(163j) Comparison Study of Riser Geometry Effect On the Solids Distribution in CFBs

The solids distribution was studied in a rectangular riser operated under a wide range conditions. Both axial and radial profiles of solids holdup show that the operating condition plays an important role in influencing the flow structure and control the flow in the rectangular riser under the same manners as that in cylindrical risers: increasing U_g and reducing G_s result in a lower solids holdup. The solids concentration profile, within the range of this study, remains low at the riser centre throughout the whole riser compared with the solids holdup in the wall region. Comparing the rectangular riser with the other cylindrical columns, it is found that the general shapes of the axial and radial profiles of solids holdup in rectangular riser are quite similar to that in cylindrical risers, but more uniform. The cause for this unusual distribution of solids holdup in current rectangular riser has been summarized into two effects, scale-up effect and the column shape effect. To the in-depth understanding，the perimeter per unit cross-sectional area is an important factor contributing significantly to the two effects mentioned above and mainly affect the solids distribution on the surface wall. In larger columns, there will be less wall surface available for the particles buildup and more particles move slowly, downwards, or stagnantly to accumulate on the wall surface to increase wall coverage. Consequently, the cross-sectional average of solids holdup is higher with the riser size increasing even under the same solids circulation rate which is a net flux escaping from the riser. This can also be agreed with the axial and radial profile of solids holdup. Thus, at the specified solids circulation rate and gas velocity, the axial solids holdup increases with the riser size increase, so as the radial solids holdup does. As a result, the perimeter per unit cross-sectional area is the main reason causing the reducing of the solids accumulation on the wall and lowering the solids holdup closer to the wall. From the other aspect, the distance between the wall and the riser centre also plays an important role in determining the distribution of solids holdup, especially in the centre region of the riser. The gas distribution is restricted by the distance between the wall and the centre since the resistance to the gas is mainly encountered at the wall due to the wall friction. Consequently, the gas velocity on the face walls is slower than in the centre. Yan (2004) also stated that the greater the distance between the wall and the centre under the same superficial gas velocity and solids circulation rate, the bigger the difference of gas velocity in these two regions becomes. Therefore, as a rectangular column, the two face walls are so close to each other that there is no significant difference between the gas velocity on the face walls and in the centre. As moving with the gas, the motion of particles on the face walls is as fast as in the centre. As a result, the solids tend not to cling to the face wall but to distribute uniformly along the depth direction. For the side walls, though they are further to the centre of the column, the area is so small that the amount of particles which are contacting with the side walls is small as well. Therefore, the rectangular geometry leads to increase the solids concentration in the centre region. It is concluded that the geometry of the riser is a factor affects the solids distribution due to the perimeter per unit cross-sectional area and the distance from the wall to the centre. These two factors result in the more uniform radial and axial distributions of solids holdup in the rectangular riser than that in three-dimensional cylindrical risers. It is reasonable to study the scale-up effect by comparing the perimeter per unit cross-sectional area, which makes lower solids holdup on the side walls in rectangular riser and less particles aggregation. The distance between the face walls and the centre of the rectangular riser is a factor contributing to the uniform solids distribution along the depth direction and is the mainly reason causing the solids holdup to be higher in the centre. In some extent, the two-dimensional and three-dimensional risers are more comparable under fast fluidization conditions since the factors from bubbles have been lessened. Generally, the solids distribution along the axial and the radial direction in rectangular riser are close to that in cylindrical risers, and the main differences have been known by the study of this paper. Reference [1] Qi, X.-B., Huang, W.-X. and Zhu, J., 2008. Comparative study of flow structure in circulating fluidized bed risers with FCC and sand particles. Chemical Engineering Technology 31, 542-553. [2] Yan, A., 2004. Hydrodynamics and scale-up of circulating fluidized beds. In: Chemical Engineering, The University of Western Ontario, London, Ontario, Canada, pp. 6-14.