Gas dispersion axial

According to bed voidage, which can be calculated from gas velocity and solids circulation rate by using Eqs. (32) to (38), the axial gas dispersion coefficient can thus be easily obtained. 

This equation indicates that axial gas dispersion coefficient increases with gas velocity and solids circulation rate. This dependence of axial dispersion coefficient on gas velocity is at variance in the available literature. 

Lou, G., and Yang, G. Axial gas dispersion in a fast fluidized bed (in Chinese), Proceedings of Fifth National Conference on Fluidization, Beijing, China, April 9-13, pp. 155-158 (1990). 

Radial and axial dispersion measurements in risers were first reported by van Zoonen [71]. With hydrogen gas as a tracer he found that radial dispersion coefficients ranged between 2.5 and 36 cmVs and axial gas dispersion coefficients were between 4,500 and 14,400 cmVs. 

Bader et al. studied both radial and axial gas dispersion using steady-state helium tracer injection [69]. Radial dispersion coefficients ranged between 25 and 67 cmVs. Axial and radial dispersion coefficients were fit to the exact solution of the following differential equation ... 

Figure 13 Influence of gas velocity on the axial gas dispersion in a turbulent fluidized bed. (From Wei et al., 2000a.)...

The axial gas dispersion tends to increase with column diameter, as shown above. If both gas and solids... 

Luo G, Yang QL. Axial gas dispersion in fast fluidized beds. Proc 5 National Fluidization Conference, Beijing, 1990, pp 155-158. 

Comparison of steady-state profiles shows that neglecting axial mass diffusion has very little effect on the temperature and concentration profiles even though the axial gradients are significant. However, Figure 16 shows that neglecting the axial thermal dispersion in the gas does affect the solution... 

For gas velocities ranging 4.1 to 6.3 m/s, the measured axial solids dispersion coefficients Ds varied from 0.39 to 0.91 m2/s for dilute solid suspension, while for the whole fast bed, in the same gas velocity range, Ds was 0.22 to 1.67 m2/s. Axial solid dispersion coefficient generally increases with both gas velocity and solids circulation rate. Milne and Berruti (1991) have also given their own model for solids mixing. 

There exists both axial and radial gas diffusion in the CFB combustor, the radial gas dispersion coefficient being much smaller than the axial. 

Therefore, for coal burning, neglecting radial gas dispersion and radial heterogeneity of solids concentration might be acceptable. Gas-solid flow can thus be simplified to a one-dimensional model with axial distributions only. 

In this paper the gas mixing in a CFB biomass gasifier is discussed on basis of radial and axial gas concentration profiles measurement in the pilot plant of ECN (100 kglhr fitel). The standard dispersion model is applied to interpret the experimental results. 

A stirred tank fermenter consists of a centrally mounted agitation system inside a cylindrical vessel. Typically, the agitation system is composed of either multiple radial flow impellers (see Fig. 4A) or a combination of radial and axial flow impellers as shown in Fig. 4B. A gas sparger is located below the radial gas dispersing impeller. The role of the bottom radial impeller... 

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