the solvent vapor pressure is equal to the ambient pressure, reducing the pressure in the tower has the effect of lowering the solvent's boiling point.
The application of the encapsulation spray drying technique is to prepare a "dehydrated" powder substance that has no water for dehydration. For example, the instant drink mix is ​​a dry spray of various chemicals that make up the drink. This technique was once used to remove water from food products; for example, in the preparation of dehydrated milk. Since milk is not encapsulated and because spray drying causes thermal degradation, milk dehydration and similar processes have been replaced with other dehydration techniques. Skimmed milk powders are still widely produced using spray drying technology worldwide, usually at high solids concentrations for maximum drying efficiency. The thermal degradation of the product can be overcome by using lower operating temperatures and larger room sizes to increase the residence time.
Recent research now shows that the use of spray drying techniques can be an alternative method for crystallizing the amorphous powder during the drying process because the temperature effects on the amorphous powder may be significant depending on the drying time of residence.
Spray drying application
Food: milk powder, coffee, tea, eggs, cereals, spices, flavorings, blood, starch and starch derivatives, vitamins, enzymes, stevia, nutracutical, dyes,
Pharmaceuticals: antibiotics, medical supplies, additives
Industry: paint pigments, ceramic materials, catalyst supporters, microalgae
Nano spray dryer
Nano spray dryer offers new possibilities in the field of spray drying. Particles can be produced in the range of 300 to 5 m with a narrow size distribution. High yields, up to 90%, can be produced and the minimum sample size is 1 mL.
References
Bibliography
- Charles Onwulata (2005). Food packed and powder . Press CRC. p.Ã, 268. ISBNÃ, 0-8247-5327-5. Ã,
Further reading
- Keey, R.B., (1992). Drying of Loose and Particulate Materials 1st ed., Taylor & amp; Francis, ISBN 0-89116-878-8
- Nutritional Food Processing Evaluation second edition (1975), Robert S. Harris, Ph.D. and Endel Karmas Ph.D. (eds)
- Cook, E.M, and DuMont, H.D. (1991) Process Drying Practices , McGraw-Hill, Inc., ISBNÃ, 0-07-012462-0
External links
- Animated Standard Spray Drying Concepts
- Spray Drying training paper
Source of the article : Wikipedia
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