Circulating fluidized beds downcomer

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... 

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 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. 

With the circulating fluidized bed used here, solid circulation measurements were carried out up to a maximum solid flux of about 120 kg/m s exceeding this value the downcomer gradually filled with particles and thus setting a limit to the measurements. 

Most fluidized bed partial oxidation processes are operated in the turbulent flow regime of fluidization. However, DuPont operated a circulating fluidized bed catalytic reactor process for maleic anhydride production in Spain, featuring regeneration of the catalyst (by oxidation) on the downcomer side of the circulating system. 

Fig. 10.5. A schematic representation of a circulating fluidized bed. The CFB loop consists of a riser, gas-solid cyclone separators, standpipe type of downcomer, and a non-mechanical solids flow control device. Reprinted from [82] with permission from...

Recently, Pallares and Johnsson [106] presented an overview of the macroscopic semi-empirical models used for the description of the fluid dynamics of circulating fluidized bed combustion units. They summarized the basic modeling concepts and assumptions made for each model together with the major advantages and drawbacks. In order to make a structured analysis of the processes involved, the CFBC unit is often divided into 6 fluid dynamical zones like the bottom bed, freeboard, exit zone, exit duct, cyclone and downcomer and particle seal, which have been shown to exhibit different fluid dynamical behavior. 

A circulating fluidized bed ean also be constructed in a concentric configuration as an internally circulating fluidized bed. The riser is usually located in the interior with the downcomer as an outer armulus (e.g., Fusey el al., 1986). Such geometries simplify construction, facilitate direct heat exchange between the riser and the downcomer, and simplify the design of high-pressure systems. 

Wirth KE. Fluid mechanics of the downcomer in circulating fluidized beds. Proceedings of the Eighth International Conference on Fluidization, Tours, France, 1995. 

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... 

In the CFB loop the pressure drop over the riser must be balanced by that imposed by the flow through its accompanying components such as the downcomer and the recirculation device. The pressure drops across the downcomer, the solids circulation and control device, and the riser are the major elements in the pressure balance around the CFB loop. The pressure drop balance is thus such that the sum of the control device, fluidized bed riser and cyclone... 

The schematic for this system, where the draft tube is operated as a fluidized bed rather than a dilute phase pneumatic transport tube, is shown in Fig. 5. A mathematical model for the system was developed by LaNauze (1976). The driving force for solids circulation in this case was found to be the density difference between draft tube and downcomer. The solids circulation rate was also found to be affected only by the distance between the distributor and the draft tube and not by the draft tube length or height of bed above it. Because of the lower velocity in the draft tube, the... 

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