Mobile impingement zone

Figure 21.32 shows the time variation of moisture content, temperature, and location for 1 mm aluminum beads covered with a thin layer of surface water and dried in an ISD with mobile impingement zones (see Figure 21.30) [52]. It can be seen that the heat transfer rate differs significantly in the period of particle deceleration and acceleration due to the different gas-particle relative velocities. Despite the stabilizing effect of oscillatory motion, this difference can be as high as 35%-40%. Because of surface water evaporation, the drying rate is controlled by the rate of external heat transfer. Thus, it is higher in the deceleration period than in the acceleration period. The particles are completely dry within 0.6 s, that is, within less than one cycle of oscillation.

Figure 21.33 presents the drying kinetics of crystalline lysine with an initial moisture content of 15.2% in an ISD with mobile impingement zones using hot air at 120°C and flow velocity of 20-23 m/s the frequency of reversing motion is 1.0-1.2 Hz [53]. Curve 1 represents drying of monodisperse crystals (0.4 mm mean diameter) at mass concentrations of 0.2-0.5 kg/kg of air. In this case, the surface water is removed within one period of motion (2-3 s). The desired moisture... 

Coaxial ISDs with mobile impingement zone as presented in Figure 21.30 are especially suitable for drying quartz and foundry sand, metal granules, polymers, and like materials with surface or loosely bound moisture. Table 21.12 gives the characteristics of an industrial unit used for drying of metal granules. 

FIGURE 21.35 ISD with mobile impingement zone for drying of crystalline lysine. 

The constraint of reduced residence time is practically eliminated in impinging streams with a mobile impingement zone (Elperin and Meltser,... 

Figure 5.2 Coaxial ISD with a mobile impingement zone.

Interestingly, the heat transfer coefficient in impinging streams with mobile impingement zones can be calculated from the standard equation for gas flowing past a sphere with particle Reynolds number based on the gas velocity in an accelerating flow duct ... 

Figure 5.7 Temporal variation of axial position, moisture content, and particle temperature in an impinging-stream dryer with a mobile impingement zone (Tj, = 250°C, o = 30 m/s, Xi = 0.11 kg/kg).

Mobile, in which the position of the impingement plane changes periodically or continuously Geometry of impingement plane/zone... 

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