Spray solidification

Several methods are known that convert droplets, formed from a melt, into solid granular products by cooling. These processes are called prilling (see Section 

In a single-stage spray dryer the process is finished when most of the moisture in the pore space has dried. The agglomerates accumulate in the lower part of the spray-drying chamber and are removed by the suction of a fan driving a dust-collection system. The agglomerates are collected in a cyclone while dust is collected in a wet scrubber. Material-laden scrubber water may be recirculated and mixed with the liquid feed. 

The combined process air exits through outlet openings in the top of the chamber. Particles still entrained in the gases are separated in a cyclone. The material collected in the cyclone can either be removed from the system for direct use or is recirculated into the fluidized bed for further agglomeration. 

The main differences in product characteristics between spray-dried and fluid bed agglomerated materials are  

light (often hollow), spherical with relatively smooth surface Larger, denser, irregularly formed with relatively rough surface 

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In the true sense of the meaning, spray solidification is not an agglomeration method (see also Section 4.2.3.3). However, because the process can be carried out in spray drier-granulators (see above) and the resulting granular material may be of special interest in pharmaceutical processing it will be mentioned at the end of this section. 

Spray solidification is used to imbed powdered drugs in fat or wax. For this purpose, the solids are suspended in the molten matrix and the liquid... 

Suspended Particle Techniques. In these methods of size enlargement, granular soHds are produced direcdy from a Hquid or semiliquid phase by dispersion in a gas to allow solidification through heat and/or mass transfer. The feed Hquid, which may be a solution, gel, paste, emulsion, slurry, or melt, must be pumpable and dispersible. Equipment used includes spray dryers, prilling towers, spouted and fluidized beds, and pneumatic conveying dryers, all of which are amenable to continuous, automated, large-scale operation. Because attrition and fines carryover are common problems with this technique, provision must be made for recovery and recycling. 

During the formation of a spray, its properties vary with time and location. Depending on the atomizing system and operating conditions, variations can result from droplet dispersion, acceleration, deceleration, coUision, coalescence, secondary breakup, evaporation, entrainment, oxidation, and solidification. Therefore, it may be extremely difficult to identify the dominant physical processes that control the spray dynamics and configuration. 

Some concerns directly related to a tomizer operation include inadequate mixing of Hquid and gas, incomplete droplet evaporation, hydrodynamic instabiHty, formation of nonuniform sprays, uneven deposition of Hquid particles on soHd surfaces, and drifting of small droplets. Other possible problems include difficulty in achieving ignition, poor combustion efficiency, and incorrect rates of evaporation, chemical reaction, solidification, or deposition. Atomizers must also provide the desired spray angle and pattern, penetration, concentration, and particle size distribution. In certain appHcations, they must handle high viscosity or non-Newtonian fluids, or provide extremely fine sprays for rapid cooling. 

Although the continuous casting of steel appears deceptively simple in principle, many difficulties are inherent to the process. When molten steel comes into contact with a water-cooled mold, a thin soHd skin forms on the wall (Eig. 10). However, because of the physical characteristics of steel, and because thermal contraction causes the skin to separate from the mold wall shortly after solidification, the rate of heat abstraction from the casting is low enough that molten steel persists within the interior of the section for some distance below the bottom of the mold. The thickness of the skin increases because the action of the water sprays as the casting moves downward and, eventually, the whole section solidifies. 

In both atomization modes, as thin unstable ligaments, and/ or sheets disintegrate into round droplets, atomization gas may plausibly be trapped into the droplets under certain conditions. For alloys with alloying elements which readily react with atomization gas, for example, oxidize to form refractory oxides, solidification may be delayed and spheroidization is prevented so that rough flakes may form. For such alloys, the atmosphere in the spray chamber must be inert and protective to avoid the formation of any refractory and to foster spheroidal shape of droplets. 

Sobolev et al)5111 conducted a series of analytical studies on droplet flattening, and solidification on a surface in thermal spray processes, and recently extended the analytical formulas for the flattening of homogeneous (single-phase) droplets to composite powder particles. Under the condition Re 1, the flattening ratios on smooth and rough surfaces are formulated as ... 

A spray is a turbulent, two-phase, particle-laden jet with droplet collision, coalescence, evaporation (solidification), and dispersion, as well as heat, mass and momentum exchanges between droplets and gas. In spray modeling, the flow of gas phase is simulated typically by solving a series of conservation equations coupled with the equations for spray process. The governing equations for the gas phase include the equations of mass, momentum and energy... 

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