Spray dryers product density

In a spray dryer, the feed material characteristics, in combination with the type of feed atomization, affect the surface characteristics, shape, density, and particle size of the product. Thin-shelled particles may shatter when they come in contact with high temperature drying gases or if the particles impact the walls and fittings in the ductwork. Of course, shattered particles are not usually a desired product (Papiagonnes, 1992). 

The initial and perhaps most critical design assumption is that both the MgO and the fly ash, which are carried out of the calciner and into the MgO product cyclone, have the same particle size distribution and density. This design (i.e., no possibility of physical separation of MgO and fly ash) is a conservative design assumption and adds complexity to the FGD process. It results in the need to recirculate large quantities of the MgO/fly ash mixture through the spray dryer and the calciner. In order to keep the MgO/ash recycle streams to a reasonable size, the mechanical collectors in the main flue gas ducts upstream from the SO2 absorber, which were used in the initial design because they are relatively inexpensive but yet remove only 80% of the fly ash, had to be replaced with the 95% efficient ESP mentioned earlier. 

The choice of dryer is more difficult when the drying step is also used to shape the product. Typical designs here are the spray dryer and the sprayed fluidized bed. If necessary, drying can be followed by a classification step in which undesired particles, for example, fines, are reyded to the feed stream for the dryer. When the desired product properties, for example, freedom from dust, pourabUity, rate of dissolution, and bulk density, are not attainable in the drying step, additional steps such as compaction and granulation must be used. 

Since the choice of the atomizer is very crncial, it is important to note the key advantages and limitations of different atomizers (centrifngal, pressnre, and pneumatic atomizers). Other atomizers, e.g., ultrasonic atomizer, can also be nsed in spray dryers (Bittern and Kissel 1999) but they are expensive and have rather low capacity. Although different atomizers can be used to dry the same feedstock, the final product properties (bulk density, particle size, flowability, etc.) are quite different and hence a proper selection is necessary. 

Most food-processing companies use spray dryers to produce powdered products. Spray drying has the ability to handle heat-sensitive foods with maximum retention of their nutritive content. The flexibility of spray-dryer design enables powders to be produced in the various forms required by consumer and industry. This includes agglomerated and nonagglomerated powders having precise particles size distribution, residual moisture content, and bulk density. As examples, spray drying of milk, tomato juice, tea extracts, and coffee is discussed. 

A typical arrangement of an open-cycle spray dryer for pharmaceuticals is shown in Figure 33.10. Gas-liquid nozzles are commonly used to spray feeds that are solutions. However, in the production of certain pharmaceuticals, such as antibiotics, the powders obtained by spray drying low-concentration aqueous solutions have low bulk density. Higher bulk density can be obtained if the feed is partly precipitated. High bulk density antibiotics are produced... 

Products produced using fluidized spray drying have a broader particle size distribution and lower bulk density than the particles produced by conventional spray dryers with a typical mean size particle size range of 150 00 pm. This process is not meant to replace conventional spray drying processes but instead is a feasible alternative for spray drying applications that require larger mean particle sizes. 

Liquid-form starting materials are commonly dried to produce a free-flowing, low bulk density powder with a size distribution using a spray dryer, or to a higher bulk density, flaky product using a drum dryer. Numerous variants of both dryer types are available, necessary... 

Spray. Direct type, continuous operation. Rotary atomizer, pressure nozzle, or two-fluid nozzle. Includes combined spray-fluid bed and spray-belt dryers Suited for large capacities. Product is usually powdery, spherical, and free-flowing. High temperatures can sometimes be used with heat-sensitive materials. Products generally have low bulk density. See comments under Liquids. Pressure-nozzle atomizers subject to erosion Requires special pumping equipment to feed the atomizer. See comments under Liquids. Not applicable unless feed is pumpable Not applicable Not applicable Not applicable Not applicable... 

Use countercurrent spray drying for products which are not heat-sensitive, but may require some degree of heat treatment to obtain a special characteristic, i.e., porosity or bulk density. In this case the final powder temperature may be higher than the dryer outlet temperature. 

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