Fluidization dipleg

The downcomer (or dipleg, or standpipe) through which solids move from bed to bed is a key component of a circulating or a multi-stage fluidized bed. It is often connected at the lower end to a mechanical or non-mechanical valve for controllable transport. The so-called V-valve is a conical (tapered) spout installed at the lower end of a fully fluidized dipleg for controlling... 

Figure 1 presents an isometric view of the pneumatically controlled downcomer. Solids drop by gravity into a fully fluidized dipleg and pass via... 

A fully fluidized dipleg to insure against bridging of solids in descent calls for a relative velocity between the gas and the solids not less than that for incipient fluidization ... 

Kwauk (1963) predicted a state of flooding, in which state the voidage of the fluidized bed increase suddenly, and as a result, the particles in the fluidized dipleg overflow from the top of the dipleg. According to the definition of flooding, it occurs in a particle-fluid system at the following condition ... 

When the values of wd or u0 in the fluidized dipleg reach the calculated values by Eq. (13) or (14), flooding occurs. Hence, the limiting discharge capacity becomes... 

A particular hazard in the operation of non-fluidized diplegs handling cohesive powders is their tendency to bridge (or to arch) unexpectedly, even though the dipleg has been correctly designed for mass flow. Powder material... 

For the annular-fast-bed design shown in Fig. 35a, solid particles elutriated from the fast bed are separated in a multi-inlet cyclone (MIC) they then descend through a mildly fluidized dipleg and are recirculated to the fast bed via a circular V-valve (CV-valve). Fluidizing air is supplied separately to the fast bed and the dipleg. 

A generic multipurpose fluidized bed is illustrated in Figure 2 (1). The soHds are contained in a vessel and gas is introduced into the system via a distributor, which is typically a drilled plate at the bottom of the vessel. A plenum chamber is provided below the distributor plate. The height of the soHds level above the distributor is called the bed height, and the vertical space above the bed height is called the freeboard. A splash zone may exist as a transition between the bed and freeboard. Cyclones, located either in the freeboard or external to the vessel, are used to remove soHds from the gas stream. Diplegs can return entrained soHds directly to the bed. 

The catalyst must be fluidized to provide an effective seal for the diplegs. Fluidization is critical without it, the diplegs cannot discharge the catalyst and will plug, with possible massive carry-over to the main fractionator. To ensure this uniform fluidization, the system uses an additional steam distributor. 

Bed components in a dense-phase fluidized bed include the gas distributor, cyclone, dipleg, heat exchanger, expanded section, and baffles, as shown in Fig. 9.4. The basic function of each component in a fluidized bed is described in the following. 

The cyclone separates solid particles from the outlet gas. Several cyclones may be combined to form a multistage cyclone system. More than one multistage cyclone system can be placed inside or outside the bed. Other types of gas-solid separators (e.g., filters) are also used for gas-solid separation although the cyclone is most widely adopted in fluidized beds. The dipleg/standpipe returns the particles separated by the cyclone into the dense bed. The outlet of a dipleg may be located in the freeboard or immersed in the dense bed. 

The uptransport spout is often designed with a trapezoidal cross-section, as shown in Fig. 34. A simplified analysis (Kwauk, 1974) will be given on the basis that the dipleg operates under the condition of incipient fluidization and the spout in moving-bed uptransport. 

For fine Geldart Group C powders that defluidize slowly, it has been shown experimentally that solids continued to be transported through the downcomer even when the fluidizing gas to the dipleg was turned off. The particulate solids move in the dense phase. This operation will be designated self-flow (Kwauk, 1974a). Under this condition, the total gas rate is zero ... 

Determination of the dimensions of the fluidized spout is based on operation experience. According to Li et al. (1982), the cone angle, 6, for smooth fluidized flow in the conical section may be taken as 6 = 10°. The minimum thickness of the spout is set by the requirement that the spout base area should not be less than the cross-sectional area of the interconnecting aperture. The diameter of the dipleg should be greater than four times the equivalent diameter of the aperture. These will ensure an aperture restriction capable of controlling solids flow through the valve. 

For the dipleg which is required to operate at incipient fluidization,... 

For practical gas and solids rates, this is possible for very fine powders, for which the terminal velocity ut is small. What happens in self-flow is that the dropping solids fed to the dipleg carry and pump down the surrounding gas, and this compressed gas carries, in turn, the solids up the spout. This is also possible for fine powders, which do not have a sharp incipient fluidization velocity, thus preventing quick defluidization. Self-flow therefore occurs in the band marked in Fig. 13 in the vicinity of e = 1. 

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