Granular Beds of Particulate Solids

Granular Beds of Particulate Solids Beds of solids like sand or coal are used as filter media to clarify water or chemical solutions containing small quantities of suspended particles. Filter-grade grains of desired particTe size can be purchasea. Frequently beds will be constructed of layers of different materials and different particle sizes. 

Rather than use a cloth, a granular medium consisting of layers of particulate solids on a support grid can be used. Downward fiow of the mixture causes the solid particles to be captured within the medium. Such deep-bed filters are used to remove small quantities of solids from large quantities of liquids. To release the solid particles captured within the bed, the flow is periodically reversed, causing the bed to expand and release the particles which have been captured. Around 3 percent of the throughput is needed for this backwashing. 

The basic fluid-bed unit consists of a refractory-lined vessel, a perforated plate that supports a bed of granular material and distributes air, a section above the fluid bed referred to as freeboard, an air blower to move air through the unit, a cyclone to remove all but the smallest particulates and return them to the fluid bed, an air preheater for thermal economy, an auxiliary heater for start-up, and a system to move and distribute the feed in the bed. Air is distributed across the cross section of the bed by a distributor to fluidize the granular solids. Over a proper range of airflow velocities, usually 0.8-3.0 m/s, the solids become suspended in the air and move freely through the bed. 

Removal of suspended solids from a liquid may be required, either to recover valuable products or to remove obnoxious particulates. Clarification of dilute aqueous suspensions is frequently performed in deep-bed granular filters, where the grains provide a surface on which the particles may adsorb. If two types of particles differ in their adsorption rate, separation of them may be accomplished by selectively adsorbing one species on the grains. Furthermore, reversibility of adsorption would... 

Fluidized bed dryers (FBDs) have found widespread applications for the drying of particulate or granular solids in the chemical, food, ceramic, pharmaceutical, agriculture, polymer, and waste management industries. More recently, they have also found special applications in the drying of slurries. Suspensions, solutions, dilute pastes, or sludges are atomized into a fluidized bed of inert particles and the dry powder is separated from the exhaust gases. 

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