Fluidized material

Flooding. When a stable rathole forms in a bin and fresh material is added, or when material falls into the channel from above, a flood can occur if the bulk sohd is a fine powder. As the powder falls into the channel, it becomes entrained in the air in the channel and becomes fluidized (aerated). When this fluidized material reaches the outlet, it is likely to flood from the bin, because most feeders are designed to handle sohds, not fluids (see Eluidization). Fimited Discharge Kate. Bulk sohds, especially fine powders, sometimes flow at a rate lower than required for a process. This flow rate limitation is often a function of the material s air or gas permeabihty. Simply increasing the speed of the feeder does not solve the problem. There is a limit to how fast material... 

Industrial, centrifugal elevators usually operate at speeds of about 75 m /min, and handle free-flowing, fine and loose materials having lump sizes of <50 mm. Sticky material can be a problem. Fine fluidizing materials often require perforations in the bottom of the buckets to vent entrapped air. Centrifugal elevator capacities range up to 370 m /h for a single row of buckets, and up to 1400 m /h for multiple rows of buckets. The buckets can be mounted on a belt or chain. 

The process of sublimation is analogous to the drying of solids so much the same kind of equipment is usable, including tray dryers (Fig. 9.6), rotary tray dryers (Fig. 9.8), drum dryers [Fig. 9.11(b)], pneumatic conveying dryers (Fig. 9.12), and fluidized beds (Fig. 9.13). The last of these requires the subliming material to be deposited on an inert carrier which is the fluidized material proper. 

The liquid and/or suspension load of the fluidizing material is locally independent. 

Fig. 7.9. Batch fluid bed spray granulator used to produce tablet granulations in the pharmaceutical industry. Air-flow necessary for fluidization is generated by a suction-fan (2) mounted in the top portion of the unit, directly driven by an electric motor. The air being used is heated to the desired temperature by an air heater (5). Prefilters remove all impurities at the air inlet (6). The material to be processed has been loaded into the material container (1). The container bottom consists of a perforated plate above which a fine mesh stainless steel retaining screen is fitted. Exhaust filters (7) mounted above the product container retain fines and dust. The granulating liquid (3) is sprayed as a fine mist through a mechanical or pneumatically actuated nozzle (4) onto the finely dispersed, fluidized material to form the desired agglomerates. (Courtesy Aeromatic AG.)...

During the late 1970s, Union Carbide developed a low-pressure polymerization process (Unipol process) capable of producing polyethylene in the gas phase that required no solvents. The process employed a chromium based catalyst. In this process (Figure 4.1) ethylene gas and solid catalysts are fed continuously to a fluidized bed reactor. The fluidized material is polyethylene powder which is produced as a result of polymerization of the ethylene on the catalyst. The ethylene, which is recycled, supplies monomer for the reaction, fluidizes the solid, and serves as a heat-removal medium. The reaction is exothermic and is normally run at temperatures 25-50°C below the softening temperatures of the polyethylene powder in the bed. This operation requires very good heat transfer to avoid hot spots and means that the gas distribution and fluidization must be uniform. 

Material is a powder that tends to be readily adhesive or becomes easily fluidized. Examples are adhesive materials such as foundry sand, pigments fluidized materials such as hydrated lime cement, talcum powder, confectionary sugar. Size —325 mesh. 

It should be noted that no screw form or length of choke section would prevent the escape of a fully fluidized material. Apart from the essential tip clearance space between the flight and the casing, there is also an unrestricted channel of flow around the helix form of the screw flight. This channel will allow the passage of a fluid-like bulk material, which is invariably under the pressure of a hydrostahc head if the material at the base of the stored contents is in a fluid state. It is quite impressive to witness a fine powder product squirting from any pinhole, crevice, or non-watertight... 

Fluidized material Particulate solid Paste/slurry sprayed onto a bed of inert particles Slurry sprayed onto absorbent particles (e.g., silica gel, biomass, etc.)... 

An important characteristic of any fluidizable material is its minimum fluidization velocity, that is the superficial velocity needed to "lift the particles. It appears that, for fine particles, this is mostly determined by the viscosity of the fluid, and by the density difference between the solid and the fluid. That is so because the flow between the small particles, at these low flow rates, is e ssentially laminar. We can use eq. (4.36), replace Ap by the weight of the bed per unit area Ap (1 - e) g /, and neglect the turbulence term. The minimum fluidization velocity u, is then... 

The intermediate material was placed into a quartz tube contained in a furnace and a stream of dry air or dry nitrogen was used to fluidize the material in the quartz tube. Then the fluidized material was calcined for several hours at 200-700°C, after which the column was cooled to ambient conditions and the calcined material was stored in an inert atmosphere until needed. 

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