Fluidization downcomer

The important design parameters for a recirculating fluidized bed with a draft tube were identified by Yang and Keaims (1978a) as the gas bypassing characteristics of the distributor plate, the area ratio between the downcomer and the draft tube, the diameter ratio between the draft tube and the draft tube gas supply, the distance between the distributor plate and the draft tube inlet, and the area ratio of the draft tube gas supply and the concentric solids feeder. The design and operation of a recirculating fluidized bed with a draft tube are discussed below. 

The pressure balance for the dense phase in the downcomer in the circulating fluidized system shown in Fig. 2 can be expressed as ... 

It was experimentally confirmed, using capacitance probes, that the solids flow down the downcomer at close to minimum fluidization, thus ebd = 0. 

Downcomer and Draft Tube Pressure Drop. Typical experimental pressure drops across the downcomer, AP, 4, and the draft tube, AP2 3, show that they are essentially similar. Successful design of a recirculating fluidized bed with a draft tube requires development of mathematical models for both downcomer and draft tube. 

Downcomer Pressure Drop. When the downcomer is less than minimally fluidized, the pressure drop can be estimated with a modified Ergun equation substituting gas-solid slip velocities for gas velocities (Yoon and Kunii, 1970), as shown in Eq. (9). 

When the downcomer is fluidized, the downcomer pressure drop can be calculated as in an ordinary fluidized bed as ... 

A straight line relationship between LJpr and Gr as shown in Fig. 8 implies that the volumetric solid loading (j) is approximately constant because Ar is constant and er can be assumed to be approximately constant when the downcomer is not fluidized. More than 85% of the gas supplied through the central No. 7 and No. 8 flows in those experiments ends up in the draft tube as can be seen from the gas bypassing data presented in Fig. 7. 

The results of an example calculation for a recirculating fluidized bed coal devolatilizer of 0.51 m in diameter handling coal of average size 1200 pm at 870°C and 1550 kPa are presented in Fig. 11. The calculation is based on operating the fluidized bed above the draft tube at 4 times the minimum fluidization velocity. It is also based on the selection of a distributor plate to maintain the downcomer at the minimum fluidization condition. If the two-phase theory applies, this means that the slip velocity between the gas and the particles in the downcomer must equal to the interstitial minimum fluidizing velocity as shown below. 

A circulating fluidized bed (CFB) photoreactor and installation of the UV light lamps is shown in Fig. 1. The CFB photoreactor composed of a riser (13 mm-i.d. x 1200mm-high, Quartz glass), cyclone, downcomer... 

The riser, gas-solid separator, downcomer, and solids flow control device are the fdur integral parts of a CFB loop. The riser is the main component of the system. In the riser, gas and solids commonly flow cocurrently upward, although they can also flow cocurrently downward. This chapter covers only the cocurrent gas and solid upward operation. In this operation, as shown in Fig. 10.1, the fluidizing gas is introduced at the bottom of the riser, where solid particles from the downcomer are fed via a control device and carried upward in the riser. Particles exit at the top of the riser into gas-solid separators. Separated particles then flow to the downcomer and return to the riser. 

Maintaining the downcomer or a standpipe under a moving bed condition allows a large pressure buildup along the downcomer. For a given quantity of particles in the downcomer, the pressure at the bottom of the downcomer is closely associated with the relative velocity between the gas and particles. The pressure drop rises with the relative velocity as the particles are in a moving packed state. The maximum pressure drop in the downcomer is established when particles are fluidized, a state which can be expressed in terms of the pressure drop under the minimum fluidization condition as... 

D. Pressure Drop Through the Solids Flow Control Devices The solids flow in the downcomer can be either in a dense fluidized state or in a moving packed state. If the particles in the downcomer are fluidized, the pressure drop through the mechanical solids flow control devices can be expressed as [Jones and Davidson, 1965]... 

Solution The maximum solids circulation rate through an L-valve is limited by the pressure drop across the downcomer under minimum fluidizing conditions. Thus, the pressure balance can be written in terms of voidage and solids inventory in the downcomer as (see Fig. 10.6)... 

Of primary significance is the location of the inflection point zi which is closely associated with the solids quantity in the downcomer and solids circulation rate in the CFB system. Consider the pressure balance across the circulating fluidized bed loop as given by... 

The pressure drops in Eq. (10.16) can be obtained by assuming that the particles in the downcomer are in the incipient fluidization state. Neglecting the solids holdup in the connecting tube between the riser and the cyclone and in the cyclone, the mass balance of solid particles in the CFB loop can be expressed as... 

The fluidized bed reactor has been used for phenol removal instead of fixed bed as most of the products formed are insoluble. The operation in packed bed reactors would lead to clogging phenomena and undesirable pressure drop [47, 88]. When deactivation of biocatalysts occurs and regeneration is needed, the liquid-solid circulating fluidized bed is a worthy alternative, as demonstrated for phenol polymerization [89]. The continuous enzymatic polymerization was carried out in a riser section and a downcomer was used for the regeneration of the coated immobilized particles. 

For successful operation of a multistage fluidized leacher/washer, the downcomer to transfer solids from any stage to the one below needs to offer sealing power against liquid bypassing and to insure steady flow of the... 

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 ... 

Various configurations have been developed for muitistaging of fluidized beds using the pneumatically controlled downcomer, as shown in Fig. 38. Figure 38A shows the fundamental concept of multistaging of fluidized beds, with a downcomer between two adjacent beds, each downcomer being supplied with a separate stream of control gas. Consequently, there are as many control gas streams as there are stages. 

Fig. 38. Configurations for multistage fluidized beds using the pneumatically controlled downcomer. [After Kwauk, 1974 Liu and Kwauk, 1980 Liu et al., 1981.]...

Kwauk, M., Particulate Fluidization in Chemical Metallurgy, Sdentia Sinica 16,407 (1973). Kwauk, M., Dense-Phase Transport Downcomers (in Chinese, unpublished), Inst. Chem. Metall., 1974-1-9, rev. 1977-2-13 (1974a). 

Fig. 5. Types of apparatus and their respective effects on axial voidage profile. 1, fast fluidized bed 2, intermediate hopper 3, cyclone 4, slow fluidized bed 5, downcomer 6, solids rate controlling device 7, solids rate measurement device 8, suspension section.

Fig. 12. Axial variation of appearance for clusters in a two-dimensional fast fluidized bed (after Bai et al., 1991). 1, riser 2, distributor 3, two-dimensional bed 4, cyclone 5, butterfly valve 6, downcomer.

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... 

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