Disengaging height

Transport Disengaging Height. When the drag and buoyancy forces exerted by the gas on a particle exceed the gravitational and interparticle forces at the surface of the bed, particles ate thrown into the freeboard. The ejected particles can be coarser and more numerous than the saturation carrying capacity of the gas, and some coarse particles and clusters of fines particles fall back into the bed. Some particles also coUect near the wall and fall back into the fluidized bed. 

Freeboard and Entrainment The freeboard or disengaging height is the distance between the top of the fluid bed and the gas-exit... 

The distance above the catalyst bed in which the flue gas velocity has stabilized is refened to as the transport disengaging height (TDH). At this distance, there is no further gravitation of catalyst. The center-line of the first-stage cyclone inlets should be at TDH or higher otherwise, excessive catalyst entrainment will cause extreme catalyst losses. 

Be sure to leave sufficient disengaging height above demisters, otherwise a healthy derate must be applied. 

Variation of k with Disengaging Height Disengaging Height Above Mesh, Inches Allowable k Value... 

Note V alues based on 12 Ib/cu ft wire mesh. Design practice normally does not c.stceed k of 0.4 even for higher disengaging height. 

Rough correlations have been made of minimum fluidization velocity, minimum bubbling velocity, bed expansion, bed level fluctuation, and disengaging height. Experts recommend, however, that any real design be based on pilot plant work. 

Chan and Knowlton (1984) also investigated the effect of system pressure on the transport disengaging height (TDH) over a pressure range of 1 to 30 bar. They found that TDH increased linearly with pressure over this range (Fig. 15). The effect of gas viscosity on TDH has not been determined experimentally by anyone thus far. 

Cyclones. According to the model presented above, Eq. (24), a minimum loss rate due to cyclone attrition requires to avoid both high inlet velocities Ue and high solids mass fluxes mc m at the cyclone inlet. The latter requirement can be fulfilled by locating the cyclone inlet above the transport disengaging height (TDH) (Kunii and Levenspiel, 1991). In addition, an enlargement of the freeboard section will reduce the amount of particles that are entrained and thus the mass flux, mc in. 

Freeboard and Entrainment The freeboard or disengaging height is the distance between the top of the fluid bed and the gas-exit nozzle in bubbling- or turbulent-bed units. The distinction between bed and freeboard is difficult to determine in fast and transport units (see Fig. 17-6). 

The disengagement height, Lj—the distance between the upper surface of the pad and the top of the drum— should be considered in design, because there will always be some reentrammeni. The higher the l, . the larger Li should be. The table on p. 78 summarizes recommended values for and L4. 

Operating parameters Voiotity "constant Disengagement height. La. in. 

Vapor-phase separation—If the disengagement droplet size. Dp4 is not known, select from the table, and calculate 14 with Eq. (10) and L vvia Eq. (11). Hating calculate the drum diameter. Consult manufacturers on the type and sue of pad. Locate the pad to allow sufficient disengagement height (see the table). 

Freeboard. Under normal operating conditions gas rates somewhat in excess of those for minimum fluidization are employed. As a result particles are thrown into the space above the bed. Many of them fall back, but beyond a certain height called the transport disengaging height (TDH), the entrainment remains essentially constant. Recovery of that entrainment must be accomplished in auxiliary equipment. The TDH is shown as a function of excess velocity and the diameter of the vessel in Figure 6.10(i). This correlation was developed for cracking catalyst particles up to 400 pm dia but tends to be somewhat conservative at the larger sizes and for other materials. 

The values of K in the preceding table are with a standard disengaging height of 10 in. The effect of other heights h is given by the equation... 

Disengaging Height (TDH) Above Bubbling Gas-Fluidized Beds. AlChE Symp. Ser., 80 (241), 24. 

Ammonia is evolved with the coal volatiles and may be therefore partially responsible for the reduction of NO that occurs in AFBC s. Because of the inhibition of free radical reactions within the bed this reaction is expected to be most important in bubbles and in the solid disengaging height above the bed. This importance of the NII3/NO reaction and its partial suppression by bed solids has been demonstrated by the injection of NH3 into... 

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