Flash and Ring Dryers

Pilot plant work is essential as a basis for full scale design. It may be directed to finding suitable velocities, temperatures and drying times, or it may employ more basic approaches. The data provided for Example 9.8, for instance, are of particle size distribution, partial pressure of water in the solution, and heat and mass transfer coefficients. These data are sufficient for the calculation of residence time when assumptions are made about terminal temperatures. 

Flash dryers are the simplest gas suspension pneumatic dryers and require the least amount of space. The basic components of a flash dryer are an air heater with a gas duct, a vertical drying column (flash tube) with an expansion joint, a feed conveyor, a venturi feed section, a cyclone collector with a discharge valve, and a system fan (Papagiannes, 1992). A fan pushes air through a heater and into the bottom of the flash tube. The wet feed enters the tube 

Various temperature-sensitive materials may be processed in this type dryer because of short material retention times and proper temperature control. Typical drying times are on the order of a few seconds. Flash dryers are most useful for moist, powdery, granular, and crystallized materials, including feeds that are wet and that are discharged from filtration equipment (Christiansen and Sardo, 2001). Because of the rapid drying process, they are often used to remove surface water, but they are not suitable for diffusion-controlled drying. Particle size of the product material is small, usually less than 500 microns, and the most suitable feed is that which can be fried, rather than a sticky material. 

Flash dryers have several advantages over more complex gas-suspension dryers such as fluid-bed or rotary dryers. They are relatively simple and take up less space, as noted earlier, hence, they require a lower capital investment. 

---

One of the innovative technologies for atomizing viscous liquids and thin pasty materials in a gas is the use of shock waves. This method could be used in spray drying to disperse the liquid feed (see Chapter 14) and thus eliminate the conventional atomizer from the system. This wiii avoid probiems with erosion in disk atomizers and clogging of singie- or two-stream spray nozzies. Potentiai applications of this idea in other types of dryers for dispersed materials include pneumatic, ring, spin-flash, and vortex dryers, in which the momentum of shock waves can enhance the hydrodynamic impact of a conventional gas-carrier. 

Ring Dryers The ring diyer is a development of flash, or pneumatic-conveyor, diying technology, designed to increase the versatility of application of this technology and overcome many of its limitations. 

The essential difference between a conventional flash dryer and the ring dryer is the manifold centrifugal classifier. The manifold pro-... 

Figure 9.12. Examples of pneumatic conveying dryers corresponding performance data are in Table 9.13. (a) Raymond flash dryer, with a hammer mill for disintegrating the feed and with partial recycle of product (Raymond Division, Combustion Engineering), (b) Buttner-Rosin pneumatic dryer with separate recycle and disinte ation of large particles (Rosin Engineering Ltd.), (c) Berks ring dryer the material circulates through the ring-shaped path, product is withdrawn through the cyclone and bag filter [Pennsalt Chemical Co.).

Barr Murphy Ltd. Victoria Ave. Westmount, Quebec H3Z 2M8 Continuous Flash Ring and Spray Dryers... 

---