SIZING DRYERS

In order to practically evaluate a design, you need to conduct test work on either a specific manufecturer s laboratory/pilot plant unit or design and build a test unit of your own. The former case is recommended because of the obvious advantages involved in making use of the manufacturer s 

The option of building your own pilot unit may be desirable if there is much test work to be performed in research and development, but the drawback is that it may be difficult to obtain a full-scale model of your own lab design without manufacturing it through a metal fabrication shop. Usually, the best option is to select a reputable manufacturer through references and rent or purchase one of their pilot or laboratory models to conduct serious test work, which can be used to scale up to a production size model. 

After conducting test work, most manufacturers are willing to explain the internal features of the design of their unit. This may require sufficient mechanical design details to remove some of the mystique surrounding the manufacturer s design. 

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For scaling up LMOX freeze-drying production, we started with examining the maximum and minimum rate of heat supply to vials from a shelf in experimental and commercial size dryers having a batch size of 2000 and 60,000 vials, respectively. As we had known from our experience that the vial in the center tray received the minimum heating and the vial in the front tray, which was bathed in... 

The control of heal traits for by this method and the relation between vacuum level and die heat transfer coefficient is shown in Figure 22. We were able to control heal transfer in the experimental size dryer as seen in Figure 2) (A to Y) and reproduce the same situalion in the commercial size dryer. Tf we managed to opti-... 

The troubles we encountered here were predictable and not so drastic as seen in the eutectic crystalline freeze-drying of CET. As a whole, the production in the commercial size dryer went well but a small number of the vials were moved to secondary drying, holding free water locally due to the variation of freezing or drying as described before. The fortuitous occurrence of product defect such as local evacuation or local high-water-content spot was unavoidable. 

Cmmbles are formed by grinding pellets to the desired sizes. Specialty feeds such as flakes can be made by mnning newly manufactured pellets through a press or through use of a double dmm dryer. The latter type of flakes begin as a slurry of feed ingredients and water. When the slurry is pressed between the hot rollers of the double dmm dryer, wafer thin sheets of dry feed are produced that are then broken into small pieces. The different colors observed in some tropical fish foods represent a mixture of flakes, each of which contains one or more different additives that impart color. 

Sodium carbonate monohydrate crystals from the crystallizers are concentrated in hydroclones and dewatered on centrifuges to between 2 and 6% free moisture. This centrifuge cake is sent to dryers where the product is calcined 150°C to anhydrous soda ash, screened, and readied for shipment. Soda ash from this process typically has a bulk density between 0.99—1.04 g/mL with an average particle size of about 250 p.m. 

FIOR Process. In the FIOR process, shown in Figure 5, sized iron ore fines (0.04—12 mm) are dried in a gas-fired rotary dryer. A skip hoist dehvers the dry fines to lock hoppers for pressurizing. The fines pass through four fluidized-bed reactors in series. Reactor 1 preheats the ore to 760°C in a nonreducing atmosphere. Reactors 2, 3, and 4 reduce the ore at 690—780°C. At higher (ca 810°C) temperatures there is a tendency for the beds to defluidize as a result of sticking or hogging of the reduced material. 

Drum Drying. The dmm or roHer dryers used for milk operate on the same principles as for other products. A thin layer or film of product is dried over an internally steam-heated dmm with steam pressures up to 620 kPa (90 psi) and 149°C. Approximately 1.2—1.3 kg of steam ate requited per kilogram of water evaporated. The dry film produced on the roHer is scraped from the surface, moved from the dryer by conveyor, and pulverized, sized, cooled, and put iato a container. 

Suspended Particle Techniques. In these methods of size enlargement, granular soHds are produced direcdy from a Hquid or semiliquid phase by dispersion in a gas to allow solidification through heat and/or mass transfer. The feed Hquid, which may be a solution, gel, paste, emulsion, slurry, or melt, must be pumpable and dispersible. Equipment used includes spray dryers, prilling towers, spouted and fluidized beds, and pneumatic conveying dryers, all of which are amenable to continuous, automated, large-scale operation. Because attrition and fines carryover are common problems with this technique, provision must be made for recovery and recycling. 

Following carbonation, the product can be further purified by screening. This screening, also used to control the maximum size of the product, is followed by dewatering (qv). Rotary vacuum filters, pressure filters, or centrifuges are used in the mechanical removal of water. Final drying is accompHshed as with natural calcium carbonate in either a rotary, spray, or flash dryer. Products having mean particle sizes from submicrometers (- O-OS fiTo) to several micrometers are available. 

Particle size distribution determines surface-to-mass ratios and the distance internal moisture must travel to reach the surface. Large pieces thus have higher critical moisture contents than fine particles of the same material dried under the same conditions. Pneumatic-conveyor flash dryers work because very fine particles are produced during initial dispersion and these have low critical moisture contents. 

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