Spherical dryers

Spherical dryers have recently become more popular in the drying of APIs, one reason being that hold-up of API in the discharge process is less than that with other types of drier. 

Spherical dryer Sometimes called the turbo, here, this is another agitated dryer with a vertical axis mixing shaft, but rotation is typically faster than in the vertical pan unit, giving improved mixing and heat transfer. The dryer chamber is spherical, with solids discharge through a door or valve near the bottom. 

The paddle with heated agitator has the shortest drying time, and the filter dryer the longest (because the bottom plate is unheated). Other types are fairly comparable. The spherical dryer would usually have a higher heat-transfer coefficient and shorter drying time than shown. 

An important application of heat transfer to a sphere is that of conduction through a stationary fluid surrounding a spherical particle or droplet of radius r as encountered for example in fluidised beds, rotary kilns, spray dryers and plasma devices. If the temperature difference T[ T2 is spread over a very large distance so that r2 = oo and 7 t is the temperature of the surface of the drop, then ... 

Spherical droplet in a liquid, mass transfer 617 Spiral heat exchangers 550 Spray dryers 393... 

The heart of the spray dryer is the atomizer where the spherical droplets are initially produced. The atomizers can be classified into three different categories ... 

Figure 14-5 shows a photograph of a good powder produced in a small (1 m ) laboratory spray dryer. We see that it consists of spherical glassy particles, of which the largest have a diameter of about 50 pm. Many of the particles are hollow - we come back to that in a moment. The flow properties of spherical particles are expected to be good if ... 

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... 

Vertical Agitated Dryers This classification includes vertical pan dryers, filter dryers, and spherical and conical dryers. 

Formation of nearly spherical lumps from a wet mixture of particulate solids by either drying or chemical reaction during tumbling typically accomplished in rotary dryers or kilns. 

The above information is all that is required to perform a preliminary sizing on a spray dryer. The temperature limit of the product is given at 170°F. Using 160 F as the outlet temperature of the dryer should allow us the safety necessary so as not to exceed the temperature limit of the dry powder (product). The fact that we get evaporative cooling while drying the spherically shaped droplets to a similarly shaped powder allows us to use an inlet temperature of about 320°F. Thus the temperature difference (AT) is 160°F. This is the primary driving force in motivating the water to leave the product due to the enormous difference in saturation equilibrium between the wet droplet/dry powder and the very hot dry air. 

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