An introduction to fluidization

Consider a bed of particulate solids or powder, say of a size similar to table salt. When a fluid, either a gas or a liquid, is passed upwards through the bed, the bed particles remain stationary or packed at low fluid velocities. This is a packed or fixed bed. If now the velocity of the fluid is increased, fhe particles will begin to separate and move away from one another the bed is said to expand. On increasing the velocity further, a point will be reached at which the drag force exerted by the fluid on a particle is balanced by the net weight of the particle. The particles are now suspended in the upward-moving stream of fluid. This is the point of minimum fluidization, or incipient fluidization, at and beyond which the bed is said to be fluidized. 

Thus fluidization is a technique which enables solid particles to take on some ot the properties ot a fluid. For example, solids fluidized by a gas will adopt the shape ot the container in which they are held and can be made to flow, under pressure, trom an oritice or overflow a weir. It the wall ot the bed is punctured by a series ot apertures aligned vertically the fluidized solids will behave just as it the bed were tilled with liquid a stream ot solids will issue trom each aperture, that trom the highest point in the bed will travel only a short horizontal distance whereas the stream trom the lowest aperture will travel turthest. Gibilaro (2001) reters to a demonstration rig in the Department ot Chemical Engineering at UCL in which a plastic toy duck buried in a bed ot sand exchanges place with a brass duck on the bed surtace when the bed is fluidized with air.  

It is possible to operate a fluidized bed in either batch or continuous mode. Strictly, most batch applications are in tact operated in semibatch mode where the solids are treated as a batch but the fluidizing medium enters and leaves the bed continuously. In the case ot gas-solid beds used in termentation (see Chapter 6), the fluidizing gas is recirculated although reactants and products flow continuously. In true continuous operation the solids may be ted into a fluidized bed via screw conveyors, weigh teeders or pneumatic conveying lines and can be withdrawn trom the bed via standpipes or by flowing over weirs. 

Essentially aggregative fluidization is a two-phase system there is a dense phase (sometimes reterred to as the emulsion phase), which is continuous, and a discontinuous phase called the lean or bubble phase. The simplitied assumption that all the gas over and above that required tor minimum fluidization flows up through the bed in the form ot bubbles is known as the two-phase theory. It the total volumetric flow ot gas is Q then 

Wilhelm and Kwauk (1948), in one of the earliest papers on fluidization, suggested that the Froude number, written for the minimum fluidizing condition, could be used to indicate the prevalent fluidization behaviour. Thus 

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An introduction to fluidized beds is given in Levenspiel s book quoted above and in his forthcoming ... 

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