Tuesday, December 18, 2007

Spray drier

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Spray drying


Spray drying is a commonly used method of drying a liquid feed through a hot gas. Typically, this hot gas is air but sensitive materials such as pharmaceuticals, and solvents like ethanol require oxygen-free drying and nitrogen gas is used instead. The liquid feed varies depending on the material being dried and is not limited to food or pharmaceutical products, and may be a solution, colloid or suspension. This process of drying is a one step rapid process and eliminates additional processing.

The liquid feed is pumped through an atomiser device that produces fine droplets into the main drying chamber. Atomisers vary with rotary, single fluid, two-fluid, and ultra-sonic designs. These different styles have different advantages and disadvantages depending on the application of the spray drying required.

The hot drying gas can be passed as a co-current or counter-current flow to the atomiser direction. The co-current flow enables the particles to have a lower residence time within the system and the particle separator (typically a cyclone device) operates more efficiently. The counter-current flow method enables a greater residence time of the particles in the chamber and usually is paired with a fluidised bed system.

Spray drying often is used as an encapsulation technique by the food and pharmaceutical industries. A substance to be encapsulated (the load) and an amphipathic carrier (usually some sort of modified starch) are homogenized as a suspension in water (the slurry). The slurry is then fed into a spray drier, usually a tower heated to temperatures well over the boiling point of water.

As the slurry enters the tower, it is atomized. Partly because of the high surface tension of water and partly because of the hydrophobic/hydrophilic interactions between the amphipathic carrier, the water, and the load, the atomized slurry forms micelles. The small size of the drops (averaging 100 micrometers in diameter) results in a relatively large surface area which 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 off in larger quantities at the processing temperatures. Loss is minimized industrially by spraying into taller towers. A larger volume of air has a lower average humidity as the process proceeds. By the osmosis principle, water will be encouraged by its difference in fugacities in the vapor and liquid phases to leave the micelles and enter the air. Therefore, the same percentage of water can be dried out of the particles at lower temperatures if larger towers are used.

The application of the spray drying encapsulation technique is to prepare "dehydrated" powders of substances which do not have any water to dehydrate. For example, instant drink mixes are spray dries of the various chemicals which make up the beverage. The technique was once used to remove water from food products; for instance, in the preparation of dehydrated milk.

Because the milk was not being encapsulated and because spray drying causes thermal degradation, milk dehydration and similar processes have been replaced by other dehydration techniques. Skim milk powders are still widely produced using spray drying technology around the world, typically at high solids concentration for maximum drying efficiency. Thermal degradation of products can be overcome by using lower operating temperatures and larger chamber sizes for increased residence times.

Sources
Nutritional evaluation of food processing second edition (1975), Robert S. Harris, Ph.D. and Endel Karmas Ph.D. (eds)
Keey, R.B., (1992). Drying of Loose and Particulate Materials, 1st, (pp504). Hemisphere Publishing Corporation,

External links
Marriott Walker Corporation, A World Leader in Spray Drying Technology.
Niro's expertise in spray drying.
Short overview on spray drying from Japanese manufacturer.
Spray drying on Hyfoma.com. Hyfoma.
Niro A/S - article on Spray Drying.
technalysis.us - CAE spray drying.
Anderson Custom Processing.
Delavan spray drying technologies.
Delavan's technical know how on spray drying.

See also
Spray Nozzle


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Spray Dryers
Anhydro spray dryers can be used for a wide range of applications. The spray drying process is a continuous operation in which almost any pumpable liquid can be converted into a free flowing powder.With more than 3,500 spray dryers installed worldwide, Anhydro combines experience and cutting-edge technology into added-value solutions for its customersSpray Dryers - Function and principle of operationThe liquid is dried, collected and delivered for further treatment without any intermediate manual handling. The spray drying process is applicable to a wide range of products and industries, and plant capacities from a few gr/h to 80 tons/h are available. Anhydro handles projects ranging from laboratory size to large industrial spray dryer processing lines and turnkey plants.Correct atomization and air distribution are the keys to the spray drying process, as both greatly influence the final powder structure and quality. Often, spray dryers are equipped with high-speed centrifugal atomizers ensuring sturdy and reliable operation. High-Pressure nozzle atomization is used especially for products where a rather coarse powder with narrow particle distribution and high bulk density is required. In the nozzle atomizer, fines are returned around one or more central nozzles in order to facilitate optimum agglomeration. Two-Fluid nozzle agglomeration is typically applied in small chambers allowing maximum flexibility in particle design and particle engineering for both super-fine and agglomerated particles.Benefits of Anhydro Spray DryersAnhydro spray dryers are available in a wide range of sizes and configurations providing:
Uniform qualityComplete control of moisture content, particle structure, particle size distribution, solubility, and wettability, and retention of natural aromas and flavours
Unbeatable performance/cost ratioEnergy efficient components, continuous and rapid drying, ease of operation and process automation provide complete control over yield and costs.
VersatilityCustomizable plant design based on many years of experience in a wide range of applications is your assurance of complete compliance with your individual requirements.
Long service lifeTop-quality, reliable components, efficient and straightforward CIP and expert engineering increase service life and availability for maximum performance
TraceabilityAutomated process control enables end-to-end traceability in compliance with current food standards and regulations
Special featuresAnhydro spray dryers incorporate a number of special design features including atomizers as well as the Anhydro Controlled Flow Nozzle for easy start-up and shut-down, short off-spec period, controlled particle size and easy process control.Fields of applicationLiquid-to-powder spray dryer products throughout the dairy, food, feed, chemical, pharmaceutical and environmental sectorsCapacity profileA few gr/h to 80 tons/hSpray Dryer Designs availableTraditional open-cycle systems in single stage or multi-stage modesClosed curcuit systems applying either inert gas, Low-Ox or super-heated steam configurationMore than 20 different chamber designs including conical bottom, flat bottom, Tall Form, Uniflow and High Body

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1 comment:

Anhydro said...

There also an article here
http://www.anhydro.com/content/us/products/dryers/spray_dryers