Temperature microwave drying

The Cs structure and dimensions (Fig. 17.26b) were established by microwave spectroscopy which also yielded a value for the molecular dipole moment p. 1.72D. Other physical properties of this colourless gas are mp -115° (or -123°), bp -6°, A//f(g,298K) —34 10kJmol [or — 273kJmol when corrected for A//f(HF, g) ]. FCIO2 is thermally stable at room temperature in dry passivated metal containers and quartz. Thermal decomposition of the gas (first-order kinetics) only becomes measurable above 300° in quartz and above 200° in Monel metal ... 

Commercial dryers differ fundamentally by the methods of heat transfer employed (see classification of dryers. Fig. 12-45). These industrial-dryer operations may utilize heat transfer by convection, conduction, radiation, or a combination of these. In each case, however, heat must flow to the outer surface and then into the interior of the solid. The single exception is dielectric and microwave drying, in which high-frequency electricity generates heat internally and produces a high temperature within the material and on its surface. 

If conduction or radiation is the predominant mode of heat transfer, the surface (and possibly the interior) moisture may literally boil regardless of the temperature or the humidity of the environment. This may be readily demonstrated by microwave drying. Thus, if control of granulation temperature is important, direct heat (convection) dryers usually offer greater control and product safety since the material s surface does not exceed the wet-bulb temperature during the steady state period. However, it will be shown later in this chapter that properly controlled dielectric drying may also be used to dry heat sensitive materials. 

Microwave drying may be monitored by. sensing the reflected power. Free solvent couples with the microwave energy and as the amount of solvent is reduced, the measured electric field increases as does the batch temperature. These factors can be calibrated to detect an endpoint. 

The clay mineral or mixture was reacted with the aqueous NaOH solution for about 20 minutes at room temperature and then was irradiated in a Microwave Drying/Digestion System (Model MDS-81, GEM Corporation, P. 0. Box 9, Indian Trail, N.C. 28079) at 500 watts (2.45 GHz) for 30 seconds to 3 minutes. The sample was then filtered, water washed, and air dried. 

Product cannot be produced by any other way. Again, the careful control of temperature combined with the unique manner in which microwave and dielectric energy couple into materials allows the drying of extremely thermolabile materials with no damage to the product. Occasionally, a unique beneficial effect may be obtained, such as the slight puffing of the pasta noodles when they are microwave dried. This allows them to be cooked more quickly. 

Another advantage of microwave is an increase in the rate of killing of bacteria due to the speedy temperature rise. Microwave drying under vacuum is becoming an established method of drying citrus fruit concentrates. 

Antti, A.L., 1992. Microwave drying of hardwood Simultaneous measurements of pressure temperature and weight reduction. Forest Prod. J., 42(6) 49-54. 

The qualitative difference in distribution of temperature, moisture content, and the drying-induced stresses in materials under convective and microwave drying is illustrated on the examples of cylindrical kaolin samples, which are assumed here to be elastic or viscoelastic. 

Figure 62.24 shows the drying curve (Figure 62.24a) and the effective and admissible stresses (Figure 62.24b) in an elastic material for microwave drying at P = 231.3 W with assumed material surface temperature = 40°C. 

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