Fluidized beds solid flow pattern

As mentioned in Section 11.3, fluidized-bed reactors are difficult to scale. One approach is to build a cold-flow model of the process. This is a unit in which the solids are fluidized to simulate the proposed plant, but at ambient temperature and with plain air as the fluidizing gas. The objective is to determine the gas and solid flow patterns. Experiments using both adsorbed and nonadsorbed tracers can be used in this determination. The nonadsorbed tracer determines the gas-phase residence time using the methods of Chapter 15. The adsorbed tracer also measures time spent on the solid surface, from which the contact time distribution can be estimated. See Section 15.4.2. 

Werdmann, C. C., and Werther, J., Solids Flow Pattern and Heat Transfer in an Industrial Scale Fluidized Bed Heat Exchanger, Proc. 12th Intern. Conf. on Fluid. Bed Comb., 2 985-990 (1993)... 

In circulating fluidized beds two main attrition sources, namely the riser and the return leg, may be distinguished. Although a lot of information is available about solids flow patterns and flow structures inside the circulating fluidized bed risers, no systematic investigations have been found in the open literature on the influence of riser geometry and flow conditions inside the riser on attrition. With respect to attrition occurring in the return leg, the work of Zenz and Kelleher (1980) on attrition due to free fall may be mentioned (cf. Sec. 4.3). 

Bader, R., Findlay, J. and Knowlton, T. M. (1988). Gas-Solids Flow Pattern in a 30.5cm Diameter Circulating Fluidized Bed. In Circulating Fluidized Bed Technology II. Ed. Basu and Large. Toronto Pergamon Press. 

Donsi, G., and L. Sesti Osseo. Gas Solid Flow Pattern in a Circulating Fluid Bed Operated at High Gas Velocity," in Circulating Fluidized Bed Technology IV (Amos A. Avidan, ed.), pp. 696-701. Somerset, Pennsylvania (1993). 

In fast fluidized beds, a proper modification to the suspension emissivity is usually needed according to the gas-solids flow pattern, such as the following correlation originally proposed by Grace (1982) ... 

Continuous fluid beds may be even more varied than batch fluid beds. The main distinction between continuous fluid beds will be according to the solids flow pattern in the dryer. The continuous fluid bed will have an inlet point for moist granular material to be dried and an outlet for the dried material. If the moist material is immediately fluidizable, it can be introduced directly onto the plate and led through the bed in a plug-flow pattern that will enhance control of product residence time and temperature control. If the moist granular material is sticky or cohesive due to surface moisture and therefore needs a certain degree of drying before fluidization, it can be handled by a backmix fluid oed, to be described later. 

The spouted bed technique has become established as an alternative to fluidization for handling particulate solids that are too coarse and uniform in size for good fluidization. Although the areas of application of spouted beds overlap with those of fluidized beds, the flow mechanisms in the two processes are very different. Agitation of particles in a spouted bed is caused by a steady axial jet and, as compared with the more random and complex bubble-induced particle flow patterns in most fluidized beds, is regular as well as cyclic. 

In an early study Rowe and Partridge [115] (see also [116]) did show that gas fluidized beds are characterized by the formation of bubbles which rise through denser bed zones of the bed and determine a gross scale gas and solid flow pattern. 

A very complex solids flow pattern will result when solid obstacles exist in the fluidized bed. The solids recirculation pattern in a cylindrical bed with a single sphere was presented by Lin, Chen, and Chao (1985), and in a 2-D bed with a single and multiple cylinders by Ai (1991). It was demonstrated that large obstacles would not only affect the local solids velocity, but also the global solids circulation patterns. 

Werdermann CC, Werther J. Solids flow pattern and heat transfer in an industrial scale fluidized bed heat exchanger. Proc 12th Intern Conf on Fluid Bed Combustion 2 985-990, 1993. 

Bader R, Findlay J, Knowlton TM. Gas solid flow patterns in a 30.5-cm-diameter circulating fluidized bed. In Basu P, Large JF, eds. Circulating Fluidized Bed Technology II. Oxford Pergamon Press, 1988, pp. 123-137. 

Hage B, Werther J. The guarded capacitance probe—a tool for the measurement of solids flow patterns in laboratory and industrial fluidized bed combustors. Powd Tech 93 235-245, 1997. 

Some recent applications of tracers in fluidized beds include measurement of solids flow patterns (163), evaluation of the RTD in an entrained flow gasifier (164), study of lateral solids mixing in a packed fluidized bed (165), investigation of gas distribution (166) and gas and solids mixing (167). 

Figure 27 Gas-solid flow patterns in fluidized beds with a tube array (A) for different times when a=45° and (B) for different settings when t=6.0 s. The settings include a square one a=0) and three triangular ones ( =30°, 45°, and 60°) from the left to right (bed width x thickness x height= 10 mm x 0.4 mm x 128 mm dp=0.1 mm,/ p=1440 kg/ m, tube diameter=40 mm). The tube material Is copper. Reprinted from Hou et al. (2015b) with permission from Elsevier.

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