Microwave dryers

Dielectric and Microwave Dryers. Dielectric, also caEed radio frequency, dryers operate in the frequency range of 1—100 MHz. Microwave dryers in the United States operate at 915 MHz and 2450 MHz (see Microwave technology). As depicted in Figure 24, a dielectric dryer may consist of two... 

Given that the introduction of solvent obviously produces wet granules, the wet granulation process includes a drying step. Drying typically occurs in a fluid bed dryer. But there are other options, such as microwave vacuum dryers. In 1994, White applied on-line NIRS to monitor moisture content and predict drying end point in two TK Fielder microwave dryers. The NIR spectral data were correlated to off-line Karl Fischer measurements which resulted in a standard error of prediction equal to 0.6% when the samples... 

Although microwave dryers may differ from one another in vessel geometry and the way microwaves are directed into the solids, no indirect radiant heating, moving solids bed dryer subclasses have been identified. 

Radiant Microwave, moving solids bed Microwave dryer... 

On-line moisture detection for a microwave vacuum dryer was the topic of a publication by White [16]. The paper described a method for determining the moisture endpoint in a microwave dryer using NIR spectroscopy. White s calibration equation used NIR absorbances measured at 1410, 1930, and 1630 nm. The NIR-predicted results were within 1% of the Karl Fischer reference values for samples with less than 6% moisture, providing a standard error of prediction (SEP) of... 

Capital equipment costs refer to the cost of the equipment and are sometimes a bit difficult to define. For example, a microwave drying system may increase the throughput of a product dramatically and thereby necessitate the purchase of additional packaging equipment, conveyors, feed systems, and the like. In that case, the overall capital outlay would be much higher than for the microwave dryer alone. 

Thermal dryers are sometimes used for predrying, but are almost always used for final drying because of the limitations of mechanical dryers. After mechanical predrying, much of the remaining water is chemically bonded to the fiber and must be evaporated. This is accomplished using several types of thermal system such as heated cans, convection ovens, and radio frequency (RF) dryers. Recently, microwave dryers have been designed for drying textiles and carpet tiles [5]. 

Although RF and microwave dryers have been used by the textile industry, the market penetration of RF dryers is much greater. The cost of the equipment has been a barrier to the use of both types of dryers, and uniformity of treatment has been a problem associated with microwave drying. As RF... 

The frequency and wavelength bands of commercially available radiant dryers are shown in Table 39.4. This table gives broad ranges for the dryers but in practice almost all IR dryers are in near-infrared range (1-10 pm), and the specific frequencies for RF and microwave dryers are allocated by international agreement, such as 13.56, 27.12, and 40.68 MHz for RF and 433.92, 896, 915, and 2450 MHz in the microwave region. 

Figure 31.16 Schematic of the experimental microwave dryer 1 —motor, 2—MW generator, 3—resonance tube, 4— spiral insert (IV section), 5—viewport, 6—quartz drum, 7—spiral insert (I section), 8—water load, 9—condenser, 10— product tank, 11—swivel, 12—slope adjuster, 13—sol tank, 14—distilled water, 15—condensate tank, 16—condensate discharge. (From Bessarabov et ah, 1999.)... 

In this section, electrical drying methods are briefly discussed - examples of applications are discussed in Chapter 10. As an example, the industrial microwave dryer exhibits similar benefits to the domestic microwave oven - the energy is taken directly to the inside of the object(s) being heated, rather than having to be conducted from the surface of the particle or body. Microwave drying can be combined with conventional (hot air) drying to finish off the process, and a combination of... 

Figure 12 Overview on granulation process consisting of extruder/granulator, microwave dryer and mill.

In contrast to the microwave dryer in a contact dryer it is very important to assure constant material flow and thus constant holding times. Therefore, the contact dryer is equipped with a spiral inside the drying pipe to forward the material with a constant speed. Additionally, the temperature of the pipe is determined at all interesting spots. 

Main advantages of this process are the excellent blending quality and temperature control associated with low energy requirements for granulation and drying and a high efficiency of the microwave dryer. 

As an example, consider the drying of bread crumbs from 27 to 5% moisture, at a rate of 1000 lb of wet bread crumbs per hour. In such a system, it would be necessary to evaporate 231.6 lb of water per hour (1000 lb of bread crumbs contain 270 lb of water, but when dried to 5% contain only 38.4 lb of water). This would require about 77 kW to dry (231.6/3.0), plus an additional 20 kW to heat to the drying temperature, for 97 kW, neglecting all heat losses. If we assume this system to have a coupling efficiency of 75% (that is, the efficiency of coupling microwaves into the product), then a system of 130 kW is required, which would cost approximately one million dollars. On the other hand, if a conventional hot air dryer is used to reduce the moisture from 27 to 12% and the microwave dryer to finish drying it to 5%, then the amount of water to be evaporated by the microwave system is only 61.6 lb, which requires 20.4 kW (61.1/3.0). Since the product is already hot, we need to only increase the output to 27 kW to account for the 75% coupling efficiency (20.4/0.75). Such a system would cost of the order of 200,000 plus the cost of the less expensive conventional dryer. 

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