Microwave-vacuum dryers

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... 

J.G. White, Online moisture detection for a microwave vacuum dryer, Pharm. Res., 11(5), 728-732 (1994). 

Double jacket, vacuum/gas assistance (AV model Aerovac Vacuum Ehyer) microwave/vacuum dryer (SP model)... 

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... 

The various modes in which microwaves are used in the industry comprised of booster (microwave-connection) and dryers (microwave vacuum dryers and microwave freeze dryers). Microwave vacuum drying of cranberries was investigated using laboratory-scale dryer operating either in continuous or pulsed mode [38]. Pulsed application of micro-wave energy was found to be more efficient than continuous application. 

Rgure 26.5 Cross-view of the microwave-vacuum dryer with revolving rack. (From Anon., 1995c.)... 

Figure 9.6 shows a laboratory microwave-vacuum dryer on the basis of hexahedron of the cavity with the dimensions of 360 mm x 340 mm x 260 mm. 

Fig. 9.6 Schematic of a laboratory microwave-vacuum dryer. 1, Vacuum pump 2, Rotating tray 3, Valve 4, Observation window 5, Drying chamber 6, Infrared thermometer 7, Vacuum gage 8, Magnetron power control unit 9, Magnetron.

To date, only microwave hot-air dryers used principally in flnish drying processes to level-off the moisture content in pasta, crackers or chips, and microwave vacuum dryers custom-made to specific product requirements, are available commercially (Ratti, 2008). Good microwave-assisted drying equipment should have a uniform distribution of the microwave field, provide a movement of the material to be dried, keep the microwave field strength in the drying chamber lower than the dielectric breakdovm strength, and possess an economic dehumidification system. 

According to a model developed by Cui et al. (2005), the temperature of sliced carrot that has been dried in a microwave-vacuum dryer can be predicted by the following equations ... 

Bremecker (1983) ttsed microwave to measure the moisture conterrt in the pharmaceuticals. Moisture can be detected online for microwave vacuum dryer itsing near Irtfrared Spectroscopy (NIS) of cottrse, if the moisture content of product is 6% or less. Thus, drying can be made a continuous process with precise control of moistrrre (White, 1994). 

As we explained in Chapter 12, motors run many devices and equipment that make our lives comfortable and less laborious. As an example, we identified a huge number of motors in various devices at home, including motors that run the compressor of your refrgerator, garbage disposer, exhaust fims, tape player in a VCR, vacuum cleaner, turntable of a microwave, hair dryer, electric shaver, computer fan, and computer hard drive. When selecting motors for these... 

Fig. 9.8 A microwave-vacuum rotary drum dryer. Adopted from Kaensupeta/. (2002).

Experimental study on drying of chilli in a combined microwave-vacuum rotary drum dryer. Drying Technol. 20(10) 2067-2079. 

Poska (1991) and Mandal (1995) have shown the comparability of physicochemical characteristics of grantrles dried in microwave vacuum processors as compared to tray dryers as well as fltrid bed dryers. 

Freeze drying has also been carried out at atmospheric pressure in fluid beds using circulating refrigerated gas. Vacuum-type vibrating conveyors, rotating multishelf dryers and vacuum pans can be used as can dielectric and microwave heating. 

Some dryers also provide heat energy to the powder mass by a jacketed vessel, thereby increasing overall heat transfer. Moisture can be removed via vacuum or hot air fluidization depending on the design of the dryer allowing for improved evaporative drying and vapor mass transfer. Fig. 13 shows the relationship between power input (W) and first-order drying rate constant in a microwave fluid-bed processor. ... 

Thermal drying caused by the vaporization of the liquid results as heat is supplied to the wet feedstock. As noted earlier, heat may be supplied by convection (direct dryers), conduction (contact or indirect), radiation, or volumetrically by placing the wet material in a microwave or radio frequency (RF) electromagnetic field. Over 85% of industrial dryers are of the convection type, with hot air or direct combustion gases as the drying medium. Over 99% of the application involves removal of water. All modes except the dielectric (microwave and radio frequency) supply heat at the boundaries of the drying object so that the heat must then diffuse into the solid primarily by conduction. The liquid must travel to the boundary of the material before it is transported away by the carrier gas (or by application of vacuum for nonconvective dryers). 

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