Vacuum dryers, described

Equipment commonly employed for the drying of solids is described both in this subsection in Sec. 12, where indirect heat transfer devices are discussed, and in Sec. 17 where fluidized beds are covered. Dryer control is discussed in Sec. 8. Excluding fluid beds this subsection contains mainly descriptions of direct-heat-transfer equipment. It also includes some indirect units e.g., vacuum dryers, furnaces, steam-tube dryers, and rotary calciners. 

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... 

A typical Mazzoni fatty acid neutralization soap plant is illustrated in Fig. 36.5.6 The operation of this plant involves the pumping of the reactants through preheaters to a turbodisperser, or high-shear mixer, where the contact of the reactants with each other initiates the formation of neat soap. The soap mass, which is partially reacted at this stage, then proceeds to the mixer where it is recirculated until the neutralization is complete. The completion of the neutralization reaction is monitored continuously by an electric potential (millivolt, mV) measurement for alkalinity. The neat soap is then dried by vacuum spray-dryers, as described earlier, to produce soap pellets ready for finishing into soap bars. 

Slurries of Tree-filtering solids that are difficult to suspend are sometimes filtered on a top-feed drum filter or filter-dryer. An example application is in the production of table salt. An alternative for slurries of extremely coarse, dense solids is the internal drum filter. In the chemical-process industry both top-feed and internal drums (which are described briefly by Emmett in Schweitzer, op. cit., p. 4-41) have largely been displaced by the horizontal vacuum filter (q.v.). 

This process essentially follows the principle just described, with an applied vacuum facilitating drying by lowering the boiling point of the solvent. If the solvent is water, a hygroscopic material such as phosphorus pentoxide is normally put in the dryer to absorb water as it comes off the sample. The sample can be heated as necessary. 

While the sensors described in the previous paragraph allow monitoring of only single vials, a balance placed directly in the vacuum chamber of the freeze-dryer allows monitoring of a group of vials the direct weight measurement enables the... 

Dryers can be continuous or batch operated. Batch dryers are less capital cost intensive and flexible. Major categories of dryers fall under four categories tray or shelf dryers, batch vacuum rotary dryers, freeze dryers, and spray dryers. The following paragraphs describe these different dryers, design and operation. 

---