Pneumatic Transport and Standpipes

Preheater. Figure 2 is a schematic of the preheat section. Ground raw shale is allowed to slide into or is pneumatically transported (a minimal volume flow) into the lower part of the preheater. A standpipe of shale serves as a resistance seal to purging gas and allows the preheater to be pressurized by transporting gas. Although a slide valve near the bottom of the standpipe should provide adequate flow control for the shale, a flapper valve, screw feeder, or rotary lock may be considered as options. 

As the gas velocity is increased beyond 17 a point is reached at which the particles are transported out of the bed altogether either in dense-phase or in dilute-phase pneumatic transport the velocity at which this occurs is called the transport velocity, C/tr- In order to maintain a constant inventory of particles in the bed at velocities in excess of 7tr it is necessary to recycle them via external cyclones and a standpipe, a geometry known as a circulating fluidized bed. A comprehensive review of the literature on circulating systems has been given by Grace et al. (1997), and the subject is also dealt with in Chapter 19 of this handbook. 

Knowlton and Hirsan (1978) established that the best position for the injection of the aeration gas is in the vertical leg (sometimes called the downcomer or standpipe) about 1.5 pipe diameters above the centreline of the horizontal leg. This is because gas is needed to reduce friction between the particles and the pip>e at the inside comer of the elbow, and if the gas is injected on or below the centreline, less is available to perform this function. This gas then streams along the top of the bed of powder lying in the horizontal leg carrying particles with it in a mode of transport that resembles dense phase pneumatic conve5ung. As the aeration gas flow is increased the depth of the flowing stream of powder increases and the mass flow rate of solids increases. Eventually the whole depth of powder may become active with the solids velocity increasing from bottom to top of the horizontal leg. 

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