Continuous operations drying

Fluidized bed dryers work best on particles of a few tenths of a mm dia, but up to 4 mm dia have been processed. Gas velocities of twice the minimum fluidization velocity are a safe prescription. In continuous operation, drying times of 1-2 min are enough, but batch drying of some pharmaceutical products employs drying times of 2-3 hr. 

A variety of measures must be taken to assure safe and continued operation. Because the natural water table is higher than the coal seams, or the seams are natural aquifers, it is necessary to pump water out of the pit or to drill weUs around the mine and pump to reduce the water table. The Rheinische Braunkohlenwerke (Rheinbraun) pumps water at a rate of 1-1.2 x 10 m /yr. Part of this water is processed to provide drinking water for Neuss and Dusseldorf. The tendency of lignite to ignite spontaneously requires care in the amount of face that is exposed, especially in naturally dry, hot, windy climates. 

The dmm dryer is a continuously operated unit that uses rotating heated dmms for the evaporation contact area. This method of drying can be used with materials high in volatiles. In several designs, the volatiles can be withdrawn and coUected for reuse, as appropriate. 

A continuously operating pneumatic conveying dryer is used for appHcations similar to the tray and compartment dryer. In this case, however, drying is performed in conjunction with grinding, as the soHds are conveyed and dried within the unit. 

Operation is continued without Dryers ore designed to oper- Drying is occomplished by Botch indirect dryers ore gen-... 

Indirect type, batch or continuous operation for pharmaceuticals such as peniciUin and blood plasma. Expensive. Used on beat-sensitive and readily oxidized materials under Liquids under Liquids under Liquids used on pharmaceuticals and related products which cannot be dried successfuby by other means. Applicable to fine chemicals under Granular solids cial cases such as emulsion-coated films under Granular solids... 

Screw conveyor and mdii ect rotaiy. Indirect type, continuous operation Applicable with dry-product recirculation Appbcable with dry-product recirculation Generally requires recirculation of dry product. Little dusting occurs Chief advantage is low dust loss. Web suited to most materials and capacities, particularly those requiring drying at steam temperature Low dust loss. Material must not stick or be temperature-sensitive Not applicable Not appbcable Not appbcable... 

Semibatch or fully continuous operation with continuous removal of a by-product gas is also common. It is an important technique for relieving an equilibrium limitation, e.g., by-product water in an esterification. The pressure in the vapor space can be reduced or a dry, inert gas can be sparged to increase Ai and lower a, thereby increasing mass transfer and lowering u/ so that the forward reaction can proceed. 

Both systems A and B can be used for continuous operation with vacuum locks. In system A (Fig. 2.53), one or two carriers are be moved into a lock (4) in front of several connected tunnels (8). The lock is evacuated and the carrier(s) are moved into the tunnels. At the same time, the equal number of carriers is moved from the tunnels into an exit lock (10). Both locks can be separated from the tunnels by two large slide valves (7). In system B (Fig. 2.54), each tray (1) passes through an entrance lock (2) into a paternoster lift (3), which moves the tray to a certain level and pushes it into the drying zone. The last tray is pushed by the entering tray into the exit paternoster lift (6), which moves the tray into the exit lock (7). These plants are illustrated in Fig. 2.55 and 2.56. 

Figure 2 graphically represents the blending of dry materials at various mixer and chopper speeds by plotting the product temperature, air temperature above the product bowl, and vacuum versus time. The mixer and chopper were in continuous operation with var iable values from the beginning of mixing untiI approximately 13 minutes into the process. At that time, the mixer and chopper were stopped and pulsed (P) for 3 seconds at 50 RPM at various time intervals. 

Another specialty area is coil coating, which involves coating metal coils by continuous operation. Modern roller systems afford speeds of up to 200 m/min. Most coils are made of cold-rolled and surface treated steel, aluminum, or alloys of the latter with manganese or magnesium. Coating systems are based on alkyd or acrylic resins, oil-free polyester, silicone-modified polyester or acrylic resin, poly(vinylidene fluoride), or poly(vinyl fluoride). Water-reducible systems, mainly based on acrylic resins, have been developed for aluminum as well as for steel coils [21-24], Drying is carried out by continuous operation in gas- or oil-heated multichamber ovens. 

Ensuring a constant feed supply is very important, because most of the biomass is only available on a seasonal basis. In such cases continuous operation of the conversion facility will require either extensive long-term storage of the feedstock or a feed reactor that is flexible enough to accommodate multiple feedstocks. Most thermal conversion processes demand a finely divided, substantially dry feed and therefore some pre-treatment is required to match the feedstock to the process. The main pre-treatment operations are [19] ... 

All filters of this type have the distinct advantage of continuous operation and, as a result, the feed and wash liquors may be fed to the equipment at steady rates. Dense, quick settling solids can be handled, and the cake thickness — and the washing and drying times — can be varied independently between quite wide limits. The cake can be flooded with wash liquor, which can be taken away separately and re-used to obtain countercurrent... 

Pumps shall be designed such that the first dry lateral critical speed is at least 20 percent above the continuous operating speed. 

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