Plate dryer

In paste drying the leather is spread on glass or porcelain plates and held in place with a low strength water-soluble paste. The plates are on a conveyor and the drying is done in a drying tunnel. The dryer usually has several temperature- and humidity-controUed zones to control the rate of drying and to prevent overdrying. 

Active Dry Yeast (ADY). The production of active dry yeast is very similar to the production of compressed yeast. However, a different strain of yeast is used and the nitrogen content is reduced to 7% of soHds compared with 8—9% for compressed yeast. The press cake made with the active dry yeast strain is extmded through a perforated plate in the form of thin strands with a diameter of 2—3 mm and a length of 3—10 mm. The strands are dried on endless belts of steel mesh in drying chambers (a continuous process) or in roto-louvre dryers (a batch process), with the temperature kept below 40°C. Drying time in drying chambers is 3—4 h and in roto-louvre dryers is 6 h or more. The final moisture level attained is 7.5—8%. 

Instant Active Dry Yeast. Instant ADY (lADY or HADY) production is similar to ADY production but requires a different strain of yeast. After pressing, the yeast is extmded into noodles 0.2—0.5 mm in diameter and 1—2 cm long and deposited on a metal screen or perforated plate in a fluid-bed air dryer. Drying time is shorter than with ADY, about 1—2 hours in practice, with a final moisture level of 4—6%. Instant active dry yeast does not require separate rehydration. It is always packaged in a protective atmosphere or under vacuum. On an equivalent soHds basis, the activity of lADY is greater than that of regular ADY, but stiU less than that of compressed yeast. 

Drum Dyers. Indirect-heat dmm dryers, like spray dryers, are usable only for materials that are fluid initially and pumpable. Drying is effected by applying a thin film of material onto the outer surface of a rotating heated dmm using appHcator roUs, spray nozzles, or by dipping the dmm into a reservoir. Usually the dmm is cast iron or steel and chrome-plated to provide a smooth surface for ease of product release by doctoring. Dmm rotational speed is such that... 

Performance Data for Direct-Heat Tray Dryers A standard two-truck diyer is illustrated in Fig. 12-48. Adjustable baffles or a perforated distribution plate is normally employed to develop 0.3 to 1.3 cm of water-pressure drop at the wall through which air enters the truck enclosure. This will enhance the uniformity of air distribution, from top to bottom, among the trays. In three (or more) truck ovens, air-reheat coils may be placed between trucks if the evaporative load is high. Means for reversing air-flow direction may also be provided in multiple-truck units. 

Vacuum-Shelf Dryers Vacuum-shelf diyers are indirec t-heated batch diyers consisting of avacuumtight chamber usually constructed of cast iron or steel plate, heated, supporting shelves within the chamber, a vacuum source, and usually a condenser. One or two doors are provided, depending on the size of the chamber. The doors are sealed with resihent gaskets of rubber or similar material (Fig. 12-49). 

Steam-heated air is the usual heat-transfer medium employed in these dryers, although combustion gases may be used also. Temperatures above 600 K are not usually feasible because of the problems of lubricating the conveyor, chain, and roller drives. Recirculation of air is in the range of 60 to 90 percent. Conveyors may be made of wire-mesh screen or perforated-steel plate. The minimum practical screen opening size is about 30 mesh. 

The plate dryer may vary in size from 5-35 vertically stacked plates with a heat-exchange area between 3.8-175 m". The largest unit available has overall dimensions of 3 m (w) by 4 m (1) by 10 m (h). Depending upon the loose-bulk density of the material and the overall retention time, the plate dryer can process up to 5,000 kg/hr of wet product. 

The plate dryer is limited in its scope of apphcations only in the consistency of the feed material (the products must be friable, free flowing, and not undergo phase changes) and diying temperatures up to 320°C. Applications include speci ty chemicals, pharmaceuticals, foods, polymers, pigments, etc. Initial moisture or volatile level can be as high as 65 percent and the unit is often used as a final dryer to take materials to a bone-dry state, if necessary. The plate dryer can also be used for heat treatment, removal of waters of hydration (bound moisture), solvent removal, and as a product cooler. 

The vacuum plate drwer is provided as pari of a closed system. The vacuum dryer has a cylindrical housing and is rated for fiill-vacuum operation (typical pressure range 3-27 kPa absolute). The exhaust vapor is evacuated try a vacuum pump and is passed through a condenser for solvent recovery. There is no purge-gas system required for operation under vacuum. Of special note in the vacuum-drying system... 

FIG. 12-78 Indirect-heated continuous plate dryer for atmospheric, gastight, or fuU-vacuum operation. (Krauss Maffei.)... 

Comparison Data—Plate Dryers Comparative studies have been done on products under both atmospheric and vacuum drying conditions. See Fig. 12-79. These curves demonstrate (1) the improvement in drying achieved with elevated temperature and (2) the impact to the drying process obtained with vacuum operation. Note that cui ve 4 at 90°C, pressure at 6.7 kPa absolute, is comparable to the atmospheric cui ve at 150°C. Also, the comparative atmospheric cui ve at 90°C requires 90 percent more diying time than the vacuum condition. The dramatic improvement with the use of vacuum is important to note for heat-sensitive materials. 

TABLE 12-30 Plate Dryer Performance Data for Three Applications... 

Vibrating-conveyor dryers are suitable for free-flowing solids containing mainly surface moisture. Retention is limited by conveying speeds which range from 0.02 to 0.12 m/s. Bed depth rarely exceeds 7 cm, although units are fabricated to carry 30- to 46-cm-deep beds these also employ plate and pipe coils suspended in the bed to provide additional heat-transfer area. Vibrating dryers are not suit le for fibrous materials which mat or for sticky sohds which may ball or adhere to the deck. 

Agitated/Monoplate Nutsche Filter/Dryer Horizontal Plate Filter/Dryer Continuous Equipment Rotary Drum Filter Centrifugal Filter Horizontal Belt Filter... 

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