Dry applications

The heavy-duty jacketed type (Fig. ll-62a) is a special custom-built adaptation of a heavy-duty vibratory conveyor shown in Fig. 11-60. Its apphcation is continuously to cool the crushed materi [from about 177°C (350°F)] produced by the vibratoiy-type caster of Fig. 11-53. It does not have the liqmd dam and is made in longer lengths that employ L, switchback, and S arrangements on one floor. The capacity rate is 27,200 to 31,700 kg/h (30 to 35 tons/h) with heat-transfer coefficients in the order of 142 to 170 W/(m °C) [25 to 30 Btii/(h ft °F)]. For heating or drying applications, it employs steam to 414 kPa (60 IbFin ). 

For estimating purposes for direct-heat drying applications, it can be assumed that the average exit-gas temperature leaving the sohds bed wih approach the final solids discharge temperature on an ordi-naiy unit carrying a 5- to 15-cm-deep bed. Calculation of the heat load and selec tion of an inlet-air temperature and superficial velocity (Table 12-32) will then permit approximate sizing, provided an approximation of the minimum required retention time can be made. 

Figure 20. Process flow scheme for batch contact drying application.

Figure 32 Two special cases of radiation heat transfer with importance to freeze-drying applications, (a) Case I Body 1 is the surroundings. (b) Case II Bodies 1 and 2 are of equal areas, and e = 1 for body 2.

The continuous-spray design is ideal for any application in which there is a relatively light loading of submicron particles or condensed organics that form a submicron flame. Ordinarily, the only other piece of equipment applicable to this type of loading would be a high-energy scrubber. Since dust resistivity has no effect on the precipitator, it can be applied successfully on many very difficult dry applications. 

Molecular sieves are synthetic zeolites that can be manufactured with extremely close control of pore size. Therefore, they can be tailored to suit specific applications. In addition to gas drying applications, molecular sieves are used for the separation of gases and vapors on the basis of molecular size and shape. Surface areas range from 350 to 1000 ft2/ft3. 

Vibrofluidization It is possible to fluidize a bed mechanically by imposing vibration to throw the particles upward cyclically. This enables the bed to operate with either no gas upward velocity or reduced gas flow. Entrainment can also be greatly reduced compared to unaided fluidization. The technique is used commercially in drying and other applications [Mujumdar and Erdesz, Drying Tech., 6, 255-274 (1988)], and chemical reaction applications are possible. See Sec. 12 for more on drying applications of vibrofluidization. 

Rotating Nozzle (rotary atomizer) fluid is fed at low pressure to the center of a rapidly rotating disk and centrifugal force breaks up the fluid. These types of nozzles are used mainly in a spray drying application. 

Bhandari, B.R., Dumoulin, H.M.J., Richard, H.M.J. (1992). Flavor encapsulation of spray drying application to citral and linalyl acetate. Journal of Food Science, 51, 1301— 1306. 

Other Liquid and Gas Drying. Applications of zeolite adsorbents in drying other industrial gases and liquids are well known and have been discussed in Refs. 34-36. Therefore, although it is an important application, it is not discussed here. 

The process conditions, such as fluid composition and temperature, often have a far greater effect on the maximum useable life than the normal dry mechanical properties of the material. A typical change is from 120°C maximum temperature for dry applications to 75°C for wet applications. 

Erosion studies have shown that the softness and resilience of elastomers handle wear better in a number of cases than hard metals (Ephithite, 1985). In dry applications, the heat dissipation is poor and can result in rapid degradation of the elastomer. However, in slurry applications the lubrication of the water changes the friction resistance greatly. 

In air-drying applications, it is important to operate the modules in a counterflow mode, usually with a small sweep flow from the residue gas. Some calculations illustrating the importance of counterflow and counterflow/sweep operation are shown in Figure 8.5. 

A recently7 developed drying application for zeolites is the prevention of corrosion in mufflers (52,55). Internal corrosion in mufflers is caused primarily7 by7 the condensation of water and acid as the system cools. A unique UOP zeolite adsorption system takes advantage of the natural thermal cycling of an automotive exhaust system to desorb the water and acid precursors. 

Dust entrained in the exit-gas stream is customarily removed in cyclone collectors. This dust may be discharged back into the process or separately collected. For expensive materials or extremely fine particles, bag collectors may follow a cyclone collector, provided fabric temperature stability is not hmiting. When toxic gases or solids are present, the exit gas is at a high temperature, the gas is close to saturation as from a steam-tube dryer, or gas recirculation in a sealed system is involved, wet scrubbers may be used independently or following a cyclone. Cyclones and bag collectors in drying applications frequently require insulation and steam tracing. The exhaust fan should be located downstream from the collection system. 

Table 12-23 outlines typical performance data from three drying applications in steam-tube dryers. 

In heating or drying applications, when cooling of the product is desired before discharge to the atmosphere, cool air is blown through a second annular space, outside the inlet hot-air annulus, and released through the louvers at the solids-discharge end of the shell. 

As a textile hand modifier, 0.5 to 2% of INTEX EMULSION 195 is applied by conventional pad/dry application depending on the degree of softness and slickness desired. 

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