Fluidisation point

Substituting e = emf at the incipient fluidisation point and for —AP from equation 6.1, equation 6.6 is then applicable at the minimum fluidisation velocity umf, and gives ... 

The minimum fluidising velocity, umf, may be expressed in terms of the free-falling velocity o of the particles in the fluid. The Ergun equation (equation 6.11) relates the Galileo number Ga to the Reynolds number Re mj in terms of the voidage < , / at the incipient fluidisation point. 

Numerical values for the sphericity are contained in the VDI-Warmeatlas [3.41] for example, fire-blasted sand has +s = 0.86. mf is the void fraction at the fluidisation point. 

The mass of the solid is the same at the fluidisation point as the operating point. It follows from this that... 

The fill height is the same as the height at the minimum fluidisation point Hmf = 1 m, from which H = 2.22 m. With (3.285) we then have... 

The height Hmt of a fluidised bed at the fluidisation point follows from the condition of constant sand mass... 

Fluidised bed This process is used for powder coating Basically, the equipment consists of a dip tank with a perforated shelf near the bottom. The powder is placed on this shelf and low pressure air is fed under the perforated shelf, resulting in a cloud of fine powder in the body of the dip tank. The article is heated to a little above the melting point of the powder and is then dipped into the fluidised bed for a short period. It is then withdrawn... 

However, on die basis of the relation between pressure drop and die minimum fluidisation velocity of particles, the point of transition between a packed bed and a fluidised bed has been correlated by Ergun41 using (17.7.2.3). This is obtained by summing the pressure drop terms for laminar and turbulent flow regions. 

Details are given of the pyrolysis of plasties waste with emphasis given to the use of a heated fluidised bed reaetor. Data are given for the pyrolysis eonditions of mixed plasties as well as gas eomposition and high- and low-boiling point fraetions. 20 refs. 

As the upward velocity of flow of fluid through a packed bed of uniform spheres is increased, the point of incipient fluidisation is reached when the particles are just supported in the fluid. The corresponding value of the minimum fluidising velocity (umf) is then obtained by substituting emf into equation 6.3 to give ... 

When the flow regime at the point of incipient fluidisation is outside the range over which the Carman-Kozeny equation is applicable, it is necessary to use one of the more general equations for the pressure gradient in the bed, such as the Ergun equation given in equation 4.20 as ... 

Niven(7) discusses the significance of the two dimensionless groups in equations 6.17 and 6.18, and also suggests that d and umf in equations 6.8, 6.9 and 6.10 are more appropriately replaced by a mean linear dimension of the pores and the mean pore velocity at the point of incipient fluidisation. 

On the assumption that equation 6.31 may be applied at the point of incipient fluidisation ... 

The formation of bubbles at orifices in a fluidised bed, including measurement of their size, the conditions under which they will coalesce with one another, and their rate of rise in the bed has been investigated. Davidson el alP4) injected air from an orifice into a fluidised bed composed of particles of sand (0.3-0.5 mm) and glass ballotini (0.15 mm) fluidised by air at a velocity just above the minimum required for fluidisation. By varying the depth of the injection point from the free surface, it was shown that the injected bubble rises through the bed with a constant velocity, which is dependent only on the volume of the bubble. In addition, this velocity of rise corresponds with that of a spherical cap bubble in an inviscid liquid of zero surface tension, as determined from the equation of Davies and Taylor ... 

From a practical point of view, one of the most challenging applications to date has been in the experimental fluidised bed shown in Figure 3, which is a scaled-down version of a polyethylene production unit, of overall height approximately 2 m, with a conical expansion section 1 m above the distributor the diameter of the lower section is 0.154 m. The bed is constructed of 316 stainless steel and is placed within a gas circulation loop allowing operation at pressures up to 20 bars. 

Ash related problems such as agglomeration occur, and exchange of the sand bed may be needed after some interval (Bridgwater, 2003). One way to overcome some problems with agglomeration is to use a mixture of biomass and peat in order to increase the melting point of the raw material in the furnace. It is also possible to use lime (contains CaO, Ca, etc.) instead of quartz in the fluidised bed reactor to improve the agglomeration temperature in the combustion atmosphere of some biomass (Natarajan et al., 1998). 

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